The trapping concept in nonlinear viscoelasticity of amorphous glassy polymers

Nonlinear stress-strain relations are derived for the viscoelastic behavior of glassy polymers. An amorphous medium is treated as an ensemble of cooperatively rearranging regions (flow units). Any unit is thought of as a point in the phase space which hops (being thermally activated) to higher energy levels in its potential well on the energy landscape. The viscoelastic behavior of a polymer is modeled as a sequence of rearrangement events occurring at random times when relaxing regions reach (in hops) some liquid-like level. We assume that external loads affect the position of the liquid-like state with respect to the energy landscape, and the descent of the reference energy level is proportional to the average mechanical energy of a flow unit. This hypothesis is verified by comparison of observations for polycarbonate in tensile relaxation tests with results of numerical simulation. Fair agreement is demonstrated between experimental data and their predictions.