The Polymer Processing Society 23rd Annual Meeting EFFECT OF MOLECULAR WEIGHT ON THE SURFACE TENSION OF POLYSTYRENE IN SUPERCRITICAL NITROGEN H. Park 1* , C.B. Park 2 , C. Tzoganakis 3 , and P. Chen 4 1* Departments of Chemical Engineering University of Waterloo, 200 University Avenue, Waterloo, Ontario, Canada N2L 3G1, - hspark@uwaterloo.ca; 2 University of Toronto, - park@me.uwaterloo.ca; 3 University of Waterloo, - ctzogan@cape.uwaterloo.ca; 4 University of Waterloo, - p4chen@cape.uwaterloo.ca. Abstract - The surface tension of polymers in a supercritical fluid is one of the most important physicochemical properties in many polymer processes such as foaming, coating, sintering and blending of polymers. This paper presents experimental results on the effect of molecular weight on the surface tension of polystyrene melt in supercritical nitrogen. The surface tension was determined by the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) method, for which a high-pressure and high-temperature cell was used to form pendant drops of the polystyrene melt. A linear relationship was found between surface tension and temperature, and between surface tension and pressure. Monodispersed polystyrene of a higher molecular weight has a higher surface tension. The surface tension dependence on temperature and on pressure is more significant for the higher molecular weight polystyrene. For a polydispersed polystyrene, high surface tension values are determined predominantly by its high molecular weight portion of polystyrene molecules. The empirical Macleod equation was also applied to relate surface tension and density. Introduction Surface tension is one of the most important physicochemical properties for polymeric materials in many engineering processes, such as foaming, suspension, wetting and blending [1]. However, experimental determination of the surface tension of a high viscous polymer has been difficult at high temperatures and pressures during the experiment [2]. The effect of molecular weight on polymer properties and processing has been well documented in the literature. Limited studies have been reported on the effect of polymer molecular weight on its surface tension [3-6]; the results were restricted to the polymers of low molecular weight and surface/interfacial surface tensions between two polymer melts. There has been no report on the molecular weight effect on the surface tension of polymers of high molecular weight in a supercritical fluid. Supercritical fluids, such as carbon dioxide and nitrogen, have widely been used as foaming agents in the production of microcellular polymer foams [7, 8]. Although the amount of a supercritical fluid dissolved in the polymer is small, it can result in dramatic changes in physicochemical properties such as glass transition temperature, viscosity, solubility and surface tension [9]. The molecular weight of polymers is an important factor in determining the surface tension of polymers in fluid phases, which in turn significantly affects the foaming process and eventual morphology of final polymer products. There are many methods to measure surface tension. Among them, the pendant drop method has many advantages because of its simple setup and versatile applications [10, 11, 12]. The pendant drop method has been used extensively for low molar mass liquids, liquid crystals and polymers [13]. This method relies on the determination of a drop profile of dense liquid in another fluid, and the surface tension of the liquid is obtained from the best fit of the Laplace equation of capillarity to the experimentally determined drop profile [14, 15]. Although the pendant drop method is theoretically simple, research on the surface tension of polymers in a supercritical fluid has been limited because of experimental difficulties in handling high viscosity polymer melts under high temperature and high pressure [16, 17]. In fact, the surface tension data are available for only a few select polymers under narrow range of experimental conditions. The primary objective of this study is to investigate the effect of the molecular weight on the surface tension of polystyrene melt in supercritical nitrogen. The effects of temperature and pressure on the surface tension are studied in monodispersed polystyrenes of two different molecular weights and a polydispersed polystyrene. A recently designed high-temperature and high-pressure sample cell is employed in the surface tension measurement to obtain a wide range of experimental conditions. Using the set of surface tension data obtained, an empirical equation approximating the surface tension of polystyrene in supercritical nitrogen as a function of temperature and pressure is developed. Experimental Materials