A wide class of semi-crystalline polymers, in which amorphous and crystalline phases coexist in steady state, is studied. Certain thermo-mechanical conditions can induce the transition from the amorphous to crystalline phase in these materials, and the amorphous phase can be expected to show a glass transition. In this paper, we propose a variant of constitutive relations to describe the relationship between the stress-strain state and the processes of crystallization and glass transition. Numerical results obtained for different types of uniform stressed-strained states show that the developed constitutive equations can provide a satisfactory description of thermo-mechanical effects associated with relaxation and phase transitions.
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