Contrasting static-to-kinetic friction transitions on layers of an autophobically dewetted polymer film using Fourier-analyzed shear modulation force microscopy
Characterization Facility, Institute of Technology, University of Minnesota, 100 Union Street SE, Minneapolis, Minnesota 55455
Fourier analysis of oscillating forces at a laterally modulated tip provides new insight into static-to-kinetic friction transitions on ultrathin polyvinyl alcohol (PVA) films. In addition to contrast in sliding friction, layers of autophobically dewetted PVA films exhibit remarkable contrast in the transition from static to kinetic friction as derived from spatially resolved Fourier analysis. These differences relate to strong adsorption of first layer to mica substrate and concomitant conformational arrest, as compared to bulk-like behavior in the second layer. The third Fourier harmonic is found to be a sensitive gauge to variable degrees of sliding as a function of both lateral drive amplitude (0.25 - 25 nm) and normal load (tensile to compressive). For a 2.5-nm drive on PVA, it is discovered that a largely static contact at compressive loads becomes a largely sliding contact at tensile loads. This finding has implications for the analysis of shear modulation force microscopy of polymers in the context of contact mechanics models, and for studies under variable sample compliance as a function of temperature or plasticizer absorption.