Pressure Dependence of Surface Tension of Polymer Melts under High Vacuum

The surface tension of polymers under ambient or high-pressure conditions has been extensively studied; however, it remains much less explored at high vacuum. Previous studies showed that the surface tension of most polymers decreases linearly when the temperature or pressure is increased. Herein, the effect of air pressure (P) on the surface tension (γ) of polymer melts under high vacuum is studied using a home-built apparatus. Under high vacuum conditions, the surface tension of polymers is found to exhibit an anomalous behavior: while γ remains a decreasing function of temperature, it increases as the air pressure is increased. The measured surface tension of several polymer samples is well described by the Hill equation, γ = γ_₀ + α · [(P/P_₀)ᴴ / (1 + (P/P_₀)^ᴴ)] with H = 0.2 in the pressure range of 10^-4 to 10⁵ N/m². The implication of this behavior on the air-polymer interaction is discussed within the context of air adsorption.