RATE-DEPENDENT ADHESION BETWEEN POLYMER AND SURFACTANT MONOLAYERS ON ELASTIC SUBSTRATES

Organic monolayers were formed on mica substrates whose detachment was rate-independent in the absence of these monolayers. The pull-off for ce under dry conditions was measured as a function of contact time (0. 01-500 s) and separation rate (0.003-100 mu m/s; corresponding lateral crack velocity approximately 0.04-1200 mu m/s) using a piezoelectric bimorph attachment to a surface forces apparatus. No time or rate depe ndence was observed for close-packed crystalline monolayers of condens ed n-octadecyltriethoxysilane (OTE) or the adsorbed glassy diblock cop olymer poly-2-vinylpyridine (PVP)-polystyrene, Rate dependence beyond a critical separation rate was observed in monolayers whose chains wer e more mobile although anchored at one end to the solid surface. For l oose-packed monolayers of cetyltrimethylammonium bromide (CTAB), the a dhesion in excess of the constant observed at low rate increased as a power law with the square root of the separation rate. For adsorbed PV P-polybutadiene, the excess adhesion increased nearly linearly with th e logarithmic separation rate. For both CTAB and PVP-polybutadiene, th e critical separation rate lessened with increasing contact time befor e detachment. The time effects are ascribed to interdigitation between the contacting layers as a result of interdiffusion over nanoscale di stances, facilitated by the low glass transition of the polybutadiene and the loose packing of the CTAB monolayers. The rate effects are asc ribed to viscoelasticity during chain pull-out. The rate dependence wa s weaker than the simple proportionality to velocity that has been exp ected theoretically for chain pull-out. The oft-proposed separation of rate-dependent adhesion into the product of bulk viscoelastic respons e and a constant surface energy is inconsistent with these findings.