Effect of grafting density of poly(2-methacryloyloxyethyl phosphorylcholine) brush on the tribological properties

High-density polymer brush on silicon wafer was prepared by surface-initiated atom transfer radical polymerization ('grafting-from' method) of 2-methacryloyloxyethyl phosphorylcholine (MPC) using CuBr / 2,2'-bipyridyl complex in methanol at 300 K. Low-density poly(MPC) brush was prepared by 'grafting-to' method using an poly(MPC) end-functionalized with alkoxysilyl group. The surface of both poly(MPC) brush was very hydrophilic; water contact angle was below 5 degree. Dynamic friction tests were carried out by sliding a glass ball (φ = 10 mm) on the surface at a sliding velocity of 90 mm/min under loading of 0.49 N at 298 K. Friction coefficient of high-density poly(MPC) brush was much lower than that of low-density one in aqueous condition. In addition, great wear resistance was observed in a high-density polymer brush. These results indicate that high-density and super-hydrophilic polymer can act as efficient lubricant to reduce the friction coefficient.