Estimation of solvent activities in polymers solutions using a group-contribution method

A group-contribution method is presented for the estimation of solvent acti vities in polymers solutions. The method is an extension of the UNIFAC, whi ch was originally derived for mixtures of ordinary liquids. The fundamental assumption is the additivity of the contributions made by the functional g roups which constitute the compounds in the mixtures. The extension to mixt ures with polymers is attractive because as show by T. Oishi, J.M. Prausnit z, Ind. Eng. Chem. Process Des. Dev. 17 (1978) 333, the solvent activities in various polymers solutions can be assumed with an uncertainty of no more than 10%, with the advantage that for the UNIFAC method no experimental da ta are required. However, this method has been mainly proven in non polar s ystems. In this paper, the method is extended to cases in which polymers an d/or solvents have polar groups and when the solid phase is a copolymer wit h a random distribution. To calculate the influence of polar group, with st ronger interactions, the solubility parameters concept is introduced. Also, a correction factor is used to take into account the distribution of repet itive units in the copolymer. Experimental results obtained by a gravimetri c method were fitted with the proposed model. The polymers used were polydi methyisiloxane (PDMS) and poly(ethylene-co-vinyl acetate) (EVA) while the s olvents were ethanol and n-hexane. The results fit the data very well for p olar-polar system, within the range of temperature investigated. However, t he equilibrium sorption data for the system n-hexane-EVA cannot be properly fitted with the UNIFAC model, mainly because the effect of vinyl acetate g roups are rejecting n-hexane and its influence overcome its own boundaries in the copolymer structure. (C) 2001 Elsevier Science B.V. All rights reser ved.