Pervaporative separation of azeotropic methanol/toluene mixtures in polyurethane-poly(dimethylsiloxane) (PU-PDMS) blend membranes: Correlation with sorption and diffusion behaviors in a binary solution system

A separation of toluene/methanol binary mixtures was pervaporated using toluene-selective polyurethane (PU)-poly(dimethylsiloxane) (PDMS) blend membranes. The separation efficiency and toluene flux from the azeotropic mixtures reached a maximum in the blend containing 20% PU content. The toluene sorption behavior from the binary mixtures into the blend was predictable using the Flory-Huggins equation with the interaction parameters (χ_iM) determined from the single solvent sorption measurements. However, the methanol solubility from the solutions was lower than predicted, indicating the less permeable component sorption was suppressed. This competitive sorption is actually beneficial in the pervaporation process since it enlarges the sorption selectivity. The diffusivity of permeants, on the other hand, demonstrated a diffusion-coupling phenomenon from the binary solutions in the blend. The diffusion coefficient of the less permeable (methanol) component was increased by the facilitation of a toluene-swollen polymer matrix. The methanol molecules diffused at a rate higher than toluene, although the pure solvent diffusivities were comparable with each other. Despite the fact that the high sorption selectivity was slightly offset by the synergistic diffusion effect, the PV performance was dominated by the sorption characteristics. The PU-PDMS (20/80) blend exhibited excellent toluene flux with reasonable separation efficiency in toluene-lean and azeotropic compositions. This research demonstrates that polymer blends is a simple way to modulate membrane's transport properties and can achieve higher performance than the pristine polymer materials.