(304d) Hydration Structure and Dynamics of Poly(2-methacryloyloxyethyl phosphorylcholine)

We explore the hydration structure and dynamics of poly(2-methacryloyloxyethyl phosphorylcholine), or pMPC, in solution using molecular dynamics simulations. Surface bound films of this biocompatible polymer have been shown experimentally to produce tribological properties that surpass those of the human synovial joint [1] and are being developed for use in artificial joints [2]. The mechanism by which such materials are thought to provide ultra-low friction coefficients has been termed “hydration lubrication” [3]. In previous work, we explored the conditions under which water favorably hydrates surface bound MPC monomers and enables such materials to utilize the hydration lubrication mechanism [4]. Here, we further characterize the molecular level structure and dynamics of this hydration water around the much larger pMPC molecules that are experimentally grafted onto substrates for biomimetic lubrication. Our results show that the charged choline groups of individual monomers tend to fold onto neighboring monomers. This has the threefold effect of i) compacting the polymer, ii) excluding significant volume where hydration shells could otherwise form around the choline groups and iii) bending the monomers at the phosphate group thus preferentially exposing the two strongly negatively charged oxygen atoms in the phosphate group to the aqueous environment. These results suggest that the predominant contribution to hydration lubrication in pMPC based materials is derived from water hydrating the phosphate group and that the choline group plays a lesser or perhaps negligible role. However, the strong association of the choline groups with neighboring monomers may serve to stabilize the overall polymer structure which would contribute to the high resistance to compression exhibited by hydrated pMPC films. Comparisons of the steady-state residence time of water molecules in several hydration shells around the polymer are computed and compared to our previous results for surface bound MPC monomers to demonstrate the effect of the structural difference on hydration.

[1]   M. Chen et al., “Lubrication at Physiological Pressures by Polyzwitterionic Brushes.” Science 2009, 323, 1698–1701.

[2]   M. Kyomoto et al., “Biomimetic hydration lubrication with various polyelectrolyte layers on cross-linked polyethylene orthopedic bearing materials.” Biomaterials 2012, 33, 4451-9.

[3]      J. Klein, “Hydration lubrication.” Friction 2013, 1, 1–23.

[4]   C. Klein, C. R. Iacovella, C. McCabe, P. T. Cummings, “Tunable transition from hydration to monomer-supported lubrication in zwitterionic monolayers revealed by molecular dynamics simulation.” Soft Matter 2015, 11, 3340-6.