A fully coupled model for diffusion-induced deformation in polymers

Abstract

Polymers that mechanically respond to the presence of a diffusing fluid/solvent have found various applications in drug delivery, tissue scaffolding, sensors and actuators. These applications involve understanding of both the diffusion process and the evolution of the deformation of the polymers during the diffusion process. For example, in a polymeric actuator one might be interested in the extent of deformation one can achieve given a solvent environment and the time in which it can be achieved. There are two key aspects in modeling such behavior. First, the displacement gradients involved are usually large, especially in problems such as “self-assembly.” Second, since the diffusion occurs in a deforming polymeric medium, an appropriate diffusion model that includes the effect of the deformed state of the body as well as the interaction between the polymeric medium and the diffusing fluid has to be considered. In effect, this results in the diffusion and equilibrium equation being fully coupled and nonlinear. In this work, we model diffusion-induced deformation in an elastic material including large deformations based on thermodynamics framework. For the chemical potential, we use the Flory–Huggins potential adapted to include the effect of stress in the polymers. Using the model, we simulate folding and bending of a rectangular polymeric strip by simultaneous solution of the diffusion equation as well as the equilibrium equation using the finite element method. Parametric studies are also conducted in order to examine the effect of material parameters on the diffusion and deformation behaviors. Finally, using the coupled diffusion–deformation model we simulate deformations of composite domains comprising of polymeric constituents with different diffusion–deformation behaviors in order to achieve various interesting “self-assembly” shapes.