Nanoscopic adhesion test for polymers by a light-lever system

Polymer nano-adhesion between a cantilever tip coated with polymer and a flat polymer film was studied by a light-lever system using an atomic force microscope. The polymer interface was adhered at a temperature above the surface glass transition temperature for a given time. Nano-adhesion force (F _nano), at which the tip was detached from the surface, was estimated from the deflection of the lever with a known spring constant. Nano-adhesion strength (G_N) was simply obtained dividing F_nano by the contact area, which was estimated on the basis of Johnson-Kendall-Roberts theory. The time evolution of the interfacial thickness was independently examined by dynamic secondary ion mass spectrometry. Interestingly, G _N increased with increasing interfacial thickness. However, it can be hardly judged whether G_N is proportional to the interfacial thickness with the exponent of 1 or 2. Then, temperature dependence of G _N was examined. Above the bulk glass transition temperature, the relation between temperature and G_N was well expressed by a Williams-Landel-Ferry type equation. This means that the nano-adhesion strength is governed by friction between segments. Once this is accepted, G_N should be proportional to the interfacial thickness with the exponent of 2.