SORPTION, DIFFUSION AND VAPOR PERMEATION OF VARIOUS PENETRANTS THROUGH DENSE POLY(DIMETHYLSILOXANE) MEMBRANES - A TRANSPORT ANALYSIS

A detailed analysis of the transport of chloroform, isomeric butanols, methanol and water vapors at 40-degrees-C through dense silicone rubb er membranes has been undertaken by the vapor permeation technique. Th e variation of fluxes versus upstream activity, coupled to sorption is otherms of the investigated compounds in silicone rubber at 40-degrees -C (determined by the swelling technique), offer the possibility to de termine the sorption and diffusion steps in the overall permeant flux. It is shown that the chloroform isotherm follows the Flory-Huggins th eory, while alcohols require a more sophisticated approach (Koningsvel d and Kleinjtens type) in order to fit the isotherms. The water sorpti on isotherm shows a typical sigmoid shape which could not be fitted co rrectly by classical sorption equations. The diffusion coefficient of chloroform in PDMS appears to be almost unaffected by solvent concentr ation in the membrane, similarly to the results obtained with other go od solvents in PDMS. Diffusion coefficients of alcohols and water show , however, a common tendency to decrease with their local concentratio n. An interpretation of the sorption isotherms based on the cluster in tegral of Zimm and Lundberg suggests some clustering behavior, which s eems consistent with the diffusion coefficient variation. The clusteri ng tendency of the solvent molecules in the PDMS matrix was related to their proton donating power, which follows the sequence: water> metha nol > 1-butanol > 2-butanol > 2-methyl-2-propanol > chloroform.