Many-body polarization and hydration forces can strongly affect the equilibrium structure and energetics of polymer phases. We compare experimental measurements of the pressure-area isotherm for Hydroxypropylcellulose (HPC) against molecular dynamics results with four different forcefield models -- united-atom, all-atom (OPLS and CHARMM) and drude-oscillator models. All forcefields display the well-known, exponentially-shaped hydration force at short range. Above a critical temperature of about 40 C and lattice spacing around 14 Ang, HPC displays a reversible, heat-induced polymer aggregation into an ordered phase driven by loss of water. Unsurprisingly, the united atom model performs worst, showing aggregation on cooling instead. For all temperatures, the OPLS and CHARMM36 all-atom forcefields produce nonuniform biphasic systems at low densities, indicative of net attractive forces at intermediate lattice spacings. In contrast, experiment shows smooth, repulsive interactions at the same separations. The CHARMM Polarizable Drude 2013 forcefield at large spacings shows homogeneous phases with repulsive forces in line with experiment. Analysis of structural data from our simulations provides several clues to help interpret these findings. Although all forcefields show similar water-water hydrogen bond numbers in the polymer phase, there are notable differences for water-HPC and HPC-HPC hydrogen bonding in the polarizable model.

Hydration and dispersion forces in hydroxypropylcellulose phase behavior

2019 Spring National Meeting

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