Molecular Aggregation States and Physical Properties of Syndiotactic

Publication Date (Web): August 9, 2017 ... stability of the δ form sPS crystals in the spherical sPS domains during uniaxial stretching were demonstr...
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Molecular Aggregation States and Physical Properties of Syndiotactic Polystyrene/Hydrogenated Polyisoprene Multiblock Copolymers with Crystalline Hard Domain Yuji Higaki,†,‡,§ Ken Suzuki,‡ Yudai Kiyoshima,‡ Tomoyuki Toda,∥ Masayoshi Nishiura,∥ Noboru Ohta,⊥ Hiroyasu Masunaga,⊥ Zhaomin Hou,∥ and Atsushi Takahara*,†,‡,§ †

Institute for Materials Chemistry and Engineering, ‡Graduate School of Engineering, and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ⊥ Japan Synchrotron Radiation Research Institute/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: Molecular aggregation structure and mechanical as well as thermal properties of novel well-defined multiblock copolymers consisting of crystalline syndiotactic polystyrene (sPS) and rubbery hydrogenated polyisoprene (hPIp) were investigated. The morphology and crystalline ordered structure of the multiblock copolymer films prepared by solvent casting from 1,2-dichlorobenzene solution depended on the volume fraction of sPS (VFsPS) and number of blocks. The multiblock copolymer films exhibited ordered morphology with low crystallinity. The crystallinity of the sPS reduced with decreasing the VFsPS. The pentablock copolymer produced more ordered morphology and less crystallinity than the triblock copolymers. The anisotropic orientation and mechanical stability of the δ form sPS crystals in the spherical sPS domains during uniaxial stretching were demonstrated. Tensile testing and dynamic mechanical analysis indicated that these multiblock copolymer films with appropriate sPS fraction are strong, tough, and elastic and thus could be potential candidates for a new type of thermoplastic elastomer with discrete crystalline hard domains.

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melt-processing, while the γ and δ forms are identified in solvent casting or solvent treatment. The α (hexagonal unit cell) and β (orthorhombic unit cell) forms are of all-trans planar zigzag conformation in their backbone. These two crystalline forms are coexisting, and the fraction depends on the thermal history. The β form is the favored type of thermodynamically stable packing, while the α form is kinetically favorable to be observed in cold crystallization. The equilibrium melting temperatures are T0m,α = 281.7 °C and T0m,β = 288.7 °C, which are extremely higher than the Tg of atactic PS of about 100 °C; thus, the semicrystalline sPS has advantage in the heat deflection temperature compared with glassy atactic PS. The γ and δ forms are produced by cocrystallization with low molecular weight substances such as toluene, 1,2-dichloroethane, and dichlorobenzene to produce polymeric clathrates; i.e., polymer crystals consist of a lattice with cavities in which specific molecules can be introduced.12 The γ and δ forms are of monoclinic unit cell with helical chain conformation, and the lattice dimension depends on the molecules trapped in the crystal. The δ form is an unstable

INTRODUCTION Thermoplastic elastomers are a class of polymers that have been used as elastic products able to be produced by melt processing.1 The glassy domains and/or crystallites are employed as cross-linking points of the network structure for rubber elasticity. The cross-linking points disappear by the glass transition or melting to cause plastic flow. Block copolymers consisting of rubbery polyolefins or polydienes and glassy polystyrene (PS) are typical thermoplastic elastomers. PS− polybutadiene−PS triblock copolymers (SBS) and PS− polyisoprene−PS triblock copolymers (SIS) are representative elastic block copolymers, and the families are commonly used in commercial products.2−8 However, the heatproof temperature is limited by the glass transition temperature (Tg) of the glassy domains. The plastic deformation of the glassy domains with low molecular weight PS occurs even at room temperature because of the stress concentration to the small spherical microdomains and less entanglement in the PS chains, resulting in the stress relaxation and fracture of the films.7 Syndiotactic polystyrene (sPS) is known as a semicrystalline stereoregular polymer.9−12 The sPS exhibits polymorphism, whereas atactic PS produces amorphous phase. Four crystal forms (α, β, γ, and δ) exist, and the crystal form depends on the crystallization process.13 The α and β forms are produced in © XXXX American Chemical Society

Received: June 8, 2017 Revised: July 17, 2017

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DOI: 10.1021/acs.macromol.7b01193 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules mesophase that transforms readily to the α, β, and γ forms by heating. Needless to say, if the block copolymers consist of the sPS instead of atactic PS, and both the morphology and crystalline structure are well-controlled, novel thermally stable thermoplastic elastomers are developed. Syndio-specific polymerization of styrene was first achieved more than three decades ago by use of homogeneous titanium catalysts.14−17 But the catalyst system cannot be applied to a controlled polymerization of diene monomers, so that the stereospecific copolymerization of styrene and dienes has been unfeasible. Hou et al. developed a new catalyst system for the copolymerization of styrene with conjugated dienes in a living, syndio-specific fashion.18−20 The cationic half-sandwich scandium alkyl species generated from the dialkyl precursors allow to produce well-defined multiblock copolymers consisting of sPS and polyisoprene (PIp) by sequential chain extension.19 The microphase separation and crystallization of sPS chains were indicated by the discrete baseline shift and melting endotherm in the DSC thermograms. This suggests that the multiblock copolymers with appropriate volume fraction of sPS could produce well-ordered morphology consisting of discrete sPS domains including thermally and mechanically stable crystals and thus yield elastomers providing a unique combination of mechanical and thermal stability. In this paper, we elaborate on the molecular aggregation structure and physical properties of the novel well-defined stereoregular sPS−hydrogenated PIp (hPIp) multiblock copolymers. We designed a series of linear multiblock copolymers with identical sPS block molecular weight to figure out the dependence of sPS volume fraction (VFsPS) on the morphology and physical properties. The PIp chains were hydrogenated to prevent oxidation of the unsaturated bonds. The morphology, crystalline structure, and thermal and mechanical properties were comprehensively explored by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), tensile tests, and dynamic mechanical analysis (DMA). The morphology evolution in the stretching process was investigated by in situ SAXS/WAXD measurements during uniaxial stretching.

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and syndio-specific fashion to give block copolymers with narrow molecular weight distribution and fine stereoregularity. The PIp chains were hydrogenated by 7−8 equiv of p-toluenesulfonyl hydrazide (Tokyo Chemical Industry, >98%) at 90 °C for 5 days in toluene to prevent oxidation and succeeding cross-linking during film preparation process.21 The hydrogenated PIp was designated as hPIp. The chemical characterization of the multiblock copolymers employed to this research was conducted by 1H NMR spectroscopy (Figure S1). The sPS content was determined by integration of the resonances. The completion of the hydrogenation was confirmed by disappearance of the alkenyl proton peaks at 4.7 ppm for 3,4-polyisoprene and 5.1 ppm for 1,4-polyisoprene. The 3,4-hPIp/1,4-hPIp ratio was determined to be 81/19 by the integration of resonances assigned to 3,4-hPIp and 1,4-hPIp. Characteristics of the multiblock copolymers used in this study are summarized in Table 1.

Table 1. Sample ID and Characteristics of the sPS-hPIp Multiblock Copolymersa ID tri_470 tri_800 tri_1800 penta_800

compositionb

Mnc

Mw/Mnc VFsPSd

100/470/100 (sPS/hPIp/sPS) 100/800/100 (sPS/hPIp/sPS) 100/1800/100 (sPS/hPIp/sPS) 100/800/100/800/100 (sPS/hPIp/sPS/hPIp/sPS)

76500 105000 136000

1.16 1.18 1.36

0.37 0.27 0.17

160000

1.37

0.17

a

The sPS-hPIp multiblock copolymers were synthesized by polymerization catalyzed by (C5Me4SiMe3)Sc(CH2SiMe3)2(THF)/[Ph3C][B(C6F5)4].19 bMolar ratio of feed monomer to Sc. cNumber-averaged molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by SEC with 1,2-dichlorobenzene as eluent at 145 °C against atactic PS standard. dVolume fractions of sPS (VFsPS) were determined by the exact sPS content obtained by 1H NMR spectra and density of PS (1.04 g/cm3) and hydrogenated PIp (0.9 g/cm3). Film Preparation. The 1.0 wt % 1,2-dichlorobenzene solutions of the multiblock copolymer were prepared by stirring at 140 °C. The solutions lost fluidity at room temperature because of the network formation through aggregation of the sPS chains. The solution was poured into a Petri dish and dried by holding at 140 °C under dry nitrogen gas flow. The film was dried under vacuum for 12 h and subsequently annealed at 120 °C under vacuum for 12 h (see Figure S2). Measurements. Differential scanning calorimetry (DSC) was performed using a EXSTAR DSC6220 calorimeter (Hitachi HighTech Science Corporation, Tokyo, Japan) in a temperature range from −100 to 300 °C at a scanning rate of 10 °C min−1 under dry nitrogen gas flow. The first heating scan data were employed to understand the molecular aggregation structure of the films. Dynamic mechanical analysis (DMA) was conducted with a dynamic viscoelastometer Rheovibron DDV-IIFP (Orientec A&D Co., Ltd., Tokyo, Japan). A rectangular shape sample was clamped in a gauge length of 15 mm. The measurements were carried out at 1, 3.5, 11, 35, and 110 Hz frequency under a dry nitrogen atmosphere at a heating rate of 2 °C min−1 over the temperature range from −100 to 300 °C. Tensile tests were carried out with a tensile tester EZ-Graph (Shimadzu Co., Ltd., Kyoto, Japan) with a 50 N load cell. A dumbbell shape elastomer film with gauge length of 16 mm was stretched at a crosshead speed of 16 mm min−1 at 298 K. Stress−strain curves in the successive elongation−recovery cycles were obtained by 1.0 strain intervals up to a strain of 5.0. The nominal strain was determined from the displacement of the crosshead. Wide-angle X-ray diffraction (WAXD) measurement was carried out at BL40XU beamline of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). The X-ray beam was focused to 2.9 μm × 3.5 μm (full width at half-maximum) at a sample position, and the wavelength of X-ray beam was set at 0.082 56 nm. A 1344 × 1024 pixel charge-coupled device (C4880-50-24A, Hamamatsu Photonics

EXPERIMENTAL SECTION

Materials. 1,2-Dichlorobenzene (Tokyo Chemical Industry, >99%) was used as received. Tri- and pentablock copolymers consisting of sPS and PIp were synthesized by sequential polymerization of styrene and isoprene with the cationic half-sandwich scandium alkyl species (Figure 1).19 The cationic half-sandwich scandium alkyl species can serve as a catalyst for polymerization of styrene and isoprene in a living

Figure 1. Chemical structures of the stereoregular sPS-hPIp-sPS triblock and sPS-hPIp-sPS-hPIp-sPS pentablock copolymers used in this study. B

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Macromolecules Co., Ltd., Shizuoka, Japan) with X-ray imaging intensifier (V4554P, Hamamatsu Photonics Co., Ltd.) (II-CCD) was used to record the diffraction patterns. The pixel resolution of the detector was 97.5 μm. The camera length was set at 131 mm. The scattering vector, q = (4π/ λ) sin θ, where 2θ is the scattering angle, was calibrated using the peak position of CeO2. The WAXD patterns were acquired by irradiating Xray from the edge. The definition of the film geometry is represented in Figure S2. Small-angle X-ray scattering (SAXS) measurement was carried out at BL03XU beamline of SPring-8.22 The wavelength of X-ray beam was set at 0.1 nm. A 3000 × 3000 pixel imaging plate detector (RAXIS-VII, Rigaku Corporation, Tokyo, Japan) was used to record the scattering patterns. The pixel resolution of the detector was 100 μm. The camera length was set at 3404 mm. The scattering vector was calibrated using the peak positions of collagen. In situ SAXS/WAXD measurements were carried out at the BL40B2 beamline of SPring-8. 2D WAXD and 2D SAXS patterns were acquired with a 1032 × 1032 pixel CMOS flat panel (FP) detector (C9728DK, Hamamatsu Photonics Co., Ltd., Japan; pixel resolution: 50 μm) and II-CCD, respectively. The wavelength of X-ray beam was set at 0.1 nm. The camera lengths were set at 73 mm for WAXD and 4000 mm for SAXS. The film was clamped at both ends with a remote control tensile testing apparatus (Sentech Co., Ltd., Osaka, Japan) equipped with a 20 N load cell with gauge length of 10 mm, which was installed on the beam path. The film is stretched symmetrically in the lateral direction, which ensures that the X-ray beam always irradiates the same position during stretching. The data collection timing was controlled by an external trigger, which enable to synchronize the IICCD and FP detectors. 2D SAXS and 2D WAXD patterns were accumulated every acquisition period of 10 s (exposure time: 2 s (SAXS), 8 s (WAXD), interval = 2 s) during the stretching with 1 mm min−1 stretching rate (0.1 strain min−1). Data processing was carried out with FIT2D software.

Table 2. Thermal Transition Temperatures and Melting Enthalpy in the Multiblock Copolymers Determined by DSC Measurementa ID

Tgb (°C)

tri_470 tri_800 tri_1800 penta_800

−16 −21 −12 −12

Tm,δc (°C)

Tm,βc (°C)

ΔHm,δd (J g−1)

270 266 209

ΔHm,βd (J g−1) 16.4 5.5

4.8

a The multiblock copolymer films were prepared from 1,2-dichlorobenzene solution by solvent casting at 140 °C. bGlass transition temperatures (Tg) were determined as the middle point on the incline of the baseline shift in the DSC profile. cMelting temperatures (Tm) were determined as the endothermic peak in the DSC profile. Tm,δ: Tm of δ form crystals; Tm,β: Tm of β form crystals. dΔHm,δ: melting enthalpy (ΔHm) of δ form crystals; ΔHm,β: ΔHm of β form crystals.

the sPS and thermal transition temperatures depended on the VFsPS and the number of blocks. The multiblock copolymers exhibited a glass transition of the hPIp matrix as a baseline shift at around −15 °C and melting endotherms in the heating scan. The Tg of the hPIp matrix depends on the fraction of 1,4- and 3,4-microstructures and the partial phase mixing of sPS chains. The Tgs shifted to low temperature with increasing VFsPS, indicating the fine phase separation. The endotherms at around 200 and 270 °C are assigned to melting transitions of the δ form and β form crystals of sPS chains, respectively. De Rosa et al. reported that crystallization of sPS through solvent casting from 1,2-dichlorobenzene solutions produces δ form at low temperature and β form at high temperature (>130 °C).11 The 1,2-dichlorobenzene molecules are incorporated in the lattice to give the δ form crystals. If the 1,2-dichlorobenzenes are removed by evaporation, the cavities remain and the lattice dimension slightly varies in the emptied δ form crystals. Melting enthalpy of the sPS crystallites and the fraction of thermally stable β form crystals decreased with decreasing VFsPS, while only the δ form crystal was observed in the low VFsPS copolymers instead of β form crystals even though the solvent casting was carried out at 140 °C (tri_1800, Figure 2c). An exothermic peak was observed at 199 °C only in the cooling scan of the high VFsPS triblock copolymer (tri_470, Figure 2a′). Because the δ form crystal is produced in the presence of solvent, the exothermic peak is assigned to the crystallization of kinetically favored α form crystals. The other triblock copolymers showed no exothermic peaks in the cooling process, indicating the absence of crystallization in the rapid cooling process. Although a weak endothermic heat flow was observed in the pentablock copolymer at 215 °C (penta_800, Figure 2d), no endothermic peak was observed in the second heating process (Figure S3). These results indicate that the crystallization of sPS chains get unfavorable with decreasing the VFsPS and the number of blocks. Molecular Aggregation Structure. Morphology and crystalline structure of the sPS phase were explored by SAXS and WAXD measurements, respectively. Figure 3 shows 2D WAXD patterns of the triblock copolymer films that were taken by irradiating X-ray from the direction normal to the edge plane. The triblock copolymer films exhibited isotropic circular Debye rings in the 2D WAXD patterns if X-ray was incident from the direction normal to the through plane, whereas weakly anisotropic diffraction patterns were observed in the edge view as shown in Figure 3. The β form crystallites show a characteristic diffraction assigned to (020) lattice plane at 4.2

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RESULTS AND DISCUSSION Thermal Properties. DSC thermograms in first heating and subsequent cooling scan for the multiblock copolymer films are shown in Figure 2, and the thermal transition temperatures are summarized in Table 2. The crystallinity of

Figure 2. (A) DSC thermograms for the sPS-hPIp-sPS triblock copolymer films [(a) tri_470, (b) tri_800, (c) tri_1800)] and the sPShPIp-sPSt-PIp-sPS pentablock copolymer film [(d) penta_800]. The solid lines and dashed lines show the first heating and subsequent cooling scans, respectively. C

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Figure 4. (A) 2D SAXS patterns and (B) sector-averaged intensity profiles of the sPS-hPIp-sPS triblock copolymer films (a) tri_470 and (b) tri_800. X-ray was irradiated from the direction normal to the edge plane.

Figure 3. 2D WAXD patterns of the sPS-hPIp-sPS triblock copolymer films (a) tri_470, (b) tri_800, and (c) tri_1800. X-ray was irradiated from the direction normal to the edge plane. The X-ray incident plane is shown in the bottom right corner.

clear scattering spots in vertical to film plane (Figure 4A), whereas the SAXS patterns acquired by irradiating X-ray from the direction normal to the through plate exhibited circular isotropic scattering (Figure S4). The parasitism scattering is attributed to the beam grazing at the edge. The anisotropic scattering implies that the periodic structure has favorable orientation vertical to the film plane. The sector-averaged intensity profiles exhibited a clear scattering peak (Figure 4B). The q/q* of the Bragg peaks, where the q* is scattering vector of the primary peak, are in the ratio of 2/1, indicating the lamellar stacked structure. The long-period (d) was simply determined from the scattering vector of the peak to be 85.9 nm according to the equation d = 2π/q. The phase diagram for linear symmetric ABA triblock copolymers indicates that cylindrical morphology is preferable when the volume fraction of A component is 0.27 (tri_800),25 whereas the morphology was assumed to be lamellar by SAXS result. It may be the metastable morphology produced probably due to the rapid solvent evaporation. Figure 5 shows 2D WAXD and 2D SAXS patterns and the evolution of sector-averaged SAXS intensity profiles of the pentablock copolymer film during uniaxial stretching. The pentablock copolymer films produced ordered morphology as shown in the SAXS patterns with multi isotropic circular scattering peaks at 0.132, 0.278, and 0.444 nm−1 (Figure 5b,c; ε: 0). The first peak is assigned to the interdomain distance (lattice factor), while the second and third peaks are assigned to the particle scattering (form factor) of the scattering bodies, respectively. As the higher-order scattering peaks were not observed for the first peak, the precise lattice assignment (bcc and/or fcc) is unable to determine, but the averaged long period is determined from the first scattering peak to be 47.5 nm. The scattering vectors of the second and third scattering Bragg peaks (qsecond and qthird) are in the ratio of qsecond/qthird = 1/1.6, that is, fully consistent with sphere form scattering. The second and third scattering peaks were well fitted by form factor of uniform sphere26

nm−1,10 while the δ form crystallites show a characteristic diffraction assigned to (010) lattice plane at 5.2 nm−1.12 The triblock copolymers with high VFsPS (tri_470) exhibited polymorphism of thermally stable β form crystals and unstable δ form crystals (Figure 3a). The degree of crystallinity decreased with decreasing VFsPS as shown in the relative diffraction intensity of crystalline ordered structure against the broad amorphous halo peak, while only the δ form crystals are produced in the triblock copolymers with low VFsPS (tri_800 and tri_1800). Because the chain length of the sPS block is identical in the triblock copolymers, the variation in crystalline form and crystallinity would be attributed to the morphology of the sPS domains. The (010) diffraction peak of the δ form crystals showed weak orientation in vertical position to the film plane, indicating the arrangement of chain axis to in-plane. Pentablock copolymers exhibited very weak diffractions assigned to δ form crystallites in the WAXD pattern (Figure 5a), indicating the less crystallinity of the sPS blocks. The semicrystalline polymer chains in crystalline diblock copolymers are less likely to crystallize because the single chain-end is confined to the microphase surface.23 In case of crystalline triblock copolymers with crystalline center block, the crystallinity drastically reduces because the chain-folding is unfavorable as the both chain-ends are confined to the microphase surface.24 Because the center sPS block in the pentablock copolymer is linked with hPIp chains at the both ends, and the block-linking points are confined at the microphase boundary, the center sPS block is unfavorable in crystallization. Since the center sPS block and end sPS blocks coexist in the hard domains, the crystallization of the end sPS blocks would have negative effect by the less crystalline center sPS block. Although we found subtle inconsistent between DSC and WAXD in the crystal form, the decline of crystallinity with decreasing VFsPS and increasing number of blocks were observed in common. Figure 4 shows 2D SAXS patterns of the triblock copolymer films that were taken by irradiating X-ray from the direction normal to the edge plane. The triblock copolymer films showed

⎡ sin(qR ) − qR cos(qR ) ⎤2 ⎥ p(q) = (Δρ)2 V0 2⎢3 (qR )3 ⎦ ⎣ D

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assigned to (010) plane of δ form crystallites converged to the equator (vertical to the stretching direction) along with the stretching (Figure 5a; ε: 0.6, 1.6). The anisotropic diffraction peak conversion of (010) plane diffraction implies that the chain axis in the δ form crystallites aligned parallel to the stretching direction. The SAXS patterns also evolved to anisotropic patterns along with the stretching (Figure 5b). The inner circular scattering changes into elliptical shape immediately followed by splitting to four arcs. Symmetric arcs exist at the outer position. The isotropic circular scattering at 0.278 and 0.444 nm−1 kept the circular pattern and peak position. The sector-averaged scattering intensity profiles in the stretching direction at different stage of stretching are shown in Figure 5c. The first scattering peak shifted to low q side along with the stretching, indicating the expansion and orientation in the lattice structure. While the first scattering peak shifted to high q side in the case of the sector-averaged scattering intensity profile in direction orthogonal to the stretching direction, indicating that the distance of the lattice plane decreased with increasing strain (Figure S6). The scattering pattern evolution is well consistent with the theoretically expected bcc and fcc lattice deformation models, whereas they are not fully consistent due to the deviation from the ideal ordered lattice.28−30 We have also investigated the scattering pattern evolution of elastomeric colloidal crystals with fcc lattice structure and observed the similar scattering pattern evolution.31 The ordered lattice was distorted in the stretching direction, and the aligned distorted lattice was recovered to the initial state by unloading through entropic elasticity of the hPIp matrix. Full analysis of the scattering patterns specifying both the deformation and orientation of the lattice is hard because of the imperfection of the lattice structure, but there is no doubt that the film deformation accompany the rearrangement in lattice structure of sPS domains and the initial lattice structure was recovered by unloading. On the other hand, the outer scattering peaks associate with the form of the scattering bodies. Because the peak position was unchanged during stretching even at large strain, the spherical domains hardly deformed by mechanical stretching. Sakurai et al. reported that the deformation of glassy atactic PS microdomains occurs by stretching due to a high extent of the stress concentration at the microdomains.7,8 The plastic deformation of the spherical domains causes stress relaxation and fracture in the elastomer films. The mechanical stability of the sPS domains would be attributed to the existence of the mechanically stable δ form crystals. As seen in the WAXD profiles, the crystalline diffraction remained even in large strain, indicating that the δ form crystals survive without significant fragmentation and melting (Figure 5a). The mechanical stability of the network junction points is undoubtedly a distinct advantage as elastomeric products. The tri_470 and tri_800 triblock copolymer films caused necking after yielding point, so that the successive molecular aggregation structure change in the elastic stretching process could not be acquired. The tri_1800 films showed uniform elastic deformation in the uniaxial stretching process, and the ordered-morphology evolution was tracked by in situ SAXS/ WAXD measurement (Figures S5c and 6b). During the stretching process, the SAXS profiles exhibited identical variation to penta_800 films. The first peak shifted to low q side in the stretching direction, and the second and third gentle shoulder peaks were invariable during the stretching. However,

Figure 5. (a) 2D WAXD patterns and (b) 2D SAXS patterns of the sPS-hPIp-sPS-hPIp-sPS pentablock copolymer film (penta_800). Xray was irradiated from the direction normal to the edge plane in WAXD, and normal to the through plane in SAXS. (c) Evolution of the sector-averaged SAXS intensity profiles in the stretching direction acquired during uniaxial stretching.

where Δρ is the excess electron density, V0 is the volume of the sphere (V0 = 4πR3/3), and R is the radius of the spheres. The sphere radius was determined by the curve fitting on the basis of the sphere form factor to be 21.2 nm. Taking into account the VFsPS and WAXD results, the spheres are spherical domains of sPS chains including δ form sPS crystals. Meanwhile, the morphology of the tri_1800 films was verified by SAXS measurement (Figure S5). The scattering pattern was similar to that of penta_800 films, but the scattering peaks were weak and broad compared with the penta_800 films indicating the less ordered morphology. As the number of blocks in multiblock copolymers increases, the microphase separation generally becomes more difficult because of the disorientation entropy loss by immobilization of block-linking points at the microphase surface.27 But, the pentablock copolymer produced preferably ordered morphology against the triblock copolymers. As the crystallization of sPS chains are unfavorable in the pentablock copolymer, the ordered morphology would remain without disrupting through crystallization of the sPS blocks. In Situ SAXS/WAXD Measurement during Uniaxial Stretching. The molecular aggregation structure change in the pentablock copolymer film during the uniaxial stretching was investigated by in situ SAXS/WAXD measurement (Figure 5). In the WAXD patterns, the weak circular crystalline diffraction E

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Macromolecules it is inadequate to refer to the morphology change in detail because of the less ordered morphology. Mechanical Properties. Typical stress−strain curves of the sPS-hPIp multiblock copolymers in uniaxial tensile test are shown in Figure 6. As seen in the triblock copolymer series

Figure 6. Cyclic stress−strain curves for the sPS-hPIp-sPS triblock copolymer films (a) tri_470, (b) tri_800, and (c) tri_1800 prepared by solvent casting at 140 °C. (d) Cyclic stress−strain curves for the sPS-hPIp-sPS-hPIp-sPS pentablock copolymer (penta_800).

(Figure 6a−c), the films got soften with decreasing VFsPS. The triblock copolymer with 0.37 VFsPS (tri_470) exhibited high elastic modulus (91.8 MPa) and low elongation at break (strain at break: 0.4), whereas the triblock copolymers with low VFsPS (tri_1800) showed typical stress−strain curves of thermoplastic elastomers such as low elastic modulus, large strain at break, and less hysteresis loss without yielding point. The poorly ordered morphology in high VFsPS triblock copolymer produces a rigid sPS network to cause hysteresis loss through fragmentation of the network. The pentablock copolymer (penta_800) also exhibited rubber elasticity with strain hardening trend. As shown in the SAXS and WAXD data, the pentablock copolymer produces more ordered morphology with less sPS crystallinity than the triblock copolymers. The loop/bridge conformation ratio as well as the regularity of the microdomain lattice associate with the mechanical properties of the multiblock copolymer elastomers.32 The isolated mechanically stable spherical sPS domains hardly cause fragmentation to show extremely low elastic modulus and complete restoration. The mechanical stability of the sPS domains is also supported by the weak stress-softening in the cyclic uniaxial tension test. Strain-hardening behavior was observed at large strain, indicating the absence of plastic deformation and mechanical stability of the network under extension of the chains bridging the cross-linking points. The pentablock structure would be preferable for toughening to prevent chain pullout fracture by bridging plural domains.33,34 Dynamic Mechanical Analysis. Viscoelastic properties of the multiblock copolymers depended on the morphology and crystalline states (Figure 7). The triblock copolymers with high VFsPS (tri_470) showed two loss modulus (E″) peaks at 11 and 106 °C at 11 Hz frequency (Figure 7a). According to the activation energies determined by Arrhenius plots, the first

Figure 7. Temperature dependence of the dynamic tensile storage modulus (E′, open red circle) and the dynamic tensile loss modulus (E″, open blue triangle) at 11 Hz frequency for (a) sPS-hPIp-sPS triblock copolymer (tri_470) and (b) sPS-hPIp-sPS-hPIp-sPS pentablock copolymer (penta_800) films.

absorption is assigned to α-relaxation of the hPIp matrix, while the second absorption is assigned to α-relaxation of the sPS domains. The α-relaxation is associated with the onset of segmental motion in the glass transition of polymers. Although no significant baseline shift was observed in the heating scan DSC profile at around 100 °C, clear E″ absorption peak was observed in the DMA profile. Because of the low sPS content, the heat capacity change at the glass transition of sPS block is not large, thereby the baseline shift is not pronounced. However, DMA analysis is generally sensitive to the activation of the thermal molecular motion through the viscoelastic response. The tri_470 triblock copolymer film kept the elasticity by network structure produced by sPS crystallites even after the glass transition of the amorphous sPS fraction. The film lost the elasticity at 250 °C owing to the melting of the sPS crystallites. The pentablock copolymer films showed single E″ peak at −2 °C that is assigned to α-relaxation of hPIp chains. After the α-relaxation of the hPIp chains, the E′ reduced rapidly then the slope got gentle, but the clear rubbery plateau was not observed. The film was significantly deformed at 80 °C, indicating that the network structure was hardly retained. Although the morphology was well-ordered in the pentablock copolymer films, the degree of crystallinity was quite low. The glass transition of the amorphous sPS induced deformation of F

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the sPS domains to make the film in a state of highly viscous flow.

ACKNOWLEDGMENTS This work was supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). This work was supported by the Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. The synchrotron radiation experiments were performed at BL40B2 (2014B1368, 2016A1020), BL40XU (2013B1442, 2014A1250, 2015B1519), and BL03XU (2013B7261) in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). Dr. Yasushi Okamoto and Mr. Takashi Aoki (DENSO CORP.) are also gratefully acknowledged for kindly providing the opportunity to conduct the experiments in BL03XU.

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CONCLUSIONS Morphology of the novel well-defined multiblock copolymers consisting of semicrystalline stereoregular sPS and rubbery hPIp, and the thermal and mechanical properties were elaborated. The morphology depended on the VFsPS and number of blocks. The morphology and crystalline structure associated with the mechanical performances. The multiblock copolymer films produced ordered morphology with low crystallinity. The crystallinity reduced with decreasing VFsPS. The pentablock copolymer films exhibited more ordered morphology and less crystallinity than the triblock copolymer films due to the inferior crystallinity of the center sPS block. The multiblock copolymer films showed stress−strain curve typical to elastomers, if the VFsPS is appropriate. The anisotropic orientation of δ form sPS crystals in the spherical domains and the mechanical stability of the semicrystalline domains during the uniaxial stretching were demonstrated. This work shows that multiblock copolymers with semicrystalline stereoregular sPS chains have considerable potential in improving the mechanical and thermal performances of conventional thermoplastic elastomers with atactic PS chains. This finding could be extended to creating new families of tougher, stronger, and thermally stable elastic materials. The compatibility of the extreme thermal stability and elasticity through control of the morphology and crystallinity is crucial hurdle for the application, and it is a possible extension of this report.

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ABBREVIATIONS sPS, syndiotactic polystyrene; hPIp, hydrogenated polyisoprene; VFsPS, volume fraction of sPS.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01193. (1) Chemical characterization of the sPS-hPIp multiblock copolymers by 1H NMR spectroscopy; (2) schematic representation of the film preparation process and definition of the film geometry; (3) DSC thermograms of the multiblock copolymers in second heating scan; (4) SAXS data of sPS-hPIp-sPS triblock copolymer films; (5) 2D WAXD and 2D SAXS patterns of the sPShPIp-sPS triblock copolymer film (tri_1800) and evolution of the sector-averaged SAXS intensity profiles during stretching; (6) evolution of the sector-averaged SAXS intensity profiles of tri_1800 and penta_800 films in the direction orthogonal to the stretching direction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81-92-802-2517; Fax +81-92-802-2518 (A.T.). ORCID

Zhaomin Hou: 0000-0003-2841-5120 Atsushi Takahara: 0000-0002-0584-1525 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.macromol.7b01193 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b01193 Macromolecules XXXX, XXX, XXX−XXX