Ingo Pinnau

Transport of organic vapors through poly (1-trimethylsilyl-1-propyne)

Poly(1-trimethylsilyl-1-propyne) [PTMSP], a high-free-volume glassy polymer, has the highest gas permeability of any known synthetic polymer. In contrast to conventional, low-free-volume, glassy polymers, PTMSP is more permeable to large, condensable organic vapors than to permanent gases. The organic-vapor/permanent-gas selectivity of PTMSP based on pure gas measurements is low. In organic-vapor/permanent-gas mixtures, however, the selectivity of PTMSP is much higher because the permeability of the permanent gas is reduced dramatically by the presence of the organic vapor. For example, in n-butane/methane mixtures, as little as 2 mol% n-butane (relative n-butane pressure 0.16) lowers the methane permeability 10-fold from the pure methane permeability. The result is that PTMSP shows a mixed-gas n-butane/methane selectivity of 30. This selectivity is the highest ever observed for this mixture and is completely unexpected for a glassy polymer. In addition, the gas mixture n-butane permeability of PTMSP is considerably higher than that of any known polymer, including polydimethylsiloxane, the most vapor-permeable rubber known. PTMSP also shows high mixed-gas selectivities and vapor permeabilities for the separation of chlorofluorocarbons from nitrogen. The unusual vapor permeation properties of PTMSP result from its very high free volume - more than 20% of the total volume of the material. The free volume elements appear to be connected, forming the equivalent of a finely microporous material. The large amount of condensable organic vapor sorbed into this finely porous structure causes partial blocking of the small free-volume elements, reducing the permeabilities of the noncondensable permanent gases from their pure gas values.

Surface fluorination of polysulfone asymmetric membranes and films

Abstract Asymmetric polysulfone membranes were surface fluorinated using a fixed set of treatment conditions and the transport of six gases was measured. After 2 and 5 min of fluorination the membranes show a significantly reduced ratio of permeability and effective skin thickness ( P/l ) and an improved selectivity for certain gas pairs like hydrogen, helium, and carbon dioxide relative to methane. There is some variability in the H 2 /CH 4 selectivity improvement observed but, in general, it increases 2–3 fold for membranes fluorinated for 5 min and 2–5 fold for those fluorinated for 2 min. In order to gain insight about the fraction of the asymmetric membrane thickness that is being fluorinated and to determine the chemical nature of the fluorinated layer, thick (76 μm) isotropic films of polysulfone were fluorinated from one side only for a wide range of times. The chemical composition and structure of the fluorinated layer were determined from bulk elemental analysis and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR). X-ray photoelectron spectroscopy (XPS) was also used to determine the chemical composition at the surface of asymmetric membranes and the thick films. The fluorinated surface of selected membranes was examined by scanning electron microscopy.

Influence of quench medium on the structures and gas permeation properties of polysulfone membranes made by wet and dry/wet phase inversion

Abstract Essentially defect-free integrally-skinned asymmetric polysulfone membranes were fabricated by a dry/wet phase inversion process using forced-convective evaporation. The choice of the quench medium is of utmost importance for the formation of gas separation membranes having high selectivities combined with high gas fluxes. The integrally-skinned asymmetric membranes were characterized by gas permeation measurements and field emission scanning electron microscopy. This study indicates that the phase separation and vitrification processes occurring during the wet phase inversion step should be as rapid as possible to generate the highest performance gas separation membranes.

Gas transport through homogeneous and asymmetric polyestercarbonate membranes

Abstract Permeabilities, solubilities and diffusivities of N2, O2, CO2 and H2 are reported for a polyestercarbonate at 35°C and 4.4 atm. Constant activation energies of permeation were determined for dense films and defect-free asymmetric polyestercarbonate membranes over a temperature range of 25–55°C. Activation energies of permeation were 10 and 20% higher for N2 and O2, respectively, in the asymmetric membrane samples as compared to the dense films. Permselectivities for the gas pairs O 2 N 2 , H 2 N 2 and CO 2 N 2 were found to be higher in the asymmetric membranes compared to those of the dense films. The detailed cause of the higher permselectivities and activation energies in the asymmetric membranes is currently not known. Possible orientation-induced increases in segmental packing density in the asymmetric membrane skin is discussed as a potential explanation for the differences between the asymmetric and dense films.