Structural origins of mechanical properties and hysteresis in SIS triblock copolymers/polystyrene blends with spherical morphology

Abstract

The mechanical response and the corresponding microstructure of polymer blends composed of styrene-isoprene-styrene block copolymer and deuterated polystyrene are studied via in situ small-angle neutron scattering (SANS) measurements during intermittent uniaxial extension. The initial blend morphologies consist of spherical PS glassy domains arranged in body-centered cubic (BCC) lattices. The spheres rearrange affinely with the macroscopic deformation up to Hencky strains ~0.35. Larger deformation is mechanically characterized by yield and strain hardening. In this regime, SANS measurements reveal a transition from a BCC crystal lattice to chevron-like patterns. Using electron microscopy, we found that the chevrons arise from a real-space structure described as spheres that have been stretched along one axis and aligned parallel to one another. Such configuration is unstable and reverts to BCC structure after stress release. After unloading the blends, remnant nano-deformation is observed, which is fully relaxed only in the transverse direction after more than 10 h. Permanent damage of the internal structure along the stretching direction impedes the full recovery of the initial sphere configuration.