Modeling the response of light-activated shape memory polymers

The aim of this paper is to model the macroscopic response of light-activated shape memory polymers (LASMPs) subject to mechanical loadings and exposure to light at certain wavelengths and frequencies. When exposed to external stimuli of mechanical, thermal, photochemical and other origins, polymers undergo microstructural changes, e.g., scission, cross-linking, crystallization, etc. These microstructural changes affect the macroscopic performance of the polymers. In this study, in order to incorporate the effect of microstructural changes on the macroscopic response of light-activated shape memory polymers, we formulate constitutive models based on the notion that the natural configuration of the body under consideration evolves during its response. The theoretical framework appeals to a multinetwork approach consisting of two microstructural networks, which are the original network and the new network formed owing to a light activation. An important distinction between the approach considered here and the usual multinetwork approaches is that there is no conversion of one network to another; instead, what we have is the formation of a second network owing to the linking of photosensitive particles that get linked due to light irradiation. Furthermore, two different constitutive models are considered. The first model assumes the two networks are isotropic. The second model takes into account the directional preference of the second network that is formed. Both these models build on the work of Sodhi and Rao, which is based on the framework developed by Rajagopal and Srinivasa. Several classical boundary value problems involving homogeneous and inhomogeneous deformations are studied. We also investigate two nonlinear constitutive relations and different loading modes. The results highlight the differences in the responses when isotropic and anisotropic models are considered.