Preparation and study of crosslinked polyurethane films to fractionate toluene-n-heptane mixtures by pervaporation

Poly (ether urethane) and poly(ester urethane) segmented copolymers (noted PEG and PCL) were prepared from aliphatic diisocyanate and ether or ester oligomer diols in one or two synthetic steps tither from end capped isocyanates (route 1) or hydroxyl (route 2) functionalised oligomers. The ratio of NCO to OH groups used for the copolymerisation led to the preparation of chemically crosslinked polyurethane films. Their microphase-separated structures were characterised by DSC and the T-g and T-m of the soft blocks was recorded. The separation properties of the polymer films were investigated by sorption, diffusion and pervaporation (PV) experiments with toluene-n-heptane mixtures. The experiments demonstrated strong toluene selectivity for all the films carried out on the whole range of concentration. The membranes prepared according to route 1 exhibited lower sorption degrees and fluxes than the ones prepared by route 2, differences that could be easily related to the lower crosslinking density achieved by route 2; on the other hand the selectivity of both membrane types was almost the same. The diffusion coefficients at infinite dilution of both pure solvents were determined from experiments carried out in the transient state and the toluene coefficient was found to slightly exceed the n-heptane one in the less crosslinked urethane networks. The preferential pervaporation of the aromatic compound was basically shown to be due to the sorption step; nevertheless in all cases the sorption selectivity was higher than the pervaporation one which means that the diffusion step was in fact more favourable to the aliphatic component. (C) 2001 Elsevier Science B.V. All rights reserved.