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 polyimi de gels with high modulus, the approach of end-crosslinking with an imide l inkage was developed. Methylenedianiline oligomer (pMDA), which has an aver age amino functionality of 2.6, was used as an amine comonomer as well as a crosslinker to react with 4,4'-(hexafluoroisopropylindene)-diphthalic anhy dride (6FDA), 4,4'-oxydiphthalic anhydride (ODPA) and 3,3', 4,4'-biphenylte tracarboxylic dianhydride (BPDA) to make different polyimide gels. High com pression moduli greater than 1 MPa were found for the ODPA/pMDA polyimide g els 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 compressio n 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 theo ry. (C) 2001 Elsevier Science Ltd. All rights reserved.