Thermomechanical behavior of shape memory polymers

Shape memory polymers (SMPs) can retain a temporary shape after pre-deformation at an elevated temperature and subsequent cooling to a lower temperature. When reheated, the original shape can be recovered. Minimal work has addressed the constitutive modeling of the unique thermomechanical coupling in shape memory polymers. Constitutive models are critical for predicting the deformation and recovery of shape memory polymers under different constraints. In this study, the thermomechanics of shape storage and recovery of an epoxy resin is systematically investigated in uniaxial tension and compression. After initial pre-deformation at a high temperature, the strain is held constant for shape storage while the stress evolution is monitored. Experimental results indicate that the polymer network can achieve near 100% strain recovery when reheated above its glass transition temperature. Three cases of heated recovery are selected: unconstrained free strain recovery, stress recovery under full constraint at the pre-deformation strain level (no low temperature unloading), and stress recovery under full constraint at a strain level fixed at a low temperature (low temperature unloading). The relationship between the final recoverable stress and strain is governed by the stress-strain response of the polymer material in the rubbery state. Due to the high stiffness in the glassy state, at temperatures below Tg, the evolution of the stress under strain constraint is strongly influenced by thermal expansion of the polymer. Based on the experimental results and the molecular mechanisms of shape memory, a one-dimensional small strain constitutive model is developed. The model is based on the storage and release of conformational entropy states during a thermomechanical process. The fraction of the material storing a temporary entropic state is a function of temperature, which can be determined by fitting the free strain recovery response. With this formulation, the model can predict stress evolution during the shape storage and fully constrained stress recovery with/without low temperature unloading. The model captures differences in tensile and compressive recovery responses caused by thermal expansion, and is used to explore strain and stress recovery responses under various external elastic constraints that would be encountered in applications of shape memory polymers.