Nano-friction of polystyrene : the role of the surface chemistry

Abstract

The purpose of this study is to analyse the influence of the chemical contribution on the nano-friction of polystyrene. The role of interfacial interactions is analysed by comparing friction between the polymer and hydrophobic AFM tip (methyl-terminated grafted layer) and hydrophilic tip (hydroxyl-terminated tip) as a function of sliding velocity and normal force. Nano-friction experiments are achieved by using atomic force microscopy (AFM) in torsion mode. Amorphous atactic polystyrene films are prepared by spin coating a solution onto a smooth and rigid substrate (silicon wafer). Two amorphous atactic polystyrene, varying by their different molecular weights are used. Experimental results show that the friction coefficients measured with hydroxylated tip are always larger than those obtained with non-treated tip (intermediate value) and hydrophobic tip (lower friction), indicating a relationship between nano-friction and interfacial interactions. The dependence of frictional force on velocity is consequent, with a great increase of friction with speed in the case of the hydroxylated tip. A higher friction is obtained for the higher molecular weight polystyrene in contact with hydroxylated tips. However, differences between both polymers become negligible in the case of non-treated and hydrophobic tips.