TIME-TEMPERATURE DEPENDENCE IN FRACTURE-BEHAVIOR OF HIGH-IMPACT POLYSTYRENE

High Impact Polystyrene (HIPS) is one of the first toughened systems i n which the brittle polystyrene becomes more ductile with the addition of an elastomer. However, it exhibits a ductile behavior only above a certain temperature and below a certain loading rate. Fracture in thi s material, like in most toughened systems, can become brittle when th e temperature is lowered or the loading rate is increased. The correla tion between temperature and loading rate seems to be controlled by th e molecular relaxation according to the Arrhenius equation. The object ive of this work is to foster the understanding of the effects of time and temperature on the fracture behavior of HIPS. The time and temper ature dependence in fracture performance has been found to be governed by the strain energy density criterion. The theory allows prediction of fracture performance at various loading rates and temperatures. The brittle-ductile transition is controlled by an energy activation proc ess. A peak in fracture energy always occurs at the transition region. This is attributed to the relaxation of the polymer macromolecules. T he time and temperature dependence of this relaxation can be predicted by the Arrhenius equation. The rise in fracture energy at high loadin g speeds is not due to the higher frequency oscillations from dynamic effect but is controlled by the critical strain energy density. (C) 19 98 Elsevier Science Ltd. All rights reserved.