The main aim of this work was the preparation and characterization of the soluble amine end-capped HBPAA and its fully imidized form, hyperbranched polyimide (HBPI), and their use as a multi-functional crosslinking agent of the linear polyimide MI.
Preparation and Characterization of HBPAA and HBPI Based on 6FDA and TTM
The amine end-capped HBPI [HBPI(6FDA-TTM)] made from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) (Fig.
The band at approximately 1,680 [cm.sup.-1] corresponding to the amide groups of the polyimide precursor (HBPAA) is absent in the HBPI spectrum.
The gas permeability coefficients [P.sub.x] in the membrane based on HBPI(6FDA-TTM) were considerably higher in comparison with those for HBPI based on 4,4'-oxydiphthalic anhydride and TTM  (33.7 x [10.sup.-17] vs 6.2 x [10.sup.-17] [m.sup.2][s.sup.-1][Pa.sup.-1] for hydrogen and 18.2 x [10.sup.-17] vs 1.6 x [10.sup.-17] [m.sup.2][s.sup.-1][Pa.sup.-1] for carbon dioxide).
The gel content in the materials was inversely related both to the molecular weight and functionality of the crosslinking agent (HBPI < TTM < DDM) and was proportional to its concentration.
The products for which 10% or 20% of the imide groups of MI reacted with the amino groups of HBPI during the formation of amide linkages show a polyimide character (e.g., a glass transition temperature comparable with that of the pure MI) and a gel content of up to 70% if the crosslinking temperature of 200[degrees]C was employed.