Interrelation between mechanical response, strain field, and local free volume evolution in glassy polymers: Seeking the atomistic origin of post-yield softening

Abstract

A set of complementary experiments are used for the first time to elucidate the interrelation between the mechanical properties, the strain field, and the free volume evolution during non-homogenous compression of a glassy polymer. Two sets of quenched and annealed polystyrene samples, having different free volume histories, are notched and exposed to compression. The variation of both the strain field and the free volume are measured on a microscopic scale via digital image correlation in case of strain and Doppler broadening spectroscopy of positron annihilation line in case of free volume measurements. Eventually, the interplay between the local evolution of free-volume, the local strain field, and the mechanical response is investigated throughout the yielding, softening and plateau regions. We found that in all stages of plastic deformation the generated local strain field is positively correlated to the global strain independent of the active mechanism of plastic deformation. Moreover, the local change of free volume is not correlated to the mechanical response of the polymer at the softening stage. Therefore, the free volume evolution should not be responsible for the intrinsic post-yield softening behavior. The easy flow in the plateau region, however, begins at a particular fraction of free volume independent to the thermal history of samples.

Keywords

• Polymer blends and alloys
• Mechanical properties
• Positron annihilation
• Digital image correlation
• Post-yield softening