The effect of interfacial tension in carbon dioxide assisted polymer processing

Supercritical fluid technology has made tremendous strides in the past decade in terms of commercial application and fundamental understanding of solution behavior. The addition of small amounts of compressed gases to polymer phases results in substantial and sometimes dramatic changes in the physical properties that dictate processing. These include viscosity, permeability, interfacial tension, and glass transition temperature. By understanding the effects of CO2 on these properties and developing techniques for incorporating CO2 in continuous processes, a wide range of opportunities open up for impacting the plastics industry. The products range from foam board insulation and high impact polymer blends to surface-modified biomedical implants and biological micro-electromechanical system (bio-MEMs) devices.; Supercritical CO2 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 break up in blending. However very limited data on this property is available in the literature for CO2-polymer systems.; A novel technique is presented to determine the interfacial tension for the polymer melts and high pressure CO2 systems by analysis on the axisymmetric pendant drop shape profile, which can simultaneously yield the density, swelling and interfacial tension results. The method avoids the "capillary effect" and the "necking effect" and provides good axisymmetry of the pendant drop, which makes it a suitable method for measuring the interfacial tension for polymer melts under high pressure CO 2 conditions.; In this work, the interfacial tension between polymer melt (PS, PP, PLGA, PMMA) and high pressure CO2, and the interfacial tension between polymer melt pairs (PS/PP) saturated with high pressure CO2 were studied using the pendant drop method in a high pressure, temperature view cell. The effects of CO2 pressure, temperature and molecular weight on interfacial tension were studied. The interfacial tension between polymer melt and CO2 was significantly depressed and decreases almost linearly as CO2 pressure increases in the pressure range up to 100 atm. The interfacial tension between polymer pairs saturated with CO2 was studied in the CO2 pressure range up to 100 atm, and was found to decrease significantly with increasing CO2 pressure and levels off at higher CO2 pressures.; The linear gradient theory combining with the Sanchez-Lacombe Equation of State was applied in predicting the surface tension or interfacial tensions for polymer melts under high pressure CO2 conditions, which correctly predicts the depression of interfacial tension by high pressure CO2 and yields reasonable agreement with experimental data. The temperature effect on the interfacial tension of polymer melts was also correctly predicted using this model.; The role of CO2 in enhancing the polymer blending process was carried out by combining the interfacial tension depression data with the viscosity reduction data. The capillary number, the most important parameter governing the drop breakage and coalescence in the blending process and thus the morphology of the blends, was used in the analysis. A highly simplified population balance model was applied to calculate the morphology evolution by only considering the droplet breakup during the mixing. The calculated results agree with the experimental data relatively well. Based on the model, the effect of CO2 on the morphology evolution was also discussed.