Craze plasticity in a series of polystyrene/polybutadiene di-block copolymers with spherical morphology

Abstract

The plastic behaviour by crazing of a series of polystyrene/polybutadiene di-block copolymers with a morphology of randomly dispersed PB spheres has been investigated at both +20 and −20°C. Materials with volume fractions of PB exceeding 0.11 were found to be quite tough in tension, with typical craze flow stresses of around 30 MPa and strains to fracture of around 0.4. Growth rates of individual crazes were studied in detail in all blends. Electron microscopy established that in the material exhibiting good toughness, crazes grew by cavitation of the spherical PB domains, followed by plastic drawing of the remaining PS carcass. The growth rates of such crazes could be modelled quite well by the advance of a cavitation front in a planar layer of a thickness equal to the PB domain diameter. When volume fractions of PB were much lower than 0.1, the craze growth rates became quite low and exhibited kinetics that fitted the interface convolution mechanism of craze growth. The good agreement between the kinetics of craze growth and uniaxial tensile craze plasticity points to the importance of craze growth kinetics in the overall kinetics of craze plasticity and in obtaining toughness.