Experimental Study and Modeling of Boundary Lubricant Polyelectrolyte Films

International audienceStrongly adsorbed polyelectrolytes are central in the conditioning properties of many personal care products. Whereas the mechanism of polymer adsorption is rather well understood, less is known about the actual mechanism of polyelectrolyte lubrication. We investigated the adsorption on mica and the lubricant properties of a strong polyelectrolyte, poly- (diallyldimethylammonium chloride), polyDADMAC, in aqueous solutions of different salt concentrations. We found that the adsorption of the polymer was enhanced and the morphology of the adsorbed layer modified by increasing salt concentration. The lubricant properties of the adsorbed polyelectrolyte layer were good at low compressions but rapidly deteriorated at larger applied pressure. A complex velocity dependence of the friction was observed, with a maximum value at intermediate velocities and hysteresis in an acceleration/ deceleration cycle. A progressive increase in separation between the rubbing surfaces with velocity was also observed. We developed a model that describes the complex behavior of friction observed, taking into account the hysteretic elastic deformation of the polymer layer under shear and the dilatency due to the elastohydrodynamic effects resulting from its low elastic modulus. Our results can help to explain the significant lasting of the lubricating properties when polyDADMAC is used in conditioners, where polymer-lubricated hair-hair contacts exhibit low shear forces