A new series of high flow PMR‐type addition curing polyimides was developed, which employed the substitution of 2,2′‐bis(trifluoromethyl)‐4,4′‐diaminobiphenyl (BTDB) for p‐phenylenediamine (p‐PDA) in a PMR‐II formulation. These thermoset polyimides, designated as 12F resins, were prepared from BTDB and the dimethyl ester of 4,4′‐(hexafluoroisopropylidene)‐diphthalic acid (HFDE) with either nadic ester (NE) or p‐aminostyrene (PAS) as the endcaps for addition curing. The 12F prepolymers displayed lower melting temperatures in DSC analysis, and higher melt flow in rheological studies than the corresponding PMR‐II polyimides. Long‐term isothermal aging studies showed that BTDB‐based 12F resins exhibited comparable thermo‐oxidative stability to p‐PDA based PMR‐II polyimides. The noncoplanar 2‐ and 2′‐disubstituted biphenyldiamine (BTDB) not only lowered the melt viscosities of 12F prepolymers, but also retained reasonable thermal stability of the cured resins. The 12F polyimide resin with p‐aminostyrene endcaps showed the best promise for long‐term, high‐temperature application at 343°C (650°F). © 1994 John Wiley & Sons, Inc.

Poly(pheny1quinoxalines) (PPQ's) are considered to have excellent potential for use as high temperature-resistant matrix resins in advanced fiber/resin composites. However, because of processability problems, their potential as high-temperature matrix resins has not been realized. Current methods used to fabricate PPQ/fiber composites consist of impregnating the reinforcement with high molecular weight PPQ polymer dissolved in a solvent or solvent mixture containing m-cresol. Because of the inordinately high viscosity of these solutions, which results from both the high molecular weight of the polymer and the use of m-cresol, complete wetting of the fiber during impregnation is difficult to achieve. The approaches used to overcome the viscosity problem are (1) to limit the polymer molecular weight by upsetting the stoichiometry of the system, or (2) to use dilute solutions. Unbalancing the stoichiometry can adversely affect the polymer thermooxidative stability and even dilute solutions of high molecular weight polymer are extremely viscous.This communication describes a new approach to circumvent some of the composite processing problems associated with the use of PPQ polymers as the matrix material. The method used to prepare high-performance PPQ/graphite fiber composites consisted of impregnating the fiber with a freshly made solution of the appropriate monomers instead of a solution of high molecular weight polymer.Impregnation of the fiber prior to appreciable polymerization completely eliminates the impregnation problems encountered with the use of high-viscosity high molecular weight PPQ solutions. It is important to note that the major part of the polymerization of the reactant mixture is conducted on the fiber during the solvent removal and final curing stages. Although a t the time of impregnation the solution is not truly monomeric, we refer to this approach as the in situ polymerization of monomers because of its similarity to the monomeric reactant approach developed in our laboratories for A-type polyimides.

The mechanical and electrical properties of pristme and bromine intercalated graphite fiber-epoxy composites were compared. The two types of composite were similar in terms of tensile modulus, tensile strength, and Poisson's Ratio. However, the interlaminar shear strength of the brominated composite was 18 percent greater than its pristine counterpart. Only slight differences were observed in flexural properties. A five-fold decrease was observed in the electrical resistivity of the brominated composite parallel to the axis of the fibers, resulting in a unidirectional resistivity of about 90 μΩ-cm. Transverse resistivity was unaffected. Both types of composite were subjected to a simulated lightning strike of 10 KJ (at a peak current of 150 kA), and the composite with the intercalated graphite exhi bited less damage.

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