Surface and interface structure and tribological properties of hydrophilic polymer brushes

The hydrophilic polymer brushes with 25-35 nm thick were prepared by surface-initiated atom transfer radical polymerization of (2,2-dimethyl-1,3- dioxolan-4-yl)methyl methacrylate (DMM), 2-methacryloyloxyethyl phosphorylcholine (MPC), 2-dimethylaminoethyl methacrylate (DMAEMA), and vinyl acetate (VAc) on the initiator-immobilized silicon wafer. PDMM, PDMAEMA, and PVAc were converted to poly(2,3-dihyroxypropyl methacrylate) (PDHMA), poly(2-methacryloxyethyltrimethylammonium iodide) (PMETA), and polyvinyl alcohol) (PVA), respectively. Macroscopic frictional properties of the hydrophilic polymer brushes were characterized by sliding a glass ball probe in air, water, and toluene under the load of 0.49 N at a sliding velocity of 90 mm/min. The dynamic frictional coefficient of the surfaces immobilized with PDHMA, PMPC, PMETA and PVA brushes were lowered in water (good solvent) compared with in dry air condition, while they increased in toluene (poor solvent). Extremely low friction coefficient of PMPC brush was observed in humid atmosphere because water molecules adsorbed into brush surface acted as a lubricant. Adhesion force measurement of high-density PMPC brush was also carried out by AFM using a colloidal probe. Larger adhesion force was observed in a poor solvent (ethanol/water = 75/25, v/v) compared with good solvent (pure water), because of strong interaction between a brush and a probe in a poor solvent. These results indicate that the frictional properties largely depend on the solvent quality and the adhesion force between the substrate and the sliding probe.