(502e) Molecular Simulation Study of the Volume Transition of Hydrogels

Hydrogels are three-dimensional networks of hydrophilic polymers. They are widely found in nature, e.g. in membranes and tissues, and are also technically broadly used, e.g. as super-absorbers or in drug delivery systems. One of their most interesting properties is their ability to swell and collapse. The swelling behaviour is determined by their network properties, namely the chemical nature of the polymer chains and their degree of cross-linking, and the external conditions, like the temperature and the composition of the solvent. In the present work, the swelling and collapsing of synthetic hydrogels is studied by molecular dynamics simulation on the basis of transferable all-atom and united-atom force fields using explicit solvent models. An efficient simulation strategy was developed that allows investigating the dependence of the volume transition of hydrogels on the parameters discussed above. It is applied to molecular models for real synthetic hydrogels + solvent systems so that simulation results can be compared to experimental data. The volume transition of hydrogels mainly depends on the nature of the polymer backbone and the solvent. The cross-linker usually only has a minor influence. This allows studying the volume transition in the hydrogel + solvent system by simulations of the computationally much less expensive polymer strand + solvent system. The volume transition of the hydrogel then corresponds to the change of the polymer strand between its coiled form in a poor solvent and its stretched form in a good solvent. The validity of the single-stand approach was tested by comparison to very large simulations with three-dimensional cross-linked networks. Molecular simulations were performed with GROMACS 4 and different transferable force fields from the literature using different water models. The swelling behaviour of synthetic hydrogels was studied with Poly(N-isopropylacrylamide) based gels. This work focuses on the influence of the temperature and solvent composition on the volume transition of the hydrogel. The results are compared to experimental data. As no force field parameters were changed, all results are fully predictive. It is shown that molecular modelling and simulation is a suitable tool for studying the volume transition in real hydrogel + solvent systems.