Thin Solid Films 519 (2011) 6629–6636

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Nanopatterns of ABA triblock copolymer thin films formed on water surface

Yuan Zhou, Xia Han ⁎, Jun Hu, Yongmin Huang, Honglai LiuKey Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China

⁎ Corresponding author: Tel./fax: +86 21 64252921.E-mail address: xhan@ecust.edu.cn (X. Han).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.04.238

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 November 2010Received in revised form 27 April 2011Accepted 30 April 2011Available online 7 May 2011

Keywords:Surface patterningPhase separationThin filmsSurface morphologyBlock copolymersAtomic force microscopy

Nanopatterns of hydrophobic triblock copolymer SEBS thin films formed on a water surface by using dropspreading casting and dilute solution casting methods have been studied. It is found that the surfacemorphologies of thin films, as well as the spreading behavior of polymer solutions on water, depend stronglyon the selectivity of the solvent and the functional group which interacts with the water subphase. Theresulting nanopatterns were examined in terms of the relative interaction parameter (Δχ) and the copolymervolume fraction (ϕ) in the solution, based on the physics of solvent annealing and evaporation.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the past decade, block copolymers have been extensivelystudied in terms of their self-assembly capabilities and microphaseseparation, which result in an extraordinarily rich and complexpanoply of morphologies. To study the surface morphology ofcopolymers, much attention has been paid to thin polymer filmsdeposited directly on solid substrates such as silicon or mica. Surfacepatterns of block copolymer films deposited on liquid substrates arealso interesting and important, though less studied. This is becausethe surfaces of liquids are highly homogeneous chemically and havevery low roughness. Additionally, unlike solids, some liquids have freeinterfaces where physical and chemical species can be introduced.

One major challenge of films on liquid substrates is the difficulty ofobtaining equilibrium morphologies. Successful samples are limited toeither amphiphilic copolymers [1–6]or hydrophobic copolymers inverydilute solutions [7–11]. Water is mostly chosen as a liquid substrate tostudy the self-assembly of amphiphilic copolymers. Eisenberg and co-workers [1] were the first to study ionic diblock copolymers at the air/water interface. They demonstrated that the quarternized polystyrene-block-poly-4-vinylpyridin (PS-b-P4VP) diblock copolymers are able toform surface aggregates spontaneously at the air/water interface, and itsself-assembled surface patterns could be controlled by varying thecompositionsof theblockcopolymers [1,2]. Additional two-dimensional

surface aggregates have been obtained from non-ionic diblock co-polymers [3,4] and more complex polymers [5,6].

It is more difficult to get equilibrium morphologies of blockcopolymers on liquid substrates when both blocks are hydrophobic.The only example in this case was for casting from very dilutesolutions to make monolayer films. Kumaki et al. [7] used theLangmuir–Blodgett technique to obtain individual polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer chains fromhighly dilute solution. Two-dimensional monolayer patterns of poly(methyl methacrylate)-block-poly(octadecyl methacrylate) (PMMA-b-PODCMA) diblock copolymer have also been achieved by thismethod [8]. Otherwise, films obtained from semidilute solutions areunlikely to be in equilibrium. Mansky et al. [9,10] demonstrated thatby depositing a drop of polystyrene-b-polybutadiene (PS-PB) diblockcopolymer solution on a water surface, the copolymer self-assembledinto a hexagonally packed array of cylinders oriented perpendicularlyto the film surface. Thick films can also result from semidilutesolutions. Han et al. [11] reported that for thick polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) triblock copolymerfilms cast on the surfaces of aqueous Gemini surfactant solutions,various microphase separation patterns were induced by differentsurfactants.

The nature of the solvent is another vital factor in patternformation. The conformations of polymer chains in dilute polymersolutions are distinct from those in more concentrated solutions. Inthe very dilute solutions, polymer coils are separative and can beconsidered to possessing single chain conformations [7], whereaspolymer chains penetrate into each other in semidilute solutions andare often used in film preparation [9–11].

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For block copolymers, the interactions between blocks and solventare more complex. The chain conformations of different blocks insolutions as well as the phase separation patterns in films depend onthe selectivity of solvent. Moreover, many research groups’ worksshow that equilibrium films can be obtained by using solventannealing and solvent-induced film morphologies [12–15]. In thiswork, the self-assembly microstructures of linear triblock copolymerSEBS (one of commercial Kraton polymers) thin films cast on waterwere studied. As a high performance thermoplastic elastomer, it has awide spectrum of end use. Its microphase separation behavior hasbeen widely investigated [15–17]. As mentioned above, it is not easyto obtain equilibrium hydrophobic block copolymer thin films onliquid substrates. We therefore adopt two simple methods, i.e., dropcasting with solvent annealing and dilute solution casting, to produceequilibrium morphologies. To investigate the interactions betweenwater substrate and copolymers, we took chemically modified SEBSby 2 wt.% of maleic anhydride randomly grafted on rubber segments,SEBS-g-MA, for comparison with the original SEBS. It is shown thatsmall amount of hydrophilic functional group in SEBS-g-MA candrastically change the spreading behavior of the polymer solution onwater, as well as the resulting surface patterns of correspondingthin films. A model based on the physics of solvent annealing andevaporation is used to explain the effects of solvent selectivity on thespreading behavior and nanopatterns formation.

2. Experimental section

2.1. Materials

Two triblock copolymers, SEBS (Kraton G-1650) and SEBS-g-MA(Kraton FG-1901), were obtained from the Shell Development Co. andused as received. The number-averaged molecular weight (Mn), thepolydispersity index (Mw/Mn,) and the weight fraction of polystyrene(PS) in SEBS were 8.78×104, 1.27 and 29%, respectively, while thosefor SEBS-g-MA were 7.91×104, 1.11 and 29%, respectively. Toluene,benzene and n-heptane, all of them are analytical grade, werepurchased from Shanghai Chemical Reagent Co., China, and wereused without further purification.

2.2. Film preparation

Triblock copolymers SEBS and SEBS-g-MA were dissolved intoluene, benzene and n-heptane to prepare 1.0% w/v and 1.0×10-3%w/v solutions, respectively (hereinafter denoted by polymer/solution,e.g. SEBS/toluene). The triblock copolymer thin films at the air/watersurface were obtained by the following two methods.

2.2.1. Drop castingA 10-μL drop of copolymer solution (1.0% w/v) was deposited from

a pipette onto the deionized water surface at room temperature. Foras-cast films, 15 minwere allowed for solvent evaporation before theywere transferred directly onto freshly cleaved mica (~1 cm2). Forsolvent annealing, the spreading thin film on the water surface wasexposed to the corresponding solvent vapors in closed vessels at roomtemperature. After a certain exposure time, the sample was removedto an ambient atmosphere, picked up onto a freshly cleft micasubstrate and dried at room temperature under vacuum for further

Table 1The selectivity and spreading coefficient for solvents (20 °C).

Solvent VS cm3/mol ΔS (MPa) 1/2 χPS-solvent (MPa) 1/2 χ

Toluene 107 18.20 0.35 0Benzene 89 18.80 0.34 0n-Heptane 147 15.10 1.08 0

measurement. The water substrates were kept static during deposi-tion and annealing.

2.2.2. Dilute solution castingThe alternative approach was to cast a 10-mL dilute copolymer

solution (1.0×10−3% w/v) onto the deionized water in a beakerwhich was then kept static for evaporation. It took several days for thesolvent to slowly evaporate, and eventually a thin film was left onwater. Subsequently, as-prepared thin films were transferred onto afreshly cleft mica substrate and dried at room temperature undervacuum for measurement.

2.3. Surface morphology observation

The AFM topography images were obtained in constant repulsiveforce mode by a tapping mode atomic force microscopy (AFM) (AJ-III,Aijian nanotechnology Inc., China) which used a triangular micro-fabricated cantilever (Mikro Masch Co., Russia) with a length of100 μm, a Si pyramidal tip, and a spring constant of 48 N m−1. Aresonance frequency in the range of 240–400 kHz was used, andresonance peaks in the frequency response of the cantilever typicallyat 330 kHz were chosen for the tapping mode oscillation. Thescanning frequencies were usually in the range of 0.6 and 2.5 Hz perline. The measurements were carried out in ambient conditions.

3. Results and discussion

3.1. Effect of solvent

To carry out the experiment, water-immiscible solvents whosedensities are less than that of water are chosen tomake sure the polymersolution can be the upper phase while water the subphase. In solution,structural characteristics of the block copolymer can be characterized bythe relative affinity of solvent for each block: the polymer-solventinteraction parameter, χP−S (P=polymer and S=solvent). For non-

polar systems, χP−S = χH + χS =VS

RTδS−δPð Þ2 + 0:34, where VS is the

molar volume of the solvent, R is the gas constant, T is the Kelvintemperature and δ is the solubility parameter for solvent or polymer [18].If χP−S is less than 0.5, the solvent is considered as a good solvent inwhich the block chains are swollen. The solubility parameters of PS andPEB are 18.60 MPa1/2 and 17.20 MPa1/2, respectively [19]. Their corre-sponding polymer–solvent interaction parameters are calculated andlisted in Table 1. Toluene is almost a neutral solvent, because bothχPS-toluene and χPEB-toluene are smaller than 0.5 and close to eachother. Benzene is slightly selective with its χPEB-benzene near 0.5.Heptane is a nonsolvent for PS blocks and therefore, a stronglyselective solvent for SEBS. In a selective solvent, one block of thecopolymer is more swollen than the other.

3.2. Effect of drop spreading casting

Spreading of a liquid B over the surface of another liquid A happenswhen the initial spreading coefficient SB/A=γA−γB−γAB is positive[20]. Here γA is the surface tension of liquid A, γB is the surface tensionof liquid B, and γAB is the interfacial tension between these twoliquids. For pure liquid-on-liquid spreading, a thin (often referred toas a monolayer) precursor film extends beyond the main body of

PEB-solvent (MPa) 1/2 Δχ (MPa) 1/2 γB[20]mN/m SB/A[20]mN/m

.38 0.03 28. 52 6.8

.43 0.09 28. 88 8.8

.61 0.47 20.14 0.2 (30 °C)

Table 2Categorization of the morphologies observed in the thin SEBS and SEBS-g-MA filmsprepared from different solutions by different methods.

Solvent SEBS SEBS-g-MA

Drop spreading Dilute solution Drop spreading Dilute solution

Toluene – Cylinder Cylinder CylinderBenzene Cylinder Sphere Cylinder Nonen-Heptane Sphere None Sphere Sphere

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the lens, driven by tension gradients. [21] For toluene, benzene andn-heptane, the spreading coefficient onwater are 6.8 erg/cm2, 8.8 erg/cm2, and 0.2 erg/cm2, respectively (Table 1). These solvents canspontaneously spread on a water surface because of the positivespreading coefficient; however, their spreading liquid films willretract to lens after the water subphase is saturated with the organicsolvent [20]. By contrast, the spreading process of a polymer solutionon water is quite different from that of a pure solvent. The presence ofthe polymer chains in solution effectively prevent the solvent fromretracting to lens, which might because the chains tend to beelongated and “threads” in the shear flow [22]. A flat spreading thinfilm is left on the surface of water after rapid evaporation of thesolvent, unlike the broken film obtained by PS–PB diblock copolymersin the former work [9].

All the spreading films are obtained by this drop casting method.The only exception is the SEBS/toluene system which forms nouniform film but sticky mass on water because of the elasticity ofSEBS. In toluene, both PS and PEB blocks of the copolymer remainswelled and soft; after solvent evaporation, PS domains harden to lockthe elastomeric part in place while PEB blocks contract freely onwater. By contrast, uniform films can be obtained by SEBS-g-MA withonly 2 wt.% maleic anhydride randomly grafted on PEB block intoluene. The functional groups hydrolyze in the water subphase,improving adhesion at interface and preventing the film shrinking. Onthe other hand, the spreading films of SEBS, with and without graftedmaleic anhydride, from selective solvents, can easily form on waterwhere the conformation of block copolymers becomes a major factor.The collapse of one block in the selective solvents seems not in favor ofthe formation of elastomers. The spreading films will not shrink anduniform thin films are obtained. All the resulted morphologies aresummarized in the Table 2.

Fig. 1. AFM phase images (1×1 μm2) of thin film SEBS-g-MA/toluene by drop spreading andweek toluene annealing.

3.2.1. Casting and annealing in neutral solventTo observe morphologies of polymer film by AFM, a spreading film

has to be transferred onto a solid substrate from the surface of water.The films kept integral when they were transferred, indicating that theviscoelastic property of SEBS copolymer was remained. The stability ofthe transferred films has also been proved by Devereaus et al. [4].

Fig. 1 shows the morphology evolution of the SEBS-g-MA/toluenespreading films. The thin films formed by spreading polymer solutionswithin several seconds, freezing in a nonequilibrium state duringsolvent evaporation. As-prepared SEBS-g-MA thin films (Fig. 1a)showed worm-like cylindrical morphology with average dimensionsin the range of 30–40 nm, which was similar to the disordered patternof SEBS spin-coated films [17,18]. Equilibrium morphologies of thethin films (Fig. 1c) could be obtained after solvent annealing, which isserved as a strong, highly directional external field to control theorientation and lateral ordering of block copolymer films [12–14]. Thesolvent annealing method is suitable to be in situ applied to thespreading films on the surface of water and to keep the system static,contrasting with thermal annealing which is likely to disturb thewater. Fig.1b and 1c suggest that the ordering behavior of filmstructures is time-dependent. After one day annealing in toluene, thedisordered worm-like cylindrical patterns in the as-cast films changedto the cylindrical nanostructures oriented parallel to the surface(Fig. 1b). However, the parallel cylinders have not been in the wellequilibrium state as the cylinders are not regularly arranged anddefects can be seen between the striped patterns. With increasingannealing time (about one week), defects could be removed and thewell-ordered morphology with the diameter of about 31±1 nm wasobtained (Fig. 1c).

3.2.2. Casting and annealing in selective solventSurface morphologies of the spreading films cast from selective

solvents are shown in Fig. 2 and Fig. 3.The as-cast SEBS/benzene spreading film is far from equilibrium

and exhibits a disorderedworm-like cylindrical morphology as shownin Fig. 2a. Benzene is a slightly selective solvent for PS blocks. Due tothe unfavorable contact of PEB and benzene, the PEB blocks tend tocollapse but the overlapping of polymer chains in the concentratedsolution stops it. Therefore, cylinders of SEBS are still observed. It isalso manifested by the equilibrium nanostructure as shown in Fig. 2b.Annealed by benzene, the SEBS spreading film revealed cylindrical

its morphology evolution. (a) as-prepared, (b) one day toluene annealing, and (c) one

Fig. 2. AFM phase images (1×1 μm2) of thin films of SEBS/benzene by drop spreading: (a) as-prepared, (b) annealed in benzene vapor for one week, and thin film of SEBS-g-MA/benzene bydrop spreading: (c) as-prepared, and (d) annealed in benzene vapor for one week.

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microdomains oriented parallel to the water surface, with averagediameter of approximately 31±1 nm.

Spherical patterns are observed in the SEBS/n-heptane spreadingfilm as shown in Fig. 3a. Its average diameter is about 31±1 nm whilethe center-to-center array spacing is about 25±1 nm. The change ofequilibrium phase structure is in expectation because of the flowerlikemicelles formed in the solution of SEBS/n-heptane. In n-heptane, astronglyPEB-selective solvent, theouter PSblocks tend to form the coresofmicelles,while themiddle PEB blocks swell to form loops and becomea shield around the PS aggregates. The existence of the flowerlikemicelles has been proved by Quintana et al. They detected that thehydrodynamic radius of SEBS particles was larger in the PEB selectivesolvent than in the PS selective solvent, favoring the flowerlikemicelles[23]. The AFM image (Fig. 3a) shows that themicelles keep shapeduringsolution spreading and solvent evaporation and result in PS spheresembedded in the PEB matrix in the film. Equilibrium structures areexpected with n-heptane vapor annealing. The density of the spheredecreases and the center-to-center array spacing increases to 35–40 nmafter annealing (Fig. 3b). The decreasing density of equilibriummicrodomains is understandable, which is in accordance with theloose structure of micelles in the PEB selective solution. In the flower-likemicelles, various chain topologies such as dangling, loop, and bridgechains that are inmore equilibriumstatewill lead tomicelleswith largersizes and abroader sizedistribution [24]. It is alsoworthmention that, in

our case, thedarkdomains in theAFMphase images are attributed to theharder PS cores of the micelles while the bright matrix is attributed tothe soft PEB coronae. This does not agree with the idea which found theenergy dissipation of the AFMoscillating tip interactingwith the surfaceshould be larger on the harder materials which therefore appear asbrighter domains [25,26]. But reverse cases attributed the darkerdomains to the harder materials and the brighter ones to the softmaterials can also be found and are consistent with our case [27,28].Actually, the relative contrast inversion is quite common in the tappingmode AFM due to the force level and operating frequency setting[26,29].

For SEBS-g-MAspreadingfilms (Figs. 2c, d, 3c andd), the PEB blocksare likely to be attracted at the interface because of the grafted maleicanhydride hydrolyzing in water. This explains why the AFM imagesdisplay no microphase separation in the as-cast spreading film asshown in Figs. 2c and 3c. For SEBS-g-MA/benzene (Fig. 2c), the PEBblocks preferentially occupy the interface, while the PS blocks are leftdangling and stretching in solutionwith less contactwith thewater. Inn-heptane, SEBS associate to yieldmicelles. By spreading onwater, theinner PS cores would like to avoid the contact with both the solventand water by staying in the PEB shield, while the outer PEB coronaehave to face the competition between the adsorption of maleicanhydride and the attraction of solvent. Thus, the formation ofthe micelles is disturbed (Fig. 3c). The effect of grafted maleic

Fig. 3.AFMphase images (1×1 μm2)of thinfilmof SEBS/n-heptanebydropspreading: (a)as-prepared, (b) annealed inn-heptanevapor foroneweek, and thinfilmof SEBS-g-MA/n-heptanebydrop spreading: (c) as-prepared, and (d) annealed in n-heptane vapor for one week.

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anhydride groups is also supported by the annealing experiments. TheSEBS-g-MA thin films with benzene annealing show defects amongstparallel oriented cylinders with the diameter of about 31±1 nm(Fig. 2d). Compared with non-grafted SEBS, the adsorption of maleicanhydride on water surface disturbs the order of the microphaseseparation structure. Fig. 3d shows the intriguing outcomeof SEBS-g-MAfilm treated by n-heptane vapor where a denser order of the sphericalmicrodomains with both the average diameter and the center-to-centerarray spacing of about 28±1 nm, is induced, in contrast to the SEBSmicrostructurewithout adsorption interaction. Thepolymer chainsmusthave rearranged to meet the competition of water adsorption and thesolvent attraction during annealing.

3.3. Effect of dilute solution casting

The thin films cast on the pure water from different dilute solutionsare also prepared. In dilute solution, the initial polymer coils areseparative. During evaporation of solvent, the polymer solutionexperience a process from dilute to concentrate before the formationof solid thin films, in which the copolymer chains overlap andmicrophase separation between PS and PEB blocks happens. Theprocess is slow enough for the kinetically controlled morphologies todisappear, as the polymer chains have time to rearrange and theequilibrium states are reached finally. In addition, unlike the spreading

films, the casting thin films no longer exhibit viscoelastic property sincethefilmsbreakupwhen they are transferred. And solvents again play animportant role in the film formation procedure and result in differentmicrophase morphologies.

The results are shown in Fig. 4. The thin film cast from 1.0×10−3%w/v SEBS/toluene solution shows cylindrical morphologies orientedparallel to the surface with the average dimension of about 54 nm,much larger than that of the corresponding spreading films (Fig. 4a).Fig. 4b shows the film cast from SEBS-g-MA/toluene dilute solution.Mixed parallel and perpendicular orientated cylinders with anaverage dimension of about 42 nm can be observed. The mixedmorphology is attributed to the presence of hydrolyzed maleicanhydride groups, similar to the SEBS-g-MA/toluene spreading film.

Fig. 4c and d shows the SEBS and SEBS-g-MA films cast from dilutebenzene solutions, respectively. In Fig. 4c, a dense spherical nanostruc-turewithaveragedimensionof about42 nmisobserved in the SEBS thinfilm. It is interesting to note that the result is totally different from thecylindrical morphologies of the SEBS/benzene spreading film (Fig. 2b),which indicates the effects of initial chain states in the film formationprocesses. In the SEBS/benzene dilute solution, the polymer chainsseparate from each other. The slight selectivity of the PS block, i.e., theswelling PS blocks and the comparatively collapsing PEB blocks,becomes the major factor of chain conformation. It seems that thechain conformation is predominant in the evaporation process till the

Fig. 4. AFM phase images (1×1 μm2) of thin films by 1.0×10−3% w/v dilute solution casting: (a) SEBS/toluene (height image), (b) SEBS-g-MA/toluene, (c) SEBS/benzene, (d) SEBS-g-MA/benzene, (e) SEBS/n-heptane, and (f) SEBS-g-MA/n-heptane.

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latter stage of film conformation, as shown by the spherical microstruc-tures. In this case, the selectivity of solvent cannot be neglected. Fig. 4dshows that water impacts the formation of SEBS-g-MA thin film, whereonly large concaves rather than ordered nanostructures are observeddue to the attraction of the grafted maleic anhydride to the watersubphase together with the repulsion to the selective solvent.

Fig. 4e shows that SEBS/n-heptane film appears to be homoge-neous only with clusters on it. The association behavior of the SEBScopolymer in the PEB-selective solvent may be the key factor. In thecase of SEBS-g-MA/n-heptane system (Fig. 4f), spherical microdo-mains appear. Though less ordered, it still suggests a key role ofthe grafted groups on the substrate in the equilibrium morphologiesformation.

4. Discussion

The above experimental results can be summarized as follows. Forthe ABA triblock copolymers, the chain conformations in A-selectivesolvent (benzene) are stable and display well-ordered nanostructures,but they are easily disturbed when functional groups on the B blocksinteract with the substrate. On the other hand, micelles in B-selectivesolvent (n-heptane) are less stable and are difficult to form orderedsurface patterns, while this can be improved by the strong adsorptionsites on B blocks. This is true for both the spreading films and the dilutesolution casting films.

We also notice the differences in the microdomains size of the thinfilms cast by two methods. It appears that the domain size of the dilutesolution castingfilms are generally larger than that of the correspondingdrop spreading films. This is probably because the polymer chainsinterpenetrate less in the dilute solution casting films. The initialconformations of the chains in the solution and the following filmformation processes will determine how polymer chains are packed inthe films. The different viscoelastic properties of the thin films areanother manifestation. The drop spreading films exhibit viscoelasticproperty of SEBS which means the polymer chains interpenetratestrongly,while the dilute solution castingfilmsdonot and the chains arecomparatively segregated.

Various solvent induced morphologies of the films are of interest aswell. As we know, the physics of dilution of a block copolymer has beenstudied [30,31]. When solvent is added to a block copolymer composedof A and B two blocks, two factors govern the phase segregationbehavior: (1) the relative interaction parameter (Δχ), which representsthe difference between A block-solvent and B block-solvent interactionparameters χA-S and χB-S, (2) the copolymer volume fraction ϕ in thesolution.

Theeffect of solvent annealing canbe illustrated inFig. 5a. Theeffectivesegregation of the block copolymer solution is χeffN~ϕ(χAB+Δχ)N=ϕ(χAB+|χA-S−χB-S|)N, where N is the total chain length of blockcopolymer. For a perfectly neutral solvent (χA-S=χB-S), the solvent isuniformly distributed in the microdomains and χeffN~ϕχABN, whichmakes ϕ the solo factor of segregation. The neutral solvent annealing is to

Fig. 5. Schematic phase diagrams showing the effect of: (a) solvent swelling, (b) solvent evaporation. Addition of a neutral solvent is illustrated by the vertical trajectory L1, whereasadding a selective solvent (L2) amounts to the decrease of the composition feff. Evaporation is the inverse process of swelling, describing by L3 (neutral solvents) and L4 (selectivesolvents) respectively. The AFM height images show the cylindrical morphology (C) of the SEBS/benzene spreading film (c) and the spherical structure (S) of the SEBS/benzenedilute solution casting film (d). Images are 1×1 μm2 scales.

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simply dilute the monomer-monomer contacts and to reduce theeffective segregation, which is illustrated by L1 in the phase diagram.The effect of dilution with a selective solvent is more complicated, asχeffN~ϕ(χAB+Δχ)N=ϕ(χAB+|χA-S−χB-S|)N is decided by both ϕand Δχ. Moreover, the degree of segregation would increase if aB-selective (i.e. χB–SbχA–S) solvent is added. And the effective volumefaction of the A block, feff~ fAϕA, would decrease to minimize theA-solvent contacts. Due to both factors, selective solvent annealingmight cause a change of the equilibrium phase structure (L2). Bycontrast, the evaporation process can be considered as an inverseprocess shown in Fig. 5b. The removal of a neutral solvent increases theeffective segregation (χeffN) of the block copolymer (L3), while theremoval of a selective solvent increases both the effective segregation(χeffN) and composition (feff) (L4).

Therefore, we are able to understand the SEBS thin filmmorphologywith the help of the above discussion. Toluene is an almost neutralsolvent for SEBS (shown in Table 1), whose annealing and evaporationcan be described by χeffN~ϕχABN. The addition of toluene only dilutes

the contacts of PS and PEB, enables the rearrangement of polymerchains, but will not change its equilibrium phase structure (L1). And theequilibrium structure (cylindrical morphology) can be obtained whenthe solvent is removed (L3). Benzene is much more complex. As aslightly selective solvent for PS blocks, the phase segregation dependsnot only on the effective segregation (χeffN) but also on the effectivecomposition (feff). In the case of spreading film, the film keeps thecylindrical morphologies (Fig. 5c). Unlike toluene, the selectivity ofbenzene causes the decrease of the effective volume faction of the PEBblock (feff~ fAϕ). However, the reduction of feff is not large enough tochange itsmorphology from the initial cylindrical one to a spherical one(L2). The film that is deposited by evaporating a dilute solution shows adifferent situation. Thepolymers start fromthedisordered state and endup in the spherical morphologies (Fig. 5d). The increase of its effectivesegregation (χeffN) and composition (feff) during evaporation isillustrated by L4 in Fig. 5b. The n-heptane systems are also decided byχeffN and feff. N-heptane is a strongly PEB-selective solvent in whichmicelles are formed. Comparing with the benzene systems, the starting

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state of the spreadingfilm is in the sphericalmorphologywindowratherthan the cylindrical one, with its end state in the same phase windowupon annealing. The dilute solution casting film also ends up with thespherical morphology.

5. Conclusions

On the surface of water, two simple casting methods were adoptedto obtain equilibrium SEBS triblock copolymers thin films from solventswith different selectivities, and the surface patterns of thin films wereobserved by AFM. For the drop spreading method, the as-cast films arekinetically trapped but equilibrium microstructures can be obtainedupon solvent vapor annealing. In the dilute solution casting films,equilibrium microstructures are also expected but display largermicrodomain sizes. The SEBS-g-MA copolymers are used to study theeffect of water as substrate, since the functional groups greatly changethe spreading behavior and the surfacemorphologies as well. Solvent isanother vital factor offilm formation,which can be described in terms ofthe relative interaction parameter (Δχ) and the copolymer volumefraction in the solution (ϕ) from theory.

Acknowledgements

This work is supported by the National Natural Science Foundationof China (Project Nos. 20806022, 20976044, and 20736002), thecreative team development project of the Ministry of Education ofChina (No. IRT0721) and the 111 Project of THE Ministry of Educationof China (No. B08021).

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