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 thesemonolayers. The pull-off force under dry conditionswasmeasured as a function of contact time (0.01-500 s) and separation rate (0.003-100μm/s; corresponding lateral crack velocity approximately 0.04-1200 μm/s) using a piezoelectric bimorph attachment to a surface forces apparatus. No time or rate dependencewas observed for close-packed crystallinemonolayers of condensedn-octadecyltriethoxysilane (OTE) or the adsorbedglassydiblock copolymerpoly-2-vinylpyridine (PVP)-polystyrene. Rate dependence beyond a critical separation rate was observed in monolayers whose chains were more mobile although anchoredat one end to the solid surface. For loose-packedmonolayers of cetyltrimethylammoniumbromide (CTAB), the adhesion in excess of the constant observed at low rate increased as a power law with the square root of the separation rate. For adsorbed PVP-polybutadiene, the excess adhesion increased nearly linearly with the logarithmic separation rate. For both CTAB and PVP-polybutadiene, the critical separation rate lessened with increasing contact time before detachment. The time effects are ascribed to interdigitation between the contacting layers as a result of interdiffusion over nanoscale distances, facilitated by the low glass transition of the polybutadiene and the loose packing of the CTABmonolayers. The rate effects are ascribed to viscoelasticity during chain pull-out. The rate dependence was weaker than the simple proportionality to velocity that has been expected theoretically for chain pull-out. The oft-proposed separation of rate-dependent adhesion into the product of bulk viscoelastic response and a constant surface energy is inconsistent with these findings.