Supercritical fluid assisted polymer processing [electronic resource] : plasticization, swelling and rheology /

by Royer, Joseph Robert

ROYER, Joseph Robert. Supercritical Fluid Assisted Polymer Processing: Plasticization, Swelling and Rheology. (Under the direction of Drs. Saad A. Khan and Joseph M. DeSimone.) The use of supercritical carbon dioxide, scCO2, as a transient plasticizer eliminates the disadvantages associated with many other industrial plasticizers. Because CO2 is a gas under atmospheric conditions, it can be used as a processing aid and then easily removed from a polymer through evaporation to obtain the original physical properties of the unplasticized polymer matrix. In addition, CO2 has been shown to be more environmentally friendly in comparison to many of the traditional organic plasticizers. However, the biggest challenge hindering the widespread use of CO2 as a plasticizer involves a lack of understanding of and data quantifying its effect on polymer swelling and the concomitant reduction in material viscosity. In this work, a three-step approach is used to investigate and quantify the physical phenomena associated with CO2-induced plasticization of polymer melts. First, a novel experimental apparatus was designed and constructed to measure equilibrium swelling, swelling kinetics and diffusion of CO2 into a polymer melt. It was found that diffusion of CO2 into PDMS exhibited Fickian behavior up until two-thirds of the equilibrium swelling value was obtained. The CO2 pressure had a negligible effect on the diffusion coefficient; however, the system temperature directly affected the diffusion coefficient. Increased pressure was found to enhance the extent of swelling whereas a maximum was observed with increasing temperature, at pressures above 15 MPa. The Sanchez-Lacombe equation of state was found to be in good agreement with the experimentally calculated variables, and thus, can be used as a predictive tool to obtain physical properties of the CO2-PDMS system. Secondly, a high pressure extrusion slit die rheometer was constructed to measure the viscosity of polymer melts plasticized with low concentrations of CO2. Polystyrene, poly(methyl methacrylate), polypropylene, low density polyethylene, and poly(vinylidene fluoride) were all investigated. CO2 was found to be an efficient plasticizer for all of these polymer materials, generally lowering the viscosity of the melt 30-80%, depending on processing conditions. Predictive viscoelastic scaling models based on free-volume principles and a prediction of Tg depression from a diluent were developed to quantify the effects of CO2 concentration, pressure and temperature on viscosity. This unique free-volume approach allows the high pressure polymer/CO2 rheology to be predicted based solely on physical parameters of the polymer melt and CO2. Therefore, only rheological measurements at ambient pressures are required to predict the high pressure polymer/CO2 solution behavior over the concentration and temperature ranges for which the models are valid. Finally, a novel high pressure magnetically levitated sphere rheometer (MLSR) was developed to further investigate the effects of CO2 on the viscosity of polymer melts. The MLSR measures the difference in magnetic intensity required to levitate a magnetic sphere in a sample fluid while the fluid is at rest and under shear. The observed change in magnetic intensity is directly proportional to the viscoelastic force imposed on the sphere by the surrounding fluid, and thus is used to calculate the fluid viscosity from a calibration of known viscosity standards. The rheometer eliminates many of the disadvantages associated with other high pressure rheometers and can operate over a wide range of CO2 concentrations at constant pressure with excellent reproducibility. This rheometer was used to measure the viscosity reduction of poly(dimethyl siloxane) by CO2. The effects of both system pressure and CO2 were investigated. The viscosity of the polymer melt could be lowered in excess of 97% of its original value at atmospheric pressure by adding a CO2 concentration of approximately 30 wt%. Additionally, experimental evidence revealed that the elevated pressure significantly increased the polymer/CO2 viscosity.