Model Development for Shape Memory Polymers

by Siskind, Ryan David

Shape memory polymers (SMPs) have garnered a considerable amount of attention in recent years due to their flexible, lightweight, and biodegradable nature. At high temperatures, SMPs share attributes with compliant elastomers and exhibit long-range reversibility. In contrast, at low temperatures they become very rigid and are susceptible to plastic, although recoverable, deformations. Their ability to withstand large, nonlinear deformations coupled with their ability to completely recover their original shape is of particular interest for biomedical, aerospace, and automotive applications. Whereas the scope of existing models involving isothermal nonlinear deformations at both high and low temperatures is broad, there is a noted lack of models which specifically deal with the transition process. This process cannot be overlooked as it is the driving factor for all thermomechanical shape memory effects in shape memory polymers. The scope of this dissertation focusses on the strain storage and release for shape memory polymers as they transition from a rubbery state to a rigid state and back. The proposed homogenized model directly addresses the so-called glass transition process at a macroscopic level using small, linear deformations and is validated against experimental data. The dependence of the glass transition phenomenon on the frozen volume fraction requires the development of a constitutive model based on material properties for the frozen fraction. This is developed in concert with the homogenized model. An industrial application of shape memory polymers in a rod system is proposed, and the accompanying evolution equation is developed and solved numerically using a semi-implicit 2-stage L-stable Rosenbrock method. To admit future implementation of isothermal nonlinear deformations in the homogenized model, existing nonlinear models are summarized.