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The effects of moisture on a polymerized monomeric reactant (PMR)-type polyimide (TriA X) and associated composites were investigated. Water uptake tests were performed on the polyimide at various temperatures and relative humidity levels to investigate moisture absorption behavior. Two-stage moisture absorption was observed, in which the first stage was diffusion controlled, whereas the second stage was moisture plasticization controlled. As exposure temperature increased, the equilibrium moisture content of the polyimide decreased, indicating an exothermic absorption process. The Arrhenius temperature dependence and moisture saturation as functions of temperature and humidity in the neat polymer were determined using curve fitting based on the published mathematical models. Long-term hydrothermal aging at 95°C was conducted on the neat polyimide and associated carbon fiber composites. Reversible hydrolytic reactions and a trace of irreversible hydrolysis were observed in the long-term exposure. The tensile ductility of the neat polyimide and the short-beam shear strength of the composites decreased with increasing aging time, while the tensile strength and modulus and thermal properties of the polyimide exhibited little change after 2000-h aging, demonstrating hydrothermal stability. The decrease in the ductility of the neat polymer after long-term moisture exposure was attributed to the network structure change, driven by hydrolysis and moisture plasticization. [ABSTRACT FROM AUTHOR]

The effects of thermal cycling on a polymerized monomeric reactant (PMR) type polyimide (TriA X) reinforced with carbon fibers were investigated. Composite specimens were subjected to 2000 thermal cycles between −54°C and 232°C. At 400-cycle intervals, laminates were inspected for microcracks, and glass transition temperature (T g) and short-beam shear (SBS) strength were measured. The composites did not exhibit microcracks after thermal cycling, although after 2000 thermal cycles, mechanical properties of the matrix declined slightly. The matrix degradation decreased the resistance to microcracking upon further loading. No effects of thermal oxidative aging were observed from thermal cycling, and thermally driven fatigue and creep were identified as the primary and secondary factors inducing mechanical degradation of the matrix. T g of the composites exhibited no change after 2000 cycles, while the SBS strength decreased slightly (3–9%). The results highlight the potential for use of TriA X composites as long-term structural components in high-temperature service environments. [ABSTRACT FROM AUTHOR]

A new polymerized monomeric reactant (PMR)-type polyimide, designated TriA X, was investigated to determine polymer structure, processability, thermal, and mechanical properties and establish the relationship between the molecular structure and those properties. TriA X is a PMR-type polyimide with an asymmetric, irregular, and nonplanar backbone. Both the imide oligomers and the cross-linked polyimides of TriA X exhibited loose-packed amorphous structures, independent of thermal processing. The peculiar structures were attributed to the asymmetric backbone, which effectively prevented the formation of closed-packed chain stacking typically observed in polyimides. The imide oligomers exhibited a lower melt viscosity than a control imide oligomer (symmetric and semi-crystalline), indicating a higher chain mobility above the glass transition temperature (Tg). The cured polyimide exhibited a Tg = 362°C and a decomposition temperature = 550°C. The cross-linked TriA X exhibited exceptional toughness and ductility (e.g. 15.1% at 23°C) for a polyimide, which was attributed to the high-molecular-weight oligomer and loose-packed amorphous structure. The thermal and mechanical properties of TriA X surpass those of PMR-15 and AFR-PE-4. [ABSTRACT FROM AUTHOR]

A new type of polyimide, designated TriA X, has been developed for high-temperature composite applications. TriA X is a polymerized monomeric reactant (PMR)-type polyimide derived from 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), 2-phenyl-4,4′-diaminodiphenyl ether (p-ODA), and phenylethynyl phthalic anhydride (PEPA). The polymer has an asymmetric, nonplanar backbone, resulting in an amorphous structure and high toughness. In this work, a TriA X resin (with degree of polymerization n = 7 in the imide oligomer) was investigated for processability and performance in carbon fiber composites. Rheological measurements were performed on an oligomer with a low degree of imidization to understand the chemo-rheology of the resin system and determine a suitable B-staging temperature. A composite molding cycle was designed, which yielded fully consolidated woven carbon fiber laminates. Void contents in panels produced with this molding cycle were <0.1% as measured by image analysis (IA) of polished sections, and <0.2% as measured by X-ray micro-computed tomography (micro-CT). Matrix-dominated mechanical properties of composites fabricated with the TriA X polymer exceeded those of PMR-15 and AFR-PE-4 composites. These mechanical properties and a measured glass transition temperature of 367°C indicate potential for use of this resin system in high-temperature composites. [ABSTRACT FROM AUTHOR]