Effect of casting solvent on the stress-hardening and stress-softening characteristics of Kraton-G 1650 copolymer films

Films of Kraton-G, a triblock copolymer of styrene-(ethene-co-butene)-styrene, have been cast from several solvents with solubility parameters in the range δ = 15.1 to 20.5 (J/m3)½ The films exhibit both stress-softening and stress-hardening behavior, but the response is found to depend on the casting solvent. The observed behavior is discussed in relation to the structure of the films and the extent of dispersion of the polystyrene phase is found to be a major factor in determining the film response to applied stress. It is also suggested that the interfacial region, which constitutes a substantial part of the matrix will be instrumental in the relaxation processes observed in the films when temperatures remain below Tg s for the polystyrene phase. Some crystallinity was developed in the elastomeric phase on stretching the films, but this is not a significant feature.