Daniel A. Scola

A study of the thermal cure of a phenylethynyl-terminated imide model compound and a phenylethynyl-terminated imide oligomer (PETI-5)

The kinetic mechanism of the thermal cure of a phenylethynyl-terminated imide model compound, 3,4′-bis[(4-phenylethynyl)phthalimido]diphenyl ether (PEPA-3,4′-ODA) and a phenylethynyl-terminated imide oligomer PETI-5 (MW 5000 g/mol) was studied. FTIR was used to follow the cure of the model compound, while thermal analyses (DSC) was used to follow the cure of the PETI-5 oligomer. The changes in IR absorbance of phenylethynyl triple bonds at 2214 cm−1 of PEPA-3,4′-ODA as a function of cure time were detected at 318, 336, 355, and 373°C, respectively. The changes in the glass transition temperature, Tg, of PETI-5 as a function of time were measured at 350, 360, 370, 380, and 390°C, respectively. The DiBenedetto equation was applied to define the relative extent of cure, x, of the PETI-5 oligomer by Tg. For the model compound, the reaction followed first order kinetics, yielding an activation energy of 40.7 kcal/mol as determined by infrared spectroscopy. For PETI-5, the reaction followed 1.5th order, yielding an activation energy of 33.8 kcal/mol for the whole cure reaction, as determined by Tg using the DiBenedetto method. However, the cure process of PETI-5 just below 90% by this method followed first-order kinetics yielding an activation energy of 37.2 kcal/mol.

Microwave syntheses of poly(ε-caprolactam-co-ε-caprolactone)

Microwave irradiation was applied to synthesize poly(e-caprolactam-co-e-caprolactone) directly from the anionic catalyzed ring opening of two cyclic monomers, e-caprolactam and e-caprolactone using a variable frequency microwave furnace, programmed to a set temperature and controlled by a pulsed power on–off system. Dielectric properties of e-caprolactam, e-caprolactone, and their mixture were measured in the microwave range from 0.4 to 3 GHz, showing that both e-caprolactam and e-caprolactone exhibited effective absorption of microwave energy to induce a fast chemical reaction. The microwave induced anionic copolymerization of e-caprolactam and e-caprolactone generated copoly(amide-ester)s in yields as high as 70%. Conventional thermal and microwave copolymerization studies were also conducted for comparison with the microwave results. These studies demonstrated that an effective and efficient microwave method to copolymerize e-caprolactam with e-caprolactone in higher yield, higher amide content, and higher Tg s, relative to the thermal process, has been developed.

Kinetics of the crosslinking reaction of a bisnadimide model compound in thermal and microwave cure processes

The kinetic studies of the crosslinking reaction of a nadic end-capped imide model compound, N,N′-(oxydi-3,4′-phenylene) bis(5-norbornene-2,3-dicarboximide), a bisnadimide, in thermal and microwave processes were investigated. The conversion of the endo isomer to exo isomer proceeds at a much lower temperature than the crosslinking reaction. The crosslinking reaction was monitored by the combined decrease in the infrared absorptions of the endo and exo isomers at 840 and 780 cm−1, respectively. The decrease in the concentration of starting materials follows first-order kinetics in the thermal and microwave processes. At the same temperatures (230 or 280°C), the crosslinking reaction proceeds at about 10 times faster in the microwave process than in the thermal process. Solid-state 13C-NMR showed no significant loss in C=C double bond resonance in the cured products by comparison with the starting material. This study provides direct evidence that the microwave process may be an efficient method to cure nadic end-capped polyimides.

Microwave irradiation of nadic‐end‐capped polyimide resin (RP‐46) and glass–graphite–RP‐46 composites: cure and process studies

Microwave energy was investigated to cure nadic-end-capped polyimide precursors (RP-46 resin) using a Cober Electronics Model LBM 1.2A/7703 microwave oven at a frequency of 2.45 GHz. Both neat resin samples and glass cloth and hybrid glass cloth–graphite cloth–RP-46 resin composites were studied. For the resin studies, the effect of various parameters, such as power level, sample size, processing temperature, time, and graphite fiber absorber, were investigated. The variables investigated with the composite study were the power level, mold material, vacuum, and low pressure. The results showed that microwave energy was effective in curing both neat resin samples and composite specimens. The presence of a small quantity of absorber (chopped carbon fiber) accelerates the cure dramatically. Moreover, soapstone mold material was found to be an efficient absorber for glass and glass–graphite composite processing, causing an effective cure in less than 1 h. Glass and glass–graphite hybrid composites with flexural strengths of 372–588 MPa (54–85 ksi) and moduli of 28.7–31.7 GPa (4.2–4.6 Msi) have been fabricated. This is equivalent to 50 to 80% of the properties of composites fabricated by conventional means.

Investigation of microwave energy to cure carbon fiber reinforced phenylethynyl-terminated polyimide composites, PETI-5/IM7

The application of microwave energy to the processing of carbon fiber reinforced phenylethynyl-terminated polyimide composites (PETI-5/IM7) was investigated and evaluated with a variable-frequency microwave furnace. The thermal and physical properties of the composites were measured by dynamic mechanical thermal analysis, thermogravimetric analysis, thermomechanical analysis, and density and composition tests. The mechanical properties were determined by 3-point-bending and short-beam-shear tests at both room temperature and 177 °C. The shear failure surfaces of both microwave- and thermally cured composites were detected with environmental scanning electron microscopy. A comparison of the thermal and microwave processes was conducted to evaluate the advantage of the microwave process. Microwave-cured composites, fabricated under various pressures at the fixed process temperatures, also were investigated. From these studies, it was concluded that microwave energy successfully was used to fabricate PETI-5/IM7 composites with higher glass-transition temperatures (by 11–16 °C) and higher retention in flexural strength, flexural modulus, and shear strength at 177 °C than those fabricated by the thermal process. Furthermore, the microwave processes required only half the time used for the standard thermal process.

A study of the kinetics of the microwave cure of a phenylethynyl-terminated imide model compound and imide oligomer (PETI-5)

The kinetic mechanism of the microwave cure of a simple phenylethynyl-terminated imide model compound, 3,4′-bis[(4-phenylethynyl)phthalimido]diphenyl ether (PEPA-3,4′-ODA) and a phenylethynyl-terminated imide oligomer (PETI-5, Mn 5000 g/mol) was studied. Dielectric properties of the model compound and PETI-5 were measured in the microwave range from 0.4 GHz to 3 GHz. FTIR was used to follow the cure of the model compound (PEPA-3,4′-ODA), while thermal analysis (DSC) was used to follow the cure of the PETI-5 oligomer. The changes in room temperature IR absorbance of phenylethynyl triple bonds at 2214 cm−1 of PEPA-3,4′-ODA as a function of cure time were measured after cure temperatures of 300, 310, 320, and 330 °C. The changes in the glass-transition temperature, Tg, of PETI-5 as a function of cure time were measured after cure at 350, 360, 370, and 380 °C, respectively. The Tg s were determined to calculated the relative extent of cure, x, of the PETI-5 oligomer according to the DiBenedetto equation. For the model compound, the reaction followed first order kinetics, yielding an activation energy of 27.6 kcal/mol as determined by infrared spectroscopy. For PETI-5, the reaction followed 1.5th order, yielding an activation energy of 17.1 kcal/mol for the whole cure reaction, as determined by Tg using the DiBenedetto method.

Modification of PETI-5K Imide Oligomers: Effect on Viscosity

A phenylethynyl (PE) end-capped oligoimide of number average molecular weight Mn 5000, known as PETI-5K was modified by: (1) varying molecular weight; (2) addition of phenylethynyl (PE) end-capped reactive diluents to PETI oligomides; (3) incorporation of 1,4-cyclohexanyl diamine with PETI-5K diamines; (4) incorporation of 2,4,5-triaminopyrimidine with PETI-5K; and (5) replacement of the PETI-5 diamines, 3,4′-ODA and APB with 2,2′-bis [4-(4-amino-phenoxy) phenyl] hexafluoropropane (4-BDAF) to determine the effects on glass transition temperature and minimum melt viscosity. All approaches caused a reduction in complex minimum melt viscosity, but the minimum melt temperatures were only sufficiently low enough for the material to have melt stability for a hot-melt coating application in methods (2), (4), and (5). The reduction in the complex minimum melt viscosity of oligomer prepared by method (4) may be due to unreacted monomers and a lower oligomer molecular weight, Mn , than calculated.

Electrochemical process for preparing continuous graphite fibre-thermoplastic composites

Abstract An electrochemical process for preparing thermoplastic matrix-graphite fibre composites has been developed. Graphite fibre-poly(maleimide- co -styrene) matrix composites were prepared using unsized AS-4 graphite fibre preforms as the working electrode. Copolymers of a wide range of N-substituted maleimides and styrene were formed directly on the surface of the graphite fibres. The rate of electrocopolymerization varied with the electrocopolymerization parameters, such as the comonomer concentration, current density, supporting electrolyte concentration, electropolymerization time, nature of the solvent, solvent/dilute sulfuric acid ratio and the structure of the maleimide. Graphite fibre preforms preimpregnated with the electropolymerized thermoplastic matrices were processed into unidirectional composite laminates. The electrosynthesized thermoplastic matrix composites showed excellent physical and mechanical properties.

Graphite-Epoxy Composites: Effects of An Applied Interphase

An applied interphase, between matrix resin and fibers in continuous fiber composites, resulted in a significant improvement in composite impact strength and fracture toughness concurrently with a moderate improvement or unchanged interlaminar shear strength. The temperature resistance of tailored interphases and composites was high. An optimum thickness was observed. The merits of such interphases and a novel means of application are discussed.

Amino-p-benzoquinone adducts and polymers as adhesion promoters for steel

Abstract Phenolic and quinonoid compounds are widely studied in biological sciences because of their ability to chelate heavy metals like iron and copper and recently have found new applications in synthetic macromolecules. Amino-p-benzoquinone polymers, poly[(2,5-hexamethylenediamino)-1,4-benzoquinone] and poly{[2,5-(2,2′-bistrifluoromethyl)-4,4′-biphenylenediamino]1,4-benzoquinone} were synthesized and evaluated as adhesion promoters for steel/epoxy joints. An improvement in the torsional shear strength of these joints was observed when these polymers were used as adhesion promoters. The durability of the adhesive bond was also improved after boiling water treatment, relative to untreated and silane-treated joints. The improvement in adhesion could be attributed to the formation of a chelate between the polyaminoquinone (PAQ) and the iron surface and a chemical reaction between the PAQ and the epoxy resin. A low molecular weight model compound, bis[2,5-(4-methylanilido)]-1,4-benzoquinone, was also used to study coupling between the epoxy adhesive and the steel surface. Electron spin resonance (ESR), atomic absorption spectroscopy and infrared spectroscopy were used to document the epoxy-coupling agent reaction and the chelate formation.