Confinement effects and the activation of molecular motion in polymer thin films at a moving nanoprobe

The application of cantilever-based, nanoprobe read/write devices for polymeric media, as well as thin-film polymer lubricants in confined geometries, will benefit from further understandings of the fundamentals of probe-polymer interactions. These interactions were studied on several polymeric systems and with several modes of scanning probe microscopy. Findings on polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), and polystyrene (PS) films were elucidated. On PDMS films, with bulk viscosities spanning several orders of magnitude, there was a solid-like to liquid-like transition at film thicknesses of approximately three times the radius of gyration. On PMMA, the dissipative molecular motions excited near the surface, manifested as a friction force, had activation energies on the order of beta processes, and further implied surface-enhanced mobility. In contrast, segmental and whole-chain motions in PS near the glass transition during scan-induced patterning had activation energies on the order of alpha processes, but implied no enhancement of mobility. Shear-modulation Fourier analysis on PS at room temperature further revealed a molecular weight dependence in the linearity of nanoprobe-polymer interactions, including transitional behavior near the critical entanglement molecular weight. This is an abstract of a paper presented at the 227th ACS National Meeting (Anaheim, CA 3/28/2004-4/1/2004).