| PMR-15 is an industrial standard polyimide for high temperature performance in aerospace applications. The objectives of this research were to develop environmentally friendly branched PMR-type polyimides with comparable thermal and mechanical properties and improved melt flow and processability compared to PMR-15. An aromatic triamine, 1,3,5-tris(4-aminophenoxy)benzene was used in the development of these branched polymers. This work consisted of three parts.; The first part was a screening study of four different diamine-based branched polyimides of varying formulated molecular weights. In these series, the aromatic triamine was utilized as the hub of the polymer molecules which were endcapped with a nadic ester acid. Thermal and mechanical properties of oligomers (resins), neat polymers, and selected carbon fabric reinforced polyimide composites were investigated. Results showed that branched polymers displayed higher glass transition temperatures (Tg's) than their corresponding linear systems due to a higher crosslink density, but had lower thermal oxidative stability (TOS), due to higher nadic ester content and the presence of ether linkages in the triamine, TAB. Mechanical properties including interlaminar shear, flexural strength, flexural modulus, compression strengths, and compression modulus were comparable with those of composite PMR-15 at both ambient and elevated temperatures (288°C).; The second part of the project focussed on isolating the effects of the aromatic triamine from nadic endcap on resins' Tg, TOS, and relative melt viscosity, using a statistical design of experiments (DOE). A pseudo classical design based upon a Face Centered Central Composite Model was used. DOE model generated response surfaces showed that incorporation of the triamine moieties into PMR-type polyimides slightly increased the Tg's of the polymers, due to higher crosslink density, but slightly decreased their thermal oxidative stability. It was also observed that added TAB enhanced melt flow (reduced melt viscosity) in rigid backbone polymers, but actually reduced melt flow in more flexible systems. Optimized resin formulations were also prepared and confirmed the models derived from the DOE results. Selected resins with optimized Tg, thermal oxidative stability, and melt flow were fabricated into carbon fabric reinforced composites, and their mechanical properties were evaluated and compared to PMR-15 and another commercial polyimide, AMB-21.; The final part of this project involved an investigation of the effects of meta-ether linkages, such as those in the aromatic triamine, TAB, and endcap effects on the melt flow of polyimide oligomers. A D-Optimal design was employed, based on the previous DOE study with additional experimental measurements. Results from this study showed that improvements in melt flow of the PMR-type polymers are strictly due to the presence of the meta-ether linkages rather than branching. In addition, the reactive nadic endcap, had little or no influence on the resin melt flow at elevated temperature. |
