Tip-sample interactions in dynamic force microscopy of polyvinyl alcohol films

Dynamic force microscopy (DFM) was performed in air on ultrathin polyvinyl alcohol (PVA) films. Nominal non-contact and intermittent solid contact regimes are identified in measurements of amplitude and phase as a function of the mean distance from the sample. Evidence suggests that the nominal non-contact regime is divided into two subregimes: true non-contact, in which only long-range attractive forces exist, and intermittent fluid contact, involving brief penetration of the tip into a fluidized zone at the tip-polymer interface. Viscous fluid damping differs among three mesoscopic film components, producing variable 'resistance' to intermittent solid contact. As a result, some components of the film are imaged in intermittent fluid contact and others in intermittent solid contact, within a single image. Approach-withdrawal amplitude hysteresis correlates with amplitude damping, and is consistent with component-specific surface fluidity (meniscus formation). Differences in surface fluidity are interpreted in terms of relative crystallinity.