Studying Surface Glass-to-Rubber Transition Using Atomic Force Microscopic Adhesion Measurements

Force-distance curves were obtained using a home-built atomic force microscope (AFM) at different temperatures (T ) 30-65 °C) and probe rates (f ) 31.25-50 000 Hz) on a 150 nm thick film of a model sample, poly(tert-butyl acrylate) (Mw ) 148K Da, Mw/Mn ) 17, and Tg ) 50 °C according to DSC). The pull-off force, Fad, at which detachment between the AFM tip and the sample occurred was measured as adhesion. By limiting the loading force, F, to ∼2.5 nN, the tip penetrated by no more than 2 nm into the sample in the glassy state. Therefore, evolution of the rheological properties of the polymer at the free surface with increasing T could be studied. In the vicinity of Tg, Fad was seen to increase rapidly with increasing T or decreasing f. Equivalence between T and f was found using time-temperature superposition in which, upon rescale of f by a temperature-dependent shift factor aT(T), a master curve Fad(aT(T) f) resulted. We showed that Fad(aT(T)f) could be fully accounted for by using an approach based on fracture mechanics of viscoelastic solids. No noticeable enhancement in the surface relaxation could be deduced according to our findings.