TEMPERATURE AND TIME EFFECTS ON THE ADHESION DYNAMICS OF POLY(BUTYL METHACRYLATE) (PBMA) SURFACES

We present a surface force apparatus (SFA) study of the effects of tim e and loading-unloading rates on the adhesion of solid polymeric surfa ces of poly(butylmethacrylate). We used the equilibrium JKR theory of adhesion or contact mechanics as a framework for analyzing the ''adhes ion dynamics'' of two surfaces during nonequilibrium (viscoelastic, pl astic) adhesion and separation. PBMA films of thickness similar to 2 m u m were prepared on curved mica surfaces by casting from a solution o f methyl ethyl ketone. Pull-off forces from adhesive contact were meas ured at different temperatures around the glass-rubber transition temp erature (T-g approximate to 25 degrees C) at different loads and conta ct times, and hysteretic loading-unloading cycles were measured at dif ferent rates. On entering the rubber regime, the effective surface ene rgies deduced from the pull-off forces increase dramatically, by up to 3 orders of magnitude above the ''equilibrium'' value, with increasin g contact time and load. Strong entanglements across the interface, pr obably through reptation, increase the effective area of contact with time, giving rise to the high pull-off forces observed. Bulk viscoelas tic deformations of the surface profiles accompany the time-dependent adhesion processes. The existence of at least two different relaxation (energy dissipating) processes, one at the molecular level and the ot her at the microscopic to macroscopic level, can be inferred from thes e experiments. The implications of the results for understanding the a dhesion, fracture strength, and crack-propagation of elastic Versus vi scoelastic materials are discussed.