Mechanisms of brittle-ductile transition in toughened thermoplastics

The objective of this work was to investigate the mechanism of brittle-ductile transition in toughened polymers. Two systems, namely, a rubber-toughened nylon 66 (Zytel ST-801) and a high impact polystyrene (HIPS), were chosen for this study. The samples were prepared by injection molding and were tested in three-point bending under various loading rates and temperatures. The brittle-ductile transition temperature (T_b–d) was determined from the observed fracture behavior as a function of temperature. Molecular relaxation temperatures of the polymers were measured by mechanical spectroscopy at various frequencies. The correlation between temperature and loading rate was estimated using the Arrhenius equation. The results show that T_b–d of Zytel ST-801 is only slightly affected by the loading rate, whereas T_b–d of HIPS strongly increases with deformation rate. It is found that for the former, within the experimental errors, an increase in T_b–d with loading rate corresponds to the shift in the secondary relaxation temperature T_b of the nylon 66 matrix. For the latter however, the increase in T_b–d is related to the glass/rubber relaxation of the polystyrene matrix. It seems that the type of molecular relaxation controlling the brittle-ductile transition corresponds to that with lower activation energy.