Scratch Resistance of New Polystyrene Nanocomposites with Ionic Liquid-Modified Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes, both neat (MWCNT) and modified (MWCNTm) by the room-temperature ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM]BF4), were added in a 1 wt% to polystyrene (PS) to obtain the new nanocomposites (PS + MWCNT and PS + MWCNTm). Friction coefficients and abrasive wear from instantaneous penetration depth, residual depth and viscoelastic recovery were determined for compression-moulded materials as a function of applied normal load and of the number of successive scratches. The new nanocomposites improve the abrasion resistance of neat PS and of the analogous PS nanocomposites containing neat and IL-modified single-walled carbon nanotubes. The lowest friction coefficient and residual depth values, after 15 scratches, under the whole range of applied loads are obtained for PS + MWCNTm, with maximum reduction under the most severe conditions. The influence of sliding direction with respect to flow was studied for injection-moulded PS + MWCNTm under multiple scratching. The most severe surface damage is observed in the transverse direction to injection flow, while the lowest friction coefficient and the highest abrasion resistance and viscoelastic recovery values are obtained in the direction parallel to injection flow, due to the higher mobility of the polymer chains and the additives. Thermal analysis (DSC and TGA), Raman spectroscopy, transmission electron microscopy, X-ray diffraction and XPS surface analysis have been used as characterization techniques. XPS shows that the IL molecules are present on the nanotube surface. According to TGA, the IL content in MWCNTm can be estimated to be of a 12 wt%. Mechanisms of surface damage are discussed upon scanning electron microscopy, 3-D surface topography, surface roughness and profilometry observations.