A thermo-viscoelastic constitutive model addressing the cyclic shape memory effect for thermo-induced shape memory polymers

The cyclic shape memory effect of thermo-induced shape memory polymers (TSMPs) is a typical thermo-mechanical process that can be affected by thermo-mechanical loading histories. During the deformation stage of the cyclic shape memory effect, polymer chains exhibit the initial dissociation of sub-entanglements, slipping and orientation with the increase in the deformation. The strain can be recovered with the internal rotation of the dihedral angle due to the enhancing motion of polymer chains during the reheating. Based on the rheological theory, a thermo-viscoelastic model is proposed to capture the cyclic shape memory effect of TSMPs. A temperature-dependent stress threshold value is introduced to reflect the slipping of polymer chains. In order to take into account how orientation affects the cyclic shape memory effect, a relationship between the strain and orientation is constructed and introduced into the evolution equations of elastic modulus, viscosity and irrecoverable strain. By comparing the experimental and simulated results at different loading levels and numbers of cycles, the proposed model is verified. The results show that the proposed model can reasonably predict the cyclic shape memory effect of TSMPs.