Supercritical CO2 Aided Processing of Thin Polymer Films Studied Using the Quartz Crystal Microbalance

Abstract

Fundamental and applied aspects of the interactions between carbon dioxide (CO₂) and different polymer systems were investigated to demonstrate the effect and performance of CO₂ during polymer processing. From a fundamental perspective, the sorption of CO₂ into a non-soluble polymer and its dependence on the different system variables were examined. Another fundamental study investigated the dissolution of a fluorinated polymer in CO₂ at different conditions. Finally, the application of supercritical CO₂ for the impregnation of additives into two different polymers was evaluated. In all these studies, the quartz crystal microbalance (QCM) was used as the primary analytical technique.

In the first part of this work, the sorption of CO₂ into poly(methyl methacrylate), PMMA, was investigated. The effect of several parameters, including pressure, temperature, film thickness, and polymer state, on the equilibrium and kinetics of the sorption process was studied. The uptake isotherms of CO₂ into PMMA were estimated from the QCM frequency change. This uptake was found to decrease with temperature and to depend on the film thickness. The presence of hysteresis in the sorption-desorpotion isotherms clearly marked the glass transition which was found to be in good agreement with previously reported values. This glass transition also affected the sorption kinetic. In the glassy state, two-stage sorption curves were observed, whereas in the rubbery stage, Fickian diffusion was evident. The results from this study were utilized to examine the reliability of Sauerbrey equation for mass calculation. By measuring the change in QCM resistance, it was found that both the thickness and the amount of CO₂ dissolved in the polymer can affect the QCM response. However, it was demonstrated that Sauerbrey equation was still applicable for films up to ˜1 μm thick.

In the next part, the dissolution of poly(dihydroperfluorooctyl methacrylate-r-tetrahydropyranyl methacrylate); PFOMA, a copolymer was studied. The dissolution process consisted of two stages: CO₂ sorption and polymer dissolution. The measured frequency was utilized to determine mass changes for both processes. In the sorption stage, the solubility of CO₂ into PFOMA was measured at different temperatures and pressures. The solubility was found to depend on both the CO₂ density and the temperature. Polymer dissolution started at pressures between 1100 and 1600 psi, depending on the temperature. The dissolution rate was found to increase as the CO₂ density increases, but has a possible dependence on the temperature. Finally, the fraction of undissolved polymer after 1 hour of CO₂ exposure was estimated. This fraction increased linearly from 20 to more than 90% with CO₂ density.

The last part in this work examined the impregnation of ibuprofen (IBU) into two biocompatible polymers: PMMA and poly(vinyl pyrrolidone), PVP. For PMMA, the amount of impregnated IBU decreased as the CO₂ density increased. The solubility parameter approach provided a possible explanation for this behavior based on the interactions among PMMA, IBU, and CO₂. High partitioning coefficients of IBU between PMMA and CO₂ were estimated, indicating a thermodynamically driven impregnation mechanism. A linear increase in the IBU uptake with the initial polymer mass was observed. This behavior could indicate uniform distribution of IBU in the polymer sample. The impregnation rate was found to have a strong dependence on the temperature. Pressure, on the other hand, did not seem to have significant effect. For the impregnation of IBU into PVP, the frequency response was significantly larger than the PMMA case. This unusual behavior can indicate that the PVP films physical properties (e.g., viscoelastic nature of the film or in the film-substrate adhesion) are affected by IBU which might add a non-gravimetric contribution to the frequency change.