Preparation of end-crosslinked polyimide gels with high moduli

Crosslinked polyimide gels with pendant hydroxy groups reported previously were found to have only a small fraction of elastically effective junctions. In order to increase the amount of effective junctions and obtain polyimide gels with high modulus, the approach of end-crosslinking with an imide linkage was developed. Methylenedianiline oligomer (pMDA), which has an average amino functionality of 2.6, was used as an amine comonomer as well as a crosslinker to react with 4,4′-(hexafluoroisopropylindene)-diphthalic anhydride (6FDA), 4,4′-oxydiphthalic anhydride (ODPA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) to make different polyimide gels. High compression moduli greater than 1 MPa were found for the ODPA/pMDA polyimide gels at their equilibrium swollen states in N-methylpyrrolidone (NMP). BPDA/pMDA gels did not swell in NMP, while 6FDA/pMDA gels swelled significantly and finally dissolved in NMP. The equilibrium swelling ratio and compression moduli for as-prepared and equilibrium swollen states were correlated to the chemical structure and initial monomer concentrations of the polyimide gels, and found to agree quite well with the prediction of the scaling theory.