Light-induced oscillating topographies in liquid crystal ... · Surface topographies and reliefs...

Light-induced oscillating topographies in liquid crystal coatings

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https://research.tue.nl/en/publications/6fe87d9c-1cdb-4266-a1c7-861f99abfb2b

Light-Induced Oscillating Topographies

in Liquid Crystal Coatings

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit

Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. F.P.T. Baaijens

voor een commissie aangewezen door het College van Promoties, in het

openbaar te verdedigen op woensdag 17 oktober 2018 om 13.30 uur

door

Matthew Hendrikx

geboren te Leuven, België

Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de

promotiecommissie is als volgt:

voorzitter: prof.dr.ir. E.J.M. Hensen

1e promotor: prof.dr. D.J. Broer

2e promotor: prof.dr. A.P.H.J. Schenning

leden: dr. D. Liu

prof.dr. E.W. Meijer

prof.dr. J.M.J. den Toonder

prof.dr. O.D. Lavrentovich (Kent State University)

prof.dr. N.H. Katsonis (Universiteit Twente)

adviseur: dr. C. Sánchez Somolinos (ICMA)

Het onderzoek of onderwerp dat in dit proefschrift wordt beschreven is uitgevoerd in

overeenkomst met de TU/e Gedragscode Wetenschapsbeoefening.

“And if I claim to be a wise man.

Well, it surely means that I don’t know.”

- Kansas (Carry On Wayward Son)

A catalogue record is available from the Eindhoven University of Technology

Library.

ISBN: 978-94-9301-472-5

Copyright © 2018 by Matthew Hendrikx

Cover design by Karel Haesevoets

Image created by ICMS Animation Studio

This research was made possible by funding from the Netherlands Foundation for

Scientific Research (NWO-TOPPUNT grant 10018944) and the European Research

Council (ERC grant agreement no. 669991).

Table of Contents

Chapter 1 Introduction 1

Chapter 2 Oscillatory deformations in glass-supported coatings 23

Chapter 3 Design of complex oscillating topographies 39

Chapter 4 Compliance-mediated topographic oscillations 55

Chapter 5 Visible light-responsive surface topographies 71

Chapter 6 Technology assessment 89

Summary 101

Samenvatting 105

Curriculum Vitae 109

Acknowledgements 113

1

Chapter 1. Introduction

ABSTRACT

Properties such as friction, wettability and visual impact of polymer coatings

are influenced by their surface topography. Therefore, control of the surface

structure is of eminent importance to tune its function. Photochromic

azobenzene-containing polymers form an appealing class of coatings of which

the surface topography is controllable by light. The topographies can be designed

to remain static or have dynamic properties, that is, be capable of reversibly

switching between different states. The topographical changes can be induced by

using linear azo polymers to produce surface relief gratings. With the ability to

address specific regions, interference patterns can imprint a variety of structures.

These topographies can be used for nanopatterning, lithography or diffractive

optics. For crosslinked polymer networks containing azobenzene moieties, the

coatings can form topographies that disappear after the light-trigger is switched

off. This allows the use of topography-forming coatings in a wide range of

applications, ranging from optics to self-cleaning, robotics or haptics.

This chapter is partly reproduced from M. Hendrikx, M.G. Debije, A.P.H.J.

Schenning, D.J. Broer, Crystals, 2017, 231.

2

1. General introduction

The surface of a material governs its contacts with its environment. Natural

surfaces often allow for multiple functions, either to maintain a healthy living

organism or simply to attract other organisms in order to procreate. A beautiful

example of maintaining a healthy environment via a clean surface is the Lotus leaf.1

This self-cleaning property relies on the nano- and micro-structuring of the leaf

surface.2 Multiple efforts have been made in creating artificial surfaces with

functional properties inspired by nature.3 An important strategy to functionalize a

surface is to apply dimension-controlled elevations; that is, topographies. Outside

nature-inspired applications, textured surfaces show promising optical properties,

both as transmissive and reflective diffraction gratings.4,5 Designing topographies on

a surface can lead to applications in photonics (such as nanostructured polarizers

and/or wave plates6,7 and antireflection coatings8–11), friction control (including

robotic fingerprints12,13), (de)wetting of surfaces and even control over cellular

adhesion and mobility.14

Surface topographies and reliefs can be created in multiple ways, either by

physically imprinting (embossing15,16) or by manipulation of the material itself using

light, temperature, pH, and/or solvent-swelling. Interestingly, light allows for a

remote, contactless approach without changing the chemical environment. This

contactless approach grants the possibility of locally addressing and changing the

material’s surface/bulk properties (including shape17–21, roughness and

wettability22,23, color22,24, etc.). In order to make materials responsive to light, usually

a light-sensitive molecule is incorporated in the polymer.22 A tremendous number

of light-responsive trigger molecules exist.25 The most commonly used molecules are

azobenzenes, first discovered in 1937.26 These molecules can undergo a reversible

trans- and cis-isomerization induced by illumination (Figure 1.1a). For the most

common azobenzene molecules, ultraviolet (UV) light irradiation induces the

isomerization from trans-to-cis, while the reverse takes place via thermal relaxation

or upon irradiation by visible light. Photo-isomerization of azobenzene results in a

large change of the molecular geometry, where the trans-isomer decreases in length

between the para-carbon atoms from 10 to 6 Å. In turn, this results in a tremendous

nanoscale force.27,28 Moreover, this geometrical change between trans- and cis-isomer

also leads to a change in dipole moment from near 0 to 3 Debye, respectively.29,30

These large geometrical changes make azobenzene one of the most interesting

Chapter 1

3

embeddable light-trigger molecules. Additionally, azobenzene molecules show

dichroism, that is, higher absorption of polarized light along the long optical axis

compared to the orthogonal axis. Through photo-isomerization with polarized light,

the azobenzene molecules can (re)orientate to a more preferred orientation. This is

the so-called ‘Weigert effect’.31–33

The design of topographies in azo polymeric materials can be achieved through

different techniques, either via a patterned laser treatment on homogeneous, flat

surfaces or by designing a heterogeneous surface prior to illumination. Mainly, these

techniques result in two surface states: the initial flat, regular state and a final

textured state. Solid, light-induced surface structures can be made in both linear and

crosslinked polymer network materials containing azobenzene molecules without

the need of a solvent (Figure 1b). Azo-containing polymers can range from azo dye

doped34,35, to azo side-chain36–38, to azo main-chain amorphous polymers, and even

azo functionalized polymer networks. The interaction of the chromophore

(substituents), liquid crystalline or amorphous nature, and the polymer skeletal

structure determines the efficiency of the topography formation.

Figure 1.1. Azobenzene-containing polymers. (a) Photo-isomerization of azobenzene. (b) Schematic

representation of azobenzene containing polymeric systems: linear polymers (top, left), side-chain

polymers (bottom, left) and polymer networks (right).

2. Surface relief grating in linear azo polymers

2.1. Light-induced permanent surface relief gratings in linear azo polymers

Soon after realizing the potential of azobenzene moieties in polymers to create

surface patterns by illumination, the interest and development of these surface

reliefs increased. In order to create surface structures with light, it is important to

understand the influence of the azobenzene additive. Both Natansohn and co-

4

workers and Tripathy and co-workers simultaneously reported the formation of

surface relief gratings (SRGs) based on the photoisomerization of azobenzene.36,37

For linear polymers containing this photosensitive molecule, it is necessary to

incorporate the dye via (non-)covalent bonds to the polymer. By mere blending, or

mixing, the interactions between the non-functionalized polymer and

photosensitive azobenzenes are not sufficient to create and sustain the light-induced

patterns.39,40 Moreover, the activation of the azobenzene molecule needs to be

addressed in a specific order to cycle between cis- and trans-isomer continuously.41

One azobenzene derivative that is very suitable for this is Disperse Red 1, where

visible light irradiation (488 nm) leads to continuous switching between cis and trans

states. Upon incorporation of the acrylate derivative in a polymeric system by

polymerization (Figure 1.2a), this continual switching is the driving force to the

creation of SRGs upon irradiation with a laser interference pattern. In Figure 1.2b, a

typical experimental setup is depicted to achieve an SRG in (an)isotropic materials.

Exposure to interfering polarized light of appropriate wavelength led to an intensity

interference pattern in the film, causing the exposed azobenzene derivatives to

undergo isomerization, leading to a realignment of the molecules (Weigert’s effect).

The resulting alignment of the trans-isomer will therefore be perpendicular to the

direction of the electrical component of polarized light (Figure 1.2c). During this

realignment, the azobenzene will preferentially align in a direction where the

excitation is minimal. This process is also called ‘photo-alignment’ (Figure 1.2d).42,43

This isomerization process is either powerful enough to move the polymer chains

and/or fluidize the polymer, leading to the formation of an anisotropic fluid state.44

It is known that during the light absorption of the azobenzene, localized heat is

generated which can enable additional mobility.45 This light-induced realignment

process creates typical SRG topographies with the maxima located in the low

intensity regions of the intensity interference pattern. The efficiency of the SRG is

determined by the writing time. This time is determined by the azo polymer’s

inscription rate. This rate is defined by the ratio of the growing diffraction efficiency

as a function of the inscription time.46 The diffraction efficiency expresses the ratio

of the power of the diffracted and the incident beam, respectively. It is common for

these materials to possess a high glass transition temperature (Tg), leading to stable

gratings at temperatures below the Tg. However, increasing the temperature above

Tg typically results in removal of the grating.47

Chapter 1

5

Different states of polarization are also used to fabricate gratings in azo polymer

films. The polarization state can be out of the plane of incidence (s, senkrecht), in the

plane of incidence (p, parallel) or even left- or right-hand circularly polarized (LCP

and RCP, respectively). Normally, the use of s-s polarization geometry leads to

weaker gratings, while the p-p and RCP-LCP geometry results in a much stronger

and clearer pattern.48–51 In contrast, research shows that the kinetics of the formation

during the polarization sensitive lithography depend largely on the azo polymer’s

molecular weight.52 Moreover, the content of the azo-based dye, the driving force of

the formation of the SRGs, determines the modulation depth that can be achieved

by interference irradiation.53 Interestingly, multiple research groups reported the

construction of topographies in azo polymers using only one laser beam rather than

using a complex optical interference pattern.54–61 SRGs can be used for multiple

applications, ranging from diffraction gratings, micro/nanostructuring, as molding

templates, to etch masks.51

Figure 1.2. Surface relief grating formation. (a) Chemical structure of poly(Disperse Red 1 acrylate).

(b) Experimental setup to write surface relief gratings in (an)isotropic azo-containing polymers. P:

polarizer, M1, M2: mirrors, BS: beamsplitter, WP: wave plate, S: azo-containing polymer, ϴ:

interference angle. (c) Typical AFM profile of a surface relief grating (SRG) topography. Reproduced

from reference 37. (d) The photo-alignment of azo-containing polymers before and after polarized light

illumination. Azobenzene moieties will align their long molecular axis preferentially perpendicular to

the electric component of the polarized light. The dashed line indicates the polarization direction of the

light.

6

Research has shown multiple types of azo polymers leading to SRGs57,62–66,

ranging from grafted polymers67, liquid crystalline polymers39,58,68–72 to reactive

components for in-situ approaches.73,74 Santer and co-workers have developed a

method that allows in-situ atomic force microscopy during the formation of SRGs in

azo polymers. In their study75, it was proven that for intensity interference patterns,

the topography maxima correspond to the position of intensity minima and vice

versa. Meanwhile, for the polarization interference pattern, they were able to reveal

the topographic extremes and correlate them with the distribution of the E-field

vector within the polarization interference pattern. Furthermore, most SRGs do not

show erasure during a second illumination below the glass transition. Most

interestingly, orthogonal recording of two gratings leads to a two-dimensional

grating. These higher dimensional gratings can also be fabricated from more easily

accessible azo-containing polymers73,76, leading even to colorless gratings via

decoupling of the azobenzene after recording.76 Moreover, using multilayered

approaches, they were able to achieve three-dimensional gratings for more complex

diffraction applications.77–79 Figure 1.3 depicts the systematic approach to achieving

a multilayer 3D grating. This technique allows for the creation of hierarchical

microstructures.

Figure 1.3. Three-dimensional surface relief gratings. (a) Schematic representation of the layer-by-

layer fabrication of the 3D structures reported by Stumpe and co-workers. (b) Microscope and

diffraction (Bertrand lens) images of hexagonal (1), tetragonal (2) and hierarchical (3) 3D structures.

Reproduced from reference 79.

In contrast with the covalent polymeric systems, SRGs can also be created via

supramolecular approaches.80–82 These type of polymers were used to systematically

lower the azo content and retrieve the minimal needed azo content to induce SRGs;

here, found to be 1 mol%.83 Hence, the inscription rate is lowered by decreasing the

Chapter 1

7

chomophore concentration. Meanwhile others have reported optimum degrees of

loading between 50 and 75 mol%, but these chromophore loading relationships are

greatly dependent on the polymer used.66,84,85 In addition to the use of hydrogen

bonds to create supramolecular azobenzene-containing polymers, Priimagi and co-

workers reported on the creation of supramolecular polymers by use of halogen

bonds.86–89 A more comparative study of different halogen bond donor

chromophores showed that the efficiency of the SRG formation is increased with

interaction strength (Figure 1.4).

Figure 1.4. Supramolecular azo polymers. (a–e) Chemical structures of the different azobenzene

modules (fluorinated (a), hydrogenated (b,c)), dipyridyl compound (d) and poly(4-vinyl pyridine)

(P4VP, (e)) used in reference 89. The label X can be either H, OH, or any halogen. (f) Diffraction

efficiency of the different fluorinated and (g) AFM surface profiles of the gratings for azobenzene

modules P4VP(n)0.1 (a) with n = 1 (X = F), n = 2 (X = Br) and n = 3 (X = I). Reproduced from reference

89.

2.2. Light-induced reversible surface relief gratings in linear azo polymers

A relief grating can undergo changes upon exposure to external triggers

depending on the molecular weight of the polymer. Reducing the chain length of the

polymer results in a decrease of the thermal stability of the grating, and above the

Tg of the polymer, the grating grooves will diminish and disappear entirely, as

discussed in Section 2.1. This section focuses on light-erasure of the gratings.

8

Generally, a second flood illumination of circular or unpolarized light leads to the

random alignment of embedded azo-chromophores. In turn, this leads to the partial

or complete erasure of the SRGs. This light-initiated erasure of the gratings is more

appealing than thermal erasure due to its capability of generating “on” and “off”

states that can be controlled remotely, affects the film locally, and on-demand.

Numerous efforts have been made to remove gratings remotely by light, but

generally resulted in no or only partial erasure.35,57,90–92 Unstable gratings that are

formed by utilizing polymers with low glass transition temperatures lead to

eventual thermal self-erasure.69,93,94 For these materials, the glass transition

temperature is typically lower than ambient temperature. This leads to an unstable

grating, which disappears over time, resulting in a flattened polymer coating in the

dark, which opens up possibilities of fabricating SRGs with temporal “on” and “off”

states dictated by light exposure. In early efforts, Jiang et al. concluded that the

erasure process is optimal when the erasing beam is polarized along the grating

vector direction.90 However, they were unable to achieve full erasure and a

topography of more than 10 nm remained after exposure. Ubukata and co-workers

reported an erasure of 91 % from a 35 nm deep SRG.91 Priimagi and co-workers

found that the erasure capabilities of their supramolecular azobenzene–polymer

complex is determined by molecular weight and its glass transition temperature

(Figure 1.5).95 Notably, they reported that there is no polarization dependency in the

erasure process of their P4VP–azobenzene complexes in contrast to the results

reported by Jiang et al.90

Figure 1.5. Reversible surface relief gratings. (a) The erasure behavior of SRGs expressed as normalized

diffraction efficiency for samples with molecular weights of 1000 (blue), 3200 (red) and 7000 (green)

g mol−1. (b) Normalized diffraction efficiency of the SRG inscription (magenta), erasure (blue) and

rewriting (red) of 1000 g mol−1 azo-containing polymer. Insets show 3D AFM images of the inscribed

(1) and erased SRG (2), with AFM surface profiles of these states shown in (c). Reproduced from

reference 95.

Chapter 1

9

Moreover, the optical erasure was performed at least 30 °C below the Tg of the

azobenzene–polymer complexes and still resulted in selective removal of the

patterns. Generally, the erasure process, or the ability to erase a SRG from a polymer

coating is expected to be related to the chemical structure, molecular weight, photo-

orientation and photo-induced mechanical changes in the materials.95

3. Surface topographies in crosslinked network coatings

3.1. Liquid crystal networks

Liquid crystals (LCs) are materials that possess an intermediate state

(mesophase) between the solid crystalline state and the isotropic liquid state, namely

the liquid crystal phase. In this phase, the material exhibits long-range orientation

or positional organization as seen for crystalline materials while maintaining their

ability to flow. Molecules containing both a rigid core and flexible (mostly aliphatic)

side- or end-chain typically show liquid crystalline properties.96 Depending on the

molecular structure, the material can express multiple LC phases. A few of these

phases are shown in Figure 1.6. LCs have anisotropic properties that are interesting

for optical applications with the most well-known being displays. Additionally, LC

materials also find applications in optics and photonics, mechanics and biomedicine.

Most LC applications rely on polymeric materials to maintain their anisotropic

properties even at temperatures well above the phase transition. Liquid crystal

polymers (LCPs) can refer to different types of polymers, namely main-chain liquid

crystal polymers (MCLCPs), side-chain liquid crystal polymers (SCLCPs) liquid

crystal elastomers (LCEs) and liquid crystal networks (LCNs) (Figure 1.6). Both

LCEs and LCNs can be created from reactive LC monomeric mixtures that can be

aligned by a variety of techniques (among them are various interfacial techniques,

mechanical14,97,98, optical99–103 and electrical104). LCEs typically have a low crosslink

density compared to LCNs, leading to a lower modulus of ca. 1–5 MPa compared to

the stronger LCNs (ca. 1–2 GPa).105 The LC monomer mixtures can be

photopolymerized with light resulting in a polymer coating. By patterning the LCN

coating with different alignments, in particular isotropic and cholesteric (chiral

nematic), topographies were generated by heating.106 It is important to note that

order within the LCN is needed to create topographies.

10

Figure 1.6. Illustration of different liquid crystal phases (nematic, smectic A and smectic C) and liquid

crystal polymers (main-chain liquid crystal polymers, side-chain liquid crystal polymers, liquid crystal

elastomers and liquid crystal networks).

Figure 1.7. Thermal-induced surface topographies in crosslinked network coatings. (a) Polarized

micrograph of the LCN coating with isotropic (black) and cholesteric domains (colored). The inset shows

the mask used. (b, c) The surface structure of the LCN coating with 140 nm topographies at 25 °C (b)

and 300 nm topographies at 200 °C (c). Reproduced from reference 106.

Chapter 1

11

3.2. Light-induced permanent surface topographies in azo crosslinked

network coatings

Permanent light-induced surface topographies have been produced in LC

polymers, more specifically in LCNs. LCs allow for a high degree of control over

molecular alignment, which is important in forming topographies. The LCNs are

created from mixtures of different reactive LC monomers in order to precisely

control the LC properties. These mixtures typically contain an azobenzene moiety

(molecule 4 in Figure 1.8c) acting both as a crosslinker and deformation trigger

molecule. Once achieving the desired alignment, the monomeric mixture can be

photopolymerized. Because of the average polyfunctionality (mono- and di-

acrylates) of the chosen monomers, this leads to crosslinked liquid crystal polymer

networks.107 The ability to design and fabricate the anisotropic coating to respond to

specific commands leads to the creation of many different directed topographies.

Most interestingly, UV exposure of pre-aligned azobenzene-containing LCN

coatings induces the creation of topographies without the need of complex optical

setups. Upon illumination, the azobenzene will undergo a trans-to-cis isomerization,

leading to a disruption of the local molecular order. This results in expansion

perpendicular and contraction parallel to the molecular alignment, known as the

director n, and thus leading to the creation of localized topographies when the

coating has been pre-patterned via illumination through a mask. Figure 1.8

illustrates the expansion of the azo-LCN polymer coating and the principle of

disruption in the alignment caused by the isomerization of the embedded azo

molecule. Liu et al. have found that the topography formation is based on the

creation of free volume inside the network. The free volume formation is enhanced

by an azobenzene mesogen that also acts as crosslinker.98,103,108 Utilizing dual

wavelength exposure, the azobenzene will undergo continuous trans–cis–trans

photoisomerization that, in turn, leads to a maximum stress of the network, resulting

in a larger surface topography.108

In contrast to the formation of linear or amorphous polymer based SRGs, which

typically require interfering coherent or masked incoherent, any type of light can

used to create surface topographies in crosslinked, liquid crystal-based polymeric

coatings. Interestingly, the optical methods for creating static or reversible LCN

topographies are nearly identical. The difference lies in the chain length of the

network. In contrast with SRGs, utilizing short ‘flexible’ chains leads stable

12

topographies in crosslinked liquid crystalline polymer coatings. The flexible short

chains “fill” the free-volume generated during illumination. Additionally, this

allows the azobenzene to rotate out of plane during the conversion from cis-to-trans.

This rotation is only possible if the rotational mobility of the azobenzene in the LCN

is sufficiently high.103

Figure 1.8. Light-responsive azo crosslinked liquid crystal networks. (a) The graphical representation

of the polymeric network with copolymerized di-acrylate azobenzene with illumination of different

wavelengths. Reproduced from reference 108. (b) Density change in a chiral nematic polymer film

containing copolymerized di-acrylate azobenzene (yellow circle) and the photoabsorber Tinuvin 328—

molecule 7—(blue circle) before, during and after UV exposure in salt brine. Reproduced from reference

98. (c) Chemical structures of the mixtures typically used to achieve glassy azo-liquid crystal networks

(LCN) coatings. Molecules 1–3 make up the LCN, 4 is the azo crosslinker, 5 and 6 are a chiral dopant

and radical scavenger, respectively, and 7 is a UV absorber.

By adding radical scavengers (molecule 6 in Figure 1.8c) to the mixture and

controlling the polymerization of the coating, Liu et al. reported stable topography

formation in azo-LCN coatings.98 Here, they used a crosslinked cholesteric liquid

crystal polymer coating, with regions selectively exposed through a mask with UV

light: the chiral nematic (cholesteric) phase is present before and after UV exposure

in Figure 1.9a. Cholesteric LC phases exhibit a reflection band determined by the

pitch of the helical structure generated by the doping of the nematic LC with a chiral

molecule; in this example, the center of the cholesteric reflection band is at 630 nm

before UV exposure. Due to the disturbance of order of the network in the areas

exposed to UV, the reflection band becomes narrower (Figure 1.9b). The effective

formation of topographies by actuation of the azobenzene is only present when

exposing areas polymerized in the cholesteric phase. When using a coating of the

same composition but is instead polymerized in the isotropic phase, there is nearly

no expression of topographies upon selective UV exposure: any topographies

formed in the isotropic crosslinked coating result solely from thermal effects.

Chapter 1

13

Interestingly, when the azobenzene is swapped for a simple UV absorber, Tinuvin

328 (molecule 7 in Figure 1.8c), the topographies formed by selective illumination

are lower in height than those generated using the azobenzene moiety (Figure 1.9c–

f). Tinuvin 328 has an absorption spectrum that coincides with that of azobenzene.98

However, Tinuvin 328 displays no large geometrical changes upon UV absorption

as is observed for azobenzene. This leads the authors to conclude that the

azobenzene isomerization is only partly responsible for the topographies, as

thermally induced deformation from the absorbed UV light also plays a role. The

crosslinked coatings containing azobenzene show very stable topographies without

observing any relaxation up to 120 °C, which persists for months when maintained

at room temperature in the dark.

This technique was recently used to investigate cell adhesion and migration

behavior to different sized surface topographies.14 Here, UV irradiation of a planar

cholesteric azo-LCN coating through a hexagonal dotted mask resulted in pillars.

Depending on the dose of the UV illumination at a temperature above the Tg,

different sized pillars were achieved by local expansion of the cholesteric coating.

The resulting pillar-like topographies ranged from 0.2–1.6 µm. The researchers used

the pillar-structured coatings to study the interactions of living cells in contact with

structured surfaces.

Stable topographies can also be prepared by locally controlling the alignment of

the liquid crystal coating prior to polymerization and then generating the stable

topographies in the film with uniform UV exposure after polymerization (that is, not

using a mask as in the previous example).13,103,104,109 Stable topographies require the

presence of radical scavengers during polymerization of the LC coating.

McBride et al. used a different technique to create stable surface topographies in

LCN materials. Topographies were formed relying on photo-activated reversible

addition fragmentation chain transfer (RAFT)-based dynamic covalent chemistry in

an azobenzene-free polymeric coating.110 By implementing a chain transfer agent

(CTA) inside the polymeric coating, the researchers could activate a nematic LCN

coating through the CTA with a UV photoinitiator. When they carried out this

process at elevated temperatures and addressed the coating with UV light exposing

the film surface through a mask, the exposed areas of the LCN coating undergo a

phase transition to isotropic, leading to a loss in order and concomitant increase in

14

height. This in turn leads to the creation of surface topographies (400 nm) that are

still present after cooling and subsequent heating.

Figure 1.9. Light-induced surface topographies in cholesteric azo crosslinked network coatings. (a)

Schematic representation of the cholesteric polymer coating before and after exposure with UV light

with change in order. (b) Transmission spectra of the chiral nematic coating before (solid line) and after

UV exposure (dashed line). (c) Surface profiles and the 3D view of the topographies made by masked

UV exposure of the cholesteric polymer coating with azobenzene moiety (black line in (c) and (d)),

Tinuvin 328 (blue line in (c) and (e)) and an isotropic coating with azobenzene (red line in (c) and (e)).

Reproduced from reference 98.

3.3. Light-induced reversible surface topographies in azo crosslinked

network coatings

Reversible surface topographies can undergo a change after initial, under

continuous, or during sequential illumination. In contrast with the permanent

surface topographies, here, the azo-LCN coating is prepared in the absence of any

radical scavenger. This limits the mobility of the azobenzene under UV illumination,

only allowing isomerization and the generation of disorder in the molecular

alignment. This highly crosslinked azo-LCN coating can create topographies upon

illumination, but the features are limited in stability. This results in a return to its

initial state post-irradiation.103 Liu et al. have proven that polymerization of a

cholesteric-based film under the effects of localized electric fields creates azo-LCN

coatings with alternating domains of planar (director parallel to the substrate)

Chapter 1

15

cholesteric and homeotropic (director perpendicular to the substrate) nematic

alignment within the same film (Figure 1.10a–c).97,104 With this alignment setup, the

coating will contain areas that expand and contract in thickness adjacent to one

another, resulting in larger differential topographical features. This makes the

coating more attractive for applications such as generation of variable friction

surfaces that can respond in the order of seconds (Figure 1.10d, e).

Figure 1.10. Light-induced gripping and releasing of azo crosslinked network coatings. (a) Graphical

illustration of the patterned coating with resulting influence of light. (b) 3D images of the coating in

the absence and presence of light with different heights (c). Reproduced from reference 97. (d, e) The

slide-off of two pattern coated glass slides in contact and balanced at an angle ß under the influence of

UV light. The samples are positioned parallel (d) and orthogonal (e) with respect to one another, leading

to different slide-off properties and friction. Reproduced from reference 104.

An example of surface topographies controlling friction was based on the self-

assembly of cholesteric azo-LCN polymer coatings. Here, the cholesteric helical axis

of the coating is aligned parallel to the substrate, leading to a so-called fingerprint

texture.12 The fingerprint texture is of particular interest as it contains multiple

alignment regions (homeotropic and planar) directly upon application, and does not

require additional electric fields or other processing states to achieve this condition.

By containing both planar and homeotropic alignment regions in the same coating,

the film will both expand and compress, respectively, leading to larger

deformations.97,104 For these types of azo-LCN coatings, it was found that the

actuation and relaxation time were both close to 10 s.108 The typical domain sizes

achievable with azo-LCN coatings range from tens to hundreds of microns, with

16

topographies varying between 0.1 and 2 µm. It was also shown by experimental and

computation results that self-assembled polydomain azo-LCN coatings could also

generate films with light-switchable surface roughness.13

4. Outline of the thesis

As can be concluded from this introduction, the generation of topographies in

azobenzene polymer surfaces by light has been extensively researched. The use of

light as a trigger to control the surface structure has the advantage of being both

contactless and able to affect the film remotely. Azobenzene containing polymers

are highly suitable for creating topographies, and it is fascinating that these small,

light-induced molecular shape changes can result in large amplified changes in the

bulk polymeric material. Despite this, in order to free the path to applications,

further research is needed. The dynamics of the structure formation is still not well

understood as well as the methods to enhance this. New switching on and off

methods as well as the use of more biology-friendly wavelengths will support

further biological applications such as cell proliferation and differentiation. Also, in

order to come into the reach of haptic applications, amplification of the height of the

structures is desired. This thesis aims at developing light-responsive azobenzene

based coatings with enhanced topographical changes. Furthermore new methods,

materials and molecules will be used to come closer to potential applications.

Chapter 2 will introduce the effects of linear polarized UV light actuation on azo-

LCN coatings with different nematic alignments. These alignments are

asymmetrically organized leading to an asymmetric hill-valley shaped topography.

Upon rotation of polarized light at different speeds, oscillatory deformation is

achieved and characterized.

Chapter 3 will discuss similar effects as discussed in Chapter 2 but now based on

different symmetrically organized alignments to affect the shape of the

topographies. The change in topological defect lines led to different topographic

responses and dynamics characterized by shape and deformation, such as

symmetric hills and valleys that oscillate in height and laterally upon actuation in

rotating linear polarized UV light.

Chapter 4 will focus on the effects of lateral stresses in patterned uniaxial planar

nematic coatings and introduces a method to express these stresses more globally.

As results of using compliant layers the topographies can be tuned to different

Chapter 1

17

heights and are able to oscillate at much faster speeds without changing the azo-

LCN chemistry.

Chapter 5 will introduce a new azobenzene derivative that allows the creation of

topographies with low intensity visible light, leading to a re- and pre-configurable

surface structures with multi-stable properties directly in sync with the molecular

kinetics. Moreover, this new design allows for the in-situ monitoring and cultivating

of living cells.

Chapter 6 will give a technology assessment on the achieved results and future

prospects of these azo-LCN coatings and their unique properties.

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23

Chapter 2. Oscillatory deformations in

glass-supported coatings

ABSTRACT

Light-induced surface topography of a liquid crystal polymer coating is

brought into a patterned oscillatory deformation. Thereto a dichroic photo-

responsive azobenzene is co-aligned with a planar oriented nematic liquid crystal

network molecules which makes the surface deformation sensitive for

polarization of UV light. Locally selective actuation is achieved in coatings with

a complex alignment pattern. Dynamic oscillation, as controlled by actuation and

relaxation kinetics of the polymer, is obtained by a continuous change the

polarization of the UV source. Of special interest is the atypical deformation at

the defect lines between the domains. The amplitude and presence of the

oscillation can be manipulated by different ratios between blue and UV light and

by varying the ambient temperature of the coating.

This chapter is partly reproduced from M. Hendrikx, A.P.H.J. Schenning, D.J.

Broer, Soft Matter, 2017, 13, 4321–4327.

24

1. Introduction

The concept of making materials smart by design, meaning that they change their

property or shape by an external stimulus has the prospective to change our daily

life in many ways. Special attention is given to responsive surfaces as they mediate

between the bulk of the material and the outside world with properties related to

friction and tribology, touch perception, capability to remove dirt or to reject liquids.

A powerful example is cell manipulation at surfaces that revealed that changes on

nanoscale can be used in cellular adhesion and motility.1,2 Studies have been

performed to produce responsive surface topographies.3–7 For example, coatings

based on liquid crystal networks (LCNs) containing azobenzene moieties have been

shown to produce dynamic topographies (see Chapter 1). The topographies form

and disappear on demand by turning light on and off, resulting in controllable

surface structures.8,9 In all these cases the surface deformation is binary; the whole

surface is in its “on” state or in its “off” state. Local variations are not possible other

than exposing through a mask. This allows for an easy remote-like actuation, which

can be utilized in different applications. The use of light as a trigger also allows for

a self-powered approach to smart materials, removing electronics and batteries from

the device itself.

Of even more interest would be coating materials that can switch their surface

topography in an oscillating manner between an ‘on’ corrugated state and an ‘off’

flat state. Oscillatory systems can be found in nature. Cardiovascular rhythm,

respiration, cell cycles and other biological rhythms are energy-driven actions

without the need for an on/off switch or trigger. These processes show a non-

equilibrium state in which the system constantly modifies its behavior to address for

a continuous change. A large effort has been made to achieve such autonomous

oscillators, unfortunately most of them rely on wet conditions. These are mainly

based on responsive hydrogels or chemical reactions leading to out-of-equilibrium

states; such as the Belousov-Zhabotinsky reaction10,11, self-oscillation12, self-walking

hydrogels13, photoregulated wormlike motion14 and binary light switching.15

Attempts to create oscillators in dry environment with an autonomous behavior

have been made.16–18 These oscillatory actuators are all freestanding polymer films

and are based on the isomerization of azobenzenes. Inducing the trans-to-cis and cis-

to-trans isomerization leads to continuous actuation and relaxation of the polymer

Chapter 2

25

films.19,20 However, coatings that change their surface topography in an oscillating

manner have not been reported.

It is the aim of the research presented here to control local actuation of selected

elements at the surface by means of polarized light. Thereto we designed coatings

with a patterned nematic organization that benefit from the dichroic properties of

azobenzene to preferentially address those elements in the surface that are aligned

parallel to the polarization of light. This approach will allow us to use a continuous

power source to obtain a patterned oscillatory response in a coating.

2. Results and discussion

Liquid crystal network coatings with a uniaxial planar and patterned director

orientation are made between two glass plates. The local director orientation is

controlled by photo-alignment layers based on a linearly photopolymerizable

polymer (LPP).21 After curing the LC mixture, the polymeric coating is obtained by

removing one of the glass substrates. The coated glass is placed in the setup depicted

in Figure 2.1. The surface topography formation is monitored by digital holographic

microscopy (DHM) upon illumination with polarized 365 nm light in a bottom-to-

top fashion and simultaneous illumination with unpolarized 455 nm light at an

angle from the top. The modulation is here reported as height change compared to

the average of the observed area, unless stated otherwise. The azobenzene moiety

has an absorption maximum around 365 nm in its trans-state and at 455 nm in its cis-

configuration. For memorizing the shape of the azobenzene moiety it is important

to notify that the elongated configuration (trans-isomer) is most sensitive for

actuation by light with its field vector parallel to its long axis. The bend configuration

(cis-isomer) is less dependent on the polarization of light. The dichroic ratio, the ratio

between absorbance parallel and perpendicular to the director, is 3.6 and 1.7 for 365

nm and 455 nm, respectively. An irradiance ratio of 0.1 between both wavelengths,

365 nm and 455 nm respectively, should result in an optimal response, as previously

published by Liu.22 This ratio was used as a basis for the following experiments.

26

Figure 2.1. Illumination setup in combination with the Digital Holographic Microscope. (a) Simplified

illustration of the illumination setup used for polarized light actuation. Blue light (455 nm)

illumination from the top and UV light (365 nm) illumination originates from the bottom while passing

through a rotating polarizer. The sample contains black lines to indicate the orientation of the director.

(b) Photograph of the setup with the 365 nm LED turned on.

Firstly, we will discuss the influence of actuating a uniaxial planar aligned

coating with polarized light parallel and perpendicular to the director. The height

change is measured with respect to the glass substrate. As visualized in Figure 2.2,

the height changes for the coating illuminated with parallel polarized light (//) are

largest. Within 60 s the photostationary state is reached for both the parallel and

perpendicular illumination. The expansion of the film is of the order of 0.75–1 %

under influence of light. This result is rather low compared to published results for

cholesteric liquid crystal phases. Moreover, due to the illumination setup, most

actuation will occur in the bottom regions of the coating, limiting the strain of the

material. Furthermore, we observed that even with perpendicular polarized UV

illumination (⊥), there still is a remarkable actuation present, which is ca. 30 %

smaller than parallel actuation. The height increase upon illumination with

perpendicular polarized light can be ascribed to the considerable absorption of UV

light related to the relatively poor dichroic ratio as well as some depolarization of

light when it penetrates into the sample. Upon rotation of polarization, the

maximum oscillation will be between the given extrema for parallel and

perpendicular exposure. The actual height oscillation is determined by the rotation

speed and the kinetics of the relaxation of the azobenzene moiety. As can be seen in

Figure 2.2, at a polarizer rotation speed of 2.5° s-1 the sinusoidal height wave

oscillated with a period of 72 seconds, as expected. The amplitude is between the

Chapter 2

27

height obtained by perpendicular and parallel illumination and the period

corresponds with the time needed to fully rotate the polarization. This result

suggests that after each rotation a photostationary state of the cis-trans azobenzene

equilibrium is reached. The oscillation is formed by continuous change in the local

ratio of absorbed UV light and blue light causing each time a different cis-trans

photostationary state and therefore a different height. Slowing the polarizer rotation

speed down does not increase the amplitude further. However, increasing the

rotation speed reduces the oscillation amplitude: doubling the rotation speed

decreases the amplitude by 17 %. The actuation height of the parallel exposure (//)

almost overlaps with the maximum of the oscillation given by the 2.5° s-1 rotation,

while the minimum overlaps with the perpendicular exposure (⊥). For increased

rotation speeds, the oscillation starts diverging from the sinusoidal shape, being the

result of a mismatch between the kinetics and the rotation speed. Increasing the

rotation speed hardly changes the response leading to the conclusion that 2.5° s-1 is

the optimal speed for these oscillations.

Figure 2.2. Height changes for the uniaxial planar nematic azo-LCN coating illuminated with polarized

light. Parallel and perpendicular polarized light actuation is depicted in dashed black, with respect to

the director, labelled // and ⊥, respectively. The solid black line represents the full height change over

time during rotating polarized UV with 2.5° s-1. The insert shows the actuations measured while

rotating the polarized UV light with 0.5° s-1, 5.0° s-1 and 2.5° s-1 between the marked parallel and

perpendicular actuation extrema. The rotation of the polarizer and the LEDs were turned on at t = 30

s. Intensity of 365 nm and 455 nm LEDs were 200 mW cm-2 and 20 mW cm-2, respectively.

Next, in order to create different simultaneous oscillations, we studied coatings

with patterned alignment. In order to achieve this, we created adjacent striped

28

domains with an orthogonal uniaxial orientation with a periodicity of 200 µm and

40 µm, respectively (Figure 2.3). We first consider the 200 µm periodic structures

with 0°/90° orientation, 0° and 90° meaning parallel and perpendicular alignment of

the director with respect to the topological defect line. The domains are orthogonal

with respect to each other while the transition (Néel wall) between both domains

forms a +½ or -½ topological defect line governed by the LC liquid elasticity prior to

polymerization. The initial state of the coating is quasi-flat after opening of the cell

with very small topographies visible around the topological defect lines due to local

stresses caused by anisotropic expansion coefficients while cooling from the

polymerization temperature to room temperature. Then the domains are

simultaneously exposed by rotating polarized UV light. Results of the topographical

actuation upon LED irradiation through a rotating polarizer demonstrate the

oscillating responses of the two different zones in the coating (Figure 2.3). In

comparison with the experiment performed with uniaxial aligned planar coatings,

we measure a difference in actuation for the two adjacent orthogonally aligned

domains, resulting in a different height changes. When measured 50 µm away from

the defect line, indicated as zone 1 and 2 in Figure 2.3, the deformations oscillate out

of phase around their initial height corresponding to what we found in Figure 2.2

for a uniaxial film. The difference between height changes of zone 1 and 2 can be

dedicated to a small tilt of the sample during the measurement.

However, the situation is different closer to the defect line. We measure in zone

3 and 4 which are at a distance less than 20 µm from the defect line. Here, we observe

much larger deformations. Figure 2.3 shows an apparent increase of the normalized

height for zone 3 and a decrease for zone 4. The increase and decrease are related to

the average. The photostationary states for the zones located close to the defect line

are completely different than observed for zones further away. Moreover, for zone

3 and 4 the oscillatory growth and descent are out-of-phase with respect to each

other. Resulting in two different oscillations, one increasing and one decreasing,

with out-of-phase characteristics while only changing the polarization of the UV

irradiation. Most interestingly, the largest deformations in this coating are

completely concentrated around the topological defect line, illustrated by the dotted

line in the schematic representation in Figure 2.3, and determined by the molecular

orientation of the adjacent domains. The largest deformations occur at the interface

of the two regions: the highest peak forms on the 90° side and the deepest valley at

the 0° side of the boundary. The lateral dimensions at which these surface

Chapter 2

29

deformations are expressed reach ca. 20 µm in both directions (See Figure 2.5). This

is a factor 10 larger than the topological defect line width, which is in the order on

1–2 µm.

Figure 2.3. Digital Holographic Microscopy results of the topological defect line actuation of a 0°/90°

aligned azo-LCN coating. Left: Schematical representation of the coating with 200 pitch domains with

zone 1–4 depicted. The thick black lines in the top right corner of each domain indicate the director

orientation. The yellow/black dashed line indicates the topological defect line. Right: Normalized height

changes monitored for the nominated zones during light exposure. Zones 1 and 2 are monitoring the

changes at a distance of more than 50 µm away from the defect line in each domain. Zones 3 and 4

monitor the changes close to the defect line on each side, less than 20 µm. The rotation of the polarizer

started at t = 20 s and the LEDs were turned on 10 s later. Rotating speed was 2.5° s-1. Intensity of 365

nm and 455 nm LEDs were 200 mW cm-2 and 20 mW cm-2, respectively.

With the oscillation mostly present in the areas at or near the topological defect

line, this leads to believe that the oscillating topographies originate from

accumulated stresses in these regions. Taking the stresses into account that the azo-

LCN develops during actuation, one can believe that one domain dominates the

stresses over the adjacent orthogonal domain. Stresses push perpendicular to the

director of the azo-LCN coating and a contraction along this director. For

perpendicular aligned domains in a 0°/90° design, this leads to the decrease of stress

in the “0° domain” and increase in the “90° domain”, as we observed.

30

In order to study this phenomenon further, a patterned coating with a pitch of 40

µm was investigated (Figure 2.4). Here, the domain sizes are in the order of the

lateral dimensions of the topographies observed during the actuation for large

pitches (200 µm). Therefore, a pure asymmetric response is observed where one

domain increases and the adjacent domain decreases in height. These patterned

oscillations are a result of the deformations created on and near the topological

defect line.

Figure 2.4. Digital Holographic Microscopy results for domains with a 40 µm pitch with a 0°/90°

design. Left: Schematic representation of the coating with 20 µm domains containing the monitored

zones 1 and 2. The thick black lines in the top right corner of each domain indicate the director

orientation. The yellow/black dashed line indicates the topological defect line. Right: Normalized height

change over time upon illumination (LEDs and rotating polarizer on at t = 30 s) of the monitored zones.

Intensity of 365 nm and 455 nm LEDs were 200 mW cm-2 and 20 mW cm-2, respectively.

Chapter 2

31

Figure 2.5. 3D images of the azo-LCN coatings as initial quasi-flat (a, c) and maximum topographic

state (b, d) of the coating with orthogonal 0°/90° patterned domain sizes of 100 µm (a, b) and 20 µm

(c, d) upon illumination.

In all previous cases, the experiments were performed at room temperature. This

resulted in rather slow cis-to-trans kinetics.23–25 To monitor the effect of temperature,

the patterned coating with a pitch of 40 µm was investigated at temperatures

between 30 °C and 80 °C. Prior to any UV illumination the sample was left in place

and allowed to relax back to its quasi-flat state for ca. 30 mins in the presence of blue

light while equilibrating at the elevated temperature. The height changes are

normalized to show the effect of the resulting light actuation (Figure 2.6a). It is

important to note that the surface starts deforming upon heating well above the glass

transition at 46 °C, measured with differential scanning calorimetry, due to thermal

expansion. These deformations are of the same order as those actuated with

polarized light at room temperature (Figure 2.6b). After equilibration of the sample

at the set temperature, the same procedure of actuation was applied. Results of

individual temperature runs are also shown in Figure 2.6a, the monitored zone is

the same for each experiment (i.e. zone 1). A first observation is that during heating

above the glass transition, the actuation is still present. Furthermore, oscillation is

also maintained. Hence, the average photostationary states around which the

oscillation occurs, measured from resting state, decrease significantly with

increasing temperature. To visualize the influence, a plot expressing the maximum

and minimum of the oscillation as a function of temperature is shown in Figure 2.6c.

It is visible that below 50 °C the absolute height and amplitude of the oscillation

increases. Upon further increasing the temperature, the absolute height starts to

32

decrease significantly, while the amplitude remains unchanged. Interestingly, a

maximum amplitude of the oscillation is reached at 50 °C. Without the presence of

any light, this temperature is close to the materials glass transition. However from

previous work, we know the glass transition lowers upon irradiation with blue light

due to photosoftening.24

Figure 2.6. Influence of temperature upon illumination of rotating polarized UV light and unpolarized

blue light. (a) Normalized height changes over time during polarized actuation with a rotating speed

of 2.5° s-1 for different temperatures for the 0°/90° design. (b-c) The topographies at t = 0 at different

temperatures (b) and the height maximum (▲) and minimum (▼) of the oscillation (c) in function of

temperature. Intensity of 365 nm and 455 nm LEDs were 200 mW cm-2 and 20 mW cm-2, respectively.

In all the measurements, the monitored zone was chosen the same and normalized. The chosen zone

corresponds with zone 1 in Figure 2.4.

The influence of the amount of blue light was investigated to determine an

optimal balance of UV and visible light at room temperature. In Figure 2.7a, the

results of the rotating linear polarized light actuation with different intensity ratios

Chapter 2

33

of blue and UV light are given. One can clearly observe the same type of trend as in

the temperature scanning run leading to believe that the increase of blue light, leads

to photosoftening of the azo-LCN coating. Upon the addition of blue light, a

maximum of response combined with oscillation is achieved for an illumination

containing 10 % of blue light compared to UV light intensity; however, this does lead

to a decrease in maximum actuation. Lower or higher values lead to the diminishing

of the oscillation or disappearance of the actuation, respectively. In case of an

intensity ratio of 1.00, the azo-LCN coating is unable to achieve any pronounced

actuation and even a disappearance of the oscillation is found. The presence of blue

light during relaxation of the coating’s topographies is important, as seen in Figure

2.7b. In the absence of blue light, the relaxation is in order of days. While, with blue

light present the relaxation of the topographies follows an exponential decay.

Figure 2.7. Influence of blue light during actuation of the azo-LCN coating. (a-b) The influence of the

ratio on actuation (a) and relaxation (b) between the intensity of blue (455 nm) and UV light (365 nm)

for the 0°/90° designed coatings with the phase snapshot of the coatings. The intensity of UV light is

calibrated to 200 mW cm-2. The comparison in the graph is made for corresponding zone 1 in Figure

2.4.

3. Conclusion

It was demonstrated that nematic azo-LCN coatings can be aligned in orthogonal

domains to create polarization selective responses. Upon illumination with rotating

linear polarized UV light while tuning the isomerization with blue light, patterned

oscillations of topographies can be achieved. The speed of rotation of polarized UV

light is optimal at 2.5° s-1. Most importantly, in all cases the topographies form

around the topological defect lines and extend ca. 20 µm laterally. Changing the

pitch of the patterned domains to match this lateral deformation resulted in a larger

34

amplitude. A maximum amplitude is found for an intensity ratio (I365/I455) of 0.10.

The temperature of the coating also determines the amplitude of the oscillation

during actuation. The best results are obtained at temperatures just above the glass

transition temperature, here 50 °C. Typical topographical deformation are in the

order of 1 % of the initial coating thickness with oscillations in the scales of

nanometers, ca. 35 % of the actuation height. Such dynamic topographies can be

imprinted on materials for haptic feedback or easy-to-clean solutions in dry and

extreme conditions by mechanical removal of sand or dust or even to control cellular

adhesion and mobility. For the latter, both pitch and defect patterns can be

optimized to result in different patterned oscillations.

4. Experimental

4.1. Materials

The azo-LCN coatings are made from a mixture of liquid crystalline acrylates and

necessary additives shown in Scheme 3.1 and was described previously in more

detail.4,26 Monomer 1 to 3 were obtained from Merck UK. Monomer 4 was custom-

synthesized by Syncom (Groningen, the Netherlands). Photoinitiator 5 was obtained

from Ciba. A typical azo-LCN composition consists of 41.4 wt% monomer 1, 20.6

wt% monomer 2, 31 wt% monomer 3, 5 wt% monomer 4 and 2 wt% photoinitiator

5. This amount of photoinitiator is chosen to produce a fully crosslinked LC polymer

film. The amount of monomer 5 is chosen higher than previously reported due to

the bottom-to-top illumination technique used. This high content allows for a higher

light intensity without harming the camera. The constituents were mixed by

dissolving in dichloromethane and stirred until a homogeneous solution was

obtained. Dichloromethane was removed under reduced atmosphere to achieve a

reactive LC monomer mixture. The photo-alignment layer, LPP ROP-108/2CP, was

obtained from Rolic. All chemicals were used as received.

Chapter 2

35

Scheme 2.1. Chemicals used to create responsive nematic liquid crystal network coating.

4.2. Fabrication of the patterned azo-LCN coating

Glass substrates (3×3 cm2) were cleaned by sonication using acetone and

propanol-2 followed by UV ozone cleaning. Photo-alignment material (linearly

photopolymerizable polymer, LPP) was spincoated onto the cleaned substrates. Two

substrates were glued together using adhesive containing 6 µm spacers. The LPP

surfaces of the thus obtained LC cells are patterned by a 2-step exposure. In the first

step, the sample was exposed with polarized light through a mask for 15 minutes.

In the second step, the mask was removed and a shorter flood exposure, 3 minutes,

was applied with light with polarization orthogonal to the first exposure. The second

exposure aligns the areas that were unaddressed but does not overwrite the

alignment achieved by the first exposure step, thus creating orthogonally aligned

pattern. The LC cells were filled with the LC monomer mixture by capillary forces

and cured at 38 °C with light > 400 nm (EXFO Omnicure S2000) followed by a short

post-cure at 125 °C for 5 minutes. Afterwards, one of the glass plates was removed

leaving a coating adhering to glass at one side and with a free surface at the other

side.

4.3. Characterization and actuation of the azo-LCN coating

The monomeric mixture and coatings are characterized with a crossed polarized

microscope (Nikon Ci Eclipse) with a thermocontrolled stage (Linkam). For the

monomeric mixture the transition temperature, nematic to isotropic, is determined

36

by cooling from isotropic liquid to nematic LC phase. Both the polymeric and

monomeric transitions were confirmed with differential scanning calorimetry (DSC

Q1000, TA Instruments). The surface of the coating is monitored with a digital

holographic microscope (DHM® R210, Lyncée Tec SA, Switzerland) equipped with

a thermocontrolled stage (Linkam) and mounted with UV (λ = 365 nm) and blue

light (λ = 455 nm) collimated LEDs (M365L3 and M455L3 respectively, Thorlabs).

The UV light originated from the UV LED was redirected with a UV mirror

(Thorlabs) and polarized with a polarizer (10LP-UV, Newport) mounted in a

rotating stage (Thorlabs), see Figure 2.1. The coating was triggered by turning the

blue and UV light LED on (typically 20 and 200 mW cm-2 respectively) together with

the rotating stage (typically 2.5° s-1). The initial resting state was monitored for 30 s

without illumination, after 20 s the rotating stage was turned on and 10 s later the

UV and blue light were turned on and monitored for ca. 25 minutes. In each

experiment, 10 full rotations of the polarizer were monitored to investigate the

development and relaxation of topographies formed by polarization selective

absorption of the azobenzene moieties. After 10 full rotations, the UV light was

turned off. Monitoring of the changes in real-time are done at 15 fps, the acquisition

rate of the holograms is set to 0.5 fps. This results in a captured hologram every 5°

of rotation. The azo-LCN coating relaxes in the presence of blue light until an

equilibrium state was reached. Post processing of the holograms was performed

with Koala software (Lyncée Tec SA) and ImageJ.

References

1 M. Nikkhah, N. Eshak, P. Zorlutuna, N. Annabi, M. Castello, K. Kim, A. Dolatshahi-

Pirouz, F. Edalat, H. Bae, Y. Yang and A. Khademhosseini, Biomaterials, 2012, 33, 9009–

9018.



3 K. Hermans, I. Tomatsu, M. Matecki, R. P. Sijbesma, C. W. M. Bastiaansen and D. J.

Broer, Macromol. Chem. Phys., 2008, 209, 2094–2099.



6 N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J.

Kumar and S. K. Tripathy, J. Mater. Chem., 1999, 9, 1941–1955.



Ed., 2012, 51, 892–896.


10 V. Petrov, V. Gáspár, J. Masere and K. Showalter, Nature, 1993, 361, 240–243.

Chapter 2

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11 H. Zhou, Z. Zheng, Q. Wang, G. Xu, J. Li and X. Ding, RSC Adv., 2015, 5, 13555–13569.

12 D. Suzuki, T. Kobayashi, R. Yoshida and T. Hirai, Soft Matter, 2012, 8, 11447–11449.

13 S. Maeda, Y. Hara, T. Sakai, R. Yoshida and S. Hashimoto, Adv. Mater., 2007, 19, 3480–

3484.

14 S. I. Shinohara, T. Seki, T. Sakai, R. Yoshida and Y. Takeoka, Angew. Chem. Int. Ed.,

2008, 47, 9039–9043.

15 S. Coleman, J. ter Schiphorst, A. Ben Azouz, S. Bakker, A. P. H. J. Schenning and D.

Diamond, Sens. Actuators, B, 2017, 245, 81–86.



17 K. Kumar, C. Knie, D. Bléger, M. A. Peletier, H. Friedrich, S. Hecht, D. J. Broer, M. G.

Debije and A. P. H. J. Schenning, Nat. Commun., 2016, 7, 11975.

18 A. H. Gelebart, G. Vantomme, E. W. Meijer and D. J. Broer, Adv. Mater., 2017, 29,

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19 Y. Yu, M. Nakano and T. Ikeda, Nature, 2003, 425, 145.

20 K. M. Lee, H. Koerner, R. A. Vaia, T. J. Bunning and T. J. White, Soft Matter, 2011, 7,

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23 J. M. Harrison, D. Goldbaum, T. C. Corkery, C. J. Barrett and R. R. Chromik, J. Mater.

Chem. C, 2015, 3, 995–1003.

24 K. Kumar, A. P. H. J. Schenning, D. J. Broer and D. Liu, Soft Matter, 2016, 12, 3196–

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25 J. Vapaavuori, A. Laventure, C. G. Bazuin, O. Lebel and C. Pellerin, J. Am. Chem. Soc.,

2015, 137, 13510–13517.


39

Chapter 3. Design of complex oscillating

topographies

ABSTRACT

Different alignment patterns in photochromic azobenzene containing liquid

crystal polymer coatings were studied and brought into oscillatory deformation

upon actuation with rotating polarized UV light. The use of photo-alignment

layers allowed for the generation of two similar alignment patterns, a symmetric

line pattern and a polarization grating pattern. Using symmetric alignment

patterns lead to oscillating topographies varying between tens to hundreds of

nanometers. The topographies are formed at certain local changes in alignment,

so-called alignment deformations. Typical hill-shaped topographies are achieved

at bend alignments, while valleys are formed at splay alignments. The actuation

with rotating polarized UV light leads to the oscillation of hills both in height and

laterally and the oscillation depends on operating temperature and illumination

intensity.

This chapter is partly reproduced from M. Hendrikx, D. Liu, A.P.H.J.

Schenning, D.J. Broer, Proc. SPIE 10735, Liquid Crystals XXII, 2018, 1073507.

40

1. Introduction

Polymeric materials responding to external stimuli with a mechanical

deformation have received great interest in recent years. With the ability to control

shape-changing properties in a more precise fashion, well-defined patterned

nanomaterials have been developed. The use of liquid crystal (LC) is attractive to

fabricate such materials.1 For example, LC polymers can be fine-tuned precisely for

the desired response by controlling their molecular alignment. Different mechanical

deformation is achieved by modulating the LC’s director in space and the so-called

order parameter in time. A reduction of the order parameter by an external stimulus

leads to a contraction along and an expansion perpendicular to the director.2 This

specific deformation can be further tuned by patterning the material.

Liquid crystal networks (LCNs) can be designed with responsive mesogens,

leading to deformations towards specific stimuli (mainly heat3, light4–7, electricity8,9

and humidity10,11). For example, azobenzene-containing LCN (azo-LCN) free-

standing films aligned in a splay pattern, gradually changing the alignment from

planar to homeotropic though the thickness, bends in the same direction regardless

of the direction of UV light with respect to the sample side.12 If the UV light is

switched off, the materials returns to its original shape. The bending deformation

can be used to the create artificial cilia working in water6 or light-induced walking

actuators that walk away from or towards to the light source.5 Three-dimensional

alignment has been achieved by photo-patterning LCNs. The resulting actuators

show for example accordion-shaped or spiky deformations.13,14 For surface-attached

azo-LCNs (coatings), the only deformation that is possible, is the z-direction leading

to the generation of surface topographies.15 By altering the alignment, large

topographies can be achieved by local expansion and contraction in the z-

direction.7,16 Work on different point defects revealed the formation of hills and

valleys depending on the LCN’s change in director17 and showed the effect of

different alignment deformations (namely splay, bend or twist) on the created

topographies. The use of these type of alignments only results in two states and the

continuous oscillatory deformation has not been reported.

Previously, we have reported on the generation of oscillatory deformation, using

0°/90° alignment (Chapter 2). Here, we focus on the generation of different geometric

light-induced oscillating topographies. We present a symmetric line pattern and a

polarization grating (PG) pattern in azo-LCN coatings supported on glass. We

investigate the shapes produced upon illumination.

Chapter 3

41

2. Results and discussion

2.1. Oscillating topographies of random symmetric boundaries with a

-45°/45° alignment

In Chapter 2, we have shown that asymmetric topographies can be achieved by

actuation of an azo-LCN that is photo-patterned with the alignment parallel (0°) or

perpendicular (90°) to the line domains with the presence of a hill and valley directly

connected (asymptotic). We now investigate a symmetric alternating line alignment

with domains at an angle of -45° and 45° with respect to the lines. It should be noted

that both the splay and bend changes between alignment coexist (Figure 3.1, bend

or splay). The two adjacent domains reach the boundary at the center and the

alignment will undergo two different elastic deformations: a bend or a splay elastic

deformation. This will lead to different types of stress created in the coating upon

UV illumination.

Figure 3.1. Illustration of the liquid crystal moieties of the -45°/45° aligned azo-LCN. Gray are inert

LC mesogens and yellow are azobenzene moieties. The backbone of the polymer is not illustrated for

simplification. Bend and splay represent the change in the alignment direction. This illustration

describes a +½ defect.

The symmetric -45°/45° aligned azo-LCN coatings were made by using a cell

having two patterned photo-alignment layers on glass. These cells were made by

exposure in two steps to achieve orthogonal aligned lines with different widths.

After filling the cell with the LC monomer mixture and curing with light, the

patterned azo-LCN coating is obtained by removal of one of the glass plates. Upon

investigation with polarized optical microscopy with the patterned lines parallel to

one of the polarizers (P and A), we observe the birefringence of the individual

domains (-45°/45°) with black defect lines. At 45°, we observe the birefringence of

the defect lines, indicating the topological changes at the boundary of the orthogonal

42

aligned domains (Figure 3.2). However, the defect lines contain small and local

defects (see insets Figure 3.2). This is due to the ability of the LCs to undergo a splay

or bend alignment, without a noticeable preference to one or the other (Figure 3.1).

Upon investigation with solely one polarizer (bottom Figure 3.2), we observed the

alignment of the dichroic azobenzene moieties. The domains with the director

parallel to the polarizer show up yellow compared to the domains with the director

orthogonal.

Figure 3.2. Polarized optical micrographs of the -45°/45° azo-LCN coating. Crossed polarizers (top, P

and A) and single polarizer (bottom, P) images show the alignment. The pitch is 50 µm. The insert in

the crossed polarized micrographs show a 3× zoom-in of the topological defect line with defects.

To analyze the oscillatory deformation of the patterned azo-LCN coatings, we

used a UV – blue light intensity ratio of 0.10 and a rotation speed of 2.5° s-1 for the

polarizer for topography formation at room temperature (Chapter 2 and ref. 18). We

monitor the center of the individual domains (zone 1 and 2, 20 µm away from the

defect line) and the topological defect line (zone 3 and 4) (Figure 3.3). For zone 1 &

2, we observe an oscillation out-of-phase near their initial height (see also Chapter

2). We observe that the topographies on the defect line are different depending on

the transition (splay or bend) between the adjacent -45° and 45° domains. Here, zone

3 (bend) creates a hill and oscillates, while the topography in zone 4 (splay) becomes

a valley upon actuation without any pronounced oscillations. The azo-LCN coating

creates either only a hill or a valley, thus a symmetric topography as result of the

symmetric alignment. Since the topographies are only pronounced on the defect line

without large lateral dimensions, changing the width of the domains led to no

further improvement of the height changes or oscillations.

Chapter 3

43

Figure 3.3. Digital Holographic Microscopy results of the -45°/45° aligned azo-LCN coating upon

actuation. Left) An illustration of the monitored -45°/45° coating with monitor zones 1–4. The dashed

line represents the topological defect line. Right) Height changes in function of time for zones 1–4.

Rotating polarizer (2.5° s-1) and LEDs (365 and 455 nm, 200 mW cm-2 and 20 mW cm-2, resp.) are

turned on at t = 30 s. Zone 1 and 2 monitor the bulk of the -45° and 45° domain, respectively. Zone 3

and 4 monitor the defect line at different locations.

To shed more light on the deformation of the defect line, the topography was

investigated in more detail (Figure 3.4). Remarkably, we observe that besides an

oscillation in height, a lateral oscillation takes place in x-direction. The clockwise

oscillating deformations are 15 nm in height and 4 µm in width (Figure 3.4b) and is

only observed at the defect lines with the bend alignment. As expected, the

clockwise rotation of the linear polarized UV light induces the in-phase deformation

of the azobenzene moieties (Figure 3.1) causing lateral clockwise stress rotations.

44

Figure 3.4. Oscillation in the -45°/45° aligned azo-LCN coating. Contour plot of the -45°/45°

topography (a) and the profiles at y = 25 µm for different arbitrary angles of linear polarized UV light

(b). The defect at x = 40 µm and y = 55 µm represents a +½ defect. The black arrows indicate the height

and lateral clockwise oscillation upon rotation of linear polarized UV light.

We investigated the thermal influence of the -45°/45° azo-LCN topography by

increasing the operating temperature from 30 °C to 90 °C (Figure 3.5). Without the

exposure to light, the surface starts deforming at temperatures above the

polymerization temperature (37 °C). The thermal deformation arises from a decrease

in order leading to the generation of topographies (Figure 3.5b). Upon illumination

with rotating linear polarized UV light, we observe that the amplitude of the

oscillation is temperature-independent above the glass transition temperature (46

°C) of the coating. However, the absolute height changes above the glass transition.

This behavior is different from the 0°/90° alignment as earlier report (Chapter 2). The

-45°/45° alignment shows lateral movement upon irradiation with rotating polarized

UV light. In other words, this leads to an oscillation in height adjacent to the center

of the topography. Moreover, the symmetric defect line also undergoes a symmetric

stress from both domains upon actuation. Increasing the temperature leads to a

larger topography. However, it appears that the lateral oscillation is restricted by the

width of the bend alignment.

Chapter 3

45

Figure 3.5. Influence of temperature on the actuation of a -45°/45° aligned azo-LCN coating. The actual

topography height as a function of time (a) and the surface profiles (b) at different temperatures for the

topography at the bend alignment transition in the -45°/45° azo-LCN coating. Solid and dashed lines

are in dark and exposed state, respectively. Rotating polarizer (2.5° s-1) and LEDs (365 nm: 200 mW

cm-2; 455 nm: 20 mW cm-2) turned on at t = 30 s.

2.2. Oscillating topographies of controlled symmetric boundaries with

alternating bend and splay alignment

In order to control the bend and splay alignment in the defect lines, we used a

polarization grating alignment to create an alternating bend and splay pattern along

the grating vector (�⃗⃗� ) (Figure 3.6). An azo-LCN with PG alignment was obtained*

by patterning brilliant yellow (BY) coated glass plates with a holographic

polarization interference pattern (pitch 15 µm). These patterned cells were filled

with the LC monomer mixture and cured. A coating was obtained by removal of one

the glass plates. Alternatively, coatings were made by spincoating and curing LC

mixture directly on one BY patterned glass plate.

* The aligned BY coated glass plates and LC cells were kindly made and supplied by

Colin McGinty (research group Philip Bos, Kent State University).

46

Figure 3.6. Illustration of the liquid crystal moieties of the polarization grating (PG) aligned azo-LCN.

Gray are inert LC molecules and yellow are azobenzene moieties. The backbone of the polymer is not

illustrated for simplification.

After creation of the azo-LCN coating with the PG alignment, the coating was

investigated with polarized optical microscopy to ensure the alignment (Figure 3.7).

Under each given angle, with respect to the polarizers (P and A), continuous blue

colored and black lines are observed along the grating vector (�⃗⃗� ) with a pitch of ca.

15 µm. This proved the desired alignment was achieved without the loss of order,

hence the blue color. We assume that the small differences in the tint of blue arise

from a difference in height, and thus thickness. We can also clearly observe an initial

topography at the two monitored zones, the bend and splay deformation alignment

of the PG alignment. This height difference can lead to a different color observed in

the micrographs.

Figure 3.7. Polarized optical microscopy images of the PG azo-LCN coating at different angles with

respect to the polarizer (P) and analyzer (A). �⃗⃗� is the grating vector.

Upon illumination, the initial topography is inverted without any oscillations (I

in Figure 3.8). After 720 s (12 min) of illumination, we observe a relative stable

oscillation mainly expressed by the bend alignment of the PG azo-LCN (II in Figure

3.8). The topographies reach to nearly 200 nm in height and show oscillations of 20

nm, thus 10 % of the total topography (Figure 3.8a). We speculate that the initial

height of ca. 20 nm results from the periodical tension induced by azo-LCN upon

cooling from the polymerization temperature (37 °C) to room temperature. The

Chapter 3

47

order of the azo-LCN is larger at room temperature leading to the generation of

topographies with maxima located at the splay alignment. In Figure 3.8b, the initial

surface topography has its extrema located at the inverse location of the final

illuminated state. During illumination, no deformation along the changing LC

director is observed while rotating the polarizer. Over the 20 s timespan plotted in

the graph, the polarizer moved 50°. If the surface would form topographies solely at

the maximal absorption locations (linear polarized UV light parallel to LC director),

the topography should travel and a travelling wave on the surface would be

observed. This means that the light-induced stresses dictate the formation of

topographies in these coatings. Exactly like the previous section 2.1 and as reported

by Babakhanova et al.17, these coatings form maxima and minima at splay and bend

alignments, respectively.

Figure 3.8. Actuation of the PG aligned azo-LCN coating. (a) Actual height of the PG azo-LCN coating

under rotating polarized UV light and unpolarized 455 nm light illumination. Surface profile of the

PG azo-LCN coating during the first 20 s of illumination. Rotating polarizer (2.5° s-1) and LEDs (365

and 455 nm, 200 mW cm-2 and 20 mW cm-2, resp.) are turned on at t = 60 s. (b) Surface profiles during

the first 20 s of illumination with initial topographies of ca. 20 nm. I and II indicate the timespans

discussed in more detail in graph Figure 3.8b and Figure 3.9, respectively.

When taking a closer look at the oscillating topography in the graph and the

different contour plots at different angles of the polarizer for one single rotation,

lateral deformations are observed (Figure 3.9). The oscillation laterally is limited to

ca. 10 % of the PG pitch, corresponding to an oscillation of 2 µm. Here, the oscillation

is counter clockwise in correspondence with the counter clockwise rotation of the

polarizer. Furthermore, we also clearly observed that the adjacent topography

maxima oscillate laterally in phase; this is due to the repetitive alignment of the PG

azo-LCN.

48

Figure 3.9. Oscillations in the PG aligned azo-LCN coating. Surface profile (a) and contour plot (b) of

the PG aligned azo-LCN coating during one full rotation of the polarizer. Arrows indicate the trajectory

of the topography over time. The black dotted line shows the change in position of the topography

maxima at the different polarizer angles. Timespan corresponds to the gray box II in Figure 3.8a.

Lastly, we tested the influence of a lower UV intensity at 50 mW cm-2 and

operating temperatures (room temperature and 50 °C). As expected, the topography

height and the corresponding oscillation at the bend alignment transition region are

smaller compared to the higher intensity (200 mW cm-2) at room temperature (Figure

3.10a). At elevated temperatures (50 °C), we observe an initial surface topography

with the maxima at the bend alignment and minima at the splay alignment (Figure

3.10b). This coincides with the topographies found with previous illumination

experiments. Upon illumination, the topographies reach the ‘stable’ state around

which oscillation occurs nearly instantly while approximately doubling the

oscillation amplitude. Meanwhile, the splay aligned area starts showing a more

pronounced oscillation. These enhanced effects are due to the actuation at

temperatures barely above the glass transition temperature (46 °C), leading to faster

actuation and relaxation kinetics of the azobenzene moieties and higher mobility in

the polymer network.

Chapter 3

49

Figure 3.10. Actuation of the PG aligned azo-LCN coating with lower intensities. (a–b) Oscillations

created in a PG azo-LCN with 50 mW cm-2 rotating linear polarized UV light and 5 mW cm-2

unpolarized blue light at room temperature (a) and at 50 °C (b). Polarizer (2.5° s-1) and LEDs are

turned on after 30 s.

3. Conclusion

Different director patterns were made in azo-LCN coatings by means of photo-

alignment layers. These patterned coatings show a specific topographic response to

UV and blue light exposure depending on the alignment pattern. For alignments

with symmetric boundaries, the topographic geometry is also symmetric. However,

depending on the alignment pattern, splay or bend, the forming topography

becomes either a hill or a valley. Depending on the alignment pattern, these

topographies can reach up to 200 nm and show lateral oscillations of ca. 2 µm in

combination with height oscillations of ca. 40 nm.

The controlled oscillating patterns are interesting for the variety of applications,

such as friction control, cell culturing, or in case of diffractive coatings, even optical

applications as topographies can improve the diffraction efficiency. Furthermore,

the alignments patterns are not limited to line structures and can be extended to

point defects leading to new oscillating surface topographies. Moreover, these

coatings with lateral displacements might show great potential as icephobic

surfaces.

4. Experimental section

4.1. Materials

The azo-LCN coatings are made from a mixture of liquid crystalline acrylates and

necessary additives shown in Scheme 3.1 and was described previously in more

detail.6,19 Monomer 1 to 3 were obtained from Merck UK. Monomer 4 was custom-

50

synthesized by Syncom (Groningen, the Netherlands). Photoinitiator 5 was obtained

from Ciba. A typical azo-LCN composition consists of 42 wt% monomer 1, 21 wt%

monomer 2, 31 wt% monomer 3, 5 wt% monomer 4 and 1 wt% photoinitiator 5. 2-

(N-ethylperfluorooctanesulfonamido) ethyl acrylate (surfactant) was bought from

BOC Sciences. The constituents were mixed by dissolving in dichloromethane and

stirred until a homogeneous solution was obtained. Dichloromethane was removed

under reduced atmosphere to achieve a reactive LC monomer mixture. The photo-

alignment layers, LPP ROP-108/2CP and brilliant yellow (BY), were obtained from

Rolic and the group of Philip Bos (Kent State University), respectively. All chemicals

were used as received unless stated otherwise.

Scheme 3.1. Chemicals used to create responsive nematic liquid crystal network coating.

4.2. Fabrication of the patterned azo-LCN coating

4.2.1. Symmetric -45°/45° alignment

Glass substrates (3 × 3 cm2) were cleaned by sonication using acetone and

propanol-2 followed by UV ozone cleaning. Photo-alignment material (LPP) was

spincoated onto the cleaned substrates. Two substrates were glued together using

adhesives containing 6 µm spacers. The LPP surfaces of the thus obtained LC cells

are patterned by a 2-step exposure. In the first step, the sample was exposed through

a line mask with polarized light at 45° to the lines for 15 minutes. In the second step,

the mask was removed and a shorter flood exposure, 3 minutes, was applied with

light with polarization orthogonal to the first exposure (135°). The second exposure

aligns the areas that were unaddressed but does not overwrite the alignment

Chapter 3

51

achieved by the first exposure step, thus creating orthogonally aligned pattern. The

LC cells were filled at 75 °C with the LC monomer mixture by capillary forces and

cured at 38 °C with light > 400 nm (EXFO Omnicure S2000) followed by a short post-

cure at 125 °C for 5 minutes. Afterwards, one of the glass plates was removed leaving

a coating adhering to glass at one side and with a free surface at the other side.

4.2.2. Polarization grating alignment

The PG patterned layers and cells were supplied by the research group of Philip

Bos (Kent State University). The BY substrates or empty cells (5 µm gap) were

patterned using a holographic exposure created by two beams with opposite

handedness circular polarization (λlaser = 457 nm, Optotronics VA-I-200-547). Both

beams were of the same intensity (4.7 mW). The BY substrates were exposed for 10

minutes. In case of the single substrate, the LC monomer mixture with the addition

of 1 wt% surfactant was spincoated at a 33 wt% solution in DCM (2500 rpm, 28 s)

and cured at 38 °C with light > 400 nm (EXFO Omnicure S2000) followed by a short

post-cure at 125 °C. For the 5 µm BY cells, the empty cell was loaded at the open

edge with crystalline LC monomer mixture at room temperature, heated to 75 °C in

a vacuum oven and left overnight under reduced pressure. The molten isotropic LC

monomer mixture filled the BY cell by capillary forces and was brought back to

atmospheric pressure. The filled BY cell was cured at 38 °C with light > 400 nm

(EXFO Omnicure S2000) followed by a short post-cure at 125 °C.

4.3. Characterization and actuation of the patterned azo-LCN coating

The monomeric mixture and coatings are characterized with a crossed polarized

microscope (Nikon Ci Eclipse) with a thermocontrolled stage (Linkam). For the

monomeric mixture the transition temperature, nematic to isotropic, is determined

by cooling from isotropic liquid to nematic LC phase. Both the polymeric and

monomeric transitions were confirmed with differential scanning calorimetry (DSC

Q1000, TA Instruments). The surface of the coating is monitored with a digital

holographic microscope (DHM® R210, Lyncée Tec SA, Switzerland) equipped with

a thermocontrolled stage (Linkam) and mounted with UV (λ = 365 nm) and blue

light (λ = 455 nm) collimated LEDs (M365L3 and M455L3 respectively, Thorlabs).

The UV light originated from the UV LED was redirected with a UV mirror

(Thorlabs) and polarized with a polarizer (10LP-UV, Newport) mounted in a

rotating stage (Thorlabs). Typical experimental procedure is described in more detail

in Chapter 2.

52

References


2 C. L. Van Oosten, K. D. Harris, C. W. M. Bastiaansen and D. J. Broer, Eur. Phys. J. E,

2007, 23, 329–336.





5 A. H. Gelebart, D. J. Mulder, M. Varga, A. Konya, G. Vantomme, E. W. Meijer, R. L.

B. Selinger and D. J. Broer, Nature, 2017, 546, 632–636.


7 D. Liu and D. J. Broer, Liq. Cryst. Rev., 2013, 1, 20–28.

8 W. Feng, D. J. Broer and D. Liu, Adv. Mater., 2018, 30, 1704970.

9 D. Liu, N. B. Tito and D. J. Broer, Nat. Commun., 2017, 8, 1526.

10 A. Ryabchun, F. Lancia, A. D. Nguindjel and N. Katsonis, Soft Matter, 2017, 13, 8070–

8075.

11 M. Dai, O. T. Picot, J. M. N. Verjans, L. T. De Haan, A. P. H. J. Schenning, T. Peijs and

C. W. M. Bastiaansen, ACS Appl. Mater. Interfaces, 2013, 5, 4945–4950.

12 C. L. Van Oosten, D. Corbett, D. Davies, M. Warner, C. W. M. Bastiaansen and D. J.

Broer, Macromolecules, 2008, 41, 8592–8596.

13 L. T. de Haan, V. Gimenez-Pinto, A. Konya, T. S. Nguyen, J. M. N. Verjans, C. Sánchez-

Somolinos, J. V. Selinger, R. L. B. Selinger, D. J. Broer and A. P. H. J. Schenning, Adv.

Funct. Mater., 2014, 24, 1251–1258.

14 S. K. Ahn, T. H. Ware, K. M. Lee, V. P. Tondiglia and T. J. White, Adv. Funct. Mater.,

2016, 26, 5819–5826.


2012, 45, 8005–8012.


17 G. Babakhanova, T. Turiv, Y. Guo, M. Hendrikx, Q. H. Wei, A. P. H. J. Schenning, D.

J. Broer and O. D. Lavrentovich, Nat. Commun., 2018, 9, 456.

18 M. Hendrikx, A. P. H. J. Schenning and D. J. Broer, Soft Matter, 2017, 13, 4321–4327.


Chapter 3

53

55

Chapter 4. Compliance-mediated

topographic oscillations

ABSTRACT

The ability to induce oscillating surface topographies in light-responsive

liquid crystal networks on-demand by light is interesting for applications in soft

robotics, self-cleaning surfaces and haptics. However, the common height of

these surface features is in the range of tens of nanometer, which limits their

applications. Here, a photo-responsive liquid crystal network coating with a

patterned director motive exhibiting surface features that oscillate dynamically

when addressed by light with modulated polarization is reported. By utilizing a

compliant intermediate layer, the surface topographies increase with a factor 10,

from roughly 70–100 nm to 1 µm. This increase in topography height is

accompanied by a superimposed dynamic oscillation with an amplitude of ca.

100 nm. These values can be translated to a 16.7 % average static strain with

3.3 % oscillations with respect to the coating thickness. Moreover, utilizing the

complying support increases the maximum rotation speeds with an in-phase

response from 2.5° s-1 up to 25° s-1. However, at this maximized rotation speed the

oscillation amplitude decreases to about half of the initial value.

This chapter is partly reproduced from M. Hendrikx, B. Sırma, A.P.H.J.

Schenning, D. Liu, D.J. Broer, Advanced Materials Interfaces, 2018, 201800810.

56

1. Introduction

The surface of a material is the communication channel between its bulk property

and the environment, or between the function of a device and the human perception.

The surface properties are largely determined by the hierarchical topographical

landscape of the material, varying from nanometers to micrometers in height and

from micrometer to centimeters in lateral direction. Controlling these landscapes in

shape and size is crucial to tune the surface properties. The ability of surface

topographies to respond to external triggers gives rise to new applications in the

field of haptics, friction control, self-cleaning (by repelling or rejecting liquids and/or

solids), structural color changes and adaptive micro-optics.1–3 The next challenge is

to achieve continuous oscillatory dynamic materials with large and fast responses.

This type of response translates to a perpetual change upon continuous triggering

and is interesting for applications such as haptics.

Polymers containing azobenzene light-responsive trigger molecules provide a

basis for investigating different topographies on surfaces.4–6 The azobenzene

molecules allow for controlled switching, based on the trans-to-cis and cis-to-trans

isomerization. These molecules can be controlled by selectively illuminating the

azobenzene’s absorption bands, being 365 nm and 455 nm, respectively.7–10

However, it remains a challenge to obtain reversible and/or dynamic topographies

with at high response speeds combined with large topographical changes. At most,

the deformations achievable are up to a micrometer at best. These large topographies

were only reported for chiral nematic liquid crystal coatings.2,11 For other glass-

supported azo polymers, topographies are smaller, reaching up to only a few

hundred nanometers.12,13 We have shown in Chapter 2 and Chapter 3 that

oscillations in glass supported coatings are achievable by changing the polarization

state of incident UV light. The azobenzene’s dichroism leads to a change in

absorption and surface topography as response to a constantly changing

polarization of UV light. We observed localized topographies near the defect lines,

where the largest stresses accumulate. However, depending on the desired

application (haptics or friction control), the achieved topographies and oscillation

amplitude can be rather small. It would be of great interest if the localized stresses

could be expressed globally to enhance the topographies. Glass as a substrate limits

and governs the topography size by suppressing lateral stresses. To express these

stresses, harder responsive layers are typically deposited on soft substrates.

Chapter 4

57

Examples showing stress releases in coatings typically show wrinkling, as two layers

of different modulus are deposited on one another.14–16

In this work, we present an approach to bridge the effects observed in

freestanding films and the applicability of surface-attached coatings. For that, we

created a layer stack with a soft compliant layer between glass and the photo-

responsive glassy coating. Different compositions of soft layers were used based on

acrylates to achieve an optimal actuation, focusing on a larger deformation and

faster response depending on the domain size.

2. Results and Discussion

The detailed preparation of the azobenzene-containing liquid crystal network

(azo-LCN) coatings supported by a soft compliant layer in between the coating and

the glass substrate is described in the Experimental section. Briefly, a non-

polymerized soft acrylate layer consisting of 2-ethylhexyl acrylate (EHA) and tetra

(ethylene glycol) diacrylate (TEGDA) has been sandwiched between an acrylate

functionalized glass plate and a premade 6 µm thick azo-LCN coating (Figure 4.1).

After photopolymerization of the soft compliant layer, a stable layer stack is

obtained. By decreasing the crosslink density (expressed in vol% TEGDA), the

modulus of the soft layer can be changed from 900 MPa to ca. 1 MPa (Appendix 4.A,

Figure 4.A1a). We found that the azo-LCN polymer has a high storage modulus of

2.6 and 1.1 GPa, parallel and perpendicular to the director, in the absence of light.

Upon illumination with UV and blue light, the modulus drops to 0.85 and 0.1 GPa

for parallel and perpendicular director upon linear polarized UV (LPUV) irradiation

parallel to the director (Appendix 4.A, Figure 4.A1b).

The azo-LCN coating is aligned in adjacent line domains with orthogonal

director. The anisotropic expansion and contraction perpendicular and parallel to

the director, respectively, induced by illumination are indicated by the red arrows

in Figure 4.1.

58

Figure 4.1. Alignment of the azo-LCN acrylate bilayer coating. (a) Polarized micrographs of the azo-

LCN acrylate bilayer at different polarization angle of the polarizer (P) and analyzer (A). �⃗⃗� is the

grating vector. (b) Graphical representation of the line pattern alignment of the azo-LCN coating (top)

and of the bilayer coating with the compliant layer sandwiched between the azo-LCN and the glass

substrate (bottom). The red arrows indicate the stress induced by UV light as result of the alignment.

The thickness of the azo-LCN coating is 6 µm and the soft layer is at least 70 µm.

In order to obtain the topographies, the azo-LCN bilayer is exposed with static

polarized UV light (365 nm) and unpolarized blue (455 nm) light at an intensity ratio

of 0.1. Earlier work has proven that this ratio leads to the largest deformation by

continuous trans-cis-trans isomerization.17 The angle of the polarizer is aligned at 90°

to fully express the stress created by the line with the director parallel to the grating

vector (Κ⃗⃗ ). This orientation led to a maximum topography height. We also expect

that part of the light will be absorbed by the 0° aligned line caused by the distribution

function of the azobenzene molecules with respect to their director. As can be seen

in Figure 4.2a, different topographical heights for 500 µm wide lines were obtained

for the various compliant layers. Upon decreasing the TEGDA content, and thus

increasing the compliance (the inverse of the tensile storage modulus), we observe

an increase in the height of the topographical feature.

From previous work on glass supported azo-LCN coatings, we know that most

of the stresses and resulting strains are expressed near and at the boundaries of the

aligned domains within a range of 20 µm. In our bilayer system, we see a similar

response of the LCN coatings on the higher crosslinked substrates (≥ 60 vol%

TEGDA, ≥ 60 MPa). However, remarkably for the compliant substrates with lower

Chapter 4

59

crosslink density (≤ 40 vol% TEGDA, ≤ 15 MPa), a clearly distinguishable

amplification of the structure height occurs. This makes us believe that the lateral

stresses are propagated throughout the soft substrate layer to the neighboring

domains. Figure 4.2a shows the profiles of the 500 µm wide domains for the different

compliant bilayers under static polarized illumination.

The profiles of each bilayer, in Figure 4.2a, are centered at x = 500 µm and a

mismatch is visible at x = 0 and 1000 µm. This observation is a direct confirmation of

the stresses being transferred and expressed through the soft substrate leading to

larger deformations with broadening and narrowing of the 0° and 90° line domains,

respectively. This change in line width is plotted in Figure 4.2b as relative domain

size (the width of the deformed topography divided by the pitch of the line pattern)

as function of the compliance. Here, we observe that the lateral change levels off at

increasing softness. The relative domain sizes of both domains are a direct mirror

image and diverge towards an expansion and shrinkage of ca. 3–4 % for the 0° and

90° domains, respectively. These values are larger than those found by van Oosten

et al. for the contraction and expansion of freestanding uniaxial aligned azo-LCN

films.18 It is important to realize that in our case the change in domain size is an

interplay of both parallel contraction and perpendicular expansion, leading to these

higher values.

Figure 4.2. Topographies of different compliant azo-LCN acrylate bilayer coatings. (a) Profiles of the

500 µm wide lines under static polarized UV light illumination at 90° (200 and 20 mW cm-2 for UV

and blue LED light, respectively). The left and right side of the graph shows the 0° and 90° aligned

domains, respectively. The profiles are centered at x = 500 µm. A mismatch is observed at x = 0 µm and

1000 µm. (b) The relative domain size (ratio between topography width and line width) of two adjacent

500 µm wide lines as function of the compliance (the inverse of the storage modulus). This graph

expresses the widening and narrowing of the 0° and 90° domain, respectively.

60

Furthermore, we believe that the thickness of the sublayer plays an important

role for less compliant layers. This thickness is chosen to be 10 times the thickness of

the azo-LCN top coating. Especially for the softest sublayers, an onset of wrinkling-

like phenomena might be observed at the larger periodicities, explaining the shift of

the maximum deformation towards the center rather than at the edge of the director

domain (ear shape structures).

Next, the polarization axis of the UV light is continuously rotated, in presence of

a stationary blue light source, stimulating the trans-cis azobenzene conversion in

specific domains. The blue light promotes the back reaction of the azobenzene. This

combination provides a continuous shift of the balance between the forth and back

reaction of azobenzene, forming a continuously changing stress between the two

domains in the coating. In Figure 4.3a, the height difference of these structures is

shown as function of time under illumination with rotating linear polarized UV and

unpolarized blue light. The definition of the height difference is given in the

Experimental Section. Upon further investigation of the shape and velocity of the

oscillation at the various compliances of the sublayer, we observed an asymmetric

phenomenon. For the harder layers, the oscillation is symmetric, as indicated by the

shape of the velocity profile in Figure 4.3b. For the softer compliant layers, we

noticed that the oscillation has an asymmetric sine wave shape. This wave shows a

longer maximum time and shorter minimum time (inverted U-shape). The velocity

profile during time tends to form a sawtooth shape.

Chapter 4

61

Figure 4.3. Oscillations of different azo-LCN compliant acrylate bilayer coatings. (a) Height difference

of the oscillations induced by rotating linear polarized UV light (200 mW cm-2) and unpolarized blue

light (20 mW cm-2) for the different compliant coatings. (b) A zoom-in (top of figure 3b) of the stable

oscillations and corresponding speed for the 20–80 vol% TEGDA compliant coatings. The oscillation

is a result of the rotation of the polarizer (2.5° s-1) and the illumination of UV and blue light LEDs (200

mW cm-2 and 20 mW cm-2, respectively). The line width is 500 µm.

In order to investigate the stress-translating effect of the compliant layer in more

detail, we studied the height difference and oscillation of different line widths

(domain sizes). For a hard layer (0.9 GPa, 100 vol% TEGDA), we found a dependency

of the lateral dimensions of the lines and concluded that smaller widths (ca. 25 µm)

lead to more expressed surface structures (Figure 4.4a). This finding is due to the

local expression of the stress near the defect. In contrast, for the softest substrate (1

MPa, 5 vol% TEGDA), we observed a large increase in topography height by

increasing domain size (Figure 4.4b). This observation confirms that the

topographies created on soft layers are an expression of the lateral stresses induced

by the adjacent domains. Figure 4.4c and Figure 4.4d show the topographies formed

for both compliant layers with the same scale. The smaller topography size in the

smaller domains (25 µm) are caused by the limited widening and narrowing of the

domain size through the limited shear applied by the smaller domain.

62

Figure 4.4. The height change expressed as function of time for line patterns with different width for

100 vol% TEGDA (a) and 5 vol% TEGDA (b). The 3D images of the surface for 100 vol% TEGDA

(c) and 5 vol% TEGDA (d). The oscillation is a result of the rotation of the polarizer (2.5° s-1) and the

illumination of UV and blue light LEDs (200 mW cm-2 and 20 mW cm-2, respectively). It is important

to note that the scale of the y-axis in graphs a and b is different by a factor of 10.

Lastly, we investigated the influence of the rotation speed of the polarizer for the

softest sublayer (1 MPa, 5 vol% TEGDA). The domains tested were 100 µm wide to

ensure a full observation of the complete area with a 10× DHM objective. The

optimal rotation speed was determined by plotting height and its derivative (speed)

as function of time with increasing rotation speed of the polarizer every two full

rotations (Figure 4.5). We observe a slight asymmetric sine wave shape (inverted U-

shape) as oscillation for all oscillations and the derived speed confirms this

asymmetricity. From the bottom graph (Figure 4.5b), it can derived that the speed of

the oscillations is asymmetric for all different speeds. Importantly, the topography

size does not diminish with increasing rotation speed. The amplitude decreases from

96 nm to 57 nm upon increasing the rotation speed to 25° s-1. Even after these high

and fast actuations, we observed no damage or delamination in the bilayer coating.

Chapter 4

63

Figure 4.5. Influence of the rotation speed on the oscillatory deformations for 100 µm wide azo-LCN

domains on 5 vol% TEGDA soft layer. The height of the topography (a) and the derivative (speed) (b)

of at different rotation speeds. The oscillation is a result of the rotation of the polarizer and the

illumination of UV and blue light LEDs (200 mW cm-2 and 20 mW cm-2, respectively). Rotation speeds

are 1.0, 2.5, 5.0, 7.5, 10, 12.5, 15 and 20° s-1. Rotation speed is increased after 2 full rotations or 4

periods of oscillation, indicated with the dashed blue lines.

3. Conclusion

The use of a compliant sublayer for supporting a patterned azo-LCN coating has

a large effect on the formation of topographical structures by light. We have

demonstrated an amplification of a factor 10 for the height of the surface

topographies and an increased oscillation speed when addressed by a polarization-

modulating light source. As result, topographies of 1 µm can be achieved in fully

planar aligned azo-LCN coatings with oscillations reaching to ca. 100 nm in

amplitude. This corresponds with a time-averaged topographic structure of 16.7 %

of the azo-LCN coating’s thickness with an oscillation amplitude of 1.67 %. In

contrast to LCN coatings directly on glass or on a stiffer sublayer, the patterned LCN

coating on a low modulus sublayer give the largest topographical heights for the

larger domain sizes (0.5 mm). Whereas for LCN coatings on a hard sublayer (0.9

GPa), the smaller domains (25 µm) perform best with respect to topography height

and amplitude. Of relevance for applications, we have observed large deformations

at much higher rotating speeds than in the absence of the soft sublayer. Increasing

the rotation speed with a factor 10 only depletes the total topographical heights with

20 nm. However, the oscillation amplitude nearly drops to half of the original value.

All these results show an interesting new design of materials to overcome the

applicability issues of glass-supported azo-LCN coatings.

64

The improvements made by only implementing a soft support layer leads

to the possibility to design new applications while having the ability to fine-tune the

response without completely changing the chemistry. The much faster oscillations

and larger topographies can further expand on applications in haptics, robotics or

even towards control of materials on surfaces.

4. Experimental

4.1. Chemicals

Azo-LCN coatings were made from a mixture of liquid crystalline acrylates and

additives, as shown in Scheme 4.1, and have been described previously in more

detail in Chapter 2. Monomers 1–3 were obtained from Merck UK. Monomer 4 was

custom-synthesized by Syncom (Groningen, the Netherlands). Photo-initiator 5 was

obtained from Ciba. The azo-LCN composition consists of 42 wt% monomer 1, 21

wt% monomer 2, 31 wt% monomer 3, 5 wt% monomer 4 and 1 wt% photoinitiator

5. The constituents were mixed homogeneously by dissolving in dichloromethane.

The solvent was removed under a reduced atmosphere to achieve a reactive LC

monomer mixture. The photo-alignment layers used are Brilliant Yellow (BY) and

PAAD-22. These materials were bought from Sigma Aldrich and BEAM Co,

respectively. BY was dissolved at 1.5 wt% in dimethyl formamide (DMF) and

PAAD-22 was diluted 3 times in DMF prior to application. The soft layers were made

from a mixture of 2-ethylhexylacrylate (EHA) and tetra (ethylene glycol) diacrylate

(TEGDA) in a volume ratio of 0.95, 0.80, 0.60, 0.40, 0.20 and 0.00, respectively, with

1 wt% of photo-initiator 5. 2-(trimethoxy silyl) propyl methacrylate is bought from

Sigma Aldrich and dissolved in ethanol at a 1 vol% concentration.

Chapter 4

65

Scheme 4.1. Chemical composition of the reactive LC monomer mixture.

4.2. Preparation of the patterned LCN coatings on soft layers

Glass substrates (3×3 cm2) were cleaned by sonication using acetone and

propanol-2 followed by UV ozone cleaning. The photo-alignment material (BY or

PAAD-22) and the 2-(trimethoxy silyl) propyl methacrylate solution were spin-

coated onto the cleaned substrates and baked for 10 minutes at 100 °C. BY or PAAD-

22 coated substrates were glued together using an adhesive containing 6 µm spacers.

The BY or PAAD-22 surfaces of the thus obtained LC cells were patterned by a two-

step exposure. For BY, in the first step, the sample was exposed to polarized light

through a mask for 5 minutes. In the second step, the mask was removed and a

shorter flood exposure, 2 minutes, was applied with light with polarization

orthogonal to the first exposure. For PAAD-22, the patterning was performed in a

two-step procedure as well. First, a flood exposure of 2 minutes in one polarization

direction is performed followed by a masked 1.5-minute orthogonal aligned

polarization exposure. These LC cells were filled with the LC monomer mixture by

capillary forces at 75 °C. The filled cells were cured at 38 °C with light > 400 nm

(EXFO Omnicure S2000). Afterwards, one of the glass plates was removed leaving a

coating adhering to glass on one side and with a free surface on the other side. This

azo-LCN coated substrate was washed with water to dissolve any remaining PAAD-

22 and dried. Afterwards, the coated substrate is glued to an acrylate modified

substrate with adhesive containing 70 µm spacers. This newly constructed cell was

filled with the EHA:TEGDA mixture and polymerized with light > 400 nm for 15

minutes at room temperature. Afterwards, the top glass plate with the alignment

66

layer was removed and any remaining PAAD-22 was dissolved in deionized water.

The desired layer stack was dried for at least 48h before use. Figure 4.6 shows a

scheme of the preparation of these layered coatings.

Figure 4.6. Schematic illustration of the preparation of the photo-patterned azo-LCN compliant bilayer.

The black lines in the photo-alignment layer and azo-LCN coating indicate the line pattern.

4.3. Characterization and actuation of the azo-LCN soft layered coating

The monomeric LC mixture and azo-LCN coating were characterized using a

crossed-polarized optical microscope (Nikon Ci Eclipse) equipped with a thermo-

controlled stage (Linkam). For the monomeric mixture, the nematic to isotropic

transition temperature is determined by cooling from the isotropic liquid to the

nematic LC phase. Both the polymeric and monomeric transitions were confirmed

using differential scanning calorimetry (DSC Q1000, TA Instruments). The

mechanical properties of the individual polymer layers were measured with a

dynamic mechanical thermal analyzer (DMTA, Q800 Dynamic Mechanical

Analyzer, TA Instruments). The soft polymers were made in a mold to obtain a

tensile bar (40×10×1 mm3). The modulus of a uniaxial aligned azo-LCN film was

measured perpendicular and parallel with respect to the LC director. The

illumination of the uniaxial aligned azo-LCN films during the modulus

measurement in DMTA was performed with a 365 nm LED (M365L, Thorlabs)

passing through a polarizer (LPUV100-MP2, Thorlabs) mounted in a rotating stage

(PRM1Z8, Thorlabs) and a 455 nm LED (M455L3, Thorlabs) at 70 mW cm-2 and 7 mW

cm-2, respectively. The surface of the azo-LCN coating was monitored using a digital

holographic microscope (DHM® R210, Lynceé Tec SA, Switzerland) equipped with

a thermo-controlled stage (Linkam) and mounted with UV (λ = 365 nm) and blue

light (λ = 455 nm) collimated LEDs (M365L3 and M455L3, respectively, Thorlabs).

Chapter 4

67

This setup was discussed earlier (Chapter 2). The objective used has a magnification

of 10×, this translated to the ability of visualizing a 500×500 µm2 region. For the

patterns of 500 µm wide, the boundary was placed in the center of the field of view

and monitoring zones were placed in each zone of 250 µm. This corresponds the half

of each domain. For smaller domains, the complete domain was monitored. The

change in height is plotted as a difference between the 90° domain and the 0°

domain. Changes of the surface were monitored in real-time. The acquisition rate of

the holograms was set to 0.5 frames per second. This resulted in a captured hologram

every 5° of rotation.

References



Ed., 2012, 51, 892–896.

3 A. Priimagi and A. Shevchenko, J. Polym. Sci. Part B Polym. Phys., 2014, 52, 163–182.



15848.

6 M. Hendrikx, A. P. H. J. Schenning, M. G. Debije and D. J. Broer, Crystals, 2017, 7, 231.




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10 K. M. Lee, M. L. Smith, H. Koerner, N. Tabiryan, R. A. Vaia, T. J. Bunning and T. J.

White, Adv. Funct. Mater., 2011, 21, 2913–2918.



12 C. Rianna, L. Rossano, R. H. Kollarigowda, F. Formiggini, S. Cavalli, M. Ventre and

P. A. Netti, Adv. Funct. Mater., 2016, 26, 7572–7580.

13 A. Bobrovsky, K. Mochalov, V. Oleinikov, D. Solovyeva, V. Shibaev, Y. Bogdanova,

V. Hamplová, M. Kašpar and A. Bubnov, J. Phys. Chem. B, 2016, 120, 5073–5082.

14 A. Agrawal, P. Luchette, P. Palffy-Muhoray, S. L. Biswal, W. G. Chapman and R.

Verduzco, Soft Matter, 2012, 8, 7138–7142.


Funct. Mater., 2015, 25, 1360–1365.

16 C. Zong, Y. Zhao, H. Ji, X. Han, J. Xie, J. Wang, Y. Cao, S. Jiang and C. Lu, Angew.

Chem. Int. Ed., 2016, 55, 3931–3935.


18 C. L. Van Oosten, K. D. Harris, C. W. M. Bastiaansen and D. J. Broer, Eur. Phys. J. E,

2007, 23, 329–336.

68

Appendix 4.A

Modulus of the individual layers and influence of light on the azo-

LCN

The influence of the EHA-content on the modulus measured with DMTA is

plotted in Figure 4.A1a. The modulus of the compliant layer lowers with increasing

EHA-content from 0.9 GPa to 1 MPa. The top layer is an azo-LCN coatings and the

modulus is measured parallel (strong direction) and perpendicular (weak direction)

to the alignment director (Figure 4.A1b) and found to be 2.6 and 1.1 GPa,

respectively. Upon illumination with polarized UV light parallel to the director and

unpolarized blue light (t1), the modulus drops rapidly to 0.8 and 0.1 GPa for the

strong and weak direction, respectively. Rotating the polarizer perpendicular to the

alignment (t2), the modulus increases to 1.4 and 0.2 GPa for the strong and weak

direction, respectively. When the polarizer is rotated constantly at 2.5° s-1 (t3), the

modulus oscillates between these values in phase with the polarizer. At t4, the

polarizer is left parallel to the alignment direction. Removing the UV light while

maintaining blue light (t5), the modulus increases rapidly to 2.4 and 1.0 GPa for the

strong and weak direction. After turning off the blue light (t6), the modulus slowly

returns to its original value.

Figure 4.A1. (a) Storage modulus of the soft layer in function of the EHA-content. (b) Storage modulus

of the azo-LCN as freestanding film upon illumination with polarized UV light (t1-6) and unpolarized

blue light with recovery by removal of the UV and blue light sequentially. ‘Parallel’ and ‘perpendicular’

describe the direction of measurement with respect to the LC director.

Chapter 4

69

71

Chapter 5. Visible light-responsive

surface topographies

ABSTRACT

Materials able to adapt to changes in their environment are becoming

increasingly important. Here, we show that fluorinated azobenzenes can be used

to create rewritable and configurable responsive surfaces that show multi-stable

topographies. These surface structures are formed and removed by using low

intensity green and blue light, respectively. Moreover, we show that these

coatings can act as a responsive bio-interface to guide cell behavior.

This chapter is partly reproduced from M. Hendrikx, J. ter Schiphorst, E.P.A.

van Heeswijk, G. Koçer, C. Knie, D. Bléger, S. Hecht, P. Jonkheijm, D.J. Broer,

A.P.H.J. Schenning, Submitted.

72

1. Introduction

Polymer coatings play an important role in our society. They protect everyday

objects from environmental influences and are widely used for aesthetic purposes,

adhesion-promotion/reduction, anti-reflection or anti-fouling. Many of these

functional properties are determined by the surface topography. Therefore, the

control of the surface structure is of eminent importance to tune its function. Photo-

responsive surfaces that are capable of converting light stimuli in topographical

changes have been a class of materials of interest1–10, ranging from oscillating

surfaces11,12, on-demand structure formation7,13,14 and friction control.15 Using light to

induce dimensional or structural changes is appealing since it can be done locally

without physical contact. Many of these responsive coatings are based on liquid

crystalline networks containing azobenzene moieties acting as versatile photo-

responsive molecules that require ultraviolet (UV) and blue light to induce trans

cis and cis trans isomerizations, respectively (see Chapter 1).16,17 However, often

harsh (UV) illumination conditions are required with intensities ranging from 100 to

1000 mW cm-2.11,18–22 Such intensities limit biomedical applications, and potentially

decrease the lifetime of materials or devices by damaging the polymer. Furthermore,

the photothermal component required to actuate these materials is often significant,

locally heating the material.19,20 Light-responsive polymers reported often only show

two states by the large mismatch between the isomerization kinetics or limited

stability of the photo-generated isomer, i.e. fast thermal back relaxation.

Intermediate states would, however, be very appealing where multiple states can be

formed and erased by two wavelengths of light. These states should be stable in the

time-frame of the application. Recently, light-responsive soft actuators

incorporating fluorinated azobenzenes have been reported that can be switched by

only visible light and, in contrast to classical azobenzenes, resulted in a bistable

actuator.13,17,31,23–30

In this chapter, we report on a liquid crystal polymer coating responsive to visible

light that doped with fluorinated azobenzene reversibly changes the surface

topography using mild illumination conditions (Figure 5.1). Re-configurable

arbitrary surface topographies were created using green light and were erased using

blue light. Multi-stable pre-configured structures were fabricated forming

differently sized topographies in the absence of a mask. Preliminary studies reveal

Chapter 5

73

that such coatings are suitable as a responsive bio-interface17,32 to guide cell behavior,

mostly in shape of the cell.

2. Results and Discussion

The light-responsive cholesteric liquid crystalline material was fabricated by

using a mixture of (meth)acrylate functionalized ortho-fluoroazobenzene30 and

liquid crystalline mono- and diacrylate monomers (see Experimental). This mixture

was aligned in plane by shear forces on a glass substrate and then photopolymerized

resulting in an 18 μm thick coating. Using a cholesteric coating allows to only induce

changes upon actuation in the direction of the helical axis.33 This leads to the optimal

generation of surface topographies. Multi-stable features where endowed on the

surface by illuminating the coating through a mask containing circular features (20

µm diameter transparent circles) with 530 nm green light (6 mW cm-2) for 35 min to

keep the dose of light required to form structures as low as possible. The fluorinated

chromophore and the illustration of the reprogrammable surface structures are

depicted in Figure 5.1.

Figure 5.1. Reconfigurable cholesteric polymer coating containing a functionalized ortho-

fluoroazobenzene. (a) Chemical structure of the visible light-responsive ortho-fluoroazobenzene that

allows surface modification upon illumination with wavelengths corresponding to blue and green colors

in a cholesteric liquid crystalline coating. (b) Schematic representation of the visible light-responsive

coating showing formation (green light), erasure (blue light) and re-configurability of surface

topographies by masked exposure.

74

Illumination inscribes the pattern of the mask in the coating resulting in a

hexagonally arranged pillars. The peak to valley height was found to be 150 nm

(Figure 5.2a, b). Mask illumination with green light of the coatings leads to local

trans-to-cis isomerization of fluorinated azobenzene molecules incorporated in the

network resulting in a local formation of protrusions in the illuminated areas (vide

infra). The structures formed in this glassy polymer corresponds to a strain of ca. 1

% of the total height. To determine the stability of the surface topography, the

sample was monitored continuously with a digital holographic microscope (DHM)

while being in the absence of blue and green light. After 12 h, no significant change

in the surface pillars was found suggesting that stable topographies were created.

This experiment was performed again, leaving the sample in the dark for 12 d.

Measurements of the same pillars revealed a reduction in pillar height of roughly 50

% and after 50 d, a loss of the features was observed. Experiments on similar pillars

were also performed at 80 °C, resulting in a gradual loss of the surface structures in

4 h (Figure 5.2c). Additionally, the structures can be erased rapidly upon irradiation

with blue light (Figure 5.2d).

Chapter 5

75

Figure 5.2. Light-induced generation of topographies in the cholesteric coating. (a-b) Surface height

measurements and corresponding 3D contour plot of cholesteric coating that was illuminated though

a mask containing circular features (20 µm), showing the formed pillars of ca. 125 nm in height. After

12 d in the dark, roughly 50 % of the initial height remains (measured on the same pillars), while after

50 d, complete loss of the structures was found. A cyclic impression is given to illustrate the

reversibility. (c) The gradual decay of the pillars in dark at 80 °C. (d) The stability of the pillars over

10 min and decay with 455 nm light (< 1 mW cm-2) at room temperature.

To gain more insight into the origin of the stable surface topographies, UV-Vis

measurements were performed to study the photoisomerization of the fluorinated

azobenzenes in the LC network (Figure 5.3). Before illumination an absorption

maximum at 470 nm corresponding to the n * transition of the trans-isomer is

visible. When illuminated with 530 nm green light, inducing the trans-to-cis

isomerization of the fluorinated azobenzene, a decrease of the absorption at 470 nm

occurs, while an increase in the n * band of the cis-isomer becomes visible (425

76

nm), indicating that the trans-isomer is partially converted to the cis-isomer. This

process was found to be slower than the back isomerization in dark, requiring

roughly 30 min to fully achieve the photostationary state (pss). Exposing the sample

using 405 nm blue light, results in back-isomerization from cis trans in less than

10 s. Interestingly, when the polymer coating was exposed to green light for 30 min

to reach the pss, relaxation in the dark was found to be associated with a thermal

half-life (t1/2) of 281 h at room temperature, as estimated by extrapolating the UV-Vis

data assuming first order kinetics (Figure 5.3e and f). This value corresponds well

with the 12 d that were found in Figure 5.2 for the surface topographies created by

green light. Increasing the temperature accelerates the back isomerization as

expected, resulting in t1/2 = 56 h at 40 °C, 15 h at 60 °C, and 3 h at 80 °C. These data

reveal that there is a correlation between the isomerization of the molecule and the

stability of the surface topographies. Please note that the decrease of the pillar size

at 80 °C is faster than isomerization, which might be caused by the enhanced

mobility of the system above the glass transition temperature (Tg = 53 °C). Most likely

after exposure to 530 nm green light, the generation of cis-isomers results in a small

decrease of the local molecular order, i.e. the order parameter, leading to expansion

along the helical axis of the cholesteric liquid crystalline coating.18 After exposure to

blue light (405 nm), the flat state is attained again, showing that the process is fully

reversible. In the dark, the disappearance of the topography follows the thermal

isomerization of the cis-isomers suggesting that other mechanisms such as rapid

trans cis trans isomerization and photothermal effects play a minor role. Our

results indicate that the changes in molecular shape of the photo-responsive

molecule in the cholesteric liquid crystalline coating generate local decrease in order

caused by the change in cis- and trans-isomer population.

Chapter 5

77

Figure 5.3. UV-Vis measurements of the cholesteric coating at room temperature illuminated with

green light (530 nm, a and c), blue light (405 nm, b and d) and left in dark for at least 60 h (e). (f)

Time-dependent spectra and change of the absorption at 422 nm of a non-structured cholesteric coating

to determine the cis decay from the azobenzene. First order kinetics are assumed to determine the half-

life time.

To determine whether these structures could be re-configured, the same flat

cholesteric liquid crystalline coating was used again. Illumination of the sample

78

through a zig-zag patterned mask for 30 min using green light (530 nm, 6 mW cm-2),

resulted in the inscription of a zig-zag based structure being present on the mask

(Figure 5.4), which were erased using < 1 mW cm-2 455 nm light instead of 405 nm

light. Illumination with 455 nm light allows to work with light that is even less

harmful than 405 nm light.34 When exposed through a hexagonal circular patterned

mask for 30 min using green light (530 nm, 6 mW cm-2), pillars were formed,

showing that the topographical features can be fully erased and rewritten. The

pillars were erased again using 455 nm light. Due to the mild illumination, no fatigue

of the coating was observed.

Figure 5.4. Reconfigurable surface topographies. Polymer coating before illumination, showing a flat

surface. Upon illumination (530 nm) through a mask containing zigzag pattern, transfer of this image

is observed, which can be erased with illumination of 455 nm and reconfigured with a hexagonal

oriented pillar structure (530 nm). This structure can be erased again with 455 nm light to be reused.

A cyclic impression is given to illustrate the re-configurability.

In order to obtain multi-stable intermediate states, pre-configured surface

topographies were fabricated. For this, the cholesteric liquid crystalline mixture was

deposited on glass and shear aligned with a mask containing a line pattern (80 µm

pitch), followed by UV light illumination through this mask. Due to depletion of the

reactive mesogens by photopolymerization, diffusion of LCs from the non-exposed

area to the exposed areas takes place.13 Subsequently, the sample with mask was

turned around and UV flood exposure was executed at the isotropic temperature,

resulting in a fully polymerized coating with a spatially modulated crosslink density

and molecular orientation. Visually, the patterned coating showed alternating red

Chapter 5

79

cholesteric and black isotropic line domains in the coating between crossed

polarizers (Figure 5.5a). The height between these alternating lines is ca. 100 nm.

First, the light-induced topographical changes are monitored with DHM by

illuminating with 530 nm light at 20 °C and 40 °C (Figure 5.5b). After 35 min, the

illumination was ceased and the material was kept in the dark for 25 min.

Subsequently, the material was illuminated with < 1 mW cm-2 of 455 nm light for 23

min. During the first illumination with 530 nm light, an increase in height difference

between the cholesteric and isotropic regions was visible. As the isomerization

kinetics are temperature dependent, the rate of forming the topographies is also

temperature dependent. The polymer coating at 40 °C shows a maximal difference

of 135 nm after roughly 17 min, while the sample at 20 °C shows a steady increase

during this time period but does not reach the maximum height. After switching off

the illumination, both regions showed no height changes, having a height difference

of 40 nm for the sample at 20 °C and 135 nm for the sample at 40 °C. Upon

illumination with 455 nm light, the polymer coating at 20 °C only recovered partially

in the timeframe of 25 min, while the sample measured at 40 °C fully recovered.

Please note that the illumination intensities are lower in this case compared to the

kinetic experiments to ensure biocompatible conditions.

To show that multi-stable surface topographies can be made, a step-wise

illumination was performed. Hereby, illumination with 6 mW cm-2 was performed

for 10 min, followed by ceasing the illumination for 10 min, which was repeated 4

times. As can be seen in Figure 5.5c, this method resulted in the formation of a height

change of 12.5 nm illuminating for 10 min, which remained after the light was

switched off. The second, third, and fourth illumination cycle showed height

changes of 20, 28, and 35 nm, respectively, all showing a stable plateau when not

illuminated. In order to form the topographies fast and monitor these a longer time,

intensities were increased to 50 mW cm-2 and 5 mW cm-2 for 530 nm and 455 nm,

respectively (Figure 5.6). During monitoring for 16 h, the height of the topographies

showed near zero decay. Upon illumination with 5 mW cm-2 of 455 nm light, the

topography fully decayed.

80

Figure 5.5. Light-induced topographies of the pre-configured cholesteric/isotropic liquid crystal

polymer coating. (a) Crossed polarizer micrograph of an alternating 40 µm wide cholesteric and

isotropic line patterned liquid crystal polymer coating indicated by respectively a red and black color

(left) and illustration of the pre-configured cholesteric and isotropic liquid crystal coating (exaggerated,

right). Light and dark orange correspond to the cholesteric and isotropic aligned lines, respectively. (b)

The height change generated in this sample at 20 °C and 40 °C by illumination with 530 nm light (6

mW cm-2), resulting in formation of height, which can be erased by subsequent illumination with 455

nm light. The vertical black lines indicate switching on and off of the LEDs. (c) Stepwise growth of the

height change, showing multi-stable behavior. “530” indicates illumination with green light, while

“dark” indicates the absence of illumination. (d) Height evolution of the preprogrammed cholesteric

coating at 20 °C with higher intensities, while turning the 530 nm (50 mW cm-2) off after 1 hour for at

least 15 hours. Increased heights were removed by illumination with 455 nm light (5 mW cm-2).

Chapter 5

81

To assess the material as a bio-coating, living cells were cultured on these pre-

configured line patterned coatings.* Figures 5.7 shows preliminary results of the cell

response on the patterned surfaces, indicating cell compatibility, as well as a

response towards the surface post actuation. The NIH 3T3 fibroblast cells were

allowed to spread on the surface, characterized and sequentially exposed to the blue

light built-in laser source of the microscope (405 nm), to remove any prematurely

induced surface structures. Monitoring the cells after this illumination for 1 h

revealed that the cells generally retained their shape. Subsequent illumination with

green light (built-in laser source, 561 nm), which induces trans cis isomerization,

resulted in a response of the cells, where it is seen that retraction occurs after

illumination for 35 min (Figure 5.7). In this case, most changes are observed within

a timescale of 2 h after illumination with green light and thus well within the limits

of how long the formed structure heights are stable.

Figure 5.6. Response of a single NIH 3T3 fibroblast cell to the topographical changes of this coating

upon illumination with 405 nm and 561 nm light. The cell adheres at the boundary of a cholesteric and

isotropic line. The cell preserves its shape upon illumination with blue light (405 nm) but retracts after

surface structures are formed with green light illumination (561 nm). Cells were loaded with both violet

and orange cell tracker dyes to enable monitoring at given wavelengths. Scale bar represents 50 µm.

3. Conclusion

We have created re-configurable visible light-responsive surfaces that show

multi-stable topographies. Photoisomerization of the incorporated ortho-fluorinated

azobenzene moieties induces the formation of surface topographies with heights

that are dictated by the ratio of trans- and cis-isomers. This results in the formation

of multi-stable visible light-responsive coatings by re- and pre-configured actuation

under mild illumination conditions. Moving away from biologically undesired

* Biological experiments and analyses with NIH cells were performed in

collaboration with dr. Gülistan Koçer at University of Twente (Group Pascal

Jonkheijm).

82

wavelengths, i.e. UV light that is harmful to cells, and using kinetically stable

photochromic molecules gives new opportunities to use these materials in biological

applications. Initial cell studies to test our materials as coating show that the surfaces

support cells and a change in the cell shape by partial contraction of the cell after

illumination can be achieved. Our materials show potential for engineered surfaces

for cell culturing or for cell proliferation studies.

4. Experimental

4.1. Chemicals

All chemicals were obtained from commercial sources and used without further

purification, unless stated otherwise. The chemical composition of the components

is shown in Scheme 5.1. The azobenzene (1) was synthesized by the Hecht group

(Humboldt-Universitat zu Berlin, Germany) according to procedure reported in

reference 30. The chiral dopant (2) was obtained from BASF. Molecules (3), (4) and

(5) were obtained by Merck. Irgacure 819 (6) was obtained from Sigma Aldrich.

Scheme 5.1. Chemical composition of the components used to create the light-responsive cholesteric

liquid crystal polymer.

4.2. Fabrication of the liquid crystal polymer coating

To create the cholesteric liquid crystal, the monomers are dissolved in THF (1:4

ratio), resulting in a total concentration of 0.25 g monomer/mL. Thin coatings where

created by evaporating the solvent on an acrylate functionalized glass slide. This

glass slide was achieved by spincoating a 1 vol% solution in water/isopropanol of 3-

Chapter 5

83

(trimethoxysilyl) propyl methacrylate on an oxygen plasma treated glass slide. On

the 4 corners of this glass slide, glue with 18 micron glass bead spacers was applied

and topped off with a fluorinated glass slide for easy removal. This glass slide was

achieved by spincoating a 1 vol% 1H,1H,2H,2H-perfluorodecyl-triethoxysilane in

ethanol. The sample was photopolymerized (Exfo Omnicure S2000) for 10 minutes

using a cut-off filter (Edmund Industrial Optics Stock No. 54516) to prevent the

azobenzene from isomerization. Subsequently, the cell is heated to 120 °C to post-

cure the material. In case of the preprogrammed surface, the fluorinated glass is

replaced with the desired mask. The sample is illuminated for 180 seconds with an

intensity of 15 mW cm-2 at room temperature, flipped over and heated to 90 °C and

polymerized for 5 minutes.

4.3. Characterization of the liquid crystal polymer coating

The Light Emitting Diode (LED) lamps used are obtained from Thorlabs

(M405L3, M455L3 and M530L3). Roughness measurements (RMS roughness 13 nm)

were performed on a NT-MDT Solver P47 Pro AFM equipped with a NT-MDT

NSG10 tip in non-contact/tapping mode to measure the topography (height and

phase); scanning by tip. UV-Vis measurements were performed on a Shimadzu UV-

3102 PC spectrophotometer with a temperature control stage (Linkam). To measure

the topographies during illumination, Digital Holographic Microscopy (DHM,

Reflection DHM® Lyncée Tec) was used equipped with the LED lamps (10–20 mW

cm-2 530 nm and < 1 mW cm-2 455 nm collimated LEDs). Creation and measurement

of the reprogrammable pillars was performed by illuminating the sample through a

mask with a collimated 530 nm LED (6 mW cm-2), followed by DHM observation in

dark and under 455 nm illumination (< 1 mW cm-2). Optical micrographs where

achieved by a Leica DM2700M equipped with polarizers and a Leica DFC320C

camera. Dynamic Mechanical Thermal Analysis (DMTA) was used to measure the

glass transition temperature, performed on a DMA Q800. The glass transition

temperature is calculated as the maximum of the tangent delta, the ratio between

loss and storage modulus.

84

4.4. NIH 3T3 cell studies†

4.4.1. Cell culture and substrate preparation for cell seeding

NIH 3T3 (ATTC® CRL-1658™) cells were cultured in basic medium (DMEM

(Sigma) supplemented with 10 % fetal bovine serum (FBS, Gibco), 2 mM L-

glutamine (Sigma), 100 U/mL penicillin and 100 µg/mL streptomycin (Sigma)) and

incubated at 37 °C and 5 % CO2 in a humidified environment. Prior to cell seeding,

the cholesteric/isotropic liquid crystal polymer substrates were sterilized with 70 %

ethanol and incubated with 100 % FBS for at least 1 h at 37 °C to facilitate protein

adsorption.

4.4.2. Characterization of cell behavior on the surfaces

Live cell imaging experiments (in situ switching experiments using confocal

microscope)

In situ switching experiments were performed using the laser lines of the

confocal microscope for substrate illumination at specific wavelengths. Cells were

seeded again on the cholesteric/isotropic liquid crystal polymer surface (with 40 µm

width/separation) and allowed to adhere and spread for 2 days. For time lapse

imaging experiments, cells on the surfaces were loaded with orange CMRA and

violet BMQC CellTracker dyes (Molecular Probes, Life Technologies) at a total

concentration of 5 µM, at 1:1 ratio in serum free basic medium (see above). Samples

were placed in a climate chamber connected to a Nikon A1 confocal microscope, to

maintain normal culture conditions (37 °C and 5 % CO2 in a humidified

environment) during the experiments.

In situ illumination of the surface was subsequently performed. First, the surface

was illuminated from the bottom using continuous 405 nm laser stimulation (with

laser intensity of 2.31 % at high scan speed) for 10 min. Then, cells were imaged

every 5 min again with 405 nm laser, using the same intensity parameters, but at a

higher resolution, for 1h. After that, the surface was again illuminated from the

bottom, this time using 561 nm laser stimulation (with laser intensity of 1.44 % at

high scan speed), for 35 min, to increase the height of the cholesteric lines. Similarly,

† Biological experiments and analyses with NIH cells were performed in

collaboration with dr. Gülistan Koçer (research group Pascal Jonkheijm, University

of Twente).

Chapter 5

85

cells were imaged every 5 min again with 561 nm laser, using the same intensity

parameters, but at a higher resolution, for 2 h. In both cases, the illumination area

was 0.403 mm2, with z-distance of 153.3 µm. All the steps were performed while

monitoring the cells in the same area.

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Chapter 5

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89

Chapter 6. Technology assessment

ABSTRACT

The impact of the research presented will be discussed and reflected upon.

The possibilities of future applications and devices are examined. Specifically, the

advantages, limitations and future prospects are highlighted.

90

1. Introduction

The work discussed in this thesis addresses the formation of dynamic surface

topographies by light. Variations were made in the alignment, the substrate and

even the light-responsive molecule itself to create different types of topographical

changes. Light and the use of changing polarization led to the generation of

oscillating topographies in these coatings. The topography dimensions were

controlled by photopatterning and with different soft compliant layers as substrates.

Multi-stable azo coatings were fabricated by introducing a fluorine atom at the ortho-

position of the azobenzene. The following sections will discuss possible applications

and future prospects of these newly developed coatings.

2. Applications and future prospects

2.1. Microfluidic systems

Through microfluidic devices, we can manipulate fluids and contents with great

precision and without the need of large installations by using a network of micro-

channels. Surprisingly, to control the regulating devices (such as pumps, valves or

mixers) an extreme complex array of external equipment is needed. It was shown

before that light-responsive hydrogels can dictate a way towards downscaling the

external size of microfluidics.1–4 These materials allow for the generation of changes

in flow and/or mixing by use of light originating from a cheap LED. This permits the

user to remotely control the microfluidic device without large installations and at

low cost.

In Chapter 2 and 3, the use of photo-alignment materials allowed for the

generation of differently shaped topographies. Coatings with different alignment

patterns led to the creation of lateral deforming topographies without the loss of

oscillating properties. Together with the results found in Chapter 4, these

topographies can be enhanced drastically and show larger lateral oscillations with

the use of an intermediate compliant layer. By implementing these materials into a

microfluidic device, the change in topography can change the flow and/or mixing of

the fluids and their contents. Moreover, the oscillating dynamics that was developed

by controlled polarization of the light source may further lead to new micro-pump

designs. In contrast to hydrogels, the use of liquid crystal based materials is

attractive as it does not require water to change its shape.

Chapter 6

91

2.2. Soft robotics and tribology

The use of robotics, being small and local, is largely growing out to be one of the

most advanced techniques in control of matter.5–10 In particular, the tribology of the

surface altering upon irradiation with light is interesting. This has led to controlled

gripping and releasing of objects upon irradiation with light locally. Such

applications need fast and large changes in surface structures. Controlling the

structures formed by light will in turn control the surface properties (mainly friction

and roughness). Liu et al. have clearly shown that the use of UV light and azo-LCN

coatings lead to changes in friction and thus releasing and gripping of objects.9,11

However, the coatings studied did not show oscillatory deformations but dynamic

on and off switching of the protrusion. It would be interesting to use oscillating

surface structures to investigate the friction properties of these coatings with light.

Moreover, recently electrical-induced surface topographies have shown very

promising results.12,13 These coatings were even able to wipe away dust by creation

of 400 nm sized topographies and oscillating rapidly. The results shown in Chapter

4 can lead to new self-cleaning properties. The topographies are of the same

dimensions and can be created and tuned rapidly by controlling the compliant layer

underneath.

2.3. Towards self-cleaning surfaces

Using nature as inspiration, living organisms and plants control their surface

properties in order to maintain a healthy life. A perfect example is the Lotus leaf.14

The ability to control the wettability of a surface as a tool for self-cleaning is of

utmost interest for potential contaminated objects, such as solar cells, skyscraper

windows, etc. With the formation of topographies on-demand with light, the

surface’s ability to attract or repel water can be altered as well. In order to enhance

the change in water contact angle (WCA), the topography change must be large

enough and the surface tension can be chemically altered. In a preliminary study, a

coating was prepared by imprinting nematic circular pillars with height of 100 nm

and a diameter of 25 µm in an isotropic sea. Before evaporation of 1H,1H,2H,2H-

perfluorododecyltriholoro silane (to chemically reduce the surface tension), the

WCA was 90°, indicating the coating is neutral (Figure 6.1). After evaporation the

WCA increased to 150°, indicating the coating become hydrophobic, nearly

superhydrophobic (WCA > 150°). We found a hysteresis of 4° of the advancing and

92

receding water contact angle, proving the superhydrophobic property (< 5°) of this

coating. The WCA changes with only ca. 4° upon UV light irradiation. However, the

actuation of the polymer azo-LCN coating is limited to a few tens of nanometer

(max. 100 nm). The compliant layers studied in Chapter 4 are a very promising

approach to finalize this self-cleaning approach. Moreover, the lateral oscillations

observed for the coatings discussed Chapter 3 could be studied for icephobic

surfaces.

Figure 6.1. Static water contact angles of an azo-LCN coating before (a) and after evaporation of

1H,1H,2H,2H-perfluorododecyltriholoro silane (b) in dark and after subsequent UV and blue light

exposure (c). Topographies in a and b are ca. 100 nm and ca. 150–170 nm in c.

2.4. Towards visible light and sunlight driven systems

In Chapter 5, the introduction of a visible light-responsive photochromic ortho-

fluorinated azobenzene dye showed the ability to create surface topographies with

very low intensities. Moreover, the same dye is able to create chaotic self-oscillation

in free standing films.15 For the coatings, we reported stable topographies that have

a half-life of ca. 12 days. For the oscillating topographies described in Chapters 2–4,

we need to actuate the azo-LCN coating by polarized UV light exposure and

unpolarized blue light. Blue light is necessary to control the up- and downward

motion of the topography. The use of this controlled back- and forward

isomerization can be used to induce oscillatory deformations in the ortho-fluoro-

azobenzene reported in Chapter 5. Here, the light actuation setup will be similar but

will swap the 365 nm LED with a 530 nm LED. However, the kinetics of the ortho-

fluoro-azobenzene system could be studied in the presence of (polarized) blue and

green light or even sun light to obtain oscillatory actuation of the azo-LCN network.

To achieve self-induced oscillations without the need for the dual light set-up, a

fast-relaxing cis-azobenzene derivative is proposed (Figure 6.2). This hydroxylated

azobenzene (AzOH) shows a broad absorption band with its maxima at 405 nm. The

thermal back reaction of the AzOH is in the order of seconds.7,16 This allows for the

Chapter 6

93

addressing of solely visible polarized light (405 nm).* Note that the oscillation is a

few nanometers as for these coatings, the illumination intensity is kept low (< 50 mW

cm-2) to avoid the risk of damaging the camera of the DHM. However, the shape of

the oscillation starts resembling a square wave leading to much faster kinetics

compared to azobenzenes used in this thesis. To enhance the response, the network

of the LCN can be altered by lowering the crosslink density; increasing the

temperature increases the actuation height. Furthermore, the topographies can be

enhanced drastically by utilizing a compliant layer as demonstrated in Chapter 4.

Figure 6.2. Molecular structure of the ortho-hydroxylated azobenzene crosslinker (AzOH) and the

corresponding actuation achieved with rotating linear polarized 405 nm light for a 100 µm wide 0°/90°

line pattern.

2.5. Light-induced dynamic wrinkling

In Chapter 4, the azo-LCN is positioned atop a compliant sublayer to enhance the

surface structures. Utilizing two different materials with a thin, hard layer atop a

* This material was synthesized by dr. Ghislaine Vantomme from the Eindhoven

University of Technology (Research group Meijer).

94

thick soft substrate, typically leads to wrinkling.17–20 The use of liquid crystal

polymers as one of these layers can induce directionality in the wrinkles. Agrawal et

al. reported this principle earlier by utilizing a liquid crystal elastomer (LCE) as

substrate with a thin polystyrene toplayer.19 This technique allowed to create

wrinkles with increasing or decreasing temperature by inducing changes in order

parameter of the LCE. However, these bilayer systems show no remote method to

create wrinkles. Therefore, light would be a more interesting method.20

Preliminary experiments show that wrinkling upon actuation with UV light can

be observed on a 1 µm thick azo-LCN coating atop of 100 µm thick low crosslinked

PDMS layer supported on glass. Wrinkling is observed in the direction

perpendicular to the alignment direction as the azo-LCN will expand perpendicular

to its LC director. Moreover, by patterning the azo-LCN, we were able to control the

wrinkling direction locally. An orthogonal aligned azo-LCN coating with the two

different wrinkling directions is depicted in Figure 6.3.

Figure 6.3. Patterned azo-LCN coating atop soft PDMS leading to directed wrinkles on demand with

light. The azo-LCN coating is patterned in two domains with orthogonal alignment. The double-headed

arrows indicate the local direct and the line in the azo-LCN indicates the boundary.

An interesting property of using wrinkles as topographies is the ability to

undergo drastic changes. We induced wrinkles in the system by adhering the azo-

LCN to the PDMS at higher temperatures. This procedure was done by curing the

PDMS in direct contact to a pre-made azo-LCN at 80 °C. Upon cooling down to room

temperature, the azo-LCN expands along and contracts perpendicular to the LC

director. This is caused by anisotropic expansion while cooling where the LCN layer

parallel to the director expands and perpendicular to that shrinks, superimposed on

a shrinking PDMS layer. Now, wrinkles are produced in the parallel direction of the

LC director. We were able to rotate the wrinkles by illumination of UV light (Figure

6.4a, c). To our surprise, the transition state during the reorientation of the wrinkles

shows square-packed hills as topographies (Figure 6.4b). The period of the wrinkles

is identical in all the wrinkling states. The next step in this research would be the

Chapter 6

95

study of these changing wrinkles in a more dynamic fashion utilizing the procedures

described in Chapters 2 – 4.

Figure 6.4. The reorientation of thermal pre-induced wrinkles (a) and the reorientation upon UV light

irradiation (c). The transition state (b) shows rectangular packed hills. The period of the wrinkles are

identical in all wrinkling states.

2.6. A zero-birefringent actuator

The largest hurdle to overcome is the birefringence of the LCs when using

uniaxial aligned azo-LCN coatings as actuator with polarized UV light. Overall,

there are only two states during the actuation with rotating linear polarized UV light

where the actuation is localized. These states only occur when the E-field vector of

the UV light is parallel or perpendicular to the LC director. In all other orientations,

the birefringence of the LC crystals depolarizes the UV light. This depolarization

causes the azobenzene to undergo trans-to-cis isomerization at unwanted locations.

For this reason, a travelling wave is not observed when utilizing the polarization

grating alignment in Chapter 3. Therefore, the use of a low to zero birefringent LC

mixture would be preferred. New monomer designs are therefore necessary with a

proposed rod-like geometry and a large polarization component perpendicular to

the long axes of the molecules.

Alternatively, cholesteric LCN coatings show no retardation or reorientation of

polarized light given the reflection band is not of similar wavelength. Patterning a

cholesteric LCN coating with azobenzene crosslinkers can lead to a zero-birefringent

approach to induce full oscillation. Upon illumination with polarized light

perpendicular to the coating, no birefringence will occur. The azobenzenes can be

aligned during polymerization of the cholesteric mixture. Using polarized light for

photopolymerization and a dichroic photoinitiator, radicals will be formed locally

every half pitch rotation of the cholesteric LC mixture. The di-functionalized

96

azobenzene will therefore diffuse towards the high radical concentrated areas.

Patterning the polarized light used for photopolymerization will lead to an azo-

patterned cholesteric LCN coating (Figure 6.5).

Figure 6.5. Principle of the patterning of azobenzene dyes in a cholesteric azo-LCN coating.

2.7. A true autonomous system

The definition of autonomous is ‘the ability to act independently and without the

influence of an external operator’. In Chapters 2 to 4, we described a continuous

changing surface structure under continuous exposure of UV light. This was

achieved by utilizing a rotating stage to physically turn a linear polarizer at set

speeds. The movement of the polarizer was needed to continuously alter the

absorption locations inside the patterned coatings. A method to create an

autonomous system, without the need of an engine to turn the polarizer, would be

the implementation of a polarization state altering device. This device must use light

as a trigger, without the need of a secondary stimulus. For this, we propose the usage

of light-induced Archimedes spirals developed by A. Martinez and I. Smalyukh.21

Here, the polarization state of light passing through the contraption changes

polarization state upon exiting. The continuous change is induced by the

realignment of a chemically attached photo-alignment layer, based on a derivative

of Methyl Red (dMR, Figure 6.6). By combining the azo-LCN with this system, a

rotating stage is not needed. However, the oscillation speeds of both systems must

be matched. For the light altering system, the oscillation is ca. 1 Hz, while the azo-

LCN oscillate at 0.014 Hz. In Chapter 4, we developed a method to enhance the

topography size as well as the oscillation speed. The compliant azo-LCN bilayer

oscillates at speeds up to 0.14 Hz. It is important to note that this is the limit of the

mechanical rotating stage and we were unable to find the material’s limit.

Chapter 6

97

Figure 6.6. (a–d) The photo-active liquid crystal contraption based on different alignment layers with

active photo-alignment layer dMR depicted in the red box. The illumination is from the top with the

initial situation non-cross polarizing (a, b). Upon exiting the polarization state the dMR will realign

perpendicular and realign the LCs with it, changing the retardation of the passing polarized light (c,

d). (e–f) The micrographs of the contraption upon illumination with polarized light between crossed

polarizer (e) and with an additional compensator (full wave plate) (f). Reproduced from reference 21.

3. Conclusion

To conclude, dynamic and oscillation light-induced topographies pave a way to

an enlightened future for materials and devices with new properties. Although the

field of light-induced deformations and topographies exists for years, the

introduction of oscillating deformations and topographies together with the control

98

over alignment22 will lead to the generation of new functional materials and devices.

It is important to overcome the hurdle of depolarization of polarized UV light. With

the shift towards more visible light and less harmful intensities, the applicability for

these light-responsive materials increases and is expected to continue to grow.

References

1 S. Coleman, J. ter Schiphorst, A. Azouz, S. Ben Bakker, A. P. H. J. Schenning and D.

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J. M. J. den Toonder and A. P. H. J. Schenning, Macromol. Rapid Commun., 2018, 39,

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J. Schenning, Chem. Mater., 2015, 27, 5925–5931.

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16 J. García-Amorós and D. Velasco, Beilstein J. Org. Chem., 2012, 8, 1003–1017.


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18 E. P. Chan and A. J. Crosby, Adv. Mater., 2006, 18, 3238–3242.

19 A. Agrawal, P. Luchette, P. Palffy-Muhoray, S. L. Biswal, W. G. Chapman and R.

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21 A. Martinez and I. I. Smalyukh, Opt. Express, 2015, 23, 4591.

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J. Broer and O. D. Lavrentovich, Nat. Commun., 2018, 9, 456.

Chapter 6

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101

Summary

Light-Induced Oscillating Topographies in Liquid Crystal Coatings

The creation of optical responsive polymer films or coatings leads to dynamic

surfaces with interesting applications. As such, control over friction (gripping and

releasing of objects), wettability (and thus self-cleaning), haptics and cell motility are

along the most studied. The ability to respond to light as an external stimulus allows

for a contact-free and on-demand method to create changes in the surface properties

of materials. In particular, the presence of light can be addressed locally. Throughout

the generation of light-responsive materials and coatings, the dynamical changes

were only obtained by on/off switching. Under continuous illumination, the material

typically resulted in a stable actuated state, while without the presence of light, the

material returned to its original state. Continuously altering the surface properties

without the removal of light has remained a challenge. The aim of this thesis is to

create oscillatory deformations in a coating without the removal of the stimulus.

Liquid crystalline (LC) materials are used with imbedded photo-responsive

azobenzene chromophores to create optical responsive coatings.

In the first chapter, we give a broad introduction to a wide range of polymers

containing azobenzenes in order to create light-responsive coatings. A distinction is

made between linear azo polymers forming static or reversible surface relief gratings

and highly crosslinked azo networks formed from liquid crystalline materials

leading to, by choice, static or dynamic surface topographies under illumination.

We introduce an azobenzene liquid crystal network (azo-LCN) to achieve

continuously altering surface topographies in Chapter 2. By uniaxial alignment, the

co-aligning azobenzene moieties show dichroic properties and lead to a larger

topography with the UV light polarized parallel compared to perpendicular. This

result is extrapolated to oscillating surface changes by rotating linear polarized UV

light. We patterned the coating in alternating lines with the orthogonal alignment

(0°/90°). This pattern led to the generation of asymptotic shaped topographies near

the boundary.

102

Next, we designed different alignments in the azo-LCN coating to create

symmetric surface topographies (hills or valleys) (Chapter 3). We found that the

topographies are determined by the alignment transition (bend or splay) between

the orthogonal aligned lines (-45°/45° or polarization grating, PG). Moreover, the

bend transition introduces a combination of oscillation laterally and in height upon

rotation of polarized UV light. Depending on the alignment pattern used, -45°/45°

or PG, the lateral oscillation of the topographies is in or out of phase.

To further enhance the results for azo-LCN coatings adhered to glass, we created

a compliant layer made up of acrylates and sandwiched it in between the azo-LCN

and glass to express the lateral shear stresses (Chapter 4). These stresses are induced

by the actuation of the top azo-LCN layer with UV light. The introduction of a

compliant layer leads to the generation of micron sized topographies with faster

kinetics.

To avoid high intensity UV light that can harm the coating and broaden the range

of applications, we studied the potential of a visible light-responsive azobenzene

dye in the LCN coating (Chapter 5). Here, green and blue light lead to the generation

and removal of multi-stable topographies, respectively, showing properties of a bio-

scaffold. The surface structures can be erased and re-configured following the

molecular isomerization kinetics.

Lastly, we discuss the impact of the research (Chapter 6). In more detail, we

focused on future applications and devices. Specifically, the advantages, limitations

and future prospects are discussed.

105

Samenvatting

Light-Induced Oscillating Topographies in Liquid Crystal Coatings

Het maken van optisch responsieve polymere films of coatings leidt tot

dynamische oppervlaktes met interessante toepassingen. Als zodanig zijn controle

over frictie (grijpen en lossen van objecten), bevochtbaarheid (en dus zelfreiniging),

haptica en celbeweeglijkheid de meest bestudeerde. De mogelijkheid om te reageren

op licht als externe prikkel laat toe om contactloos en op commando veranderingen

te creëren in de oppervlakte-eigenschappen van materialen. Specifiek, het instralen

van licht kan lokaal gedaan worden. Doorheen de generaties van licht-responsieve

materialen en coatings zijn dynamische veranderingen enkel behaald door het aan-

en uitschakelen van de lichtbron. Onder continue belichting bevindt het materiaal

zich in een stabiele geactueerde toestand, terwijl zonder belichting het materiaal

terugkeert naar zijn originele toestand. Het continue veranderen van oppervlakte

eigenschappen zonder het licht te verwijderen blijft een uitdaging. Het doel van deze

dissertatie is het creëren van oscillerende deformaties in een coating zonder het

verwijderen van de prikkel. Vloeibaar kristallijne (‘liquid crystal’, LC) materialen in

combinatie met ingebedde azobenzeen chromoforen worden gebruikt om optisch

responsieve coatings te maken.

In het eerste hoofdstuk geven we een brede introductie met een wijde selectie

van polymeren die azobenzenen bevatten om licht-responsieve coatings te creëren.

Een onderscheid is gemaakt tussen lineaire azo-polymeren die statische of

reversibele oppervlaktereliëfroosters (‘surface relief grating’, SRG) vormen en dicht

gecrosslinkte azo-netwerken gemaakt van vloeibaar kristallijne materialen die,

afhankelijk de keuze, statische of dynamische oppervlaktetopografieën vormen

onder belichting.

We introduceren in Hoofdstuk 2 een azobenzeen vloeibaar kristal netwerk

(‘liquid crystal network’, azo-LCN) die onder continue belichting veranderende

oppervlakte topografieën vormt. Door uni-axiale uitlijning tonen de mee-uitlijnende

azobenzeen eenheden dichroïtische eigenschappen dat leidt tot een grotere

topografie als het UV-licht parallel is gepolariseerd in vergelijking met loodrecht

gepolariseerd licht. Dit resultaat is geëxtrapoleerd tot een oscillerende

106

oppervlakteverandering door het draaien van lineair gepolariseerd UV-licht. We

structureerde de coatings in alternerende lijnen met loodrechte uitlijning (0°/90°).

Dit patroon leidde tot de generatie van asymptotisch gevormde topografieën nabij

de grenslijn.

Daarna hebben we verschillende uitlijningen ontworpen in de azo-LCN coating

die symmetrische topografieën (pieken en dalen) creëerden (Hoofdstuk 3). We

bevonden dat de topografieën bepaald worden door de uitlijningstransitie (‘bend’

of ‘splay’) tussen de loodrecht uitgelijnde lijnen (-45°/45° of polarisatie ‘grating’, PG).

Bovendien introduceerde de ‘bend’ transitie een oscillatie zowel lateraal als in de

hoogte onder belichting van roterend gepolariseerd UV licht. Afhankelijk van het

patroon (-45°/45° of PG) dat gebruikt werd was de laterale oscillatie van de

topografieën in of uit fase.

Om de resultaten van de azo-LCN coatings op glas te verbeteren, hebben we een

meegaande (zachte) laag gemaakt van acrylaten en deze geplaatst tussen het azo-

LCN en het glas om de laterale schuifspanningen uit te drukken (Hoofdstuk 4). Deze

spanningen zijn geïnduceerd door de actuatie van de bovenste azo-LCN laag door

UV-licht. De introductie van de meegaande laag leidt tot de generatie van

micrometer grote topografieën met snellere kinetiek.

Om schade aan de organische coating door het hoog intense UV-licht te

vermijden en meer toepassingen toegankelijk te maken, bestudeerde we de

mogelijkheden van een zichtbaar licht-responsieve azobenzeen kleurstof in de LCN

coating (Hoofdstuk 5). Hier leiden groen en blauw licht respectievelijk tot de

vorming en verwijdering van multi-stabiele topografieën die eigenschappen

vertonen van een biologisch substraat. De oppervlaktestructuren kunnen

verwijderd en aangepast worden volgend aan de moleculaire isomerisatiekinetiek.

Tot slot bespreken we de impact van dit onderzoek (Hoofdstuk 6). In meer detail

focussen we op toekomstige toepassingen en apparaten. Met name worden de

voordelen, beperkingen en toekomstperspectieven besproken.

109

Curriculum Vitae

Matthew Hendrikx was born on July 3, 1991 in Leuven,

Belgium. After graduating from high school at Sint-Jan

Berchmanscollege (SJB) in Genk, Belgium, he started a

Bachelor of Science in Chemistry at Hasselt University,

Belgium. He received his BSc degree in 2012. Afterwards,

he pursued a Master of Science degree in Chemical

Engineering and Chemistry at Eindhoven University of

Technology (TU/e), the Netherlands. He received his

MSc/ir. degree in 2014 on his research in piezoelectric

discotic liquid crystal polymers in the group 'Supramolecular Polymer Chemistry'

under supervision of prof.dr. R.P. Sijbesma. He completed his study with a research

internship abroad at the Commonwealth Scientific and Industrial Research

Organisation (CSIRO), Clayton, Australia. Upon return, he started his PhD research

at Eindhoven University of Technology (TU/e) in the research group 'Stimuli-

responsive Functional Materials and Devices' under supervision of prof.dr. A.P.H.J.

Schenning and prof.dr. D.J. Broer. The most important results of his PhD research

are described in this thesis.

Publications related to this work

M. Hendrikx, A.P.H.J. Schenning, M.G. Debije, D.J. Broer. Light-triggered formation

of surface topographies in azo polymers. Crystals, 2017, 7, 231.

M. Hendrikx, A.P.H.J. Schenning, D.J. Broer. Patterned oscillating topographical

changes in photoresponsive polymer coatings. Soft Matter, 2017, 13, 4321-4327.

(Cover article)

M. Hendrikx, A.P.H.J. Schenning, D. Liu, D.J. Broer. Compliance-mediated

topographic oscillation of polarized light triggered liquid crystal coating. Advanced

Materials Interfaces, 2018, 201800810. (DOI:10.1002/admi.201800810)

M. Hendrikx, D. Liu, A.P.H.J. Schenning, D.J. Broer. Oscillatory dynamic surface

structures in patterned liquid crystal network coatings. Proceedings of SPIE 10735,

Liquid Crystals XXII, 2018, 1073507. (DOI:10.1117/12.2320576)

110

M. Hendrikx, J. ter Schiphorst, E.P.A. van Heeswijk, G. Koçer, C. Knie, D. Bléger, S.

Hecht, P. Jonkheijm, D.J. Broer, A.P.H.J. Schenning. Re- and pre-configurable multi-

stable visible light responsive surface topographies. (Submitted)

M. Hendrikx, T.J.A. Van Cleef, D.J. Broer, M.G. Debije, A.P.H.J. Schenning. Surface-

driven, dynamic, reversible photo-induced wrinkling in liquid

crystal/polydimethylsiloxane bilayers. (In preparation)

Other publications

M.K. McBride, M. Hendrikx, D. Liu, B.T. Worrell, D.J. Broer, C.N. Bowman.

Photoinduced plasticity in cross-linked liquid crystalline networks. Advanced

Materials, 2017, 29, 1606509.

G. Koçer, J. ter Schiphorst, M. Hendrikx, H.G. Kassa, P.E.L.G. Leclère, A.P.H.J.

Schenning, P. Jonkheijm. Light-responsive hierarchically structured liquid crystal

polymer networks for harnessing cell adhesion and migration. Advanced Materials,

2017, 29, 1606407.

G. Babakhanova, T. Turiv, Y. Guo, M. Hendrikx, Q. H. Wei, A.P.H.J. Schenning, D.J.

Broer, O. D. Lavrentovich. Liquid crystal elastomer coatings with programmed

response of surface profile. Nature Communications, 2018, 9, 456.

F.L.L. Visschers, M. Hendrikx, Y. Zhan, D. Liu. Liquid crystal poymers with motile

surfaces. Soft Matter, 2018, 14, 4898-4912.

J.J. Haven, M. Hendrikx, T. Junkers , P.J. Leenaers, T. Tsompanoglou, C. Boyer, J. Xu,

A. Postma, G. Moad. Elements of RAFT Navigation. Reversible Deactivation Radical

Polymerization: Mechanisms and Synthetic Methodologies, 2018, 77-103.

(DOI:10.1021/bk-2018-1284.ch004) (Book chapter)

Awards

Lyncée Tec 4D Application contest 2016 (Laussane, Switzerland) – 1st price

Dutch Polymer Days 2017 (Lunteren, the Netherlands) – Poster price (Technology)

PhoSM 2018 (Tampere, Finland) – Honorable mention, presentation

113

Acknowledgements

“In a world of talkers, be a thinker and a doer.” – Destin Sandlin (SmarterEveryDay)

To finalize and complete this thesis, I would like to thank everyone who helped

and supported me. Eerst en vooral, Dick, dank u! Graag wil ik je bedanken voor alle

vruchtvolle meetings, geweldige anekdotes, al je hulp en bezorgdheid als het even

niet goed of juist wel goed ging en ik de weg toch wel even kwijt was (wat meer dan

eenmalig gebeurde). Zonder uw hulp was dit nooit gelukt. Datzelfde geldt ook voor

Albert, zonder de kleine binnenloopmomentjes en korte updates zou het laatste

hoofdstuk er heel anders uit hebben gezien. Danqing without your help, knowledge

and know-how, it would’ve probably taken me years to get a fingerprint alignment.

All the discussions we had and all the help you’ve given me in and around the lab

has boosted the quality of this thesis to a level I am proud of. Moreover, without

your help, we would’ve never won the Lyncée Tec Application Award! Thank you

all for your help.

Prof. dr. Bert Meijer, dank u om aan het begin van mijn onderzoek me sterk op

de proef te stellen met gerichte en ondersteunende vragen tijdens de Polymers In

Motion meetings. Dit gaf me aan het begin toch wel het gevoel dat de wereld breder

is dan enkel het project dat voor me lag. Ook bedank ik u en het ICMS voor alle

faciliteiten waar ik gebruik van kon maken. Daarnaast wil ik u ook bedanken voor

het deelnemen van mijn commissie. Prof. dr. Jaap den Toonder, aan u ook een

welgemeende dank u. We hebben korte projecten samen gehad en u hebt vaker

deelgenomen aan de verdediging van mijn studenten. Ik wil u dan ook bedanken

dat u nu deelneemt aan mijn verdediging. Prof. dr. Oleg Lavrentovich, thank you

for being a part of my committee and for all the help during our collaboration. Prof.

dr. Natalie Katsonis, I would like to thank you for being part of my committee and

taking the time to read and comment my thesis. Lastly, dr. Carlos Sanchéz-

Somolinos, thank you for all the wonderful discussions and meetings we had. I

would also like to thank you for being a part of my committee.

Ik moet toegeven dat het werk er nooit hetzelfde zou hebben uitgezien zonder

de geweldige stafleden van SFD. Marjolijn, dank je om me altijd te helpen wanneer

ik het administratief niet meer zag zitten en me wegwijs te maken op meerdere

114

momenten. Tom, samen met Marjolijn zijn jullie echt de backbone van SFD en zijn

labs. Ondanks dat ik nooit bestellingen bij je kon plaatsen, kon ik altijd terecht voor

meerdere labgerichte problemen en een gezellige babbel. Michael, thank you for all

of your time I was allowed to waste talking about everything (mainly baseball and

other sports). We did have our fair share of scientific meetings and discussions.

Without your English and scientific support, the review would’ve never looked so

sleek. I would also like to thank your wife, Audrey Debije – Popson, thanks for all

the moments I could barge in with English spelling, grammar, and style questions.

Johan en Kees, jullie hebben me vaak verder geholpen met de goede suggesties.

Dank daarvoor!

Jeroen, dank u voor de samenwerking de afgelopen jaren. Zonder jouw

‘motivatie’ waren we nooit aan gefluorineerde azo begonnen. Gelukkig had je nog

geduld om samen achter de DHM te gaan zitten en te priegelen. Want ja, toen de

mooie resultaten kwamen, moesten er nog meer metingen komen, dus op naar

Twente! Veel succes met je bedrijf Lusoco. Dat komt zeker goed. Gulistan, thank you

for all the effort you put into getting the cell studies done. I really appreciate it. Ellen,

ook jij hebt hier hard bij geholpen, zonder je geduld aan de UV-Vis waren we nu nog

bezig. Matt McBride, thanks for that wonderful first year. You had us going

everywhere, West-Vleteren, Blankenberge, Interlaken. I’ll never forget the amazing

moments and great bike rides / hikes. I’ll see you on Strava! Greta, you came over to

SFD during one of my busiest moments. I had 3 students, just came back from

holiday, was writing a review and a paper. Yet, you got me motivated to go to the

lab. The work we did together was great. The energy you put into all of the analysis

and measurements was truly amazing. Thanks for the great time! Ling, we had our

fair share of discussions and every time I got new results, you were eager to give

advice to improve it. Thanks for all your help and fruitful discussions!

I would also like to thank other people that helped me achieving the results

discussed in this thesis. Those people are the wonderful students I was allowed to

guide; Ilona, Laurie, Burcu, Deepak, Yorick and Tim. All of you will recognize the

wonderful work you’ve done in this thesis one way or the other. Thanks for teaching

me how to be a better mentor and good luck will all your future endeavors!

Koen, Katie, Sander, Esther, thank you all for the fun and amazing parties,

afternoons, weekends, … Playing and winning ‘kegelen’ will remain a challenge until

Acknowledgements

115

we get that beloved price! The Eurovision Song Contest clearly became a traditional

evening of food and celebration.

Dirk-Jan, Anne Hélène, Jody, Nina, Marcos, Luiza, Hitesh, Monali, Berry en

Sylvie, thanks for the amazing parties and moments. The pool parties, barbecues (at

the lake) and so on were really the best! Also, thanks for all the help you’ve given

me during this PhD! Best of luck to all you!

To all the former and current SFD members: Thank you! Anping, Berry, Lihua,

Wajid and Jeroen (Sol), thanks for the amazing time in the office. All the best of luck

in the future! Sarah, we started at the same time and now finish a week apart. Thanks

for all the fun moments in and around SFD. Rob, Marina, Ellen, Gilles, Xinglong,

Alberto, Simon, Davey, Fabian, Yuanyuan, Wei, Stijn, Marc, Wanshu, Shaji,

Xiaohong, Jeffrey, Huub, My, Ghislaine, Evelien, Ting, Laurens, Xiao, all the

members of ICMS and MST, and all those I’ve woefully forgotten, thank you for

making TU/e a nice place to be!

Karel en Laurens, we kennen elkaar al jaren. Samen in SJB werd het al snel

duidelijk dat we verder gingen studeren, en tot mijn verbazing zijn we nog steeds

bezig. Nu komt er voor ons drieën toch stilaan een einde aan. Laurens, je doet het

goed in Duitsland en Karel, je bent er ook bijna! Nog even doorbijten en we zijn

alledrie dr. Al is er maar één van ons drie die daadwerkelijk een leven kan redden.

Karel, bedankt voor al de tijd die je in de cover en het binnenwerk hebt gestoken

tijdens je verhuis. Ik ben echt trots op wat het is geworden, dank u!

Aan mijn ouders, bedankt voor alle hulp en steun door de jaren heen.

Thuiskomen was altijd leuk. Jullie steun wanneer het even minder ging was fijn en

gaf me weer moed om er tegenaan te gaan. Jullie vroegen vaker wat ik nu precies

deed en probeerde dan ook met ideeën te komen om toch op een of andere manier

te helpen. Dank jullie voor alles! Of course, I would like to thank all the rest of my

family both in Belgium as in the USA for their support and help.

There is one more person I really need to thank. Dora, you stood by my side when

I started my masters in Eindhoven, you never left my side even when I was in

Australia for 6 months. And during the last 4 years, you were the rock I could count

on. Thank you for everything!

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