S.X. Yao, J. Lee, J.K. Lee, M.B.G. Jun, P.C. Lee
University of Vermont,
Keywords: gas, supercritical Fluid (SCF), diffusivity, solubility, optical transmission
Summary:Supercritical fluids (SCFs) and gases are widely used in polymer processing. For example, plastic foam products used for light-weight structural components, heat insulation, and food packaging can be fabricated by nano-/micro-cellular foaming processes with gases and/or SCFs. A SCF is also used as a carrier in polymer impregnation, a process of depositing a solute into polymer. Many properties such as solubility, diffusivity, rheological properties, plasticization behavior, surface tension, and crystallization of polymers can be affected by the dissolved SCFs and gases at high temperatures and pressures. Among these, solubility and diffusivity are two critical parameters which determine the final structural properties. The measurements of gas/SCF solubility in polymers can be categorized into four methods: gravimetric, piezoelectric, manometric, and chromatographic techniques. A gravimetric method measures gas/SCF solubility in polymers by monitoring sample weight changes due to gas/SCF sorption over time. A piezoelectric method uses piezoelectric crystals to investigate the gas sorption behaviors. The vibration frequency of a piezoelectric crystal depends on the weight change and can be measured accurately. A manometric method measures solubility by measuring the pressure or volume of a gas absorbed in or desorbed from a sample at an equilibrium state. The last method is a chromatographic method. However, these measurement methods have some drawbacks such as complexity and a high cost. To overcome these problems, a method of using an optical property to predict the solubility and diffusivity of a gas/SCF in polymers were investigated in this study. Amorphous polymers such as Polycarbonate (PC), Polystyrene (PS), and Cyclic Olefin Polymer (COP) with CO2 were used in this study. These films were saturated with supercritical CO2 under 5.5 MPa for 24 hours in a pressure cell and then the films were taken out from the pressure cell for optical transmission intensity and mass change measurements under room temperature and atmosphere pressure conditions. During the measurements, CO2 diffused out of the films due to the pressure drop which led to significant transmission intensity and mass changes over time. A similar CO2 desorption trend between the optical transmission intensity and mass changes was observed. The diffusivity coefficients from both types of measurements were calculated and compared.