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SYNOPSISThe effect of gamma irradiation, in acetylene, on large diameter oriented polyethylene has been investigated. The dependency of gel content, melting point, and crystallinity on dose was evaluated. The gel content dose relationship showed two regions, indicating that an effective network is formed at low doses, 60 kGy. Irradiating the oriented rod in acetylene was found to more than double the initial tensile modulus. This improvement in the tensile properties resulted in improved creep-strain resistance for the material. These improvements in the mechanical properties were attributed to crosslinks formed both in the amorphous regions and a t the surfaces of the crystalline regions. At high doses > 180 kGy degradation of the oriented polymer resulted in a deterioration in mechanical properties.

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SynopsisThe dependency of fracture on the rate of strain of a range of polyethylene samples was measured in three point flexural tests at 23°C. The range of strain rates from lo-' to reciprocal seconds was examined. The J integrals at the initiation of the cracks were determined. This enabled the interrelationship between fracture initiation and the test rate to be established. The dependency follows a power law relation. The values of the exponents in the power law equations, derived from experiment, were found to be higher than the values calculated using established criteria for crack initiation. The difference is postulated to be due to the strengthening of the fibrils ahead of the crack because of the orientation of the fibrils in that region.