The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

Abstract

The rate of initiation and growth of cracks in linear high-density polyethylene with different molecular weights was observed in single-edge-notched tensile specimens under plane strain condition as a function of applied stress, notch depth and temperature. The initial rates of crack initiation all have the form of Cσm a0nexp (-Q/RT) or AKpexp (-Q/RT) where σ = stress, a0 = notch depth and K= stress intensity factor. For the different molecular weights, m, n, P and Q are almost the same where m=5, n=2, P=4.7 and Q=115 kJ mol-1, but the constants C and A varied as (-Mw--Mc)-1 where -Mc is a limiting molecular weight for sudden fracture. A molecular model based on tie-molecules has been used to explain the dependence on -Mw. The effect of -Mw on the fast-fracture strength at low temperature and the relationship to tie-molecules have also been investigated. Quantitative relationships between the concentration of tie-molecules and the fracture behaviour have been obtained. © 1988 Chapman and Hall Ltd.