Web Release Date: April 30,
Studying Surface Glass-to-Rubber Transition Using Atomic Force Microscopic Adhesion Measurements
Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Received August 30, 1999
Revised Manuscript Received February 22, 2000
Abstract:
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 Tgbulk = 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 Tgbulk, 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 aTAFM(T), a master
curve Fad(aTAFM(T) f) resulted. We showed that Fad(aTAFM(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.
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