THE DEPENDENCE OF SLOW CRACK-GROWTH IN A POLYETHYLENE COPOLYMER ON TEST TEMPERATURE AND MORPHOLOGY

The slow crack growth resistance was measured in an ethylene-octene copolymer as a function of the morphological changes produced by varying the thermal history. Morphology was varied by annealing the quenched state at temperatures between 86-degrees-C and the melting point. The slow crack growth behavior was measured by the lifetime of a notched tensile specimen under a constant load. In general, the lifetime exhibited a maximum at a critical value of the annealing temperature. This critical annealing temperature decreased with a decrease in the temperature at which the lifetime was measured. The former result is understandable in terms of the increase in crystal strength as the annealing temperature is increased and the decrease in the number of tie molecules when more material is melted as the annealing temperature increases. The latter result depends on the relationship between crystal size and the effect of testing temperature. Differential scanning calorimetry data played a key part in analyzing the results.