Dissipative particle dynamics modeling of hydrogel swelling by osmotic ensemble method.

An osmotic ensemble method for dissipative particle dynamics (DPD) is developed for simulating the swelling behavior of polymer networks in aqueous solvent under constant solvent chemical potential, number of polymer beads, pressure, and temperature conditions. We apply a Langevin piston method to control the pressure of the polymer-solvent mixture. Chemical potential equilibrium is achieved via Monte Carlo insertions and deletions of solvent beads based on the total free energy change of the gel. The osmotic ensemble simulation produces swelling kinetics of hydrogels in excellent agreement with that obtained by previous methods but significantly reduces computational costs. The results show gel swelling as a result of the mechanical balance between osmotic pressure induced by the mixing of the polymer and solvent and elastic force originated from the network deformation. The simulations also elucidate the influence of solvent conditions and network topology on the degree of swelling. The bulk modulus of the model gel is probed at different solvency and its behavior is consistent with the prediction of Flory-Rehner theory. The osmotic ensemble DPD will permit the study of mechanical properties of hydrogels in mesoscale simulations and can be extended to model other complex fluid systems in chemical equilibrium under isothermal-isobaric conditions.