THE EFFECTS OF SHORT CHAIN-BRANCHING AND MOLECULAR-WEIGHT ON THE IMPACT FRACTURE-TOUGHNESS OF POLYETHYLENE

The impact fracture toughness of eight grades of polyethylene, five homopolymers (high density polyethylene) covering a range of molecular weights from 66 000 to 311 000, and three copolymers (linear low density polyethylene, LLDPE) with branch contents from 15 to 21 per 1000 C, has been evaluated at -20-degrees-C. It is proposed that the increased toughness of LLDPE is due to the influence of branches on morphology and tie-molecule concentration. This promotes the formation of a fibrillated damage zone in which greater energy is absorbed to initiate a crack than unbranched polyethylenes. The process for fracture toughening is similar for homopolymers, which demonstrate increasing toughness with molecular weight. The mechanisms are the same as those proposed by others for slow crack growth.