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

We explore the hydration structure and dynamics of poly(2-methacryloyloxyethyl phosphorylcholine)50, or pMPC50, in solution and grafted to silica using molecular dynamics simulations with an emphasis on isolating the contributions of the key chemical moieties. 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 a simplified monolayer analogue to the brush structures, water hydrates surface bound MPC monomers and enables such materials to utilize the hydration lubrication mechanism as monolayers experience shear; a clear cross-over, as a function of compression, is found between regimes dominated by hydration lubrication and those where MPC-MPC interactions dominate [4]. Previous results [5] showed that the charged choline groups of individual monomers tend to fold onto neighboring monomers yielding solvent facing, highly hydrated phosphoryl groups.

Here, we further analyze the effect of the two outer chemical moieties, the choline and phosphoryl groups. For bulk solutions, we compare the behavior of pMPC to poly(2-methacryloyloxyethyl phosphate) and show that the removal of the choline group reduces hydration around the polymer. When bound to a substrate, the monolayer analogue to the experimental system did not permit significant inter-chain associations via the choline groups. Simulations of much larger pMPC50 brush layers demonstrate that the choline moiety serves to stabilize the material through non-covalent cross-links between individual strands in the layer. Preliminary results strengthen the previous hypothesis [5] that the choline moiety is responsible for stabilizing the polymer while the phosphoryl moiety contributes to the hydration lubrication mechanism.

[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, P.T. Cummings, C. McCabe, “Tunable transition from hydration to monomer-supported lubrication in zwitterionic monolayers revealed by molecular dynamics simulation” Soft Matter 2015, 11, 3340-3346.

[5]  W.L. Roussell, C. Klein, C.R. Iacovella, P.T. Cummings, C. McCabe, “Molecular Origins of the Ultra-Low Friction Exhibited by Biocompatible Zwitterionic Polymer Brushes” Young Scientist, May 2015

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