Molecular weight effects in adhesion

Adhesion of poly(dimethylsiloxane) cross-linked networks to silicon oxide surfaces was studied using the Johnson-Kendall-Roberts method of contact mechanics. An increase in adhesion hysteresis was observed in the order of increasing molecular weight between cross-links. The log/log plots of energy release rates, G, vs molecular weights (M_n), for different crack propagation rates (-da/dt), gave a family of straight lines, with their slopes increasing with increasing -da/dt. When a plot of the slopes vs crack propagation rates was extrapolated to -da/dt = 0, it was found that the slope at zero velocity is equal to 0.47, in agreement with the 1/2 power predicted by de Gennes. On the other hand, when the network was kept for 30 min at maximum load before unloading, the scaling factor was 1/3. This decrease in scaling factor is attributed to the molecular weight dependence of network relaxation rates. The results suggest that the polymer chain in a network, when in contact with a surface under conditions close to equilibrium, behaves like a chain in a polymer melt near a surface.