Strain-Induced Deformation of Glassy Spherical Microdomains in

Apr 13, 2017 - Shogo Tomita† , Isao Wataoka†, Noriyuki Igarashi‡, Nobutaka Shimizu‡, Hideaki Takagi‡, Sono Sasaki†, and Shinichi Sakuraiâ€...
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Strain-Induced Deformation of Glassy Spherical Microdomains in Elastomeric Triblock Copolymer Films: Time-Resolved 2d-SAXS Measurements under Stretched State Shogo Tomita,† Isao Wataoka,† Noriyuki Igarashi,‡ Nobutaka Shimizu,‡ Hideaki Takagi,‡ Sono Sasaki,† and Shinichi Sakurai*,† †

Department of Biobased Materials Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

‡

S Supporting Information *

ABSTRACT: We have found extremely low efficiency of the elastomeric properties for SEBS (polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene) triblock copolymers having short polystyrene (PS) block chains. Since the SEBS specimens form spherical PS microdomains embedded in the matrix of the rubbery poly(ethylene-co-butylene) (PEB) chains, they exhibit elastomeric properties (thermoplastic elastomer film). However, it was found that the stress was dramatically decreased with time when the specimens were stretched and fixed at strain of 4.0. Furthermore, they showed macroscopic fracture with very short-term duration (400 s to 2 h). To reveal the reason for such low efficiency, we conducted time-resolved two-dimensional small-angle X-ray scattering (2d-SAXS) measurements for the SEBS triblock copolymer films under stretched state at strain of 4.0. Upon stretching, the strain-induced deformation (not fracture) of glassy microdomains was observed. In addition, the deformation of glassy microdomains was found to proceed as time elapsed. Since this deformation of the glassy PS microdomains is considered to result in such the low efficiency of the elastomeric properties, characteristic times related to the deformation and the stress relaxation were evaluated from the change in strain of the glassy microdomains and from the stress relaxation curves, respectively. Then, good agreements of the characteristic times were found, and therefore it was concluded that the deformation of the glassy microdomains has a strong correlation with the stress relaxation and therefore with the fracture of the elastomeric film specimen.

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INTRODUCTION Triblock copolymer comprising hard and soft segments is a useful material having a wide range of mechanical properties from elastomeric to plastic aspects. Although they depend on the fraction of the soft and hard segments, a more important factor is that the mechanical properties can be altered by morphology, such as cylinder, lamellae, and double-gyroid. It has been reported that the shape of the stress−strain curves was much different when the morphologies were controlled for the identical sample.1−3 This result clearly suggests that the mechanical property has its origin in the connectivity of the glassy microdomains with respect to the stretching direction (SD) for the triblock copolymers having minor composition of the hard segments. When triblock copolymer forming glassy cylinders is uniaxially stretched to a direction parallel to which glassy cylinders are oriented, the Young’s modulus becomes high as compared with the case of stretching perpendicular to the glassy cylinders.4−7 Therefore, orientation of microdomains is important for lamellae and cylinders cases. On the other hand, for triblock copolymers with cubic symmetry such as gyroid or spherical microdomains, the effect of the orientation © XXXX American Chemical Society

may be trivial. The more important thing to be considered is the size of grains for these cases because the grain boundaries are mechanically weak points. Little has been revealed for this aspect. In case that triblock copolymers form glassy microdomains in a rubbery matrix, they can be utilized as thermoplastic elastomer films because the glassy microdomains play a role of the physical cross-links for the rubbery chains. Therefore, ease to be stretched and 100% recovery of the stress when released are anticipated. For this requirement, the glassy spherical microdomains are supposed to be eternally hard. However, it has been suspected that the glassy spheres deformed upon stretching of block copolymer films.8−11 The deformation phenomenon was first detected by Seguela and Prud’homme, in which the deformation of the particle scattering peak was seen in the two-dimensional small-angle X-ray scattering (2d-SAXS) patterns measured for a polystyrene-block-polybutadiene-block-polystyrene Received: December 28, 2016 Revised: April 1, 2017

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DOI: 10.1021/acs.macromol.6b02797 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules triblock copolymer specimen.12 Although the 20% of the deformation of the particle scattering peak can be seen in this literature, it was not clearly mentioned in the text. Very recently, we have found that the polystyrene (PS) spherical microdomains are very easily deformed (changing its shape from spherical to prolate ellipsoid) when the spherical microdomains are tiny for SEBS (polystyrene-block-poly(ethyleneco-butylene)-block-polystyrene) triblock copolymers having short PS block chains. Actually, in the previous paper,13 we have reported experimental results of simultaneous measurements of 2d-SAXS patterns and stress−strain curves for two kinds of the SEBS samples (SEBS-8 and SEBS-16), which strongly suggested the deformation (not fracture) of glassy spherical microdomains during stretching. The deformation of the PS microdomains was detected by the 2d-SAXS measurements as shifts of the particle scattering peaks toward lower and higher q regions in directions parallel (q∥) and perpendicular (q⊥) to SD, respectively. But, the shifts of the particle scattering peaks can also be understood with another possibility that anisotropic microdomains already existed in the initial specimen (before stretching), and these would be oriented with their major axes parallel to SD. Furthermore, it was observed that the 2d-SAXS pattern with the deformed (elliptic) peak of the particle scattering changed to the pattern with an almost roundshaped particle scattering peak upon unloading from the stretched specimens of SEBS-8 and SEBS-16. Although this fact was interpreted as the unorienting process of PS prolate spheroids (eternally deformed PS spheres), at a first glance it might imply that once-deformed PS spheres recovered the original spherical shape. Of course, this idea implies in turn the elastic deformation aspect of the glassy PS, and therefore it was concluded that this idea is not acceptable in terms of the discussion made in the previous paper. However, to conclude definitely the deformation of the glassy PS spheres, more rigorous evidence is required. For this purpose, we examine the temporal change in the 2d-SAXS pattern for the specimens under uniaxially stretched state, for which the stress relaxation is observed. If the deformed glassy PS microdomains would be able to recover its original shape when unloaded, the deformed PS microdomains would be relaxed somewhat according to the stress relaxation. Namely, it is expected for this case that the 2d-SAXS pattern would exhibit the change from the elliptic peak of the particle scattering to less elliptic one, in the case of the elastic change of the PS microdomain shape. To check this possibility by conducting the time-resolved 2d-SAXS measurements for the specimen under the stretched state is the purpose of the present study.

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sample was kindly provided by Kaneka Corporation, Japan. Note that the SIBS sample contains completely alternating ethylene−butylene moieties so that such a chemical structure is the same as SEBS without any residues of the unhydrogenized double bond in the main chain. Therefore, the SIBS sample is much stable against heat.14 Solutions of these samples with a concentration of 5 wt % are prepared by using toluene as solvent for the SEBS-8 and SIBS samples. As for the SEBS-16 sample, a mixture of dichloromethane (DM) and n-heptane was used, which leads to formation of spherical microdomains in the SEBS-16 as-cast film, while the thermodynamically equilibrium morphology is cylindrical. The mechanism of the formation of nonequilibrium spheres was described in the previous papers in detail.13,15 Solvent evaporation from the solutions of SEBS-8, SEBS-16, and SIBS was fully performed for at least 1 week at room temperature. In order to equilibrate the spherical structure, the SEBS-8 and SIBS as-cast films were further subjected to thermal annealing at 140 °C for 3 days. As for the SEBS-16 as-cast film, we did not conduct the thermal annealing because it induces the transformation from the nonequilibrium spheres to equilibrium cylinders. Therefore, the reason why the thermal annealing was not conducted for the SEBS-16 as-cast film is that spherical microdomains are required for the purpose of this study. As is already reported in our previous publication,13 the glass transition temperature (Tg) of PS (Tg values estimated by differential scanning calorimetry are listed together in Table 1) is much lower than 100 °C, which is a value for PS with sufficiently high molecular weight. The reason for such lower values may be explained by the fact that the molecular weights of PS block chains in these samples were low. However, it should be noted that the Tg for SEBS-16 was lower than that for SEBS-8, although the molecular weight of PS in SEBS-16 was higher than that in SEBS-8. This is because the SEBS-16 specimen was not thermally annealed (as-cast specimen), and an unfavorable effect of solvent evaporation may remain in the specimen, while the other specimens (SEBS-8 and SIBS) were thermally annealed for a long time. Stress Relaxation Measurement. The thermally annealed SEBS-8 and SIBS films and the as-cast film of SEBS-16 were cut into rectangles with a dimension of 2.5 × 40 mm2. Dot markers were marked on the film specimen with a gap of 1.0 mm. The specimens were stretched at room temperature by using a TENSILON/UTM-11-5H (ORIENTEC Co., Ltd., Toyoshima, Tokyo, Japan) with a stretching rate of 100 mm/min until the interval of dots reached 5.0 mm (strain = 4.0). Then, stress relaxation curves for all the specimens were measured as a function of time. Here, the time zero was defined as a moment at which the strain had reached 4. Time-Resolved SAXS Measurements for Specimens under Stretched State. All the specimens for the 2d-SAXS measurements under stretched state were prepared in the same manner for the stress relaxation measurements. The specimens were clamped by a couple of jaws of a handmade stretching apparatus. The rectangular specimens were quickly stretched until the gap of the dots became 5.0 mm (the strain of 4.0). For the SEBS-8 and SEBS-16 specimens, time-resolved 2d-SAXS measurements were conducted at room temperature at BL-10C of Photon Factory in High Energy Accelerator Research Organization (Tsukuba, Ibaraki, Japan) with an exposure time of 10 s. The beamline was composed of a approximate toroidal mirror, monochromator, four-quadrant slits, and a two-dimensional detector (PILATUS 2M, DECTRIS Ltd., Baden, Switzerland), on which X-ray was focused. The wavelength of X-ray was 0.15 nm, and the sample-to-detector distance was 1.5 m. As for the SIBS specimen, a Nano-Viewer (RIGAKU Co., Ltd., Akishima, Tokyo, Japan) was used for the SAXS measurement. The Nano-Viewer was composed of a rotating-anode X-ray generator with an electron gun consisting of a tungsten filament, Osmic Confocal Max Flux Mirror (CMF, Rigaku Co., Ltd.) for focusing X-ray beam, three collimation pinhole slits (0.7, 0.8, and 0.8 mm ϕ for the first, second, and third slits, respectively) to adjust the beam size and to remove parasitic scattering, a vacuum chamber, and a two-dimensional detector (PILATUS 100 K, DECTRIS Ltd.). The wavelength components other than Cu Kα (wavelength λ = 0.154 nm) were eliminated using a Ni filter. The sample-to-detector distance was set to 1.0 m. Note here that the focal

EXPERIMENTAL SECTION

Samples. SEBS-8, SEBS-16, and polystyrene-block-polyisobutylene-block-polystyrene (SIBS) triblock copolymers were also used in the present study. The number-averaged molecular weight (Mn), Mn of the PS block chains, polydispersity index of molecular weight (Mw/Mn), and volume fraction of PS are listed in Table 1 for all the samples, where Mw is weight-averaged molecular weight. The SIBS

Table 1. Characteristics of Samples sample code

Mn

Mn,PS

Mw/Mn

ϕPS

Tg,PS (°C)

SEBS-8 SEBS-16 SIBS

6.7 × 104 6.6 × 104 1.0 × 105

3.1 × 103 6.0 × 103 7.5 × 103

1.04 1.03

0.08 0.16 0.12

60.6 55.9 70.6 B

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Macromolecules length of the CMF mirror is 900 mm and that the distance from the mirror to the 2d-detector (PILATUS 100 K) was 2000 mm in the Rigaku NANO-Viewer SAXS collimation. This means that the X-ray beam on the 2d-detector is defocused. Therefore, measured 2d-SAXS patterns were smeared. However, the extent of smearing is considered to be trivial so that desmearing of the 2d-SAXS pattern was not conducted. The unfavorable effects due to this matter on the data analyses (determination of values such as peak positions of the particle scattering) are considered to be trivial as well. The 2d-SAXS measurement was conducted with an exposure time of 5 min for the SIBS specimen.

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RESULTS AND DISCUSSION Figure 1a shows the stress relaxation curves for all the specimens under stretched state with strain of 4. Note that each

Figure 2. 2d-SAXS patterns measured for the SEBS-8 film (a) before the stretching, (b) 52 s and (c) 572 s elapsed from the quick elongation of the specimen at strain of 4.0, and for the SEBS-16 film (d) before the stretching, (e) 50 s and (f) 3230 s elapsed from the quick elongation (strain = 4.0), and for the SIBS film (g) before the stretching, (h) 5 min and (i) 12 h elapsed from the quick elongation (strain = 4.0).

intensity is not according to the linear scale but to the logarithmic scale. Before stretching (Figures 2a, 2d, and 2g) the particle scattering peak appeared in a high q region (0.52 < q < 0.74 nm−1), while the lattice peaks appeared in a low q region, where q is the magnitude of the scattering vector q, defined as q = |q| = (4π/λ) sin(θ/2) with λ and θ being the wavelength of X-ray and the scattering angle, respectively. Note here that body-centered cubic (BCC) lattice peaks are clearly observed only for SEBS-8 because this specimen was fully annealed. Although the SIBS specimen was also fully annealed, the BCC lattice peaks are not so clear. The reason exists in difference in the mobility of PS block chains arising from the different molecular weights of PS (it is larger in SIBS than in SEBS-8). The BCC lattice peaks were found to be highly deformed upon stretching in Figures 2b, 2c, 2e, 2f, 2h, and 2i. The particle scattering peaks also deformed from the round shape into elliptic ones for the SEBS-8 and SEBS-16 specimens upon stretching. To examine further shifts of the particle scattering peaks with time, 1d-SAXS profiles in q∥ and q⊥ were extracted from the measured 2d-SAXS patterns by conducting sectoraveraging within an anomalously designated sector area (see the definition of such a sector area in the Supporting Information). Figures 3−5 show 1d-SAXS profiles in the q∥ and q⊥ directions for the SEBS-8, SEBS-16, and SIBS specimens, respectively, in which panels c and d highlight the change in the particle scattering peak as a function of time. For SEBS-8 and SEBS-16, not only the strain-induced shifts of the particle scattering peaks were evident (upon stretching), but also the particle scattering peak further shifted to lower q region to a little extent in the q∥ direction under the stretched state (Figures 3c and 4c). Such a tendency was also seen for SEBS-16. These results clearly indicate that the spherical microdomains were deformed upon the stretching and the deformation was proceeding even when the specimen was kept under the stretched state. From the SAXS results, the size of the deformed PS microdomains was evaluated. For this purpose, the model calculation of the 1d-SAXS profiles was conducted by using prolate spheroids as described in the previous paper13 (also see the Supporting Information of the present paper for the details).

Figure 1. (a) Stress relaxation curves for SEBS-8, SEBS-16, and SIBS specimens stretched at strain of 4.0. Stress relaxation curves with linear time scale for (b) SEBS-8 and (c) SEBS-16, in which red lines represent fitting results.

stress relaxation curve was normalized with the stress at the time equaled to 0 s. As clearly seen, SEBS-8 and SEBS-16 show considerable decreases in the stress as compared with SIBS. Furthermore, the SEBS-8 specimen was fractured at 380 s, which is considerably faster than that of SEBS-16 (at 8.0 × 104 s). The relaxation time τstress was evaluated by using the relatioship σ(t) ∝ exp(−t/τstress). As shown in Figure 1b, the semilogarithmic plot of σ/σ0 vs time shows linearity (red straight line) for SEBS-8 in the terminal regime just before fracture. Then, from the slope of the straight line the relaxation time was evaluated. As for SEBS-16, a similar tendency can be seen in the plot of Figure 1c. However, the time regime is not just before the fracture. Instead, we selected this time region to match it for the deformation of PS microdomains (shown later in Figure 7b). Thus, evaluated values of τstress are summarized in Table 2. Figure 2 shows 2d-SAXS patterns measured for all the specimens before stretching and under stretched state at given time elapsed. Note here that the gray scale of the scattering Table 2. Comparison of Characteristic Times Related to the Stress Relaxation and the Deformation of PS Microdomains for SEBS-8 and SEBS-16 sample code

τstress (s)

τdef (s)

SEBS-8 SEBS-16

1.27 × 103 3.41 × 104

1.87 × 103 1.06 × 104 C

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Figure 3. A series of 1d-SAXS profiles extracted from 2d-SAXS patterns measured at given time elapsed from the quick elongation of the specimen at strain of 4.0 for the SEBS-8 specimen: (a, c) parallel and (b, d) perpendicular to the stretching direction; (c, d) highlighting the particle scattering peaks at 52 and 572 s in q∥ and q⊥ directions.

Figure 5. A series of 1d-SAXS profiles extracted from 2d-SAXS patterns measured at given time elapsed from the quick elongation of the specimen at strain of 4.0 for the SIBS specimen: (a, c) parallel and (b, d) perpendicular to the stretching direction; (c, d) highlighting the particle scattering peaks at 5 min and 12 h in q∥ and q⊥ directions.

shown in Figures 3a, 4a, and 5a, where the results of the model calculation were indicated with red curves. Note here that the deviation of the results of the model calculation for the q⊥ direction is pronounced in the low q-range (q < 0.55 nm−1) for the SEBS-8 and SEBS-16 specimens (see Figures 3b and 4b). Although the reason is not clear at present, in such a lower q-range the scattering from grain structures may appear.16 As a matter of fact, the peak can be identified around q = 0.3 nm−1 for SEBS-16 (Figure 4b) which might reflect the size and shape of grain. On the other hand, no such peak in the low q-range (q < 0.55 nm−1) was observed in Figure 4a, which indicates that the scattering from grain structures in the q∥ direction might exist in much lower q-range. Such contrasted facts imply that the grain size is much smaller in the direction perpendicular to SD than in the parallel direction. This kind of anisotropic shape of grains may be resulted by the uniaxial stretching. Although the scattering from such a grains structure should be taken into account to perfectly reproduce the 1d-SAXS profiles, this is beyond the scope of the present paper. In Figure 6, averaged major radius R maj (filled symbols) and minor radius R min (open symbols) used for the model calculation of the scattering function of the prolate spheroids are plotted as a function of time for (a) SEBS-8, (b) SEBS-16, and (c) SIBS. Since it is found that there is an allowance in R maj and R min to conduct model calculation to give the 1d-SAXS profile which reproduces well the experimentally obtained profile, this allowance range is shown in Figure 6 as an error bar for each symbol. For SEBS-8 and SEBS-16, only R maj increased with time, while R min remained constant. To compare the changes in R maj among the different specimens, strain of microdomains, γmaj = R maj/R̅ − 1, was evaluated and plotted as a function of time in Figure 6d. For SEBS-8, γmaj took a constant value of 0.15 until 92 s elapsed and turned into the

Figure 4. A series of 1d-SAXS profiles extracted from 2d-SAXS patterns measured at given time elapsed from the quick elongation of the specimen at strain of 4.0 for the SEBS-16 specimen: (a, c) parallel and (b, d) perpendicular to the stretching direction; (c, d) highlighting the particle scattering peaks at 50 and 3270 s in q∥ and q⊥ directions.

Here, complete orientation of the prolate spheroids with their long axes parallel to SD was assumed. The model calculation reproduced well the 1d-SAXS profiles for the q∥ direction as D

DOI: 10.1021/acs.macromol.6b02797 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. Plots of ln γmaj(t) as a function of time for the (a) SEBS-8 and (b) SEBS-16 specimens.

Figure 6. (a−c) Changes in symbols) as a function of respectively. (d) Changes in function of time for SEBS-8 (squares).

the plot of ln γmaj(t) vs time for (a) SEBS-8 and (b) SEBS-16, from which τdef = 1.87 × 103 s for SEBS-8 and τdef = 1.06 × 104 s for SEBS-16 were evaluated. Since the longest relaxation time of PS homopolymers even with low molecular weight has been estimated to be extremely high (about 3 years; see Supporting Information) at room temperature, such low values of τdef evaluated are surprising. Furthermore, the fact that the glassy PS microdomains are so easy to be deformed is hardly understood, as stated in our previous paper.13 Table 2 compares the values of τdef with these of τstress for the SEBS-8 and SEBS-16 specimens. It was found that τdef and τstress agrees within an order of magnitude. This fact suggests their strong correlation. However, close examination draws somewhat strange outcomes, as follows. The R maj gradually increased from 8.0 to 8.4 nm under stretched state for SEBS-8 until 600 s elapsed (Figure 6a). The extent of the additional deformation, 0.4 nm, is trivial as compared with the distance between the nearest-neighboring spheres in the ⟨111⟩ direction under stretched state, which was estimated to be 135 nm from the estimated value of d110. (This equals 2π/q* where q* denotes the position of the first-order lattice peak. Note here that the peak was hidden by the beam stopper in the q∥ direction. However, the peak position q* can be estimated according to the method reported in the Supporting Information of ref 13.) Therefore, it can be roughly estimated that the elongated PEB chains in the SEBS-8 specimen under stretched state can relax with only 0.6% strain by the occurrence of the deformation of the PS microdomains at 600 s elapsed. Since the extent of the strain relaxation of the elongated PEB chains is so trivial, the deformation of the PS microdomains cannot account for the stress relaxation, nor the fracture of the specimen. The clue for better understanding may exist in the criteria of the observation of the particle scattering peak in the 2d-SAXS pattern. Since the peak can be only observed when the particle shape and the size are almost uniform (the size distribution is monotonically sharp), tiny fragments (generated as a result of the fracture of the glassy PS microdomains), if any, cannot be seen by the 2d-SAXS measurement. Namely, the SAXS results cannot exclude the possibility of existence of such tiny PS fragments. The fact that R maj increased with time while R min staying at a constant value implies the formation of microvoids in the PS microdomains. This in turn reminds that fracture possibility increases with time elapsed. Therefore, the characteristic time τdef may be correlated with the stress relaxation, giving similar value of τstress.

R maj (filled symbols) and R min (open time for SEBS-8, SEBS-16, and SIBS, strain of PS domains, γmaj and γmin, as a (circles), SEBS-16 (triangles), and SIBS

abrupt increase beyond 92 s. Similarly, for SEBS-16 γmaj stayed almost constant at 0.125 until 500 s and then turned into the increase afterward. As stated in the previous paper,13 the strain-induced shift of the particle scattering peaks can be explained by the deformation of PS microdomains of which the major axis perfectly oriented parallel to SD. The orientation of the anisotropic PS microdomains is considered to be supported by the stress acting on the PS microdomains. Therefore, it is expected that the orientation of the deformed microdomains should be gradually randomized, since the stress decreases under the stretched state. According to this scheme, the particle scattering peaks should shift with time so as to retrieve its initial shape (round shape) under the stretched state due to the gradual randomization of the orientation. However, it is completely opposite to the experimentally observed fact that the particle scattering peaks gradually shifted with time toward lower and higher q regions in the q∥ and q⊥ directions, respectively. Therefore, we now can definitely conclude that the PS microdomains were initially spherical before stretching and deformed into anisotropic shape upon stretching. Furthermore, the deformation was found to proceed with time under stretched state. As for SIBS, the stretch-induced deformation of PS microdomains was observed (Figure 4), while no further shift of the particle scattering peak was observed under stretched state. This fact may be correlated to low stress relaxation for the SIBS specimen. The difference in the onset time of increasing γmaj values (92 s for SEBS-8 and 500 s for SEBS-16) indicates the difference in the deformability of the glassy microdomains for these specimens. To discuss more rigorously the time scale related to the deformation of PS microdomains, we evaluated characteristic time of the deformation (τdef), based on the changes in γmaj by using γmaj(t) ∝ exp(t/τdef). Figure 7 shows E

DOI: 10.1021/acs.macromol.6b02797 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Notes

Finally, we discuss the stress-relaxation behavior being correlated with the deformation behavior as analyzed by the 2d-SAXS measurements under the stretched state. To interpret the difference in the stress relaxation behavior for the specimens, it should be reminded that the degree of the stress concentration on PS microdomains is higher for the smaller microdomains. In other words, the degree of the stress concentration is more significant for SEBS-8 than SIBS. Therefore, we considered that the high stress concentration on PS microdomains leads to fracture of them. When a PS microdomain is locally fractured by the high stress concentration, the stress further becomes higher for other PS microdomains which are distant from the fractured microdomain. Such local stress concentration may lead to a further fracture of other microdomains. Since the fractured microdomains no longer play a role of physical cross-linking points, the stress rapidly decreased. The difference in τstress may be ascribed to the frequency of occurrence of the fragmentation of the microdomains. Namely, the fragmentation of PS microdomains occurs more frequently for the specimen with the smaller PS microdomains on which the stress highly concentrated. Since PS microdomains formed by SEBS-8 was smaller than that in the SEBS-16 specimen, the stress is explained to decrease more rapidly for SEBS-8. Thus, this fact that the macroscopic fracture of the specimen took place much earlier for SEBS-8 may be similarly accounted for.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by Grant-in-Aid for Scientific Research C with Grant 25410226 and Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks” (No. 15H00742) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The SAXS experiments were performed at BL10C, Photon Factory of High Energy Research Organization (KEK), Japan (Approval 2015G591).

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CONCLUSION We conducted the time-resolved 2d-SAXS measurements for triblock copolymer films under stretched state at strain of 4.0 in order to make sure the deformation of the glassy PS spherical microdomains, which has been reported in our previous paper.13 The deformation was definitely confirmed since the shift of the particle scattering peak was found to continuously proceed toward the direction of an increase in the major radius of the prolate ellipsoid with time under the stretched state where the stress was decreasing. Furthermore, good agreements of the characteristic times evaluated from the SAXS results (deformation) and stress relaxation were found, and therefore it was concluded that the deformation of the glassy microdomains eventually resulted in the fracture of the elastomeric film specimen.

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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.6b02797. The sector averaging method with an anomalous sector range to obtain the 1d-SAXS profile from the 2d-SAXS pattern; calculation of 1d-SAXS profiles for a prolate spheroid; evaluation of the longest relaxation time for the PS homopolymer with the molecular weight of Mw = 2630 at room temperature (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 075-724-7864; Fax 075-724-7547 (S.S.). ORCID

Shogo Tomita: 0000-0002-9885-1422 Shinichi Sakurai: 0000-0002-5756-1066 F

DOI: 10.1021/acs.macromol.6b02797 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (15) Tomita, S.; Urakawa, H.; Wataoka, I.; Sasaki, S.; Sakurai, S. Complete and Comprehensive Orientation of Cylindrial Microdomains in a Block Copolymer Sheet. Polym. J. 2016, 48, 1123−1131. (16) Myers, R. T.; Cohen, R. E.; Bellare, A. Use of Ultra-Small-Angle X-ray Scattering To Measure Grain Size of Lamellar Styrene-Butadiene Block Copolymers. Macromolecules 1999, 32, 2706−2711.

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DOI: 10.1021/acs.macromol.6b02797 Macromolecules XXXX, XXX, XXX−XXX