Strain-Induced Deformation of Glassy Spherical Microdomains in Elastomeric Triblock Copolymer Films: Time-Resolved 2d-SAXS Measurements under Stretched State

We have found extremely low efficiency of the elastomeric properties for SEBS (polystyrene-<i>block</i>-poly­(ethylene-<i>co</i>-butylene)-<i>block</i>-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-<i>co</i>-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

Application of a Strained Natural Rubber at High Temperatures

Robustness of the natural rubber (NR) crystal at high temperatures is important because vehicle tires are easily subjected to high temperatures due to friction. To understand the robustness of crystals in the strained NR, as its plausible application to high temperatures, changes in the following structural parameters as a function of temperature were examined in this study by conducting two-dimensional wide-angle X-ray diffraction measurements: crystal lattice constants (a, b, c, and β), unit cell (volume, thermal expansivity, and orientation factor), degree of crystallinity, and crystallite (size, volume, and number density in the strained NR specimen). As a result for a vulcanized NR specimen subjected to a constant strain of 6, thermal shrinkage of the c-axis length was found in the heating process, while both a- and b-axis lengths were found to increase with an increase in the temperature. Furthermore, the volume of the crystal lattice was found to increase with the temperature, while the orientation degree of the crystal lattice remained unchanged. The degree of crystallinity was found to gradually decrease with temperature from the beginning of heating. As for the crystallite, the size along the stacking direction of the (200), (120), and (201) planes decreased with temperature, while that of the (002) plane (i.e., along the c-axis direction) was found to increase. These results indicate that crystallites grow in the NR main chain direction, while they are subjected to melting in the other perpendicular directions upon heating. However, it was found that the crystallite volume and the number density of crystallites in the strained NR specimen continuously decreased with the increasing temperature. The thermo-reversibility of all the structural parameters was also examined experimentally