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 Cσ ^m a ⁿ₀ exp (−Q/RT) or AK ^pexp (−Q/RT) where σ = stress, a ₀ = 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 (¯M _w−¯M _c)⁻¹ where ¯M_c is a limiting molecular weight for sudden fracture. A molecular model based on tie-molecules has been used to explain the dependence on ¯M _w. The effect of ¯M _w 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.