Synthesis and gas transport property of polyimide from 2,2′-disubstituted biphenyltetracarboxylic dianhydrides (BPDA)

A series of dianhydride monomers, 2,2′-disubstituted-4,4′,5,5′-biphenyltetracarboxylic dianhydride (substituents = phenoxy, p-methylphenoxy, p-tert-butylphenoxy, nitro, and methoxy) were synthesized by the nitration of an N-methyl protected 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and subsequent aromatic nucleophilic substitutions with aroxides (NaOAr) or methoxide. These dianhydrides were polymerized with various aromatic diamines in refluxing m-cresol containing isoquinoline to afford a series of aromatic polyimides. The effects of varying 2,2′-substituents of the dianhydride (BPDA) moiety on the properties of polyimides were investigated. It was found that polyimides from the dianhydrides containing phenoxy, p-methylphenoxy, and p-tert-butylphenoxy side groups possessed excellent solubility and film forming capability whereas polyimides from 2,2′-dinitro-BPDA and 2,2′-dimethoxy-BPDA were less soluble in organic solvent. The soluble polymers formed flexible, tough and transparent films. The films had a tensile strength, elongation at break, and Young’s modulus in the ranges 102–168 MPa, 8–21%, 2.02–2.38 GPa, respectively. The polymer gas permeability coefficients (P) and ideal selectivities for N2, O2, CO2 and CH4 were determined for the –OAr substituted polyimides. The oxygen permeability coefficient (PO2) and permselectivity of oxygen to nitrogen (PO2/N2) of the films were in the ranges 3.4–11.3 barrer and 3.8–4.6, respectively. The gas permeability typically increased with increasing free volume in the order of tert-butylphenoxy substituted PI > methylphenoxy substituted PI > phenoxy substituted PI.