Anisotropic plastic deformation of glassy polymers containing process-induced molecular orientation
Paul Buckley, Junjie Wu
University of Oxford
UK

Keywords: constitutive model, glassy polymers, anisotropy

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The work to be presented forms part of wider collaboration between six UK universities: Micro-scale Polymer Processing, aimed at improving the ability to understand and predict quantitatively the solid-state properties of polymers, in terms of molecular parameters and process history.
Specifically, we describe here a new anisotropic constitutive model, for modelling finite deformations in amorphous polymers into which molecular orientation has been introduced during the process history. The model is based on the previously published, generic, glass-rubber constitutive model framework for amorphous polymers, subsequently elaborated for the glassy state to include plastic-strain-induced softening, and molecular weight effects. In the new work, rate-limiting thermal activation events are recognised to be inherently anisotropic at the molecular level. This may be expressed in terms of rate-dependent anisotropic viscoplasticity theory. The effect of residual molecular orientation, introduced during processing above Tg, is to cause preferential alignment of the relaxing units. Integration over the orientation distribution then predicts the development of anisotropy in the finite deformation, nonlinear viscoelasticity and viscoplastic flow of the resulting glassy polymer. The model captures naturally the distinctive features of oriented glassy polymers known from previous work. In the context of the collaborative project, however, it has been possible to test the model against experimental results from an unusually thorough study of the properties of atactic polystyrene.
Working in collaboration with other partners, who carried out synthesis and structural and melt-rheological characterisations, we have studied experimentally the yield and plastic deformation of a series of monodisperse and polydisperse polystyrenes in the glassy state, in isotropic and anisotropic form. Anisotropic tape samples were prepared by extrusion and draw-down at various temperatures and rates, to achieve varying degrees of molecular orientation. The latter was determined by other partners in the project, using small angle neutron scattering and birefringence. Increasing orientation as seen by these techniques was found to cause increase in the yield stress and post-yield strain-stiffening, in tension parallel to the extrusion direction. The constitutive model correctly captured the observed effects.
The solid-state constitutive model described, when combined with orientation predictions from a molecularly-based constitutive model for the melt, now promises the possibility of an entire predictive modelling chain - from molecular architecture, through processing, into solid-state product performance.