Effects of sulfonate incorporation and structural isomerism on physical and gas transport properties of soluble sulfonated polyimides

Abstract

This study presents the effects of sulfonate incorporation and structural isomerism of soluble sulfonated polyimides (SPIs) for potential membrane applications. The fluorinated dianhydride (6FDA) and three commercially sulfonated diamines, 2,4,6-trimethyl-3,5-diaminobenzenesulfonic acid (tDSA), 2,5-diaminobenzenesulfonic acid (pDSA), and 2,4-diaminobenzenesulfonic acid (mDSA), were used to synthesize SPIs in a one-step polycondensation. The SPIs are soluble in common organic solvents and have good mechanical properties as represented by tensile strength of 88.4-115.8 MPa and elongation at break of 10.3-15.2%. The resulting SPI membranes exhibit low dielectric constants (similar to 2.61) and linear thermal expansion coefficients (CTE) ranging from 54.6 to 71.7 ppm/K. These SPI membranes have fractional free volume (FFV) of 0.119-0.148. The sulfonation incorporation induced closer inter-chain packing, smaller FFV, larger CTE, higher average refractive indices (n(av)), better transparency and higher ideal selectivities for almost all gas pairs for their membranes. The para-linked TEA-pSPI membrane exhibited a larger inter-chain d-spacing, larger FFV, larger CTE, smaller nav and slightly higher permeability coefficients than the corresponding meta-linked TEA-mSPI membrane. This study guides molecular architecture for improving particular membrane performance by introducing sulfonation groups as well as adjusting either para- or meta-linkages into polymeric main chains.