Poly(dimethylsiloxane) crosslinked in different conditions. Part II. Pervaporation of water–ethyl acetate mixtures

Abstract

Dense membranes made of PDMS crosslinked in different conditions were studied in vacuum pervaporation of water–ethyl acetate mixtures. The decrease in both water and ester permeation fluxes with the increase in crosslinker/polymer ratio or in crosslinking temperature can be partly attributed to the decrease in the sorption of the components in the membranes. The selectivity to ester in the pervaporation of the mixtures is lower than that in sorption, but increases faster with the crosslinking extent. Both water and ester fluxes show similar variations with the ester content in the mixture as with the ester sorption extent: increases in the fluxes with the ester content in the homogeneous water-rich region, constant fluxes in the two-phase region, and increase in both fluxes in the homogeneous ester-rich region. This behaviour indicates that: (i) ester activity is the driving force for ester pervaporation and (ii) water permeation is mainly due to an enhanced sorption of water with increased ester contents. Using the solution–diffusion model, we show that the diffusion coefficient of ethyl acetate in a PDMS membrane is constant, and the flux variation is entirely due to the change in sorption extent with the ester content. The increase in the permeation flux of water when its activity in the mixture decreases is not due to the water–ester transport coupling, but to a synergy in sorption.