Surface properties of polyethylene: Effect of an amphipathic additive

Abstract

Previous studies have shown that small amounts of amphipathic (i.e., polar non-polar) molecules, in particular oleamide, cause a marked lowering of the coefficient of friction between thin films of polyethylene. In this paper the surface chemical properties of the aging film and the nascent film (i.e., during the flat film extrusion process) have been studied. The wettability (contact angle) and friction of the aging film at room temperature show that the friction is reduced only when sufficient additive is present to form a weakly held monomolecular layer. This monolayer is formed by almost complete exudation of the additive from the bulk. Contact angle measurements show that the molecules become oriented on the surface such that the polar groups are in contact with the polyethylene and the hydrocarbon chains project into the air. The stability of the monolayer to water condensation is much improved by flame treatment of the film immediately on extrusion. On preparing film by extrusion through a water-quenching bath it has been found that water adheres more easily to films containing oleamide. By suitable adjustments, the water bath was modified to form a “dynamic” Langmuir trough. Contact angle measurements on the emerging film and studies on the effect of sweeping the water surface around the emerging film show that the surface tension of the water is lowered by the amphipathic molecule being quantitatively stripped off the film as it emerges through the water/air interface. From surface pressure/area relationships, the surface concentrations of the spreading molecules are calculated. It is found that there is a surface concentration about one hundred times greater than would be expected from a uniform distribution of the additive throughout the polymer. This large surface excess is approximately proportional to the bulk concentration and implies a pronounced adsorption at the polymer melt/metal or polymer melt/air interface.