A thermoviscoelastic constitutive model for crosslinked semicrystalline two-way shape memory polymers

The paper proposed a thermodynamically consistent 3D constitutive framework for crosslinked semi-crystalline polymers, explicitly addressing the interplay between crystallization/melting-induced phase transitions and viscoelasticity under complex thermomechanical cycles. By integrating a modified theory of multiple natural configurations with a revised time-temperature superposition principle (TTSP), the model captures both one-way and two-way shape memory effects (1W/2W-SMEs). An experimental campaign on crosslinked ethylene-vinyl acetate (EVA) with controlled crosslink densities combines multi-rate DSC analysis, frequency-sweep DMA, and coupled stress-strain-temperature protocols to quantify the dynamic evolution of crystallinity and viscoelastic moduli. Crucially, a modified WLF-Arrhenius hybrid equation is introduced to describe nonlinear stress relaxation mechanisms, validated through master curve construction from TTSP tests. The model successfully predicts recovery behaviors in free 1W-SMEs and stress-rate hysteresis in 2W-SMEs, demonstrating its capability to bridge molecular-scale phase transitions to macroscopic uniaxial responses. This work provides a unified tool for designing programmable soft actuators and smart devices.