Friction coefficient polymer brushes

The effect of polymer brushes in different lubrication regimes is seen in Fig. 4a and b, in which the Stribeck curves obtained from pin-on-disk and MTM measurements are plotted, respectively. The coefficient of friction is plotted against speed multiplied by viscosity for all HEPES-glycerol mixtures both in the presence and the absence of the polymer. As expected, the effect of polymer in reducing the friction is predominantly seen in the boundary-lubrication regime. The effect of polymer on the friction is also extended to the mixed-lubrication regime... 

Figure 5.11 shows the dynamic friction coefficient of PMMA brushes of various thicknesses in air, cyclohexane, and toluene. Compared with the friction coefficient of the polymer brush in the dry state, the value decreased in both organic solvents... 

To evaluate the wear resistance of the PMMA brush, a continuous friction test for 600 s was performed by sliding a stainless steel ball on substrates covered with 20 nm of thick polymer brush or cast film under a load of 0.49 N at a sliding rate of 90 mm/min (fig. 5.12). In the early stage of the friction test, the friction coefficient of the brush surface increased to 0.5 from 0.25, but there was no further increase during continuous friction, as shown in fig. 5.12a. On the other hand, the friction coefficient of the cast film continuously inaeased with friction cycles (fig. 5.12b), and the surface polymer gradually peeled away to form debris ( wear elements ), some of which was adsorbed on the surface of the probe ( transfer particles ) or left in the wear track, interfering with the smooth sliding of the probe. Finally, the PMMA layer in the wear track had completely peeled off. 

Figure 5. Friction force measmed between two brushes of PLL-(20)-g[3.5]-PEG(2) after rinsing with 150 mM KCl, moimted with a drop of polymer solution. The sample showed a moderate increase in friction with load, leading to a fiiction coefficient of fi = 0.003.

Previous studies involving the surface forces apparatus have shown that sliding surfaces covered with grafted polymer chains under good solvents, such as polystyrene under toluene, can display extremely low coefficients of friction [4,5]. To our knowledge, such brush-solvent systems have not been previously investigated in an aqueous context, nor have they been tested under macrotribological conditions. 

---