Theoretically Informed Coarse-Grained Simulations of Polymer Nanogels

Nanoscale finite-sized polymer networks (nanogels) are smart responsive materials that undergo large reversible volume changes with moderate changes in environmental conditions such as temperature, pH, light, and electric field. We develop a coarse-grained model of nanogels in terms of experimentally measurable physical quantities, and perform a theoretically informed Monte Carlo simulation that combines ideas from both the particle and continuum approaches of polymer physics. The elastic interactions are treated through beads connected by harmonic springs (``particles''), and the van der Waals and electrostatic interactions are treated by weighted densities (``fields''). Our simulations predict high degrees of swelling and a discontinuous volume phase transition in ionic nanogels, in contrast to moderate swelling and a continuous volume phase transition for the non-ionic case. We analyze the effects of mesh-size, polymer charge fraction, ionic strength, and solvent quality, on the swelling behavior of nanogels. A comparison is made with the results of a simplified continuum model, where the electrostatic interactions are treated using the Poisson-Boltzmann approximation.