Teddy M. Keller

Phthalonitrile cure reaction with aromatic diamines

Phthalonitrile monomers can be polymerized thermally in the presence of small amounts of curing agents into thermosetting polymers. The thermosets exhibit outstanding thermo-oxidative stability, display good mechanical properties, and offer promise as matrices for composite applications. The phthalonitrile cure reaction is typically accomplished with an aromatic diamine, 1,3-bis(3-aminophenoxy)benzene (m-APB), added in the range of 1.5–2% by weight of the monomer in the melt phase. This article addresses the cure reaction with a sulfone-containing diamine, bis[4-(4-aminophenoxy)phenyl] sulfone (p-BAPS), which shows lower volatility as determined from thermogravimetric studies (TGA) compared to m-APB at the processing temperatures typically employed for phthalonitrile cures. Rheometric studies conducted to monitor the viscosity increase during a cure reaction suggest that the cure reaction with m-APB is faster compared to the reaction with p-BAPS. Even though differences are seen in the initial cure rates, the final cured products are similar in terms of the glass transition temperatures and thermal and oxidative stabilities.

Phthalonitrile polymers: Cure behavior and properties

This article compares the cure behavior and properties of phthalonitrile polymers derived from three different monomers, namely, 4,4′-bis(3,4-dicyanophenoxy)biphenyl, 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane and 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane. Rheometric measurements with monomer melt in the presence of an aromatic diamine curing agent reveal that the rate of the cure reaction differs for the three monomers. The rate is dependent on the concentration of the curing agent. The glass transition temperature advances with increasing extent of cure and disappears upon postcure at temperatures in excess of 350°C. Based on thermogravimetric analysis, the thermal stability of all three polymers are comparable, whereas the fluorine-containing resin shows the best oxidative stability at elevated temperatures. Microscale calorimetric studies on all three polymers reveal that the char yields are high and the total heat release upon exposure to 50 kW/m2 flux for each polymer is low, compared to other thermosets. Flexural strength ranges between 80–120 MPa. The water uptake under ambient conditions is less than 3% by weight after submersion in water for seven months.

Amine-Cured Bisphenol-Linked Phthalonitrile Resins

Abstract Neat polymerization of bisphenol-linked phthalonitrile monomers, which contain no active hydrogen atoms, is extremely difficult and requires several days of continuous heating at 260–290°C before a viscosity increase becomes evident. In the presence of a nucleophilic compound such as an organic amine, the cure time and temperature can be greatly reduced. The amine-cured polymers are more thermally stable than their neat cured counterparts. Preliminary results indicate that the amine causes no significant changes in the mechanical properties of the cured polymer. The bisphenol-linked phthalonitrile resins are particularly appealing as matrices for composite formulations due to the projected low material cost, the greatly improved processability, and the nonreactivity of the prepolymer at ambient temperature.

Low-melting Phthalonitrile Oligomers: Preparation, Polymerization and Polymer Properties:

A series of low-melting phthalonitrile oligomers were prepared in which variable-length multiple aromatic ether linkages interconnect the terminal phthalonitrile units. These materials were designed to address the need for a processable resin system with good high-temperature properties. The melt-processable oligomers are obtained using a modified-Ullman ether reaction between a bisphenol and a dihalobenzene to form a hydroxyl-terminated oligomeric intermediate that is endcapped by reaction with 4-nitrophthalonitrile. Viscosity measurements show that the phthalonitrile oligomers are polymerized at a moderate temperature (200°C) using the typical aromatic diamine curing additives, bis[4-(4-aminophenoxy)phenyl]sulfone and 1,3-bis(3-aminophenoxy)benzene. The oligomeric phthalonitrile/diamine mixtures exhibit a low complex melt viscosity (0.01-0.1 Pa s) at 200°C. Differential scanning calorimetric analysis is used to follow the polymerization as the oligomeric phthalonitrile/diamine mixtures are heated to elevated temperatures. Thermal and dynamic mechanical properties of thermally-cured oligomeric phthalonitrile polymers are systematically evaluated and compared with those of two other high temperature thermosetting phthalonitrile polymers, 4,4~-bis(3,4-dicyanophenoxy)biphenyl and 1,3- bis(3,4-dicyanophenoxy)benzene. After thermal treatment at 425°C for 8 h, the oligomeric phthalonitrile polymers exhibit char yields of 70% when heated to 1000°C in flowing nitrogen and decomposition temperatures in excess of 500°C when heated in either flowing nitrogen or air. Rheometric measurements indicate that the fully cured oligomeric phthalonitrile polymers do not soften or exhibit a glass transition temperature upon heating to 450°C. Overall, studies on the phthalonitrile oligomers and the corresponding polymers reveal an attractive combination of processability, thermal and thermo-oxidative stability and good dynamic mechanical properties for these materials

Tensile and fracture properties of a phthalonitrile polymer

Abstract The properties of a polymer prepared from an aromatic diether-linked phthalonitrile resin, 4,4′-bis(3,4-dicyanophenoxy)biphenyl, and cured with 1,3-bis(3-amino phenoxy) benzene are reported. The resin is easily processed from the melt of the monomer in a controlled manner as a function of the amine content and processing conditions. The resulting polymers show an excellent retention of mechanical properties following inert atmosphere post-cure to 375°C and oxidative ageing at 315°C for 100 h. Results are reported for the effects of cure, post-cure and ageing conditions on the tensile and fracture properties of these polymers.

Linear diacetylene polymers containing bis(dimethylsilyl) phenyl and/or bis‐(tetramethyldisiloxane) carborane residues: Their synthesis, characterization and thermal and oxidative properties

A series of inorganic-organic linear diacetylenic hybrid polymers (5a–e) were prepared by the polycondensation reaction of 1,4-dilithiobutadiyne with 1,4-bis(dimethylchlorosilyl)benzene and/or 1,7-bis(tetramethylchlorodisiloxane)-m-carborane. Their structures were characterized using FTIR, and 13C and 1H NMR spectroscopies, and their thermal and oxidative properties were evaluated by DSC and TGA analyses. The hybrid polymers exhibited solubility in common organic solvents and were viscous liquids or low melting solids at room temperature. Broad prominent exotherms, attributed to reaction of the diacetylenic units, were observed by DSC in the 306°C to 354°C temperature range. When 5a–e were analyzed by TGA to 1000°C under nitrogen, weight retention between 79 and 86% were obtained. Ageing studies, performed at elevated temperatures in air on a thermoset and a ceramic obtained from polymer 5b, showed this system to have excellent thermal and oxidative stability.

Oxidative protection of carbon fibers with poly(carborane–siloxane–acetylene)

Abstract Linear carborane–siloxane–acetylenic polymers have been synthesized as precursor materials for thermosets and ceramics for composite applications up to 500 and 1500°C, respectively, in an oxidizing environment. The novel linear polymers have the advantage of being extremely easy to process and convert into thermosets or ceramics since they are either liquids at room temperature or low melting solids and are soluble in most organic solvents. Carbon fibers coated with poly(carborane–siloxane–acetylene) forms a protective barrier against oxidation at elevated temperatures. The novel polymer when used as a matrix material (ceramic) was found to protect the carbon fibers from oxidative breakdown. Boron appears to be the key to the unique oxidative stability of the composite compositions. Oxidative studies were performed by thermogravimetric analyses.

Thermally and oxidatively stable thermosets derived from preceramic monomers

Monomers 1,3-bis(4-phenylethynylphenyl)tetramethyldisiloxane and 1,7-bis(4-phenylethynylphenyltetramethyldisiloxyl)-m-carborane were synthesized and compared with bis(4-phenylethynylphenyl)dimethylsilane as potential preceramic precursors. These monomers were heated to free flowing liquids above 100°C and thermally polymerized above 300°C to form heat-resistant thermosets or ceramic residues. The ceramic yields for the silane (13%) and siloxane (30%) were much lower than that for the carborane (64%) monomer. The thermoset and ceramic made from the carborane monomer were the best thermally and oxidatively stable materials. After curing, the thermoset had a weight loss of only 6% and after pyrolysis, the ceramic residue had no additional weight loss up to 1000°C in air.

Synthesis and characterization of multiple phenylethynylbenzenes via cross-coupling with activated palladium catalyst

Abstract Several compounds containing multiple phenylethynyl substituents were prepared and polymerized to thermosets through the acetylenic units. Pyrolysis of the polymers afforded high carbon yields. The acetylenic-based compounds were synthesized from the palladium-catalysed coupling reaction of phenyl-acetylene with multiple-brominated benzenes. Inconsistencies were observed in the reaction yields with time, which were attributed to poisoning of the palladium. A procedure was developed for the activation of palladium using magnesium. The activated palladium gave consistent high yield of cross-coupled products.

Cure kinetics of a multisubstituted acetylenic monomer

Abstract A polyfunctional arylacetylenic monomer, 1,2,4-tris(phenylethynyl)benzene, thermally polymerizes by a free-radical mechanism to a highly crosslinked structure of interest as a precursor matrix for carbon/carbon composites. The polymerization reaction was characterized by Fourier transform infra-red spectroscopy and differential scanning calorimetry (d.s.c.). The disappearance of the acetylenic stretching band at 2212 cm −1 was used successfully to monitor the cure reaction. The cure reaction follows first-order kinetics with an overall activation energy of 135 kJ mol −1 . Dynamic d.s.c. analysis carried out to estimate the activation energy of the cure reaction yields a value of 137 kJ mol −1 .