Isochoric glass transitions, polymers

The constitutive model used to describe large plastic deformations of glassy polymers involves a separate formulation for temperatures above and below the glass transition Tg, since the underlying deformation mechanisms are different. In either regime, the formulation is based on the decomposition of the rate of deformation into an elastic part and a plastic part Z)P so that 0 = 0° + D. By assuming an isotropic yield stress, the isochoric plastic strain rate is given by the flow rule... 

Isoharic (constant pressure) and isochoric (constant volume) glass transitions in polymers were first observed for bisphenol A polycarbonate (62). A molecular dynamics study of such transitions in a model amorphous polymer has also been reported (63). This study shows that the glass transition is primarily associated with the freezing of the torsional degrees of freedom of polymer chains (related to chain stiffness), which are strongly coupled to the degree of freedom associated with the nonbonded Lennard-Jones potential (related to interchain cohesive forces). 

---