Two phenylethynyl phthalic anhydride‐capped imide oligomers, AFR‐PEPA‐2 and AFR‐PEPA‐8, with molecular weights of 1 601 and 4 699 g · mol−1, respectively, were synthesized and characterized. The AFR‐PEPA‐N oligomers show higher glass transition temperatures and higher thermal decomposition temperatures than phenylethynyl‐terminated imide PETI‐5. After curing for 1 h at 390 °C, AFR‐PEPA‐2 and AFR‐PEPA‐8 have Tgs of 370 and 358 °C, respectively. AFR‐PEPA‐N oligomers demonstrated lower minimum complex melt viscosities than PETI‐5 due to the presence of CF3 group in the backbone structure. 1 601 g · mol−1 AFR‐PEPA‐2 imide oligomer has a complex melt viscosity of 10 Pa · s at 340 °C, and 4 699 g · mol−1 AFR‐PEPA‐8 imide oligomer has a complex melt viscosity of 227 Pa · s at 371 °C. AFR‐PEPA‐N film's crystal morphology was observed using polarized optical microscopy and the AFR‐PEPA‐8 oligomer did not show crystallinity. AFR‐PEPA‐2 film exhibits semicrystalline behavior and the crystallinity does not disappear until the film is cured above 375 °C.

Thermal-processing structure-property relationships for polyetherimide (PEI), poly(4,4Ј-oxydiphenylene pyromellitimide) (POPPI), and phenylethynyl-terminated imide (PETI-5) composite matrices are reported from a fundamental perspective. For thermoplastic PEI, deformation and failure depend primarily on free volume as evidenced by moisture-absorption, mechanical-property, and mass-density changes as a function of annealing. The deformation of POPPI can be divided into the following three regimes as a function of annealing temperature: (1) physical aging-induced glassy state free-volume decreases, (2) thermally activated microvoid collapse, and (3) chemical degradation. In the case of PETI-5, macroscopic defects, free volume, and polymer morphology control deformation. The effects of residual crystallinity on deformation are reported, and it is shown that mechanical toughness can be significantly decreased upon annealing below the glass-transition temperature.

Prediction of the cure-induced Tg increases and associated matrix and composite mechanical property deterioration of BMI systems in real service environments is attempted by network structure interrelations with mechanical and thermal properties as a function of composition (initial monomer ratio) and time-temperature cure cycles. Tensile and flexural properties of four BMI compositions at six different cure cycles (or degree of cure) have been determined at three temperatures, 230C, 1 770C, and 250'C, and correlated to Tg and density of the systems. Systematic studies on hygrothermal durability of bismaleimide (BMI) and various polyimide (PI)-carbon fiber composites and neat resins were conducted. The combined effects of moisture and thermal exposures such as hygrothermal spikes up to 250'C and hygrothermal aging under various time-temperature-moisture conditions including accelerated aging at saturated steam environment (1 60'C and 1 10 psi) on microscopic damage, polymer-matrix physical structural state, and residual properties of those composites and neat resins are presented. The onset of blistering in moist K3B PI C fiber composites occurred at 2290C during the thermal spiking. It is evident that the hygrothermal performance stability is one of the prime guidelines in future aerospace applications, especially for PI composites. The physical and chemical structural state of PI matrices such as K3B and AFR700B as a function of hygrothermal exposure are discussed in terms of hydrolytic chemical degradation, moisture vapor-induced physical damage, and molecularly locked-in water. Those structural states are characterized by systematic weight monitoring 2H NMR, and various thermal, mechanical property measurements using D20 water environment.

Transverse microcracks are present in carbon fiber/bismaleimide (BMI) cros: composite laminates composed of 4, 4′‐bismaleimidodiphenylmethane (BMPM)/diallyl bisphenol A (DABPA) matrices after standard cure and fabrication condit and grow in width upon subsequent postcure. This investigation characterizes cure‐induced microcracking in terms of the critical fundamental macroscopic croscopic, and molecular damage mechanisms and thresholds, and a cure cycle modification that prevents microcrack formation under standard processing conditions tions for [0°/90°]s laminates is examined. A unique in‐situ technique is utilized which cure of the laminate is performed inside the chamber of an environim scanning electron microscope (ESEM), allowing for (i) physical observation of microcrack crack growth and formation mechanisms and (ii) characterization of microcracking onset time‐temperature thresholds. The cure cycle modification that prevents microcracking is an extended initial cure time at 177°C prior to higher temperature; cure regimes. Effects of this modification are examined through network structure property‐processing interrelationships by way of (i) dynamic mechanical analysis (DMA), (ii) optical and electron microscopy, (iii) differential scanning calorimetry (DSC), and (iv) our previous work on carbon fiber/bismaleirnide composites. The aforementioned analysis it was concluded that an extended initial cure time 177°C prior to higher temperature cure steps prevents microcracking under standard; fabrication postcure conditions for [0°/90°]s laminates; no microcracking observed until an additional postcure of 6 h at 300°C. This microcrack resist was independent of initial BMPM:DABPA monomer stoichiometry for the monomer ratios examined and associated with an improved fiber‐matrix interface and lower composite residual stress.

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