Highly Soluble Polyimides Containing Di-tert-butylbenzene and Dimethyl Groups with Good Gas Separation Properties and Optical Transparency

Abstract

A rigid aromatic diamine monomer containing di-tert-butylbenzene and dimethyl groups, 3,3′-dimethyl-4,4′-diaminophenyl-3″,5″-di-tert-butyltoluene, was successfully synthesized by a simple coupling reaction using 3,5-di-tert-butylbenzaldehyde and o-toluidine as starting materials. A series of novel polyimides (PI 3a–3c) with large pendant groups were prepared with the obtained diamine monomer and three different commercial aromatic dianhydrides (3,3′,4,4′-biphenyltetracarboxylic dianhydride, 4,4′-oxydiphthalic anhydride, and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride) by one-step high temperature polycondensation. The prepared polyimides exhibited high solubility and good membrane forming ability: they could be dissolved not only in some high boiling solvents such as DMF, NMP, DMAc, and m-Cresol at room temperature, but also in some low boiling solvents such as CHCl3, CH2Cl2, and THF. Their solubility in most solvents could exceed 10 wt%, and the flexible membranes could be obtained by casting their solutions. The prepared membranes exhibited good gas separation properties. The permeability coefficients of PI 3c for CO2 and O2 were up to 124.6 and 42.8 barrer, respectively, and the selectivity coefficients for CO2/CH4 and O2/N2 were 14.7 and 3.3, respectively. The membranes had light color and good optical transmission. Their optical transmittance at 450 nm wavelength was in the range of 67%–79%, and the cutoff wavelength was in the range of 310–348 nm. They also had good thermal properties with glass transition temperature (Tg) values in the range of 264–302 °C. In addition, these membranes possessed good mechanical properties with tensile strength ranging between 77.8–87.4 MPa, initial modulus ranging between 1.69–1.82 GPa, and elongation at break ranging between 4.8%–6.1%.