Thermo-mechanical behavior of shape memory polymers

Shape memory polymers (SMPs) are materials that can recover a large predeformed shape in response to environmental stimuli, such as temperature and light. This ability of recovering its original shape makes SMPs suitable materials for everyday applications such as smart fabrics, biomedical devices with minimum invasive surgery and deployable structures. Design of SMP devices involves the need for strain and stress predictions to achieve a satisfactory performance under the required conditions. For a thermally induced amorphous SMP, the pre-deformation and recovery of the shape require the SMP to traverse its glass transition temperature (Tg) to complete the shape memory (SM) cycle. The recovery step can be achieved under constrained or free conditions, depending on whether the constraint needed to produce the deformation is held or removed. The SM behavior is due to the dramatic change in the molecular chain mobility during the transition between the glassy and the rubbery states. Thermo-mechanical experiments that mimic the SM cycle were performed. Due to simplicity and uniform deformation conditions a compression configuration was used. A new improved design for compression platens that provides a more uniform temperature distribution on the SMP sample and a faster response to the prescribed temperature was proposed and utilized. Two SMP material systems, a tert-butyl acrylate-based and an epoxy-based, were used in this work.;The tert-butyl acrylate-based SMP was subjected to the SM cycle with constrained recovery conditions. Experimental results show that the stress response is highly affected by the temperature rate during the recovery step. Two constitutive mechanical models are developed to predict the mechanical behavior observed during the uniaxial thermo-mechanical tests. First, a three-dimensional phase evolution with viscoplastic behavior model is developed and implemented using ABAQUS. Numerical simulations show good agreement with the isothermal compression data. Second, a one-dimensional model that considers non-equilibrium structure relaxation and viscoelastic behavior is introduced improving the prediction from the uniaxial compression thermal relaxation tests. It is shown that the observed stress response is a combined effect of these two phenomena. Next, a commercially available epoxy-based SMP material was subjected to the SM cycle with free recovery conditions. Results show that the recovery is strongly dependent on the pre-deforming and the recovery temperatures. Faster recovery is obtained by pre-deforming at a lower temperature and recovering at a higher temperature. This epoxy-based SMP was then used as a matrix to manufacture SMP composites. In order to study the temperature and the reinforcement orientation effects, tensile tests were performed on SMP composite samples. Results show an increment on the modulus of elasticity for the SMP composites.