IM7/LARC™ MPEI-1 Polyimide Composites

LARC™ MPEI-1 (Langley Research Center™ modified phenylethynyl imide-1) phenylethynyl containing aromatic polyimide, is based on the reaction of biphenyl dianhydride (BPDA), 3,4′-oxydianiline (3,4′-ODA), 1,3-bis(3-aminophenoxy)benzene (APB), 2,4,6-triaminopyrimidine (TAP) and 4-phenylethynyl phthalic anhydride (PEPA), presumably resulting in a mixture of linear, branched and star shaped phenylethynyl containing imides which was evaluated as a matrix for high-performance composites. The poly(amid acid) solution of MPEI-1 in N-methypyrrolidinone was synthesized at 35% and 42% solids. Unidirectional prepreg was fabricated from these solutions and Hercules IM7 carbon fibre utilizing NASA-Langley’s multipurpose prepreg machine. The temperature-dependent volatile depletion rates, thermal crystallization behaviour and resin rheology were characterized. Based on this information, a composite moulding cycle was developed which yielded well consolidated, voidfree laminates. Composite mechanical properties such as short beam shear strength, longitudinal and transverse flexural strength and flexural modulus, longitudinal tensile strength and notched and unnotched compression strengths were measured at room temperature (RT) and elevated temperatures. These mechanical properties are compared with those of IM7/LARC™ PETI-5 composites.

Adhesive and composite properties of LARCTM-8515 polyimide

LARCTM-8515 (Langley Research Center-8515) is an aromatic polyimide based on 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and an 85:15 molar ratio of 3,4'-oxydianiline (3,4'-ODA) and 1,3-bis(3-amnophenoxy)benzene (APB). This material is currently under evaluation as an adhesive and composite matrix resin for use in high-performance aerospace applications. The synthesis and development of the copolymer as well as studies of the effect of molecular weight on properties will be discussed. LARCTM-8515 has been evaluated as an adhesive, and titanium to titanium tensile shear strengths will be presented. Unidirectional prepreg was' made utilizing the Langley multipurpose tape machine. and the thermal and rheological properties and the solvent/volatile depletion rates of the poly(amide acid)/NMP resin system were determined. This information was used to successfully design a moulding cycle for composite preparation. Composite laminates were moulded under 200 psi, which consistently yielded good consolidation quality as measured by C-scan, acid digestion and optical photomicrography. Composite mechanical properties measured included short-beam shear strength and flexural strength and modulus at RT, 93, 150 and 177C. and compression strength (ITRI) and open hole compression (OHC) strength at RT and 177 C (wet).

Automated Tow Placed LARC™-PETI-5 Composites

LARC™-PETI-5 is a PhenylEthynyl-Terminated Imide resin developed at NASA Langley Research Center (LARC) during the 1990s. It offers a combination of attractive composite and adhesive properties. IM7/LARC™-PETI-5 composites exhibit thermal and thermo-oxidative stability typical of polyimides, superior chemical resistance and processability, excellent mechanical properties, toughness and damage tolerance. It was selected for study in the High Speed Research program aimed at developing technologies for a future supersonic aircraft, the High Speed Civil Transport, with a projected life span of 60 000 h at a cruise speed up to March 2.4. Robust autoclave processing cycles for LARC™-PETI-5 composites have been thoroughly designed and demonstrated, which involved hand lay-up of solvent-ladened ‘wet’ prepregs. However, this type of processing is not only costly but also environmentally unfriendly. Volatile management and shrinkage could become serious problems in the fabrication of large complex airframe structural subcomponents. Robotic tow placement technology utilizing ‘dry’ material forms represents a new fabrication process which overcomes these deficiencies.

This work evaluates and compares mechanical properties of composites fabricated by heated head automated tow placement (dry process) with those obtained by hand lay-up/autoclave fabrication (wet process). Thermal and rheological properties of the robotically as-placed uncured composites were measured. A post-cure cycle was designed due to the requirement of the PETI-5 resin for a 370 °C/1 h hold to reach full cure, conditions which cannot be duplicated during heated head robotic placement. Mechanical properties such as 0° flexural strength and modulus, open hole tensile and compressive strength and moduli, reduced section compression dogbone compressive strength, and modified zippora-medium small (MZ-MS) tensile and compressive properties were obtained on the post-cured panels. These properties compared favourably with those obtained from the wet process.

IM7/LARC™-IAX-3 Polyimide Composites

LARC™-IAX-3 (Langley Research Center™-improved adhesive experimental resin-3) aromatic polyimide, based on oxydiphthalic anhydride, 3,′4-oxydianiline (3,′4-ODA) and 1,4-phenylenediamine ( p-PDA), was evaluated as a matrix for high-performance composites. Four poly(amide acid) solutions in either N-methypyrrolidone or γ-butyrolactone, end-capped with phthalic anhydride to various theoretical molecular weights, were synthesized. Unidirectional prepreg was fabricated from each of the four resins utilizing NASA-Langley’s multipurpose prepreg machine. The temperature-dependent volatile depletion rates, the thermal crystallization behaviour and the resin rheology were characterized. Based on this information, a composite moulding cycle was developed which consistently yielded well consolidated void-free laminate parts. Composite mechanical properties such as short beam shear strength, longitudinal and transverse flexural strength and flexural modulus, longitudinal tensile strength and notched and unnotched compression strengths were measured at room temperature (RT) and elevated temperatures. Similar properties were obtained independent of the carrier solvent used during matrix resin synthesis. These mechanical properties were superior to those previously measured for IM7/LARC™-IA and IM7/LARC™-IAX composites. The enhanced mechanical properties were attributed to the substitution of 25% 3,′4-ODA by p-PDA in the LARC™-IA imide backbones.

Processing and Properties of IM7/LARC ™ -RP46 Polyimide Composites

LARC™-RP46 resin system is a PMR type polyimide and is prepared by replacing methylenedianiline in the PMR-15 composition with 3,4′-oxydianiline. This resin system retains the same processing characteristics as PMR-15 but also offers enhanced fracture toughness. Rheological measurements were conducted on pre-imidized LARC™-RP46 moulding powder subjected to various ramp and hold temperature schemes. Adequate flow properties were found with theoretical (formulated) molecular weight 6 ≤1500 g mol−1. Critical transition temperatures for optimizing the process cycle were identified. They included the resin softening point, the imidization reaction peak, the isomerization reaction peak and the gelation point. Utilizing this information, 1.72 × 10 6 Pa (250 psi) cure cycles were designed for B-staged (dry) and unstaged (wet) prepregs. Composite laminates were fabricated which exhibited excellent consolidation and a void content below 0.1–0.2% as measured by image analysis.

IM7/LARC™-RP46 exhibited higher composite mechanical properties than IM7/PMR-15. Short-beam shear strength, flexural strength and flexural modulus were measured at room temperature, 93, 150 and 177 °C. Composite engineering properties were also obtained including longitudinal tension, logitudinal compression, interlaminar shear, short block compression, open hole compression (OHC) and compression strength after impact (CAI). Excellent longitudinal tensile and compressive strengths were obtained and the CAI strength was 40% higher than that for PMR-15. Over 80% retention of all RT strengths were noted at 177 °C.

Processing and Properties of IM7/LARC™ -Si Polyimide Composites

LARC™-SI (NASA Langley Research Center-Soluble Imide) is an aromatic thermoplastic polyimide. LARC™-SI is synthesized from equimolar amounts of oxydiphthalic anhydride (ODPA), 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride (BPDA) and the equivalent amount of 3, 4′-oxydianiline (3, 4′-ODA). Phthalic anhydride (PA) was used as an endcapper to control molecular weight. A 30% solid LARC™-SI solution (in NMP/Xylene: 9/1 v/v) with 3% stoichiometric imbalance was made into unidirectional long-fibre-reinforced prepregs. Thermal properties, volatile depletion behaviour and resin rheology were thoroughly characterized. Using this information, two composite moulding cycles were developed that consistently yielded well consolidated, void-free laminates. Composite mechanical properties such as short-beam shear strength, longitudinal and transverse flexural strength and flexural modulus, longitudinal tensile strength and notched and un-notched compression strengths, fracture toughness, open-hole compression strength and compression after impact (CAI) strength were measured at room temperature (RT) and elevated temperatures. LARC™-SI composite exhibited very good toughness and damage tolerance. The interlaminar fracture toughness and the CAI strength were measured at 1.72 kJ m−2 at 35.2 GPa respectively.

IM7/LARCTM-IA polyimide composites

LARCTM-IA (Langley Research Center-Improved Adhesive) aromatic polyimide, based on oxydiphthalic anhydride and 3.4'-oxydianiline, was evaluated as a matrix for high-performance composites. Six poly(amide acid)solutions in N-methylpyrrolidone (NMP), end-capped with phthalic anhydride to various theoretical molecular weights, were synthesized and their molecular weights and molecular weight distributions determined, Importantly, high concentrations of low-molecular-weight species were found in all the offset compositions. Except for the 1% offset polymer, all fully imidized films failed a solvent resistance test which involved immersion in acetone, methyl ethyl ketone, toluene, dimethylacetamide and chloroform for 1 min followed by a fingernail crease. Unidirectional prepreg was fabricated from each of the six resins by both standard drum winding procedures and the LARC multipurpose prepreg machine. The consolidation cycle developed previously for IM7/LARCTM-ITPI composites was found to be equally applicable for IM7/LARCTM-IA composites since both materials are similar and were prepared in and prepregged from NMP. An optimal end-capped resin composition was identified (4% stoichiometric imbalance) by using, as a screening tool, initial composite mechanical properties (short-beam shear strength, longitudinal flexural strength and flexural modulus) at room temperature, 93, 150 and 177°C. Composite engineering properties for the 4% offset composition were obtained, including longitudinal tension, transverse flexural, longitudinal compression, interlaminar shear, short block compression, open hole compression and compression strength after impact. Notably, the CAI strength was 303.2 MPa (44 Ksi) showing that the LARCTM-IA composites have good damage tolerance.

A minor modification of LARCTM-IA polymer backbone which did not alter the consolidation cycle, designated as LARCTM-IAX, improved solvent resistance measurably. Mechanical properties of IM7/LARCTM-IAX composites were shown to be comparable to those exhibited by the baseline IM7/LARCTM-IA composites.

Processing and Properties of IM7/LARCTM-SCI Composite

LARCTM-SCI (Langley Research Center - Semi-Crystaline polyImide) is an aromatic polyimide based on 3,4′-oxydianiline and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride. This polyimide was synthesized and evaluated for use as a neat resin and a matrix resin for advanced composites. Three 30% w/w solids polyamic acid/ N-methypyrrolidone solutions were prepared using 2, 3 and 4% stoichiometric imbalances end-capped with phthalic anhydride to provide polyimides with theoretical number average molecular weights of approximately 22 800, 15 100 and 11 400 g mol−1 respectively. Unidirectional IM7 carbon fibre prepreg was prepared from these three resins using the Langley multipurpose tape machine. Thermal and rheological properties and the solvent/volatile depletion rates along with crystallization kinetics were characterized for the resin scraps taken from the prepreg tapes. Processing characteristics of the LARCTM-SCI resin were thoroughly understood from these results, and a workable moulding cycle was designed for this composite. Composite laminates were moulded at 410 °C at either 200 or 300 psi, which consistently yielded good consolidation and high-quality panels as measured by C-scan, acid digestion and optical photomicrography. The composite mechanical properties were also obtained. Short beam shear strength was 15 ksi at RT. Longitudinal flexural strength was 295 ksi at RT and 200 ksi at 177 °C. Excellent fracture toughness of 6.9 in-lb/in2 was obtained. Excellent values of transverse flexural strength and longitudinal tensile strength indicated a good translation of fibre properties into the composite. Un-notched longitudinal compression strength of 163 ksi was comparable to typical thermoplastic composites. An open hole compression strength of 55 ksi suggested good damage tolerance for this composite.

Multi-level control of conductive nano-filament evolution in HfO2 ReRAM by pulse-train operations

Precise electrical manipulation of nanoscale defects such as vacancy nano-filaments is highly desired for the multi-level control of ReRAM. In this paper we present a systematic investigation on the pulse-train operation scheme for reliable multi-level control of conductive filament evolution. By applying the pulse-train scheme to a 3 bit per cell HfO2 ReRAM, the relative standard deviations of resistance levels are improved up to 80% compared to the single-pulse scheme. The observed exponential relationship between the saturated resistance and the pulse amplitude provides evidence for the gap-formation model of the filament-rupture process.