Structure and interaction of polymer thin films with supercritical carbon dioxide
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An understanding of colloid stability in CO2 as well as the interaction of CO2 with polymer thin films is necessary for the intelligent design of CO2-based processes for future materials applications. In-situ spectroscopic ellipsometry (SE) was used to measure the thickness and optical properties of nanoscale poly(dimethylsiloxane) (PDMS) and poly(methyl methacrylate) films exposed to compressed CO2. Both the sorption and CO2-induced dilation of the thin films were measured simultaneously with SE and deviations between the thin films and the corresponding bulk films may be attributed to excess CO2 at the free interface as well as the influence of film confinement and the compressible nature of CO2 on the orientation and mobility of the polymers. SE was also used to measure sorption equilibrium and kinetics and CO2-induced dilation of polyimide (6FDADAM:DABA 2:1) thin films to determine how a gas separation membrane’s structure affects its susceptibility to CO2-induced plasticization. Both thermal annealing and chemical crosslinking reduced the polymer dilation to prevent large increases in the CO2 diffusion coefficient at high CO2 pressures. The CO2 permeability and polymer free volume strongly depend on the annealing temperature, and different effects are observed for the crosslinked and uncrosslinked membranes and for the thick and thin membranes. Neutron reflectivity (NR) and SE were used to characterize the structure of end-grafted dPDMS brushes on SiOx wafers exposed to compressed CO2. NR revealed two distinct regions in the segment density profile as a function of distance from the surface. The thickness and volume fraction profiles for the brush change much more with solvent quality than has been seen in previous studies with incompressible solvents, due to the high asymmetry in the intermolecular interactions, as well as the large compressibility and free volume differences between the polymer segments and the solvent. Turbidity versus time measurements were used to determine the critical flocculation densities (CFDs) of silica collids in CO2 that were sterically stabilized with end-grafted poly(1H, 1Hdihydroperfluorooctyl methacrylate) (PFOMA). The CFDs occurred above the UCSD for the corresponding finite molecular weight stabilizer in bulk CO2 and corresponded more closely with the Θ-density. The CFDs decreased (greater stability) when temperature was increased or the PFOMA molecular weight was decreased.