Keiichi Akabori

Time- and space-resolved fluorescence study on interfacial mobility of polymers

Segmental mobility of a typical amorphous polymer, polystyrene, at the interface with a solid substrate was examined noninvasively by fluorescence lifetime measurement using evanescent wave excitation. Glass transition temperature (Tg) was discernibly higher at the interface than in the bulk. Measurements at different incident angles of excitation pulses revealed that Tg became higher the closer to the interface. This is the observation for a Tg gradient of polymers at the interface.

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 (Fnano), 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 (GN) was simply obtained dividing Fnano 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, GN increased with increasing interfacial thickness. However, it can be hardly judged whether GN is proportional to the interfacial thickness with the exponent of 1 or 2. Then, temperature dependence of GN was examined. Above the bulk glass transition temperature, the relation between temperature and GN 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, GN should be proportional to the interfacial thickness with the exponent of 2.