Preparation of High-Density Polymer Brush with Multi-helical Structure

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Preparation of High-Density Polymer Brush with Multi-helical Structure Tomoki Kato, Masanao Sato, Hitoshi Shimamoto, Kiyu Uno, Kazutoshi Yokomachi, Yuko Konishi, Kazutaka Kamitani, Maiko Nishibori, Noboru Ohta, Ryohei Ishige, Kevin Lee White, Nobuyuki Otozawa, Tomoyasu Hirai, and Atsushi Takahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04167 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Preparation of High-Density Polymer Brush with Multi-helical Structure Tomoki Kato†, Masanao Sato†, Hitoshi Shimamoto†, Kiyu Uno§, Kazutoshi Yokomachi§, Yuko Konishi§, Kazutaka Kamitani§, Maiko Nishibori¶, Noboru Ohta‡, Ryohei Ishige⊥, Kevin L. White†, Nobuyuki Otozawa†,‖, Tomoyasu Hirai*†,§,‡, Atsushi Takahara*†,§,‡ †

Graduate School of Engineering, §Institute for Materials Chemistry and Engineering, ‡

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ¶

Faculty of Energy and Material Sciences, Kyushu University, 6-1 Kasugakoen, Kasuga,

Fukuoka 816-8580, Japan ‡

Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo, 679-5198, Japan

⊥

Department of Chemical Science and Engineering Tokyo Institute of Technology 2-12-1 O-

okayama Meguro-ku, Tokyo 152-8552, Japan ‖

Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku Yokohama-shi Kanagawa 221-

8755, Japan

KEYWORDS:

Polymer

brush,

stereocomplex,

GIWAXD,

multi-helical

structure,

stereoregularity.

ACS Paragon Plus Environment

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ABSTRACT: It is well-known that mixture of isotactic and syndiotactic PMMA forms a stereocomplex consisting of multi-helical structure in which an isotactic chain is surrounded by a syndiotactic chain. Here we report the basic structure of the stereocomplex formed when the syndiotactic PMMA chains are tethered to a silicon substrate and form a high-density polymer brush. The influence of geometric confinement was investigated by preparing the high-density polymer brushes on a flat and spherical substrate. In both cases, mixing the untethered isotactic PMMA with the grafted syndiotactic PMMA led to the formation of a stereocomplex with multi-helical structure. Static contact angle measurements showed hindered surface mobility at the outermost surface of the polymer brush, indicating that the stereocomplex forms a crystalline structure. A syndiotactic polymer brush with substituted fluoroalkyl groups was prepared to increase the contrast for grazing incidence wide-angle Xray diffraction (GIWAXD) measurements. The GIWAXD results verified that the stereocomplex forms a crystalline structure oriented perpendicular to the substrate with relatively low degree of orientation.


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1. INTRODUCTION High-density polymer brushes, wherein the end of the polymer chain is tethered to a solid substrate, are stretched perpendicular to the solid substrate in good solvents, owing to the excluded volume effect, which acts between the neighboring polymer chains, and the high osmotic pressure.1-2 The relation between the molecular motion of the polymer chain and the properties of the polymer brushes has been also studied.3-5 Though polymer brushes are prepared using both grafting-from and grafting-to methods, high-density polymer brushes are exclusively prepared using the former approach. Most high-density polymer brushes are composed of conformationally disordered polymer chains. Biopolymers, which consist of α-amino acid, saccharides, or nucleotides, form a helical structure. Since the first report of high-density polymer brushes consisting of polypeptides, various types of polymer brushes consisting of polypeptide derivatives, which also form a helical structure, have been reported.6-8 However, little attention has been paid to controlling the helical structure of polymer brushes prepared using vinyl monomers. In previous study, we had prepared a concentrated syndiotactic polymethyl methacrylate (PMMA) brush using the surface-initiated living anionic polymerization method and had found that PMMA formed a helical structure, with C60 being encapsulated within the helical cavity.9-10 However, the synthesis of high-density polymer brushes that form a multi-helical structure has not been realized either in the polypeptide or the vinyl polymer system. It is widely accepted that when syndiotactic PMMA (st-PMMA) and isotactic PMMA (itPMMA) are mixed, a multi-helical structure, that is, a so-called “stereocomplex” is formed.1115

It is thus logical to assume that, if this principle were to be applied to high-density

polymer brushes with well-controlled stereoregularity, a high-density polymer brush with a multi-helical structure would be formed (Figure 1). In this work, we demonstrate that the


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high-density st-PMMA polymer brush forms a stereocomplex with untethered it–PMMA chains and characterize the molecular aggregation structure.

Figure 1. Schematic illustration of polymer brush with multi-helical structure.

2. EXPERIMENTAL SECTION 2-1. Materials All the solvents and chemicals used in this study were employed without further purification with the exception of methyl methacrylate (MMA) (Wako Pure Chemical Industries, Ltd., 98.0%), tert-butyl methacrylate (t-BuMA) (Wako Pure Chemical Industries, Ltd., 98.0%), 2,2,2-trifluoroethyl methacrylate (FEMA) (Tokyo Chemical Industry Co., Ltd., 98.0%), toluene (99.5%), and dichloromethane (CH2Cl2) (Wako Pure Chemical Industries, Ltd., 99.0%). Chloroform (99.5%), dimethyl sulfoxide (DMSO) (99.0%), methanol (MeOH) (99.5%), tetrahydrofuran (THF) (97.0%), acetonitrile (99.5%), hydrogen peroxide (H2O2) (30.0% in water), and calcium hydride were purchased from Wako Pure Chemical Industries, Ltd. Triethylaluminum (AlEt3) (1.1 M in toluene), 1,1-diphenyl ethylene (DPE) (98.0%), triethoxysilane (97.0%), 5-hexen-1-ol (95.0%), ammonium hydride (28% in water), iodomethane (CH3I) (99.5%), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (98.0%) were purchased from Tokyo Chemical Industry Co., Ltd. tert-Butyl lithium (t-BuLi) (1.65 M in n-


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pentane) and sec-butyl lithium (sec-BuLi) (1.00 M in n-hexane) were purchased from Kanto Chemical Industry Co., Ltd. Karstedt’s catalyst (in xylene) and 2-bromoisobutyryl bromide (98%) were acquired from Sigma-Aldrich Co. LLC. Prior to use, MMA, t-BuMA, and FEMA were purified twice by distillation from CaH2 and AlEt3. CH2Cl2 was purified 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. The silicon (111) substrates were treated using a piranha solution, that is, a H2O2 (30%)/H2SO4(70%) (v/v) mixture, at 383 K for 1 h (caution: may cause explosion in contact with organic materials!). Silica particles with a diameter of 200 nm (MP-2040) were donated by Nissan Chemical Industry.

2-2. Measurements The 1H (400 MHz) and 13C (100 MHz) nuclear magnetic resonance (NMR) spectra of the polymers were recorded in CDCl3 using a Bruker Avance-400 spectrometer. The numberaverage molecular weight (Mn) and polydispersity index (PDI) values of the polymers were determined by size-exclusion chromatography (SEC) performed using a HLC-8120GPC (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 the eluent at a flow rate of 0.6 mL min-1, and the measurements were performed at 313 K. Monodisperse PMMA was used as the standard. Thermogravimetric analysis (TGA) was performed using a Seiko SII-EXSTAR TG/DTA6220 thermobalance. A 5 mg sample was loaded in an aluminum pan and heated at a rate of 10 K min-1 while using nitrogen as the purge gas. Differential scanning calorimetry (DSC) measurements were performed using a DSC6220 system (SII NanoTechnology Inc.) with 5 mg samples. The measurements were performed at heating rate of 10 K min-1with nitrogen as the purge gas at flow rate of 30 mL


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min-1. The thickness of the polymer brush layers was determined by spectroscopic ellipsometry performed using a MASS-103FH system (Five lab.). Grazing incidence wideangle X-ray diffraction (GIWAXD) measurements were performed on beamline BL40B2 at the SPring-8 facility in Hyogo, Japan. The incident X-ray wavelength (λ) was 0.1 nm, and the incidence angle, αi, was 0.12°. The incidence angle was selected such that it lay between the critical angles 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-rays, s1 is a vector parallel to the diffracted X-rays, and |s0| = |s1| = 1/λ. The diffraction peaks both in the in-plane and the out-of-plane directions were detected using an imaging plate (IP). The sample-to-IP detector distance was calibrated using silver behenate as the standard and fixed at 754 mm. The wettabilities of the samples were evaluated through time-dependent static contact angle measurements performed using a Theta T-200 system (Auto3, Altech Co., Ltd.).

2-3. Modification of silicon wafer and silica particles with 2-bromo-2-methylpropionyloxyhexyltriethoxysilane (BHE) 2-Bromo-2-methyl-propionyloxyhexyltriethoxysilane (BHE) was prepared using a previously reported procedure.16 First, 2-bromoisobutyryl bromide was added slowly to a flask containing dry CH2Cl2 and 5-hexen-1-ol at 273 K. The mixture was stirred at 303 K for 12 h. The mixture was then passed through a filter paper, and the filtrate was dried using an evaporator. The residue was purified by extraction using CH2Cl2 and water. The resulting solution was dried using magnesium sulfate, and the residue was distilled under high vacuum. The obtained compound and triethoxysilane were mixed in a flask purged with argon. Next, Karstedt’s catalyst was added to the flask, and the mixture was stirred at 303 K for 12 h. The product was purified by vacuum distillation to obtain BHE, which is a transparent liquid.


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For the surface modification of a silicon wafer using BHE, two solutions, namely, an aqueous EtOH/NH3 solution (44.5/10, wt%/wt%) and an EtOH/BHE mixture solution (44.5/1, wt%/wt%), were prepared. These two solutions were mixed just before use. Next, the solution mixture and a silicon wafer were placed in a petri dish and kept for 6 h. Then, the BHE-modified substrate was washed with ethanol several times and subsequently dried under high vacuum. The surfaces of silica particles were modified with BHE using the following process. First, 13.0 g of a 40 wt% aqueous dispersion of 200 nm diameter silica particles was placed in a flask, and 100 mL of ethanol was added to the flask using a dropping funnel. The mixture was heated to 313K. Next, 8.93 g of ammonium hydroxide (28%) and 180 mL of an ethanol solution were slowly added to the flask using a 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. The silica particles were purified by washing with ethanol several times and were subsequently collected by centrifugation. Finally, the particles were dried using a high-vacuum oven.

2-4. Synthesis of syndiotactic PMMA (st-PMMA) brushes on silicon wafer and silica particles St-PMMA brushes were prepared on a silicon wafer and silica particles using a previously reported method.9 To prepare the polymer brush on a silicon wafer, the BHE-modified wafer and 20 mL of dry CH2Cl2 were transferred to a Schlenk flask, and the flask was cooled to 195 K. AlEt3 was added to the flask, followed by t-BuLi, after allowing for stirring for 10 min. Next, the mixture was stirred for 1 h at 195 K. Then, 4.0 mL of MMA was added to the reactor using a cannula under 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 MeOH, while the silicon wafer was washed with a Soxhlet extractor for 24 h using CHCl3 as the eluent. This


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reaction yielded a free polymer not connected to the substrate. The primary structure of the polymer brush was evaluated through 1H NMR and SEC measurements, which were performed on the free polymer (Mn: 33,000, PDI: 1.06, racemo-racemo (rr): meso-racemo (mr): meso-meso (mm) = 88:12:0). Previously, the primary structure of the polymer brush was measured directly by cleaving the polymer brush from the silica particles. The values obtained using free polymer were comparable to the cleaved structure and provide a reliable evaluation of the primary structure of the polymer brush.9 To prepare the polymer brush on silica particles, 1.0 g of the BHE-modified particles and 20 mL of CH2Cl2 were added to a Schlenk flask, and the mixture was sonicated to disperse the particles. The mixture was then cooled to 195 K. Next, AlEt3 was added to the flask, followed by t-BuLi, after allowing for stirring for 10 min. The mixture was then stirred for 1 h at 195 K. Next, 4.0 mL of MMA was added to the reactor using a cannula under 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 MeOH, while the particles were washed with CHCl3 and collected by centrifugation (Mn: 32,000, PDI: 1.08, rr: mr: mm = 88:12:0). The procedure was repeated until no signals were observed in the 1H NMR spectrum of the concentrated CHCl3 solution. The graft densities of the polymer brushes formed on the silicon wafer and silica particles were calculated using Eq. 1 and Eq. 2,

wafer =

A

particle =

(1)

n

n

(2)


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where ℎ, , , , and are the film thickness, density of PMMA in the dry state (1.19 g cm3

), Avogadro’s number, mass of the polymer, and diameter of the silica particles, respectively.

17

The primary and secondary structure of the polymer brushes does not affect the value of σ.

2-5. Synthesis of st-PMMA-r-PFEMA brush on silicon wafer The BHE-modified wafer and 20 mL of dry CH2Cl2 were transferred to a Schlenk flask. The flask was subsequently cooled to 195 K. Next, AlEt3 was added to the flask, followed by t-BuLi after allowing for 10 min of stirring. The mixture was then stirred for 1 h at 195 K. Then, 4.0 mL of a mixture containing 3.34 g of MMA (31.3 mmol) and 0.78 g of FEMA (4.66 mmol) was added to the reactor via a cannula under 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 MeOH, while the silicon substrate was washed with a Soxhlet extractor for 24 h using CHCl3 as the eluent (2.94 g, 75% yield). The primary structure of the brush was evaluated using 1H and 13C NMR and SEC measurements, which were performed on the free polymer. Figure S1 shows the

1

H NMR and

13

C NMR spectra.

Generally, the

stereoregularity of poly methacrylate derivatives is evaluated based on a signal related to the

α-methyl group in the 1H NMR spectrum. However, signals related to the side chains of PFEMA are also observed in this region and overlap with the signal of the α-methyl group. Thus, the stereoregularity of st-PMMA-r-PFEMA was evaluated using

13

C NMR

measurements. The signal at approximately 17 ppm could be assigned to the α-methyl group. The ratio of the integral intensity for this region was converted into rr, mr, and mm. Moreover, the chemical compositions of PMMA and PFEMA were evaluated based on their 1

H NMR spectra. Signals associated with –O-CH3 (PMMA) and –O-CH2- (PFEMA) groups

were seen at 3.56 ppm and 4.31 ppm, respectively. The ratio of PMMA and PFEMA was 8 :


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2 (mol/mol). (Mn: 36,000, PDI: 1.42, rr: mr : mm = 86 : 14 : 0).

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1

H NMR (400 MHz,

CDCl3): δ 0.82-0.99 (m, 3H, CH3), δ 1.77 (m, 2H, CH2), δ 1.85 (m, 2H, CH2), δ 3.56 (s, 3H, OCH3), δ 4.28 (m, 5H, -CH2CF3).

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C NMR (100MHz, CDCl3): δ 177.7, 177.6, 177.5, 177.4,

175.8, 175.6, 124.4, 121.6, 61.1, 60.8, 54.1, 45.1, 44.9, 44.8, 44.6, 18.7, 16.7.

2-6. Preparation of isotactic PMMA (it-PMMA) via living anionic polymerization Isotactic PMMA (it-PMMA) was prepared using a previously reported method.18 First, 40 mL of toluene distilled from DPE/sec-BuLi was placed in a Schlenk flask and subsequently sec-BuLi and DPE were added to the flask, such that the sec-BuLi/DPE ratio was 1:4. The mixture was stirred at room temperature for 24 h. Then the mixture was cooled to 195 K and t-BuMA was added to the flask under vigorous string. The mixture was kept at 195 K for 24 h, and an excessive amount of MeOH was added to quench the reaction. The reaction mixture was poured into MeOH, and the precipitate was collected. The poly(t-BuMA) sample so obtained was placed in a flask and dissolved in a MeOH/THF mixture as solvent. Then, a small amount of 14 N HCl was added, and the mixture was refluxed at 353 K for 24 h. The reaction mixture was then poured into acetone, and poly(methacrylic acid) was collected. The polymer was dissolved in DMSO and placed in a flask. Subsequently, CH3I and DBU were added to the flask. The mixture was stirred overnight. The reaction was quenched by adding 1 N HCl, and the mixture was poured into a MeOH/H2O mixture as solvent. The residue was filtered and collected. The target, namely, it-PMMA, was obtained as a white powder (Mn: 6,000, PDI: 1.24, rr:mr:mm = 0:3:97).

2-7. Preparation of stereocomplex in bulk state


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To prepare the stereocomplex, st-PMMA and it-PMMA were dissolved separately in an acetonitrile/water (9:1) mixture solution, and the two solutions were mixed. A precipitate immediately formed and was filtered and collected. Figure S2 shows the DSC thermogram for the 1st heating run for the sample as well as the wide-angle X-ray diffraction (WAXD) analysis results. A stereocomplex consisting of st-PMMA-r-PFEMA and isotactic PMMA was also prepared.

For this, st-PMMA-r-PFEMA and it-PMMA were dissolved separately in

acetonitrile/water (= 9:1) mixture solution, and the two solutions were mixed. Figure S3 shows the DSC thermogram for the 1st heating run for the sample as well as the WAXD analysis results.

3. RESULTS AND DISCUSSION St-PMMA brushes were prepared on silica particles and silicon wafer (Table 1) using the surface-initiated living anionic polymerization method. To confirm whether the high-density polymer brush formed a stereocomplex, the silica particles with the st-PMMA brush (Mn = 32,000, PDI = 1.08) were mixed in an acetonitrile/water (= 9:1) mixture solution containing it-PMMA (Mn = 6,000, PDI = 1.24).

The particles were subsequently collected using

centrifugation. Relatively large diameter silica particles were used to reduce the effect of curvature on the graft density. The graft density can be evaluated as a dimensionless surface occupancy.

The effect of curvature is negligible in this condition (see supporting

information). DSC thermograms of the 1st and 2nd heating runs for the particles are shown in Figure 2a. The st-PMMA brush immersed in the it-PMMA solution showed two endothermic peaks, designated as Tm1 and Tm2, at 443 K and 463 K, respectively, during 1st heating run. These


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peaks were also observed in the case of the stereocomplex in bulk, and could be assigned to the crystalline structures formed owing to fringed micellar growth and lamellar growth, respectively.19 These endothermic peaks vanished, and a base-line shift corresponding to the glass transition temperatures of the st-PMMA brush was observed at 399 K in 2nd heating scan. These results indicate that it-PMMA interpenetrated the high-density st-PMMA brush, leading to the formation of a stereocomplex. Moreover, the stereocomplex was deformed and underwent phase separation during the heating process. Generally, the polymer chains in a polymer brush formed on a flat substrate are extended and densely packed as compared to those of a brush formed on a spherical substrate. To confirm if the high-density st-PMMA brush can form a stereocomplex with free it-PMMA, the silicon wafer with the st-PMMA brush was placed in an it-PMMA acetonitrile/water (=9:1) mixture solution. The brush was subsequently washed using acetonitrile.

Table 1. Characteristics of primary structures of polymer brushes formed on silicon substrates

Sample

Substrate

Tacticity rr Mn b (%)a

PDI b

Thickness (nm) c

Graft density (chains nm-2)

PMMA320

Si Particle

88

32,000

1.08

-

0.39

PMMA330

Si Wafer

88

33,000

1.06

21.9

0.48

a

Stereoregularity of PMMA was evaluated from 1H NMR spectra. bMolecular weight and PDI were determined using SEC; calibration was performed using PMMA as standard. c Thickness was determined using ellipsometry.


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Figure 2. a) DSC curves for 1st and 2nd heating runs for stereocomplex of PMMA brush, b) GIWAXD patterns for stereocomplex of PMMA brush, and c) line profile along equatorial axis. The molecular aggregation structure was evaluated using GIWAXD measurements. Figure 2b and c show the two-dimensional (2D) GIWAXD patterns and line profiles along the equatorial axis of the PMMA brush after it was immersed in the it-PMMA solution. Here, we assumed that the scattering vector q = (4π/λ)sinθ, where θ, is the Bragg angle. Though the contrast of each diffraction is very weak, characteristic diffraction peaks can be observed at q = 2.79 nm-1, 7.92 nm-1, 9.84 nm-1, and 10.8 nm-1; these are in keeping with the diffraction peaks of the stereocomplex prepared in bulk. Based on previous report, these peaks could assigned to the domain spacing of a cylindrical structure forming the stereocomplex (q = 2.79 nm-1, d = 2.25 nm); the pitch of the outermost helical structure in the stereocomplex, formed by st-PMMA (q = 7.92 nm-1, d = 0.79 nm); the pitch of the inside of the double-helical structure formed by it-PMMA (q = 9.84 nm-1, d = 0.63 nm); and the distance between the stand it-PMMA chains in the helical structure (q = 10.8 nm-1, d = 0.58 nm), respectively (Figure 3).14 The results indicate that the it-PMMA readily interpenetrated in the highdensity st-PMMA brush on the flat substrate, forming a stereocomplex.


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Figure 3. Schematic illustration of a) top and b) side view of stereocomplex. Figure 4. Time dependence of static water contact angles of polymer brush and stereocomplex. The surface wettabilities of the st-PMMA brush and the stereocomplex formed by the stPMMA brush were determined using time-dependent static contact angle measurements, which were performed using water droplets (Figure 4). The static contact angle of the stereocomplex decreased monotonically with time. On the other hand, in the initial stage, the static contact angle of st-PMMA brush showed an exponential decay. These observations can be attributed to the surface reorganization caused by a change in the local orientation of the carbonyl group in PMMA and the evaporation of water, respectively.20-21 These results indicate that the surface reorganization of the stereocomplex is restricted compared to the stPMMA brush. Taking the DSC results into account, it appears that the restriction in surface reorganization is due to the presence of a crystalline structure when the stereocomplex is formed.


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To evaluate the molecular aggregation structure of the polymer brush in detail, it is necessary to improve the contrast of the GIWAXD patterns. The contrast of the GIWAXD patterns for the stereocomplex formed by the polymer brush was relatively weak because of the low electron density of PMMA. Thus, to improve the contrast of the GIWAXD patterns, FEMA, which has a higher electron density, was introduced into the polymer brush. The difference in the electron densities of MMA and FEMA, ρe can be estimated using Eq. 3

=

(3)

where ne, NA, ρd and M are the number of extranuclear electrons, Avogadro’s number, density of the bulk state, and molecular weight, respectively. The values of ρe was estimated to be 5.82×1022 electrons cm-3.

This value is much higher than that for polystyrene and

polyisoprene (3.40×1022 electrons cm-3), which are widely utilized. A st-PMMA-r-PFEMA brush was prepared on silicon wafer by living anionic polymerization in the presence of AlEt3 (Figure 5). The MMA/FEMA ratio in the polymer brush was 8:2. A free polymer consisting of

st-PMMA-r-

Figure 5. Synthesis strategy for st-PMMA-r-PFEMA brush using surface-initiated living anionic polymerization method.


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PFEMA was also obtained during the polymer brush preparation process. This free polymer was mixed with it-PMMA, and the molecular aggregation structure was evaluated using DSC and WAXD measurements. approximately 450 K.

The mixture showed a distinct endothermic peak at

Further, its diffraction patterns were similar to those of the

stereocomplex formed by st-PMMA and it-PMMA (Figure S3). This consistent with prior work by Hatada et al., who found that random copolymers of st-PMMA with polymethacrylate derivatives formed a stereocomplex with it-PMMA if the MMA content is higher than 70%.22 The results confirm that the free st-PMMA-r-PFMA it-PMMA forms stereocomplex with it-PMMA. The protocol used to prepare the stereocomplex was also employed for synthesizing the st-PMMA-r-PFEMA brush, whom molecular aggregation structure was evaluated using GIWAXD measurements.

Figure 6. a) Different-contrast GIWAXD patterns of stereocomplex formed in st-PMMA-rPFEMA brush, b) line profile along equatorial axis, and c) schematic illustration of stereocomplex in polymer brush.


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Figure 6a shows the 2D GIWAXD patterns for the sample with different contrasts while Figure 6b is the line profile along the equatorial axis. Interestingly, the brush exhibited diffractions similar to those the stereocomplex prepared by mixing st-PMMA brush and itPMMA. Based on the results for the bulk and the brush, the peaks could be assigned to the domain spacing of a cylindrical structure forming the stereocomplex (q = 2.74 nm-1, d = 2.29 nm); the pitch of the outermost helical structure in the stereocomplex formed by the stPMMA-r-PFEMA brush (q = 8.02 nm-1, d = 0.78 nm); the pitch of the inside of the double helical structure formed by it-PMMA (q = 9.93 nm-1, d = 0.63 nm); and the distance between the st-PMMA-r-PFEMA brush and the it-PMMA chains (q = 10.9 nm-1, d = 0.58 nm), respectively. The diffraction peak at q = 2.74 nm-1 is clearly concentrated along the equator, while the diffraction peak at q = 8.02 nm-1 confirms that the stereocomplex in the polymer brush is oriented almost perpendicular to the substrate but the degree of orientation ordering is not high (Figure 6a, Figure S4).

4. CONCLUSIONS We have successfully prepared a high-density polymer brush with a multi-helical structure attributable to the formation of a stereocomplex. It-PMMA could be interpenetrated in a high-density PMMA brush, resulting in the formation of the stereocomplex.

The

stereocomplex had a crystalline structure, which restricted surface reorganization when a water droplet was deposited on it. The contrast of the GIWAXD patterns of the polymer brush could be improved by introducing FEMA in the brush. Finally, it was confirmed that the multi-helical structure is oriented almost perpendicular to the substrate but the degree of orientation ordering is not high.


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ASSOCIATED CONTENT Supporting Information. 1

H NMR, DSC and WAXD measurements results for st-PMMA, st-PMMA-r-PFEMA, and

stereocomplex are provided in Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *(Tomoyasu Hirai) Telephone: +81-92-802-2516. Fax: +81-92-802-2518. Email: [email protected]. *(Atsushi

Takahara)

Telephone:

+81-92-802-2517.

Fax:

+81-92-802-2518.

Email:

[email protected].

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. This work was performed under the Cooperative Research Program of "Network Joint


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Research Center for Materials and Devices.

The X-ray diffraction measurements were

performed at the BL02B2 and BL40B2 beam lines of SPring-8 under proposal numbers 2014B1285, 2016B1227.

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SYNOPSIS

“For Table of Contents use only”


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Preparation of High-Density Polymer Brush with Multi-helical Structure Tomoki Kato, Masanao Sato, Hitoshi Shimamoto, Kiyu Uno, Kazutoshi Yokomachi, Yuko Konishi, Kazutaka Kamitani, Maiko Nishibori, Noboru Ohta, Ryohei Ishige, Kevin L. White, Nobuyuki Otozawa, Tomoyasu Hirai, and Atsushi Takahara


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Preparation of High-Density Polymer Brush with Multi-helical Structure

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