Gas transport properties of siloxane polyurethanes

Abstract

AbstractPolyurethanes (PUs) were synthesized from toluenediisocyanates (TDIs) and a polymeric diol having polydimethylsiloxane and polyoxyethylene blocks of the ABA type, ended with OH groups. Prepolymers, prepared in toluene solution using ratios [NCO]/[OH] ≥ 2, were crosslinked with triisopropanolamine (TIPA) (ratio [OH]/[NCO] = 1.1) (two‐step process). PUs were also obtained with a one‐step process using, contemporaneously, TDI, block copolymer, and, as crosslinking agent, TIPA or the glyceride of ε‐hydroxyhexanoic acid. Polydimethylsiloxane (PDMSO) was prepared as a reference material. The course of the reaction between block copolymer and TDI was studied by differential scanning calorimetry in the absence and presence of benzoyl chloride (BzCl). Without BzCl, with ratios [NCO]/[OH] > 2, uncontrolled crosslinking side reactions occur. The properties of the PU films obtained with the two methods were studied both for the density of crosslinking and for gas transport properties. The two‐step polymers are less crosslinked than the others and are characterized by higher diffusion coefficients and by higher permeability to gases. The permeability order is 10−9 (N cm3 cm−1 cm−1 cm Hg−1 s−1) for CH4, O2, CO, and N2 and is 10 times higher for CO2. The selectivity for the couple O2/N2 is higher than that obtained with PDMSO films. Considerable selectivities are shown for the couples CO2/N2 and CO2/CO. © 1995 John Wiley & Sons, Inc.