Transport of liquid hydrocarbons in the polyurethane-based membranes

The sorption and diffusion of liquid hydrocarbons have been studied in a series of polyurethanes (PU) based on poly(oxytetramethylene) diol (PTMO) extended with 2,4-tolylene diisocyanate (TDI) and 4,4′-bis (2-hydroxyethoxy) biphenyl (BHBP). The effects of the soft and hard segment length, and the penetrant structure, on the transport behaviour were investigated using immersion sorption method. The sorption rate curves were generally Fickian for all the PU/benzene systems. For n-hexane and cyclohexane as penetrants, various non-Fickian anomalies were observed for PUs with shorter soft segments (PTMO-1000, 650). The values of the diffusional exponent indicated that system was tending towards Fickian on decreasing the hard segment length or increasing the soft segment one, on decreasing the penetrant size or increasing its flexibility, and on increasing temperature. Diffusion coefficients (D) were determined from the initial slope of the sorption curves by the small-time method for the PU/benzene systems, and for the PTMO-2000-based PUs. The variation in D was found to depend strongly on the length of the PU hard and soft segments, and on the penetrant structure. The PU/penetrant interaction parameter (χ) was estimated from the results of solvent equilibrium uptake. The values of this parameter indicated that strong interaction existed for the PU/benzene and for all the PTMO-2000-based PU/solvent systems. The variations in sorption capability among PUs in those systems were found to be solely the effect of different ability of the hard domains to restrain membrane swelling. For other systems, no particular difference in normalised sorptivity among structurally different PUs was observed. The activation energy for diffusion, and the thermodynamic parameters were also calculated and discussed.