Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces

The nanotribological properties of hydrophilic polymer brushes are conveniently analyzed by lateral force microscopy (LFM). However, the measurement of friction for highly swollen and relatively thick polymer brushes can be strongly affected by the tendency of the compliant brush to be laterally deformed by the shearing probe. This phenomenon induces a "tilting" in the recorded friction loops, which is generated by the lateral bending and stretching of the grafts. In this study we highlight how the brush lateral deformation mainly affects the friction measurements of swollen PNIPAM brushes (below LCST) when relatively short scanning distances are applied. Under these conditions, the energy dissipation recorded by LFM is almost uniquely determined by stretching and bending of the compliant brush back and forth along the scanning direction, and it is not correlated to dynamic friction between two sliding surfaces. In contrast, when the scanning distance applied during LFM is relevantly longer than the brush lateral deformation, sliding of the probe on the brush interface becomes dominant, and a correct measurement of dynamic friction can be accomplished. By increasing the temperature above the LCST, the PNIPAM brushes undergo dehydration and assume a collapsed morphology, thereby hindering their lateral deformation by scanning probe. Hence, at 40 °C in water the recorded friction loops do not show any tilting and LFM accurately describes the dynamic friction between the probe and the polymer surface.