Characterization of polyimide gels crosslinked with hexamethylene diisocyanate

Abstract

Characterization of the crosslinked structure has been carried out for soft polyimide gels prepared from a soluble high-performance fluorine-containing polyimide consisting of 4,4′-(hexafluoro-isopropylindene)diphthalic anhydride and 3,3′-dihydroxy-4,4′-diaminobiphenyl crosslinked with hexamethylene diisocyanate. Weight-average molecular weight (Mw=9.28×104), second and third virial coefficients (A2=0.82×10−3molml/g2, A3=6.42×10−3molml2/g3) of the linear polyimide in N-methylpyrrolidone at 24°C were determined by a static laser light scattering measurement. The average molecular weight between crosslinking points Mc, was calculated based on both swelling and viscoelastic measurements of the polyimide gels. As for the equilibrium swelling data, a concentration-dependent form of the Flory–Huggins parameter (i.e. χ=χ1+χ2φ) has been applied to calculate the Mc. The values of Mc from equilibrium swelling and viscoelastic measurement have been found to be in agreement. By comparing Mc with the results of 1H-NMR, the percentages of effective and ineffective linkages have been evaluated quantitatively for all the polyimide gels. It has been found that the elastically effective crosslinking is less than 20%. Results from classical and improved Flory–Rehner theory have been compared and we believe that the improved theory gives a more reasonable description of Mc at higher crosslink density.