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 bulklike 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.
Bibliographical noteFunding Information:
Support from the Characterization Facility (Institute of Technology, University of Minnesota), the Center for Interfacial Engineering (a National Science Foundation Engineering Research Center), and Molecular Imaging are gratefully acknowledged.
- Friction force Microscopy
- Polyvinyl alcohol
- Shear modulation