Shape-memorizable styrene-butadiene block copolymer. I. Thermal and mechanical behaviors and structural change with deformation

Abstract

Thermal properties in the range from room temperature to 150°C, mechanical properties from room temperature to 80°C, and structural changes by drawing and contraction at 80°C followed by crystallization have been studied in a crystalline styrene-butadiene block copolymer, which has the property of shape memory, using differential scanning calorimetry (DSC), mechanical analysis, wide-angle x-ray diffraction (WAXD), and smallangle x-ray scattering (SAXS). This copolymer has the crystal transformation temperature, the melting temperature of the trans- 1,4-polybutadiene domains, and the higher glass transition temperature of the polystyrene domains. When a high strain is adopted for the deformation at 80°C (i.e., between the melting temperature of the polybutadiene [PB] domains and the glass transition temperature of the polystyrene regions) and crystallization conditions with fixed ends are employed, a fibrillar structure with a better regularity of long spacings and a high orientation of crystals forms. When the drawn sample is allowed to contract at 80°C, the high contraction or the shape recovery appears. Nevertheless, crystallization after contraction presents essentially the same supermolecular structure as that before contraction. It is suggested that the molecular chains of polybutadiene were inhibited from flowing freely by the glassy polystyrene molecules and that there must be some structural units separated by amorphous domains that contribute to the elongation and contraction at the high temperature.