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Kinetics and mechanism of equifunctional 4,4′‐(N,N′‐bismaleimide)‐diphenylmethane/2,2′‐diallyl‐bisphenol A (BMDM/DABPA) and model (phenylmaleimide/2,2′‐diallylphenol) (PMI/AP) systems have been studied in the temperature range 140–400 °C using IR‐, 1H‐ and ­13C‐NMR spectroscopy, gas chromatography–mass spectrometry GCMS), differential scanning calorimetry (DSC) and isothermal calorimetry. It was established that the cure mechanism consists a unique combination of step‐wise and chain polymerization and polycondensation reactions: step‐wise “ene” addition reaction of allyl group to maleimide one and consecutive/parallel chain polymerization of maleimide and propenyl groups generated by first reaction. The latter reaction is the main crosslinking reaction. The second source of crosslinking is a dehydration reaction of phenol groups that proceeds with mandatory participation as one of the component 1:1 adduct (product of step‐wise polymerization). Homopolymerization of maleimide groups proceeds autocatalytically under the action of free radicals generated by thermal decomposition of maleimide‐propenyl groups' donor–acceptor pairs. Steric hindrance in 2,2′‐diallyl‐bisphenol A prevents the reversible Diels–Alder reaction but this reaction proceeds in model systems. Some thermodynamic and kinetic parameters of the reactions are determined. Copyright © 2003 John Wiley & Sons, Ltd.

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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.

Pavement distress occurs through a variety of mechanisms, but it is always controlled by the adhesive and cohesive performance of the asphalt binder. Although the causes of pavement failures are known, the precise mechanisms by which they occur remain to be understood. Observation of the fracture morphology of asphalt concrete can provide some information in this respect. The fracture morphology of asphalt concrete is dependent on the morphology of the binder. A network structure was observed in thin asphalt binder films and the fracture morphology and engineering properties of asphalt concrete were found to be dependent on the network morphology of the asphalt binder. Addition of polymers to asphalt binders causes changes in the nature of the network structure, and its effect can be qualitatively determined by characterizing the fracture morphology. Styrene butadiene styrene (SBS), styrene ethylene butylene styrene (SEBS), styrene butadiene rubber (SBR) latex and an epoxy-terminated reacting polyolefin (Elvaloy AM) were used in this study. A quantitative method to determine the effect of polymer modification on the fracture properties of asphalt concrete is the J-contour integral fracture toughness measurement. An experimental protocol to measure the critical J-integral fracture toughness ( J1 c) was developed and the low temperature (-10°C) J1 c values were determined for SEBS and Elvaloy AM-modified asphalt concrete at three different concentrations.

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.