A phenylethynyl terminated imide (PETI) oligomer from the reaction of 3,3',4,4'-biphenyltetracarboxylic dianhydride, an 85:15 molar ratio of 3,4'-oxydianiline and 1,3-bis(3-aminophenoxy)benzene and 4-phenylethynylphthalic anhydride as the end-capper at a theoretical number average molecular weight (Mn) of - 5,000 g/mol was evaluated as a composite resin matrix. Unidirectional prepreg was made by coating an N-methylpyrrolidinone solution of the amide acid oligomer onto unsized IM7. The thermal and Theological properties and the solvent/volatile depletion rates of the amide acid/NMP system were determined. This information was used to successfully design a molding cycle for composite fabrication. Composites molded under 1.38 MPa at 371°C consistently yielded good consolidation as measured by C-scan and optical photomicrography. Composite mechanical properties at various temperatures included short beam shear strength, flexural strength and modulus, longitudinal compression strength and modulus (IITRI), longitudinal tensile strength and modulus and open hole compression (OHC) strength. These composites exhibited excellent resin dominant properties. The OHC strength values, in particular, suggested that PETI composites have excellent damage tolerance.
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.
Dynamic dielectric analysis (DDA) has been used to study curing polymer systems and thermoplastics. Measurements have been made over a frequency range of six decades. This wide range of frequencies increases the amount of information which can be obtained. The data is analyzed in terms of the frequency dependence of the complex permittivity ε*, specific conductivity σ(ohm−1 cm−1), and the relaxation time τ, parameters which are characteristic of the cure state of the material and independent of the size of the sample. Dynamic dielectric measurements have been used to monitor polymer processing in UDEL‐P1700, LARC‐160, polyphenyl quinoxaline (PPQ), and Epon 828 cured with Agent U. Dynamic dielectric measurements have been correlated with viscosity for the polysulfone thermoplastic UDEL‐P1700 and with viscosity and ultrasonic measurements on the DGEBA type epoxy Epon 828 cured with Agent U. The experimental results suggest that when ionic processes dominate the dielectric response, the intensive property σ is a good monitor of the resin's viscosity. The results show that the dielectric relaxation time τ can be used to monitor the state of the system and the extent and rate of the reaction. Solvent evolution can also be easily observed.
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.
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.
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