Constitutive Modeling And Experimental Verification Of Shape Memory Polymers

Shape memory polymers have been widely used in the medical, textile, machinery, chemicals, and other fields because of their excellent recoverability, light weight, good manufacturability, the low tailorable switch temperature, operability of recovery behavior and excellent biocompatibility. Since the essence of the shape memory materials stress and strain response behavior under certain conditions, so building the constitutive equation of shape memory polymers in different conditions not only can predict the mechanical behavior of materials to optimize shape memory performance but also has important significance for indicating the shape memory mechanism of shape memory polymers.In the present research, the constitutive equations, which describe the behavior of stretching at isothermal condition and shape memory cycle, were developed by combining three elements models and on the base of phase transition theory. In order to verify the constitutive equation of shape memory polymers, the most common thermal induced shape memory polyurethane was selected as the experimental subject and the testing was conducted by tensile tests at different temperatures(small strain and large deformation), stress relaxation tests, dynamic mechanical tests, the shape memory cycle tests under different pre-strain(5% and 10%). Then, we used the experimental results to fit the parameters. On the basis of the results, we compared the experimental data at different temperature with the theoretical prediction results and meanwhile the numerical simulation results of shape memory cycle were compared with experimental data. Details are as the follows:First, the frozen segment within the shape memory polymers can break barrier and movement under the external force, which lead to the transition from the initial phase to the active phase. In order to describe the transition process, the three element model was introduced, in which the Maxwell model is used to describe the mechanical behavior of the initial phase and nonlinear spring(following Neo-Hookean law) indicates stress-strain response relationship of the active phase. The variation law of viscous strain rate is used to describe the phase transition process and the viscous strain rate is approximated by the relationship between the relaxation time and the external force. And on this basis, a nonlinear viscoelastic constitutive equation in the process of stretching was constructed. Shape memory polyurethane was selected as the experimental subject and parameters fitting results and the constitutive equations were used to predict the shape memory polyurethane tensile stress-strain curve at 311.15 K and 323.15 K. The results show that the theoretical predictions and experimental data are in good agreement in the initial stage of stretching and the stress of theoretical prediction slightly below experimental data with the increase of time. And, the theoretical predictions and experimental data have good consistency for the whole.Finally, it assumes that the shape memory polymers changes from the active phase to the frozen phase with the decreases of temperature based on the theory of phase transition driven by temperature, which the active phase is composed of movable segment and the frozen phase is composed of frozen segment. In the present study, the frozen fraction was introduced to describe the phase transition process. Since the frozen phase and active phase are not connected in series and parallel, so the coefficient k was introduced to describe the connection mode. A constitutive equation was constructed by using three element model to describe the behavior of the frozen and active phase. The constitutive equation was used to predict the shape memory process under different pre-strain. And shape memory polyurethane was selected as the experimental subject with the least square method to fit parameters. The results show that the constitutive equation not only can predict the thermo-mechanical behavior in shape memory cycle but also can describe stress relaxation in the cooling process under constant strain and creep phenomenon in strain free recovery process.