Surface-initiated controlled polymerization as a convenient method for designing functional polymer brushes: From self-assembled monolayers to patterned surfaces

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Progress in Polymer Science 37 (2012) 157– 181

Contents lists available at ScienceDirect

Progress in Polymer Science

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urface-initiated controlled polymerization as a convenient methodor designing functional polymer brushes: From self-assembled

onolayers to patterned surfaces

urore Oliviera,b, Franck Meyera, Jean-Marie Raqueza, Pascal Dammanb,hilippe Duboisa,∗

Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials & Polymers CIRMAP, University of Mons – UMONS,lace du Parc 20, B-7000 Mons, BelgiumLaboratory of Interface and Complex Fluids, Center of Innovation and Research in Materials & Polymers CIRMAP, University of Mons – UMONS,lace du Parc 20, B-7000 Mons, Belgium

r t i c l e i n f o

rticle history:eceived 5 January 2011eceived in revised form 26 April 2011ccepted 26 April 2011vailable online 28 June 2011

eywords:icropatterned surfaces

elf-assembled monolayerolymer brushesacromolecular engineering

a b s t r a c t

Surface-functionalization mediated through “grafting from” methods is of considerableinterest as means to tailor the chemical and physical properties of functional substratesin a reliable way. The resulting polymer brushes, obtained by a “grafting from” strategy,are composed of grafted polymer chains tethered from one of their extremities to a sur-face by a covalent bond. Tuning the molecular parameters of these polymeric brushessuch as the nature of monomer, the grafting density, and the chain length as well asthe design of micropatterned structures enables delicate modification of the propertiesof these substrates, paving the way to the development of functional surfaces. In thisreview, we highlight recent and most important approaches to form monolayers and tosubsequently elaborate homogeneous and heterogeneous coatings of polymer brushes by

urface-initiated polymerizationGrafting from” technique

surface-initiated polymerization. The control of initiator molecule assembly is particu-larly important for the final configuration of polymer brushes. We report the creation ofhomopolymers and block copolymers using major controlled polymerization techniquesas well as lithographic techniques aiming at the design of polymeric (micro- or nano-)patterns.

© 2011 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1582. From monolayers to homogeneous coatings of polymer brushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

2.1. Generalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1582.2. SAMs obtained from solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

2.2.1. Ring-opening polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
2.2.2. Nitroxide-mediated polymerization . . . . . . . . . . . . . .2.2.3. Reversible addition-fragmentation chain transfer2.2.4. Atom transfer radical polymerization . . . . . . . . . . . .
∗ Corresponding author. Tel.: +32 65 37 34 80.E-mail address: [email protected] (P. Dubois).

079-6700/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.progpolymsci.2011.06.002

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

dx.doi.org/10.1016/j.progpolymsci.2011.06.002

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158 A. Olivier et al. / Progress in Polymer Science 37 (2012) 157– 181

2.3. “Mixed” SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652.3.1. Variation of initiator density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1662.3.2. Variation of initiator type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1672.3.3. “Y” molecule assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

2.4. Gradient deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1672.5. SAMs obtained from the gas phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

3. From patterned monolayers to heterogeneous coatings of polymer brushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693.1. Patterned polymer brushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

3.1.1. The printing technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1703.1.2. Direct writing technique using scanning probe microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1713.1.3. Particle beam lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

3.2. Binary-patterned brushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1764. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177. . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction

Controlled surface modification is regarded as a veryappealing (attractive) strategy to tailor the chemical andphysical properties of a substrate such as UV stabil-ity/transparency, biocompatibility, wettability, wear andcorrosion resistance. The coating with specific functionalproperties has thereby arisen as an essential pathwayto engineer, develop and manufacture new technologyproducts in all industrial sectors [1]. In the same way,organic/inorganic hybrid materials have attracted everincreasing attention due to their applications includinganti-corrosion, self-cleaning coating, adhesion, etc. Self-assembled monolayers (SAMs) represent an interestingapproach to tailor the surface chemistry over a wide rangeof metallic substrates. The spontaneous organic assemblyof small molecules on surface offers a convenient, flexi-ble and simple system to modify the intrinsic propertiesof the surface. However, the main drawback of SAMs liesin some mechanical and chemical instability due to thevery limited thickness of the resulting coatings (a few nm)[2]. A relevant technique to overcome these restrictionsconsists of the preparation of thicker organic films suchas polymer brushes. This relatively new class of materialsis characterized by an assembly of polymer chains teth-ered by one of their extremities to a surface by a chemicalbond. The surface functionalization with polymer brushescan be performed according to two strategies, namelythe “grafting to” and “grafting from” techniques. The sec-ond method, also called “surface-initiated polymerization”(SIP), takes advantage of a better control over the typeof grafted polymers, the surface-grafting density and thechain-length. Therefore, in such “grafted from” coatings,a chemical amplification of the monomer from surface-bound polymerization initiators is achieved. The resultingmaterials will be endowed with a large array of character-istic features owing to the versatility of recently developedcontrolled/living polymerization mechanisms applicableto a wide variety of (functionalized) monomers.

This review will focus on the preparation of functional
SAMs on surfaces such as gold or silicon as well as itsimplementation to the development of grafted polymerbrushes. An overview of the most relevant techniquesreported to form homogeneous or (micro and nano) pat-
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

terned SAMs will be first presented. Then, the formation ofpolymer brushes starting from such functionalized SAMswill be described using controlled/living polymerizationtechniques. A special attention will be paid to the differentconfigurations/architectures obtained by (co)polymerizingpurposely selected (co)monomers. Finally, the researchprogress achieved over the past few years involvingsurface-initiated polymerization from initiator precursorsimmobilized on given substrates will be highlighted.

2. From monolayers to homogeneous coatings ofpolymer brushes

2.1. Generalities

Self-assembly, in a general sense, may be defined as thespontaneous formation of hierarchical structures from pre-designed building blocks, involving multiple energy scalesand degrees of freedom [3]. Furthermore, this concept isvalid for many natural processes as the formation of cellmembranes, the folding of polypeptide chains into protein,etc. In the 1980s, it was discovered that organo-sulphuriccompounds spontaneously react with noble metals likegold. After further investigations, the creation of mono-layer with ordered arrangement was achieved by theself-assembly of molecules with a substrate; this method isalso called Self-Assembled Monolayer (SAMs). This innova-tion in the field of surface science has created convenient,flexible, and simple tools to develop applications in manyareas as surface wetting (i.e. hydrophilic/hydrophobic),non-fouling properties, electrochemistry, molecular elec-tronics, etc. [4].

The typical feature of precursors capable of interactingwith the substrate lies in the association of three spe-cific entities: head group, end-group, and backbone (Fig. 1).Each one represents an important factor on the forma-tion of SAMS. Head-group must have an affinity for thesurface (metal, oxide, mica, etc.) like thiols with gold orchlorosilanes with ITO. The backbone is typically made upof alkyl chain (CnH2n), which gives rise to densely packed
monolayers. In the case of electronic applications, thiol-derived SAMs with interesting functionalities within thebackbone such as terthiophenes, azo-groups and tetra-cyanoquinodimethane have accordingly been investigated

A. Olivier et al. / Progress in Polymer

[gorutpwtos(r

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5]. A wide range of functions available in the case of end-roups (CH3, COOH, etc.) has shown significant influencesn the surface properties such as wettability, protein-epelling, etc. Interestingly, this terminal group can be alsosed as initiating sites for surface-initiated polymeriza-ions, i.e. by “grafting from” method. This gives access toolymer brushes with reliable properties by comparisonith SAMs due to much increased thickness. The choice of

he polymerization mechanism is influenced by the naturef the end-group. The initiator can be a free-radical, an ionicite, or can be involved in either controlled ring-openingmetathesis) polymerizations or reversible deactivationadical polymerizations.

A large variety of synthetic routes for the gener-tion of polymer brushes through the “grafting from”rocess has been established upon controlled poly-erizations: cationic and anionic polymerizations [6,7],

ing-opening polymerizations (ROP) [8,9], ring-openingetathesis polymerizations (ROMP) [10,11], atom transfer

adical polymerizations (ATRP) [12,13], nitroxide mediatedolymerizations (NMP) [14,15], and polymerizations viaeversible addition-fragmentation chain transfer (RAFT)16,17].

Due to the large number of reports devoted to theynthesis of polymer brushes from homogeneous mono-ayers, currently obtained by immersion of the substraten initiator solution, we decided to distinguish the stud-es in function of the polymerization mechanisms, i.e.he four major polymerization mechanisms involved inIPs: ring-opening polymerization, nitroxide-mediatedolymerization, reversible addition-fragmentation chainransfer polymerization, and atom transfer radical poly-

erization. Subsequently, different strategies to producemixed” SAMs and their related influence on the configu-ation/conformation of the grown polymer brushes will beiscussed. A paragraph devoted to the gradient deposition

s proposed as well.Finally, after a large description of SAMs obtained from

olution and the subsequent “grafting from” polymeriza-ions, the self-assembly of molecules directly from gashase will be presented.

.2. SAMs obtained from solution

The most common protocol for preparing, e.g., a thiol-ased SAM starts with the immersion of a clean substrate

nto a dilute (∼1–10 mM) solution of the desired moleculeor several hours at room temperature. The growth rate of

Science 37 (2012) 157– 181 159

the monolayer is affected by a large number of experimen-tal parameters such as solvent, temperature, concentrationand immersion time.

This paragraph was limited to the description of self-assembly of monolayers containing exclusively one kind ofmolecule.

2.2.1. Ring-opening polymerizationRing-opening polymerization (ROP) is currently carried

out in the synthesis of high molecular-weight aliphaticpolyesters. ROP of lactones has been investigated over thelast 40 years, due to its versatility in producing a varietyof biomedical polymers in a controlled manner. Dependingon the nature of the initiator and catalyst, ROP proceedsvia either cationic, anionic, or “pseudo” anionic reactionmechanisms. However, cationic [18] and anionic [19] poly-merizations do not allow the preparation of high molecularweight polyesters due to the occurrence of side-reactionssuch as deprotonation, inter- and intra-molecular transes-terification reactions, etc. [20]. Coordination-Insertion ROP,also called “pseudo” anionic ROP is the best way to con-trol the growth of the chains and the molecular weightdistribution. This technique proceeds through the coor-dination of the monomer to the active species, followedby the insertion of the monomer into the metal–oxygenbond [21]. This ROP mechanism uses alcohol as initiatorand most often the degree of polymerization is dependenton the monomer/alcohol ratio, leading to polyester chainsthat are end-capped by hydroxyl group(s). Over the pastfew years, a number of reviews reporting the synthesis ofaliphatic polyesters in a control manner by ROP of lactoneshave been made available [22,23,24].

There are a limited number of studies devoted tosurface-initiated ROP of cyclic monomers. Most worksare dedicated to the study of an homogeneous coatingof grafted polyesters, especially on physically adsorbedpolyesters on substrates [25,26].

Choi and Langer reported surface-initiated ROP of l,l-lactide as a cyclic ester monomer promoted by tin(II)octoate (Sn(Oct)2) catalyst to produce biodegradablepoly(l,l-lactide) (PLA) brushes on both gold and silicon sub-strates [27]. A SAM of thiol terminated by hydroxyl groupsdeposited on gold and a SAM of silane terminated by aminegroups on silicon (Si/SiO2) substrate were respectively usedto initiate the polymerization of l,l-lactide (Fig. 2b). Thepolymerization takes place at 40 ◦C on gold substrate andat 80 ◦C on silicon substrate for 3 days. No sacrificial initia-tor was added to the medium. Ellipsometric measurementsrevealed that PLA brushes grown on Si/SiO2 surfaces weremuch thicker than those grown on gold surfaces with in situgrown film thickness of about 70 and 12 nm for silicon andgold substrates respectively. Authors suggested that thetemperature can play an important role in the polymeriza-tion, explaining the difference of thickness between thesePLA brushes.

In the same way, the synthesis of poly(p-dioxanone)(PPDX) was investigated by ROP of p-dioxanone (1,4-
dioxan-2-one, PDX) from gold and silicon oxide sur-faces [28]. As far as the gold substrate was con-cerned, a SAM of 1-mercaptoundec-11-yl-tri(ethyleneglycol) (HS(CH2)11(OCH2-CH2)3OH) was self-assembled


rolacto
Fig. 2. SI-ring opening polymerization of (a) �-cap
by immersing the substrate in an ethanolic solution ofthis compound. The distribution of thiol on the surfacewas subsequently characterized by ellipsometry. Then,the ROP of PDX was performed in toluene at 55 ◦C for24 h with tin(II) octoate as catalyst. In a second part, thegrowth of poly(p-dioxanone) PPDX brushes on a siliconoxide substrate was investigated. First, a monolayer of (N-triethoxysilylpropyl)-O-poly(ethylene oxide)urethane wasdeposited on a plasma-cleaned surface through immersionin an ethanolic solution of this precursor. Then, the poly-merization of PDX from the hydroxyl group was performedunder similar conditions, namely in toluene at 55 ◦C for 1 hwith Sn(Oct)2 as catalyst. After analyses by FTIR and XPS,ellipsometry measurements determined a polymer brushthickness of 46 nm, much thinner than the polymer brushesgrown from gold substrate (h ∼ 90 nm). Authors suggestedthat the poor solubility of the PPDX chains in toluene or acongestion of the growing polymer chains could accountfor the limited thickness of the polymeric film.

Most papers reported so far have described the ring-opening polymerization of cyclic esters using organometal-lic catalysts such as Sn(Oct)2, Sn(OTf)2 and AlEt3. However,although tin(II) octoate is approved by the Food and DrugAdministration (FDA), the development of metal-free sys-tems offers an interesting opportunity for the design ofenvironmentally-friendly materials.

With this in mind, Yoon et al. studied the synthesis
of biodegradable poly(�-caprolactone) (PCL) (Fig. 2a) andPPDX polyesters from a gold surface using an enzymeas catalyst [29]. The surface was first functionnalizedwith a hydroxyl group through the self-assembly of 1-
ne, (b) lactide and (c) glutamate from flat surface.

mercaptoundec-11-yl-tri(ethylene glycol) onto the goldsubstrate. Subsequently, ROP of �-caprolactone (CL) inthe presence of Lipase B (Novozym-435 from candidaantarctica) occurred in toluene at 55 ◦C for 24 h. The ellip-sometry measurement revealed a thickness of 93 nm.Further investigations confirmed that the PCL film arisesfrom a chain growth onto the substrate starting fromhydroxyl-terminated surface. Finally, the same synthesisconditions were applied to the PPDX film formation andreached a thickness of around 11 nm.

Wieringa et al. successfully grew poly(l-glutamate)brushes from amine-functionalized silicon wafers andquartz slides (Fig. 2c) [30]. The cyclic monomers usedwere N-carboxy anhydrides (NCA) of �-benzyl and �-methyl l-glutamates (cyclised amino acids) that undergoROP in the presence of amine groups. This polymerization isextremely versatile, and allows the incorporation of a widevariety of side chains into the resulting polymer brushes.

Recently, another catalytic option was investigatedfor the formation of PCL brushes through the useof 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) [31]. Thisorganocatalyst has already demonstrated its ability to poly-merize cyclic esters in a fast and controlled manner insolution. This system offers the opportunity to operate theROP of CL at room temperature. In this temperature, theAu–S bond is stable. The use of organometallic catalyst suchas Sn(Oct)2 imply high temperature, at least 40 ◦C for its
activation.
A SAM of hydroxylated thiol was first deposited on thegold substrate after immersion in an ethanolic solutionof 11-mercapto-1-undecanol. The polymerization reaction

Polymer

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A. Olivier et al. / Progress in

as initiated with TBD as catalyst without adding anyacrificial initiator. It revealed that the thickness for theesulting PCL brushes reached a maximum value after 6 h,amely an average height of 13 nm. It was supposed thathe steric hindrance provided by neighbouring chains couldrevent the formation of thicker polymer films. Anotherart of this work concerned the morphological analysis ofCL crystals on the substrate by Atomic Force MicroscopyAFM) (Fig. 3).

According to the conditions aforementioned, ROPf l,l-lactide (L-LA) was studied directly from theydroxyl groups of the 11-mercapto-1-undecanol mono-

ayer adsorbed onto gold surfaces. In this case, 1,8-iazabicyclo[5.4.0] undec-7-ene (DBU) was used as thereferred organocatalyst [32]. By comparison with TBDsed in ROP of �-CL, DBU catalyst has already proved its effi-iency for the ROP of L-LA in solution. Therefore, metal-freeOP of L-LA initiated from gold substrate was investigatedt ambient temperature in chloroform, i.e. a good solventf PLLA. A dramatic change in polymerization kinetics wasbserved with respect to the PCL/TBD system since theLLA film covered the entire gold substrate after only 1 h.he analysis by TM-AFM showed an almost homogeneousurface and the thickness reached a value of 9 nm, as deter-ined by ellipsometry.

.2.2. Nitroxide-mediated polymerizationNitroxide-mediated polymerization (NMP) is based on a

eversible deactivation process by coupling an active chain-nd radical with a nitroxide leaving group [33]. The firstxample of polymer brushes synthesis was reported byusseman et al. through NMP of styrene according to a

grafting from” approach [14]. The authors prepared anctive surface of alkoxyamine by reaction of a chlorosi-ane derivative with the silanol groups of silicon wafers in
he presence of triethylamine as catalyst. The polymeriza-ion was mediated through nitroxy moieties (TEMPO) tobtain polystyrene (PS) brushes. A predetermined amountf “free” alkoxyamine initiator was added to the reaction
ig. 3. TMAFM height (a) and phase (b) images (5 �m × 5 �m) of PCL brushes obtcale for height and phase images is respectively 40 nm and 70◦ [31] (reproduced

Science 37 (2012) 157– 181 161

mixture to control the polymerization process. This “sacri-ficial” initiator addition creates an overall concentration ofnitroxides in the polymerization mixture, which controlsthe chain growth of the anchored and soluble initiators.Homo- and co-polymers grafted on surface were preparedand thick brushes (up to 80 nm) were finally obtained.

Within the framework of the preparation of hydropho-bic non-reconstructing surfaces, the surface-initiatedreversible deactivation radical polymerization of fluori-nated monomers was investigated from a silicon oxidesubstrate (Fig. 4) [34]. A TEMPO-based initiator wasanchored onto the surface through a functionalizedtrichlorosilyl derivative. Then, the reaction was performedin the presence of “sacrificial” initiator both to controlthe polymerization process and to determine the chainlength of grafted polymer by comparison with the com-pound grown in solution. Authors prepared homo- andblock copolymer brushes by combining styrene and fluo-rinated styrenic derivatives bearing a semifluorinated sidegroup. In the case of diblock copolymers, it was proven thatthe first polymer was able to initiate a new polymeriza-tion giving rise to the second polymeric film. As far as thefilm thickness is concerned, ellipsometry measurementsshowed that values are in the range 10–50 nm. Finally,near-edge X-ray absorption fine structure (NEXAFS) anal-ysis revealed an orientation of fluorinated side chains thatcould be correlated to the surface stability upon exposureto water.

As a counterpart of alkoxyamine initiators, NMP can becarried out using an initiator/radical derivative bimolec-ular system [35]. This strategy was described for theformation of poly(N-butyl acrylate) (PBA) brushes withtwo different azo initiators (AMCl and ATCl) in the presenceof SG1 counter radical (SG1: acyclic �-phosphonylatednitroxide, N-tert-butyl-N-(-1-diethylphosphono-2-2-
dimethyl)propyl nitroxide) (Fig. 5). The chlorosilylderivatives were attached to the silicon wafer afterimmersion of the substrate in a toluene solution of AMClor ATCl, but AMCl gave rise to a monolayer of initiator in
ained by grafting from on gold surface with TBD at room temperature. Z with permission from Elsevier).


ated mo
Fig. 4. SI-nitroxide mediated polymerization of fluorin
contrast with the multilayer formed with the highly reac-tive trichlorosilyl-based ATCl. As far as SG1 is concerned,the nitroxide allowed a control over the chain growthfrom both the immobilized and “sacrificial” initiators.Number-average molecular weights (M ) values are in the
n
range of 3700–20700 g/mol, which corresponds to a PBAthickness of 4–14 nm. A second part of this work concernedthe elaboration of block copolymers by re-initiation of

Fig. 5. Chemical structure of the SG1 stable counter radical (a) and th

nomers from silicon surface via TEMPO based initiator.

PBA growth from macromolecular chains grafted on flatsubstrate. This strategy revealed its efficiency to finelytune the surface properties and could be extrapolated tothe re-initiation of chain growth with different monomers.

The same approach has recently been applied to the
preparation of poly(4-vinylpyridine-b-styrene) (P4VP-b-PS) block copolymer brushes [36]. This system was usedfor the infiltration of nanoparticles (NPs) in the polymeric
e functionalized initiator (b) ATCl (R = Cl) and AMCl (R = CH3).


Fig. 6. Cross-sectional HAADF-STEM (high annular angle dark field-scanning transmission electron microscopy) images of P4VP-b-PS brushesif(

bTtioom(ib

2p

(ammt

stb

nfiltrated with PS-covered Au NP after annealing with CH2Cl2 vapor: Sur-ace coverage of PS for (a) 1.79 chains nm−2 and (b) 0.95 chains nm−2 [36]reproduced with permission from the American Chemical Society).

rushes (Fig. 6). First, the P4VP block was grown from aEMPO initiator anchored to a Si/SiO2 substrate. The reac-ion took place in toluene in the presence of a sacrificialnitiator at 130 ◦C for 3 h. This gave rise to a P4VP layerf 21 nm. Subsequent styrene polymerization was carriedut from the P4VP brushes to produce a diblock copoly-er of 37 nm thickness. Subsequently, gold nanoparticles

Au NPs) were infiltrated in the polymer brushes and TEMmages clearly show for the first time the morphology ofrush-particle composites.

.2.3. Reversible addition-fragmentation chain transferolymerization

Reversible addition-fragmentation chain transferRAFT) polymerization involves a series of reversibleddition-fragmentation steps based on the transfer of aoiety, e.g., dithioester moiety between active and dor-ant species. Indeed, this dynamic equilibrium provides

he uniform growth of polymer chains.
The key to a successful RAFT polymerization is the
uitable choice of a so-called RAFT agent or RAFT chainransfer agent (CTA). These are thiocarbonylthio specieselonging to the following general families of compounds;

Fig. 7. Preparation of homo-poly(styrene) brushes under SI

Science 37 (2012) 157– 181 163

dithioesters, xanthates, dithiocarbamates, and trithiocar-bonates [16,37]. This compound was composed by twospecial groups called “Z and R” with different functions.

The Z group controls the reactivity of the RAFT-agentand possesses two fundamental roles. First, it determinesthe general reactivity of the C S bond towards the radicaladdition. Secondly it affects the lifetime of the intermediateradical, due to the addition of a radical species across theC S bond.

As far as the R group is concerned, its structure adjustsfinely the overall reactivity and thus the ability to effec-tively mediate the polymerization in a controlled manner.The R group also plays a key-role in the polymerizationprocess. Firstly, the R group must be a good free radical(homolytic) leaving group. Secondly, the radical generatedfrom the homolytic dissociation must be able to initiatethe radical polymerization (in the case of small moleculeRAFT agents) or to simply add (macro)monomers in thecase of macro RAFT-agents. The structure of R group hasa large effect on the polymerization kinetics and on thepolymerization control [37].

Baum and Brittain [38] prepared polystyrene,poly(methyl methacrylate), and poly(N,N′-dimethylacrylamide) (PDMA) brushes under RAFTconditions using silicate surfaces modified with surface-immobilized azo-initiators (Fig. 7). The polymerizationtook place at temperatures up to 90 ◦C within 48 h, givingrise to a controlled thickness of polymer brushes. RAFT wasalso applied to synthesize PS-b-PDMA and PDMA-b-PMMAblock copolymer brushes that exhibited reversible surfaceproperties upon treatments with selective solvents. Fur-thermore, the addition of small amounts of untetheredradical initiator (AIBN) to the system was necessary toensure a control growth of the chains.

As previously mentioned, the RAFT agent can eitherbe attached via the R- or Z-groups but the R-group path-way allows a control over chain growth only at lowconversions [39]. For this reason, Stenzel et al. studiedthe polymerization of stimuli-responsive glycopolymerbrushes using the Z-group approach [40]. In this case,the second block grows underneath the first block in
a controlled manner. The functionalization of the sili-con substrate with RAFT initiator was performed in twosteps. 3-aminopropyl methoxy siloxane was first immobi-lized on the surface and the amino-containing multilayer
-reversible addition-fragmentation chain Transfer.

Polymer
164 A. Olivier et al. / Progress in
was allowed to react with 3-benzylsulfanylthiocarbonylsulfanylpropanoyl chloride to form the RAFT agent. Thesubsequent homopolymerization of N-isopropyl acry-lamide (NIPAAm) and N-acryloyl glucosamine (AGA)afforded the preparation of PAGA and PNIPAAm brushes,respectively, with thicknesses in the range from 5 to30 nm. After characterization by contact angle measure-ments, the preparation of stimuli-responsive glycopolymerbrushes was studied from preformed PAGA film usingNIPAAm as the second comonomer. It is worth not-ing that the RAFT agent remained in close contact withthe silicon surface. This could prevent the chain growthdue to increased steric hindrance, as previously reported[41]. However, the growth of PNIPAAm block from PAGAbrushes operated similarly comparing with the initial PNI-PAAm brushes. Contact angle measurements confirmedthe presence of PNIPAAm blocks beneath the PAGAbrushes.

The functionalization of a substrate with a RAFTagent represents a challenging step. Therefore, differentstrategies aiming at the post-modification of an ATRPinitiator (vide infra) were studied [42]. Rowe-Konopackiand Boyes anchored the RAFT initiator in two steps.First, the (11-(2-bromo-2-methyl)propionyloxy)undecyl-trichlorosilane) (BMP TCS) ATRP initiator was attachedto the silicate substrate followed by the conversion ofthe bromine end group of BMP TCS to a dithiobenzoateRAFT chain transfer agent (CTA). Subsequently, PMMA andPS brushes were polymerized and PMMA-b-PS, PMMA-b-PDMAEMA, and PS-b-PMMA diblock copolymer brusheswere formed. The authors demonstrated the role of a freeRAFT CTA in the control of the polymerization. Indeed,PS and PMMA homopolymer brushes have thicknesses of15–17 nm with “free” RAFT CTA and 90–130 nm without“free” RAFT agent but the growth was not controlled. More-over, PS and PMMA films prepared without RAFT CTA didnot allow the formation of diblock copolymers as evi-denced by contact angle and Grazing angle attenuated totalreflectance-Fourier transform infrared (GATR-FTIR) spec-troscopy.

2.2.4. Atom transfer radical polymerizationIn comparison with all the controlled/living polymeriza-

tion systems, atom transfer radical polymerization (ATRP)more likely appears as the most robust technique. ATRPprocess results from the homolytic cleavage of the alkylhalogen bond (R-X) by a transition metal in the lower oxi-dation state to generate an alkyl radical R• and a transitionmetal in a higher oxidation state. The as-formed radicalsinitiate the polymerization by adding themselves at thedouble bond of a vinyl monomer or reacting with the halo-gen atom of the oxidized metal to regenerate the R-X.This method offers the possibility to polymerize a largevariety of monomers using various conditions (in water,at ambient temperature, etc.) while having high kinet-ics. ATRP is experimentally more accessible in comparisonwith living anionic or cationic polymerizations, which cur-
rently require rigorously dry conditions. Control over thepolymerization is obtained through dynamic equilibriumbetween the active radicals and dormant alkyl halides,yielding low polydispersity (Mw/Mn) for the resulting poly-
Science 37 (2012) 157– 181

mer chains and the re-initiation from macroinitiator for thesynthesis of block copolymers.

The first surface-initiated polymerization with ATRPtechnique (SI-ATRP) was reported in 1997 by Huang andWirth. They successfully grafted poly(acrylamide) (PAM)brushes from benzyl chloride-derivatized on silica gel usingCu(bpy)2Cl as catalytic system [43]. The polymerizationwas sufficiently well-controlled that the porous silica sur-face was uniformly covered by a polymer film.

Since this pioneering work, the number of scientificaccounts regarding ATRP from flat surface has neverstopped increasing. Many reviews have been dedicatedto the description of the essential works in this field[44,45,46].

Due to the presence of a low concentration of initiat-ing groups assembled on flat substrate, different strategieshave been elaborated in order to grow polymer chainsin a controlled manner. In 1998, Ejaz et al. described atechnique that consisted in the addition of free moleculescapable of initiating the polymerization at the beginningof the polymerization [47]. The presence of a “sacrificialinitiator” produces an adequate concentration of per-sistent radical, leading to controlled chain growth. Thedetermination of the molecular weight of grafted poly-mer chains on surface are facilitated by analysis of freechains formed in solution. Husseman et al. demonstratedthe good correlation between the molecular weights ofthese “free polymer chains” and the thickness of polymericbrushes [14]. In 1999, a second method lay in the addi-tion of “deactivator complexes”, CuII, at the beginning ofthe reaction. As a counterpart of the “sacrificial initiator”strategy, the presence of deactivator species increases theinitial concentration of CuII. This complex promotes thedynamic exchange between active radicals and dormantoligo/polymeric halides towards dormant species. Furtherevidences for the control of polymerization was providedby the synthesis of block copolymers based on eithermethyl or tert-butyl acrylate from the polystyrene layer[48]. The effect on the brush growth rate of adding acti-vator and deactivator species has been extensively studied[49,50]. Despite this fact, the impact of these strategieson the polymerization control is still under investigation[51,52].

ATRP reaction is considered as a multicomponentsystem. The control over the reaction is consequentlydependent on all the parameters used in the polymeriza-tion such as the nature of the catalyst complex, the solvent,etc. These components have been extensively studied insolution [53,54] and on surface [55–57].

In 1999, the first block copolymer brush synthesized bySI-ATRP was reported by Matyjaszewski et al. [48]. A PS-b-PtBA diblock copolymer brush was accordingly producedvia SI-ATRP from 2-bromoisobutyrate-derivatized siliconwafers. The tuning of surface properties was achievedby the acidolysis of tert-butyl acrylate group from thepolystyrene brushes. This modification resulted in the for-mation of poly(acrylic acid) extremity as revealed by a large
decrease of water contact angle from 86◦ to 18◦.
In addition to diblock copolymer brushes, SI-ATRPhas been used to prepare tri-block copolymer brushes[58,59]. Tomlinson et al. demonstrated the successful

Polymer

posatfbbppeatob

dotcqdgmcadoc

ctoPottcp

FagmS


reparation of multiblock copolymer brushes composedf up to three (PMMA-b-PHEMA) or (PMMA-b-PDMAEMA)equences [60]. The capacity of a macroinitiator to reiniti-te a homopolymer brush was also studied. Authors foundhat the synthesis of block copolymer by the “graftingrom” technique is directly influenced by the nature ofoth macroinitiator and monomer. Multiblock copolymerrushes composed of PMMA and PHEMA were easily pre-ared, but the synthesis of PMMA-b-PDMAEMA brushesroved to be much more difficult. The authors discov-red that while chains terminated with PDMAEMA did notppreciably reinitiate the radical polymerization of MMA,he chain-ends remained intact. The reinitiation efficiencyf PMMA homopolymer as well as multiblocks of PHEMA--PMMA was also discussed.

Chang et al. have recently established the con-itions to obtain mixed-charge copolymerizationf the positively charged 11-mercapto-N,N,N-rimethylammonium chloride (TMA) and negativelyharged 11-mercaptoundecylsulfonic acid (SA) [61]. Theuantification of protein adsorption was used in order toescribe the molecular arrangement of mixed chargedroups present along the polymer chains. Using XPSeasurements, the charge ratio of TMA and SA in the

opolymer brushes was estimated from the ratio of thetomic percentages of nitrogen and sulphur. The authorsemonstrated that the reduction of protein adsorptionn the poly(TMA-co-SA) surfaces was associated with theharge ratio (Fig. 8).

Another approach to create block copolymers wasonsidered by Baker et al. They combined two con-rolled polymerization techniques in order to produceriginal brush morphology. They produced brushes ofHEMA by ATRP from the bromide extremity of SAMsn gold substrate (Fig. 9) [62]. Then, the ROP of lac-ide was carried out to grow poly(lactide) brushes using
he hydroxyl groups of PHEMA layer as initiators. Thisopolymer synthesis led to “bottle-brush” morphology—aolymer brush where each anchored polymer chain has
ig. 8. Adsorption of 1 mg/mL of three human proteins (HSA, �-globulin,nd fibrinogen) in buffer containing 0.01 M NaCl on poly(TMA-co-SA)rafted surfaces as a function of solution pH at 23 ◦C from SPR measure-ents [61] (reproduced with permission from the American Chemical

ociety).

Science 37 (2012) 157– 181 165

bottle brush architecture. Several polymer brush struc-tures each with backbone different chain lengths werecreated by controlling the polymerization time. In the sameway, Rowe et al. investigated the formation of diblockcopolymers by ATRP and RAFT polymerization techniquesfrom a silicon wafer [63]. Therefore, the (11-(2-bromo-2-methyl)propionyl-oxy) undecyltrichlorosilane initiatorwas deposited on the silicon wafer by immersion ofthe substrate in a toluene solution of this precursor.Then, this monolayer allowed the formation of well-defined poly(styrene) (PS) or poly(methyl acrylate) (PMA)brushes by ATRP. In a second step, the ATRP initia-tor at the outer surface of the film was converted intoRAFT agent by atom transfer addition (ATA) reactionusing two different systems, namely Cu(0)/CuBr/PMDETAor tin(II) 2-ethylhexanoate/CuBr/PMDETA. Subsequently,the surface-initiated RAFT polymerization of NIPAAm,N,N′-(dimethylamino)ethyl acrylate (DMAEA) and acrylicacid afforded the preparation of PS-b-PNIPAAm, PMA-b-PDMAEA and PS-b-PAA polymer brushes. The sampleswere characterized by ellipsometry, contact angle andGATR-FTIR revealing the controlled character of the RAFTpolymerization. Further experiments revealed a change ofstimuli-responsive behavior of both blocks upon solventtreatment. The formation of tethered PS-b-PMMA brushescombining reverse-ATRP and ATRP was reported by Sodgeet al. [64]. Azo-functional trichlorosilane molecules self-assembled onto silicon substrate, triggering the synthesisof PS brushes using reverse ATRP. A second block was thenpolymerized from the extremity of the first brush underATRP conditions. Each step of the process was scrupu-lously analyzed by ATR-FTIR. Recently, Xu et al. describedthe synthesis of poly(5-ferrocene-triazolyl methacrylate)on gold surface by a combination of SI-ATRP and “clickchemistry” in a “one pot” condition [65]. Authors showedthat ferrocene-containing polymers have unique redoxand catalytic properties and demonstrated the reversibleloading–unloading step of the �-cyclodextrin polymer viahost–guest interaction.

In addition to the control over the architecture, manystudies have led to homogeneous coatings of graftedpolymer chains in order to widen the range of possi-ble applications. Remarkably, in 2010, numerous reportsdevoted to the synthesis of polymer brushes from siliconand gold surface have described huge progress in tar-geted applications such as non-fouling coating [66–70,61],cell adhesion/detachment [71,72] fabrication of ultrahy-drophobic surface [73], chemical sensoring [74] and guid-ing neuronal growth [75]. In addition to these applications,the physico-chemical properties of thermo-and pH respon-sive polymer brushes have been investigated. This providednew detailed pieces of information on the phase transitionbetween the extended and the collapsed forms [76–78].

2.3. “Mixed” SAMs

The co-adsorption of two different molecules (i.e. RSH
and R′SH) on surface gives rise to so-called mixed SAMs.The proportion of each component in the mixed monolayeris strongly influenced by several parameters such as thenature of the end-group, the length of alkyl chains and the


ive poly
Fig. 9. Description of bottle brush formation by success
solubility of the thiols in the solvent [79,80]. In the casewhere two thiol molecules are composed of identical alkylchain length and no particular end-group, the SAM com-position is almost identical to the composition in solution.The key-parameters (initiator density, type of initiator, gra-dient deposition, etc.) will be discussed in the forthcomingparagraphs.

2.3.1. Variation of initiator densityJones et al. reported the first systematic study of the

influence of initiator density on surface-initiated polymer-ization [81]. SI-ATRP of methyl methacrylate and glycidylmethacrylate were carried out from a mixed monolayer oftwo thiols where the quantity of each thiol was known.Only one thiol bearing the bromoisobutyrate end-groupwas able to surface-initiate the synthesis of the polymerchains. A linear relationship between the initiator density
and the thickness of the polymer brush was established.Authors showed that the density of initiating sites stronglyinfluenced the rate of the chain growth and the morphologyof the resulting polymer film.
merization of Hydroxyethyl methacrylate and Lactide.

Liu et al. have recently presented a new simulation studyand established the influence of surface-initiated poly-merization rate and initiator density on the properties ofpolymer brushes [82]. Respectively, Ma et al. showed thatprotein repulsion on poly(oligo(ethylene glycol) methylmethacrylate) brushes is a function of both the film thick-ness and polymer surface density [83]. They demonstratedthat the best coating against protein adsorption was thesynthesis of low density brushes.

Moreover, Ishida et al. showed that the grafting densitycould also influence the phase transition behavior of PNI-PAAm [78]. Using AFM in a liquid cell (water) at varioustemperatures, authors demonstrated that the transitionbehavior of the grafted PNIPAAm chains from a brush-like to a mushroom-like morphology was dependent onthe grafting density: The images change abruptly fromessentially featureless structure to structures with domains
across the LCST for the low-density surface, whereas thischange becomes less abrupt with increasing polymer graftdensity. This behavior was also confirmed by quartz crystalmicrobalance (QCM-D) experiment.

A. Olivier et al. / Progress in Polymer Science 37 (2012) 157– 181 167

ushes fr

2

bPbsGlwwmafwmo

2

tdt(atemt(isflpim

ZmP

Fig. 10. Scheme of the procedure to obtain binary mixed br

.3.2. Variation of initiator typeZhao et al. described a novel method to synthesize

inary mixed homopolymer brushes [84]. The growth ofMMA and subsequently PS graft chains were performedy surface-initiated polymerization from a mixed SAM onilica substrate containing ATRP and NMP initiator (Fig. 10).ood control over the polymerization was attested by the

inear evolution of brush thickness with the moleculareight. A series of binary brushes consisting of PMMAith molecular weight of around 26,000 g/mol and PS ofolecular weights ranging from 3800 to 38,000 g/mol were

chieved. The reorganization of the two brushes under dif-erent solvent treatments (dichloromethane, cyclohexane)as studied. In contrast to the previous study, gradientixed SAMs of ATRP-silane and NMP-silane were created

n silicon wafer [85].

.3.3. “Y” molecule assemblyTo overcome the possible phase separation from

he mixed initiator SAMs and subsequent formation ofomains from the same polymer brush, Zhao et al. syn-hesized a difunctional ATRP/NMP initiator-based silaneY-silane) [86]. Y-silane ensures the mixing of both ATRPnd NMP initiators at the molecular level and consequentlyhe good mixing of the resulting polymers (Fig. 11). Santert al. studied mixed PS/PMMA brushes with two differentodes of attachment; conventional and Y-shaped [87]. In

he conventional mixed polymer brushes, grafting densityand therefore the local composition) fluctuated, resultingn a strong domain memory effect. For Y-brushes, the oppo-ite case was observed. Authors demonstrated that smalluctuations in the grafting density were amplified by thehase separation and nucleated the location of the domains

n the mixed brush. Good correlation between the experi-ents and simulation was found.
In addition to the synthetic aspects developed by
hao et al., the effect of chain length, the solvent treat-ent and its influence on the conformation of mixed

MMA/PS brushes were studied [86]. Based on the same

om monolayer containing a mixing of two initiators types.

polymerization conditions, they investigated in detail thesolvent effect on the surface morphology (Fig. 12) [88].When mixed brushes were composed of fixed PMMA,Mn and a systematic increase of Mn (PS), the watercontact angle value changed from 74◦ to 91◦ after treat-ment in chloroform. Seventy four degrees correspond tothe value for pure PMMA surface. AFM revealed a rel-atively smooth morphology for all samples. Regardingbinary brushes composed of smaller Mn (PS) than PMMA,ordered nanoscale domains were observed after glacialacetic acid treatment. In this case, PMMA chains formeda shell around the PS core. Authors completed their studyon these mixed brushes by combining selective solventtreatment and thermal annealing. Special attention waspaid to the brushes with PS molecular weight slightlylower than that of PMMA [89]. Wang et al. determinedthe conditions to synthesize binary homopolymer brushesby combining NMP of styrene and living cationic ring-opening polymerization of 2-phenyl-2-oxazoline [90]. Thereversible self-adapting surface property of mixed brusheswas observed when the chains were subjected to differentsolvents.

In the same way, block copolymer brushes with differ-ent architectures (linear, Y-shaped and comb-like) werestudied by dissipative particle dynamics under differentgrafting densities and lengths of coil blocks. This studydemonstrated the influence of the block arrangements onthe surface structures of copolymer brushes [91]. Li et al.noted that the exposed results could provide the route tothe design polymer brushes with desired surface structuresand thus desired properties.

2.4. Gradient deposition

Gradient brushes are brushes wherein the chemical
composition of the chain (grafting density and molec-ular weight) as well as the physicochemical propertiesgradually changes along the substrate. The preparation ofgradient brushes can be achieved by a modification of the


obtain b
Fig. 11. Experimental procedure to
initiator monolayer or the grafted polymer chains. Someexamples are described below.

Jiang et al. focused on a new concept in order todevelop polymer thin film gradients on substrates by waterconcentration gradient [92]. Specifically, a silicon waferfunctionalized with a monolayer of initiator moleculewas placed in a two-layer system of water (layer 1) andpoly(ethylene glycol) methacrylate (PEGMA) (layer 2) poly-merization solution. Water gradually diffused into thePEGMA monomer layer. This diffusion formed a monomerconcentration gradient across the surface of the monolayer,leading to PEGMA brushes (Fig. 13b) featuring a molecular
weight gradient across the film.
The gradual variation of PMMA molecular weight acrossa surface was envisioned by Tomlinson et al. [93]. A sil-icon substrate containing ATRP initiator was vertically

Fig. 12. Effect of solvent on rearrangement

inary mixed brushes from a Y-SAM.

positioned in the reactor chamber and covered by the poly-merization solution. The gradient was created thanks tothe aspiration of the liquid by a micropump. The authorsshowed that the brush thickness evolved as a function ofsubstrate position.

For the elaboration of grafting density gradientbrushes, the monolayer of initiator was carefully con-trolled (Fig. 13a). Wang et al. prepared a gradient ofhexadecanethiol molecule on gold surface using an elec-trochemical method [94], that implies desorption of somemolecules from homogeneous coating. Then, bare goldwas refilled by ATRP initiator and the polymerization
of NIPAAm was performed giving rise to PNIPAAm den-sity gradients. In a separate study, Liu et al. generatedthe monolayer gradient using a linear temperature gra-dient heated stage [95]. An old methodology for the
of PMMA/PS binary mixed brushes.


F cular wc

ppeb

btteiwb

om[AsopBnbetitwos

2

tsitsd

ig. 13. Scheme of grafting density gradient polymer brushes (a), molehemical composition gradient polymer brushes (d).

reparation of gradient polymerization initiator pro-osed by Chaudhury and Whitesides was used by Wut al. and led to graft density gradient poly(acrylamide)rushes [96,97].

The preparation of mixed gradient binary brushes haseen investigated by Wang et al. (Fig. 13c) [90]. ATRP ini-iator was uniformly fixed on gold surface before initiatinghe radical polymerization of NIPAAm. Then an in-planelectrochemical potential gradient was applied, imply-ng desorption of PNIPAAm. The exposed bare gold areas

ere back-filled with ATRP initiator to generate PHEMArushes.

Considering the synthesis of tapered copolymer brushesf methyl methacrylate (MMA) and 2-hydroxyethylethacrylate (HEMA), Xu et al. used SI-ATRP (Fig. 13d)

98]. The polymerization of MMA was performed from anTRP initiator monolayer uniformly adsorbed on siliconubstrate. This method consists in the gradual additionf HEMA to a MMA reaction mixture in order to formolymer brushes with a chemical composition gradient.ased on microfluidic technique, Xu et al. introduced aew way to produce gradient in a statistical copolymerrush composition [99]. Two syringes containing differ-nt polymerization solution were connected separately tohe inlets of a microfluidic passive mixer. Both polymer-zation solutions were then mixed and introduced insidehe channel connected to the bottom of a sample whichas uniformly covered by the monolayer of initiator. More-

ver, the effect of block length on solvent response was alsotudied [100].

.5. SAMs obtained from the gas phase

The self-assembled monolayers of thiol or silane deriva-ives from the gas phase have been the subject of severaltudies [80,101]. In contrast to the monolayer formation
n the liquid phase, only few studies have mentionedhe combination of self-assembly in the vapor phase andurface-initiated polymerization. Brown et al. presented aetailed study of ATRP initiator assembly on surface and
Fig. 14. Atom transfer radical polymerization initi

eight film gradient brushes (b), mixed gradient binary brushes (c) and

its influence on the growth of poly(oligo(ethylene gly-col) methacrylate) brushes [102]. The monolayer formationrequired to start the ATRP of a monomer was envisionedboth from liquid and gas phase (Fig. 14). Authors provedsimilar reactivities of each monolayer for the polymerbrush growth.

Regarding the gradient formation of polymerization ini-tiator on silica substrates, the diffusion of ATRP initiatorfrom the gas phase has been evaluated several times. Oncethe surface-initiated polymerization is performed, a gradi-ent in grafting density of polymer brushes from mushroomto brush [97,90] or a gradient in molecular weight can beestablished [103]. The arrangement of initiator moleculeson the surface is essential for this kind of polymerstructure.

Additionally, vapor phase deposition is sometimes usedas a step in the process of patterning creation [83,104].Indeed, Jonas et al. deposited mono-chlorosilane ATRP ini-tiator from gas phase on bare area of patterned siliconsubstrate created by e-beam irradiation on PMMA resistfilm [104]. The combination of microcontact printing ofnon-functional molecule and subsequently the silane vapordeposition were evaluated by Ma et al. before the synthesisof polymer brushes [83].

3. From patterned monolayers to heterogeneouscoatings of polymer brushes

During the last two decades, many techniques havebeen developed to selectively position organic moleculesand then to obtain well-defined patterned substrates atthe (sub)micrometer scale. They are divided into severalcategories such as printing techniques, direct writing tech-niques, photolithography and particle beam lithography.The description of surface-initiated polymerization fromfresh patterned monolayers was extensively reported in
scientific literature and is summarized below. Subsequentto the polymerization of unique brushes, we devote aspecial paragraph to the production of binary-patternedbrushes.
ator assembly in vapor phase on Si wafer.


attern (om the R

Fig. 15. Microscopic view (magnification 50x) of the naked PDMAEMA p(circular pattern diameter: 10 �m) [132] (reproduced with permission fr

3.1. Patterned polymer brushes

3.1.1. The printing techniqueAmong the printing techniques, we can distinguish

different approaches such as micromolding in capillar-ies (MIMIC) [105,106], microtransfert molding (mTM)[107,108] and microcontact printing (�CP) [109]. Thesestamping methods involve the direct patterning or thedeposition of the ink molecule on the substrate usingelastomeric material. Microcontact printing is probablythe most versatile and cost-effective method for the gener-ation of patterned SAMs with lateral dimension ≥100 nm[110].

Microcontact printing was first developed by Kumar andWhitesides in 1993 [111]. It is an efficient technique forthe patterning of large-area surfaces (planar and curved).The principle of the technique is simple and compara-ble to printing ink with a rubber stamp on paper. First,poly(dimethylsiloxane) (PDMS) stamp is impregnated witha solution of “ink molecules”, and thereafter, placed in con-tact with the substrate. The �CP approach requires a stampthat can make direct molecular contact with the metal andan ink that can bind sufficiently strongly to the metal [112].The contact between ink and surface is driven by adhe-sion forces [113]. Once on the surface, the molecules diffusefrom the bulk of the stamp to the interface between PDMSand the surface. The molecules are transferred in a patterndefined by the topography of the stamp with a minimumfeature size of 40 nm [114,115]. One of the main advantagesof this technique is that a large variety of “ink molecules”can be deposited by �CP method. This approach has beenused successfully performed for molecules such as alka-nethiols [116,117], silanes [118], biomolecules [119] andnanoparticles [120].

PDMS is considered as the conventional stamp mate-rial in soft lithography and has shown great performancein micrometer-scale processes [121]. In some particularstudies, a rigid polyurethane–acrylate polymer is used asstamp support to moderate the deformation and distor-tion of the material during the printing [122]. Campos et al.
evaluated the performance of another material based onpoly[(3-mercaptopropyl)methylsiloxane] (PMMS) [123].Recently, Kaufmann and Ravoo have reviewed �CP sys-tems in which the stamp, the ink and the substrate
left) and after selective anchoring of CNTs on polymeric brushes (right)oyal Society of Chemistry).

are a polymer [124]. To produce topographically pat-terned surfaces, �CP can be used to directly print theinitiator [125–127] or print a non-initiating derivativeand then backfill with initiator precursors [128–130].These strategies are mainly performed in order to pro-duce patterned polymer brushes. Using the same idea,Kelby and Huck recently elaborated a way to producefree-standing Au-polyelectrolyte brush bilayer objects[131]. Poly(methacryloxyethyltrimethylammonium chlo-ride) (PMETAC) brushes were grown from thiol end-groupspreviously deposited on gold surface by �CP. After SI-ATRP,the metals (Au and Cr) around the brushes were removedby chemical etching. Authors created free standing Au-PMETAC brush bilayer objects in order to quantify themechanical stresses present in stimulus-responsive poly-electrolyte brushes. Olivier et al. focused on the �CP of thiolinitiator on gold surface, followed by the polymerization byATRP of N,N′-dimethyl aminoethyl methacrylate (Fig. 15),yielding PDMAEMA brushes [132]. The selective adsorptionof carbon nanotubes (CNTs) on a pH-reversible PDMAEMApatterned gold surface was investigated. In acidic condi-tions, CNTs were selectively adsorbed onto the polymerbrushes due to ammonium-� interactions. The reversibilityof the process was demonstrated by successive treatmentsin both alkaline and acidic solutions.

In contrast to thiol-gold surface, the patterning strat-egy of trichlorosilane molecules involved an additionalstep. Silicon surface requires activation by plasma oxi-dation in order to increase the proportion of hydroxylgroup and immobilize the initiator molecules throughmicrocontact printing. Following this patterning method,Farhan and Huck focused on PNIPAAm growth [12]. Chenet al. demonstrated another way to deposit initiatormolecules. Authors have shown the successful attachmentof �-bromoundecyltrichlorosilane initiator to carboxylicacid extremity of a molecule previously attached ongold surface by �CP (Fig. 16) [133]. The chemical acti-vation of the surface was mediated by hydrogen bondattachment of the initiator. Subsequently, the amplifi-cation of NIPAAm into patterned polymer brushes was
effectively achieved. Although most microcontact printingefforts have been conducted with small-molecule “ink”,Edmonson et al. examined the feasibility of micropat-terning with cationic macroinitiator onto clean silicon


F SAM fs .

wydda

petwoTfbticbTNstapWpt

3m

oenn

g

ig. 16. Schematic representation regarding the patterning of an MHAubsequent amplification into polymer brushes via surface-initiated ATRP

afers [134]. They focused on the SI-ATRP of hydrox-ethylmethacrylate from �-bromoester initiator sites andemonstrated perfect control of the brush growth. Well-efined micrometer-sized PHEMA stripes were producednd reached a height value of 54 nm.

Concerning the formation of hierarchically structuredolymer brushes, Wang et al., following the study of Zhout al., presented a new strategy of multi-step microcon-act printing [135,136]. An initial concentration of thiolsas used to print the first pattern, followed by a sec-

nd deposition using another concentration and stamp.he concentration was adapted by dilution using non-unctional thiol. Subsequently, poly(glycidyl methacrylate)rushes were grown. Authors demonstrated the impor-ance of the printing order and the concentration ofnitiator solution. The procedure elaborated by Zhou et al.onsisted in the patterning of a gold surface with a thiolearing a bromide end-group as initiator for SI-ATRP [136].hen, the bromine group was deactivated by reaction withaN3. The next initiator SAM was self-assembled onto the

urface and the second brush was selectively grown inhis area. Tertiary and quaternary brushes were generatedccording to the same experimental protocol, varying theolymer thickness and the pattern design. In contrast toang’s study, Zhou et al. synthesized different types of

olymer brushes and generated binary, tertiary and qua-ernary brushes.

.1.2. Direct writing technique using scanning probeicroscopy

Patterning by direct writing of the chemical reagentsn specific regions of substrate is divided into sev-ral categories [137] including dip-pen nanolithography,
anoshaving, nanografting, anodization lithography andanooxidation or ink-jet printing.
In 1999, Mirkin’s group introduced a new nanolitho-raphic method called Dip-Pen Nanolithography (DPN)

ollowed by hydrogen-bond mediated attachment of BTS initiator, and

[138]. DPN is a direct-write scanning probe-based lithog-raphy in which an AFM tip is used to deliver chemicalreagents via capillary forces, directly to nanoscopic regionsof a targeted substrate. It was also shown that this tech-nique offers the ability to pattern multiple chemical species(like �CP) with sub-100 nm alignment [139]. The resolu-tion of patterning depends on the volume of meniscus,scan speed, surface chemistry, temperature and ambienthumidity [138].

Liu et al. in 2003 reported the first combination of DPNand surface-initiated polymerization technique in orderto produce polymer brushes [10]. 10-(exo-5-norbornen-2-oxy)decane-1-thiol molecules were deposited on a goldsurface by bringing a tip coated with norbornenylthiolin contact with the substrate. Subsequently, the pat-terned surface was backfilled with an inactive thiol,namely 1-decanethiol. In order to amplify norbornenyl-functionalized monomers in this specific area usingring-opening metathesis polymerization, the substrate waspreviously activated by immersion in a Grubbs catalystsolution. Line and dots arrays of polymer brushes werebuilt. In the same way, Ma et al. described the first SI-ATRPfrom ATRP initiator deposited by DPN [140]. The fabricationstrategy is similar to the previous one. In addition to thepatterning of �-mercaptoundecyl bromoisobutyrate usingAFM-tip, gold surface was backfilled with non-functionalmolecules and the growth of oligo(ethylene glycol) methylmethacrylate was performed.

Zapotoczny et al. highlighted a new nanofabricationstrategy based on the tip-assisted deposition of goldnanowires on hydride-terminated silicon [141]. Disul-fide initiation-transfer-termination (Iniferter) agents wereselectively immobilized on the designated substrate. Linear
poly(methacrylic acid) brushes, with a width from severalhundred to 20–30 nm and a controllable height, were syn-thesized by photopolymerization of methacrylic acid fromdisulfide extremity.

Polymer
Lastly, Liu et al. focused on the patterning of ATRPinitiator on 16-mercaptohexadecanoic acid (MHA) pas-sivated gold surface by DPN [142]. First, the AFM tipwas immersed in ethanol solution of �-mercaptoundecylbromoisobutyrate. Subsequently, the initiator inked-tipwas in contact with the MHA monolayer. Under high load(>10 nN), the molecules of the SAM were mechanicallycleaved away by the AFM tip, and the initiator moleculeswere transferred onto the uncovered area of the sur-face. Authors combined two patterning techniques; thenanoshaving and DPN in order to produce microstructures,nanodots and nanolines of ATRP initiator. Then, the SIP ofNIPAAm, 2-(methacryloyloxy)ethyl trimethylammoniumchloride and N,N,N,N,N-pentamethyldiethylenetriaminewere successfully produced from these patterns(Fig. 17).

Before considering the combination of nanoshaving andDPN as did Liu et al., other groups highlighted the procedureof nanoshaving SAM in order to generate polymer brushes.Kaholek et al. first described the selective removing of thiolfrom precoated gold surface with SAM of octadecanethiol(ODT) using AFM tip [143,144]. Large normal forces (50 nN)and high scan speeds (20 �m/s) were employed to removethe non-functional thiols and to create a pattern of straight“trenches” on the substrate. After that, the surface wasexposed to ATRP initiator solution in order to selectivelyanchor the molecules on bare area of the substrate. Finally,the line pattern of thiol initiator was chemically ampli-fied by ATRP at room temperature, creating nanopatternedPNIPAAm brushes.

The difference between the patterning of a spin-coatedpolymer and a grafted polymer brush after mechanicalnanoscratching using AFM lithography has been consid-ered by Hirtz et al. [145]. Authors investigated three kindsof polymers categorized in two groups, the crystallinePS and PNIPAAm and the soft-viscous poly(N-butyl acry-late) (PNBA). A substantial difference between the twopolymer depositions was observed. Very well structuredpolymer brushes were obtained using grafted polymerbrushes, whereas moderate results were obtained afternanoscratching of spin-coated polymer films.

Recently, Morsch et al. performed ATRP graftingof PMMA brushes onto a pulse plasma-depositedpoly(vinylbenzyl chloride)/poly(N-acryloylacrosinemethyl ester) bilayer [146]. Scanning probe lithogra-

Fig. 17. Schematic illustration of the nanofabrication of polym

Science 37 (2012) 157– 181

phy was employed to selectively remove the upper layerand then underlying halide initiator of poly(vinylbenzylchloride) initiator sites, which readily undergoes localizedATRP of methacrylate. The molecular scratch-card tech-nique has successfully demonstrated the ability to createnanoscale polymer brush structures.

Lee et al. elaborated another way to pattern a SAMusing AFM anodization lithography, a form of field-inducedscanning probe lithography [147,148]. Anodic oxidepatterns were generated on octadecylmethyldiethoxysi-lane (ODMS) SAM-coated silicon surface. Then, specificmolecule was covalently linked to the patterned area,which was able to successively attach Grubbs catalyst.Finally, the SI-ROMP of either cyclooctatetraene or 5-ethylidene-2-norbornene from patterns with a line widthof about 200 nm or a dot diameter of about 75–100 nm wasachieved.

Based on a similar method, Benetti et al. succeededin forming silicon oxide nanopatterns by AFM-assistedscanning probe oxidation (SPO) (Fig. 18) [149]. By apply-ing negative bias voltages to a gold coated AFM-tip incontact with a monolayer of octadecyltrichlorosilane, sil-icon oxide nanopatterns of different sizes and shapes wereobtained. Selective functionalization of patterns (dots andlines) with ATRP initiator and subsequent polymerizationof HEMA were carried out. The lateral resolution of patternsconfirmed the isolated grafting of a few tens of macro-molecules. The electrochemical-oxidation process of silanemonolayer has been previously demonstrated by Beceret al. using a copper TEM grid [150].

As far as the ink-jet printing is concerned, this approachrelies on the creation and release of droplet of fluids on solidsurface on demand [151]. The final pattern is formed whenthe solvent evaporates. This technology can be adapted todeposit solutions of alkanethiols on metal surfaces to gen-erate SAM patterns with features of around 100 �m in size[152]. Polymer patterning can be also achieved by the depo-sition of some solvent drops or a reactive molecule ontopolymer substrate that can selectively etch the substrate[153]. The size of the droplets determines the resolu-tion of the technique. Usually, the smallest droplet size is
limited to a value of about 10 �m, but some techniqueshave improved the resolution [154,155]. Recent progressin the understanding of the inkjet printing process wasrealized in many fields of applications including organic
er brushes by Dip-Pen Nanodisplacement lithography.


HEMA b

tc

pitSoApimoicnat

3

dep1

fbctvip

aatleas

i

Fig. 18. Preparation of nanopatterns of P

hin-film transistors, light-emitting diode (LEDs) and solarells [156].

In this way, Sankhe et al. reported the use of ink-jetrinting for precise placement of thiol-terminated ATRP

nitiator molecules on gold substrate for developing pat-erned and graduated soft surface [157]. Subsequently, theI-ATRP of methyl methacrylate and the characterizationf resulted polymer brushes were carried out by FTIR andFM. Recently, Emerling et al. elaborated a new method toattern polymer brushes on the micrometer scale [158]. An

nkjet printer was used to deposit droplets of acid onto aonolayer of ATRP initiator. Consequently, the ester bond

f the initiator was cleaved by the acid, giving rise to annactive molecule. Afterwards, the SI-ATRP of MMA wasarried out and used as a good way to check the effective-ess of hydrolysis. Many parameters such as the nature ofcid, the concentration and the reaction time were con-rolled in order to obtain the most perfect pattern.

.1.3. Particle beam lithographyLithography can be achieved with several kinds of irra-

iation including UV–vis (UltraViolet–Visible) light, X-Ray,lectron and ion-beams. These tools are able to generateatterns with a resolution varying from micrometers to00 nm.

One of the major advantages of lithographic techniquesor generating patterns is that the resolution is determinedy the size of the beam applied to the SAMs. However, theost of the equipment and infrastructure required is rela-ively high. Many studies have been devoted to patterningia these lithographic techniques using several types ofrradiation. Two important methods are described below;hotolithography and electron beam lithography.

Electron beam lithography (EBL) was developed soonfter the discovery of scanning electron microscope in 1955nd arose as an attractive alternative to fabricating nanos-ructures by X-Ray and UV-lithography [159]. Electronithography offers higher patterning resolution because thelectron beam can be readily focused to a diameter of
pproximately 1 nm. Electron beam lithography is inten-ively used in both resist-based and chemical approaches.
As far as the chemical approach is concerned, electronrradiation has been applied to a variety of different SAMs.

rushes using scanning probe oxidation.

Some studies show the possibility to induce the cleavage ofC–S bonds, but also some side-reactions such as desorptionof H2, cleavage of the C–H bonds in methyl and methyleneentities, cross-linking and formation of C C bonds [160].Patterned film of grafted polymer was fabricated for thefirst time, by the combination of an electron beam andcontrolled polymerization [161]. The initiator monolayerof silane derivatives was bombarded by the electron beam,which decomposed the molecules. Methyl methacrylatemonomer was then polymerized via ATRP in order to pro-duce patterned brushes with 175 nm of lateral resolution.

Schmelmer and co-workers have chemically modi-fied a more exotic SAM to produce an azo-polymerinitiator. Consequently, brush patterns with lateral res-olution approaching 70 nm have been produced by theconversion of nitro-based SAMs into amine functionali-ties [162] (Fig. 19). This was followed by diazotizationand coupling with malonodinitrile. The monolayer of 4′-azomethylmalonodinitrile-1,1′-biphenyl-4-thiol (cAMBT)was finally exposed to a styrene solution and synthesizedthrough radical polymerization for 6 h in toluene at 80 ◦C.Later, the same authors reperformed the experiments thistime using surface-initiated photopolymerization [163].The SIP of styrene was not initiated by thermal decom-position of the 4′-azo function, but by photolysis of theasymmetric azo function from cAMBT. Based on the samepatterning strategy, Steenackers et al. performed comple-mentary experiments and proved the linear increase ofbrush thickness with the UV irradiation/polymerizationtime [164]. Moreover, defined grafting-density gradientwas prepared and controlled by electron-beam chemicallithography (EBCL). Gradiated structures with a fine con-trol of the shape, size, position and thickness of PNIPAAmpatterns were elaborated by He et al. [165]. Authors high-lighted the control of surface topography by electron doseand polymerization conditions.

Ballav et al. performed EBCL not on aromatic, butaliphatic SAMs (Fig. 20) [166]. A primary octadecanethiollayer was irradiated by an electron beam, causing struc-
tural defects. In this study, the patterning was achieved viaan irradiation-promoted exchange reaction (IPER) in whichirradiated molecules reacted with 11-aminoundecanethiolhydrochloride. Subsequently, the esterification of the


Fig. 19. (a) Irradiation through a mask, (b) conversion of the terminalnitro group in amine group and (c) diazotization and coupling with mal-onodinitrile gives a SAM that bears an asymmetric azo-initiator.

amine end-group by bromoisobutyryl bromide was car-ried out, creating the ATRP initiator extremity for thepolymerization of NIPAAm. Authors highlighted the manyadvantages of using aliphatic templates instead of aro-
matic ones. Besides the use of commercially availablecompounds, the required irradiation dose was much lower.
Patterns with variable shapes, including complicatedgradients with nanometer lateral resolution were also

Fig. 20. Preparation of polymer brushes using electron-beam chemicallithography on aliphatic SAMs.


Fb

eeoltsrssnraNud

ttpbhw

g[3sfi

cial photoresist layer was then removed by simply washing

ig. 21. E-beam patterning process using a resist film of PMMA, followedy SIP of NIPAAm.

laborated by Schilp et al. [167] using a similar strat-gy. Furthermore Vieu et al. [168] showed that a carefulptimization of EBL processes could push the resolutionimits of the technique well below 10 nm. Development ofhe “resist” has become an important factor in achievinguch high resolution. PMMA is one of the most popularesist systems. For instance, Ahn et al. [169] demon-trated the possible combination of gold patterned andurface-initiated polymerization (SIP) to form micro- andanostructures of PNIPAAm. A first layer electron-sensitiveesist film of PMMA (∼130 nm thick) was spin-coated onto

clean silicon surface and annealed at 160 ◦C for 20 min.anometric spots (around 3 nm diameter) were formednder electronic beam irradiation, followed by successiveepositions in organic solution as shown in Fig. 21.

The same authors later reapplied the strategy, using pat-erns of 2,2′-azobisisobutyronitrile (AIBN)-type photoini-iator in order to initiate the copolymerization of weaklyolyelectrolytic brushes such as poly(NIPAAm-co-NaMAA)y photopolymerization [170]. They demonstrated theeight increase of nanopatterned ionized polymer brushesith increasing pattern feature size.

Jonas et al. combined electron beam lithography andas phase silanation in order to nanopattern silicon wafer104]. Arrays of circular holes of varying diameter (from
5 nm to 5 �m) were created in 100 nm thick PMMA filmspin-coated on silicon wafer using an electron beam. Beforexing monochlorosilane with �-bromoisobutyrate end-
Science 37 (2012) 157– 181 175

group, the bottoms of the holes were cleaned by a shortexposure to oxygen plasma. Subsequently, PMMA resistwas removed by an acetone soxhlet. After the backfill-ing of bare silicon, the poly(2-(2-methoxyethoxy)-ethylmethacrylate) (PMEO2MA) was grown from the pattern ofATRP initiator.

The photolithography process consists of a transfer ofgeometric shapes on a mask to the metallic surface such assilicon wafer. Photolithographic patterning can be achievedeither on SAMs or on photoresistive materials that can bedefined as an organic polymer sensitive to ultraviolet light.

Based on this strategy, Iwata et al. synthesized well-defined poly(2-methacryloyloxyethyl phosphorylcholine)(PMPC) brushes by SI-ATRP from patterned ATRP initia-tor obtained from the UV illumination of homogeneousmonolayer through a TEM grid [171]. The silicon waferwas previously treated with 3-(2-bromoisobutyryl) propyldimethylchlorosilane to form a monolayer that acts asinitiators for ATRP. The photoirradiation led to the decom-position of the initiator in the exposed areas. Subsequently,SI-ATRP of sodium methacrylate took place only on areasthat were not illuminated. Tugulu et al. synthesizedpoly(methacrylic acid) (PMAA) brushes by SI-ATRP frompatterned ATRP initiator derived using the same strategy[172]. In this case, the authors used the photolithographicpatterned PMAA brushes as a way to microstructure cal-cite films that are an exact 3D replica of the PMAAbrush. Mineralization experiments with the micropat-terned PMAA brushes were carried out and observed withAFM and reflected light microscopy. Recently, Mathieuet al. demonstrated the possibility to pattern a homoge-neous octadecylsiloxane monolayer with a focused beamof an Ar laser at � = 514 nm [173]. The methyl end-groupsof the stripes were aminated and subsequently coupledwith �-bromoisobutyryl bromide in order to trigger thepolymerization of NIPAAm. The lateral dimensions of poly-mer structures were from several micrometers down to thesub-100-nm range.

Prucker et al. immobilized an azo initiator on siliconsubstrate and placed a TEM grid with quadratic holes incontact with the monolayer [174]. The photopolymeriza-tion of styrene took place in the illuminated areas. Thesample was then carefully washed in order to remove anynon-bonded polymer.

Another approach to pattern polymer brushes wasdescribed by Husemann et al. [175]. Initially, poly(tert-butyl acrylate) brushes were synthesized from a layerof initiating groups on silicon surface (Fig. 22). Subse-quently, a solution of polystyrene containing bis(tert-butylphenyl)iodonium triflate (8 wt% wt PSt) was spin-coated onto the top of the brush layer to give a 1-�m-thicksacrificial photoresist layer. The surface was illuminated(� = 248 nm) through a mask, resulting in the photogener-ation of acid in specific areas of the polystyrene overlayer.At elevated temperature, photogenerated acid diffused intothe polymer brush layer and deprotected the tert-butylester groups to create poly(acrylic acid) chains. The sacrifi-

with an appropriate solvent. In this case, the authors elab-orated a way to produce binary patterned brushes, whichare extensively developed later in this document. On the

176 A. Olivier et al. / Progress in Polymer

Fig. 22. Illustration of the strategy to prepare binary polymer brushes viasacrificial photoresist layer and lithographic tool.

other hand, Fan et al. combined for the first time SI-ATRP and molecular assembly patterning by lift-off (MAPL)techniques in order to create micropatterning of graftedpolymer [176]. A photoresist was spin-cast and illuminatedthrough a mask by UV. After the development step, ATRPinitiator was immobilized between the circular domainsof photoresist. After removal of the physisorbed layer, SI-ATRP was performed from chemically adsorbed initiator inorder to polymerize methyl methacrylate macromonomerswith oligo(ethylene glycol) (OEG) side chains. The authorsdemonstrated the feasibility of this strategy to produce sur-face with cell-adhesive and cell-resistant regions.

Zhou et al. exploited another strategy that con-sisted in the covering of the entire gold surface withpoly(hydroxyethyl methyacrylate) brushes synthesized bygrafting-from technique [177]. Subsequently, the polymerlayer was passivated by a reaction with NaN3, and etchedwith UV irradiation through a TEM grid. After this treat-ment, the organic layer (initiator and polymer brush) onthe exposed area was completely removed. In this case, themicropatterning of grafted polymer was realized directlyon the polymer and not on the initiator layer.

3.2. Binary-patterned brushes

The formation of binary-patterned brushes has beenless widely studied than patterned homopolymer brushes.The papers report essentially their synthesis without much
attention to their properties. This field remains extremelyopen in terms of applications with the possibility tocarefully design grafted surfaces supplemented by a finemodulation of polymeric species. The first synthesis of
Science 37 (2012) 157– 181

binary-patterned polymer brushes was reported by Tovaret al. in 1995 [178]. Homogeneous PS grafted film wasobtained by thermally induced free radical polymerization.The patterning of the layer was established using appropri-ate mask and deep-UV ablation. Subsequently, bare areasof silicon surface were backfilled with initiator and usedto promote a second polymerization. A few years later,Husemann et al. investigated for the first time the combi-nation of NMP and a photolithographic approach (Fig. 22)[177] in order to develop binary-patterned brushes. Thesynthesis of tert-butyl acrylate was achieved from an ini-tiator anchored on silicon surface. As previously explained,a sacrificial photoresist layer deposited on top of polymerbrushes was exposed to 248-nm radiation through a mask.The resulting photogenerated acid modified the intrinsicnature of the poly(tert-butyl acrylate) to give poly(acrylicacid) brushes.

Maeng et al. investigated the formation of particu-lar binary-patterned brushes comprised of PS-b-PMMAmicropatterns on a polystyrene layer [179]. The polymer-ization of styrene was realized from a mixed aromaticamide monolayer composed of active and inactive end-groups. The patterning of the PS layer was carried out withan electron beam through a TEM grid, which led to thecleavage of the bromide end group of PS chain. The binarybrushes are obtained after the synthesis of PMMA from theremaining bromide group.

Zhou et al. exploited another option, which consistedof passivating homogeneous grafted polymer chains withNaN3/DMF, followed by the UV irradiation of the layer[177]. After the complete removal of organic materials onthe exposed area, a second initiator deposition followed bya subsequent polymerization was carried out.

Xu et al. demonstrated the feasibility of preparingbinary-patterned brushes using two types of initiatorcapable of developing different controlled polymeriza-tions. First, the immobilization of ATRP and NMP initiatorson a specific area of a resist-patterned Si (1 0 0) sur-face was performed [180]. Then, the SI-NMP of styreneand SI-ATRP of sodium 4-styrenesulfonate were initi-ated from the corresponding areas. Later, binary-patternedbrushes were micropatterned using photomask and inthe absence of any microlithographic resist used in theprevious report [181]. This time, the ATRP initiator wasput down on the specific areas followed by the poly-merization of sodium 4-styrenesulfonate. Subsequently,the RAFT initiator was assembled and used to synthe-size poly(hydroxyethyl methacrylate). Recently, Olivieret al. immobilized ATRP initiator using microcontact print-ing technique and performed the synthesis of PDMAEMAbrushes, a stimuli-responsive polymer. Then, the samplewas immediately immersed in a ROP initiator solution inorder to backfill the bare gold area. Finally, the ROP of l,l-Lactide was achieved between two PDMAEMA areas. Thecombination of a semi-crystalline and a stimuli-responsivepolymer in a binary-patterned brushes system was therebyconsidered for the first time [31].

Liu et al. established a unique method to producebinary-patterned brushes that is not based on any irradi-ation technique (Fig. 23) [182]. Authors deposited a thinlayer of polystyrene on a layer of macroinitiator previously


Fc

araacwdtptwicniboaTius

[2] Zhou F, Huck WTS. Surface grafted polymer brushes as ideal

ig. 23. Schematic diagram of the fabrication of binary polymer brushesreated by capillary force lithography and SIP.

nchored on a surface. By the way of capillary force lithog-aphy (CFL), a PDMS mold was placed over the PS filmnd the complete assembly was annealed in an oven with

temperature superior to the glass temperature. After aooling step, the PDMS mold was peeled off the surfacehich generated patterned PS structures due to selectiveewetting of the PS film. Consequently, a part of macroini-iator layer was available and able to synthesise the firstolymer brush. Then, PS patterns were removed by solventreatment and the second polymer brush, poly(PEGMeMA)as produced. Using a similar strategy, Konradi et al.

ntroduced an initiator system that can be used for twoomplementary patterning pathways [183]. A homoge-eous monolayer of chlorosilane derivative containing azo

nitiator end-group was chosen for its possibility to cleaveoth thermally and photochemically. The substrate previ-usly covered by the first monomer solution (methacryliccid) was in direct contact with a different TEM grid.he first polymer brush was grown through photochem-
cal initiation in the unprotected areas. After extraction ofngrafted polymer with good solvent, a second monomerolution (hydroxyethyl methacrylate) was placed on the
Science 37 (2012) 157– 181 177

substrate. The still-intact initiator molecules were thencleaved thermally at 60 ◦C to cover the uncoated area withthe second polymer brush.

In term of applications, Liu et al. prepared binary pat-terned brushes directly via microcontact printing [184].One type of brush is composed of poly(oligoethylene gly-col) methacrylate (POEGMA) terminated with a hydroxylgroup that is able to attach biomolecules. Anothertype of POEGMA brush is terminated with a methoxylgroup that has non-fouling behavior. The careful con-trol over interactions of biological organisms withsynthetic interfaces is particularly important for bio-analytical applications. Otherwise, Liu et al. elaboratedbinary patterned polyelectrolyte brushes from con-tact printing initiator monolayer for selective electro-less deposition (ELD) of metals [185]. Authors syn-thesized positively charged poly(methacryloyloxy)ethyl-trimethylammonium chloride (PMETAC) and negativelycharged poly(methacryloyl ethyl phosphate) (PMEP)brushes. After synthesis, catalytic-active metal ions wereloaded and used for ELD of two different metals, i.e. Cu andNi.

4. Conclusions

In summary, we discussed recent advances in surface-initiated polymerization (SIP) from gold and silicon surface,emphasizing polymer brush growth from homogeneousand heterogeneous monolayers. This issue encompassesthe design of micro- and nanopatterned structures throughvaried lithographic techniques as well as the formation ofpolymers and block copolymers. Owing to the rich vari-ety of polymerization techniques, this contribution hascovered the main features of controlled chain growthused in SIP and further accomplishments regarding thecontrol of the density, chain length and nature of molec-ular brushes. Finally, the ability to create well-definedarchitectures with specific functional coatings opens newperspectives in materials science and provides new chal-lenges for chemists’ imaginations.

Acknowledgements

The authors are grateful to the “Région Wallonne” andEuropean Community (FEDER, FSE) in the frame of “Pôled’Excellence Materia Nova” for their financial support.CIRMAP thanks the “Belgian Federal Governement OfficePolicy of Science (SSTC)” for general support in the frameof the PAI-6/27. A. Olivier thanks F.R.I.A. for her PhD the-sis grant. J.-M. Raquez is “chargé de recherches” from theF.R.S.- FNRS.

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Surface-initiated controlled polymerization as a convenient method for designing functional polymer brushes: From self-assembled monolayers to patterned surfaces