Photo-Induced Creep of Network Polymers
We investigate the photo-induced creep behavior of a network polymer that can relax stress upon irradiation at particular wavelengths (Scott et al., 2006; Scott et al., 2005). The underlying mechanism is a photochemical-mediated structural rearrangement of the polymer network through polymer chain cleavage and reformation which results in macroscopic stress relaxation. Previous work on this material focused on photo-induced stress relaxation experiments where the overall deformation of the material is fixed. In that case, evolution of the network structure occurs in a single configuration of the body. In this paper, we investigate simultaneous light-stimulated material evolution and deformation under a creep setup. In contrast to stress relaxation, under photo-induced creep, evolution of the network occurs in different configurations as network relaxation is necessarily accompanied by further deformation of the body. Hence, photo-induced creep involves additional complexity because the newly reformed networks have different stress-free configurations and states of deformation. To model the light-stimulated activity, we decompose the cross-linked network into three parallel networks: an initial, a reformed, and a newly formed network. These three networks are distinguished by their volume fractions, stress-free configurations, and states of deformation. Together, the combined mechanical behavior of all three networks describes the mechanical response of the material. The model is used to predict photo-induced creep behavior under different applied nominal stresses and light intensities. Generally, the model performs well in predicting the overall creep behaviors as well as capturing the trends produced by changing the light intensity and applied nominal stress.