Nanocomposite Elastomers Composed of Silica Nanoparticles Grafted

Jul 31, 2019 - Nanocomposites composed of monodisperse silica nanoparticles (SiNPs) modified with an elastic comb-shaped block copolymer brush were ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Nanocomposite Elastomers Composed of Silica Nanoparticles Grafted with a Comb-Shaped Copolymer Brush Hitoshi Shimamoto,† Chao-Hung Cheng,† Kazutaka Kamitani,‡ Ken Kojio,†,‡,§ Yuji Higaki,*,∥ 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 ∥ Department of Integrated Science and Technology, Faculty of Science and Technology, Oita University, 700 Dannoharu, Oita 870-1192, Japan Downloaded via UNIV AUTONOMA DE COAHUILA on July 31, 2019 at 13:53:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Nanocomposites composed of monodisperse silica nanoparticles (SiNPs) modified with an elastic comb-shaped block copolymer brush were produced for the first time. Comb-shaped polymer brushes consisting of glassy poly(methyl methacrylate) branches and a rubbery poly(butyl acrylate) backbone were grafted from 100 nm diameter SiNPs by surface-initiated atom transfer radical polymerization. The comb-shaped polymer brush-grafted SiNPs produced self-standing films with a facecentered cubic ordered lattice structure via solvent-casting. The composite films showed rubber elasticity because of the interparticle rubbery polymer brush boundary phase. The ordered lattice exhibited significant alignment and distortion in response to the macroscopic film strain, and then reproduced the initial ordered lattice after unloading.

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with cohesive subgroups show elastic recovery.19 However, the mechanical stability is insufficient to cause significant elastic deformation due to the weak and/or dynamic networking. The composite film with a linear block copolymer brush consisting of a particle surface-binding rubbery inner chain and glassy outer chain promotes the strength at break and Young’s modulus of the composite films, whereas the block copolymer brush network is easily broken to cause plastic deformation.20 A random copolymer brush consisting of methyl methacrylate and butyl acrylate yields elastic recovery to the composite films, whereas significant hysteresis loss occurs probably due to the large glassy domains.21 Comb-shaped block copolymers composed of a rubbery chain backbone with glassy branch chains show outstanding rubber elasticity with mechanical toughness.22−24 The combshaped block copolymers exhibit peculiar morphologies depending on the volume fraction of branch chains and the degree of branching, and the mechanical properties are different from those of linear block copolymer analogues.25−27 The rubbery main chain would bridge multiple hard domains because the glassy branch chains are introduced to distinct hard domains to avoid conformational entropic loss through chain-looping, leading to an efficient network.28 Herein, we propose a mechanically robust elastic nanocomposite colloidal crystal with rubbery comb-shaped polymer brush-grafted silica nanoparticles (SiNPs). Poly(methyl

INTRODUCTION Colloidal crystals are an ordered array of monodisperse particles. The well-ordered colloidal lattice structure offers peculiar photonic properties including the Bragg reflection of visible light with a specific wavelength corresponding to the periodic lattice spacing to exhibit structural color.1,2 Colloidal crystals are involved in many applications including optical filters,3−6 laser devices,7 and coloring materials.8−14 However, colloidal crystals are usually fragile and lose their ordered structure by mechanical stress, leading to the deterioration of optical performances. Therefore, mechanical durability of the ordered lattice is required for practical application of the colloidal photonic crystals. Uniform spherical nanoparticles modified with a polymer brush form a well-organized lattice through self-assembly.15−18 The interparticle distance is controllable by tuning the grafted chain length, and the optical properties including the refractive index and absorbance are adjustable with solvents. Meanwhile, single-component nanocomposite self-standing films with a well-ordered colloidal crystal structure are produced through solvent-casting. The composite colloidal crystal films exhibit structural color, while the color changes with mechanical stress because the lattice deformation leads to the variation of the selective reflection wavelength.19 The mechanochromic property is viable for soft-sensors of mechanical stress. However, plastic deformation of the colloidal lattice in the composite films is still a concern. Various approaches for mechanically durable composite colloidal crystals on the basis of the polymer brush design have been reported.19−21 Composite colloidal crystal films with rubbery brush chains © XXXX American Chemical Society

Received: May 5, 2019 Revised: July 13, 2019

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DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

branches and a poly(butyl acrylate) main chain (PBA-g-PMMA) were grafted on the SiNPs by the surface-initiated atom transfer radical polymerization (SI-ATRP) method.28 The typical procedure is as follows: BHE-modified SiNPs (0.5 g), anisole (8.58 mL), BA (10.25 g, 80 mmol), EBIB (2.0 mg, 0.01 mmol), PMDETA (69 mg, 0.4 mmol), and the PMMA macromonomer (1.53 g) were added to a 50 mL flask. The solution was stirred for 60 min under an argon gas flow. CuBr (57 mg, 0.4 mmol) was added to the flask, and the solution was stirred for 10 min under argon gas flow. The solution was heated at 90 °C, and vigorously stirred for 24 h. The particles were collected by centrifugation, and then purified by five cycles of centrifugation and redispersion in THF × 2, THF/methanol (1:1) with traces of hydrochloric acid × 2, and THF. The obtained particles were dispersed in toluene and stored in a refrigerator. The monomer conversion was not precisely determined because the yield of product polymers is not simply consistent with monomer conversion. Film Preparation. The PBA-g-PMMA brush-grafted SiNP toluene dispersion (0.2 g mL−1) was poured into a poly(tetrafluoroethylene) dish, and then the toluene was slowly evaporated over 7 days to give a self-standing film. The film was vacuum-dried for 24 h at room temperature, and subsequently hot-pressed at 150 °C for 3 h to give a composite film with 0.2 mm thickness. Characterization. 1H NMR (400 MHz) spectra were recorded in CDCl3 using a Bruker Avance-400 spectrometer. The number-average molecular weight (Mn) and polydispersity index (PDI) were determined by SEC using an HLC-8120GPC (TOSOH) equipped with three columns (TOSOH TSK gel super H 2500, TSK gel super H 4000, and TSK gel super H 6000) and an RI-2031 plus refractive index detector. THF was used as an eluent with 0.5 mL min−1 flow rate at 40 °C. Thermogravimetric analysis (TGA) was performed using an EXSTAR TG/DTA6200 thermobalance (SII NanoTechnology Inc.) at a heating rate of 10 °C min−1 over nitrogen gas flow. Differential scanning calorimetry (DSC) measurements were conducted with a DSC6220 (SII NanoTechnology Inc.) at a heating/cooling rate of 10 °C min−1 over nitrogen gas flow. Temperature-dependent dynamic viscoelastic functions were obtained using a dynamic viscoelastometer RHEOVIBRON DDV-IIFP (ORIENTEC Co., Ltd.). The measurement was carried out at frequencies of 1, 3.5, 11, 35, and 110 Hz under nitrogen gas flow at a heating rate of 1 °C min−1. The minimum tension was set to 20 mN. Scanning electron microscopy (SEM) observation was performed with JSM-7900F (JEOL) at an accelerating voltage of 0.5 kV. The samples were coated with a thin layer of osmium. Mechanical properties were measured using a tensile tester EZ-Graph (Shimadzu Co., Ltd.) with a 10 N load cell at room temperature. The dimensions of the samples were 0.2 mm × 5 mm × 30 mm. The gauge length was 15 mm, and the elongation rate was 0.2 strain min−1. Ultrasmall-angle X-ray scattering (USAXS) measurements were carried out at SPring-8 BL05XU beamline. The X-ray wavelength was 0.177 nm. The X-ray beam size was 150 μm × 150 μm. PILATUS 1 M (DECTRIS, Ltd., pixel size of 172 μm × 172 μm) was used as a detector. The camera length was 3951 mm. The scattering vector was calibrated using the peak position of collagen. In situ USAXS measurement during stretching deformation was performed using a tensile testing apparatus (JUNKEN MEDICAL Co., Ltd.) equipped with a 200 N load cell. The dimensions of the samples were 0.2 mm × 5 mm × 30 mm. The initial length was 10 mm, and the elongation rate was 0.3 min−1. USAXS patterns were accumulated for every acquisition period of 5 s (exposure time: 0.5 s) during stretching. Data processing was carried out by ImageJ31 and FIT2D32 softwares.

methacrylate) (PMMA) and poly(butyl acrylate) (PBA) were adopted as glassy branches and a rubbery backbone of the comb-shaped polymer brush, respectively. The effect of the comb-shaped copolymer brush architecture on the morphology and mechanical performance of the nanocomposites is addressed by comparing with a random copolymer brush.

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EXPERIMENTAL SECTION

Materials. 4-Chloromethyl styrene (Tokyo Chemical Industry Co., Ltd., 90%), isopropylamine (iPrNH2, Wako Pure Chemical Industries, Ltd., 99.0%), n-butyl acrylate (BA, Wako Pure Chemical Industries, Ltd., 98.0%), and ethyl α-bromoisobutyrate (EBIB, Tokyo Chemical Industry Co., Ltd., 98%) were purified by distillation from CaH2 before use. Methyl methacrylate (MMA, Tokyo Chemical Industry Co., Ltd., 99.8%) was purified by distillation twice from CaH2 and triethylaluminium. Tetrahydrofuran (THF, Wako Pure Chemical Industries, Ltd., 97.0%) was dried by distillation twice from the sodium/benzophenone complex under nitrogen gas and secbutyllithium (sec-BuLi)/1,1-diphenyl ethylene complex on a vacuum line. All other reagents were purchased from Kanto Chemical Co., Inc., Wako Pure Chemical Industries Ltd., or Sigma-Aldrich Co. LLC., and used without further purification. SiNPs with an average diameter of 100 nm dispersed in water (40 wt %) were kindly supplied by Nissan Chemical Industries. The surface initiators, 2-bromo-2-methyl-propionyloxyhexyl triethoxysilane (BHE),29 BHE-modified SiNPs,29 and N-isopropyl-4-vinylbenzylamine (PVBA),30 were prepared following the previous report. Synthesis of PMMA Macromonomers. The reaction was performed under an argon atmosphere. Briefly, LiCl (0.25 g, 6.0 mmol) was added to a Schlenk flask and vacuum-dried at 100 °C for 12 h. THF (120 mL) and PVBA (0.26 g, 1.5 mmol) were added to the flask. The mixture was cooled to −78 °C. sec-BuLi (1.17 mL, 1.03 M, 1.2 mmol) was added, and the solution was stirred for 30 min. MMA (6.0 g, 60 mmol) was added, and the solution was stirred for 1 h. Methanol was added to the flask to quench the reaction. The solution was poured into a methanol/water (9:1) mixture. The precipitated polymer was filtered, and then vacuum-dried for 24 h at 40 °C. The resulting polymer was characterized by size exclusion chromatography (SEC) and 1H nuclear magnetic resonance (NMR) (Table 1). Preparation of PBA-g-PMMA Brush-Grafted SiNPs. Combshaped block copolymers composed of poly(methyl methacrylate)

Table 1. Characterization of Copolymer Brushes PBA-g-PMMA comb-shaped block copolymer PMMA branch

sample IDa MG-10 MG-24 MG-32 RA-23

Mnb −1

(g mol ) 6000 5000 5000

P(BA-r-MMA) random copolymer

PDI

Mnb −1

(g mol )

1.06 1.05 1.05

130 000 128 000 100 000 86 000

PDIb

volume fraction of MMA (%)c

average number of branch pointsd

1.44 1.55 1.48 1.64

9.7 24.2 31.9 22.7

2.3 6.5 6.7

a Sample identification: “MG” indicates comb-shaped block copolymers composed of PMMA branches and a PBA main chain (PBA-gPMMA). “RA” indicates random copolymers of MMA and BA (P(BA-r-MMA)). The abbreviation consists of the MG-volume fraction of PMMA in the comb-shaped block copolymer brush and the RA-volume fraction of MMA in the random copolymer brush. b Determined from SEC measurements. cCalculated from the 1H NMR integration ratio using densities of 1.159 g mL−1 for PMMA and 1.080 g mL−1 for PBA. dCalculated according to the expression of (Mn of PBA-g-PMMA) × (weight fraction of PMMA in PBA-gPMMA)/(Mn of PMMA branch chain).

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RESULTS AND DISCUSSION Preparation of PBA-g-PMMA Brush-Grafted SiNPs. A PMMA macromonomer was synthesized by living anionic polymerization with the sec-BuLi/PVBA initiation system (Scheme 1a).33 A slightly excess mole equivalent PVBA was introduced to prevent the initiation from free sec-BuLi. The solution color changed from colorless to green by mixing secBuLi and PVBA, indicating the formation of the initiation B

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of (a) the PMMA Macromonomer, (b) SiNPs with the PBA-g-PMMA Brush, and (c) SiNPs with the P(BA-r-MMA) Brush

complex. The solution turned colorless by subsequent MMA addition, indicating the polymerization. The methanol/water (9:1) mixture was adopted for the precipitation of the PMMA macromonomer instead of hydrocarbon solvents to remove the LiCl salt efficiently. The existence of the styryl group at the chain end of the PMMA macromonomer was confirmed by 1H NMR signals at 5.2, 5.7, and 6.7 ppm (Figure 1a). The PMMA macromonomer exhibited a narrow unimodal peak in the SEC curve with PDI < 1.1 (Figure 1b). Although the Mn depends on the amount of water contamination and subsequent initiator deactivation, almost identical PMMA macromonomers were obtained reproducibly. PBA-g-PMMA brush-grafted SiNPs were prepared via SIATRP by the grafting-from approach (Scheme 1b). P(BA-rMMA) brush-grafted SiNPs were also prepared by copolymerization of BA and MMA in a manner similar to that of the control sample (Scheme 1c, see the Supporting Information (SI)). The Mn, PDI, and average branch number of the combshaped copolymer brushes were determined from free polymers that were obtained in the grafting-from process (Table 1). The composition ratio of PBA/PMMA was calculated from the integration of signals at 3.6 ppm assigned to −OCH3 protons in PMMA and 4.0 ppm assigned to −O− CH2− protons in PBA (Figure 1a). The PBA-g-PMMA showed a symmetric unimodal SEC curve, indicating successful polymerization without significant side reactions (Figure 1b). Because the hydrodynamic radius of comb-shaped copolymers is inconsistent with those of the linear polymer analogue with the same molecular weight, the Mn values determined by SEC would slightly deviate from the absolute molecular weight. The PDI increased from 1.44 to 1.55 and then decreased to 1.48 with increasing PMMA branch content. The number density of

Figure 1. (a) 1H NMR spectra and (b) SEC curves for the PMMA macromonomer and free PBA-g-PMMA (MG-32).

the branching point increases with an increasing volume fraction of PMMA branch chains. Copolymerization of the PMMA macromonomer leads to a broadening of the molecular weight distribution because the molecular weight asymmetry in the monomer components is involved in the copolymers.34 C

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules This tendency is remarkable in the case of a low PMMA branch chain content, whereas it diminishes if the copolymer chain includes sufficient PMMA branch chains. In addition, the hydrodynamic radius of gyration in comb-shaped copolymers usually get smaller in comparison with that of their linear analogues. The contraction factor depends on various factors including the degree of polymerization, branching density, and chain rigidity. Therefore, it is rational that the molecular weight distribution does not monotonically increase with an increasing volume fraction of PMMA branch chains. The consistency of chain structure between the polymer brushes on nanoparticles and free polymers in the polymerization solution was verified in a previous report.35 However, in the case of comb-shaped copolymer brushes, it is rational that the Mn, PDI, and chain architecture including the number of PMMA branches are inconsistent with those of free polymers. The bulky PMMA macromonomer has disadvantage in approaching the propagating chain ends. This effect is encouraged at the propagating chain ends of the polymer brush, where the neighboring chains are crowded. In addition, the comb-shaped copolymer brush chains may involve a gradient of the monomer composition along the main chain, because the accessibility of PMMA macromonomers to the propagating chain ends at the vicinity of the particles is inferior to that of BA. Besides, PBA-g-PMMA comb-shaped copolymer brush-grafted SiNPs with a higher PMMA branch content were not obtained through the SI-ATRP process probably because of the steric inhibition by the bulky PMMA branch beside the propagating chain end. MG-32 was the sample with the highest PMMA branch content that we have synthesized. It should be noted that the steric hindrance of the bulky PMMA branch would lead the reduction of the grafting density. The bulky PMMA branch chains would inhibit the approach of monomers toward the propagating chain ends, leading to the reduction of effective propagating radicals in a unit area. As a result, the graft densities of MG series would be lower than that of RA-23, whereas the graft density reduces with an increase in the PMMA branch fraction. Meanwhile, the branch chains in the comb-shaped block copolymers promote the extension of the main chain because of the excluded volume effect beside the branching points. Ordered Structure of PBA-g-PMMA Brush-Grafted SiNPs in the Composite Films. The as-prepared composite films through the solvent-cast process were subjected to thermal aging under hot-pressing at 150 °C for 3 h to form the ordered lattice. The lattice defects decreased through thermal annealing. The composite films were thermally stable in the moderate temperature range for organic polymers. The composite colloidal crystal films exhibited a pale blue interference color (Figure 2a, left panel). The surface morphology of the as-cast composite films was observed by SEM (Figure 2a, right panel). The film surface was covered with close-packing structure by particles, while the undulated topographic geometry existed. Figure 2b shows 1D USAXS profiles of the composite films. The scattering intensity profiles were reproduced by the paracrystal model of the colloidal face-centered cubic (fcc) lattice that is a convolution of the fcc structure factor36,37 and uniform sphere form factor.38,39 The diameter of silica nanoparticles was determined to be 114 nm. In the case of a composite film with a PBA-g-PMMA brush, the d111 lattice plane spacing increased with an increase in the polymer brush weight fraction (Table 2). The polymer brush weight fraction

Figure 2. (a) Picture and SEM image of the nanocomposite film with PBA-g-PMMA brush-grafted SiNPs obtained by solvent-casting (MG24). (b) 1D USAXS profiles of the nanocomposite films.

Table 2. Characterization of the Composite Film

a

sample ID

SiNPs weight fractiona (%)

d111b (nm)

SiNPs diameterb (nm)

MG-10 MG-24 MG-32 RA-23

21 23 28 23

181 165 159 173

114 114 114 114

Determined by TGA. bDetermined by SAXS.

roughly associates with the thickness of the polymer brush layer. The steric repulsion of the polymer brushes leads to the expansion of the interparticle distance. Although the PBA-gPMMA brush chains are partly interdigitated at the shell boundary region to some extent, the penetration depth is limited. Because of the excluded volume effect of the neighboring brush chains and branch chains, the main chain of the PBA-g-PMMA brush would take a substantially extended conformation. Since the chain dimension depends on the molecular weight of the comb-shaped copolymer, the chain length of branch chains, the degree of branching, and the grafting density, the interparticle distance would not be simply in reverse proportion to the weight ratio of the SiNPs. Molecular Aggregation Structure of the PBA-gPMMA Brush in Composite Films. Figure 3 shows the temperature dependence of the dynamic storage modulus, E′, the dynamic loss modulus, E″, and loss tangent, tan δ, of the MG-24 film at 1 Hz. A strong E″ absorption was observed at −41 °C, whereas a weak absorption was observed at 75 °C.

Figure 3. Temperature dependence of dynamic viscoelastic properties for MG-24 at 1 Hz. D

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The absorptions at −41 and 75 °C are assigned to the αrelaxations of the PBA-rich and PMMA-rich phases, respectively. The composite film exhibited a storage modulus of 1 GPa at 1 Hz below the α-relaxation temperature of PBArich phases, whereas it decreased to 3 MPa after the αrelaxation. Meanwhile, the rubber elasticity was lost above the α-relaxation temperature of PMMA-rich phases. These results clearly indicate that the rubbery PBA segments in the main chain and glassy PMMA branches are microphase-separated into two phases. The MG-24 film exhibited a rubbery plateaulike behavior, but the E′ value gradually decreased with increasing temperature. Furthermore, the tan δ peak at −32 °C exhibited a broad shoulder at the higher temperature side. This shoulder was much broader than that of the PMMA-b-PBA-bPMMA linear triblock copolymer.40 The downward slope in the E′ curve and the broad shoulder in the tan δ curve indicate that the rubbery PBA chains and glassy PMMA chains are partially phase-mixed. MG-10 films were elongated at around 90 °C (Figure S5). Because the PMMA volume fraction is lower than that of MG-24, the PMMA-rich hard domains would be defective to cause softening at low temperatures. SAXS and USAXS are powerful tools to figure out phaseseparated polymer morphologies. However, in the case of SiNP-based composite films, the large electron density contrast between polymer brushes and SiNPs prevents the exploration of morphology in the polymer brush continuous boundary phase. Mays et al. investigated the morphology of PBA-gPMMA comb-shaped block copolymers by atomic force microscopic observation.36 They found that the comb-shaped block copolymers with relatively short PMMA branches (Mn = 5300) showed obscure phase boundaries compared with the copolymers with longer PMMA branches (Mn = 11 700). We tried to unravel the morphology by SAXS measurement. The free MG-30 PBA-g-PMMA comb-shaped block copolymer films, which correspond to the polymer brushes in MG-24 and MG-32, showed a very broad peak, whereas the free RA-32 P(BA-r-PMMA) random copolymer films exhibited no significant scattering peaks (Figures S6 and S7). The broad scattering peak is attributed to disordered, but weakly aggregated PMMA-rich domains. Namely, the PBA-g-PMMA comb-shaped block copolymers cause phase separation, but the PMMA domain morphology and the interdomain distance are disordered. Because the PMMA branch chains are randomly distributed in the PBA main chain, the comb-shaped polymers are an ensemble of asymmetric polymer chains. The nonuniform polymer chains are unlikely to produce an ordered morphology with long-range order.26,41 Moreover, the multiple high-molecular-weight branch architecture delays the kinetics to reach the equilibrium morphology. Besides, molecular dynamics calculation suggests that the segregation of branch chains depends on the chain length.42 The comb-shaped polymer brush would produce a similar morphology nearby the particle interface in the composite colloidal crystal films, although the interaction with hard SiNPs and chain confinements may vary the morphology to a certain extent. Mechanical Properties of Composite Films. Figure 4a shows the continuous uniaxial tensile stress−elongation curves of the composite films. Composite films with the PBA-gPMMA brush were stretched over 150% elongation at break without yielding, and their maximum tensile strength increased with increasing PMMA content. The stress−elongation curves were similar to those silica-filled rubbers.43 The tensile behavior involves the dispersion of silica particles in the

Figure 4. (a) Continuous and (b) cyclic stress−elongation curves by uniaxial deformation for the composite films with comb-shaped and random copolymer brush-modified SiNPs.

rubbery matrix without significant aggregation. The tensile mechanical properties can be controlled by adjusting the PMMA content. MG-24 showed elastic recovery in the cyclic tensile test, whereas MG-32 exhibited greater mechanical strength and maximum stress at break than MG-24. On the other hand, RA-23 showed poor tensile strength. Figure 4b shows cyclic uniaxial stress−elongation curves of the composite films. RA-23 exhibited significant softening and hysteresis loss, and the residual strain rapidly increased with increasing maximum strain in the cycles. The normalized residual strains with respect to the maximum strain are shown in Figure S8f. This large plastic deformation could be attributed to the irreversible slippage as well as the collapse of networks in the polymer brush matrix. In contrast, MG series showed an elastic recovery superior to that of RA-23. The elastic recovery was also better than those of previously reported composite colloidal crystals composed of polymer brush-grafted nanoparticles with cohesive subgroups.19 The PMMA hard domains in MG-24 are mechanically robust enough to keep the network structure even at high elongation, whereas the random copolymer brushes produce weak networks that collapse immediately with mechanical stress. The mechanical robustness of the composite film with the PBA-g-PMMA brush would be attributed to the multibridging of the PBA chains over multiple PMMA hard domains. The lattice structure deformation during the uniaxial elongation was investigated by in situ USAXS measurement. Figure 5 shows the evolution of USAXS patterns of MG-24 and RA-23 films at various strains. At the initial state (0%), MG-24 showed isotropic circular scattering assigned to the isotropic SiNP lattice structure. The scattering pattern turned to an anisotropic diffuse pattern with conversion, indicating the orientation of the lattice. The sector-averaged 1D scattering intensity profiles of MG-24 along with the stretching direction are shown in Figure 6. The first and second scattering peaks shifted to the low q side, indicating the expansion of the lattice plane to the stretching direction. In contrast, the scattering E

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. 2D USAXS pattern evolution of (a) MG-24 and (b) RA-23 films during uniaxial stretching.

well-defined comb-shaped block copolymer brush. The PBA-gPMMA brush produced a rubbery interparticle boundary phase to give an elastic network to the colloidal crystal. The PBA-gPMMA comb-shaped block copolymer brush caused phaseseparation to give glassy PMMA-rich domains without longrange order, yielding a robust network. The composite films exhibited elastic recovery over 100% maximum strain. The efficient elasticity is attributed to the multibridging of the rubbery segments over glassy domains in the comb-shaped polymer brushes. SiNPs are arranged in a well-ordered fcc lattice structure in the composite films to exhibit interference color. The ordered colloidal crystal deformed through the entropic elasticity of the polymer brush boundary phase and the local lattice structure recovered without significant distortion, whereas the lattice slippage occurred above the threshold stress depending on the mechanical robustness of the polymer brush boundary phase. The mechanically robust composite colloidal crystals offer the potential for elastic mechanochromic polymer nanocomposites.

Figure 6. Evolution of the sector-averaged USAXS intensity profiles of MG-24 film in the stretching direction of the film during continuous uniaxial stretching. The sector-averaged region is shown in the left-most panel of Figure 5a. The film elongations are shown beside the corresponding profiles.

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peaks derived from the sphere form factor in the high q region showed no shift of fringe and conversion, because the hard sphere of SiNPs hardly deforms by the stretching. The scattering pattern evolution was highly consistent with those of previously reported colloidal crystals with a cohesive polymer brush.44 Other nanocomposite films (MG-32 and MG-10) showed features similar to MG-24 (Figure S10). RA-23 also showed a similar scattering pattern change with elongation, but the lattice deformation and peak conversion were not significant. The scattering peak no longer shifted from 20% strain to break. This result suggests that the lattice deformed in response to the film strain until 20%, but the lattice slippage occurred under further elongation. The anisotropic SAXS pattern in MG-24 and RA-23 during elongation almost recovered to the initial pattern after the loading−unloading process (Figures S11 and S12). Therefore, the lattice deformed through the entropic elasticity of the interparticle polymer brush boundary phase by the applied stress, and the local lattice structure recovered without significant residual strain. However, the lattice slippage occurred above the threshold stress depending on the mechanical robustness of the polymer brush boundary phase to cause plastic deformation of the films.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00927.

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(1) Preparation of P(BA-r-MMA) brush (RA-23), free (unbound) PBA-g-PMMA (free MG-30), and free (unbound) P(BA-r-MMA) (free RA-32); (2) paracrystal model for the random-oriented fcc lattice; (3) DSC and TGA curves of the composite films; (4) viscoelastic properties of the composite films; (5) 1D SAXS profiles of free PBA-g-PMMA and P(BA-r-MMA) films; (6) stress−strain curves in the cyclic tensile test of the composite films; (7) 2D USAXS patterns and sectoraveraged 1D USAXS intensity profiles in the in situ USAXS measurements of the composite films during uniaxial elongation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +81-97-554-7895 (Y.H.). *E-mail: [email protected]. Fax: +81-92-802-2518. Tel: +81-92-802-2517 (A.T.).

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CONCLUSIONS Nanocomposite elastomers composed of SiNPs modified with comb-shaped elastic polymer brushes were proposed. PMMA macromonomers with narrow PDI were incorporated into polymer brush chains by the grafting-from approach to yield a

ORCID

Ken Kojio: 0000-0002-6917-7029 Yuji Higaki: 0000-0002-1032-4661 Atsushi Takahara: 0000-0002-0584-1525 F

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Author Contributions

Crystal with Solvent and Redox Tunability: A Step towards Photonic Ink (P-Ink). Adv. Mater. 2003, 15, 503−507. (12) Takeoka, Y.; Watanabe, M. Controlled Multistructural Color of a Gel Membrane. Langmuir 2003, 19, 9554−9557. (13) Stein, A.; Li, F.; Denny, N. R. Morphological Control in Colloidal Crystal Templating of Inverse Opals, Hierarchical Structures, and Shaped Particles. Chem. Mater. 2008, 20, 649−666. (14) Iwasaki, T.; Tamai, Y.; Yamamoto, M.; Taniguchi, T.; Kishikawa, K.; Kohri, M. Melanin Precursor Influence on Structural Colors from Artificial Melanin Particles: PolyDOPA, Polydopamine, and Polynorepinephrine. Langmuir 2018, 34, 11814−11821. (15) Ohno, K.; Morinaga, T.; Takeno, S.; Tsujii, Y.; Fukuda, T. Suspensions of Silica Particles Grafted with Concentrated Polymer Brush: A New Family of Colloidal Crystals. Macromolecules 2006, 39, 1245−1249. (16) Ohno, K.; Morinaga, T.; Takeno, S.; Tsujii, Y.; Fukuda, T. Suspensions of Silica Particles Grafted with Concentrated Polymer Brush: Effects of Graft Chain Length on Brush Layer Thickness and Colloidal Crystallization. Macromolecules 2007, 40, 9143−9150. (17) Huang, Y.; Morinaga, T.; Tai, Y.; Tsujii, Y.; Ohno, K. Immobilization of Semisoft Colloidal Crystals Formed by PolymerBrush-Afforded Hybrid Particles. Langmuir 2014, 30, 7304−7312. (18) Nakanishi, Y.; Ishige, R.; Ogawa, H.; Sakakibara, K.; Ohno, K.; Morinaga, T.; Sato, T.; Kanaya, T.; Tsujii, Y. USAXS Analysis of Concentration-Dependent Self-Assembling of Polymer-Brush-Modified Nanoparticles in Ionic Liquid: [I] Concentrated-Brush Regime. J. Chem. Phys. 2018, 148, No. 124902. (19) Williams, G. A.; Ishige, R.; Cromwell, O. R.; Chung, J.; Takahara, A.; Guan, Z. B. Mechanically Robust and Self-Healable Superlattice Nanocomposites by Self-Assembly of Single-Component ″Sticky″ Polymer-Grafted Nanoparticles. Adv. Mater. 2015, 27, 3934− 3941. (20) Huang, Y. C.; Zheng, Y.; Sarkar, A.; Xu, Y. M.; Stefik, M.; Benicewicz, B. C. Matrix-Free Polymer Nanocomposite Thermoplastic Elastomers. Macromolecules 2017, 50, 4742−4753. (21) Jiang, F.; Zhang, Y. Q.; Wang, Z. K.; Wang, W. T.; Xu, Z. H.; Wang, Z. Q. Combination of Magnetic and Enhanced Mechanical Properties for Copolymer-Grafted Magnetite Composite Thermoplastic Elastomers. ACS Appl. Mater. Interfaces 2015, 7, 10563−10575. (22) Uhrig, D.; Schlegel, R.; Weidisch, R.; Mays, J. Multigraft Copolymer Superelastomers: Synthesis Morphology, and Properties. Eur. Polym. J. 2011, 47, 560−568. (23) Uhrig, D.; Mays, J. Synthesis of Well-Defined Multigraft Copolymers. Polym. Chem. 2011, 2, 69−76. (24) Staudinger, U.; Weidisch, R.; Zhu, Y.; Gido, S. P.; Uhrig, D.; Mays, J. W.; Iatrou, H.; Hadjichristidis, N. Mechanical Properties and Hysteresis Behaviour of Multigraft Copolymers. Macromol. Symp. 2006, 233, 42−50. (25) Zhu, Y. Q.; Burgaz, E.; Gido, S. P.; Staudinger, U.; Weidisch, R.; Uhrig, D.; Mays, J. W. Morphology and Tensile Properties of Multigraft Copolymers with Regularly Spaced Tri-, Tetra-, and Hexafunctional Junction Points. Macromolecules 2006, 39, 4428− 4436. (26) Beyer, F. L.; Gido, S. P.; Buschl, C.; Iatrou, H.; Uhrig, D.; Mays, J. W.; Chang, M. Y.; Garetz, B. A.; Balsara, N. P.; Tan, N. B.; Hadjichristidis, N. Graft Copolymers with Regularly Spaced, Tetrafunctional Branch Points: Morphology and Grain Structure. Macromolecules 2000, 33, 2039−2048. (27) Xenidou, M.; Beyer, F. L.; Hadjichristidis, N.; Gido, S. P.; Tan, N. B. Morphology of Model Graft Copolymers with Randomly Placed Trifunctional and Tetrafunctional Branch Points. Macromolecules 1998, 31, 7659−7667. (28) Duan, Y. X.; Thunga, M.; Schlegel, R.; Sehneider, K.; Rettler, E.; Weidisch, R.; Siesler, H. W.; Stamm, M.; Mays, J. W.; Hadjichristidis, N. Morphology and Deformation Mechanisms and Tensile Properties of Tetrafunctional Multigraft Copolymers. Macromolecules 2009, 42, 4155−4164. (29) Ohno, K.; Morinaga, T.; Koh, K.; Tsujii, Y.; Fukuda, T. Synthesis of Monodisperse Silica Particles Coated with Well-Defined,

The manuscript was written through contributions of all authors, and all authors have approved the final version of the manuscript. A.T. and Y.H. conceived and directed the project. H.S. performed the experiments and analyzed the results. C.H.C., K.K., and K.K. contributed to the SAXS experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. This work was supported in part by the “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” (MEXT). USAXS experiments were performed at BL05 in SPring-8 with the approval of RIKEN. Part of SAXS and USAXS experiments were performed at BL05XU, BL40XU, BL19B2, and BL20XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2018A1008, 2018A1030, 2018B1035, 2018B1024, 2018B1036, 2017A1028, 2017A1029, 2017B1016, 2017B1025, 2016A1031, 2016A1018, 2016B1034, 2016B1035). We gratefully acknowledge Dr Taiki Hoshino, Dr So Fujinami, and Dr Tomotaka Nakatani for their assistance with the USAXS measurements.

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ABBREVIATIONS SiNPs,silica nanoparticles; PBA-g-PMMA,poly(butyl acrylate)graf t-poly(methyl methacrylate); P(BA-r-MMA),poly(butyl acrylate-random-methyl methacrylate)

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REFERENCES

(1) Hiltner, P. A.; Krieger, I. M. Diffraction of Light by Ordered Suspensions. J. Phys. Chem. A 1969, 73, 2386−2389. (2) Okubo, T. Polymer Colloidal Crystals. Prog. Polym. Sci. 1993, 18, 481−517. (3) Park, S. H.; Xia, Y. N. Assembly of Mesoscale Particles over Large Areas and Its Application in Fabricating Tunable Optical Filters. Langmuir 1999, 15, 266−273. (4) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Self-Assembly Motif for Creating Submicron Periodic Materials - Polymerized Crystalline Colloidal Arrays. J. Am. Chem. Soc. 1994, 116, 4997−4998. (5) Lu, Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. Synthesis and SelfAssembly of Au@SiO2 Core-Shell Colloids. Nano Lett. 2002, 2, 785− 788. (6) Xia, J. Q.; Ying, Y. R.; Foulger, S. H. Electric-Field-Induced Rejection Wavelength Tuning of Photonic Bandgap Composites. Adv. Mater. 2005, 17, 2463−2467. (7) Furumi, S.; Fudouzi, H.; Miyazaki, H. T.; Sakka, Y. Flexible Polymer Colloidal-Crystal Lasers with a Light-Emitting Planar Defect. Adv. Mater. 2007, 19, 2067−2072. (8) Fudouzi, H.; Xia, Y. N. Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers. Langmuir 2003, 19, 9653−9660. (9) Gu, Z. Z.; Kubo, S.; Qian, W. P.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. Varying the Optical Stop Band of a ThreeDimensional Photonic Crystal by Refractive Index Control. Langmuir 2001, 17, 6751−6753. (10) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Gornitz, E. Ordered Arrays of Large Latex Particles Organized by Vertical Deposition. Langmuir 2002, 18, 3319−3323. (11) Arsenault, A. C.; Miguez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. A Polychromic, Fast Response Metallopolymer Gel Photonic G

DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules High-Density Polymer Brushes by Surface-Initiated Atom Transfer Radical Polymerization. Macromolecules 2005, 38, 2137−2142. (30) Seki, A.; Ishiwata, F.; Takizawa, Y.; Asami, M. Crossed Aldol Reaction Using Cross-Linked Polymer-Bound Lithium Dialkylamide. Tetrahedron 2004, 60, 5001−5011. (31) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods 2012, 9, 671−675. (32) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan. High Pressure Res. 1996, 14, 235−248. (33) Zeng, F. Q.; Shen, Y. Q.; Zhu, S. P. Synthesis of StyrenicTerminated Methacrylate Macromonomers by Nitroanion-iInitiated Living Anionic Polymerization. Macromol. Rapid Commun. 2001, 22, 1399−1404. (34) Goodwin, A.; Wang, W. Y.; Kang, N. G.; Wang, Y. Y.; Hong, K. L.; Mays, J. All-Acrylic Multigraft Copolymers: Effect of Side Chain Molecular Weight and Volume Fraction on Mechanical Behavior. Ind. Eng. Chem. Res. 2015, 54, 9566−9576. (35) Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Polystyrene and Poly(3-vinylpyridine)-Grafted Magnetite Nanoparticles Prepared through Surface-Initiated Nitroxide-Mediated Radical Polymerization. Macromolecules 2004, 37, 2203−2209. (36) Matsuoka, H.; Tanaka, H.; Hashimoto, T.; Ise, N. ElasticScattering from Cubic Lattice Systems with Paracrystalline Distortion. Phys. Rev. B 1987, 36, 1754−1765. (37) Matsuoka, H.; Tanaka, H.; Iizuka, N.; Hashimoto, T.; Ise, N. Elastic-Scattering from Cubic Lattice Systems with Paracrystalline Distortion. II. Phys. Rev. B 1990, 41, 3854−3856. (38) Narayanan, T. Soft-Matter Characterization; Borsali, R., Pecora, R., Eds.; Springer: Dordrecht, 2008; Chapter 17, pp 899−952. (39) Hamley, I.; Castelletto, V. Soft-Matter Characterization; Borsali, R., Pecora, R., Eds.; Springer: Dordrecht, 2008; Chapter 20, pp 1021−1081. (40) Tong, J. D.; Leclere, P.; Doneux, C.; Bredas, J. L.; Lazzaroni, R.; Jerome, R. Morphology and Mechanical Properties of Poly(methylmethacrylate)-b-Poly(alkylacrylate)-b-P oly(methylmethacrylate). Polymer 2001, 42, 3503−3514. (41) Qi, S. Y.; Chakraborty, A. K.; Wang, H.; Lefebvre, A. A.; Balsara, N. P.; Shakhnovich, E. I.; Xenidou, M.; Hadjichristidis, N. Microphase Ordering in Melts of Randomly Grafted Copolymers. Phys. Rev. Lett. 1999, 82, 2896−2899. (42) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics - Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (43) Ito, M.; Isago, H.; Suzuki, N. ESR Study of Silica-Filled SBR with Different Rubber/Filler Interactions. J. Appl. Polym. Sci. 2008, 108, 1385−1392. (44) Ishige, R.; Williams, G. A.; Higaki, Y.; Ohta, N.; Sato, M.; Takahara, A.; Guan, Z. B. In Situ Ultra-Small-Angle X-ray Scattering Study under Uniaxial Stretching of Colloidal Crystals Prepared by Silica Nanoparticles Bearing Hydrogen-Bonding Polymer Grafts. IUCrJ 2016, 3, 211−218.

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DOI: 10.1021/acs.macromol.9b00927 Macromolecules XXXX, XXX, XXX−XXX