Nanotribology on a Polymer Network Film

G. Haugstad
Center for Interfacial Engineering, 187 Shepherd Laboratories
University of Minnesota, Minneapolis, MN 55455

W. L. Gladfelter
Department of Chemistry
University of Minnesota, Minneapolis, MN 55455

R. R. Jones
Medical Products Division, E. I. du Pont de Nemours and Co.
Staton Road, Brevard, NC 28712


Abstract

Scanning force microscopy (SFM) of topography, frictional force and stiffness on thin gelatin films reveals an entire spectrum of tribological behavior. Images display two distinct phases of gelatin whose characteristics relate to the degree of crystallinity. Dissimilar regimes of velocity- and and load-dependent friction and wear on each phase indicate glassy, rubbery or melt behavior. Of fundamental importance is the finding that energy transferred to the film in the vicinity of the sliding SFM microasperity modifies film response. Moderate frictional heating melts the highly-crystalline phase, but reversibly induces rubbery behavior on the partially-amorphous phase. More extreme frictional heating melts the latter and allows the liberated molecules to reassociate irreversibly into the highly-crystalline phase. This relatively slow process (minutes) is imaged in real time on the submicron scale. Relaxation from rubbery to glassy behavior upon termination of perturbative scanning is extremely slow (hours) and also is characterized in frictional images. In this case the imaging process itself hastens the relaxation, however, apparently by providing energy to activate some relaxational processes.