Atomistic modelling of plastic deformation of glassy polymers

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Abstract
A detailed atomistic approach has been used to investigate the kinematics of plastic deformation in glassy atactic polypropylene to 20% strain. The microstructural stress-strain behaviour was found to consist of smooth reversible portions bounded by irreversible sharp stress drops indicating plastic rearrangement of the structure. Averaging the stress-strain behaviour over an ensemble of 1-815 nm microstructures showed a yield point in the neighbourhood of 5-7% strain. The transformation shear strain for plasticistructural rearrangements was found to be broadly distributed, averaging 1-5% shear strain with a standard deviation of 2-6% shear strain. Combining this result with the activation volume measurements of common glassy polymers showed the size of the plastically transforming region to have a diameter of about 10 nm, thus involving several thousand segments. The transformation shear strain was independent of the system size. Scrutiny of the molecular segment motions associated with plastic rearrangements showed no recurring simple kinematical configurations and no correlation of the local atomic strain to topological features of the chain.