IM7/LARCTM-IA polyimide composites

Abstract

LARC^TM-IA (Langley Research Center-Improved Adhesive) aromatic polyimide, based on oxydiphthalic anhydride and 3.4'-oxydianiline, was evaluated as a matrix for high-performance composites. Six poly(amide acid)solutions in N-methylpyrrolidone (NMP), end-capped with phthalic anhydride to various theoretical molecular weights, were synthesized and their molecular weights and molecular weight distributions determined, Importantly, high concentrations of low-molecular-weight species were found in all the offset compositions. Except for the 1% offset polymer, all fully imidized films failed a solvent resistance test which involved immersion in acetone, methyl ethyl ketone, toluene, dimethylacetamide and chloroform for 1 min followed by a fingernail crease. Unidirectional prepreg was fabricated from each of the six resins by both standard drum winding procedures and the LARC multipurpose prepreg machine. The consolidation cycle developed previously for IM7/LARC^TM-ITPI composites was found to be equally applicable for IM7/LARC^TM-IA composites since both materials are similar and were prepared in and prepregged from NMP. An optimal end-capped resin composition was identified (4% stoichiometric imbalance) by using, as a screening tool, initial composite mechanical properties (short-beam shear strength, longitudinal flexural strength and flexural modulus) at room temperature, 93, 150 and 177°C. Composite engineering properties for the 4% offset composition were obtained, including longitudinal tension, transverse flexural, longitudinal compression, interlaminar shear, short block compression, open hole compression and compression strength after impact. Notably, the CAI strength was 303.2 MPa (44 Ksi) showing that the LARC^TM-IA composites have good damage tolerance.

A minor modification of LARC^TM-IA polymer backbone which did not alter the consolidation cycle, designated as LARC^TM-IAX, improved solvent resistance measurably. Mechanical properties of IM7/LARC^TM-IAX composites were shown to be comparable to those exhibited by the baseline IM7/LARC^TM-IA composites.

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