A molecular dynamics simulation study of polymer dynamics in aqueous poly(ethylene oxide) solutions

We have performed molecular dynamics simulations of aqueous solutions of poly(ethylene oxide) (PEO) in order to investigate the influence of water on polymer dynamics as a function of solution composition. Simulations were performed on 12 repeat unit PEO chains (530 Da) at 318 K covering a composition range (polymer weight fraction omega (p)) from 0.17 to 1.0. It was found that addition of a small amount of water to a PEO melt dramatically affects polymer dynamics on all length scales, leading to a maximum in relaxation times in the range omega (p) = 0.78-0.90, with the greatest effect occurring for smaller length-scale motions. With further dilution relaxation times decreased dramatically, with the greatest effect occurring for the largest length-scale motions. Water slows the rate of conformational transitions in PEO compared to the melt, an effect that sets in rapidly with initial dilution and saturates at omega (p) approximate to 0.5. Initial addition of water leads to an increase in the heterogeneity of the rate of conformational transitions, with a maximum in the range omega (p) = 0.78-0.90, followed by a decrease in heterogeneity with further dilution. A maximum in the non-Gaussianity of PEO atom displacements was observed in the same composition range. On the segmental scale, water was found to engender a monotonic decrease in relaxation anisotropy and an increase in the efficacy of conformational transitions in effecting PEO segmental relaxation. Finally, water was found to first lead to a slight increase and then to a dramatic decrease in the longest relaxation times of the solution, including viscosity, with dilution.