Structure and interaction of polymer thin films with supercritical carbon dioxide.

摘要

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 CO ₂ 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 (6FDA-DAM: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 d-PDMS brushes on SiO_x 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 colloids in CO₂ that were sterically stabilized with end-grafted poly(1H, 1H-dihydroperfluorooctyl 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.