A series of addition cure poly(imide siloxane) resins were synthesized, incorporating various concentrations of alpha, omega-bis(3-aminopropyl) poly(dimethyl-diphenylsiloxanes), and alpha, omega-bis(p-aminophenyl) poly(dimethylsiloxane) into the formulated imide oligomer. Both carbon and glass fiber textile laminates were fabricated using amic acid and polymerization of monomer reactants (PMR) approaches. The cured composite laminates were subjected to an accelerated thermo-oxidative aging environment of 400 degrees C for 100 h in air. Physical, thermal, and mechanical properties were evaluated to determine the structure-oxidative stability interrelationships. In general, composite mechanical properties were found to increase with increasing siloxane concentration in the matrix. Composite thermo-oxidative durability (measured via mass loss and mechanical property retention after oxidative aging) was improved through incorporation of diphenyl and diphenyl-dimethyl siloxane segments into the imide oligomer backbone up to similar to 35% by weight aminosiloxane. Oxidative stability was found to be mostly dependent on the degree of phenyl substitution on the silicon atoms in the siloxane blocks, as compared to the moiety attaching the amine groups to the siloxane block.

Phenylethynyl terminated poly(imide siloxanes) with various amounts of alpha, omega-bis(3-aminopropyl) polydimethylsiloxane, 1,3-phenylenediamine, and 4,4'-(1,3-phenylenediisopropylidene) bisaniline reacted into the imide oligomer backbone were prepared. Prepregs and subsequent composite laminates were fabricated using T650-35/8HS carbon textiles, and aged under conditions of 329 degrees C for 1000 h in air and 60 degrees C at 95-100% relative humidity. Thermo-oxidative stability and hygrothermal durability were characterized and evaluated as a function of imide oligomer molecular structure using optical microscopy, dynamic mechanical analysis, physical property, measurements, and mechanical testing. Mechanical property retention after 1000 hours aging at 329 C in air improved as the concentration of siloxane in the imide oligomer backbone increased for the formulation range of 5-20% siloxane by weight, and carbon fiber reinforced poly(imide siloxane) composites were found to exhibit good hot/wet mechanical durability, with strength retention of at least 75% after 1000 hours of exposure.

In this study, a novel conduction heating-based thermal cycling apparatus combined with large deflection bending is developed and utilized to identify the critical controlling parameters for microcracking of [90/0]1s, M40J/ PMR-II-50 high modulus carbon fiber/polyimide composite laminate under synergistic environmental conditions. The synergistic test involves four controlling parameters namely, average in-plane mechanical strains (0 and 0.488%), thermal cycling temperature amplitudes (—196—23 °C and —196—250°C), number of thermal cycles (1 and 8), and heating rate (1 and 4°C/min). The 2k factorial design is used for the four factors to provide their quantitative primary and interaction effects on crack density with a minimum number of experiments. The experimental results indicate that the number of thermal cycles is the primary controlling factor (41%), while the thermal cycling temperature amplitude (25%) or the in-plane strain (22%) is the secondary factor. The number of thermal cycles also exhibits a significant interaction effect on the development of microcracks when it was combined with either the temperature amplitude of thermal cycling (7%) or mechanical in-plane strain (5%).

In this study, the effects of thermal cycling combined with mechanical loading on the microcracking of M40J/PMR-II-50 are investigated. Characterization of the failure mechanisms are conducted based on the critical parameters which cause composite microcracking, as presented in Part I. Based on the test results in Part I, the tests with intermediate in-plane lamina strain (0.175—0.350%) and an increased number of thermal cycles are added. Elevated temperature thermal cycling (23—250 ° C) is also added to the original test plan to investigate the thermal cycling temperature amplitude effect on microcracking of the composites. Observations indicate that the elevated temperature exposure under mechanical loads causes an easy fiber/matrix debonding. Subsequent exposure to cryogenic temperatures results in fiber/matrix debonding due to the high thermal stresses associated with fiber/ matrix thermal expansion mismatch. Crack propagation under cryogenic exposures is shown to be dominant with an increasing number of thermal cycles, especially when combined with high temperature exposure associated with high amplitude of cyclic thermal stresses.

Thermal residual stresses, internal pressure stresses, and acceleration stresses during launch were evaluated and quantified for cryogenic composite fuel tank design. Both failure initiation and progression of graphite/epoxy laminate system (IM7/977-2) [0/90/90/0/0/90](s) and graphite/BMI laminate system (IM7/5250-4) [0/90/90/0/0/90](s) were investigated using the non-isothermal classical laminate and plate theory (CLPT) and the maximum stress failure criterion. The thermal residual stresses in the transverse direction are the dominant stresses on each ply in the launch stage. After initial ply cracking, through-the-thickness temperature change of a laminate related to fuel leakage as well as a laminate stiffness matrix change was applied to the progressive failure analysis. The fuel leakage-based progressive analysis shows a higher number of initial ply cracking does not necessarily mean a higher chance of matrix cracking in all plies. The graphite/BMI laminate has such an advantage as transverse thermo-mechanical resistance over the graphite/epoxy laminate at an initial exposure to -253 degrees C and 1500kPa. In terms of complete laminate matrix cracking, however, the graphite/epoxy laminate is more resistant to transferring stresses to other plies than the graphite/BMI laminate.

Structure-thermal property interrelationships are characterized and reported for organic/inorganic addition cure polyimide composite matrices based on 3,3',4,4'-benzophenone tetracarboxylic dianhydricle, the reactive terminal group 4-phenylethynyl phthalic anhydride, and stoichiometric controlled diamine ratios of 1,3-phenylenediamine, 1,4-phenylenediamine, or 4,4'-(1,3-phenylenediisopropylidene) bisaniline, combined with bis(p-aminophenoxy) dimethyl silane or an alpha, co-bis(3-aminopropyl) polydimethylsiloxane oligomer. Polymerization of monomer reactants resin solutions, carbon fiber prepregs and composites, and imidized oligomers are characterized to relate molecular chemical structure and morphology to glass transition temperature, processing characteristics, thermodynamic properties, and thermal stability. Glass transition temperature, thermal decomposition temperature, and char yield were found to increase with increasing siloxane block length in the imide backbone. As the concentration of inorganic component in the imide oligomer backbone increased, the cured glass transition temperature decreased. Char yield and thermal decomposition temperature were observed to decrease as the inorganic component concentration increased. Incorporation of bis(p-aminophenoxy) dimethyl silane into the imide oligomer structure did not provide any significant advantages over traditional polyimides relative to thermal properties or composite processing, but aminosiloxanes improved composite toughness, prepreg tack, and composite processability. (c) 2007 Wiley Periodicals, Inc.

Two phenylethynyl phthalic anhydride-capped imide oligomers, AFR-PEPA-2 and AFR-PEPA-8, with molecular weights of 1601 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 degrees C, AFR-PEPA-2 and AFR-PEPA-8 have for T(g)s of 370 and 358 degrees C, respectively. AFR-PEPA-N oligo- AL mers demonstrated lower minimum complex melt viscosities than PETI-5 due to the presence of CF3 group in the backbone structure. 1601 g (.) mol(-1) AFR-PEPA-2 imide Af oligomer has a complex melt viscosity of 10 Pa-s at f 340 degrees C, and 4 699 g (.) mol(-1) AFR-PEPA-8 imide oligomer has a complex melt viscosity of 227 Pa-s at 371 degrees 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 degrees C.

The thermal cure reactions of phenylethynyl terminated AFR-PEPA-4 oligomer and a model compound N-phenyl-[4-(phenylethynyl) phthalimide] were investigated. The kinetics analysis of the thermal cure of AFR-PEPA-4 was determined using DSC, with modified DiBenedetto equation. The activation energy of thermal cure reaction of AFR-PEPA-4 oligomer is 34.1 kcal/mol with the kinetic order of one, when the reaction conversion is less than 80%. The activation energy of thermal reaction of N-phenyl-[4-(phenylethynyl) phthalimide] is 41.5 kcal/mol with the kinetic order of 0.95. The cure reaction of AFR-PEPA-4 imide oligomer can be described as a fast first-order reaction stage for the formation of polyenes followed by a slow diffusion-controlled crosslinking reaction stage. (c) 2006 Wiley Periodicals, Inc.

Thermal analysis of phenylethynyl end-capped imide oligomer AFR-PEPA-4 was performed to characterize cure reaction, thermal stabilities and semicrystalline behavior of AFR-PEPA-4 oligomer and its cured polyimide. Cured AFR-PEPA-4 polyimide showed high T g s up to 418°C. Both AFR-PEPA-4 oligomer and polyimide exhibit excellent thermal stabilities comparable to PETI-5 polyimides. AFR-PEPA-4 imide oligomer has a T m of 330°C and exhibits spherulite crystalline morphology in the film. The crystallinity in AFR-PEPA-4 films could not be regenerated under any annealing conditions after the initial melt.

The characterization of electron beam (E-beam) curing of diglycidyl-ether of bisphenol A-diaryliodonium hexafluoroantimonate epoxy resin-initiator system is reported as a function of (i) diaryliodonium hexafluoroantimonate catalyst (initiator) concentrations of 0.1-10 parts per hundred (phr) and (ii) total electron beam doses of 5-150 kilogray (kGy). The in situ E-beam temperature of the resin is monitored as a function of dose-time characteristics. The degree of cure is monitored after radiation exposure by Fourier transform infrared spectrometry (FTIR) and the glass transition temperatures (Tg) by differential scanning calorimetry (DSC). The degree of cure and cure rate increased with total dose exposure and initiator concentration. The maximum cure rate occurred at 5 kGy exposure and, thereafter, decreased as reactive species concentration decreased. The maximum in situ E-beam temperature of 76°C was recorded for the resin containing 10 phr of initiator, with a maximum degree of cure of 94% and a glass transition temperature of 86 C, indicating that the cure reactions under E-beam are glassy state diffusion controlled. The resin glass transition temperatures are considerably lower than the thermally cured glass transition temperatures of 170 C because of H2O termination reactions at the lower E-beam cure temperatures that result in a poor cross-linked network. In addition, the diaryliodonium hexafluoroantimonate catalytic activity for epoxide cationic polymerization is retarded by H2O. E-beam exposure causes the diaryliodonium hexafluoroantimonate to dissociate into active catalytic species, such as HSbF6, well below 100°C compared to catalytic thermal induced dissociation near 200°C. The E-beam cure reaction rate is modeled as a function of degree of cure and dose exposure by a standard autocatalytic kinetic model.

A novel conduction heating apparatus that combines thermal loading with large deflection bending is introduced, and its effectiveness for thermo-mechanical stress analysis was investigated. By clamping composite specimens (M40J/PMR-II-50, [0,90]s, a uni-tape cross-ply) on the radial sides of half cylinders having two different radii (78.74mm and 37.96mm), three different in-plane strains including a no strain condition were applied to the composites. Three different thermal loading experiments, 1) 23°C to −196°C to 250°C, 2) 23°C to 250°C, and 3) 23°C to −196°C were performed as a function of mechanical in-plane strain levels. The apparatus was excellent enough to generate cracks related to the in-plane stresses (or strains) on plies. The quadratic failure criteria solution based on the thermal residual stresses shows a good agreement with the experimental results at low temperatures, but does not supply a good agreement at high temperatures. A weak adhesion of fiber/matrix interface at high temperatures (250°C) might cause the de-bonding at the interface and subsequent exposure to −196°C caused the intensive crack propagation.

The objective of this research was to determine the effect of thermal cycling on the development of microcracks in bismaleimide (BMI)-carbon fiber composites (5250-4 RTM/IM7 4-harness satin weave fabric). By clamping composite specimens on the radial sides of half cylinders having two different radii (78.74 and 37.96 mm), two different strain conditions with respect to the neutral axis (0.406 to 0.406% and -0.843 to 0.843%) were applied to the composites. Three different thermal cycling experiments: (1) -196 to 250°C, (2) 23°C to (i) 150°C, (ii) 200°C, (iii) 250°C, and (3) -196 to 23°C were performed as a function of stress, number of thermal cycles, heating or cooling rate, and humidity conditions. An in situ monitoring microscope was used to observe the microcrack development during the experiment. The results suggest that there is a higher probability of microcracking with increasing number of thermocycles, higher prestrain, and humidity. The principle findings are that the full cycles from 196 up to 250°C cause the most significant microcrack development. Observations indicate that the high-temperature portion of the cycle under load causes fiber–matrix interface failure. Subsequent exposure to higher stresses in the cryogenic temperature region results in composite matrix microcracking due to the additional stresses associated with the fiber–matrix thermal expansion mismatch.

The hygrothermal model has been developed to predict the temperature evolution, epoxy conversion ratio, the glass transition temperature increase and associated resin yield stress rise, and absorbed moisture vapor pressure in ISbF 6 -catalyzed DGEBA epoxy resin systems during e-beam-induced polymerization on the effects of four different initiator concentrations: 0.1,1,3, and 10 phr. This paper summarizes the modeling procedure of e-beam-induced cure kinetics, and the results of local temperature and degree of cure rise within the sample as a function of dose. Dynamic characteristics of Tg y Pv rise and their modeling procedures together with an attempt at optimizing e-beam cure process will be presented in a subsequent paper. These papers provide a methodology to generate the overall integrated model for the e-beam, fast cure process of composites and the consequences upon process control in terms of thermal control, moisture-induced void elimination, and minimization of inherent composite processing stresses that had not been previously addressed by composite process model studies. An autocatalytic model was chosen to describe the cure kinetics for this study and provided excellent agreement with experimental results. The numerical results showed that the cure reaction of ISbF 6 –DGEBA epoxy resin is diffusion controlled, but long lived reactive species allowed for measurable increase in conversion after e-beam irradiation (postcure effects). Higher initiator concentration results in higher degree of cure at a specific dose, causing higher temperature rise and larger temperature and conversion ratio gradients within the sample under given experimental conditions.

Electron-beam (E-beam) curing of 4,4'-bis-maleimidodiphenylmethane (BMPM)BMI-1,3-tolyl/o,o'-diallylbisphenol A (DABPA)-based bismaleimide (BMI) systems and their mixing, with various reactive diluents, such as N-vinylpyrrolidone (NVP) and styrene, were investigated to elucidate how temperature, electron-beam dosage, and diluent concentration affect the cure extent. The effect of free-radical initiator on the cure reactions was also studied. It was found that low-intensity E-beam exposures cannot cause the polymerization of BMI. High-intensity E-beam exposures give high reaction conversion attributed to a high temperature increase, which induced thermal curing. It was shown that the dilution and activation of NVP in BMI cause a more complete BMI cure reaction under E-beam radiation. BMI/NVP can be initiated easily by low-intensity E-beam without thermal curing. FTIR studies indicate that about 70% of the reaction is complete for BMI/NVP with 200 kGy dosage exposure at 10 kGy per pass. The sample temperature only reaches about 75degreesC. The free-radical initiator, dicumyl peroxide, can accelerate the reaction rate at the beginning of E-beam exposure, but does not affect the final reaction conversion. The increase of the concentration of NVP in the BMI/NVP systems increases the reactive conversions almost linearly. (C) 2004 Wiley Periodicals, Inc.

A hygrothermal thermoset resin-cure model has been developed, in conjunction with experimental data, for the e-beam processing of composites in terms of e-beam dose-time sequences in order to achieve full resin cure, minimum residual stresses and avoidance of moisture-induced cavitation within the resin system. This paper, as Part II of this study, summarizes dynamic characteristics of Tg y Pv rise and their modeling procedures together with an attempt at optimizing the e-beam-cure process. The modeling procedure of e-beam-induced cure kinetics, and the results of local temperature and degree of cure rise within the sample as a function of dose were presented in Part I.

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 140-400degreesC using IR-, H-1- and C-13-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 (C) 2003 John Wiley Sons, Ltd.

Future space transportation systems require single stage to orbit (SSTO) launch vehicles that are reusable. Such vehicles require lightweight, tough, low cost structures of which the health and safety are well characterized. The utilization of new, high performance polymeric matrix fibrous composites (PMFC) for the liquid hydrogen (LH2) and liquid oxygen (LOX) cryogenic fuel tank containment structures will allow critical weight reduction needed for the SSTO launch vehicles. We will present our studies to develop and characterize non-autoclave processes for the production of such cryogenic liquid containment composite structures.Our studies on the relations between the processing parameters, the resultant physical and chemical structure and the performance of Electron beam (E-beam) cured PMFC's will be described. The areas we will present are:(1) Processing of composites by E-beam curing that addresses experimental investigation of processing parameters. Development of a stress chemo-hygro-thermomechanical cure model.(2) Identification, characterization and development of new, innovative non-autoclave composite fabrication processes.(3) Characterization of composite cryogenic containment performance and damage formation mechanisms.(4) Materials developed for enhanced cryogenic damage resistance. The primary materials development issue is to reduce the DeltaT between the composite processing cure temperature and cryogenic service environment temperature in order to minimize composite thermal expansion mismatch stress build-up and associated microcrack development. We will present our studies to develop solid-state, low temperature, radiation curable resins that will allow a decrease in DeltaT, which at present is near 500degreesC.