| The objective of this research is to understand the relationship between polymers in bulk solution and at a surface. In particular, we are interested in studying, in situ, the adsorption of poly(dimethylsiloxane) (PDMS) from supercritical carbon dioxide (SC-CO2) onto a gold (Au) surface using Surface Plasmon Resonance (SPR) spectroscopy. By using a compressible supercritical fluid as the solvent, pressure variation becomes an important and simple way to "tune" the solvent quality and therefore vary the bulk free energy from good to nonsolvent behavior. These adsorption studies have potential for playing a role in some interesting industrial applications such as the prevention of reactor fouling and the coating of inner surfaces of porous media.; Cloud point curves for liquid-liquid (L-L) phase separation for the PDMS/SC-CO 2 system were measured to provide a phase diagram over which to study the adsorption process. The cloud point curves for various PDMS samples in SC-CO2 were obtained as a function of pressure, temperature, and molecular weight using a variable-volume high pressure cell of our design. The system was observed to be thermodynamically stable in the one-phase region. As predicted by theory, the coexistence curves shifted to higher pressures as the difference in the molecular size of the two components (PDMS and SC-CO 2) increased. In addition, there were no strong concentration effects over the range studied (0.1--3wt% PDMS/SC-CO2).; The adsorption behavior of PDMS from SC-CO2 was determined using the same high pressure cell with the SPR technique. The SPR method provided information about the optical and physical properties of the Au film, the adsorbed PDMS layer, and the bulk solution. This information was obtained by measuring the reflectivity as a function of incident angle for a glass/Au/Air, a glass/Au/SC-CO2, and a glass/Au/PDMS/PDMS-SC-CO2 solution system. The experimental values were then fitted using multi-layer methods and non-linear least squares fitting. From the experiments performed, it was noted that the thickness of the adsorbed PDMS layer could, in fact, be "tuned" by varying pressure. The results obtained demonstrate the adsorption of PDMS from SC-CO2 to a Au surface and indicate an increase in adsorption as phase separation is approached as well as complete wetting at the coexistence curve and within the two-phase region. |
