MINIMAL COARSE-GRAINED MODELS FOR SIMULATING POLYMERS INTERACTING WITH SITE-DEPENDENT SHORT-RANGE POTENTIALS

Abstract

Monte Carlo simulations are performed to study the self-assembly of a dilute system of spherocylinders interacting with square-well potential. The interactions are defined between randomly placed sites on the axis of the spherocylinder, akin to the interacting groups on a rigid rodlike molecule. This model therefore also serves as a minimal coarse-grained representation of a system of low molecular weight or stiff polymers with contour lengths significantly lower than the persistence length, interacting predominantly with short-range interactions (e.g., hydrogen bonding). The spherocylinder concentration, square-well interaction strength and range, and fraction of interacting sites are varied to study the phase behavior of the system. We observe the formation of dispersed, bundled, and network configurations of the system that may be compared with previous atomistic simulation results of weak polyelectrolytes. Monte Carlo simulations developed above is then extended to a study of self-assembly of a mixture of spherocylinders, labelled A and B. Here, the smaller spherocylinder (B) may correspond to a small molecule drug and the larger spherocylinder (A) may correspond to a low molecular weight or stiff polymer. The simulation may also represent a mixture of low molecular weight or stiff polymers. Simulations are performed for different lengths, concentrations, composition (A/B ratio), and interaction strengths of A and B. To mimic different nature of interactions between A and B, we consider four different situations: 1. Case “ALL”: Interactions between AA, AB, and AB pairs of spherocylinders. 2. Case “NO AB”: Interactions between AA and BB pairs of spherocylinders. 3. Case “No BB”: Interactions between AA and AB pairs of spherocylinders. 4. Case “Only AB”: Interactions between AB pair of spherocylinder. We obtain a rich phase diagram in terms of the variables studied, which may be useful in designing polymer-drug formulations. Taken together, these two studies provide a minimal coarse-graining framework that may be trained using atomistic simulations of polymer solutions and polymer-drug formulations, which are typically limited to low molecular weights of polymers, i.e., in the rodlike regime and small molecule drugs.