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Synthesis and Morphological Study of Thick Benzyl MethacrylateStyrene Diblock Copolymer Brushes Selvaraj Munirasu,†,‡,# Raghuraman G. Karunakaran,†,‡ J€urgen R€uhe,‡ and Raghavachari Dhamodharan*,† † ‡
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India Chemistry and Physics of Interfaces, Department of Microsystems and Engineering, University of Freiburg, Georges-K€ohler-Allee 103, 79110, Freiburg, Germany
bS Supporting Information ABSTRACT: We demonstrate, for the first time, the synthesis of model poly(benzyl methacrylate) [P(BnMA)] brushes of very high thickness (>300 nm) on silicon wafer. P(BnMA) brush is also synthesized from the surface of silica nanoparticles, from a covalently anchored initiator monolayer, using ambient temperature ATRP. The kinetic studies and block copolymerization from the surface anchored P(BnMA)-Br macroinitiator showed that the polymerization was controlled in nature. AFM, ellipsometry, and water contact angle were used for the characterization of the polymer brush. The grafting density of the P(BnMA) brush, formed by immersion in a dilute monomer solution, was relatively less (∼11% less) in comparison to that obtained by immersion in neat monomer under similar conditions. The P(BnMA)-Br macroinitiator brushes were used to synthesize P(BnMA-b-S) diblock copolymer brushes by the ATRP of styrene at 95 °C. The P(BnMA-b-S) brushes showed stimulus response to a selective solvent and various nanopatterns were observed according to the composition of the block copolymer.
’ INTRODUCTION The morphology of block copolymers in the melt or in solution is well-established and this can be used to prepare a variety of materials using the advantages of the self-assembly of block copolymers.14 It has been shown that block copolymers can be used as template materials for formulating nanopatterns for microelectronic, optical, and biomimetic applications. For example, ultra-high-density nanowire arrays,5 nanoscopic SiO2 posts,6 organically modified aluminosilicate mesostructures7 were prepared using block copolymers as template material. Nanoporous materials with spherical and gyroid cavities were synthesized by selective removal of PDMS moiety in PS-PDMS diblock copolymer,8 and nanostructured carbon arrays were prepared using poly(acrylonitrile) block copolymer.9 Nanotubes were prepared by the self-assembly of a triblock copolymer followed by selective photo-cross-linking and ozonolysis.10 The application of block copolymers in lithography has also been reported.11 An interesting situation occurs if such block copolymers are tethered to a solid surface at one end. If the block copolymers are attached at one end to a solid surface in such a way that they stretch away from the surface, additional forces such as the stretching of the molecules and the polymersurface interactions come into play when compared to the situation in which they are free and not attached to the surface. It has been shown, theoretically, that such block copolymers tethered on planar surface would result in the formation of checkerboard, “dumbbell”, flowerlike, “garlic”, and “onion” like morphologies.12 These interesting structures can be obtained by fine-tuning various internal parameters such as the block length, the interaction energy between the blocks, chain architecture, grafting density, and external parameters such as solvent, temperature, and so forth. Brittain et al. showed that various nanopatterns r 2011 American Chemical Society
can be fine-tuned by using different solvents on the diblock copolymer brush, anchored to a planar surface.13,14 An interesting solvent-induced nanopattern of mixed brushes was also reported.15 Depending on the molecular weight of the two individual blocks and the miscibility between the two, rich phase behavior was observed.16 Although reports dealing with the synthesis of diblock copolymer brushes using controlled radical polymerizations (CRP) are available in the literature, most of them are limited to polymers of low molecular weight, an important parameter that can influence the morphology of a block copolymer. One of the drawbacks of atom transfer radical polymerization (ATRP) is its limitation with respect to the synthesis of polymers of high molecular weight. This limitation makes it difficult to compare the properties of a polymer brush synthesized by conventional free radical polymerization and ATRP. The molecular weight distribution in a CRP method such as ATRP is determined by the ratio of the propagation rate to the initiation rate, assuming that conditions resulting in loss of end group, bimolecular termination, and all other side reactions are minimal. In order to synthesize a model polymer brush system with high graft density and high molecular weight, two strategies are available. One of them is to bring down the probability of side reactions by minimizing the loss of the end group by carrying out the polymerization at low temperature or choosing appropriate solvent and reaction conditions, and so forth.17 An alternative approach would be to perform a polymerization reaction, which has a very high rate of polymerization. We have shown that these Received: July 26, 2011 Revised: September 18, 2011 Published: September 19, 2011 13284
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Langmuir limitations can be overcome by the atom transfer radical polymerization of benzylmethacrylate (BnMA) and tert-butyl acrylate using the CuBr/PMDETA system, which proceeds very rapidly, even at ambient temperature. In this system, we could obtain high molecular weight polymer with high conversion without losing control over the molecular weight distribution (MWD).1820 We have also shown that several monomers such as cyclohexyl methacrylate,21 carbazole methacrylate,22 and isobornyl methacryatle23 can be polymerized rapidly at ambient temperature by SET-RAFT,24 a controlled radical polymerization involving initiation by SET mechanism and propagation by RAFT mechanism. Taking advantage of the unusually high rate of polymerization of one of these monomers, namely, benzyl methacrylate (BnMA) monomer as the example, we report on the synthesis of thick benzyl methacrylate polymer brushes on the order of a few hundred nanometers on planar silicon wafer as well as on silica nanoparticles, for the first time, as represented in Scheme 1. The P(BnMA) brushes thus-synthesized possess active but dormant end groups, and these are used as macroinitiators to generate poly(benzylmethacrylate-b-styrene) diblock copolymer brushes. This diblock copolymer brushes phase separate into very specific nanopatterns, if a selective solvent is used. The effect of block length of the BnMA segment on the film morphology is also studied, and the results are compared with the reported morphologies for the diblock copolymer in the bulk state.3
’ EXPERIMENTAL SECTION Materials. Benzylmethacrylate (BnMA), CuBr (99.999%), N,N,N0 , N00 ,N00 -pentamethyldiethylenetriamine (PMDETA), ethyl-2-bromoisobutyrate (EBiB), dimethyl formamide (DMF) (dry), anisole (dry), styrene, allyl alcohol, tetraethoxy silane, 2-bromoisobutyryl bromide, triethylamine, dimethylchlorosilane, and hexachloroplatinic acid (H2PtCl6) were purchased from commercially available sources (Sigma-Aldrich and Fluka Germany). BnMA was passed through a basic alumina column to remove any traces of acidic impurity. Styrene was stirred with CaH2 and distilled under reduced pressure. After purification, both the monomers were stored in the freezer maintained at 20 °C. All the other reagents were used as received. Synthesis of Silica Nanoparticles (SiO2 NP). Silica nanoparticles were synthesized by Stober’s process.25 5.6 mL of ammonium hydroxide solution was added to 44.4 mL of ethanol placed in a roundbottomed flask and stirred vigorously. To this mixture, 3 mL of tetraethoxysilane was added and it was stirred for another 2 h. After the completion of the reaction, the silica nanoparticles were separated using a high-speed centrifuge and dried. Immobilization of ATRP Initiator on Silicon Wafer and SiO2 NP Surface. The ATRP initiator, 2, namely, (3-(2-bromoisobutyryl)propyl)dimethylchlorosilane synthesized by the hydrosilylation of 2-bromo-2-methyl propionic acid allyl ester, 1, was immobilized onto silicon wafer, 3, as reported earlier by us (Scheme 1).17,26 The ATRP initiator, 2, was immobilized on SiO2 NP as follows. 2.0 g of silica nanoparticles were taken in a dry Schlenk tube and 50 mL of dry toluene was added. To this mixture, 0.5 mol of the initiator, 2, in toluene was added along with 3 mL of triethylamine and stirred overnight. The product was centrifuged and rinsed with acidified wateralcohol mixture (1:1 v/v, pH = 4), with wateralcohol mixture (1:1 v/v), then with alcohol and finally with ether. The resulting product, namely, ATRP initiator immobilized silica nanoparticle, 4, was dried.
Growth of P(BnMA) Brushes from Si Wafer—Surface Initiated ATRP of BnMA. In a dry Schlenk tube, equipped with a stir bar and a Teflon stand (to isolate the wafers), CuBr (10 mg, 0.07 mmol) was added. The required amount of BnMA (100 mL,
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590 mmol) and the ligand, PMDETA (14.5 μL, 0.07 mmol) were added. Oxygen was removed through three freezepumpthaw cycles. The required amount of sacrificial initiator, EBiB (10 μL, 0.07 mmol), was added under nitrogen atmosphere, as it is known that the presence of added initiator or deactivator molecule is required to grow brushes in a controlled manner.2729 In the meantime, silicon wafer substrates (4 2 cm2), 3, were dried using high vacuum in another Schlenk tube and maintained under nitrogen atmosphere. To study the kinetics of brush growth, a set of 6 to 10 initiator immobilized substrates were added to the same reaction solution. After the addition of the sacrificial initiator into the Schlenk tube containing the ATRP reaction mixture, the wafers were transferred immediately under the flowing nitrogen atmosphere. After the desired period of time, samples (silicon wafer and a small amount of the solution containing free polymer samples for the GPC measurement) were taken out periodically for analysis. Great care was taken during the removal of the substrate in order to prevent the entry of oxygen. In the case of solution ATRP, the solvent (anisole — 25% v/v) was added along with the monomer. The solution was stirred at room temperature (23 ( 2 °C). One sample of the wafer and an aliquot from the solution (for the analysis of the free polymer by GPC) were taken periodically for the kinetic studies. After completion of the polymerization reaction, the wafers were thoroughly rinsed using toluene and extracted in a Soxhlet setup with chloroform for 24 h. After drying the wafers, at room temperature in nitrogen stream, the thickness of the P(BnMA) brushes were measured (by ellipsometry). The free polymer solution was stirred with neutral alumina in order to remove the metal complex, filtered, and used as such for GPC measurements without any further purification steps. The reaction medium remained homogeneous and was light green in color throughout the polymerization.
Synthesis of P(BnMA-b-S) Diblock Copolymer Brush from Si Wafer. The procedure for the ATRP of styrene from the PBnMA-Br macroinitiator, 5, was similar to that used for the homopolymerization of BnMA. In this case, wafers with the PBnMA-Br macroinitiator were taken instead of initiator immobilized wafers. For a typical block copolymerization, three samples with different thickness of the PBnMABr macroinitiator were placed in a clean, dry Schlenk tube. To this, the required amount of CuBr (31 mg, 0.22 mmol), styrene (50 mL, 436 mmol), solvent (DMF — 5% by volume with respect to the monomer), and sacrificial initiator, EBiB (32 μL, 0.22 mmol) were added. After three freezepumpthaw cycles, the ligand, PMDETA (46 μL, 0.22 mmol) was added at room temperature. A homogeneous solution with a green color was observed. It was then placed in an oil bath maintained at 95 °C for 18 h. After cooling to room temperature, the wafers, 6, were rinsed with the solvent as described for the homopolymerization. The free polymer formed from the sacrificial initiator was used for the GPC measurements. Surface-Initiated ATRP of BnMA from Silica NPs. Ambient temperature ATRP of BnMA was carried out using silica whose surface was previously immobilized with ATRP initiator. In a dry Schlenk tube, 0.5 g of silica with surface-anchored ATRP initiator, 4, was taken along with CuBr (0.016 g; 0.112 mmol), PMDETA (0.0194 g; 0.112 mmol), ethyl 2-bromoisobutyrate (0.0218 g; 0.112 mmol), and benzyl methacrylate (5.921 g; 33.6 mmol) in the ratio of 1:1:1:300. Anisole (50 v/v) was added to the system. Then, the system was carefully degassed to remove the oxygen (three freezepumpthaw cycles) and polymerization reactions were performed in an oil bath maintained at 30 ( 2 °C.
Synthesis of P(BnMA-b-S) Diblock Copolymer Brush from Silica NPs. The procedure for the ATRP of styrene from the P(BnMA)-Br macro initiator was the same as the homopolymerization of BnMA. P(BnMA)-Br macro initiator grafted silica nanoparticles, 7, were taken in a clean, dry Schlenk tube. To this, the required amount of CuBr (0.22 mmol), CuBr2 (0.11 mmol), styrene (0.11 mol), solvent (DMF — 10% volume by volume with respect to the monomer), and sacrificial initiator, ethyl 2-bromoisobutyrate (0.22 mmol) were added. 13285
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Scheme 1. Schematic Representation of the Synthesis of ATRP Initiator, Immobilization onto Silicon Wafer and SiO2 Nanoparticle Surfaces, Surface-Initiated Polymerization of P(BnMA)-Br Macroinitiator and P(BnMA-b-S) Diblock Copolymer Brush
After three freezepumpthaw cycles, the ligand, PMDETA (0.22 mmol), was added at room temperature. It was placed in an oil bath maintained at 90 °C for 8 h. After cooling to room temperature, the P(BnMA-b-S) grafted silica particles were subjected to Soxhlet extraction in toluene for 18 h.
Characterization of Polymer Brush and the Free Polymer. The molecular weight of free polymer was measured using an Agilent GPC setup with software from PSS. DMF was used as the eluent and linear PMMA standards from PSS were used for the calibration. The thickness of the polymer brushes were measured using Ellipsometer 13286
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Langmuir (DRE -X-2C, Dr. Riss Ellipsometerbau) operating with a 632.8 nm He/Ne laser at a 70° incident angle. Three measurements, per spot, were performed at three different spots per sample. Contact angle measurements were carried out on a Dataphysics OCA20 instrument possessing at telescopic goniometer interfaced with a CCD camera. Water placed in a Hamilton syringe with a flat-tipped needle was used as the probe liquid. At least three measurements were made per specimen of Si wafer. The specific surface area and pore volume of the samples were estimated from nitrogen adsorption studies at 77 K using a SORPTOMETRIC (Micromeritics ASAP 2020 Porosimeter) instrument. Prior to the adsorption of nitrogen, the samples were degassed at 423 K for 12 h. Thermogravimetric analysis (TGA) measurements were performed using Mettler Toledo 851e TGA/SDTA (Switzerland), under nitrogen atmosphere, at a heating rate of 10 °C/min. For diffused reflectance infrared (DRIFT) and IR measurements, JASCO Fourier transform infrared spectrometer 410 (Japan) was used. Transmission electron microscopic measurements were carried out with a LEO CEM 912 transmission electron microscope applying an acceleration voltage of 120 KeV. Samples were prepared by applying a drop of the particle solution in THF/toluene to a carbon-coated grid. XPS analysis was performed on a Physical Electronics 5600 spectrometer equipped with a concentric hemispherical analyzer and using an Al Kα X-ray source (15 KeV, filament current 20 mA). The samples were investigated under ultra-high-vacuum conditions at (109108 mbar). Spectra were taken at a 45° takeoff angle with respect to the surface. AFM measurements were performed using Nanoscope III (Digital Instruments, Santa Barbara, CA) operating in tapping mode. For the investigation of the stimulus responsive behavior of the diblock copolymer brushes, the substrates with the surface-attached diblock monolayers were exposed to the respective solvent for five minutes, before measurement.
’ RESULT AND DISCUSSION The reaction scheme for the initiator synthesis, immobilization on the surface and the polymerization are shown in Scheme 1. The details of the synthesis and anchoring of the ATRP initiator to the silicon wafer surface was reported by us, earlier.17,26 The immobilization of the ATRP initiator monolayer on the silicon wafer was carried out under dry condition, using toluene as the solvent. Triethylamine was used as the catalyst and acid scavenger. Due to the deposition of the initiator monolayer, the water contact angle increased from 18 ( 3° to 70 ( 1°. The thickness of the initiator monolayer, as measured by ellipsometry is found to be 15 ( 3 Å. The presence of bromide group in the initiator was confirmed by XPS (data not shown).17 To study the kinetics of the surface-initiated ATRP of BnMA, the procedure reported earlier was adopted.18,19 The bulk ATRP of BnMA was carried out at ambient temperature with the following ratio of the monomer, initiator, and catalyst: BnMA/ EBiB/CuBr/PMDETA = 8466:1:1:1. The thickness of the PBnMA brush obtained as a function of reaction time is shown in Figure 1. It can be seen from this data that the thickness increases linearly with respect to time, as expected for the controlled polymerization. The molecular weight of the free polymer (initiated by the sacrificial initiator) is plotted as a function of reaction time in Figure 2. From this figure, it can be inferred that the molecular weight of the free polymer also showed a linear increase with respect to time and, significantly, the polydispersity remained less than 1.25 throughout the polymerization despite the very high degree of polymerization. The monomer was used in very large excess in comparison to the initiator (surface as well as free), and thus, a linear plot of the molecular weight versus time is a reasonable representation of
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Figure 1. P(BnMA) brush thickness versus time for the bulk ATRP of BnMA, at ambient temperature. Reaction condition: BnMA/EBiB/ CuBr/PMDETA = 8466:1:1:1. The open triangles represent values obtained in separate reactions.
Figure 2. Number average molecular weight Mn (filled triangle) and polydispersity Mw/Mn (empty triangle), as obtained by GPC, versus reaction time for the bulk ATRP of BnMA, at ambient temperature. For the reaction conditions, see Figure 1.
Figure 3. GPCs of the free polymer formed in bulk ATRP, at ambient temperature. Molecular weights of the polymers were calculated using linear PMMA standards and DMF as the eluent.
the controlled nature of the ATRP. The GPC of the free polymer formed from sacrificial initiator is shown in Figure 3. From this kinetic data, it is evident that the surface-initiated ATRP as well as the solution polymerization proceed in a controlled manner. One of the advantages associated with the formation of thick polymer brush is its physical appearance, i.e., color. When the thickness 13287
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Langmuir is ∼100 nm, we could estimate the thickness of the brush roughly by its color in comparison with spin-coated films of P(BnMA) of known thickness. The color of the P(BnMA) brush changes from purple/blue (∼100 nm) to golden yellow to pink to green color (more than 300 nm). The brush was homogeneous throughout the wafer size (4 2 cm2) and pinhole was not seen in all the brushes that varied in thickness from 5 to 350 nm. The roughness of the brushes was less than 1.5 nm for all the samples, as estimated by AFM measurement. However, it is observed that beyond certain monomer conversion, when molecular weight of the free polymer reaches close to a million, it was extremely difficult to remove the substrate from the reaction medium as it turned highly viscous. In order to check the reproducibility of this result, we duplicated the reaction under identical conditions. The initiator immobilization was also done in separate reaction to check whether there is any discrepancy in the kinetics. The ATRP of BnMA was carried out for the same-target degree of polymerization. The empty triangles in the kinetic study are taken from this separate reaction (Figure 1). These data show that the desired thickness of the brush can be obtained with precision. It should be noted that all reactions described so far were obtained by the bulk ATRP of BnMA. When the polymerization was carried out in anisole, essentially the same behavior was observed with lower rate of polymerization. The GPC traces of the free polymer formed in solution ATRP, thickness of the polymer brush versus number average molecular weight of the polymer formed in solution polymerization and the number average molecular weight of the polymer formed in solution polymerization versus time of polymerization are given in Supporting Information as SI Figure 1, SI Figure 2, and SI Figure 3, respectively. The advancing water contact angle of the silicon wafer following the growth of the PBnMA brush is found to be 85 ( 1° (Supporting Information SI Table 1). Growth of P(BnMA) Brushes from SiO2 NP Surface. In order to establish that the polymerization of BnMA was taking place in a controlled fashion and to assess the molecular weight of the polymer brush present on the surface vis-a-vis that formed in solution (from the free initiator), polymerizations were also carried out from identical initiating moiety, under similar conditions, from silica nanopartilces. Since silica nanoparticles possess a large surface area in comparison to silicon wafer, it should be possible to degraft the polymers anchored to its surface and analyze its molecular weight characteristics with that of the free polymer formed in solution. The synthesis of silica nanoparticles is described in the Experimental section. The surface area of the silica nanoparticles was found to be 25 ( 5 m2/g (from BET measurements). The AFM image of synthesized silica nanoparticles is shown in the Supporting Information (SI Figure 4). The X-ray photoelectron spectrum (XPS) survey scan of unmodified silica nanoparticle is shown in Figure 4a. This shows signals corresponding to the presence of silicon [152 eV, Si(2s); 100 eV, Si(2p)] and oxygen [533 eV, O(1s)] atoms, respectively. The analysis of elemental composition through multiplex spectrum revealed the following composition: C(1s) 4.72, O(1s) 45.41, Si(2p) 25.02, Si(2s) 24.85 (the actual composition will have to be recalculated with respect to one peak of Si, namely, either Si2p or Si2s). The XPS survey spectrum of the silica nanoparticles following the immobilization of the ATRP initiator is shown in Figure 4b. This reveals additional peaks corresponding to carbon [287 eV, C(1s)] and bromine [189 eV, Br(3p); 72 eV, Br(3d)] atoms. The analysis of elemental composition
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Figure 4. X-ray photoelectron survey spectrum of silica nanoparticle (a), after immobilizing the ATRP initiator (b) and after the surfaceinitiated polymerization of BnMA (c).
through multiplex spectrum revealed the following composition: C(1s) 48.94, O(1s) 24.68, Si(2p) 22.14, Br(3d) 4.24. On the basis of the structure of the initiating group that is anchored to the Si NPs, a ratio of 1:9 is expected for the Br/C atomic ratio. However, the actual atomic composition suggests that part of the Si NP is not functionalized and that adventitious carbon has adsorbed on to the surface as result of surface functionalization. However, the XPS spectral data confirms the presence of the ATRP initiator on the surface of silica. This was confirmed, independently, by DRIFT IR of the Si NPs that revealed the presence of the carbonyl group from the initiator at 1725 cm1. The graft density, σ, of the ATRP initiator immobilized silica nanoparticles (using the known density and surface area of the silica nanoparticles) was determined from elemental analysis. The graft density, σ, was found to be 678.8 μmol g1, which was calculated using eq 1,30 where gsilica is the element content, Msilica is the molecular weight of the immobilized fragment, and Z is the number of elements in the immobilized fragment. σ ¼ 1 gsilica =ðZ Msilica Þ
ð1Þ
In order to graft P(BnMA) brushes using surface-confined ATRP initiator, the initiator-immobilized silica nanoparticle, 4, was subsequently subjected to copper-mediated ambient temperature ATRP of BnMA. The ATRP of benzyl methacrylate was carried out at ambient temperature from the modified silica nanoparticles. Ethyl-2-bromoisobutyrate was added as the free initiator to control the polymerization. Control experiments were carried out under identical polymerization conditions but with no initiator on the silica surface. From these experiments, it could be inferred that the entire free polymer formed is essentially from the free initiator and that the polymer that had adsorbed onto the unmodified surface could be removed by extraction procedures. The DRIFT spectrum of P(BnMA) grafted silica particles, 7, using the ATRP initiator is shown in Figure 5. This shows an intense band at 1726 cm1 corresponding to the carbonyl peak of P(BnMA) suggesting the presence of P(BnMA) on the silica surface. 13288
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Langmuir After the polymerization, the P(BnMA) grafted silica was subjected to Soxhlet extraction in THF for 18 h to remove the free P(BnMA). In order to degraft the polymer from the surface of silica particles, the polymer modified silica particles, 7, was suspended in HF and stirred overnight. The polymer cleaved from the surface was separated and dissolved in THF and subjected to GPC measurements. The free polymer and the degrafted polymer were subjected to GPC analysis to determine Mn and Mw/Mn. The Mn and Mw/Mn of the free polymer formed in solution from the free initiator, as well as the degrafted polymer are plotted as a function of polymerization time in Figure 6. A linear increase in the molecular weight (Mn) of the poly(benzyl methacrylate) is observed with polymerization
Figure 5. DRIFT spectrum of P(BnMA) brush grafted silica nanoparticles.
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time and polydispersity (Mw/Mn) values remained fairly low and constant as in the case of P(BnMA) grafted to Si Wafer. This confirms that the polymerization is well-controlled and the linearity in the plot implies that the concentration of the propagating species is constant throughout the polymerization reaction. The 1H NMR spectrum of poly(benzyl methacrylate) cleaved from the surface and the TGA of the P(BnMA) grafted SiO2 NPs are provided in the Supporting Information (SI Figures 5 and 6). The X-ray photoelectron survey spectrum of P(BnMA) grafted silica nanoparticles is shown in Figure 4c. It can be seen from this figure that the signal corresponding to carbon and oxygen atoms from the P(BnMA) are prominently seen at 287 eV [C(1s)] and 534 eV [O(1s)] suggesting that polymerization had proceeded successfully from the surface-anchored ATRP initiator. The analysis of elemental composition through multiplex spectrum revealed the following composition: C(1s) 80.33, O(1s) 17.43, Si(2p) 2.12, Br(3d) 0.12. On the basis of the structure of P(BnMA), the ratio of the C and O expected is 11:2, and thus for the carbon composition of 80.33% (assuming that it arises exclusively from the polymer), only 12.36% is expected from the oxygen. This implies that about 2% SiO2 (from unfunctionalized Si NPs) contributes additionally to the XPS elemental analysis. The transmission electron microscope (TEM) images of silica nanoparticles (prepared from dispersion in tetrahydrofuran) shows that the particles are agglomerated, as seen in Figure 7a. The TEM images of P(BnMA) grafted silica nanoparticles are shown in Figure 7b,c. It can be observed that the particles with polymer layers are attached to one another perhaps due to
Figure 6. Plot of molecular weight and PDI of the free polymer (a) and degrafted polymer (b), with polymerization time.
Figure 7. TEM images of silica nanoparticle (as synthesized) (a), after grafting P(BnMA) brush (b) and (c). 13289
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solvent-induced plasticization. This is because the glass transition temperature, Tg, of poly(benzylmethacrylate) is approximately 54 °C, and during the extraction procedure, the polymer brushes were subjected to slightly elevated temperature and solvent, which could result in the softening of the polymer. This, in turn, could result in the attachment of one polymer layer with another. This is also evident from the AFM image of P(BnMA) grafted silica particles (SI Figure 7). After grafting poly(benzylmethacrylate) brushes on the surface, it is found that the particles are well-dispersed in most of the organic solvents, which is important for commercial applications. Block Copolymerization of Styrene from P(BnMA)-Br Macroinitiator. The important parameters, which decide the morphology of the block copolymers, apart from incompatibility (immiscibility) between the blocks, are the total degree of polymerization of the block copolymer (N), and the composition (fraction of either one) of the block fA. Here, fA = NA/N, where NA is the average number of repeat units of monomer A in the diblock copolymer.1,3,14 However, reports that deal with the effect of N and NA for the diblock copolymer brush is limited. This is related to the limitation associated with the synthesis of polymers of very high molecular weight anchored to the surface through one end. This is especially true for normal ATRP. In order to get reasonable differences in the thickness of one fraction of the block, we investigated PBnMA-Br macroinitiator of intermediate thickness, synthesized by solution ATRP. Styrene was chosen as the second monomer because of the known incompatibility of the styrene block with the methacrylate block. To elucidate the phase separation behavior arising out of the block length, the number average molecular weight of the PBnMA block was varied, while the PS block length was kept constant. The polystyrene block length could not be varied as easily as the benzyl methacrylate block due to the limitation in synthesizing PS brush of high molecular weight by normal ATRP. For this, three different thicknesses of P(BnMA)-Br macro initiators, namely, 37, 57, and 79 nm, were used to synthesize P(BnMA-b-S) diblock copolymer brush (Table 1). CuBr/ PMDETA catalyst was used for the ATRP of styrene from P(BnMA)-Br macroinitiator. The solvent, DMF (5% by volume with respect to the monomer), was added in order to carry out the polymerization under homogeneous condition. The reactions were carried out for similar period (of polymerization) for all three brushes to get a styrene block of similar thickness. The results from the block copolymerization are shown in Table 1. All three samples showed increase in thickness, and from this result, it is evident that the end groups are intact following the polymerization of BnMA, at ambient temperature polymerization. The molecular weight of the free polymer formed in the
solution was 56 250 with a MWD of 1.30. The reaction time and the DP of styrene were adjusted with the intention of varying fA from 1/2 to ∼1/4, to investigate the effect on the morphology of diblock copolymer brush. The diblock copolymer of P(BnMA-b-S) was also prepared from the macro initiator [P(BnMA)-Br] grown from the SiO2 NP surface. For this purpose, P(BnMA)Br [Mn = 20 300 g/mol] grafted silica nanoparticles were subjected to copper-mediated ATRP of styrene in DMF. After completion of the reaction, the P(BnMA-b-S) brush grafted silica was subjected to Soxhlet extraction in toluene for 18 h and the degrafted P(BnMA-b-S) was subjected to GPC measurements (SI Figure 8). Morphology Study by AFM. Tapping-mode AFM was used to study the morphology of the diblock copolymer brush. When the brushes were exposed to a good solvent for both blocks, such as chloroform or dichloromethane, smooth and featureless morphology was seen (Figure 8a showed in high magnification, Z = 50 nm/division for all the images). When it is exposed to a selective solvent such as acetone, which is good solvent for P(BnMA) and a θ solvent for PS (Mn > 50 000), at room temperature, it showed phase separation. As reported in the literature for the diblock copolymer in the bulk state, it showed a lamellar morphology when fA ≈ 1/2 (Figure 8b).3 When the thickness of P(BnMA) block increased from 37 to 57 nm, the morphology changed from lamella to beaded cylinder (Figure 8c). When the thickness of P(BnMA) is 79 nm compared to 30 nm for the PS block, the morphology completely changes to spherical structure (Figure 8d). These morphology changes are interestingly consistent with the morphologies predicted and reported for the diblock copolymer in the melt.3 It has been shown, theoretically, that block copolymers tethered on planar surface would result in the formation of checkerboard, “dumbbell”, flowerlike, “garlic”, and “onion” like morphologies if the various internal parameters such as the block length, the interaction energy between the blocks, chain architecture, grafting density, and external parameters such as solvent and temperature are fine-tuned.12 Brittain et al. showed that various nanopatterns can be fine-tuned by using different solvents on the diblock
Table 1. Thickness Data of P(BnMA)-Br Macroinitiator Brush and P(BnMA-b-S)-Br Diblock Copolymer Brush as Assessed by Ellipsometry
a
macroinitiator P(BnMA)-Br
P(BnMA-b-S)
increased thickness
thickness in nm
diblock copolymer
of styrene block in nm
(no. of monomer unita)
thicknessb in nm
(no. of styrene unita)
37 (533)
71
34 (540)
57 (794)
93
36 (540)
79 (1047)
108
29 (540)
Calculated based on MW of free polymer formed. b CuBr/PMDETA as catalyst and EBiB as the sacrificial initiator for styrene ATRP, at 95 °C, in 5% v/v DMF.
Figure 8. AFM height images of P(BnMA-b-S) diblock copolymer brush. The thickness of the styrene block was a constant, 34 nm, for all the samples; after exposure to choloroform (37 nm of PBnMA) (a); after exposure to acetone with the PBnMA brush thickness being 37 nm (b), 57 nm (c), and 79 nm (d), respectively. (Z-axis scale bar: 50 nm). 13290
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copolymer brush, anchored to a planar surface.13,14 Solventinduced nanopatterns of mixed brushes was also reported.15 Molecular weight effects and the role of miscibility between the two individual blocks was also reported.16 In our study, for the specific system investigated and under the conditions studied, the tethering of a diblock copolymer by one end to a surface appears to result in morphologies that are typically seen for diblock copolymers in bulk (spherical, cylindrical, and lamellar structures that vary with composition of the block) and no specific effect due to the anchoring of one end to the surface is not seen. More detailed experimental studies are required than what is reported in the present study to observe the theoretically predicted structures block copolymers tethered on planar surfaces.12 The size of all three nanopatterns reveals an interesting feature in section analyses (Supporting Information SI Figure 9). Since the block length of the styrene segment is the same for all three samples and only the PBnMA block varies in thickness, this effect was observed in the nanopattern size. The feature sizes, i.e., horizontal distance between the morphology of all three samples is in the region of 30 nm. These interesting features suggest that it is possible to arrive at different nanopatterns by keeping one block length constant. Grafting Density. Apart from the possibility of obtaining high brush thickness, the significant advantage of using the “grafting from” method over “grafting to” is the grafting density of the polymer brush. The polymer brush with high grafting density by the “grafting from” method is quite different from the polymer brush achieved by the “grafting to” method.31,32 The grafting density, σ, of the surface-attached chains can be calculated according to eq 2, if it is assumed that the molecular weight of the free polymer formed in solution is similar to the molecular weight of the surface-attached polymer17 and if the dry film thickness (t) is known33,34 σ ¼ tN A F=M n
ð2Þ
where NA is Avogadro’s number and F is the density of the polymer, and in the case of P(BnMA), it is 1.179 g/cm3. The assumption of the similarity of the molecular weights of the free and surface-attached polymers seems reasonable, as both follow the same growth kinetics.17 The average grafting density for all the samples prepared by bulk ATRP was around 0.59 μmol/m2 or 0.36 chains/nm2. It may be noted that this equation does not predict the marginal increase in graft density with the increase in molecular weight of the free polymer formed in solution. This could be due to one of the two following factors: the thickness of the dry polymer brush, as determined by ellipsometry, may not be accurate at lower brush thickness, or it could be due to the use of inaccurate molecular weight for the polymer in the brush form. The second reason appears reasonable due to the known fact that, during the initial phase of the growth of the brush, inadequate generation of Cu(II) could lead to less controlled polymerization and therefore higher molecular weight. It is interesting to note that, in these brushes, the polymer chains of more than 600 000 g/mol are anchored to the surface with an average anchor distance of about 1.7 nm (Figure 9). The grafting density of P(BnMA) obtained by solution polymerization is noticeably smaller, about 11% less, compared to that was realized in bulk polymerization. Previously, we reported that the grafting density of the polymer brush on the spherical particle surface is directly related to the rate of polymerization for various
Figure 9. Grafting density (σ = tNAF/Mn) of PBnMA homopolymer brush synthesized by bulk and solution ATRP, at ambient temperature.
monomer systems.35 However, our findings need to be assessed by independent theoretical study that might explain this observation. On the basis of our previous findings on the spherical magnetite particles35 and the current study on the planar surface, it is evident that the rate of polymerization may have a direct influence on the grafting density of the polymer brush. Since many properties of the polymer brush depend on the grafting density, more studies are indeed needed to ascertain this fact. From the dry film thickness increase and the molecular weight of the free PS formed in solution, the grafting density of the second block also can be estimated according to eq 2 and the calculated values are close to that obtained for the P(BnMA)-Br brush (about 0.6 μmol/m2). These data suggest that nearly all of the P(BnMA)-Br macroinitiators initiated the polymerization of styrene. The graft density, σ, for the P(BnMA) grafted silica nanoparticles were calculated using eq 1 and it was found to be equal to 0.47 chains/nm2 (Mn = 30 700 g/mol). This value is more than that observed for silicon wafer in view of the greater surface density of silanol groups on silica nanoparticles in comparison to silicon wafer. An ideal P(BnMA) brush could be formed if the graft density is in the vicinity of 0.75 chains/nm2, assuming the van der Waals diameter of P(BnMA) to be 10 Å (or) 1 nm. Thus, the polymer brush formed on silica NPs is as close to the ideal polymer brush as has been reported so far, in terms of graft density. It was observed that such Si NPs are well-dispersed in organic medium (Supporting Information SI Figure 10).
’ CONCLUSION In this work, we have successfully demonstrated the growth of thick P(BnMA) brushes from the surface of silicon wafers as well as silica nanoparticles by the surface-initiated ambient temperature ATRP of BnMA using CuBr/PMDETA catalytic system. The polymerization proceeds in a controlled fashion, exhibiting first-order kinetics with polymerization time, and the polydispersity values remained fairly low and constant. When polymerization parameters such as the monomer to initiator ratio and time for the polymerization are properly adjusted, thick polymer monolayers of more than 300 nm can be obtained on the silicon wafer surface by this method. This renders them among the thickest non-cross-linked brushes obtained by normal ATRP for hydrophobic monomers and makes them ideal polymer brushes for further studies on polymer brushes. The block copolymer monolayers from P(BnMA) and PS show phase separation on a nanoscale, which makes them interesting examples for nanostructured ultrathin polymer films.16 The polymer brush synthesized through bulk polymerization shows higher grafting density compared to that synthesized through solution polymerization 13291
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Langmuir suggesting that the rate of polymerization has a significant effect on the grafting density on the polymer brush, as reported earlier by us. On the basis of the findings of this work that the brush thickness can be controlled through the control of variables associated with polymerization kinetics such as temperature, monomer concentration, and catalyst concentration and the fact that functional groups can be introduced through postpolymerization modification of P(BnMA) and P(BnMA-b-S) brushes, enough possibilities exist to synthesize a variety of model polymer thin films ranging in character from purely hydrophobic (as synthesized) through hydrophilic (base hydrolysis) to lipophobic (transesterifaction with fluorinated alcohols).
’ ASSOCIATED CONTENT
bS
Supporting Information. Contact angle of unmodified and modified silicon wafer, GPC of the free polymer formed in solution ATRP, thickness of the polymer brush versus number average molecular weight of the polymer formed in solution polymerization, number average molecular weight of the polymer formed in solution polymerization versus time of polymerization, AFM images of silica nanoparticles, 1H NMR of P(BnMA) cleaved from the surface of silica nanoparticles, thermogram of poly(benzylmethacrylate) grafted on to silica nanoparticles, AFM images of P(BnMA) grafted silica nanoparticles, GPC traces of the degrafted P(BnMA-b-S), AFM section analysis of the diblock copolymer brushes (after exposure to acetone), and photographs of silica nanoparticle dispersion in chloroform. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses #
KAUST, Saudi Arabia.
’ ACKNOWLEDGMENT The Department of Science and Technology, India, is thanked for the research support that enabled the ATRP research programme at IIT Madras. Munirasu thanks CSIR (Council of Scientific and Industrial Research), India for fellowship in India [9/84/(326)/2001-EMR-I]. We thank Volkswagen Research foundation for the financial support. We thank Dr. Oswald Prucker for helpful discussion. Dr. Svetlena Santer and Dr. Alexey M. Kopyshev is thanked for the TEM and AFM measurements. This paper is dedicated to the memory of the corresponding author’s father Late Sri. Raghavachari Narayana Iyengar. ’ REFERENCES
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