Physics of strain hardening in glassy polymers

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Some glassy polymers with high molecular weights exhibit a strain hardening regime at large deformation. This regime is very important because SH limits the ductility of polymer glasses, so its microscopic understanding is an important scientific challenge. The Long group, one of the French collaborators in this joint project, has developed a microscopic theory for plasticity, according to which relaxation takes place by reorganizations on a scale of about 5 nm. The stored elastic energy on this scale corresponds to a reduction of free energy barriers which enables plastic flow. This group has developed a companion numerical tool for calculating plastic flow in 3D, with a spatial resolution the 5 nm scale which allows for calculating the distribution of relaxation times and its evolution under applied strain, during ageing and rejuvenation for any thermo-mechanical history. It is based upon a dynamical equation regarding the evolution of a tensorial order parameter S on the monomer scale. The orientation enhances the monomer-monomer interactions which leads to an increase of the free energy barriers. The reorientation dynamics on the monomer scale and its relaxation appear to be key for describing the physics of strain hardening. In this project, a joint experimental and theoretical approach will be developed to study the local orientation and its relaxation behavior under various thermo-mechanical loading histories by X-ray scattering, NMR and Differential Scanning Calorimetry. Solid-state NMR plays an important role in this regard, as it will provide a quantitative insight into the orientational ordering at the molecular level.