Impact of Dispersity and Hydrogen Bonding on the Lubricity of Poly(acrylamide) Brushes

The effects of chain-length dispersity and monomer type on the lubricity of acrylamide-based polymer brushes have been examined in aqueous media. The polymer brushes used have been synthesized by surface-initiated, atom-transfer radical polymerization of N,N-dimethylacrylamide (DMAM), N-hydroxyethyl acrylamide (HEAM), and N-isopropylacrylamide. Polymerization reactions have been performed in the presence and also in the absence of CuX2 and tEtAmX (X = Cl or Br), in order to achieve polymer brushes with substantial differences in their chain-length dispersities. While an influence of the dispersity on the lubricity of the polymer brushes can be observed, it is convoluted with the competing phenomena of hydrogen bonding and increased water content. The most hydrophilic, p(HEAM) brush exhibits the highest friction coefficient (mu) of approximate to 0.04 compared to approximate to 0.004 in the case of the p(DMAM) brush. Such a large difference is presumed to originate from the hydrogen bonding between the tribological countersurface (plasma-oxidized poly(dimethylsiloxane)) and the p(HEAM) chains, as evidenced by the substantial reduction in mu when the friction measurements are performed in 5 m urea solution instead of pure milli-Q water.