Modeling the mechanics of light activated shape memory polymers

Shape memory polymers (SMP's) are a relatively new kind of smart materials and have significant technological applications ranging from biomedical devices to aerospace technology. First generation SMP's relied on changes in temperature to fix the temporary shape and have been studied quite extensively in the past. In the last few years a new generation of SMP's have been synthesized in which the temporary shape is fixed by exposure to light at specific wavelengths (typically in the Ultraviolet, UV, range). Exposure to light at certain wavelengths causes photosensitive molecules, which are grafted on the polymer chains comprising the material, to form covalent bonds. These bonds act as crosslinks and are responsible for the temporary shape. On exposure to light at a different wavelength these bonds cleave and the material returns to its original shape. Our research focuses on modeling the mechanics associated with such light activated shape memory polymers (LASMP's) undergoing complex deformations. The modeling is done using a framework based on the theory of multiple natural configurations taking into consideration the different aspects of modeling this material, which include developing a model for the original virgin network and for the other networks with different stress-free states, formed due to exposure to light. In addition to this, we also model the initiation and the formation of the light activated networks and the reverse transition resulting in the dissolution of these networks. Anisotropy in the mechanical response is also incorporated into the model. The model is then used to simulate results for specific boundary value problems, such as uni-axial extension and inflation of a cylinder.