Abstract
The NASA-developed polyimide, PMR-15 is the frontrunner in the race to develop a matrix resin for composites intended for high temperature applications. The properties of PMR-15 depend on the polymer structure. Variations in the structure such as molecular weight, molecular weight distribution and crosslink density, will affect the ultimate mechanical and thermal properties of the material. The polymerization of PMR-15 is complex. Competing reactions in the initial imidization stage, e.g. anhydride formation, can lead to variations in the structure of the oligomers which will affect processing characteristics. This study examines the structure of the prepolymers produced by the thermal imidization of the monomer mixture under different time/temperature conditions. Techniques used include NMR, FTIR, GPC, mass spectrometry and dynamic mechanical spectroscopy. The polymer structures are complex with imide, amide, anhydride, ester, amine and salt all being detected in varying amounts depending on the thermal treatment. Differences in prepolymer molecular weight and molecular weight distribution are also observed. Variations are also seen in prepolymer viscosity and also in volatile evolution at elevated temperatures. Such differences could have a profound effect on processing characteristics.
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