AbstractPMR‐15 resin solutions consist of nadic ester (NE), the dimethyl ester of 3,3′,4,4′‐benzopheno‐netetracarboxylic acid (BTDE), and 4,4′‐methylenedianiline (MDA) dissolved in methanol in a 2.000/2.087/3.087 molar ratio. Other chemical species may be present as a result of impurities in the monomers or reaction products which form upon aging of the solution. The effect of these other chemical species on the resin cure chemistry and composite properties is only partially understood. The composition of 50 wt PMR‐15 resin solutions was measured by reverse phase, high performance liquid chromatography (HPLC). The major impurity in the monomers was found to be monoester and tetraacid impurities in the BTDE solution. These impurities could be eliminated by recrystallizing the dianhydride from which the BTDE was made. Triester formation was not a problem because of the high rate of esterification of the anhydride compared to that of the carboxylic acid. When the PMR‐15 monomer solution was aged at room temperature, the concentration of BTDE remained constant. The concentration of NE and MDA decreased as their reaction products formed. The amide‐acid formed quickly but remained at a small concentration. The monoimide and bisimide concentrations increased monotonically during the entire aging time. When the PMR‐15 resin was stored as a dried film, imidization of NE and MDA still occurred,
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