Marjorie Schmitt

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

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.

Adhesion and Friction Properties of Polymers at Nanoscale: Investigation by AFM

Friction and adhesion of model elastomers are quantified at two different scales: at nanoscopic scale, using atomic force microscopy (AFM) and at macroscopic scale, using a tack test and a translation tribometer. The objective is firstly to find a correlation between nanoscale adhesion and friction, by comparing the influence of structural parameters such as crosslinking degree and presence of free chains. The scope is also to verify if friction and adhesion behaviors are comparable at both scales. Experimental results underline the major role of molecular parameters such as degree of crosslinking and length und content of free and pendant chains. However, their effect on nano and macroscale properties is different. Conceptual schemes are proposed to describe the complex interfacial molecular mechanisms, especially chain adsorption onto the tip which seems to govern nanoscale friction and adhesion. Further friction studies, performed on polymers in contact with hydrophilic and hydrophobic tip and substrate allow us to underline the complex competition between interfacial interactions and polymer surface rheological behavior.

Structuring the Surface of Crystallizable Polymers with an AFM Tip

In this chapter, we demonstrate that atomic force microscopy (AFM) is a well-suited technique for manipulating polymers, in particular for inducing crystallization and for controlling crystal melting. As examples, we present results on the crystallization and melting behavior of highly asymmetric semi-crystalline block copolymers where the crystallizable block is confined in nanometer size mesophase domains and on lowering the crystal nucleation barrier by deforming polymer chain conformations via friction (rubbing) with an AFM tip. In the here discussed studies, the main focus is on questions concerning primary nucleation, morphological reorganization, crystal growth under confinement, non-equilibrium annealing processes, etc., which still remain unsolved despite numerous theoretical and experimental works. As will become clear from these examples, AFM offers unique possibilities for manipulations of polymers at a molecular scale, which affect their behavior on macroscopic scales up to sizes visible with the naked eye.

Nano-Wear of Polystyrene in Contact With Hydrophilic and Hydrophobic Substrates: Influence of Interfacial Interactions

The aim this study is to analyse the influence of interfacial interactions in the nano-wear behaviour of polymers. Studies will be focussed on the analysis of the transfer layer induced by the friction of a polystyrene cylinder in contact with a flat and smooth substrate. In order to change the nature of interfacial interactions, two different substrates are used. The first one is a hydrophilic silicon wafer (hydroxylated by a piranha treatment) and the second one is a hydrophobic silicon wafer, obtained by a chemical grafting with a CH3 terminated silane. Friction experiments are performed with a translation tribometer which measures the tangential force between the polymer and the substrate for controlled normal force and friction rate. The transfer layer is analysed using atomic force microscopy (AFM) and IR spectroscopy (reflection mode) Tentative correlations between transfer layer characteristics and interfacial properties are proposed.Copyright