The swelling behavior of ionic polymers in the presence of diffusion and chemical reactions

During the past few decades, ionic polymers have attracted much attention due to their unique properties, i.e., a stimulus-sensitive swelling behavior. The use of ionic polymers in biotechnology and medicine is increasingly attractive for applications such as controlled drug delivery, biomimetic actuators, and chemical valves. The ability to model and predict the swelling behavior of polymers and mass flux with respect to external environmental conditions is very important in such applications.; The focus of this dissertation is on developing a continuum model for the behavior of a swollen solid and diffusion of a fluid through it in the presence of chemical reactions. To model diffusion through an ionic/nonionic polymer in the presence of mechanical deformation, we develop a variational procedure based on a thermodynamic framework for a swollen solid subject to constraints. This thermodynamic framework was mainly developed by Rajagopal and Srinivasa as a general thermodynamic framework for inelastic behavior of materials. First, a model for the mechanical behavior of a rubber-like solid in the presence of mass diffusion is developed using the variational procedure. By assuming a specific form for the Helmholtz potential function, we obtain an equilibrium equation that is identical to the equation suggested by Flory, Huggins, and Rehner. For the non-equilibrium problem, we assume a specific form for the rate of dissipation function and obtain the relation between pressure difference and mass flux. The numerical results show very good agreement with experimental data for the diffusion of organic solvents through a rubber slab. Hysteresis response of the swollen solid due to time-dependent loads is also simulated by the model.; The variational procedure is also applied to pH-sensitive ionic polymers, which not only yields the equations for the deformation of a swollen solid and mass flux, but also the equations for chemical equilibrium. As a specific example, an anionic polymer containing carboxylic acid in a solution of NaOH is investigated and its behavior in solutions of different pH and surrounding pressures is predicted. The numerical results show good agreement with the experimental data.