Friction and Surface Dynamics of Polymers on the Nanoscale by AFM

Abstract

In this article the measurement and understanding of friction forces and surface dynamics ofpolymers on the one hand and the importance of molecular relaxation processes and viscoelasticityin polymers for advanced micro- and nanoscale applications on the other hand are discussed. Particularattention is paid to the nanoscale (surface) analysis by scanned probe microscopic approaches, includingatomic force microscopy (AFM), as a means to assess molecular relaxation processes that operateat a given temperature. Established AFM approaches, including lateral force and force modulationmicroscopy, are introduced and more recently developed techniques, such as torsional resonant modes,are briefly sketched. On the basis of the discussion of the techniques to measure friction and toprobe surface dynamics of polymers on the nanoscale, illustrative examples are reviewed. The examplesdiscussed address in particular the determination of values of the glass transition temperature (T g) and the difference of T gassessed in the bulk vs. at the free surface of polymers. Confinement and thin film effects on T g, but also on sub-T gtransitions and chain dynamics, are treated in detail. Finally, the mapping of multiphase systemsand anisotropic friction receive attention.

A wrinkling-based method for investigating glassy polymer film relaxation as a function of film thickness and temperature

“…The plateau value ( E a ≈ 350 kJ mol −1 ) has a magnitude comparable to previously reported bulk values of the segmental relaxation time of PS in the glassy state. 43,44 In contrast, for films having h f < 100 nm, both the entropy and enthalpy of activation decrease as the films becomes thinner, making τ appreciably smaller than its bulk value. Our observations are remarkably consistent with nano-hole filling experiments in PS films, 6 where a similar reduction of E a is observed in the glassy state.…”

Collapse from the top: brushes of poly(N-isopropylacrylamide) in co-nonsolvent mixtures

“…However, measurements by AFM based friction force microscopy (FFM) provide quantitative tribological information concerning polymer gra properties, accompanied by surface morphology imaging. [41][42][43]51,52 For instance, greatly enhanced lubrication for poly(2-(dimethylamino)ethyl methacrylate) was observed at low pH, which was attributed to the formation of a repulsive, highly charged, hydrated cushion. 53 In a recent study, 54 the frictional properties of high density PMPC brushes under co-nonsolvency conditions were described and substantially increased friction coefficients were observed in ethanol/water (90% v/v) mixtures.…”

Thermo-mechanical properties of the cell surface assessed by atomic force microscopy

“…Friction force measurements are performed via scanning the tip back and forth over certain distances or areas. On a horizontal flat surface, the "frictional force" is determined by taking half the difference between lateral forces acquired in the forward and backward directions of scanning (Ogletree et al, 1996;Schónherr et al, 2008). The average between lateral forces obtained while scanning back and forth provide information on surface topography.…”