The change of glass transition temperature under general stress state in amorphous materials

Abstract

The phenomenon of the change in glass transition temperature (Tg) upon externally applied stresses in amorphous materials has been known for a long time. However, most of the existing works focus on the effects of hydrostatic stress state, while the effect of a general stress state on glass transition behaviors remains largely unknown. In this work, molecular dynamics (MD) simulations are first performed on typical amorphous polymers and alloys to probe the change of Tg under different stress states. It is found that Tg changes linearly with pure hydrostatic stress, while under shear stress, Tg changes nonlinearly with the stress magnitude. This further leads to an asymmetric change in Tg under uniaxial tension and compression loading conditions. Based on the above observations, a theoretical model based on the free volume theory and Eyring model is proposed to quantify the change of Tg under general stress states. Comparison with simulation results and existing experimental data reported in the literature demonstrate that the theoretical model can capture the stress dependence of Tg in amorphous materials. Potential applications in tuning Tg through external stress and the sensitivity of Tg to hydrostatic and shear stresses for different amorphous materials are subsequently discussed. The current work reveals the dependence of Tg on general stress states, pointing out potential routes to tune the mechanical properties of amorphous materials.