Thin Solid Films 539 (2013) 201–206

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Morphology response by solvent and vapour annealing usingpolystyrene/poly(methyl methacrylate) brushes

Iñaki Zalakain, Nikolaos Politakos, Raquel Fernandez, Haritz Etxeberria, Jose Angel Ramos,Mª. Angeles Corcuera, Iñaki Mondragon, Arantxa Eceiza ⁎‘Materials + Technologies’ Group, Dpto Ingeniería Química y Medio Ambiente, Escuela Politécnica, Universidad País Vasco/Euskal Herriko Unibertsitatea, Pza Europa 1,20018 Donostia-San Sebastián, Spain

⁎ Corresponding author. Tel.: +34 943017185; fax: +E-mail address: arantxa.eceiza@ehu.es (A. Eceiza).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.05.097

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 July 2012Received in revised form 11 May 2013Accepted 13 May 2013Available online 30 May 2013

Keywords:Polymer brushesPolystyrenePoly(methyl methacrylate)Atomic force microscopy

The aim of this work is to evaluate the switching behaviour of polymer brushes. Polystyrene (PS) andpoly(methyl methacrylate) (PMMA) were grafted onto a silicon surface by radical polymerization wheremolecular weight of PMMA to be higher than molecular weight of PS. The morphology of polymer brusheswas switched by modifying the surrounding environment using solvent and vapour annealing treatments.As selective solvent for PMMA acetic acid was chosen and cyclohexane at 55 °C for PS. Using acetic acid assolvent or its vapours PS/PMMA brushes adopted dimple-like morphology composed mostly by PMMAchains resulting in contact angle values (~74° and 76°, respectively). In the case of cyclohexane, surfacetopography changed and the outermost layer was occupied by PS and PMMA chains. In this case, the topog-raphy adopted a slightly rough surface. In addition, PS/PMMA showed a smoother surface after treated withcyclohexane vapour annealing due to poor solubility of PS in cyclohexane at room temperature. Contact anglevalues changed from 85° with cyclohexane solvent annealing to 80° with cyclohexane vapour annealingsuggesting that switching behaviour did not completed.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nanometer-scale structures production has great interest in thenanotechnology for use in different applications. There are twomain ways to achieve this kind of structures with nano-scales dimen-sion: the top-down and the bottom-up methods [1]. In the case of thetop-down, the most common technique is the photolithography. Inthis case patterns are generated by using lithographic masks andstructures are assembled by adopting the shape of the masks. Thismethod has physical limitations because it is difficult to obtain fea-tures below 100 nm [2–4]. For that reason, bottom-up methodshave gained interest. Promising materials to achieve nanostructurein the range of 10–100 nm are block copolymers [5–7]. The abilityof the block copolymers to self-assembly has been used to producetemplates of periodic arrays of holes and dots in silicon wafer surfaces[8,9]. However, block copolymers are not suitable in some applica-tions because they are not chemically bonded to the surface. Byusing these nanostructures in solution, the block copolymer filmwill remove from the substrate. In these conditions, polymer brushesare promising candidates, because they are linked by covalent bondsto the substrate [10–12]. Polymer brushes can be used also as substratesfor block copolymer specific arrangement. Changing substrate-filmand air-film interactions, variations in block copolymer orientation

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morphologies can be achieved. Thus, the interaction substrate-filmcan be modified and the block copolymer microdomains orientationcan be controlled inducing different rearrangements [9].

Polymer brushes consist of polymeric chains anchored covalentlyonto the surface with high grafting density. In the most of cases, thebrushes are composed by two different homopolymers. These poly-mers show different properties which depend on the environment.The switching behaviour can be carried out by using solvents, vapours,changes in humidity, pH and temperature or for polyelectrolyticbrushes when an external electric filed is applied [10,11]. Polymericchains under favourable conditions adopt extended configurationswhereas under non-favourable conditions the polymeric chainsadopt collapsed configurations. Thus, if the polymeric brushes areunder polar environment influence, the hydrophilic chains adopt ex-tended configuration oriented in the outermost layer of the filmwhile the hydrophobic chains collapse in the bottom of the surface.Therefore, the surfacewill showhydrophilic behaviour (Fig. 1a). In ad-dition under non-polar environment the hydrophobic chains wouldadopt extended configuration whereas the hydrophilic chains wouldcollapse. In this case the surface will show hydrophobic behaviour(Fig. 1b). The switching behaviour of polymeric brushes in differentenvironments leads to different structures and is reversible manytimes because the polymers are grafted to the substrate with covalentbonds [10,11].

Polymer brushes can be obtained by two different methods:“grafting to” and “grafting from” [10–13]. In the “grafting to” method,

Fig. 1. Schematic illustration of polymer brushes in different environment a) structurein selective solvent for PMMA and non-selective for PS, and b) in selective solvent forPS and non-selective for PMMA.

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an inorganic surface with appropriate reactive groups is reacted witha previously synthesized polymer. One of the advantages of “thegrafting to” method is the polymer’s monodipersity and the easyway to graft different kinds of polymers onto different substratesbut two major drawbacks of this method are related to diffusionproblems and the low grafting density [10,14]. In the “graftingfrom” method, an initiator is immobilized on a surface and polymer-ization is carried out directly from the substrate. In the “graftingfrom” method, the viscosity of the system is lower and monomer dif-fusion is better than the “grafting to” method. For this reason, byemploying this method, higher grafting densities can be obtained.One disadvantage of this method is the difficulty to control well thepolymerization conditions. So the properties of these brushes arestrongly connected with the less homogeneity of the achieved poly-mers which are usually polydisperse. Monodisperse polymers canbe obtained by using living radical polymerization and atom transferradical polymerization but reaction is more complex to be wellcontrolled [15,16].

In the “grafting from” method, during the polymerization twokinds of radicals are generated: one kind of radical is bonded to thesurface which can generate surface grafted polymer and the otherradical which is present in the solution generate free polymer. Someauthors suggest that grafted and free polymers have similar molecu-lar characteristics. The free polymer can be used to determine molec-ular characteristics of the grafted polymer [17].

In the present work, poly (methyl methacrylate) (PMMA) as polarand polystyrene (PS) as non-polar homopolymer were chosen toevaluate their response under different environments. 4,4-azobis(4-cyanopentanoic acid) (ABCPA) initiator was bonded to a siliconwafer substrate using 3-glycidoxypropyl trimethoxysilane (GPS).Polystyrene and poly (methyl methacrylate) chains were grown bymeans of radical polymerization.

2. Experimental section

2.1. Materials

The monomers that were used in the present work were styreneand methyl methacrylate and distilled under reduced pressure overcalcium hydride, in order to further purify from impurities. Siliconwafers (100) were purchased from Si-Mat (Germany) and employedas surface for the grafting. Toluene, tetrahydrofuran (THF), 1,4-dioxane,dichloromethane and dimethyl sulfoxide (DMSO)were used as solvent.

The organosilane used was GPS. Pyridine was used as received. Theselected initiator, ABCPA, was stored at 4 °C and used as received with-out further purification. All reagents were purchased from Aldrich(Germany).

2.2. Graft polymerization

In order to carry out the graft polymerization of PS and PMMA, theselected initiator must be chemically anchored to the substrate. Foranchoring the initiator to the surface the method employed bySidorenko was followed [10]. A piece of 2 × 2 cm2 was cleaned withdichloromethane in an ultrasonication bath for 15 min at 30 °C. After-wards the silicon wafer was immersed for 25 min at 80 °C in a mix-ture of water/ammonia aqueous solution (25% v/v) and hydrogenperoxide (30% v/v) with a volume ratio 6:1:1. In this way nativeoxide (SiO2) was removed and hydroxyl groups (–OH) were createdin the silicon wafer surface. The silicon wafer was rinsed severaltimes with ultrapure water (resistivity) and dried under nitrogenflow.

Firstly, organosilane was immobilized on the surface to subse-quently anchor the initiator. The silicon wafer was introduced in asolution containing 5 wt.% GPS in toluene for 8 h at 80 °C. Then, thesilicon wafer was cleaned with methanol and dried under nitrogenflow. Secondly, organosilane functionalized silicon wafer was intro-duced in a solution of 2 wt.% ABCPA in DMSO with a catalytic amountof pyridine and the mixture was maintained for 5 h at 50 °C undernitrogen (N2) atmosphere. The sample with the attached initiatorwas rinsed several times with THF and dried under nitrogen flow.

The PS and PMMA radical polymerizations were carried out in twosteps through thermal initiation at 60 °C. ABCPA is a thermal initiatorwith a half-life of 10 h at 60 °C. Initiator slow decomposition allowedperforming both polymerization reactions [18]. Polymerization ofstyrene took place in a 1,4 dioxane solution (1:1 v/v) at 60 °C for17 h under inert (N2) conditions. Ungrafted polymer was precipitatedin methanol and non grafted polymer was removed from the siliconwafer surface rinsing several times with THF. This allowed removingall not covalently attached polymer chains formed in the solution.After that, the silicon wafer was dried with nitrogen flow. After PSpolymerization, PMMA was grown from the surface with residual ini-tiator which was not activated during the first reaction. The siliconwafer was introduced in a solution of methyl methacrylate (1:1 v/v)in 1,4 dioxane at 60 °C for 8 h under inert conditions. In order toremove the ungrafted polymer the same procedure as in the PS casewas followed. As a reference, brushes of PS and PMMA homopoly-mers were also prepared using the same polymerization conditions.

2.3. Acetic acid solvent treatment

After polymerization, PS/PMMApolymer brushes were immersed inacetic acid for 30 min at room temperature. The sample was removedfrom the solvent and quickly dried under a nitrogenflow.Water contactangle measurements and atomic force microscopy (AFM) were used toanalyze the sample hydrophobicity and topography.

2.4. Cyclohexane solvent treatment

The sample was immersed in cyclohexane at 55 °C for 30 min.Afterwards the sample was removed from the solvent and immedi-ately dried under a nitrogen flow followed by surface characterizationwith contact angle measurements and AFM.

2.5. Acetic acid vapour treatment

The sample vapour annealing was carried out to check the vapourinfluence in the brushes topography. The polymer grafted wafer wasplaced in a petri dish and then was put inside an Erlenmeyer at

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room temperature which was filled by acetic acid on the bottom.Then, the Erlenmeyer was closed and the ambient was saturatedwith the vapour. The sample was kept inside for 24 hours. Afterthat, brushes were characterized by contact angle measurement andAFM.

2.6. Cyclohexane vapour treatment

Polymer brushes were exposed to saturated cyclohexane vapoursin a closed vessel keeping at room temperature for 24 h. The samplewas removed from the vessel, followed by water contact angle mea-surements and atomic force microscopy measurements to analyzethe sample topography.

The switching behaviour of PS/PMMA brushes in response to dif-ferent environments was evaluated by means of water contact anglemeasurements using Data Physics OCA 20 contact angle systems.Measurements were performed at room temperature. In order toobtain representative results five independent measurements wereperformed for each system. Measurements on the surface were car-ried out after 1 min to achieve the equilibrium of the water drop.The surface morphology resulting from different treatments wasinvestigated by atomic force microscopy. AFM images were obtainedoperating in tapping mode with a scanning probe microscope(Nanoscope IV, Dimension 3100 from Digital Instruments, Veeco)equipped with an integrated silicon tip/cantilever having a resonance

Fig. 2. AFM images of PS/PMMA brushes after solution treatment with: a

frequency of ~300 kHz, from the same manufacturer. In order toobtain repeatable results, different regions were scanned. Similarimages were obtained demonstrating the reproducibility and homo-geneity of the results. The molecular weight of the free polymerswas analyzed by size exclusion chromatography (SEC) with aPerkinElmer chromatograph equipped with a binary pump and arefractive-index (RI) detector. Tetrahydrofuran was used as a solventand separation was carried out through four columns packed withparticle gels of different nominal pore sizes. Elution rate was of1 mL min-1 at 30 °C. The molecular weights were based on a calibra-tion curve from monodisperse polystyrene standards.

3. Results and discussion

PS/PMMA brushes were synthesized by radical polymerizationusing azo-initiator. The attached azo-initiator produces two differentradicals: radicals bonded to the substrate which create surface graftedpolymer, and other radicals which diffuse to the solution and produceungrafted (free) polymer. Some aliquots of ungrafted polymer wereextracted during reaction to be analyzed by SEC. Molecular weightof the polymer brushes after complete polymerization was analyzedby SEC. The free polymer obtained from the solution was assumedto have similar molecular weight to grafted polymer as it is referredin literature [19,20]. There is some controversy about this issue.Some authors considered that molecular weight of the grafted

) acetic and b) cyclohexane. Height (left) and phase (right) images.

Table 1Contact angle values after treatments.

Brushes Treatment Contact angle (Θ) Image

PS – 93°

PMMA – 64°

PS/PMMA AA solution 74°

PS/PMMA Cyclohexane solution 85°

PS/PMMA AA vapour 76°

PS/PMMA Cyclohexane vapour 80°

Fig. 3. AFM images of PS/PMMA brushes after vapour annealing with a

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polymers are smaller than for the free polymer [21,22]. However,other authors reported the same values for grafted and ungraftedpolymers [23,24]. There are some kinetic schemes for the grafting po-lymerization which suggest free and grafted polymer molecularweights show similar weight [25,26]. We will consider that there isa relationship between grafted and ungrafted polymer molecularweight and it can be used as reference to estimate molecular weightof grafted polymer. SEC analysis showed that number molecularweights (Mn) at the end of the reaction for ungrafted PMMA and PS(and therefore for grafted polymers) were 157,000 and 116,500 g/mol,respectively. Taking into account that molecular weight is directlyrelated with chain length, it was assumed that PMMA chains werelonger than PS chains.

It is well known that incompatible polymer brushes show phaseseparation on a nanometer scale depending on the surrounding envi-ronment and brushes distribution within the monolayer are found tochange with the solvent to which the brush is exposed [10,27,28].When a polymer chain is exposed to a selective solvent, it swellsadopting an extended conformation and it is located on the outer-most layer. On the contrary, when a polymeric chain is exposed to anon-selective solvent, it collapses in the bottom of the monolayer.In this work, acetic acid was selected as a selective solvent forPMMA and non-selective solvent for PS, and cyclohexane was select-ed as a selective solvent for PS and non-selective solvent for PMMA

) acetic acid) and b) cyclohexane height (left) and phase (right).

Fig. 4. Cross-sectional profile comparison of solution and vapour treatment with a) acetic acid and b) cyclohexane.

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(solubility parameters: δCyclohexane = 16.9 MPa1/2; δAcetic acid. =20.7 MPa1/2; δPS = 17.52 MPa1/2, δPMMA = 22.69 MPa1/2 [29]). Aftersolvent treatments, the wafer was quickly dried under nitrogenflow. The time employed for drying was much smaller than the nec-essary for switching morphology. Thus, it is possible to assume thatthe morphology in dry state is the same as under solvent [18]. Thesame solvents were employed for analyzing switching brushesbehaviour with vapour annealing treatment.

Because acetic acid is a selective solvent for PMMA and anon-selective solvent for PS, when PS/PMMA brushes are immersedinto acetic acid solution it is expected that PMMA chains tend toorientate in the outermost layer. In Fig. 2a, dimple-like morphologyis observed from AFM image. Lighter and darker regions in the topo-graphic image are related with elevations and depressions. In thephase image, phase contrast among different areas on the micro-graphs is caused by differences in material stiffness. From the AFMphase image, it is believed that both polymers are located in theoutermost layer. Such behaviour can be detected by contact anglemeasurement (Table 1).

The contact angle measurement for the PMMA/PS brushes was~74°. In addition the contact angle value measured for the PMMAhomopolymer brushes was ~64°. This value suggests that mostlyPMMA segments were present on the surface. Taking into accountAFM image and contact angle value, it was assumed that the outer-most layer was composed by mainly PMMA chains but with also PSchains. However, it should be noted that the application of watercontact angle measurements for analysis of surface composition islimited and not conclusive in this case due to the high surface rough-ness of dimple structures.

Then, PS/PMMA was treated with cyclohexane at 55 °C for 30 minleading to smoother topography. The phase image of the polymerbrushes did not show clear contrast between the two phases(Fig. 2b). The measured contact angle value was 85°, indicating achange on the surface composition (Table 1). A value of 93° was mea-sured on a surface consisting from PS homopolymers brushes, higherthan it was obtained with this treatment. As it is mentioned above,taking into account that PMMA chains were longer than PS chains,it is expected that they affect the outermost layer of the polymerbrushes. Although PS chains should be located on the top, they werenot able to completely overlap PMMA chains. Therefore, the AFMimages and contact angle value suggest that the surface is occupiedby both polymers.

The influence of the other surrounding environments in thepolymer brushes was examined using vapour annealing treat-ments. It was expected that vapour annealing should induce simi-lar morphologies than those treated with solutions. Same solventswere chosen to verify this assumption. Sample was kept for 24 hin an acetic acid saturated media and the surface was observedby AFM (Fig. 3a). AFM images show a topography which wasalmost the same as shown in Fig. 2a. Slight differences can be

observed between both treatments. Both surface showed dimple-like morphology but PS/PMMA brushes showed smaller domainsusing vapour annealing. Brushes surface contact angle measure-ment was 76°, a value quite similar to the measured with solutiontreatment (Table 1).

In order to study the influence of liquid and vapour treatment inPS/PMMA brushes surface, both cross sections were compared(Fig. 4a). Acetic acid solution treatment show height differencesbetween features around ~8 nm meanwhile using acetic vapourthe brushes displayed cross section height differences of ~6 nm.Taking into account the contact angle and topographic analysis, theresults suggested that the treatments with acetic acid solution aswell as vapour induce equal switching behaviour in the PS/PMMAbrushes surface.

However, cyclohexane vapour treatment changed totally the brushessurface as it is shown in Fig. 3b. AFM image shows a smooth surfacemorphology with few amounts of dark regions. Comparing solutionand vapour treatments for height and phase images, it can be observedthat the surface changed from rough to smooth. Contact angle resultsfor vapour treatment show a value of 80° meanwhile using cyclohexanesolution the value was 85° (Table 1). Cyclohexane vapour treatmentchanged the PS/PMMA surface to more hydrophilic than with solutiontreatment. Comparing sections after liquid and vapour treatments(Fig. 4b), the domains height differences of the morphology was halved,from ~3.2 nm to ~1.5 nm. In spite of high vapour pressure of cyclohex-ane at room temperature, the differences in surface morphology be-tween solution and vapour treatment could be explained because ofthe poor solubility of PS in cyclohexane. PS is soluble in cyclohexane attemperatures above 45 °C. So, cyclohexane vapours could not inducemovement of the PS chains in order to switch PS/PMMA brushesmorphology.

4. Conclusions

In this study, PS/PMMA brushes with different molecular weight(Mn PMMA >Mn PS) were synthesized by radical polymerization ontoa silicon wafer. Affinity of brushes to different selective treatmentdemonstrated different surface morphologies which they werestudied by AFM and contact angle measurements. Acetic acid solutiontreatment (selective for PMMA and non-selective for PS) triggereddimple-like morphology using solution and vapour annealing. However,for cyclohexane solution treatment (selective for PS and non-selectivefor PMMA) a rough surface was observed, nevertheless its vapour arenot be able to switch the surface.

Acknowledgments

Financial support is gratefully acknowledged from the Basque Coun-try Government in the frame of Grupos Consolidados (IT776-13) and

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from the University of Basque Country (PES09/12). The authors alsothank the technical and human support provided by SGIker (UPV/EHU).

Dedication

This article is dedicated to Professor Dr Iñaki B. Mondragon, whopassed away just after his contribution to this work and who foundedthe research group “Materiales + Tecnologías” (GMT) in 1988.

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