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 rubb er-toughened nylon 66 (Zytel ST-801) and a high impact polystyrene (HI PS), were chosen for this study. The samples were prepared by injectio n 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 corre lation between temperature and loading rate was estimated using the Ar rhenius equation. The results show that Tb-d of Zytel ST-801 is only s lightly affected by the loading rate, whereas Tb-d of HIPS strongly in creases with deformation rate. It is found that for the former, within the experimental errors, an increase in Tb-d. With loading rate corre sponds to the shift in the secondary relaxation temperature T-b of the nylon 66 matrix. For the latter however, the increase in Tb-d is rela ted 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.