NMR-STUDY OF PMR-15 PREPOLYMERIZATION STEPS

Variations in PMR-15 composites properties could be due to differences in their chemical compositions occurring during the curing cycle. Var iations in the composition of the PMR-15 resin were studied for four d ifferent curing cycles, to determine the influences of parameters such as the heating rate or the presence of a temperature stage. The resin mixture was sampled at the same four different temperature points in each cycle and analyzed mainly by H-1- and C-13-NMR. Quantitative anal ysis of the different chemical species were realized by deconvolution of NMR spectra. At a given temperature, the resin contains the same mo lecules but in proportions which depend on the curing conditions and w hich reflect differences in the chemical reactivity of the monomeric s pecies. We have precisely defined the temperature ranges at which each of the key reactions (amidization, imidization, exoendo isomerization ) occurs. An imine condensation product of NE and MDA has been identif ied and shown to be present in the resin mixture even at low temperatu re. As a result of this study, two possible causes of resin alteration during ageing can be put forward: the presence of nonimidized BTDE mo ieties and the endo-exo isomerization reaction which will result in de formation and, possibly, microcracking, if reticulation of the end-cap ped nadimides is not total. (C) 1994 John Wiley & Sons, Inc.