11 Surface and Particle Modiﬁcation via the RAFT Process ... · two different CTAs,...

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c11 September 6, 2007 17:56 Char Count=

423

11Surface and Particle Modification via the RAFT Process:Approach and Properties

Yu Li, Linda S. Schadler, and Brian C. Benicewicz

11.1Introduction

Surface modification of materials is of great importance, as it can alter the propertiesof the surface dramatically and thus control the interaction between materials andtheir environment. Due to the wide applications of polymers in many areas, forexample adhesion, lubrication, friction and wear, composites, microelectronicsand biotechnology [1–5], surface modification by polymers is gaining increasingattention [6–8]. Generally, there are two ways to achieve the surface modificationof materials with polymers: physisorption and covalent attachment. Comparedwith the physisorption method, covalent attachment can avoid the desorption issueand provide a robust linkage between the introduced polymer chains and materialsurfaces.

Polymer grafting techniques provide a versatile tool to covalently modify thesurface of materials. These techniques can be categorized into ‘grafting to’ and‘grafting from’. In the ‘grafting-to’ technique, the polymer, bearing an appropri-ate functional group, reacts with the material surfaces to form chemically attachedchains. However, due to the steric hindrance imposed by the already-grafted chains,it becomes increasingly difficult for the incoming polymer chains to diffuse to thesurface, which intrinsically results in low surface graft densities. In the ‘grafting-from’ technique, the initiators are initially anchored on the surface and then subse-quently used to initiate the polymerization of monomer from the surface. Becausethe diffusion of monomer is not strongly hindered by the existing grafted polymerchains, this technique is more promising to achieve high graft densities.

The recent development of controlled polymerization techniques includingcationic, anionic, ring-opening metathesis and controlled radical polymerizations(CRP) makes it possible to provide considerable control over both the structure ofthe polymer to be grafted onto the materials surface and surface graft densities.The combination of these polymerization methods with polymer grafting tech-niques has been successfully used as an approach to modify various surfaces with

Handbook of RAFT Polymerization. Edited by Christopher Barner-KowollikCopyright C! 2008 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31924-4

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424 11 Surface and Particle Modification via the RAFT Process

a variety of functional polymers. As a relatively newer CRP technique, reversibleaddition–fragmentation chain transfer (RAFT) polymerization has been success-fully applied to the controlled polymerization of various monomers under a widerange of conditions to prepare polymer materials with predetermined molecularweights, narrow polydispersities and advanced architectures [9–12]. RAFT poly-merization is performed under mild conditions, is applicable to a wide range ofmonomers and does not require a catalyst. Due to these advantages, the RAFTtechnique has recently received substantial attention in the area of surface modifi-cation with polymers. Since the first report of applying this technique to surface-initiated graft polymerization on a solid surface in 2001 [13], the RAFT techniquehas been utilized in the surface modification of various substrates, including in-organic/organic particles [13–37], flat silicon wafers [38–43], clay [44–46], flat goldsurfaces [47, 48], gold nanorods [49], glass slides [50], carbon nanotubes [51–57],cellulose [58–61], rigid plastic [62, 63] and polymer films [64–68]. This chapter fo-cuses on the approaches that have been used to modify various surfaces via theRAFT process as well as the physical and molecular properties of the resultingmaterials.

11.2Approach

11.2.1‘Grafting-to’ Approach

The ‘grafting-to’ approach provides a convenient way to modify the surface of mate-rials by utilizing an end-functionalized polymer chain reacting with an appropriatelytreated substrate. As the grafted chains are preformed in this technique, their typesand structures can be carefully designed via various polymerization methods. As aversatile CRP technique, RAFT is compatible with almost all of the conventionalradical polymerization monomers, which allows for the preparation of a wide rangeof polymers with well-defined structure. Because RAFT polymerization follows adegenerative chain-transfer mechanism in which thiocarbonylthio compounds actas chain-transfer agents (CTAs), polymers prepared by this technique usually beardithioester or trithiocarbonate end groups that can be easily reduced to thiols. Thehigh affinity of thiols for the surfaces of metals, in particular gold, makes it possibleto modify various metal substrates with well-defined polymer chains prepared viaRAFT.

Lowe et al. [31] developed a facile one-step process to prepare (co)polymer-stabilized transition metal nanoparticles based on Au (HAuCl4 sol), Ag (AgNO3), Pt(Na2PtCl6•6H2O) and Rh (Na3RhCl6). In this process, the (co)polymers employedas stabilizers, including poly(sodium 2-acrylamido-2-methyl propane sulfonate)(PAMPS), poly[(ar-vinylbenzyl)trimethylammonium chloride] (PVBTC), poly(N,N-dimethylacrylamide) (PDMA) and poly[3-(2-N-methylacrylamido]-ethyl dimethylammonio propane sulfonate-b-N,N-dimethylacrylamide) (PMAEDAPS-b-PDMA),

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11.2 Approach 425

Transitionmetal

complexNaBH4

H2OSH

S

S

SS

S

SS

SC

Scheme 11.1 Preparation of polymer-stabilized transition metalnanoparticles. (Reproduced with permission from [31]. Copyright 2002American Chemical Society.)

were synthesized by aqueous RAFT polymerization. The subsequent reduction ofthe dithioester end groups of these (co)polymer chains and a metal complex or metalsolid occur simultaneously in aqueous media, giving a series of polymer-stabilizedtransition metal nanoparticles (Scheme 11.1). Transmission electron microscopy(TEM) was used to examine the metal nanoparticles after stabilization. Comparedwith those nanoparticles obtained by the reduction performed in the absence of theRAFT-synthesized (co)polymer, the polymer-stabilized metal nanoparticles wereextremely stable. Sumerlin et al. [47] further extended this work to the modificationof gold films, in which the reduction of the dithioester end-capped (co)polymerswas performed in the presence of the gold substrates. Attenuated total reflectanceFourier transform infrared spectroscopy and atomic force microscopy (AFM) con-firmed the presence of the monolayer (co)polymers on the surface of gold films.Chemical bonding of the thiol end groups to the surface of gold films was evidencedby the fact that the (co)polymers remained immobilized after thorough rinsing withsolvent.

Using a similar approach, Spain et al. [32] prepared biologically active goldnanoparticles stabilized with multivalent neoglycopolymers synthesized via RAFT.Shan et al. [34] prepared amphiphilic gold nanoparticles grafted with a mixtureof RAFT-prepared poly(N-isopropylacrylamide) (PNIPAM) and polystyrene (PS)chains with two different ratios. These amphiphilic gold nanoparticles showed dif-ferent behaviors at the air–water interface in Langmuir monolayer experiments,and the contact-angle measurements revealed that PS and PNIPAM chains graftedon the surface of the gold cores appeared to be phase separated.

Shan et al. [17] also employed three methods in the preparation of PNIPAM-monolayer-protected clusters (PNIPAM-MPC) of gold nanoparticles (Scheme 11.2),in which three types of PNIPAMs were used: RAFT-prepared PNIPAMs bear-ing dithiobenzoate end groups, RAFT-prepared PNIPAM end capped with a thiolgroup obtained through hydrazinolysis and thiol-functionalized PNIPAM obtainedthrough a conventional radical polymerization and subsequent modification. It wasfound that the one-step method was facile in controlling the sizes of gold clusters

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One-step way:

Cumyl-orcpa-RAFT-PNIPAM+HAuCl 4

LiBEt3H

THFCpa-PNIPAM-MPCCumyl-PNIPAM-MPC

Two-step way:

Cumyl-RAFT-PNIPAMH2N-NH2

Ethanol

PNIPAM-SH+

Disulfide

+H AuCl4/THF

LiBEt3HCumyl-PNIPAM-MPC

Three-step way:

NIPAM+ACPA

PNIPAM-COOH+ Cysteamine

EDACPNIPAM-SH

+H AuCl4/THF

LiBEt3HPNIPAM-MPC

Scheme 11.2 Schematic representation of three ways to preparePNIPAM-MPCs. (Reproduced with permission from [17]. Copyright 2003American Chemical Society.)

with reasonably narrow size distributions compared to the other two methods. Thepresence of the PNIPAM disulfide caused a broad size distribution of MPCs in thetwo-step method, and separate gold clusters could not be prepared by the three-stepmethod due to a certain amount of dithiolated PNIPAM that acted as a cross-linkingagent.

Recently, an interesting study by Duwez et al. [48] showed that dithioesters ortrithiocarbonates can be directly chemisorbed on gold substrates (Scheme 11.3)without the need for reduction into thiols. Polystyrenes prepared by RAFT withtwo different CTAs, benzyldithiobenzoate (BDTB) and dibenzyl trithiocarbonate(DBTTC), were successfully grafted onto gold substrates via the chemisorptionof the dithioester and trithiocarbonate end groups. This strategy simplifies theconventional procedures and avoids the formation of disulfides, resulting fromthe coupling between two thiol-functionalized polymer chains, which may cause abroad size distribution of the grafted chains [17].

In a more recent study, Hotchkiss et al. [49] modified the surface of gold nanorodsby RAFT-prepared polymers, including poly[2-(dimethylamino)ethyl methacrylate](PDMAEMA), poly(acrylic acid) (PAA) and polystyrene (PS), with or without theuse of reducing agents (Scheme 11.4). TEM and UV–vis spectroscopy results con-firmed that both reduced and nonreduced RAFT-prepared polymers were covalently

Scheme 11.3 Chemisorption configuration of the BDTB (left) andDBTTC (right). (Reproduced with permission from [48]. Copyright 2006American Chemical Society.)

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11.2 Approach 427

Scheme 11.4 Proposed mechanism describing synthesis, reduction andimmobilization of RAFT-prepared PDMAEMA on a gold surface.(Reproduced with permission from [49]. Copyright 2007 AmericanChemical Society.)

attached to the gold nanorods, and the thickness of the grafted polymer varied from3 to 14 nm, depending on the polymer and grafting conditions used.

Instead of grafting polymer chains onto the substrate surfaces via the sulfur-metalbond, Guo et al. [24] used a different approach to prepare glycopolymer-modified sil-ica gel particles. The surface-attached monomers were first prepared by modifyingthe surface of silica gel particles with !-methacryoxypropyltrimethoxysilane, whichwas reacted with RAFT-prepared glycopolymers via radical exchange in the pres-ence of 2,2"-azobisisobutyronitrile (AIBN) (Scheme 11.5). The subsequent cleavageof the acetyl groups of the grafted polymer gave the silica gel particles modifiedwith well-defined lactose-carrying polymer.

Although the ‘grafting-to’ approach provides a convenient way to modify thesubstrate surface with well-defined RAFT-prepared polymers, the inherent problemassociated with this approach is the limitation of surface grafting density. Thediffusion barrier established by the already-grafted polymer chains makes it difficultfor the new polymer chains to access the reactive sites on the substrate. Thus theamount of the grafted polymer chains was limited, which usually resulted in lowgrafting densities and film thickness. To overcome this problem, great attentionhas been paid to the modification of material surfaces via surface-initiated RAFTpolymerization.

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Si CH2CH2CH2 O CO

CCH3

CH2

SS C

CH3

CH2

C OOCH2

CH2

O O OO

AcO

AcO

AcO

OAcAcO

CCH3

CH3n

CCH3

CH2

C OOCH2

CH2

O O OO

AcO

AcO

AcO

OAcAcO

CCH3

CH3

Si CH2CH2CH2 O CO

CCH3

CH2R

nAIBN

OAc

OAc

Scheme 11.5 Schematic illustration of grafting glycopolymers ontosilica gel particles. (Reproduced with permission from [24]. Copyright2006 American Chemical Society.)

11.2.2Surface-Initiated RAFT Approach

Surface-initiated RAFT polymerization has been widely explored as an approach tomodify the material surfaces due to its ability to precisely control the structure ofthe grafted polymer chains with a low-to-high range of grafting densities. In thisapproach, there are two general routes to prepare surface-grafted polymer chains,including using (1) a surface-anchored initiator with free CTA in solution and (2)a surface-anchored CTA with appropriate initiation method. In both cases, thepolymer chains are able to grow from the surface of materials rather than diffuseto the surface against the concentration gradient of the existing grafted polymers.Thus compared to the ‘grafting-to’ approach, surface-initiated RAFT polymerizationis a more promising approach to construct dense and thick polymer layers on thesurface of materials.

11.2.2.1 Grafting-From Surface-Anchored InitiatorsThe immobilization of initiators on the material surfaces can be achieved by varioustechniques, including chemical reaction, plasma discharge and high-energy irradi-ation. The subsequent polymerization from these surface-anchored initiators in thepresence of free CTA can generate surface-grafted polymer chains with uniformstructure and adjustable length.

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11.2 Approach 429

H2C CR

R'

S

S

(CH2)11 OCN

NN CN

O

(CH2)11 OCN

O

AIBN, solventR'

R

SS

Chain-transfer agent(CTA)

+‘free’ polymer

Scheme 11.6 A general process of surface-initiated RAFTpolymerizations from a surface-anchored azo initiator. (Reproduced withpermission from [38]. Copyright 2002 American Chemical Society.)

Baum and Brittain [38] utilized RAFT to graft PS, polymethylmethacrylate(PMMA), PDMA and their copolymers from silica substrates using a surface-anchored azo initiator (Scheme 11.6). A silane coupling agent was used to im-mobilize the azo initiator on the silicate surfaces. 2-Phenylprop-2-yl dithiobenzoatewas used as a free CTA in solution to control the graft polymerization. It was foundthat addition of free initiators was needed to achieve an effective polymerizationrate, and increasing the concentration of free initiators produced a thicker polymerlayer but decreased the control over the polymerization. A linear increase of filmthicknesses with sequential monomer additions was observed, indicating the liv-ing characteristics of the grafted polymer chains prepared by this surface-initiatedRAFT approach. Both Mn and Polydispersity index (PDI) of the PS and PMMAhomopolymers cleaved from the surface of silica gel were comparable to those ofthe corresponding free polymers generated in solution, suggesting that the prop-erties of the surface-grafted polymer chains can be estimated by analyzing the freepolymer. Tensiometry tests showed that compared to a typical PS overlayer, PShomopolymer brushes made by RAFT showed a lower water contact angle, whichwas attributed to the dithioester end group.

Zhai et al. [39] used a similar azo initiator to prepare polybetaine brushes from thesurface of hydrogen-terminated Si(100) substrates by surface-initiated RAFT poly-merization. The azo initiator was immobilized on the Si H surface in three steps.An alkyl ester was first immobilized on the surface under UV irradiation, which wasreduced to a hydroxyl group and then coupled with a carboxylated azo initiator by es-terification. A free initiator was also used in solution. The thickness of the polymerfilms increased linearly with the time of polymerization. Yu et al. [40] further ex-panded this approach by synthesizing poly(4-vinylbenzyl chloride) (PVBC) brushesfrom the same substrate. The free polymer generated in solution was analyzed toestimate the molecular weight of the surface-grafted polymer. A linear relationshipbetween the film thickness and Mn of the free polymer was observed, and thePDI of the free polymer was approximately 1.2–1.3. The surface-grafted PVBC wasfurther functionalized to give the Si-g-viologen surface with redox-responsive prop-erties. Chen et al. [67] used a similar strategy to graft polymer brushes of PMMAand poly[poly(ethylene glycol) monomethacrylate] (PPEGMA) from poly(vinylidenefluoride) (PVDF) surfaces. The azo-initiator coverage on the PVDF surface was de-termined by reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH) and estimated tobe approximately 0.68 units nm#2. The molecular weight of the grafted PMMAbrushes was calculated from the thickness of polymer brushes and the estimated

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initiator coverage, which was comparable to the molecular weight of free polymerformed in solution. Compared to the native PVDF film, a more hydrophilic surfacewas generated after surface grafting of PEGMA and PMMA.

Bae et al. [68] applied microwave plasma to modify the surface of poly-(dimethylsiloxane) (PDMS) substrates with maleic anhydride. The introduced car-boxylic groups were used to immobilize the azo initiators by condensation reactions,followed by surface-initiated RAFT polymerizations of N,N-dimethylacrylamide(DMA), styrenesulfonate (SS), and (ar-vinylbenzyl) trimethylammonium chloride(VBTC) on PDMS surfaces. Subsequently, a layer-by-layer process of alternatelydepositing RAFT-prepared PSS and PVBTAC homopolymers on the surface ofPVBTAC-grafted PDMS was applied to create stable and highly hydrophilic sur-faces on the PDMS substrates.

Xu et al. [43] utilized an interesting strategy to micropattern spatially well-definedbinary polymer brushes on the Si(100) surface via a combination of surface-initiatedatom-transfer radical polymerization (ATRP) and RAFT (Scheme 11.7). The ATRPinitiator was first immobilized on the Si(100) surface via UV-induced hydrosilyla-tion through a photomask, which was used to prepare sodium 4-styrenesulfonate(NaSS) polymer (PNaSS) brushes. The azo initiator for RAFT polymerizationwas immobilized to the unhydrosilylated SiO2 domains using a silane couplingagent, which was then used to prepare poly(2-hydroxyethyl methacrylate) (PHEMA)

Scheme 11.7 Schematic diagram illustrating the process ofnonlithographic micropatterning of a silicon surface by a combination ofsurface-initiated ATRP and RAFT. (Reproduced with permission from[43]. Copyright 2006 Royal Society of Chemistry.)

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11.2 Approach 431

brushes. The thicknesses of PNaSS and PHEMA brushes, determined by AFM,were estimated to be 21.3 and 25 nm, respectively.

Rather than using a surface-anchored azo initiator, Pirri et al. [50] prepared poly-mer brushes of N,N-dimethylacrylamide (DMA), glycidyl methacrylate (GMA) andpoly(DMA-b-GMA) from the surface of glass slides and silica beads by depositinga thiol-bearing organosilane. With a radical initiator in solution, S• radicals wereformed on the surface by radical exchange, which were able to initiate the RAFTpolymerization of monomers in the presence of a free CTA. Surfaces modifiedby poly(DMA-b-GMA) brushes bearing oxirane groups showed superior perfor-mance in oligonucleotide hybridization experiments compared to those coated withnonpolymeric self-assembled monolayers containing the same functional group.Barner et al. [15, 36] grafted PS from cross-linked poly(divinylbenzene) (PDVB) coremicrospheres by thermally induced RAFT polymerization. The core microsphereswere prepared by precipitation polymerization. The residual double bonds locatedat the surface of core microspheres facilitated the growth of the polymer chainsfrom the surface by radical capture of oligomers and monomers. 1-Phenylethyldithiobenzoate was used as a free CTA in solution. A rapid increase in the aver-age particle volume was found during the early stages of polymerization, whichwas attributed to polymer chains that grow from both the surface and the outerlayer of the microspheres. After this initial stage the particle volume increase wasslower and linear with reaction time. The PDIs of the free polymer in solution werebelow 1.2 for all reaction times. Using the same approach, Joso et al. [21] graftedpoly(n-butyl acrylate) (PBuA) and poly(N,N-dimethyl acrylamide) from PDVB coremicrospheres. Cumyl dithiobenzoate was used as the RAFT agent.

In addition to chemical deposition, high-energy radiation and plasma are alsoconvenient and powerful tools to generate initiating sites on the surface of sub-strates. Barner et al. [62] applied !-initiated RAFT polymerization to graft PS froma polypropylene solid phase at ambient temperature. Since initiating radicals can begenerated both on the polypropene (PP) surface and in the PS chains by ! radiation,two distinct grafting regimes were observed. In the first regime, polymer chainsgrew in a grafting layer, in which the surface was not completely covered by poly-mer chains. In the second regime, the surface was completely covered with polymerchains, and new polymer chains grew from radicals generated in the already-graftedpolymer chains. It was believed that the growing surface-grafted polymer chainsare in a dynamic equilibrium with free polymer chains in the solution. The freepolymers in the solution were analyzed to estimate the chemistry of surface-graftedpolymer chains. The results showed that Mn of the free polymers increased linearlywith conversion, and the PDI remained below 1.2 throughout the entire polymer-ization. In a later report, Barner et al. [63] expanded this approach by graftinga comonomer system of styrene (St) and m-isopropenyl-",""-dimethylbenzyl iso-cyanate (TMI) from a PP solid phase. Two different grafting regimes were alsoobserved.

By utilizing an O2-plasma treatment, Yoshikawa et al. [65] were able to pre-pare high-density PHEMA brushes on the surface of poly(tetrafluoroethylene-co-hexafluoropropylane) (FEP) films (Scheme 11.8). Peroxides were first introduced

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O2 plasma

FEP

FEP

(a) In good solvent for FEP (b) In nonsolvent for FEP

Nonswelling

RAFT polymerizationwith

Swelling

S

CH3

X

CH3

C S

nf f

Scheme 11.8 Schematic illustration of the graft polymerization on thepoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) film in (a) goodsolvent and (b) nonsolvent for FEP. (Reproduced with permission from[65]. Copyright 2005 American Chemical Society.)

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11.2 Approach 433

onto the surface as initiating moieties by the O2-plasma treatment. The density ofperoxides on the surface was determined by a radical-scavenging method, whichincreased with increasing plasma-treatment time and reached a constant value ofabout 10 peroxides nm#2. Surface-initiated RAFT polymerization of HEMA fromthe plasma-treated FEP film was conducted in a nonsolvent for FEP at 40 $C toavoid the growth of polymer chains from deep within the swollen FEP film surface.N,N-dimethylaniline was added to accelerate the decomposition of the peroxidesvia a redox process. V-70 was used as a free initiator in solution. The graft den-sity was estimated to be about 0.3 chains nm#2. The results of the contact angleand electron spectroscopy for chemical analysis measurements indicated that thePHEMA chains were densely grafted to a considerably thin (%10 nm) boundarylayer. To intentionally swell the FEP film surface, a graft polymerization was alsoattempted at 80 $C, which resulted in the growth of polymer chains from the innerpart of the swollen film as well as the surface, giving high-density polymer brusheswith ill-defined structure.

Yu et al. [66] reported the synthesis of comb copolymer brushes from plasma-treated poly(tetrafluoroethylene) (PTFE) films via a combination of surface-initiatedRAFT approach with ATRP. The PTFE film was subjected to 90 s of radio frequencyAr plasma pretreatment to introduce peroxides on the surface with a density ofabout 0.3 units nm#2. Poly(glycidyl methacrylate) (PGMA) brushes were first syn-thesized by surface-initiated RAFT polymerization from the immobilized peroxides.The ATRP initiators were then introduced by reacting 2-bromo-2-methylpropionicacid with the epoxy groups in the PGMA side chains. The subsequent surface-initiated ATRP of hydrophilic monomers, including poly(ethylene glycol) methylether methacrylate and sodium 4-styrenesulfonate, produced comb copolymerbrushes on the surface of PTFE films. In a recent report by Wang et al. [27],plasma irradiation was also used to introduce peroxides on the surface of Fe3O4

magnetic nanoparticles (MNP). The subsequent surface-initiated RAFT polymer-ization of St and AA on the plasma-treated MNP produced core-shell Fe3O4-g-PSand Fe3O4-g-AAc nanoparticles. These surface-modified nanoparticles showed ex-cellent dispersibility and stability in organic solvents.

Generally, in addition to the initiator immobilized on the surface of substrates, afree initiator is also added in the above investigations. Because of the low concen-tration of initiating sites on the surface, which can be terminated by trace amountsof impurities present in the reaction mixture, it has been found that the addedfree initiator can act as a scavenger for the impurities to facilitate the growth ofthe grafted polymer chains [38]. The addition of free initiator can also result in theformation of ungrafted polymer in solution. Researchers have shown that the Mn

and PDI of the grafted polymer chains agreed closely with those of the ungraftedpolymer chains [38]. In these cases, the characterization of the ungrafted polymerprovides a convenient method to estimate the properties of the grafted polymer,especially those polymer chains that are difficult to separate from the surface ofsubstrates. However, the disadvantage, which is also derived from the presence ofthe large amount of ungrafted polymer in the final product, is the requirementof additional isolation and purification procedures after polymerization.

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Pn M

Pn S

S

Z

M

S

S Pn

Z

Z-group approach

R-group approach

Reversible activation

Reversible activation

Pn

Scheme 11.9 Comparison of R-group and Z-group approaches forsurface-initiated RAFT polymerizations.

11.2.2.2 Grafting-From Surface-Anchored CTAsAn alternative way to modify the surface of materials via surface-initiated RAFTpolymerization is grafting-from surface-anchored CTAs, which generally can beaccomplished through either the R-group or Z-group approach (Scheme 11.9). Inthe R-group approach, the RAFT agent is attached to the substrate surface via itsleaving and reinitiating R group. The solid substrate acts as part of the leaving Rgroup, and thus the propagating radicals are located on the terminal end of thesurface-grafted polymer, which facilitates the growth of grafted polymer chains.This approach resembles a ‘grafting-from’ approach. In the Z-group approach, theRAFT agent is attached to the surface via its stabilizing Z group. Because the RAFTagent is permanently attached to the surface, this approach resembles a ‘grafting-to’approach. The polymeric radicals always propagate in solution before they attach tothe surface of substrate via the chain-transfer reactions with attached RAFT agents.

11.2.2.2.1 R-group Approach Tsujii et al. [13] reported the first application ofsurface-initiated RAFT polymerization in the modification of silica particles viaan R-group approach. An ATRP macroinitiator was first prepared on the surface ofsilica particles, which was subsequently converted to a terminal RAFT moiety byreacting with 1-phenylethyl dithiobenzoate in the presence of CuBr via an atom-transfer addition (ATA) reaction. The conversion of this reaction was estimated tobe 70% by UV–vis absorption spectroscopy. The surface-initiated RAFT polymeriza-tion of St from the immobilized RAFT moiety was carried out at 110 $C with a freeRAFT agent in solution. The addition of the free RAFT agent in solution could notonly control the free polymerization in the bulk phase but also keep the graft poly-merization under control at high conversions. After polymerization, the grafted PSchains were cleaved from silica particles by treating with HF and analyzed by GPC.The results revealed that the grafted radicals predominantly undergo bimoleculartermination at an unusually high rate. The enhanced recombination was attributedto the fast migration of radicals on the surface by sequential chain-transfer reactions,

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11.2 Approach 435

Scheme 11.10 Comparison of the key processes in (a) the ATRP- orNMP-mediated and (b) RAFT-mediated graft polymerizations.(Reproduced with permission from [13]. Copyright 2001 AmericanChemical Society.)

which is not observed in an ATRP system (Scheme 11.10). It was also observedthat the surface graft density had a critical value of about 0.08 chains nm#2 be-low which the surface migration of radicals hardly occurred. Rowe-Konopacki andBoyes [30] used a similar strategy to prepare a series of diblock copolymer brushes,including PMMA-b-PDMAEMA, PMMA-b-PS and PS-b-PMMA on the surface offlat silicon substrates. A modified ATA reaction was applied to convert a surface-immobilized ATRP initiator to a RAFT agent. The addition of Cu(0) was criticalto achieve effective conversion of the ATA reaction. Diblock copolymer brusheswere then synthesized from the surface-immobilized RAFT agent via sequentialsurface-initiated RAFT polymerization. The addition of free CTA in solution wasalso required to control the growth of polymer brushes. Due to the low concen-tration of polymer on the surface the grafted polymer was unable to be degraftedfor direct characterization. The free polymer in solution was isolated and analyzedby GPC to estimate the properties of the grafted polymer, which showed a narrowpolydispersity and predictable molecular weight.

Although well-defined polymer brushes were successfully prepared on the sur-face of silicate substrates in the above studies, large amounts of free polymerwere also produced in the final products due to the free RAFT agents requiredin the polymerization, which required laborious purification steps. To overcomethis problem, Li and Benicewicz [22] used a different strategy to synthesize poly-mer brushes on the surface of silica nanoparticles. A RAFT-silane agent wasfirst prepared in three steps, which was then reacted with the surface of silica

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Br

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Karstedt's catalyst

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Scheme 11.11 Synthesis procedures for attaching RAFT agent ontosilica nanoparticles. (Reproduced with permission from [22]. Copyright2005 American Chemical Society.)

nanoparticles, providing a surface-immobilized RAFT agent (Scheme 11.11). Theamount of RAFT agent on the modified silica nanoparticles was determined quan-titatively by UV–vis spectroscopy. By varying the silane concentration utilized fordeposition, various graft densities of RAFT agent on silica nanoparticles, rangingfrom 0.15 to 0.68 units nm#2 were prepared. Using these surface-immobilizedRAFT agents, homopolymer and block copolymer brushes of PS and PBuA wereprepared on the silica nanoparticle surfaces via surface-initiated RAFT polymer-ization without the addition of free RAFT agents in solution. A low AIBN:CTAratio (<0.1) was used to minimize radical recombination and the amount of freepolymer derived from the radicals formed by AIBN decomposition. The polymeriza-tions were conducted at low conversion range (<20%) to avoid possible gelation orinterparticle polymeric radical coupling. After polymerization, the grafted polymerchains were cleaved from the silica particles by treating with HF and characterized.The results showed that the grafted polymer chains had narrow polydispersitiesand predictable molecular weights, indicating that the surface-immobilized RAFTagents participated in the polymerization with a high activity. Polymerization re-tardation was observed for the surface-initiated RAFT polymerization of both PSand PBuA, which was ascribed to the localized high RAFT-agent concentration.Preliminary work with PS-modified fumed silica prepared at 16.6% monomer con-version using the same approach showed that the fraction of ungrafted polymerestimated by TGA was only about 9%. However, due to the lack of tertiary R-groupstructure, the surface-immobilized RAFT agent used in this work could not beused in controlling the polymerization of methacrylate monomers. Hence, in asubsequent investigation by Li et al. [25], a more versatile RAFT agent containinga 4-cyanopentanoic acid dithiobenzoate (CPDB) moiety was immobilized on thesurface of silica nanoparticles and used to prepare both PS- and PMMA-grafted sil-ica nanoparticles (Scheme 11.12). Amino-group-functionalized silica nanoparticleswere first prepared by reacting 3-aminopropyldimethylethoxysilane with silica parti-cles. An initial attempt of directly reacting CPDB with amino-group-functionalizedsilica nanoparticles via condensation failed due to the aminolysis of the dithioben-zoate group of CPDB. Therefore, the carboxyl group of CPDB was first activatedby reacting with 2-mercaptothiazoline. Due to the ability of mercaptothiazoline-activated amide bond to selectively consume the amino groups in the pres-ence of dithiobenzoate groups, the subsequent reaction of activated CPDB with

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11.2 Approach 437

SHN

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Scheme 11.12 Synthesis procedures for anchoring CPDB moieties ontosilica nanoparticles. (Reproduced with permission from [25]. Copyright2006 American Chemical Society.)

amino-group-functionalized silica nanoparticles successfully produced CPDB-anchored silica nanoparticles with variable graft density. Surface-initiated RAFTpolymerizations of methyl methacrylate (MMA) and styrene (St) were mediated byCPDB-anchored silica nanoparticles without the addition of free CTA in solution,producing surface-grafted polymers with narrow polydispersities and predictablemolecular weights. It was found that the rate of surface-initiated RAFT polymer-ization of MMA from the CPDB-anchored silica nanoparticles was much higherthan that of the MMA polymerization mediated by free CPDB, and polydispersitiesof the PMMA cleaved from the silica nanoparticle surfaces were much narrowerthan those of the PMMA prepared using free CPDB as the RAFT agent, which wereattributed to the unique structure and steric environment of the surface-anchoredintermediate macro-RAFT-agent radical and also the localized high RAFT-agentconcentration effect. Polymerizations mediated by a hybrid CPDB system, consist-ing of both free RAFT agent and surface-anchored RAFT agent, showed that thefree polymer had a higher initial molecular weight than the grafted polymer thatconverged at high conversions. HPLC equipped with a C18-coated silica columnwas used to isolate and quantitatively characterize the ungrafted PMMA polymer.The results showed that the amount of ungrafted polymer was generally very low,only around 5 wt % of the total polymer prepared up to 16% conversion and lessthan 15 wt % of the total polymer prepared at 22% monomer conversion.

Zhang et al. [46] grafted PS chains from the surface of layered silicatesby RAFT polymerization. A RAFT agent, 10-carboxylic acid-10-dithiobenzoate-

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decyltrimethylammonium bromide, was first synthesized and intercalated intomontmorillonite (MMT) via electrostatic attraction. The resulting CDDA-intercalated MMT was used to mediate the RAFT polymerization of St at 110 $Cwith AIBN in solution. GPC results revealed that the grafted PS chains had predi-cable molecular weights and narrow polydispersities. Also, the molecular weightsand polydispersities of the free PS chains in solution were similar to those of thegrafted PS chains, indicating that the intercalated MMT did not restrict the diffusionof propagating radicals and dormant chains. The obtained PS/MMT nanocom-posites had an exfoliated structure and higher thermal stability compared toneat PS.

Many other substrate surfaces in addition to silicate substrates can be modifiedusing surface-initiated RAFT polymerization by attaching the appropriate CTAthrough the R group. Skaff and Emrick [20] grafted a series of homopolymersand copolymers from the surface of CdSe nanoparticles via surface-initiated RAFTpolymerization. A phosphine oxide ligand containing a trithiocarbonate moietywas first prepared, which was then anchored to a conventional tri-n-octylphosphineoxide (TOPO)-covered CdSe nanoparticle through ligand-exchange chemistry. Graftpolymerizations of various monomers from the CTA-functionalized nanoparticleswere achieved at 70 $C. It was found that common free-radical initiators includingAIBN and benzoyl peroxide could induce the degradation of nanoparticles quicklyat 70 $C, which could be attributed to the susceptibility of CdSe nanoparticlesto free-radical degradation. Therefore, di-tert-butylperoxide was selected as a free-radical initiator, which has lower radical yield. The number-average molecularweight of the grafted polymer ranged from 9000 to 49 000 g·mol#1 and PDIs weregenerally below 1.3. The unique optical properties of the CdSe nanoparticles werewell maintained after graft polymerization. TEM analysis of a composite thin filmcast from the PS-grafted CdSe nanoparticles revealed that the CdSe nanoparticleswere uniformly dispersed throughout the matrix.

Hu et al. [14] grafted linear thermally sensitive PNIPAM chains onto a sphericalPNIPAM/hydroxyethyl acrylate (HEA) copolymer microgel. The hydroxyl groupbearing NIPAM/HEA microgel was first prepared by dispersion polymerization.Then, "-butyl acid dithiobenzoate was immobilized on the surface of NIPAM/HEAmicrogel by esterification. The subsequent RAFT polymerization of PNIPAM fromthe CTA-immobilized microgel was conducted with AIBN in solution, resulting in acore-shell nanostructure. It was observed that the thickness of the grafted PNIPAMlayer first decreased in the low-temperature range 25–32 $C and then increased inthe high-temperature range 32–35 $C, which was related to a coil–globule–brushtransition of linear grafted PNIPAM chains. Using a similar strategy, Raulaet al. [16] synthesized gold nanoparticles grafted with PNIPAM by surface-initiatedRAFT polymerization. CPDB was attached to gold nanoparticles by reacting withthe 11-mercapto-1-undecanol ligands on the surface. The resulting CPDB-anchorednanoparticles were used to mediate the RAFT polymerization of NIPAM. Thegrafted PNIPAM was removed from the particle surfaces by treating with I2 inCH2Cl2/ethanol. The molar mass and polydispersity of the grafted PNIPAM chainsdetermined by GPC were 21 000 and 1.17, respectively. The optical properties of

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11.2 Approach 439

Scheme 11.13 Synthesis of cellulose CTA for reversibleaddition–fragmentation chain transfer polymerization and their use tomediate St polymerization. (Reproduced with permission from [59].Copyright 2005 American Chemical Society.)

these PNIPAM-modified gold nanoparticles varied with changes in environmentaltemperature and particle concentration.

Roy et al. [59, 69] utilized surface-initiated RAFT polymerization to graft PSfrom a cellulose substrate (Scheme 11.13). The hydroxyl groups of the cellulosewere first treated with 2-chloro-2-phenylacetyl chloride and then converted to athiocarbonylthio RAFT agent using a Grignard reagent. Based on the results ofelemental analysis, the average loading of RAFT agent on the cellulose substrateswas calculated to be 1.9 mmol·g#1. From this cellulose-bound RAFT agent, St waspolymerized in the presence of AIBN in solution. The graft ratio ranged from 11to 28 wt %, depending on different polymerization conditions. The grafted PSwas cleaved form cellulose backbone by treating with HCl solution. The cleavedPS chains from a sample with 28 wt % graft ratio were analyzed by size-exclusionchromatography, which gave values of Mn = 21 000 g·mol#1 and PDI = 1.1. Contact-angle measurements revealed a dramatic increase in hydrophobicity of the PS-modified cellulose surface compared to untreated cellulose surface. Using the same

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strategy, they also grafted poly[2-(dimetylaminoethyl) methacrylate] from cellulosefiber. They found that the addition of a free RAFT agent in solution could increasethe graft ratio. The maximum graft ratio was obtained when the molar ratio of freeRAFT agent to cellulose-bound RAFT agent was 1.5:1.0 [58].

Using a surface-immobilized RAFT agent, Cui et al. [51] were able to graft PS fromthe surface of multiwalled carbon nanotubes (MWNTs). Carboxylic acid groupswere first introduced to the surface of MWNTs by treating with nitric acid. The car-boxylic acid group functionalized MWNTs were then treated with thionyl chlorideand 2-hydroxyethyl-2"-bromoisobutyrate to form bromoisobutyrate-functionalizedMWNTs, which were further reacted with a Grignard reagent to produce a surface-immobilized RAFT agent attached to the MWNT substrate via its R group (Scheme11.14). Polymerization of St was conducted in THF at 100 $C, using AIBN as a free-radical initiator. In later investigations, the same strategy was used to modify thesurface of MWNTs with PMMA-b-PS block copolymer [56] and a series of aqueoussoluble polymers [52, 53, 55, 57]. In a graft polymerization of PNIPAM mediated bythe RAFT-agent-functionalized MWNTs [52], the molecular weight of the graftedPNIPAM increased linearly with monomer conversion and the molecular-weight

Scheme 11.14 Synthesis of RAFT-agent-functionalized MWNTs.(Reproduced with permission from [51]. Copyright 2004 Elsevier.)

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11.2 Approach 441

distribution was around 1.3. On the basis of this strategy, Hong et al. [54] alsoreported a new method to graft functional polymers and small molecules ontoMWNTs, without significantly altering their surface structure. MWNTs were firstslightly functionalized with RAFT agents to maximally maintain the surface struc-ture. The graft density of RAFT agents was estimated to be approximately 1.5 RAFTagents per 1000 carbon atoms. From these RAFT-agent-immobilized MWNTs, analternating copolymer of poly(styrene-alt-maleic anhydride) was grafted. The in-troduced highly reactive maleic anhydride groups could further react with varioushydroxyl or amino groups that were contained in functional polymers and smallmolecules to achieve the functionalization of MWNTs.

11.2.2.2.2 Z-group Approach In comparison with the substantial application ofthe R-group approach to the surface modification of various substrates, relativelylittle attention has been paid to the Z-group approach. In the Z-group approach,the RAFT agent is located close to the surface throughout the polymerization andchain-transfer reactions between propagating polymer radicals, and attached RAFTagents must occur near the surface of the substrates. Thus, these reactions canbe severely hampered due to the steric hindrance of the neighboring attachedpolymer chains, which makes it difficult to prepare high-density grafted polymer.However, since the propagation of the polymer chains occurs only in solutionin the Z-group approach, the polymer chains attached on the surface are alwaysdormant, which excludes the bimolecular termination of grafted radicals oftenobserved in the R-group approach. Recently, a few reports showed that this approachholds a unique advantage in constructing well-defined homopolymers and blockcopolymers grafted on the surface of solid substrates [23, 28, 33].

Perrier et al. [23] utilized a Merrifield-supported RAFT agent s-methoxycarbonylphenymethyl dithiobenzoate (Mer-MCPDB) and silica-supportedMCPDB (Si-MCPDB) to mediate the RAFT polymerization of methyl acrylate (MA).MCPDB was attached to the surface of Merrifield resin via its Z group in two steps.The chlorobenzyl functional groups on the resin were first reacted with sodiummethoxide and elemental sulfur to form a sodium dithiobenzoate salt, which wasthen converted to Mer-MCPDB by treating with methyl-"-bromophenylacetate.The same procedures were used to prepare Si-MCPDB except that a silane wasfirst used to introduce a chlorobenzyl functional group to the surface of the resin.The RAFT polymerizations of MA were conducted at 60 $C in the presence ofAIBN. After polymerization, excess AIBN was added to cleave the grafted poly-mer chains via radical exchange. In a polymerization of MA mediated by a Mer-MCPDB, GPC was used to characterize both the grafted polymethylacrylate (PMA)chains and free PMA chains. The results revealed that the grafted PMA chainshad lower polydispersity compared to the free PMA chains and did not show theGPC hump at high molecular weights due to termination by combination. It wasfound that the use of free RAFT agents in solution could not only help to increasethe control over polymerization but also help to reduce the amount of free poly-mer chains in solution. When a free CTA (MCPDB) was used with a ratio freeCTA:supported CTA = 1:1, the fraction of free polymer chains decreased from

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Scheme 11.15 Synthetic route to polymer-grafted silica particles byZ-supported RAFT polymerization. (Reproduced with permission from[28]. Copyright 2006 American Chemical Society.)

51 to 38% in the Mer-MCPDB-mediated polymerization of MA and from 90 to48% in the Si-MCPDB-mediated polymerization of MA. As the free polymer chainscan be removed by simple filtration and all grafted chains are attached to thesolid substrates via the RAFT agent, this strategy provides a way to modify thesurface of substrates with polymer chains that can be considered ‘truely’ living.Zhao and Perrier [28] further extended this approach to the modification of sil-ica particles with various homopolymers and block copolymers. Silica-supported3-(methoxycarbonylphenylmethylsulfanylthiocarbonysulfanyl)-propionic acid (Si-MPPA) was first prepared by a two-step reaction (Scheme 11.15). The surfacedensity of RAFT agent was estimated to be 0.388 molecule of CTA nm#2. Si-MPPAwas then used to mediate the RAFT polymerization of various monomers withAIBN and free MPPA in solution. The grafted polymer chains were cleaved bytreating with n-hexylamine and analyzed by GPC. In a RAFT polymerization of MAmediated by Si-MPPA, it was found that the molecular weights of free and graftedpolymers were similar at low conversion but differed with increasing conversion.At high conversion (>40%), the molecular weights of free polymers in solutionwere much higher than those of grafted polymers, which was attributed to the in-creased shielding effect of the grafted polymer chains with increasing conversion.Compared to free polymers produced in solution, the grafted polymers had betterdefined structure with a lack of GPC shoulders and obvious tailings correspondingto irreversible termination. A different free RAFT agent (CPDB) was also utilized to

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11.2 Approach 443

mediate the RAFT polymerization, which afforded better defined grafted polymerthan using MPPA due to the slower polymerization rate of CPDB-mediated RAFTpolymerization in solution. The homopolymer-grafted silica particles were alsosubjected to a RAFT chain-extension polymerization of a second monomer, whichproduced well-defined diblock copolymer-grafted silica particles. Using a similarstrategy, Nguyen and Vana [37] attached a cumyl dithiobenzoate to the surface ofsilica particles via its Z group. The resulting silica-supported RAFT agent was usedto mediate the RAFT polymerization of MMA and St.

Stenzel et al. [42] used the Z-group approach to grow temperature-responsiveglycopolymer brushes from flat silicon wafers. 3-Aminopropyl trimethoxysi-lane was first used to introduce amino functional groups to the surface ofsilicon wafers. The resulting amino-functionalized surface was reacted with3-benzylsulfanylthiocarbonyl sulfanylpropanyl chloride to produce a surface-immobilized RAFT agent. The surface-initiated RAFT polymerizations of N-acryloyl glucosamin (AGA) and NIPAM were conducted at 60 $C with a radicalinitiator and free RAFT agent in solution. The RAFT process in solution was mon-itored to estimate the chemistry of the RAFT polymerization on the surface. Themolecular weight of the polymer in solution increased linearly with conversionclose to the theoretical value and the molecular-weight distributions remained nar-row throughout the polymerization. Both the PNIPAM and PAGA layer thicknesseswere found to increase linearly with monomer conversion. The PAGA-grafted sur-face was subjected to a chain-extension polymerization with NIPAM to prepareblock copolymers grafted onto the silicon wafers. They found that the PNIPAMblock grew approximately at the same rate independent of the initial size of theRAFT agent or the size of the PAGA block. This result was contrary to that fromearlier studies using the Z-group approach to synthesize star polymers, in whichthe growth of the second block was found to be very difficult due to the increasingsteric hindrance [70]. A possible explanation for this result may involve a highlyentangled macroradical within the brush layer that cannot diffuse away from thereactive site. Consequently, to allow further brush growth, monomer must diffuseinto the brush, which is not severely affected by the increasing steric hindrance.

In the above studies, a silane agent was used to introduce functional groups ontothe surface that facilitated the subsequent attachment of the RAFT agent via its Zgroup. Peng et al. [41] explored a slightly different approach to immobilize the RAFTagent on the surface of silicon wafers (Scheme 11.16). A UV-induced hydrosilylationwas used to immobilize a 4-vinylbenzyl chloride (VBC) monolayer on the Si H

Si HUVVBC

Si (CH2)2Cl (1) S, NaOMe, THF

(2) Methylbromophenylacetate, THFSi (CH2)2

S

S

Ph

OMeO

Si-MCPDB surfaceSi-H surface Si-VBC surface

Scheme 11.16 Synthesis of surface-immobilized RAFT agents via Si Cbonds. (Reproduced with permission from [41]. Copyright 2006American Chemical Society.)

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surface via robust Si C linkages. The resulting Si VBC surface was coupled witha RAFT agent in a two-step reaction. The RAFT polymerizations of MMA andHEMA were conducted at 70 $C with AIBN and a free RAFT agent in solution.An approximately linear increase in thickness of the grafted PMMA and PHEMAbrushes on the surfaces with polymerization time was observed. The free polymersrecovered from the reaction solution showed narrow polydispersities. The PHEMAand PMMA brushes were also subjected to a chain-extension polymerization of(2-dimethyl-amino)ethyl methylacrylate. The formation of a diblock copolymer-grafted surface was confirmed by X-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectrometry.

11.3Properties

11.3.1Surface Structure and Properties

The RAFT technique provides a versatile way to modify the surface of different ma-terials with surface-attached polymers. By controlling the type, structure and graftdensity of the surface-attached polymers, surface hydrophophicity/hydrophilicityof substrate materials can be significantly modified, which further affects otherproperties including adhesion, wettability, compatibility and solubility [71, 72].AU: Is ‘hy-

drophophicity’OK as given? Or,should it bechanged to‘hydrophobicity’?

Due to the great promise of stimuli-responsive materials in many fields includ-ing nan1otechnology, biochemistry and materials science, substantial attentionhas been given to the modification of material surfaces with stimuli-responsivepolymers via the RAFT process. Typically, the surface-attached stimuli-responsivepolymers can be mixed polymers, block copolymers or functional homopolymers(especially PNIPAM). Under external stimuli such as pH, temperature or solvency,the properties of these polymers can be affected through either structure rearrange-ment or conformational change, which is very useful for controlling the surfaceproperties of materials.

Baum and Brittain [38] prepared diblock copolymer brushes attached on silicatesubstrates using RAFT polymerizations. The rearrangement behavior of diblockcopolymer brushes upon treatment with different solvents was observed via ten-siometry. Upon treatment with methylcyclohexane at 35 $C, the advancing watercontact angle of a PS-b-PDMA brush (where the PS block is adjacent to the silicatesurface) increased from 42 to 65$. Treatment of the same sample with tetrahydro-furan (THF)/ H2O (1/1, v/v) at 35 $C reversed the contact angle back to the originalvalue. For the PDMA-b-PMMA brush, the advancing water contact angle decreasedfrom 66 to 58$ after the sample was treated with THF/H2O (1/1, v/v) at 35 $C andreturned to the original contact-angle value after treatment with dichloromethane.These results were reproducible over several cycles of solvent treatment.

Sumerlin et al. [47] modified gold surfaces with a PMAEDAPS-b-DMA copolymervia a grafting-to approach where the DMA block is adjacent to the gold surface.

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11.3 Properties 445

Water contact-angle measurement results showed that the contact angle for thePMAEDAPS-b-DMA sample was 29.1$, which was nearly identical to that of thePDMA-modified gold (30.5$). This result was interesting because the outer block isexpected to be PMAEDAPS. To gain further insight, the contact angle was measuredfor a gold film modified with MAEDAPS homopolymer, which was determined tobe 41.5$, indicating the surface was more hydrophobic than the gold modifiedwith the block copolymer. By combining these two results, it was concluded thatthe relatively hydrophobic PMAEDAPS block causes the block copolymer to adopta conformation such that the more hydrophilic PDMA block is exposed to theaqueous environment. Rearrangement of the blocks most likely occurred when thesample was treated with deionized water during the rinsing step that immediatelyfollowed the immobilization procedure.

Yoshikawa et al. [65] prepared a high-density PHEMA brush on the surface ofFEP film by surface-initiated RAFT polymerization. The contact angle # on the filmsurface was measured both in water with an air bubble and in air with a waterdroplet (Fig. 11.1). In the former measurement, it was found that all the PHEMA-grafted samples gave a # of 26$, which was equal to the value for the pure PHEMAsurface, indicating the uppermost surface was totally coved with PHEMA. In thelater measurement, when L > 20 nm (L is defined as the layer thickness), samplesshowed the same # value as the pure PHEMA film. For samples with L < 20 nm, #

increased with the decrease of L, suggesting the surface was not totally covered witha pure PHEMA layer. This was ascribed to the arrangement on structure of FEP andPHEMA chains at the surface. In air (hydrophobic environment), a rearrangementoccurred to allow the more hydrophobic FEP chains to move to the outer surface.When L & 20 nm, this surface rearrangement was completely suppressed.

PNIPAM is one of the most studied stimuli-responsive polymers and exhibitsa lower critical solution temperature (LCST) in water at approximately 32 $C. Atthe LCST, PNIPAM undergoes a volume-phase transition, causing a change froma hydrophilic to a hydrophobic state, which results in dramatic changes in phys-ical properties [73]. Hu et al. [14] grafted linear PNIPAM chains onto a sphericalPNIPAM/HEA copolymer microgel. In the low-temperature range 25–32 $C, theyfound that the grafted PNIPAM layer thickness decreased, which was related tothe coil–globule transitions of linear grafted PNIPAM chains. While in the high-temperature range 32–35 $C, the layer thickness increased linearly with the graftingdensity. This was attributed to a strong steric repulsion among the chains graftedon the shrunken MG core, which forced the tethered PNIPAM chains to stretch intoa brushlike conformation on the surface (Fig. 11.2). Raula et al. [16] polymerizedNIPAM from the surface of gold nanoparticles using the RAFT technique. An aque-ous PNIPAM-attached gold nanoparticles solution was gradually heated above theLCST of PNIPAM. They found that both the absorption maximum of the surfaceplasmon (!max) and the absorbance intensity (Iabs) at 650 nm decreased stronglyduring the collapse of PNIPAM around 34 $C (Fig. 11.3). The shift of the !max tolower wavelengths indicated that the surroundings of the surface of the gold corebecame less hydrophilic, as the surface was covered by the collapsed PNIPAM. Thedecrease in the surface plasmon band was due to the shielding of the gold surface

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(a)

(b)

L (nm)

!! (d

egre

e)!

(deg

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Fig. 11.1 Plot of ! versus L (a) in water withair bubbles and (b) in air with water droplets:(!) pristine FEP film, ($) 120-s plasma-treated FEP film, (") PHEMA-spin-cast FEPfilm and PHEMA-grafted FEP film withplasma-treatment time of (") 60 s, (#) 80 s

and (•) 120 s. The broken lines show thecontact angles of the pure PHEMA surface.(Reproduced with permission from [65].Copyright 2005 American ChemicalSociety.)

by collapsed PNIPAM, which was observed to be reversible as the temperature wasdecreased back to 20 $C.

11.3.2Interfacial Properties in Polymer Nanocomposites

Nanoparticles have been used extensively as fillers in polymer nanocompositesto improve the mechanical, thermal, electric and optical properties. The largesurface area of nanoparticles has the ability to affect a large volume fractionof the matrix polymer. Therefore, interfacial interactions between nanoparticlesand the matrix polymer are especially important in determining the properties of

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11.3 Properties 447

Fig. 11.2 Schematic of the coil–globule–brush transition of linearPNIPAM chains grafted on a thermally sensitive microgel. (Reproducedwith permission from [14]. Copyright 2002 American Chemical Society.)

polymer nanocomposites. The RAFT technique provides a versatile tool to tailor thesurface of nanoparticles and thus enables researchers to design interfaces betweennanoparticles and the matrix polymers with several levels of control over chemistry,chain length, chain density and layer thickness. Li and coworkers [22, 25] developeda RAFT polymerization method capable of growing PS with molecular weights (M)of up to 150 000 g·mol#1, polydispersity of less than 1.2 and graft densities rangingfrom 0.05 to 0.8 chains nm#2 on the surfaces of silica nanoparticles. Using thesenanoparticles with controlled interfaces as nanofillers in polymer nanocomposites,Bansal et al. [74, 75] studied the relationship between local interface behavior andthermomechanical properties of polymer nanocomposites. To study the wettingof SiO2-g-PS surfaces by PS matrices with various molecular weights, films ofSiO2-g-PS nanoparticles were spin-cast onto Si wafers, which were previously

T (°C)

! max

(nm

) I650 (a.u.)

Fig. 11.3 Changes in the !max of the surface plasmon and the Iabs at650 nm of the aqueous MPC-PNIPAM solution as a function oftemperature. (Reproduced with permission from [16]. Copyright 2003American Chemical Society.)

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PS thin film SiO2-g-PS layer

Fig. 11.4 Optical micrographs of the surface of PS films atop a layer ofSiO2-g-PS nanoparticles. The matrix molecular weight is varied from(a) 44 200 g·mol#1, where complete wetting is seen, to (b) 92 000g·mol#1, where partial dewetting is seen and finally to (c) 252 000g·mol#1, where the PS film completely dewets and forms islands [74].

etched with an oxygen plasma for 15 min to remove organic impurities. A layerof narrow-polydispersity ‘free’ PS was then placed on top of this layer. Figure 11.4shows optical micrographs of PS films of various molecular weights sitting atop afilm of SiO2-g-PS nanoparticles (M = 110 000 g·mol#1, the grafting density (# ) wasabout 0.27 chains nm#2). It can be seen that the PS films with a lower molecularweight (M = 42 000 g·mol#1 had a homogeneous surface that indicated that the PSfilm wets the surface of the SiO2-g-PS nanoparticles. As the M of PS film increasedto 92 000 g·mol#1, partial dewetting was observed. Complete dewetting of the PSfilms was apparent when the M of the PS films increased to 250 000 g·mol#1,evidenced by the many small PS islands on the SiO2-g-PS surface. These resultssuggested that the wetting behavior of nanoparticles can be controlled by varyingthe ratio of the M of free polymer to that of the grafted polymer on the surface ofnanoparticles. To further evaluate the effect of polymer-nanoparticle wetting behav-ior on the thermomethanical properties of polymer nanocomposites, they studiedthe glass transition temperature (Tg) of SiO2-g-PS/PS nanocomposites. As shown inFig. 11.5, the crossover from wetting to autophobicity was observed whenMmatrix/Mgraft ' 0.7. For Mmatrix/Mgraft < 0.7, the polymer matrix wetted thenanoparticles and the Tg of the nanocomposites increased. For Mmatrix/Mgraft >

0.7, dewetting was observed and the Tg decreased with increasing SiO2 concen-tration. A possible explanation for these phenomena was that the low M matrix,which strongly wets the particles, interdigitates with the brush, creating a stronginterface. The brush chains extend further and thus, the grafted PS not only losesconformational entropy but also occupies a greater volume fraction of the nanocom-posite. Both of these effects lead to reduced mobility of the polymer chains in thenanocomposite and could explain the observed increase in Tg. For the high M case,where dewetting occurs, the interface acts akin to a free polymer surface therebyresulting in a reduced Tg.

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11.4 Conclusions 449

0 1 2 3 4 5

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11.4Conclusions

The employment of the RAFT process to modify the surface of materials has at-tracted increasing interest in recent years. The versatility of the RAFT techniqueprovides the ability to modify the material surfaces via various grafting approaches.By varying the combinations of grafting approach with substrate, surface modifica-tion can be achieved by grafting a wide range of polymer chains with considerablecontrol over chain structure, chain length and grafting density.

The increasing demand for materials with novel surface properties will continu-ously direct researchers to design sophisticated polymer structures on the surfaceof materials, which requires a further understanding of the polymerization mech-anism and kinetics. Although the kinetics of RAFT polymerization in solution hasbeen extensively studied, it has been found that the kinetics of RAFT polymerizationon the surface of solid substrates can be quite different from that in solution, whichwas ascribed in early studies to the unique steric environment of the intermediatemacro-RAFT-agent radical on the substrate surfaces. Further investigations are stillneeded to understand how the experimental factors involved, such as RAFT-agentsurface density and monomer steric structure, affect the polymerization kinetics.Also, while the ‘toolbox’ of synthetic methods has expanded in the last several years,

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improvements in the levels of control (e.g. degree of polymer attached to the surfacevs free polymer) are still needed.

Finally, while substantial attention has been given to the synthetic approachesvia the RAFT process, extensive studies of the properties of the interface andresulting nanocomposite materials are still at an early stage. A clear understandingof the structure and properties will help to develop many potential applications invarious fields and may also, in turn, guide researchers to design novel structuresby selecting the most efficient approaches [76].

11.5Abbreviations

AFM Atomic force microscopyAGA N-acryloyl glucosaminAIBN 2,2"-AzobisisobutyronitrileATRP Atom-transfer radical polymerizationBDTB BenzyldithiobenzoateCPDB 4-Cyanopentanoic acid dithiobenzoateCRP Controlled radical polymerizationsCTA Chain-transfer agentDBTTC Dibenzyl trithiocarbonateDMA N,N-dimethylacrylamideDPPH 2,2-Diphenyl-1-picrylhydrazylFEP Poly(tetrafluoroethylene-co-hexafluoropropylane)GMA Glycidyl methacrylateHEA Hydroxyethyl acrylateHEMA 2-Hydroxyethyl methacrylateLCST Lower critical solution temperatureMA Methyl acrylateMer-MCPDB s-Methoxycarbonylphenymethyl dithiobenzoateMMA Methyl methacrylateMMT MontmorilloniteMNP Magnetic nanoparticlesMWNT Multiwalled carbon nanotubeNaSS Sodium 4-styrenesulfonateNIPAM N-isopropylacrylamideRAFT Reversible addition–fragmentation chain transferPAA Poly(acrylic acid)PAMPS Poly(sodium 2-acrylamido-2-methyl propane sulfonate)PBuA Poly(n-butyl acrylate)PDI Polydispersity indexPDMA Poly(N,N-dimethylacrylamide)PDMAEMA Poly[2-(dimethylamino)ethyl methacrylate]PDMS Poly(dimethylsiloxane)

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PDVB Poly(divinylbenzene)PGMA Poly(glycidyl methacrylate)PHEMA Poly(2-hydroxyethyl methacrylate)PMA PolymethylacrylatePMMA PolymethylmethacrylatePNIPAM Poly(N-isopropylacrylamide)PNIPAM-MPC PNIPAM-monolayer-protected clustersPP PolypropenePPEGMA Poly[poly(ethylene glycol) monomethacrylate]PS PolystyrenePTFE Poly(tetrafluoroethylene)PVBC Poly(4-vinylbenzyl chloride)PVBTC Poly[(ar-vinylbenzyl)trimethylammonium chloride]PVDF Poly(vinylidene fluoride)Si-MCPDB Silica-supported MCPDBSi-MPPA Silica-supported

3-(methoxycarbonylphenylmethylsulfanylthiocarbonysulfanyl)-propionic acid

SS StyrenesulfonateSt StyreneTEM Transmission electron microscopyTHF TetrahydrofuranTOPO Tri-n-octylphosphine oxideVBC 4-Vinylbenzyl chlorideVBTC (ar-Vinylbenzyl)trimethylammonium chloride

11.6Acknowledgment

The authors gratefully acknowledge support through the Nanoscale Science and En-gineering Initiative of the National Science Foundation under NSF Award NumberDMR-0117792.

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