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) (M w = 148K Da, M w/M n = 17, and T g bulk = 50 °C according to DSC). The pull-off force, F ad, 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 T g bulk, F ad 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 a T AFM(T), a master curve F ad(a T AFM(T) f) resulted. We showed that F ad(a T AFM(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.