THE INFLUENCE OF SHORT-CHAIN BRANCHING ON THE CREEP-BEHAVIOR OF ORIENTED POLYETHYLENE, AND ITS EFFECT ON THE EFFICIENCY OF CROSS-LINKING BYELECTRON-IRRADIATION

Studies have been made of the creep behavior of oriented (15:1) polyet hylenes containing 0.4 and 1.3 butyl branches per 1000 C atoms. Increa sing the branch concentration reduces significantly the creep strain a nd the equilibrium strain rate. The data have been fitted to an establ ished model comprising two thermally activated processes in parallel, relating to the amorphous network at low stress, and the crystal phase at high stress. Analysis based on this model indicates the similarity between branching, entanglements, and crosslinks on the creep respons e. The creep behavior of electron-beam-irradiated materials shows that increasing the branch concentration makes the polyethylene more susce ptible to main-chain scission, indicated by increased creep flow rates at higher stress, consistent with previous rubber elasticity studies. Irradiation in an acetylene atmosphere with low (< 1 Mrad) doses is s hown to reduce the creep rates at all accessible stresses, and this at tributed to an increase in crosslinking compared with scission. (C) 19 94 John Wiley & Sons, Inc.