The critical molecular weight for resisting slow crack growth in a polyethylene

An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The slow crack growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to slow crack growth was negligible for the three fractions whose M_w was less than 1.5 × 10⁵. For the fourth fraction with M_w greater than 1.5 × 10⁵, the resistance to slow crack growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 10⁵ is required to create the number of tie molecules that is necessary to produce a high resistance to slow crack growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.