Authors: G. Haugstad
Affilation: University of Minnesota, United States
Pages: 675 - 678
Keywords: scanning probe, polymer, nanotribology
Oscillatory methods in scanning probe microscopy have enabled nanoscale tribology and rheology investigations on polymeric systems. Among these methods, shear modulation force microscopy has been used to probe phenomena such as the stick-to-slide transition, time-delayed viscoelastic response, and near-surface glass transition, all at the nanoscale. Given that fundamental polymeric responses often are being studied, it is imperative to understand shear modulation dynamics at a fundamental level. To this end we examine nonlinear dynamic behavior under nanoshear modulation via Fourier analysis of the real-time response. The methodology exploits the third harmonic of response as a gauge of nonlinearity. One important finding is that at typical shear modulation drive amplitudes of a few nanometers, the response can be highly nonlinear depending on the conformational mobility of the polymer as well as its thermo-kinetic state (e.g., glassy or rubbery). Increases in compliance (with temperature or plasticization) or increases of load can change the nature of contact from sliding to static. We exploit this transition to elucidate the origins of contrast in friction force microscopy of nanostructured, autophobically dewetted polyvinyl alcohol films that exhibit variable aspects of confinement.