Development of first-principle based methods for predicting the structures and properties of solvated polymers

Abstract

Water soluble polymers such as polyethylene glycol (PEG) are critical components of industrial processes ranging from drug delivery to water purification. However, the understanding of the microscopic structure of these polymers in water and of the thermodynamic and osmotic properties of the mixtures is limited because available experimental techniques (such as SLS and SANS) give little information about conformations and provide even a radius of gyration (Rg) only in dilute limit (< ~5 wt% PEG). Computer simulations employing Monte Carlo (MC) and molecular dynamics (MD) techniques can provide a microscopic understanding of the molecular structure; however such approaches have difficulties in predicting the equilibrium polymer configurations and in obtaining entropies and free energies directly from the MD. Here, we develop the scaled effective solvent (SES) method to predict the equilibrium ensemble of polymer configurations, which we illustrate for the case of a 20 kDa PEG (455 monomers) at a 25 wt% PEG aqueous solution (3,339 waters per PEG chain). We evaluate the free energy and entropy of the members of this ensemble including explicit water, validating that it leads to average sizes (Rg) observed experimentally and that all members of the ensemble have favorable free energies. This method will be useful for predicting the mechanical and physical properties of high molecular amorphous polymers as well as ensembles of polymer chains in polymer melts and in various solvents. Also, the equilibrium configurations of a 20 kDa PEG immersed in water with 25 to 60 wt% PEG obtained by using the SES method are used to predict the osmotic pressure as a function of PEG concentrations. From this work, we find that interfacial water adjacent to the ether oxygen leads to reduced entropy depending on the concentration. Thus, the thermodynamics depends on the internal surface area and local structure of these interfacial waters which in turn determines properties such a...