Constitutive model for photo-mechanical behaviors of photo-induced shape memory polymers

Light-activated polymers are an exciting class of materials that respond mechanically when irradiated at particular wavelengths. Recent demonstrations include two novel polymers developed by Scott et al (2006) and Lendlein et al (2005). In these polymers, photochemistry alters the microstructure of the cross-linked polymer network, which is further translated as light-induced deformation and when properly used light-induced shape memory effect. In this work, we develop a model framework to simulate the photomechanical response of light-activated polymer systems. This framework breaks down the observed macroscopic photomechanical phenomenon into four coupled sets of underlying physics, which occur throughout the material during irradiation and mechanical deformation. In the context of this framework, a basic photomechanical phenomenon involves simultaneously modeling photophysics, photochemistry, chemomechanical coupling, and mechanical behavior. Furthermore, network alteration are accounted for through the parallel decomposition of the cross-linked network into two components, an original network and a photochemically altered network, which allows to capture the observed photomechanical behaviors demonstrated in these materials. One of the principal strengths of this model framework is its generality as it can be applied to light activated polymer systems with fundamentally different of photophysics, photochemistry, and chemomechanical behaviors simply by choosing different field equations for the four sets of physics specific to a material system.