Hierarchical description of deformation in block copolymer TPEs

We report on the deformation behavior of commercially relevant lamellar and cylindrical tri-block copolymers poly (styrene-b-ethylene-co-butylene-b-styrene) (SEBS) with two different compositions. The structural changes that occur at various length scales have been studied using a simultaneous small-and wide-angle X-ray scattering (SAXS/WAXS) during uni-axial tensile deformation of the polygrain samples. SAXS provides information about changes that occur upon deforming the glassy cylindrical or lamellar PS domains. WAXS, on the other hand, is sensitive to the crystallographic structure of the rubbery mid-block. Deformation calorimetry has been used to determine the energetics involved. The combined results from the various techniques indicate that the deformation takes place in three stages. First, at small strains, dilation occurs in the rubbery phase. At intermediate strains, the hard lamellar or cylindrical domains undergo micro-buckling, which is associated with a downturn in the stress-strain curve. Finally, we interpret that at higher strains, the bent lamellar/cylindrical domains rotate in the stretching direction resulting in a significant shear on the rubbery mid-block. This in turn leads to strain-induced crystallization in these materials. Although we could not prove it by WAXS, deformation calorimetry (which is more sensitive than the WAXS) was utilized to show its presence.