University of Groningen Janus particles Yang, Qiuyan · 2016. 6. 2. · General Introduction 11...

University of Groningen

Janus particlesYang, Qiuyan

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Chapter 1

General Introduction

Part of this chapter was submitted as a review article:

Qiuyan Yang and Katja Loos. “Janus Nanoparticles inside Polymeric Materials: Interfacial Arrangement toward Functional Hybrid Materials” submitted

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1.1 Janus particles

The quest for new smart materials with engineered properties and desired functionalities has driven scientists into the domain of nanotechnology over the past 20 years. The hope is that, by reducing more and more the scale at which matter is manipulated, unprecedented control over material properties can be achieved. One of the most promising research topics in nanotechnology is the bottom-up design of materials[1,2] in which desired engineered building blocks are used to create new materials by their spontaneous self-assembly. Significant attention, therefore, has been devoted to the preparation of various types of building blocks, such like anisotropic nanoparticles. Theoretical works have shown that anisotropic particles could be very useful for controlling molecular recognition and self-assembling processes, which are one of the more intriguing and challenging aspects in current materials science.

The synthesis of Janus-type nanoparticles (Figure 1.1) is currently a route that is extensively investigated.[3] A Janus Particle (JP) is defined as possessing different surface chemical/physical compositions on two sides of the particle and was initially named after the double-faced Roman God Janus - who was usually represented with two heads placed back to back so that he could look in two directions at the same time. The field of JPs’ synthesis in laboratory was inspired by de Gennes who called attention to them in his Nobel Prize address.[4]

Figure 1.1. Different Types of Janus particles: Spherical (a), two types of

cylindrical (b,c), and disc-shaped (d,e) JPs. (f−k) Various kinds of dumbbell-

shaped JPs with asymmetric or snowman character (f), symmetric appearance (g,k), attached nodes (h), and eccentric encapsulation (i). (l) Janus vesicles or capsules. Reprinted with the permission from ref [3]. Copyright 2013 American Chemical Society.


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1.1.1 Properties of Janus particles

Janus nanoparticles possess a number of unique properties related to their asymmetric structure and/or functionalization. They are very attractive materials for numerous applications,[5-7] such as biological sensors,[8,9] drug delivery,[10-13] optical sensing devices,[14-18] nano/micromotors,[19-22] two-phase stabilizers,[23-25] and electronic displays.[26-30] In several examples in the literature, it is evident that when researchers make clever use of the inherent non-centrosymmetric character and the resulting physical characteristics, new materials with novel properties can be obtained. For example, various groups have already incorporated magnetic nanoparticles (NPs) or magnetic/metallic caps to investigate the self-assembly behavior of JPs in magnetic [31-35] or electric fields.[36-38]

Another class of this self-assembly behavior of JPs is the so-called Pickering effect at the fluid-fluid or fluid-air interface.[23-25] De Gennes, who called attention to JPs in his Nobel Prize address, described the spontaneous monolayer arrangement from JPs at, for instance, water and air interfaces as “breathable skin”. He predicted the possibility of the matter exchange between the two phases, which has recently been realized by using JPs as interfacial catalyst emulsifiers.[39-42]

The main reason of that JPs are able to stabilize emulsions and show their superior long-term stability more effectively than homogeneous particles can be explained by their strong adsorption energy to fluid interface. Binks and Fletcher theoretically studied the energy to detach a single JP from an oil-water interface, assuming a flat interface, as shown in Figure 1.2.[43] Total surface free energy (E) for a JP at the interface as a function of the angle β (Figure 1.2a) was given by: For β ≤ α

E β =2πR2 γ AO 1+ cos α +γ PO cos β - cos α +γ PW 1- cos β -1

2γ OW sin

For β ≥ α

E β =2πR2 γ AO 1+ cos β +γ AW cos α - cos β +γ PW 1- cos α -1

2γ OW sin

where R is the particle radius and γ(AO), γ(PO), γ(AW), γ(PW), and γ(OW) are the interfacial energies of the apolar-oil, polar-oil, apolar-water, polar-water, and oil-water interfaces, respectively. They weighted the average contact angle, plotted as the abscissa in Figure 1.2a, by the relative areas of the polar and apolar particle surface regions according to:

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1 cos 1 cos2

Figure 1.2. Geometry of a Janus particle at an oil-water interface with parameters α and β, which represent positions of the Janus boundary and the interface, respectively. Reprinted with permission from ref [43]. Copyright 2001 American Chemical Society.

The amphiphilicity of JPs can be “tuned” by the magnitude of the difference between the two contact angles θA and θP, ∆θ = (θP–θA)/2. Zero amphiphilicity (corresponding to homogeneous particles) corresponds to ∆θ = 0. Strongest amphiphilicity is expected when ∆θ = 90°. According to their calculation on the desorption energy of a JP for the fluid interface in Figure 1.2b, it was shown that increasing the particle amphiphilicity through ∆θ increases the strength of particle adsorption up to a maximum of three-fold for θaverage of 90°.[43] In addition, JPs maintain their strong adsorption at average contact angles approaching 0 or 180° where the surface activity of the nonamphiphilic (homogeneous with ∆θ = 0)


13

particles is low.[43] Further extensive theoretical simulations and experimental observations indicate that the interfacial activity of the JPs can vary depending on several parameters, including morphology, and distribution of the spatial domains, and that the JPs show an enhanced interfacial activity compared to the corresponding homogeneous particles, regardless of the synthesis and interfacial activity characterization methods.[44-50]

1.1.2 Preparation of Janus particles

Due to the benefit of unique properties related to asymmetric structure and/or functionalization of Janus particles, researchers from various fields were attracted to the investigation of Janus particles’ preparation and properties. To date, a wide variety of techniques have been developed to produce nanoparticles comprised of both inorganic materials, such as ceramics, metals, oxides, salts, etc., and of polymers. Crucial issues in the fabrication of Janus materials, including high productivities and uniformities of asymmetric features, were mostly determined by the synthesis pathways. Synthesis approaches of zero-dimensional Janus materials with micro- or nano-structure can be generally categorized into a direct dual supplied method and an indirect template-assisted method. The direct dual-supplied method involves the formation of droplets consisting of two immiscible materials in a liquid or molten form. Biphasic particles with diameters on the order of tens of micrometers were then co-ejected via a spinning disk or a microfluidic system (Figure 1.3a).[51] The continuous ejection process in the direct dual-supplied method demonstrates the efficient production of Janus particles with moderate uniformities in terms of particle sizes and hemispheric features (Figure 1.3b).[52] Another direct preparation method of Janus particle is based on the self-assembly of block copolymers. Müller’s group [53-56] prepared cross-linked Janus polymeric nanoparticles based on the self-organization of triblock terpolymers. Their pioneering work took advantage of the wide variety of complex morphologies with a high degree of spatial control that can be obtained spontaneously by the self-organization of terpolymers during film casting, depending on the chemical nature and molecular weights of the different blocks (Figure 1.3c).[57]

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Figure 1.3. Overview of direct preparation methods of Janus particles. (a) Microfluidic system; (b) Electrospinning using a bi-phasic nozzle; (c) Self-assembly of triblock terpolymers. Reprinted form Ref [57]. Copyright the Royal Society of Chemistry 2008.

The indirect template-assisted method addresses the chemical or physical modifications of the hemispheric surfaces of existing monodisperse particles. Particle embedding on substrate surfaces is required to conceal one hemispheric surface and to modify the other exposed hemisphere with chemical functionalities or geometric shapes. Particle adsorption and embedding are usually conducted on 2D flat surfaces (Figure 1.4a).[58-60] The major drawback of this strategy is that the amount of particles is extremely limited and does not allow their use in larger scale application studies. Nevertheless, due to its simplicity, this approach is still in use now.


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Figure 1.4. Overview of indirect template-assisted methods of Janus particles. (a) 2D flat soft substrate. Reprinted with permission from ref [58]. Copyright 2010 American Chemical Society. (b) Liquid/liquid interface template. Reprinted with permission from Ref [61]. Copyright the Royal Society of Chemistry 2008. (c) Fiber polymer substrate. Reprinted with permission from ref [63]. Copyright 2010 American Chemical Society.

To overcome this limitation, spherical substrates offer an opportunity to increase the surface-to-volume ratio. For example, the effective fabrication of Janus silica particles has been demonstrated by using suspended wax microparticles as embedding vehicles by Granick and coworkers (Figure 1.4b).[61, 62] The concept essentially transfers the two-dimensional technique into a solution phase and uses the high internal interface of an oil (wax)–water emulsion to achieve higher mass

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fractions of Janus particles. In a first step, they created a Pickering emulsion of wax and water using silica particles as stabilizers at high temperature. After cooling down of the emulsion and a purification step, the particles were immobilized at the solidified interface. The key step of this process is the immobilization of the particles at the interface and the suppressed rotational diffusion of the particles at the solidified interface. The Janus particles can then be obtained after functionalization of one side with aqueous phase chemistry and filtration at higher temperatures. Another example using polymer fiber as scaffolds for immobilization of the particles to prepare Janus particles was presented by Kuo and coworkers (Figure 1.4c).[63 , 64 ] They reported the fabrication of binary and ternary Janus particles with even and uneven surface-functionalities by particle embedding and gas-phase silanization of the exposed surface.

A multitude of reviews has recently reported details on the fabrication of Janus particles [3, 57, 65-69] and therefore a comparison of all these preparation methods in this thesis is not necessary. In brief, these abundant methods offer us more options to achieve Janus particles, but we should always keep in mind that all these approaches have their own merits and limitations and choosing the most suitable one is critical in achieving the type of Janus particle needed.

In this thesis a facile masking technique for large scale production with stable particles spacing is developed based on spherical polymer matrixes as masking templates. This process holds several advantages:

a) in comparison to two-dimensional flat substrates or one-dimensional fibres, a spherical polymer matrix provides a high amount of surface area (per unit volume) for particle embedment, and thus results in a higher yield of Janus particles.

b) polymer substrates can easily be softened with proper treatment and turn back rigid. This amazing property of polymeric substrates demonstrates superior stability for particle spacing as compared to interface substrates suspended in liquid media.

c) the solid property of polymeric substrates can also simplify the purification and separation by simple liquid washing processes which is a known challenge for preparation methods based on immobilizing particle on a liquid/liquid or liquid/air interface.


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1.2 The role of carbon dioxide

To embed target nanoparticles inside the spherical polymer substrate and thus to protect them from further surface modification, the polymeric substrates should be softened without losing their spherical shape and the embedding degree should also be tunable over a wide range for the method to be applicable. Carbon dioxide (CO2) is one of the best candidates for this purpose with various merits.

Figure 1.5. Phase diagram for carbon dioxide. Beyond a specific temperature and pressure (the critical point) carbon dioxide becomes a supercritical fluid, a state that is neither a gas nor a liquid, but has properties of both. Reprinted with permission from ref [70]. Copyright © 2000, Rights Managed by Nature Publishing Group.

CO2 is a clean and versatile solvent and a promising alternative to noxious organic solvents and chlorofluorocarbons. It has attracted particular attention as a supercritical fluid in the synthesis as well as processing areas for polymers due to the following properties (Figure 1.5) [70]:

(1) CO2 is non- toxic, non-flammable, chemically inert, and inexpensive. A large amount is available as a by-product from NH3 and ethanol industries and refineries.

(2) Supercritical conditions are easily achieved: TC = 31 °C and PC= 73.8 bar.

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(3) The solvent may be removed by simple depressurization. (4) The density of the solvent can be tuned by varying the pressure. (5) Many polymers become highly swollen and plasticized in the presence of

CO2. CO2 is a good solvent for many non-polar (and some polar) molecules with low molecular weight.[71] It is a very poor solvent for most high molecular weight polymers under readily achievable conditions. Very few polymers have shown a good solubility in pure CO2 under mild conditions like certain amorphous fluoropolymers and silicones.[ 72- 77 ] Though the solubility of most polymers in supercritical CO2 (sc CO2) is extremely low, the solubility of sc CO2 in many polymers is substantial. The concentration of dissolved CO2 in polymers mainly depends on the processing temperature and pressure. The dissolved CO2 causes a considerable reduction in viscosity due to an increase in free volume of the polymer. Thus, less energy is consumed during the process. The dissolved CO2 also alters some other physical properties and for instance implies a reduction in density and an increase in diffusion coefficient. Therefore, it has a tremendous potential as a plasticizer in polymer processing. It has been reported that properties such as the glass transition temperature and the elastic modulus of polymers in the region nanometers beneath the free surface may be significantly lower than the bulk values due to higher chain mobility and faster chain relaxation.[78, 79] Once the polymer is exposed to a gas such as CO2 under pressure, the mobility of polymer chains, especially those near the surface, can be greatly enhanced due to the increased free volume resulting from the adsorption of gas in the polymer, leading to a lower glass transition temperature and lower elastic modulus.[80] For example, although the Tg of PS at atmospheric pressure is 105 °C, this value decreases as a function of CO2 pressure.[81,82] A decrease of Tg of roughly 0.5 to 1 °C for every bar of pressure applied (the difference can be attributed to different definitions of glass transition in publications).[81, 82, 83] Chen and his co-workers were able to show that an increase of pressure to 70 bar causes a decrease in Tg of 32 °C; above this temperature Tg becomes pressure independent up to 110 bar.[84] This concept has been successfully applied to bond polymer structures with micro- and nanoscale patterns. Abranowitz and coworkers[ 85 ] have shown the ability of CO2 to ‘‘weld’’ polystyrene (PS) colloidal crystals and have followed the kinetics of the process using in situ measurements of Bragg diffraction. PS photonic crystals were treated in sc CO2 at temperatures ranging from 32 to 40 °C up to 256 bar. Upon exposure to CO2, the welds grew tangentially from the points


19

of contact in all directions. After 6 h of exposure to CO2 at 32 °C and 256 bar, film formation was achieved. Near the critical point, an anomalous sorption and swelling of CO2 into PS latex was observed. The irregularities were explained as a consequence of the Gibbs excess adsorption that results from the high compressibility of CO2 in this region. Cabaňas and coworkers[86] have studied the effect of sc CO2 on PS latex particles decorated with different acrylic groups on their surface and cross-linked PS latex particles. Selecting the composition of the block copolymer and the pressure and temperature conditions of the CO2 treatment, the coalescence of 3D latex arrays can be fine-tuned. Above all, the sorption of CO2 can result in plastifying effects in polymers systems at quite low temperature ranges and therefore the surface of polymers can be softened in a controllable way according to the CO2 conditions applied. These softening properties of CO2 allows polymeric substrates to be good candidates for templates to mask rigid particles and this represents an excellent alternative to current masking fabrication methods of Janus particles. This CO2 assisted approach is quite suitable to prepare relatively hard Janus particles of which the shape won’t be influenced during softening process of the polymer substrates. Actually, various kinds of materials can meet this requirement, including inorganic particles, metallic particles, relatively hard polymeric particles as well as other soft organic particles that have a strong resistance to CO2.

1.3 Janus nanoparticles inside polymeric materials - towards functional hybrid materials

Incorporating NPs into polymeric materials is not only a practical pathway to developing engineered plastics with increased mechanical, optical, electrical, magnetic, and other properties. This technique is also one of the most attractive ways to obtain well-defined structures at different length scales by controlling spatial organization of NPs inside polymers. The term “polymer-nanoparticle nanocomposites” was already being practiced since the early 20th century but just attracted broad interest until the 1990s.[87 , 88 ] In the past 20 years, to produce nanocomposites for practical uses with engineered, desired, and tailored properties, extensive effort has been made to comprehensively understand the structure-property relationship in the polymer-NPs mixture.[89-99] Control over the location and spatial organization of NPs inside polymers however is essential to generate highly ordered NP-based functional devices including plasmonic waveguides, photonic crystals, optical lenses, memory storage devices, nanoelectronic circuits, photovoltaics, and batteries. Both experimental

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and theoretical results indicate that the dispersion and location of NPs inside polymers, block copolymers and polymer blends is governed by a delicate balance between enthalpic and entropic contributions, which rely on the properties of both polymer matrices (chemistry and rigidity) and particles (selectivity, size, and shape). Even though several general strategies based on the understanding of complex polymer-particle interaction have been proposed, precisely controlling the location of NPs in polymer matrices remains an obstacle in fabricating polymer-NP functional materials.[88] Due to the unique combination of amphiphilicity with the particle character, JPs are also reported to strongly adsorb and orient to the interface. Their use is promising in achieving precise organization of multicomponent NPs at the interface or surface of polymer matrixes with a high degree of order. These types of highly ordered hybrids have potential applications in flexible sensors,[100-104] tunable plasmonic nanostructures for surface-enhanced Raman scattering,[105-108] ultrafast switches and organic memory devices,[109,110] and advanced photovoltaic devices.[111,112] For example, in analogy to the emulsification of fluid mixture, JPs are also expected to strongly attach at the interface inside polymer matrices, either in polymer blends or block copolymers. Previous thorough studies of the interfacial behavior of JPs at fluid-fluid interface are not only essential for further practical application of JPs as solid stabilizers, but is also helpful for a fundamental understanding how JPs interact with polymeric interfaces, even though polymer interfaces cannot be simply understood as a fluid-fluid interface. The flexibility and chemical properties of long-chain polymers should be considered as well.

1.3.1 Janus nanoparticles as compatibilizers for the polymer blend

Using molecular simulations, Estridge and Jayaraman[ 113 ] compared the interfacial activity of different types of compatibilizers inside polymer blends, including diblock copolymer grafted nanoparticles (DBCGPs), diblock copolymers, and different homopolymers grafted Janus nanoparticles (JPs). They showed that JPs have the largest desorption energy due to their deeper penetration of the grafted beads into the A and B domains (Figure 1.6a), and the lowest average interfacial tension of the compatibilized blend (γ) normalized by blend without compatibilizers (γ0) at all volume fraction of compatibilizng agent ϕ considered (Figure 1.6c).[113] Unlike DBCGPs and diblock copolymers, Janus NPs localized directly at the interface, allowing all the A (B) homopolymers grafted on one hemisphere of the particle to interact with the A (B) domain of the blend (Figure 1.6b) at all volume fractions of JPs considered.


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Figure 1.6. (a) Energetic penalty for leaving the interface. These are representative snapshots of compatibilizer configurations at the interface and within each domain, and the desorption energies associated with each case. (b) Average distance of the compatibilizer from the interface versus volume fraction ϕ (same x-axis as c). (c) Reduction in interfacial tension. Error bars are standard error.[113] Reprinted with permission from ref [113]. Copyright 2015 American Chemical Society.

While a number of experimental and theoretical reports have demonstrated its advantage to improve the miscibility of immiscible binary polymers, homogeneous NPs still face complications in their use to stabilize polymer blends. Depending on variances in the surface chemistry, preferential wetting of homogeneous NPs inside one component can often occur. Interfacial adsorption can only be achieved if the difference between the interfacial tension values for the particles with each component is less than the interfacial tension of pure polymer blends themselves.[114,115] JPs, however, are expected to exhibit a higher surface activity than homogeneous NPs and hence greatly reduce the interfacial tension to achieve interfacial location.

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By taking the homogeneous surface modified NPs as a reference for comparison, numerical simulations by Guo and co-workers[116] found that Janus nanospheres significantly hamper domain growth, and the average size of domains is smaller at later stages of the phase separation process (Figrue 1.7). Also, the estimated domain size at saturation for the JPs-filled systems was found to be smaller than that for the homogeneous nanparticles-filled systems,[116] leading them to the conclusion that Janus nanospheres can be used as a more effective emulsifying or stabilizing agent than homogeneous nanospheres for immiscible polymer blends.

[116]

Figure 1.7. Two regions selected from the snapshots of 25HS2.0 (a) and 25JS2.0 (b) systems at t ¼ 1600s, which encompass a typical configuration that is seen throughout the samples. The yellow circle, which is drawn to guide the eye, outlines the nanoparticles that are not equatorially adsorbed at interfaces. The green, gray, pink, and cyan colors correspond to homopolymers A, homopolymers B, p and q hemispheres of Janus nanospheres, respectively. (c) The mean square displacement of nanospheres in the 25JS2.0 and 25HS2.0 systems. Reprinted with permission from ref [116]. Copyright the Royal Society of Chemistry 2012.

Experimentally, Virgilio and Favis investigated the effect of the affinity of JPs and found that the JPs consisting of PS and poly(methyl methacrylate) (PMMA)

spherical caps are interfacially active and locate at the interface of a phase separated high-density polyethylene (HDPE) and polypropylene (PP) blends.[117]


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Walther, et al.[118] also confirmed that organic Janus particles can be used to efficiently compatibilize polymer blends under high-shear conditions. Their results showed a constant decline of the domain size of the dispersed phase with the increased addition of JPs, independent of the polystyrene (PS) and poly(methyl methacrylate) (PMMA) blend composition. In the transmission electron microscopy (TEM) images (Figure 1.8), JPs were observed to be almost exclusively located at the interface of the PS and PMMA domains, and only a negligibly small fraction was “lost” as clusters or micellar aggregates in one of the components. They considered this a significant improvement compared to BCP compatibilizers as a direct consequence of the high interfacial activity and increased adsorption energy at interfaces of JPs.[118]

Figure 1.8. TEM images of Janus particles and their adsorption at the blend interface of a PS/PMMA blend obtained for (a) 10 wt% JP in an 8/2 PS/PMMA blend and for (b) 20 wt% JP in a 6/4 PS/PMMA blend.[118] Reproduced with permission from Ref [118]. Copyright 2008 American Chemical Society.

For industry-scale application, Bahrami, et al. subsequently used 200g of JPs to compatibilize several kilograms of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) (SAN) (Figure 1.9).[ 119 ] The scaled-up experiments indicated that the fraction of JPs in the range of 2-5 wt% was the optimum amount necessary for sufficient droplet stabilization of PPE/SAN blends. The addition of only 0.5 and 1 wt% JPs was not able to provide the required interfacial coverage to stabilize the interface. As a result, the blends were not mixed well. When large amount of JPs was added, excess content JPs inside blends on the contract would create additional interface to form double emulsion morphology.[119]

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Figure 1.9. Schematic representation of processing of polymer blends using JPs as a compatibilizer during extrusion of the polymer melt. JPs (black dots) compatibilize and stabilize PPE droplets (yellow) within the SAN matrix (gray). Reprinted with permission from ref [119]. Copyright 2014 American Chemical Society.

Remarkably, due to the combination of strong interfacial affinity and the Pickering effect, JPs still quantitatively adsorbed at the blend interface in these industry-scale blending experiments, despite the harsh shear-processing conditions (Figure 1.9).[118, 119] Soon afterward, they also achieved controlled blend morphologies by demixing in solvent-cast films by adjusting the content of JPs and the composition ratio between PS and PMMA.[120] Interfacially trapped structures in the final blends showed good resistance to coarsening during several days of annealing well above the glass transition temperature of the components.

JPs in the examples above show their superiority over other methods for compatibilizing immiscible polymer blends. They can reduce the micellization problems of diblock copolymer stabilizer and also the difficulty of achieving interfacial adsorption encountered by homogeneous particle surfactants. As long as Janus inorganic particles can be prepared at industrial scale, it will be a quite promising approach to achieve hybrid materials with desired functional properties by simply mixing them with properly chosen polymer.


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1.3.2 Janus nanoparticles for ordered interfacial arrangement inside block copolymer scaffolds

To generate highly ordered NP-based functional materials, control in the location and spatial organization of the NPs inside polymers is essential. The ability to control the length, spatial, and orientational organization of block copolymer (BCP) morphologies (from spherical to cylindrical, bicontinuous, and lamellar structures, Figure 1.10)[121-123] makes BCP materials particularly attractive as templates for manipulating the spatial location of inorganic NPs at various length, from micro to nano scales. Several reviews have summarized the progress made in understanding and controlling NP distribution in BCP-ordered structures.[87-99]

Of particular interest is controlling the assembly of inorganic or metallic NP, at the interface between different phase domains of the BCPs, to achieve a high degree of order and even responsive behaviors to certain external stimuli.[ 124 ] However, interfacial location of NPs, and the overall morphology within BCPs, depends on a delicate balance between the enthalpy of NP insertion and the entropy related to particle conformation and translation, which are subjected to various interacting factors. These include properties of both the BCPs (chemistry, conformation, and molecular weight) and the particles (selectivity, size, shape, and concentration).

1.3.2.1 Enthalpic Effects

Enthalpic interactions rely on the surface chemistry of NPs and BCPs. With suitable grafting, polymeric ligands on the NP surface can change the particle surface chemistry and effectively control NP-BCP enthalpic interactions – and thus the location of NPs inside BCPs, either in one specific domain or at the block interface. For example, extensive studies were performed on the surface chemistry effect of polymeric ligands grafted NP inside BCPs. It was shown that NPs, by modifying with A or B homopolymers, were preferred to selectively localize within the A or B microdomains, respectively.

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Figure 1.10. Diblock copolymer melt state phase behavior. (a) Theoretical predictions by self-consistent field theory where N is the degree of polymerization, c is the Flory interaction parameter, f is volume fraction of one of the blocks. CPS: close packed spheres, H: hexagonally packed cylinders, Q229: body centered spheres, Q230: double-gyroid, L: lamellae and DIS: disordered. Reprinted with permission from Ref [121]. Copyright 1996 American Chemical Society. (b) Experimental results for PS-b-PI. Reprinted with permission from Ref [122]. Copyright 1995 American Chemical Society. (c) Schematic illustration of microstructures of diblock A-b-B (where block A is consists of monomers ‘a’ while B is made of monomers ‘b’) on increasing the volume fraction of the B block. Reprinted with permission from Ref [123]. Copyright the Royal Society of Chemistry 2014.


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Figure 1.11. (upper) The spatial organization of Au NPs coated by a mixture of low molecular weight thiol end-functional PS/P2VP can be controlled in PS-b-P2VP. Varying the PS and P2VP surface compositions (FPS) on the Au NPs allows placing the NPs into PS or P2VP microdomains or at the interface. (lower) Adsorption energies for 4 nm radius r113andom copolymer-coated NPs (solid line) and amphiphilic NPs (dotted line) are plotted as a function of FPS. Reprinted with permission from Ref [129]. Copyright 2007 American Chemical Society.

However, according to Kramer and his co-workers’ observations, the surface coverage of polymer ligands on NPs can change this selectivity of NPs inside BCPs and, consequently, drive a shift in the spatial organization of NPs from inside the selective polymeric domain of the BCP matrix to the interfacial regions with the ligand areal density decrease. PS-thiol (PS-SH) ligands-coated Au NPs were identified to have a critical areal chain density ΣC below which NPs favorably

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adsorbed to the interface. The ΣC decreased from 3.1 to 0.9 chains/nm2 as the Mn of PS-SH chains increased from 1.5 to 13 kg/mol. [125-127]

For the purpose of interfacial localization of NPs inside BCPs with enhanced stability, particles with amphiphilic surface properties are the preferred choices, such as NPs grafted with a mixture of ligands, random copolymer-anchored NPs, or Janus-type NPs.

Kim et al. reported that gold particles, which are coated by a mixture of low molecular weight PS and poly(2-vinyl pyridine) (P2VP) thiols, were observed to segregate at the interfaces between the PS-P2VP blocks over a broad range of PS fractions (FPS) from 0.1 to 0.9 (Figure 1.11).[128, 129] In stark contrast, Au NPs with surfaces covered by a random copolymer of styrene and 2VP with FPS = 0.40 remained in the P2VP domain and far away from the interface. However, the coated Au NPs became clearly segregated along the PS/P2VP interface when the fraction increased to 0.52, which is consistent with the results in Figure 1.11.[129] Afterward, Kim and Matsen developed a quantitative theoretical method to examine the effect of grafted brushes on the equilibrium distribution of spherical NPs inside the BCP lamellar phase by implementing self-consistent field theory and a new multi-coordinate-system scheme.[ 130 ] Their conclusions agreed with experimental results: mixed brushes are significantly more effective than the random-copolymer brushes for positioning NPs at the interface. However, the preference for the interface was much stronger than expected from simple surface-tension consideration, especially when the mixture-grafted particle had a majority of PS chains on the surface (Fps = 0.9).

To explain the results, Kim et al hypothesized that chains of the mixed brush segregate to opposite hemispheres, creating JPs, as long as an appropriate time scale is involved. Once the particles adsorb at the interface, the ligands can rearrange on the Au NP surface, leading to strengthened adhesion and a pinning to the interface. They confirmed this possibility by simple calculations on the adsorption energy of random copolymer-coated NPs to the interface (Figure 1.11).

[129] Similar phenomena were also observed in polymer blends interfaces from small-angle neutron scattering (SANS) results. It was revealed that two polymer ligands of high molecular weight showed phase separation to form a Janus-type shell on the NP surface, whereas those of low molecular weight formed either a mixed or partly demixed shell. [131]

The question arises, then, whether it is still necessary to use JPs to capture interfaces, especially when mixed ligands or random copolymer-grafted NPs can do the same job and are even easier to prepare. It can be discussed from several aspects. First, as discussed in various examples above, [113,128-131] JPs theoretically


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show higher interfacial adsorption energy than random copolymer-grafted NPs (or mixed polymer ligands-coated NPs without phase separation to form the Janus type) and thus are more preferentially located at the interface. Second, even though ligand exchanges on Au NPs are likely to form Janus-type particles at the polymer interface, it is not always true for other types of metallic or inorganic NPs, which endow hybrid materials with optical, magnetic, or electronic properties that Au NP-based hybrid materials cannot achieve. Actually, gold NPs with mixed ligands were still observed to be dispersed within one of the micro domains in PS-b-P2VP when the ratio of dodecanethiol to 11-mercapto-1-undecanol ligand was changed from 1:1 to 3:1.[ 132 ] Also, coated by a ligand mixture with a molar ratio approximately 5:1 of dodecanethiol:PS-SH, Au NPs showed an affinity for PS-rich domains but not localized at the interface in poly-(styrene-b-isoprene-b-styrene) triblock copolymer thin films.[133] In these cases, the entropic effects surpassed enthalpic ones playing the main role on the location of NPs inside BCPs. However, limited by the rare experimental studies on the incorporation of JPs in BCP scaffolds, it is difficult to draw a firm conclusion on which type of particle is better for interfacial location. To compare and thus reach a conclusion more thorough studies on JPs are needed.

1.3.2.2 Entropic Effects

From the point of view of the enthalpy effect – that is, interaction between NPs and BCPs – JPs with two chemically different grafting compartments tend to adsorb at the interface inside BCPs. These benefit from their higher interfacial activity and their stronger amphiphilicity compared to homogeneous polymer-grafted NPs. However, their ordering and interface-centered position inside BCP composites is not simply determined by their surface chemistry, but usually governed by an intricate balance of enthalpic and entropic interaction. Therefore, entropic effects relying on the size, shape, and chain properties of BCPs should also be considered in designing new materials with regard to the orientation and off-center position at the interface.

JP Size Wang et al. undertook an investigation on the effect of JP size on NP distributions in BCP scaffolds, in the framework of self-consistent field theory/density functional theory (SCFT/DFT). They discovered that the JPs’ capability to stick to the interface is dependent on their sizes. Larger JPs were found to be strongly attached to the interface with a higher value of the orientational order parameters. However, when the JPs are smaller, or one spherical cap becomes smaller than the other one, they are able to migrate from the interface to the domain of BCPs.[134]

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It is remarkable to note that the conclusion on size-dependent interfacial location of JPs in BCP scaffolds from Wang et al. seems opposite to that of homogeneous NPs inside BCPs addressed by both theory and modelling.[127,135,136] For example, it was found that relatively smaller homogeneous particles (with the ratio of NP diameter, dNP, over the BCP domain size, L, being smaller than 0.2) were located in the interfacial regions of a poly(styrene-b-ethylene propylene) (PS-b-PEP) diblock copolymer, whereas relatively large NPs (dNP/L > 0.3) were localized in the interior of the PEP domains.[88] There is, in fact, no conflict to observe completely different locations of larger JPs and homogeneous NPs in the BCP matrix. Both systems indicate that favorable enthalpic interaction and minimal loss in the conformation entropy determine the particles’ positions. As examples, for larger particles, Janus types locate at the interface due to their amphiphilicity, and homogeneous ones are sequestered in the interior of one selective polymeric block. For smaller homogeneous particles, the stretching required by the polymers to circumvent the spheres is less significant. Hence, it is the other entropy effect, named as the translational entropy of the particles, that dominates the system’s behavior and causing the prferential location of homogeneous small particles at the interface.[137] However, the random distribution, but not interfacial arrangement, of smaller JPs in BCP scaffolds[134] is hardly to explained in this way and therefore more reasonable theoretical explanations and experimental results are required.

JP Shape Yan et al. conducted simulation investigation on a mixture of a diblock copolymer and JPs with different shapes.[138] As shown in Figure 1.12, JPs, independent of their shapes, exclusively segregate to the interface between two lamellae in the equilibrium state. This reveals that the enthalpic effects, which rely on the surface chemistry of NPs and BCPs, overcome the entropic effects from the conformational transition of polymer chains under their simulation conditions.[138]

Benefiting from such strong enthalpic interactions between polymer blocks and the sphere surface, Janus spheres remain anchored at the interface during the whole dynamic assembling process of the nanocomposites and also during the dynamic shear process.[138] However, it is interesting to note that the orientation of these JPs with anisotropic shapes, with respect to the interface, can be controlled upon changing their surface and shape, especially for Janus rods and disks. It was also suggested that JP-containing diblock copolymers could allow better processing due to enhanced shear dynamics and feasible viscosity changes. [138] Therefore, by using Janus-type NPs with surface and shape architectures, even more interfacial, stable, functional, and hybrid composites can be obtained with oriented structures and responsive behaviors to certain external stimuli.


31

Figure 1.12. Equilibrium self-assemblies formed by various NPs in symmetric diblock copolymers. The volume fraction of each type of NP is 0.15. The interface between phases A and B is shown in yellow, and phase A is displayed as half-transparently blue. Phase B is fully transparent. The types of NPs are (a) HS (homogeneous sphere with radius Rs = 2rc), (b) JS (Janus sphere Rs = 2rc), (c) JR1 (one type of Janus rod with a radius of the bottom face Rr = 1.5rc and height Lr = 9rc), (d) JR2 (another type of Janus rod with the same size of JP1), (e) JD1 (one type of Janus disk with a radius of the bottom face Rd = 3.5rc and thickness Ld = 2rc), and (f) JD2 (the other type of Janus disk with the same size as that of JD1). The schematic diagrams at the right bottom of images b-f illustrate the orientation of JPs with respect to the interface. Reprinted with permission from Ref [138]. Copyright 2010 American Chemical Society.

BCP Properties In addition to the entropic effect caused by JPs themselves, BCPs’ properties can play an important role in the interfacial location of JPs as well, by influencing polymer chain conformational entropy or particle translational entropy. To study the influence of the BCP architecture on the precise position of JPs with regard to the interface, systematic computer simulations and theoretical analyses were conducted by co-assembling symmetric JPs into asymmetric diblock copolymer scaffolds of varying molecular architectures. This was done by simply

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increasing the length of the B block segment (while the length of the A block segment remained the same). [139] For systems presenting a lamellar phase with flat-phase interfaces, JPs tend to move to A domains for the longer B segments, of which the extended conformation makes it difficult for the JPs to be positioned in the B segment.[139] However, for even larger B segment systems, where the phase interface curves (i.e. cylindrical BCP morphologies), the increase of B segments also causes JPs to shift off-center from the interface but turn to the B domains.[139] In this case, the topology mismatching between JPs and curved polymer interfaces at the mesoscale accounts for the JPs’ position transition.[139] The semiflexible block’s stiffness was also observed to be able to regulate the off-center distribution of symmetrical JPs with respect to the phase interface, featured by a roughly 35% deviation from the interface to the utmost extent (as shown in Figure 1.13).[140]

Despite these theoretical studies, establishing more detailed knowledge regarding the effect of JNPs on their positions in BCP scaffolds is still needed to create nanocomposites with desired structures. Experimental studies, which are still particularly rare, presumably due to the more complicated synthesis required. Hence additional efforts in large-scale synthesis and interfacial behavior studies of JNPs for the design of these smart materials are needed.


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Figure 1.13. (a-d). Representations of the self-assembly of Janus NPs in AB diblock copolymers with flexible A block but with B block of various stiffness: (a) Ka = 0 kBT, (b) Ka = 40 kBT, (c) Ka = 100 kBT, and (d) Ka = 300 kBT. at each Ka, the top snapshot shows the self-assembled morphologies where the cyan and pink chains denote blocks A and B, and sites P and Q of every JP are shown in red and yellow, respectively. The bottom snapshot highlights the detailed position of a JP at the interface and the conformation of the polymer chains around it. The green and

yellow lines mark the phase interface and the equator of the NP (e1−e5). The

density profiles of blocks A (cyan) and B (pink), and the JP (red) are shown for various stiffnesses of block B: (e1) Ka = 5 kBT, (e2) Ka = 40 kBT, (e3) Ka = 60 kBT, (e4) Ka = 100 kBT, and (e5) Ka = 400 kBT. Reprinted with permission from ref [140]. Copyright 2015 American Chemical Society.

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1.4 Scope of this thesis

This thesis focuses on a novel technique to prepare Janus particles from spherical PS templates with the assistance of CO2. The scheme of this approach is shown in Figure 1.14, of which experimental details are discussed in Chapter 2 – 4 of this thesis. Briefly, with the assistance of CO2, polymer particles can be softened to allow a part of the rigid particle to be embedded inside their surface. With the embedded side being protected by a polymer substrate, further surface modification can be made on the other exposed side of rigid particles to form Janus-type NPs with subsequent removing polymer templates by dissolving or thermal degradation. In addition to the advantages mentioned in Section 1.1.2, the ability to tune the degree of welding by controlling the treatment conditions of CO2 affords great control of the geometry (Janus balance) of the Janus particles over a wide range.

Figure 1.14. Designed preparation process of Janus particles: (a) assembly of spherical superstructures; (b) embedment with the assistance of CO2, the embedding degree can be controlled by adjusting CO2 conditions; (c) further surface modification; (d) releasing from the PS substrate.

In Chapter 2, we focus in detail on the coupling of target particles and PS templates (Figure 1.14a). Theoretically, target particles (for Janus particles) can be easily attached to the surface of polymer spheres driven by either chemical or physical interactions. Here, we use carboxyl end group functionalized PS as the masking substrates. Silica particles with amino functionalized surfaces are successful coupled onto PS templates via electrostatic interaction and further chemical reaction between carboxyl and amino groups. Several examples of PS/silica composites with different sizes are shown in this chapter. Other examples with different materials of target particles are also discussed.

Chapter 3 reports the embedment control of target rigid particles on the surfaces of spherical polymer particles which can be softened in the presence of CO2 (Figure 1.14b) According to the CO2 condition applied, the embedding depth of


35

rigid particles on the surface of polymer substrate are tuned to range from several hundred to several nanometers. It is quite easy to observe and measure the embedment depths above a hundred nanometers by scanning electronic microscope (SEM) but would be a challenge to recognize much smaller sizes (sub 20 nm). Therefore, another technique, atomic force microscope (AFM), is chosen to study the embedment of rigid nanoparticles which are dispersed on PS films.

In Chapter 4 surface modifications of the target particles (Figure 1.14c) are performed on the exposed side (with the embedded side being protected by the polymer substrate) to form Janus-type NPs after polymer templates are dissolved in organic solvents or degraded by thermal heating (Figure 1.14d). Both gold nanoparticles (Au NPs) and ferroferric oxide nanoparticles (Fe3O4 NPs) are used to label the exposed hemisphere of silica target particles to achieve metallic or magnetic Janus particles. The exposed hemispheres are also suitable to be modified by other chemicals, like polymers. Different Janus silica particles with further surface modification by poly(2-vinylpyridine) or poly(acrylic acid) are also prepared in Chapter 4.

The distribution and self-assembly behavior of as-prepared Janus nanoparticles inside BCP scaffolds is studied in Chapter 5. Diblock copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) with a fraction of PS roughly around 0.5 are found to show a lamellar morphology both in bulk and thin films after proper solvent vapor annealing treatment. The distribution of Janus particles with PS and P2VP surface modification on the respective other sides inside the lamellar morphology (bulk or thin films) are investigated in a detail.

Various types of Janus particles with different functional materials is promising to be prepared via the developed approach, such as Janus-type Ag, TiO2, Fe3O4, nanoparticles. The size, Janus balance and surface functionality of Janus particles can be tailored in a wide range according to practical requirements. With proper design of the system (type of JNPs and polymers) functional hybrid nanostructures and devices with engineered, desired, and tailored properties toward real-life applications can be easily obtained. These types of hybrid materials based on the precise location of JNPs at the interface of polymeric materials can combine the functional properties of NPs with the mechanical properties of polymer matrixes. For example, multi-layer based devices are expected to be achieved via this way, such as flexible sensors, tunable plasmonic nanostructures for surface-enhanced Raman scattering, organic memory devices and photovoltaic hybrid devices.

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