Studying surface glass-to-rubber transition using atomic force microscopicadhesion measurements

Force-distance curves were obtained using a home-built atomic force microsc ope (AFM) at different temperatures (T = 30-65 degrees C) and probe rates ( f = 31.25-50 000 Hz) on a 150 nm thick film of a model sample, poly(tert-bu tyl acrylate) (M-w = 148K Da, M-w/M-n = 17, and T-g(bulk) = 50 degrees C ac cording 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 l oading force, F, to similar to 2.5 nN, the tip penetrated by no more than 2 nm into the sample in the glassy state. Therefore, evolution of the rheolo gical 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 rapidl y with increasing T or decreasing f. Equivalence between T and f was found using time-temperature superposition in which, upon rescale off by a temper ature-dependent shift factor a(T)(AFM)(T), a master curve F-ad(alpha(T)(AFM )(T) f) resulted. We showed that F-ad(alpha(T)(AFM)(T)f) could be fully acc ounted for by using an approach based on fracture mechanics of viscoelastic solids. No noticeable enhancement in the surface relaxation could be deduc ed according to our findings.