Effect of Carbon Dioxide on the Interfacial Tension of Polymer Melts

Supercritical CO 2 is a promising solvent for application in polymer blending and foaming. Interfacial tension is a key parameter in determining the bubble nucleation and growth rates, as well as droplet breakup in blending. However, very limited data are available in the literature for CO 2 −polymer systems. In this work, pendant drops were imaged in a high-pressure, high-temperature view cell for determining the interfacial tension between polystyrene (PS) and CO 2 (210−230 °C and pressure up to 140 atm), and between polystyrene and two different molecular weight polypropylenes (PP) (both saturated with CO 2 at 220 °C and pressures up to 165 atm). The interfacial tensions in polymer/CO 2 and polymer/polymer systems decrease significantly with increasing CO 2 pressure and then level off at higher CO 2 pressures. Density gradient theory combined with an equation of state was applied in predicting the interfacial tension between PS and CO 2 starting from only the surface tension data of pure PS at the temperature of interest. The theory captures well the magnitude of the depression at high pressure, however, it severely underpredicts the interfacial tension in the low pressure region below 60 atm.