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Full PaperMacromolecularChemistry and Physics

Random Poly(methyl methacrylate- co -styrene) Brushes by ATRP to Create Neutral Surfaces for Block Copolymer Self-Assembly

Hui Liu, Colm T. O’Mahony, Fabrice Audouin, Claudia Ventura, Michael Morris, Andreas Heise*

Random copolymer brushes of styrene and methyl methacrylate (MMA) on silicon wafers by atom transfer radical polymerization (ATRP) are synthesized using CuCl/CuCl 2 /HMTETA. It is found that with increasing amount of styrene the thickness of the brush layer could no longer be well controlled by the amount of free (sacrifi cial) initiator in the reaction. At constant con-centration of free initiator a constant thickness is obtained for various ratios of MMA to styrene. Within 30–70% MMA in the monomer feed the composition of the free polymer cor-responds well to the monomer feed ratio, displaying a water contact angle in agreement with the theoretical value for a random copolymer. These copolymers are shown to create a neutral surface directing spin-coated poly(styrene- b -MMA) into a perpendicular lamellae orientation.

1. Introduction

The decoration of surfaces with polymer brushes is a versa-tile approach to modulate surface properties. In particular, the use of radical polymerization offers almost unlimited possibilities due to the availability of numerous monomers with a large variety of properties and functional groups.

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H. Liu , F. Audouin , C. Ventura , A. Heise School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland E-mail: andreas.heise@dcu.ie H. Liu College of Chemistry and Chemical Engineering, State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, P. R. China C. T. O’Mahony , M. Morris Department of Chemistry, University College Cork, Cork, Ireland M. Morris Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin, Ireland

Most effi cient is the “grafting from” approach in which a polymer chain is grown from a surface tethered initiator. Following this approach, Prucker and Rühe [ 1 , 2 ] applied free radical polymerizations for the synthesis of numerous polymer brushes. With the advent of controlled radical polymerization (CRP) surface initiated radical polymerization became an area of increased attention as with these tech-niques not only the polymer composition can be adapted by the choice of monomer but also the thickness of the grafted polymer layer can be varied. Considering that three main CRP techniques can be employed, that is, reversible addition–fragmentation chain transfer (RAFT), nitroxide-mediated polymerization (NMP), and atom transfer radical polymerization (ATRP), and combined with a great variety of monomers and surfaces, this resulted in a large number of research papers in the last decade. Comprehensive reviews on the topic were published by Brittain and co-workers [ 3 ] for surface initiated polymerizations from silica nanoparticles and Klok and co-workers [ 4 ] on surface initi-ated CRP.

Our interest lies in the development of polymer brush layers for the self-assembly of poly(styrene- b -methyl

nelibrary.com DOI: 10.1002/macp.201100531

Random Poly(methyl methacrylate- co -styrene) Brushes by ATRP . . .

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MacromolecularChemistry and Physics

methacrylate) (PS- b -PMMA) block copolymers into nanosized lamellar structures on silicon wafers for electronic applications. [ 5 , 6 ] A polymer brush (or other form of surface chemistry control) is a requirement for these PS- b -PMMA thin fi lms on silicon substrates in this application. This is because the PMMA block forms a wetting layer at the polymer substrate interface due to the polar interaction of the PMMA with the native SiO x layer and the lamellae form parallel to the sub-strate surface plane, not a perpendicular orientation as required. [ 7 ] We hypothesized that a perpendicular ori-entation of lamellae can be expected when prepared on a polymer brush neutral to both components, as would be the case for random copolymer brushes from styrene and MMA. The use of an in situ growth method for pro-ducing neutral surfaces for control of orientation rather than the use of random copolymer attachment usually used [ 7 ] include: control of attachment and grafting den-sity, control of thickness, ability to affect precise control of the styrene to MMA ratio thereby controlling surface energy more precisely and fi nally to integrate the brush chemistry into an integrated process within a manufac-turing environment.

As we intend to control the thickness of the grafted copolymer layers, we chose ATRP for the synthesis of the brushes. Despite the large body of literature on surface grafting by ATRP to our knowledge poly(styrene- co -MMA) brushes have only been reported once from carbon nano-tubes. [ 8 ] In fact, there are also only very few reports on the synthesis of this copolymer in solution ATRP. One reason is that the ATRP reaction conditions applied to MMA and styrene are signifi cantly different and in some respect contrary. Generally speaking, MMA is a very fast reacting monomer that is prone to early termination, which is usually overcome by halogen transfer of the ATRP active end-group to Cl and/or the addition Cu 2 + deactivator. [ 9 ] Styrene, on the other hand, has a comparably slower reaction kinetic and the addition of deactivator or con-version of the end-group is reducing the reactivity dra-matically. [ 10 , 11 ] Sawamoto and co-workers [ 12 ] synthesized random copolymers of styrene and MMA with controlled composition and molecular weights as macroinitiators for block copolymers using Ru catalyst. Brar and Puneeta [ 13 ] carried out a detailed 2D NMR investigation on the copoly-mers of styrene and MMA using CuBr/ N , N , N ′ , N ′ ′ , N ′ ′ -pentamethyldiethylenetriamine (PMDETA) as the catalyst/ligand. The same catalytic system was used by Semsa-radeh and Abdollahi [ 14 ] in a kinetic study of the copoly-merization initiated from a poly(vinyl acetate) macroini-tiator. Random copolymers of MMA and styrene were also synthesized by active generated electron transfer (AGET) ATRP using a FeBr 3 catalyst. [ 15 ] Higher selectivity of the iron-based catalyst was found for the styrene unit during the polymerization.

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In the present work, we report on the optimization of the grafting reaction of random poly(MMA- co -styrene) from silicon wafers by ATRP. Thickness and composi-tional control of the resulting copolymer brushes were systematically investigated through ellipsometry, contact angle, and 1 H NMR. Preliminary results show that these polymer brushes can indeed be used as neutral surfaces and direct the microphase separation of PS- b -PMMA fi lms into lamellar structures.

2. Experimental Section

2.1. Materials

Silicon wafers were purchased from Aldrich and cut into 1 cm 2 pieces using a diamond cutter. Methyl methacrylate (MMA, 99%, Aldrich) and styrene (99%, Aldrich) were distilled over CaH 2 under reduced pressure before use. The ATRP silane initiator (11 ′ -trichlo-rosilylundecyl), 2-bromo-2-methylpropionate, was synthesized according to a previously reported procedure. [ 16 , 17 ] Poly(styrene- b -MMA) ( Mn = 37 000 g mol − 1 each block) was obtained from Polymersource of Canada. All other chemical reagents were pur-chased from Aldrich or Fisher and used as received.

2.2. Immobilization of ATRP Initiator on Silicon Wafer

Silicon wafers were washed with freshly prepared Piranha solu-tion (70/30, v/v, concentrated H 2 SO 4 /30% H 2 O 2 ) at room tempera-ture for 3 h and rinsed with deionized water until the washing solution reached pH = 7. Afterward, the silicon wafers were dried with a stream of nitrogen and then placed into a fl ask containing 24 mL (0.226 mol) dry toluene, 120 mg (0.264 mmol) ATRP initi-ator and 1 mL (6.918 mmol) triethylamine. After 24 h, the silicon wafers were removed from the mixture and rinsed with dichlo-romethane, acetone, methanol, and chloroform. Finally, the wafers were dried under vacuum.

2.3. Synthesis of Random Poly(methyl methacrylate-co-styrene) Brushes by ATRP

Typical polymerization procedure: An ATRP initiator immobi-lized silicon wafer was added to a Schlenk tube containing CuCl (6.0 mg, 0.06 mmol), CuCl 2 (2.0 mg, 0.015 mmol), toluene (1.6 mL, 15 mmol), DMF (0.12 mL, 1.5 mmol), MMA (1.068 mL, 10.0 mmol), styrene (0.573 mL, 5.0 mmol), and ethyl-2-bromoisobutyrate (EBiB, 11 μ L, 0.075 mmol). The tube was degassed by nitrogen bubbling for 1 h before injecting 1,1,4,7,10,10-hexamethyltri-ethylene-tetraamine (HMTETA, 20.4 μ L, 0.075 mmol) by syringe. The Schlenk tube was then placed into an oil bath at 125 ° C for 16 h. Monomer conversion was calculated from the 1 H NMR spec-trum of the mixture before and after reaction using DMF as the internal standard. [ 18 ] After the reaction, the solidifi ed reaction mixture was dissolved in dichloromethane, and passed through a column of alumina to remove the Cu catalyst. Precipitation of the resulting solution gave the free random copolymer, which was analyzed using GPC and 1 H NMR. The silicon wafer was purifi ed

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by ultrasonifi cation in dichloromethane (DCM), methanol, and tetrahydrofuran (THF) to remove any physically adsorbed polymer and residual copper catalyst.

2.4. Methods

The monomers conversion in the ATRP grafting reaction was measured using a Bruker Avance 400 NMR spectrometer using deuterated chloroform as the solvent and DMF as the internal standard. The composition of the free random copolymer was also determined from 1 H NMR spectrum according to the lit-erature. [ 13 ] Molecular weights and polydispersity indices of the copolymer were characterized by gel permeation chromato-graphy (GPC) performed on an Agilent 1200 series equipped with two PL Gel 5 μ m Mixed-C 300 × 7.5 mm columns at 40 ° C. Tetrahy-drofuran (THF) was used as an eluent at a fl ow rate of 1 mL min − 1 . Molecular weights were calculated based on polymethylmeth-acrylate (PMMA) standards. The thicknesses of the polymer layers on silicon wafers were measured by a Jobin Yvon Horiba ellipsometer equipped with a 632.8 nm He–Ne laser at a 70 ° inci-dent angle. The following optical constants (refractive index n , extinction coeffi cient k ) were used: for Si, n = 3.865, k = 0.020; for ATRP initiator layer: n = 1.500, k = 0; for PMMA, n = 1.4914, k = 0; for styrene, n = 1.5917, k = 0; for the random copolymer, n was linearly calculated according to the composition of copolymer, k = 0. [ 19 ] Three measurements were carried out for each silicon wafer and the average thickness was recorded. The static water contact angles were determined using a FTÅ200 contact angle measuring system at room temperature, and the contact angle was automatically calculated by the software. Poly(styrene-b-MMA) block copolymer (12 wt% in toluene) was spin-coated at 3000 rpm onto a substrate modifi ed with a styrene/MMA graft copolymer ( f MMA = 0.5; thickness 46 nm) and after drying vacuum annealed at 170 ° C for 6 h. An atomic force microscope (PARK

Macromol. Chem. Phys. © 2012 WILEY-VCH Verlag Gm

Scheme 1 . Synthesis of poly(MMA- co -styrene) polymer brushes (top)assembly on copolymer brushes.

XE-100 AFM) was used for studying the surface structure of the samples. The microscope was operated in NC (non-contact) mode under ambient conditions using silicon microcantilever probe tips with a force constant of 60 000 N m − 1 . Fast Fourier transfor-mation (FFT) of the topographic images were used to measure the degree of alignment and the presence of defects/non-regular patterns. SEM images were obtained using Hitachi S4800 cold emission microscope operating at 10 kV. To minimize charging effects, samples were coated with a thin layer of Au/Pd sput-tering methods. Scanning probe microscopy measurements were performed using a DME DS-50 dual scope in tapping mode.

3. Results and Discussion

Silicon wafer were functionalized with ATRP initiator fol-lowing a literature procedure. [ 16 ] For the grafting reaction CuCl/CuCl 2 /HMTETA was chosen as a catalytic system (Scheme 1 ). [ 20 ] The choice of copper chloride is based on the halide exchange principle. [ 21 , 22 ] The mixed halogen system R-Br/CuCl leads to a faster initiation, slower propagation and a better control of molecular weight and distribu-tion. [ 23 ] To examine the infl uence of the reaction conditions on the grafting results fi rst the individual monomers were investigated followed by the grafting of the copolymers. Free (sacrifi cial) initiator ethyl-2-bromoisobutyrate (EBiB) was added to all polymerizations to allow control over the grafting process and the thickness of the polymer brushes assuming a correlation of the molecular weight and com-position of the free and grafted (co)polymer. [ 24 , 17 ] When this procedure was applied to the MMA grafting at 90 ° C the number-average molecular weight ( Mn ) of the resulting

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and concept of directed poly(MMA- b -styrene) block copolymer self-

Random Poly(methyl methacrylate- co -styrene) Brushes by ATRP . . .

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Table 1. Experimental results of MMA ATRP from silicon wafers using EBiB as free initiator. [MMA]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 216:0.8:0.2:1.0:x. T = 90 ° C, t = 16 h.

[EBiB] a) [mmol L − 1 ]

x Conversion b) Mn c)

[g mol − 1 ]

PDI c) Thickness d) [nm]

Contact angle [ ° ]

20 1.13 77% 46 800 1.4 30.5 72

40 2.26 99% 30 600 1.1 27.6 72

60 3.39 93% 28 300 1.3 26.4 72

80 4.52 99% 22 400 1.1 23.7 73

100 5.65 99% 17 400 1.1 17.1 73

a) EBiB concentration refers to the molar concentration of EBiB in toluene; b) Monomer conversion was measured by 1 H NMR using DMF as an internal standard; c) Mn and PDI were determined by GPC using PMMA standards; d) Measured by ellipsometry.

Figure 1 . Effect of reaction temperature on molecular weight and polydispersity of free polystyrene obtained from sacrifi cial initia-tion in surface grafting ATRP from silicon wafers. ( t = 16 h, [styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 216:0.8:0.2:1.0:1.0).

free PMMA decreases with the increasing concentration of free initiator (Table 1 ). The low PDIs (1.1–1.4) indicate that the reaction is well controlled. Consequently, the thickness of PMMA layer on silicon wafers decreases linearly with the increase of EBiB (Table 1 ), which shows that the thickness of PMMA layer can be well controlled by varying the free initiator concentration. It was also found that the contact angle of PMMA brushes remains constant at 72 ° regardless of the thickness of the grafted PMMA layer. These results are in agreement with the literature. [ 25 ]

The same experiments were carried out with styrene at 90 ° C and at 125 ° C. The effect of reaction tempera-ture on the free polystyrene obtained in the presence of the silicon wafer using CuCl/CuCl 2 /HMTETA is shown in Figure 1 . As can be observed, polystyrene with lower molecular weight and PDI is obtained when the tempera-ture is 90 ° C; however, the styrene conversion is only 10% after 16 h. While a higher conversion (95%) was reached at 125 ° C, the resulting polymer has fairly wide molecular weight distribution (1.5).

The grafting of styrene was thus carried out at 125 ° C. Although decreases with increasing free initiator concen-tration, both the high PDI of the resulting free polystyrene and the irregular relationship between polymer layer thickness and the EBiB concentration show a reduced con-trol of the styrene ATRP grafting (Table 2 ). Nevertheless,

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Table 2. Experimental results of styrene ATRP from silicon wafer 216:0.8:0.2:1.0:x. T = 125 ° C, t = 16 h.

[EBiB] a) [mmol L − 1 ]

x Conversion b) [%]

Mn

[g m

25 0.5 55.7 86,2

50 1.0 94.6 35,0

75 1.5 96.3 20,1

100 2.0 98.3 19,5

125 2.5 98.6 11,0

a) EBiB concentration refers the molar concentration of EBiB in toluean internal standard; c) Mn and PDI were determined by GPC using PM

contact angles of the polystyrene brushes are largely constant around 85 ° . This experimental result confi rms the limited control of styrene ATRP using CuCl/CuCl 2 /HMTETA. Nevertheless, these conditions appear to be

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using EBiB as free initiator. [styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] =

c) ol − 1 ]

PDI c) Thickness d) [nm]

Contact angle [ ° ]

00 2.3 30.0 90

00 1.4 65.0 86

00 1.7 28.9 85

00 1.4 25.3 85

00 1.6 10.0 85

ne; b) Monomer conversion was measured by 1 H NMR using DMF as MA standards; d) Measured by ellipsometry.

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Figure 2 . The effect of monomer ratio on the thickness of p(MMA- co -styrene) brushes on silicon wafers with different EBiB con-centration ( T = 125 ° C, t = 16 h, [MMA + styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 200:0.8:0.2:1.0:x; � [MMA]:[styrene] = 2:1, � [MMA]:[styrene] = 1:2).

Table 3. MMA:St = 2:1 ATRP on Si wafer (CuCl/CuCl 2 /HMTETA). [MMA]/[St]/[CuCl]/[CuCl 2 ]/[HMTETA]/[EBiB] = 134/67/0.8/0.2/1.0/x. T = 125 ° C, 16 h in toluene.

[EBiB] a) [mmol L − 1 ]

x Conversion MMA b) [%]

Conversion styrene b) [%]

f PMMA c) [%]

Mn d)

[g mol − 1 ]

PDI d) Thickness e) [nm]

25 0.5 96 84 66 50 200 1.8 52

50 1.0 93 92 68 42 800 1.3 46

75 1.5 98 98 67 33 800 1.4 36

100 2.0 98 97 66 22 300 1.3 25

125 2.5 99 98 64 19 900 1.6 20

a) EBiB concentration refers the molar concentration of EBiB in toluene; b) Monomer conversion was measured by 1 H NMR using DMF as an internal standard; c) The composition of the free copolymers was determined by 1 H NMR by comparison of the aromatic styrene sig-nals and the MMA methoxy signal; d) Determined by GPC using PMMA standards; e) Measured by ellipsometry.

the best compromise for both monomers and were thus applied for the copolymerization experiments.

The synthesis of copolymer brushes was systematically investigated by varying the concentration of free EBiB and the MMA to styrene ratio. The results of the experi-ments with a ratio of MMA to styrene 2:1 are summa-rized in Table 3 . Noticeable is that the MMA conversion is relatively constant (93%–99%), while the styrene con-version slightly increases with increasing free initiator concentration (84%–98%). However, the MMA fraction in the free copolymer remains almost constant around 0.66, which is in good agreement with the monomer feed ratio. As in the case of the MMA grafting reaction the and the corresponding layer thickness linearly decrease with increasing EBiB concentration (Figure 2 ). When the MMA to styrene ratio was inverted (1:2) similar trends were observed (Table 4 ). However, in agreement with the reduced control of the styrene polymerization, the thickness of the copolymer is no longer controlled by the free initiator. This suggests that the correlation between the free polymer formed in this reaction and the surface grafted polymer might be lost due to steric or mechanistic reasons for both thickness and composition. Figure 2

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Table 4. MMA:St = 1:2 ATRP on Si wafer (CuCl/CuCl 2 /HMTETA). [MMA]/[St]/[CuCl]/[CuCl 2 ]/[HMTETA]/[EBiB] = 67/134/0.8/0.2/1.0/x, 125 ° C, 16 h.

[EBiB] a) [mmol L − 1 ]

x Conversion MMA b) [%]

Conversion styrene b) [%]

f PMMA c) [%]

Mn d)

[g mol − 1 ]

PDI d) Thickness e) [nm]

25 0.5 99 94 30 41 700 1.6 31

50 1.0 99 96 30 30 800 1.5 29

75 1.5 98 96 39 25 700 1.5 31

100 2.0 98 96 39 22 800 1.5 33

125 2.5 95 91 37 14 100 1.5 42

a) EBiB concentration refers the molar concentration of EBiB in toluene; b) Monomer conversion was measured by 1 H NMR using DMF as an internal standard; c) The composition of the free copolymers was determined by 1 H NMR by comparison of the aromatic styrene sig-nals and the MMA methoxy signal; d) Determined by GPC using PMMA standards; e) Measured by ellipsometry.

Random Poly(methyl methacrylate- co -styrene) Brushes by ATRP . . .

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Figure 3 . Molar fraction of MMA ( � ) and polydispersity index ( � ) of free p(MMA- co -styrene) (PDI) in the ATRP surface polymeriza-tion as a function of the MMA feed ratio. ( T = 125 ° C, t = 16 h, [MMA + styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 200:0.8:0.2:1.0:1.0).

Figure 4 . Experimental ( � ) and theoretical (dashed line) contact angle and thickness ( � ) of random poly(MMA- co -styrene) brushes on silicon wafers as a function of the MMA feed ratio. ([MMA + styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 200:0.8:0.2:1.0:1.0, T = 125 ° C, t = 16 h).

Figure 5 . GPC traces of free poly(MMA- co -styrene) at different MMA feed ratios. ([MMA + styrene]:[CuCl]:[CuCl 2 ]:[HMTETA]:[EBiB] = 200:0.8:0.2:1.0:1.0. T = 125 ° C, t = 16 h).

visualizes the trend of decreased control over the layer thickness when the molar feed ratio of MMA to styrene was changed from 2:1 to 1:2.

As our interest is in the synthesis of graft copolymers, which are neutral in the spin-on and self-assembly of PS- b -PMMA, a series of copolymerization with different styrene/MMA molar feed ratios at constant monomer to EBiB ratio (200) was carried out under the same experimental condi-tions. As depicted in Figure 3 , a linear relationship between the MMA fraction in the free poly(MMA- co -styrene) and the MMA feed was obtained. The increasing PDI with higher styrene concentrations clearly suggests that under the applied conditions the reaction is less controlled for the styrene polymerization, which is in agreement with the results discussed above. It is very diffi cult to determine whether the composition of the grafted copolymer also follows the monomer feed ratio but evidence for the quality of the polymer brushes can be obtained from con-tact angle and thickness measurements.

Figure 4 shows the experimental and theoretical contact angle of random p(MMA- co -styrene) brushes on silicon wafers with increasing MMA fraction in the monomer feed. Because styrene is more hydro-phobic than MMA, the contact angle decreases with the increasing MMA unit in the copolymer. The dash line is the theoretical results according to linear calculation θ = m MMA θ PMMA + m styrene θ polystyrene , where m MMA and m styrene are the molar fraction of MMA and styrene in copolymer, respectively; θ PMMA (72 ° ) and θ polystyrene (86 ° ) are the con-tact angle of PMMA brushes and polystyrene brushes on silicon wafers, respectively. As can be seen, a good agree-ment is observed between the theoretical and experi-mental contact angle when the molar fraction of MMA varies from 0.3 to 0.7. Deviation from the linearity occurs

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when a large excess of one of the monomers is used, sug-gesting a different composition of the grafted copolymer compared to the theoretical value. However, the thick-nesses of random copolymer brushes remain almost con-stant around 45 nm (Figure 4 ) even when the monomer feed changes from 0.2 to 0.8. The graft thickness is closely related to the molecular weight of the free copolymer and as can be seen in Figure 5 the GPC traces of the free copolymers are centered around the same elution volume regardless of the monomer ratio. It is also noticeable that the molecular weight distribution becomes narrower

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Figure 6 . Top: AFM topography (left) and phase images (right) of a 12 wt% poly(styrene- b -MMA) symmetrical diblock copolymer (37 000 g mol − 1 each block) on a random equimolar styrene/MMA copolymer brush. Lower: Tilt SEM images of same block copolymer after etch to remove PMMA block.

with increasing MMA feed ratio, indicating better control of copolymerization when MMA feed ratio increases. This result is in agreement with the decreasing polydispersity shown in Figure 3 .

The effectiveness of the substrate-grafted copolymer for structural orientation of a symmetric block copolymer layer was examined by deposition of a PS- b -PMMA sym-metrical diblock copolymer (37 000 g mol − 1 each block). A 12% by weight toluene solution of the block copolymer was spin-coated onto the substrate modifi ed with a sty-rene/MMA graft copolymer ( f MMA = 0.5; thickness 46 nm). The substrate was dried and vacuum annealed at 170 ° C for 6 h to facilitate microphase separation of the blocks. If the brush layer shows good neutralisation of the sur-face (i.e., interacts equally with both blocks) rather than favoring one block, vertically (to the substrate surface plane) aligned lamellar structures should be observed. Representative SEM and AFM data of the block copolymer layer are shown in Figure 6 . SEM data were collected after reactive ions etch to selectively remove the PMMA block

Macromol. Chem. Phys© 2012 WILEY-VCH Verlag Gm

as detailed elsewhere. [ 26 ] As can be seen, the expected lamellar structure is well developed and a typical “fi nger-print” pattern with no preferred alignment of the struc-ture (as shown by the Fourier transform in the inset in Figure 6 ) is obtained. It can further be seen in the SEM images that the structure is well developed with a uni-form lamellar size. It can therefore be concluded that the in situ developed brush has provided a neutral surface for block copolymer orientation. Further, the brush is robust enough to survive deposition and thermal treatments used in structure development and all withstands the reactive ion etching used to generate a porous structure at the surface which can be used for development of device structures. [ 5 ]

4. Conclusion

The synthesis of random copolymer brushes of styrene and MMA on silicon wafer by ATRP has been investigated.

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Random Poly(methyl methacrylate- co -styrene) Brushes by ATRP . . .

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Owing to the intrinsically different reaction kinetics of both monomers, CuCl/CuCl 2 /HMTETA was applied for the copolymerization of these monomers. At constant concen-tration of free initiator a constant thickness was obtained for various ratios of MMA to styrene. Within 30%–70% MMA in the monomer feed, the composition of the free polymer corresponded very well to the monomer feed ratio displaying a water contact angle in agreement with the theoretical value for a random copolymer. Within these limits it is thus possible to obtain surfaces with properties adjustable by the monomer feed ratio. Preliminary experi-ments show that the obtained random styrene/MMA graft copolymers can be used to create a neutral surface directing spin-coated poly(styrene- b -MMA) into a perpen-dicular lamellae orientation. Further detailed studies on the infl uence of graft layer thickness and other parameters on the block copolymer self-organization will be reported in a forthcoming paper.

Acknowledgements : This project was fi nancially supported by Science Foundation Ireland (Grant No. 07/IN.1/B1792) and China Scholarship Council (Grant No. 2008107444). H.L. also express his gratitude to the support of the China Postdoctoral Science Foundation (Grant No. 20080440987, 200902475) and Hunan Postdoctoral Scientifi c Program (Grant No. 2009RS3026).

Received: September 22, 2011; Revised: November 9, 2011; Published online: December 2, 2011; DOI: 10.1002/macp.201100531

Keywords: ATRP; block copolymers ; polymer brushes ; self-assembly

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