Microporous polyimides containing bulky tetra-o-isopropyl and naphthalene groups for gas separation membranes

Herein, the microporous structure and gas transport property of a new family of polyimides (PIs) containing bulky tetra-o-isopropyl and naphthalene groups prepared via a microwave-assisted polymerization process are reported. The polyimides were prepared from a naphthalene and isopropyl containing diamine, i.e., 4,4'-(naphthalen-1-ylmethylene)bis(2,6-diisopropylaniline)) (i.e., BAN-3) obtained via a one-step electrophilic substitution reaction, reacting with three commercial dianhydrides. Analysis of fractional free volume (FFV) in conjunction with positron annihilation lifetime spectroscopy (PALS) showed that the resultant BAN-3 based PIs had high FFV values of over 0.19 featuring a bimodal size distribution of microcavities with average diameters in the range of 8.0-9.0 angstrom and 5.6-6.3 angstrom, respectively. Wide angle X-ray diffraction (WXRD) further confirmed that the introduction of naphthalene and tetra-o-isopropyl groups effectively enhanced polymeric backbone rigidity and disrupted chain packing, leading to high gas permeabilities in all three PI films. For example, the PI-3F film had high H-2 and CO2 permeability values of 722 and 849 barrer, respectively while maintaining comparable selectivities for H-2/CH4 and CO2/CH4 gas pairs with other reported PIs of the same dianhydride. In addition, the PI-3B film displayed excellent gas separation performance for O-2/N-2 gas pair approaching the 2008 Robeson upper bound.