Relationships between chemical structures and solubility, diffusivity, andpermselectivity of 1,3-butadiene and n-butane in GFDA-based polyimides

The solubility, diffusivity, and permselectivity of 1,3-butadiene and n-but ane in seven different polyimides synthesized from 2,2-bis (3,4-carboxyphen yl) hexafluoropropane dianhydride (6FDA) were determined at 298 K. The infl uence of chemical structures on physical and gas permeation properties of 6 FDA-based polyimides was studied. Solubility of 1,3-butadiene in 6FDA-based polyimides can be described by a dual-mode sorption model. 1,3-Butadiene-i nduced plasticization is considered to be associated with the increasing pe rmeabilities of 1,3-butadiene and n-butane and the decreasing permselectivi ty of 1,3-butadiene vs, n-butane in the mixed gas system containing a high concentration of 1,3-butadiene. It was found that controlling the solubilit y of 1,3-butadiene in an unrelaxed volume in 6FDA-based polyimides is very important to maintain the high permselectivity of 1,3-butadiene vs. n-butan e in the mixed gas system. Changing the -C(CF3)(2)- linkage to a -CH2-, -O- linkage, removing methyl substituents at the ortho position of the imide l inkage, and changing the p-phenylene linkage to an m-phenylene linkage in t he main chains in some 6FDA-based polyimides are effective to decrease frac tional free volume and restrict the solubility of 1,3-butadiene in the unre laxed volume of a polymer matrix. The 6FDA-based polyimides restricting the solubility of 1,3-butadiene in an unrelaxed volume exhibit high separation performance in the 1,3-butadiene/n-butane mixed gas system compared with c onventional glassy polymers and, therefore, are potentially useful membrane materials for the separation of 1,3-butadiene and n-butane in the petroche mical industry. (C) 1999 John Wiley & Sons, Inc.