Brian J. Jensen

Poly(arylene ethers)

Abstract Several new arylene ether homopolymers and copolymers have been prepared by the nucleophilic displacement of aromatic dihalides with aromatic potassium bisphenates. Polymer glass transition temperatures ranged from 114 to 310°C and a few of the polymers were semicrystalline. Two ethynylterminated poly(arylene ethers) were synthesized by reacting hydroxy-terminated oligomers with 4-ethynylbenzoyl chloride. Heat induced reaction of the acetylenic groups provided materials with good solvent resistance. The chemistry, physical and mechanical properties of the polymers are discussed.

Effect of Molecular Weight on Processing and Adhesive Properties of the Phenylethynyl-Terminated Polyimide LARC™-PETI-5

Abstract Three different molecular weight versions of the phenylethynyl-terminated polyimide LARC™-PETI-5 were synthesized. The materials synthesized had theoretical number average molecular weights of 2500, 5000, and 10000 g mol. Differential Scanning Calorimetry (DSC) was performed on the dry powder form of these materials to establish cure conditions which result in high glass transition temperatures. Lap shear specimens were prepared from adhesive tape made from each material and with the thermal cure conditions determined from the DSC data. The tensile shear data established which processing conditions provided the best adhesive strengths. Titanium tensile shear strengths as high as 52.6 MPa (7630 psi) at RT and 35.2 MPa (5100 psi) at 177°C were determined. Processing temperatures as low as 316°C and pressures as low as 0.17 MPa (25 psi) resulted in good adhesive properties. The tensile shear properties of these materials were unaffected by hydraulic fluid. The molecular weight of LARC™-PETI-5 has an...

Fiber optic sensors for health monitoring of morphing airframes: I. Bragg grating strain and temperature sensor

Fiber optic sensors are being developed for health monitoring of future aircraft. Aircraft health monitoring involves the use of strain, temperature, vibration and chemical sensors to infer integrity of the aircraft structure. Part 1 of this two part series describes sensors that will measure load and temperature signatures of these structures. In some cases a single fiber may be used for measuring these parameters. Part 2 will describe techniques for using optical fibers to monitor composite cure in real time during manufacture and to monitor in-service integrity of composite structures using a single fiber optic sensor capable of measuring multiple chemical and physical parameters. The facilities for fabricating optical fiber and associated sensors and the methods of demodulating Bragg gratings for strain measurement will be described.

Chemistry and properties of a phenylethynyl-terminated polyimide

As part of an ongoing effort to develop processable, high-performance resins for aerospace applications, a phenylethynyl-terminated imide (PETI) oligomer designated LaRC MT PETI-1 was developed. This reactive oligomer has a number-average molecular weight of 6300 g/mol and a T g of 218°C. Upon curing the reactive oligomer at 350°C for 1 h, a tough material with a T g of 249°C was obtained. The properties of cured PETI-1 in the form of composites, adhesive specimens, thin films, and neat resin moldings are excellent. The synthesis, characterization, and mechanical properties of this polyimide are discussed.

Polyimides with pendent ethynyl groups

Abstract Three novel diamines containing pendent ethynyl, hexynyl and phenylethynyl groups were synthesized and used to prepare polymers and copolymers. When heated to 250–350°C, the ethynyl groups react to form crosslinks. After curing, the homopolymers were brittle owing to the high crosslink density. Copolymers prepared using 10mol% ethynyl-containing diamine and 90mol% of diamines with no ethynyl groups were relatively tough, forming creasible films. After a thermal cure, the copolymers became insoluble and exhibited high Tg values and good thin film properties.

Adhesive Properties of Cured Phenylethynyl-Terminated Imide Oligomers

Abstract As part of a program to develop structural adhesives for high performance aerospace applications, phenylethynyl-terminated imide oligomers are under evaluation. Imide oligomers with different molecular weights and compositions endcapped with either 4-(3-aminophenoxy)-4′-phenylethynylbenzophenone (3-APEB) or 4-phenylethynylphthalic anhydride (PEPA) have been prepared and characterized. These oligomers exhibit excellent processability. After heating to 350°C for 1 hour, the terminal phenylethynyl groups have reacted to provide chain extension, branching and crosslinking. The cured polymers exhibit excellent solvent resistance and high mechanical properties as neat resins and in various adhesive forms (tensile shear, sandwich flatwise tension and climbing drum peel specimens). The chemistry and properties of these phenylethynyl-terminated imide oligomers are discussed.

Synthesis and Characterization of Modified Phenylethynyl Imides

As an ongoing effort to develop structural adhesives for high-performance aerospace applications, recent work has focused on phenylethynyl terminated imide (PETI) oligomers. The work reported herein involves the synthesis and characterization of a series of phenylethynyl containing oligomers designated LARC™ MPEI (modified phenylethynyl imide). These oligomers presumably contain mixtures of linear, branched and star-shaped molecules. The fully imidized polymers exhibited minimum melt viscosities as low as 600 poise at 335 °C, significantly lower than equivalent molecular weight linear materials. Ti/Ti lap shear specimens processed at 288 °C under 15 psi showed tensile shear strengths as high as ∼6000 psi and ∼5200 psi at ambient temperature and 177 °C respectively. The chemistry and properties of these new MPEIs are presented and compared with an optimized linear PETI, LARC™ PETI-5.

Crosslinking in Phenylethynyl-Terminated Polyimides

Stress relaxation of cured phenylethynyl-terminated polyimides was used to estimate their equilibrium rubbery moduli. The corresponding concentrations of elastically effective chains derived from classical rubber elasticity theory were less than one-fifth what would be expected if every chain end participated in a crosslink. A model is proposed to allow the calculation of the average crosslink functionalities.

High Temperature VARTM of Phenylethynyl Terminated Imides

Depending on the part type and quantity, fabrication of composite structures using vacuum-assisted resin transfer molding (VARTM) can be more affordable than conventional autoclave techniques. Recent efforts have focused on adapting VARTM for the fabrication of high temperature composites. Due to their low melt viscosity and long melt stability, certain phenylethynyl terminated imides (PETI) can be processed into composites using high temperature VARTM (HT-VARTM). However, one of the disadvantages of the current HT-VARTM resin systems has been the high porosity of the resultant composites. For aerospace applications, the desired void fraction of less than 2% has not yet been achieved. In the current study, two PETI resins, LaRC PETI-330 and LaRC PETI-8 have been used to make test specimens using HT-VARTM. The resins were infused into ten layers of IM7-6K carbon fiber 5-harness satin fabric at 260 or 280 °C and cured at temperature up to 371 °C. Initial runs yielded composites with high void content, typically greater than 7% by weight. A thermogravimetric-mass spectroscopic study was conducted to determine the source of volatiles leading to high porosity. It was determined that under the thermal cycle used for laminate fabrication, the phenylethynyl endcap was undergoing degradation leading to volatile evolution. This finding was unexpected as high quality composite laminates have been fabricated under higher pressures using these resin systems. The amount of weight loss experienced during the thermal cycle was only about 1% by weight, but this led to a significant amount of volatiles in a closed system. By modifying the thermal cycle used in laminate fabrication, the void content was significantly reduced (typically ∼ 3% or less). The results of this work are presented herein.

Modulated FT-Raman Fiber-Optic Spectroscopy: A Technique for Remotely Monitoring High-Temperature Reactions in Real-Time.

A modification to a commercial FT-Raman spectrometer is presented for the elimination of thermal backgrounds in FT-Raman spectra. The modification involves the use of a mechanical chopper to modulate the CW laser, remote collection of the signal via fiber optics, and connection of a dual-phase digital signal processor lock-in amplifier between the detector and the spectrometers collection electronics to demodulate and filter the optical signals. The resulting modulated FT-Raman fiber-optic spectrometer is capable of completely eliminating thermal backgrounds at temperatures exceeding 370 °C. In addition, the signal/noise of generated Raman spectra is greater than for spectra collected with the conventional FT-Raman under identical conditions and incident laser power. This is true for both room-temperature and hot samples. The method allows collection of data using preexisting spectrometer software. The total cost of the modification (excluding fiber optics) is ∼