Modeling the One-Dimensional Transient Deformation of An Elastic Photopolymer

Motivated by the need for implants that can be adjusted precisely and non-invasively, we analyze the light-induced shape change of a photopolymer consisting of an elastomeric polymer matrix swollen in liquid macromer solution (photosensitive short chain polymer with photoinitiator). When a concentration gradient of macromer is created by spatially-resolved photopolymerization, diffusion of free macromer chains induces swelling in irradiated regions and shrinkage elsewhere. Using mixture theory, a branch of continuum mechanics, we model the one-dimensional reaction-diffusion-deformation of the material as a nonlinear partial differential equation. Important attributes are the magnitude of the shape change and the time required for it to occur. We find that the most important material parameter affecting both of these is M*, the ratio of molar mass between network crosslinks to molar mass of macromer chains. An open network (large molar mass between cross-links) gives the largest shape change and the fastest equilibration time, all other parameters fixed. Other parameters that enter weakly are the macromer-network interactions (χ) and the solvent quality of the macromer for the network (α). The deformation modeled agrees with experiments and previous equilibrium swelling models.