Dynamics of lubricious, concentrated PMMA brush layers studied by surface forces and resonance shear measurements

We employed surface forces and resonance shear measurement (RSM) for studying the structure and properties of typical concentrated polymer brushes (CPBs) of poly(methylmethacrylate) (PMMA) in toluene, which are known to show very low friction. The surface forces measured between the silica surfaces bearing PMMA brush layers showed a steric repulsive force at distances between the silica surfaces of less than ca. 1050 nm (D_onset). Upon retraction after compression of the PMMA brush layers, no adhesive force was observed. This indicated that the interpenetration of the polymer chain was not induced by the normal load. Based on the resonance shear measurement, the elastic (k₂) and damping (viscous) (b₂) parameters, which represent the dynamic properties, of the PMMA brush layers were obtained by analyzing the resonance curves. At distances below the D_onset, the b₂ value significantly increased and slightly decreased at the higher normal loads, and the k₂ value monotonically increased with increasing load. These k₂ and b₂ values were greater than those obtained for a PMMA brush layer and a bare silica surface (PMMA-silica). This indicated that the mobility of the polymer chains for the PMMA-PMMA brush layers was more suppressed compared to that for the PMMA-silica, due to the interpenetration of the polymer chains. The interpenetration of the polymer chains, commonly not observed for CPBs, could be most probably induced by the application of both the normal load and oscillating shear motion. With increasing shear amplitude on the compressed PMMA-PMMA brushes (at L = 0.84, 1.34 and 4.28 mN), the b₂ value first increased then decreased whilst the k₂ value monotonically decreased. These tendencies can be explained by the change from the sticking condition due to interpenetration (high k₂), small sliding under interpenetration (increase in b₂, decrease in k₂), and then smooth sliding by pulling out of interpenetrated polymer chains (decrease in b₂ and k₂). The obtained results indicated that the operating conditions are quite important for using polymer brush layers as tribological materials because they can exhibit both a high and low friction depending on the conditions such as the load and shear amplitude.