Precise Synthesis of Poly(methyl methacrylate) Brush with Well

Mar 1, 2016 - Densely grafted polymer brushes, which consist of end-tethered chains that are predominantly oriented perpendicular to the substrate, sh...
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Precise Synthesis of Poly(methyl methacrylate) Brush with WellControlled Stereoregularity Using a Surface-Initiated Living Anionic Polymerization Method Masanao Sato,† Tomoki Kato,† Tomoyuki Ohishi,‡ Ryohei Ishige,‡ Noboru Ohta,∥ Kevin L. White,‡ Tomoyasu Hirai,*,†,‡,§ and Atsushi Takahara*,†,‡,§ †

Graduate School of Engineering, ‡Institute for Materials Chemistry and Engineering, and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo 679-5198, Japan ABSTRACT: Densely grafted polymer brushes, which consist of endtethered chains that are predominantly oriented perpendicular to the substrate, show a number of unique functional properties due to their highly organized structure. Controlling the stereoregularity of the main chain while maintaining a high grafting density offers further opportunities to increase the density of functional sites through the formation of cavities within the brush structure. Concentrated poly(methyl methacrylate) (PMMA) brushes with well-controlled stereoregularity and polydispersity index (PDI) were prepared on flat and spherical substrates using a surface-initiated living anionic polymerization method in the presence of triethylaluminum (AlEt3). The syndiotactic PMMA brush forms with helical chains and is able to encapsulate C60 molecules within the formed cavities.

1. INTRODUCTION Polymer brushes are end-tethered polymer chains grafted on a substrate at sufficient density that the orientation of the main chain with respect to the substrate is altered due to steric interactions between neighboring chains.1,2 Compared to the base polymer, polymer brush shows a range of unique permanent properties due to the geometric confinement of the chains along the surface via either a covalent3−5 or noncovalent bond.6,7 The properties are closely related to the graft density. Polymer brushes with graft density higher than 0.1 chains/nm2 are generally described as being in the concentrated polymer brush regime, which is characterized by the major of chains being oriented perpendicular to the substrate.8 The highly organized structure results in a number of unique surface and physical properties9−15 but is challenging to achieve with many polymerization strategies due to steric hindrance between neighboring chains. Surface-initiated polymerization methods have been developed to prepare a number of concentrated polymer brush systems with wellcontrolled molecular weight (Mn) and polydispersity index (PDI) on flat16,17 and spherical substrates.18,19 The concentration of functional groups can potentially be further enhanced by incorporated functional molecules inside the brush structure. However, this would likely require additional control of the conformation of the polymer brush, which has so far not been achieved. Polymers with well-controlled stereoregularity show a number of distinct properties compared with atactic one. In the case of syndiotactic poly(methyl methacrylate) (PMMA), © XXXX American Chemical Society

the chains exhibit helical structure in organic solvent and are capable of encapsulating functional molecules, such as fullerenes in the cavities.20,21 If syndiotactic PMMA chains grafted at high density show a similar helical conformation, then this strategy offers an approach to introduce a high density of functional molecules on a target substrate, as shown in Figure 1. Living anionic polymerization of methyl methacrylate (MMA) had been used to prepare PMMA with well-controlled Mn and narrow PDI.22 The stereoregularity can be further

Figure 1. Schematic illustration of polymer brush with well-controlled stereoregularity encapsulating fullerene molecules within helical cavities. Received: December 26, 2015 Revised: February 19, 2016

A

DOI: 10.1021/acs.macromol.5b02773 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules controlled through the introduction of a Lewis acid.23 Surfaceinitiated living anionic polymerization has been previously used to prepare various polymer brushes on various substrates including silicon wafer,24 gold surfaces,25 and silica particles.26 However, few works have focused on the preparation of PMMA brush based on this method. So far, the preparation of a concentrated PMMA brush with well-controlled stereoregularity has not been achieved. In this work, we report the preparation of a novel syndiotactic PMMA brush using the surface-initiated living anionic polymerization method in the presence of a Lewis acid. We discuss the effect of stereoregularity on the ability of the brush to encapsulate functional molecules.

(IP). The sample-to-IP detector distance was calibrated with silver behenate standard and fixed at 406 mm. 2.3. Synthesis of Preinitiator (BHE). 2-Bromo-2-methylpropionyloxyhexyltriethoxysilane (BHE) was prepared using the same procedure as previously reported.19 Briefly, 2-bromoisobutyryl bromide (24.0 mL, 195 mmol) was slowly added to the flask with 75 mL of dry CH2Cl2 and 5-hexen-1-ol (20.0 mL, 150 mmol) at 273 K. The mixture was stirred at 303 K for 12 h. The mixture was subsequently passed through a filter paper, and the filtrate was dried using an evaporator. The residue was purified by extraction using CHCl3 and water. The solution was then dried by magnesium sulfate, and the residue was distilled under high vacuum. The obtained compound (10 g, 40.2 mmol) and triethoxysilane (22.2 mL, 120 mmol) were mixed in a flask purged with argon. Karstedt’s catalyst (54.0 μL) was added to the flask and stirred at 303 K for 12 h. The product was purified by vacuum distillation to obtain BHE, which is a transparent liquid (12.1 g, 73% yield). 1H NMR (400 MHz, CDCl3): δ 4.15 (t, 2H, CH2O), 3.80 (q, 6H, CH3CH2OSi), 1.91 (s, 6H, CCH3), 1.61−1.71 and 1.32−1.47 (m, 8H, CH2), 1.21 (t, 9H, CH3CH2OSi), 0.62 ppm (t, 2H, SiCH2). 13C NMR (100 MHz, CDCl3): δ 172.0, 66.5, 58.7, 56.3, 32.9, 31.1, 28.6, 25.8, 23.0, 18.6, 10.7 ppm. 2.4. Synthesis of Polymer Brush on Silicon Substrate. A 1.5 cm2 of silicon substrate immobilized with BHE (0.07 mL)/dry toluene (0.3 mL) mixture were put into an autoclave heated to 423 K and held for 4 h. The BHE modified substrates were then washed by ethanol several times and subsequently dried at high vacuum. The substrate and 20 mL of dry CH2Cl2 were transferred to a Schlenk flask. The flask was subsequently cooled to 195 K. AlEt3 (0.92 mL, 1.0 mmol) was then added to the flask, followed by t-BuLi (0.063 mL, 0.10 mmol) after allowing 2 min stirring.27 The mixture was stirred for 1 h at 195 K. 2.0 mL of MMA was added to the reactor via cannula with vigorous stirring. The reaction was maintained at 195 K for 24 h, after which MeOH was added to the flask. The solution was poured into hexane, while the silicon substrate was washed by Soxhlet extractor for 24 h using THF as an eluent. 2.5. Synthesis of Polymer Brush on Silica Particles. 13.0 g of a 40 wt % solution of 200 nm diameter silica particles in water and 100 mL of ethanol were added to a reaction flask using a dropping funnel. The mixture was heated to 313 K, and then 8.93 g of (28%) ammonium hydroxide and 180 mL of ethanol were slowly added to a reaction flask via dropping funnel over a 6 h period. Then, 1.90 g of BHE and 10 mL of ethanol were added to the flask using a dropping funnel. The reaction was maintained at 313 K for 17 h. Then, the silica particles were purified by washing with ethanol several times and were subsequently collected by centrifugation. The particles were dried using a high-vacuum oven. 1.0 g of the BHE modified particle and 20 mL of CH2Cl2 were added to a Schlenk flask and sonicated to disperse the particles. The mixture was then cooled to 195 K. AlEt3 (2.70 mL, 2.8 mmol) was added to the flask, and after a few minutes, t-BuLi (0.18 mL, 0.29 mmol) was added. 4.0 mL of MMA was added to the reactor via cannula with vigorous stirring. The reaction system was maintained at 195 K for 24 h. MeOH was added to the flask to quench the reaction. The solvent was poured into hexane, and the particles were washed by CHCl3 and collected by centrifugation. The procedure was repeated until no signals were observed in 1H NMR spectrum of concentrated CHCl3 solution.

2. EXPERIMENTAL SECTION 2.1. Materials. All solvents and chemicals for synthesis were used without further purification, except for methyl methacrylate (MMA, Wako Pure Chemical Industries, Ltd., 99.5%) and dichloromethane (CH2Cl2) (Wako Pure Chemical Industries, Ltd., 99.0%). Hydrogen peroxide (H2O2, 30.0%), toluene (99.5%), methanol (MeOH), calcium hydride (CaH2), and chloroform (99.5%) were purchased from Wako Pure Chemical Industries, Ltd. Sulfuric acid (98.0%) was purchased from Nacalai Tesque Inc. Triethylaluminum (AlEt3) (1.04 M in n-hexane) and tert-butyllithium (t-BuLi) (1.60 M in n-pentane) were purcheased from Kanto Chemical Co., Inc. Triethoxysilane (97.0%), 5-hexen-1-ol (95.0%), and ammonium hydroxide (28% in water) were purchased from Tokyo Chemical Industry Co., Ltd. Karstedt’s catalyst (in xylene) and 2-bromoisobutyryl bromide (98%) were acquired from Sigma-Aldrich Co. LLC. Prior to use, the MMA was purified twice by distillation from CaH2 and AlEt3. CH2Cl2 was purified by using a glass contour solvent dispensing system (Nikko Hansen & Co., Ltd., Osaka, Japan) under a high-quality argon gas atmosphere and was subsequently distillated from CaH2. Silicon (111) substrates were treated using piranha solution, a mixture of H2O2 (30%)/H2SO4 (70%) (v/v), at 383 K for 2 h (caution: may cause an explosion in contact with organic material!). Silica particles with diameter of 200 nm (MP-2040) were kindly donated by Nissan Chemical Industry. 2.2. Measurements. 1H (400 MHz) and 13C (100 MHz) nuclear magnetic resonance (NMR) spectra were recorded in CDCl3 using a Bruker Avance-400 spectrometer. The number-average molecular weight (Mn) and polydispersity index (PDI) of polymers were determined by size-exclusion chromatography (SEC) using a HLC8120GPC (TOSOH) with three columns (TOSOH TSK gel super H 2500, TSK gel super H 4000, and TSK gel super H 6000) and a RI2031 plus refractive index detector. THF was used as an eluent with 0.5 mL min−1 flow rate at 313 K. A monodisperse PMMA was used as standard. Thermogravimetric analysis (TGA) was performed using a Seiko SII-EXSTAR TG/DTA6220 thermobalance. A 5 mg sample was loading in a platinum pan and heated at rate of 10 K/min with nitrogen as purge gas. The DSC measurements were performed using a DSC6220 (SII NanoTechnology Inc.). The heating rate was 10 K/ min for a sample of 5 mg in an aluminum pan. Nitrogen was used as purge gas at a flow rate of 30 mL min−1. The thickness of the polymer brush layers was evaluated using of spectroscopic ellipsometry using a MASS-103FH (Five lab.). X-ray photoelectron spectroscopy (XPS) measurements were performed using an XPS-APEX (Physical Electronics Co. Ltd.) at 1 × 10−6 Pa with a monochromatic Al Kα X-ray source of 150 W. Grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements were performed on the BL40B2 beamline at the SPring-8 facility in Hyogo, Japan, using an incident X-ray wavelength (λ) of 0.1 nm and incidence angle, αi, of 0.12°. The incidence angle was selected to be between the critical angle of the polymer thin film and the substrate. The scattering vector, q = (4π/λ) sin θ, where θ is the Bragg angle, is defined as 2π(s1 − s0), where s0 is a vector parallel to the incident X-ray, s1 is the vector parallel to the diffracted X-ray, and |s0| = |s1| = 1/λ. Diffraction peaks from both inplane and out-of-plane directions were detected using an imaging plate

3. RESULTS AND DISCUSSION 3.1. Preparation of Syndiotactic Polymer Brush and Characterization of Primary Structure. Surface-initiated living anionic polymerization in the presence of AlEt3 was performed on a silicon substrate (Figure 2). The chemical composition of the substrate was characterized using XPS. The XPS wide scan spectra before and after reaction are shown in Figure 3a. The analytical depth of XPS can be estimated as 3λ sin θ, where λ and θ are the inelastic mean-free path of the photoelectron and the emission angle, respectively. The inelastic mean-free path of C1s photoelectrons was B

DOI: 10.1021/acs.macromol.5b02773 Macromolecules XXXX, XXX, XXX−XXX

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effect of curvature on the polymerization reaction. Panels b and c in Figure 3 show the 1H NMR spectra and SEC curves, respectively, of the free polymer and cleaved polymer. The results are summarized in Table 2. Interestingly, the Mn, PDI, and stereoregularity of the free and brush polymers are almost identical. 3.2. Graft Density of PMMA Brush on Silicon Wafer and Silica Particles. The graft density of polymer brush on the silicon wafer and silica particles, film thickness for brush on the silicon wafer, and mass of the polymer on the silica particles were measured using an ellipsometry and TGA. The graft densities of the polymer brush on the silicon wafer, σsubstrate, and silica particles, σparticle, were calculated using eqs 1 and 2, respectively.29

Figure 2. Synthesis strategy of PMMA brush with well-controlled stereoregularity.

calculated using Ashley’s equation as 2.9 nm.28 The emission angle was set to 45°, which provides elemental information limited to a surface depth of 6.2 nm. The Br3d peak could only be observed from the BHE-modified silicon substrate. Moreover, Si2s and Si2p peaks were clearly observed, which shows that the formed BHE layer is much thinner than the analytical depth. On the other hand, the peaks are not present for the polymer brush prepared on the substrate. The integral intensity ratio of C1s to O1s shown in brush sample was 72:28, which is in good accordance with the theoretical ratio of C1s to O1s in PMMA (71:29). These results indicate that a PMMA brush was successfully prepared on a silicon substrate using surfaceinitiated living anionic polymerization in the presence of AlEt3. The reaction was carried out with an excess amount of t-BuLi and AlEt3. The Mn, PDI, and stereoregularity evaluated from the “free polymers” not attached to the substrate are summarized in Table 1. Free polymers with narrow PDI and well-controlled stereoregularity were obtained from the living anionic polymerization method. In order to elucidate the relationship between the primary structure of the free polymer and polymer brush, it is most desirable to directly cleave the polymer brush from the silicon wafer. However, it is difficult to obtain an adequate amount of polymer to characterize the primary structure due to the small surface area of the polymer brush on the silicon substrate. To overcome this problem, the polymer brush was prepared on the surface of 200 nm diameter silica particles and subsequently cleaved and collected. Relatively large diameter particles were used to minimize the

σsubstrate = hρNA /M n

(1)

σparticle = w/{πd 2(M n /NA )}

(2)

where h, ρ, NA, w, and d are the film thickness, density of PMMA in dry state (1.19 g/cm3), Avogadoro’s number, mass of the polymer, and radius of silica particle, respectively. As Mn of both free and brush polymers were nearly identical, Mn of the free polymer was used for the calculation. It is important to note that the equation do not reflect any change in dimension associate with conformation of the polymer chains. Generally, it is assumed that polymer brushes with grafting density larger than 0.1 chains/nm2 are within the concentrated polymer brush regime.1 A polymer brush with helical conformation would be expected to occupy a larger lateral distance than a predominantly linear chain, which should reduce the threshold for concentrated brush behavior. Both polymer brushes on flat and silica particles show a graft density ranging from 0.2 to 0.6 chains/nm2 (Tables 1 and 2). The results strongly support that concentrated polymer brushes with well-controlled PDI and stereoregularity were obtained on both flat and spherical substrates using the technique presented here. We suggest that in addition its role as a Lewis acid to form “ate” complex at the PMMA active chain end and to complex with the ester group in methyl methacrylate (MMA) monomers, both of which serve to increase stereoregularity,30−32 the AlEt3 also serves as a

Figure 3. (a) XPS spectra of PMMA brush and BHE. (b, c) 1H NMR spectra and SEC chart of cleaved PMMA and free PMMA, respectively. C

DOI: 10.1021/acs.macromol.5b02773 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Summary of Molecular Characteristics of Polymer Brush Prepared on Silicon Substrate tacticityb (%) sample

Mna

a

PDI

PMMA60 PMMA200 PMMA400 PMMA470 PMMA830

6000 20000 40000 47000 83000

1.09 1.06 1.06 1.07 1.13

rr

mr

mm

thicknessc (nm)

graft density (chains/nm2)

88 90 90 88 88

12 10 10 12 12

0 0 0 0 0

5.3 10.3 18.6 30.4 30.0

0.63 0.37 0.33 0.46 0.26

a

The molecular weight and polydispersity index were determined using SEC, which was calibrated PMMA standards. bStereoregularity of PMMAs was evaluated 1H NMR spectra. cThe thickness was determined using ellipsometry.

Table 2. Primary Structure of Free Polymer and Polymer Brush on Si Nanoparticles free polymer

polymer brush

sample

Mna

PDIa

rrb

mrb

mmb

Mna

PDIa

rrb

mrb

mmb

graft density (chains/nm2)

PMMA170 PMMA220

17000 22000

1.11 1.08

86 88

14 12

0 0

17000 24000

1.15 1.10

86 85

14 15

0 0

0.63 0.37

a

The molecular weight and polydispersity index were determined using SEC, which was calibrated PMMA standards. bStereoregularity of PMMAs was evaluated 1H NMR spectra. cGraft density was determined using TGA measurements.

dehydrating agent in the reaction system. This dual role makes it possible to synthesize a PMMA polymer brush with wellcontrolled PDI and stereoregularity. 3.3. Formation of the Inclusion Complex of PMMA Brush/C60. Syndiotactic PMMA has been previously reported to form a gel when the polymer is dissolved in aromatic solvents and annealed at 383 K for several minutes.33,34 The gel formation was shown to result from the encapsulation of the solvent within the cavity formed by the helical polymer chains. Moreover, when the C60 toluene solution was used, C60 was encapsulated in the cavity instead of the solvent.20 In order to evaluate whether this phenomenon occurs in the system reported here, free PMMA400 was dissolved in toluene and annealed at 383 K for 10 min. Indeed, the solution containing PMMA synthesized with AlEt3 formed gel (Figure 4a), but

The value obtained for the PMMA400 was 1.74 nm. According to the literature, syndiotactic PMMA forms helical structure with about 1 nm diameter cavities, which is able to encapsulate C60 molecules with 1.01 nm diameter. Hence, the PMMA400 brush should be able to accommodate the functional molecules; GIWAXD measurements were performed to evaluate the structure of the polymer brush. The equatorial GIWAXD patterns and line profiles of the PMMA400 brush, the PMMA400 brush after immersion in the C60 toluene solution, and C60 are shown in Figure 5. The PMMA400 brush shows a broad peak centered at q = 8.9 nm−1, which indicates an amorphous structure. After immersion in the C60 toluene solution, a new characteristic diffraction peak at q = 3.2 nm−1 was observed in the in-plane direction. This result indicates that a vertically oriented ordered structure with respect to the silicon wafer is formed. Yashima et al. similarly observed the emergence of a new diffraction peak at q = 3.7 nm−1 that was shown to be due to the encapsulation of C60 molecules within helical cavities of the syndiotactic PMMA.20 Taking these into account, it appears that the syndiotactic PMMA400 brush on the silicon substrate forms a helical structure and is able to encapsulate C60 molecules in the helical cavity, leading to a vertically oriented ordered structure with respect to the silicon wafer (Figure 5e). The thermal properties of the PMMA brush with C60 were also investigated using DSC measurements. Figure 6 shows the second heating scan of PMMA brush on silica particle, first heating scan for PMMA brush on silica particles treated by C60 toluene solution for 10 min, and second heating scan for the PMMA brush on silica particles treated by C60. In the case of PMMA brush sample, there is a clearly baseline shift corresponding to a glass transition temperature (Tg) at 404 K. The value was slightly higher than that of free polymer (401 K). This is attributed to the suppression of molecular motion for the concentrated polymer brush due to steric hindrance caused by neighboring polymer chains and the substrate. The first heating scan for PMMA brush treated by C60 toluene solution shows a Tg and small endothermic peak (Tm) at 412 and 463 K, respectively. This result supports that the C60 molecules are encapsulated into the PMMA brush and form a semicrystalline structure. Interestingly, the melting peak is not present the second heating scan, which suggests that after heating above Tg, the functional C60 molecules are released

Figure 4. Photograph showing (a) PMMA toluene solution and (b) PMMA/C60 complex gel.

when AlEt3 was not used, it did not. Free PMMA400 was dissolved in a C60 toluene solution and annealed at 383 K for 10 min into two layers, purple and colorless, following centrifugation (Figure 4b). These results are in good accordance to the literature20 and well support that the PMMA400 forms a helical structure and encapsulates C60. To determine if a similar encapsulation effect occurs for the grafted syndiotactic PMMA400 prepared here, the polymer brush prepared on silicon substrate was immersed in C60 toluene solution and heated at 383 K for 10 min. The ability of the PMMA400 brush to form an inclusion complex in the bulk state can be roughly estimated based on the distance between polymer chains (D), as evaluated from the expression35 σ = 1/D2

(3) D

DOI: 10.1021/acs.macromol.5b02773 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. GIWAXD patters PMMA brush on silicon substrate (a) before and (b) after immersion in C60 solution. (c) C60 patterns for reference. (d) GIWAXD line profiles of PMMA brush on silicon substrate before and after immersion in C60 solution. Line profile for C60 included for reference. (e) Schematic illustration of PMMA brush/C60 complex.

ordered structure oriented perpendicular to the substrate. The C60 molecules are also anticipated to show some order perpendicular to the substrate when confined within the cavity of the concentrated PMMA brush. This approach to produce materials with an increased density of molecular is expected to have use in challenging functional applications such as organic electric materials.

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AUTHOR INFORMATION

Corresponding Authors

*(T.H.) E-mail: [email protected]. *(A.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 6. DSC curves for second heating run of the PMMA brush, first heating run of the PMMA brush/C60 complex, and second heating run of the PMMA brush/C60 complex.

ACKNOWLEDGMENTS This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We also acknowledge support from the World Premier International Research Center Initiative (WPI) MEXT, Japan, and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. Part of this work was supported by the Impulsing Paradigm Change through Disruptive Technologies (ImPACT) Program. The Xray diffraction measurements were performed at the BL02B2 and BL40B2 beamlines of SPring-8 under proposals 2013B1171, 2014A1228, 2014A1222, and 2014B1286.

from the PMMA brush. These thermal properties are consistent with prior reports20 and strongly support that C60 molecules are encapsulated into the concentrated PMMA brush and are responsible for the formation of a crystal structure.

4. SUMMARY Novel concentrated PMMA brushes with well-controlled PDI and stereoregularity were synthesized on the basis of surfaceinitiated living anionic polymerization in the presence of AlEt3. The polymer brushes were synthesized not only on the silicon substrate but also on silica particles using this approach. 1H NMR and SEC measurement revealed that the stereoregularity, Mn, and PDI of the free polymer and polymer brush were the same. The syndiotactic PMMA brush forms a helical structure and was able to encapsulate C60 within the cavities formed by the end-tethered chains. The concentrated polymer brush shows a crystal structure resulting from the emergence of an

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REFERENCES

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Macromolecules

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DOI: 10.1021/acs.macromol.5b02773 Macromolecules XXXX, XXX, XXX−XXX