Simultaneous measurements of small-angle X-ray scattering with stress-strain tests were conducted to reveal changes in microphase-separated structures being correlated with changes in the mechanical properties upon the uniaxial stretching of an elastomeric block copolymer film containing glassy spherical microdomains embedded in the rubbery matrix. For this purpose, high brilliant synchrotron X-ray source was utilized at SPring-8, Japan, and cycled stretching-and-reversing process was conducted three times. While the tensile modulus decreased to be less than the half for the second cycle of the stretching, affine deformation for the changes in the microphase-separated structures was confirmed not only for the second but also for the third cycles. The Mullins effect (stress-induced softening) may be attributed to stress-induced disentangling of the soft-segment chains between two adjacent glassy spherical microdomains (as physical crosslinking points). It was found that the number density of the active network chain (νe ) is more than 22-times higher than that of the soft-segment chains between two adjacent glassy spherical microdomains (νe°) in the virgin sample. Such a big number of the active network chain suggests that the soft-segment chains between two adjacent glassy spherical microdomains are heavily entangled. Furthermore, it was found that the ratio νe /νe° is still around 10 for the second and third stretching cycles, suggesting that appreciable number of entanglements can be relaxed while comparable number of entanglements cannot be. The latter is so referred to as “trapped entanglement due to crosslinking”.
