A molecular model for yield and flow in polymethylmethacrylate

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Abstract
A molecular model to account for the yield behaviour of glassy polymers was proposed in an earlier paper (Bowden and Raha 1974) in terms of the nucleation and growth of sheared regions analogous to dislocation loops. The previously suggested shape and mode of growth of these sheared regions are now shown to be the most favourable energetically. Shear modulus and compressive yield stress data obtained on carefully conditioned identical samples of polymethylmethacrylate have enabled the model to be more rigorously tested. The Burgers vector (b) of the sheared regions is calculated from the experimental data and is approximately constant at 3·5 A from - 120° to 80°C. Assuming this constant value for b, the compressive yield stress as a function of temperature is predicted using the measured values of shear modulus. The theoretical and experimental results are found to agree closely, supporting this approach to the problem of yield in glassy polymers.

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