A thermodynamic framework for the modeling of crystallizable shape memory polymers

Shape memory polymers are a relatively new class of materials that have the ability to retain a temporary shape, which can be reset to the original shape with the use of a suitable trigger, typically an increase in temperature. The temporary shapes can be very complex and the deformations involved large. These materials are finding use in a large variety of important applications; hence the need to model their behavior. In this paper, we develop constitutive equations to model the thermo-mechanical behavior of crystallizable shape memory polymers. Crystallizable shape memory polymers are called crystallizable because the temporary shape is fixed by a crystalline phase, while return to the original shape is due to the transition of this crystalline phase. The modeling is carried out using a framework that was developed recently for studying crystallization in polymers and is based on the theory of multiple natural configurations. In this paper we formulate constitutive equations for the original amorphous phase and the semi-crystalline phase that is formed after the onset of crystallization. In addition we model the transition of the crystalline phase to capture the return of the polymer to its original shape. These models for shape memory effects in polymers have been developed within a full thermodynamic framework, extending our previous work in which the models were developed within a mechanical setting [G. Barot, I.J. Rao, Constitutive modeling of the mechanics associated with crystallizable shape memory polymers, ZAMP 57 (4) (2006) 652-681]. The model is applied to the problem of inflation and extension of a hollow cylinder. The results are consistent with what has been observed in experiments.