Orientation of Aromatic Ion Exchange Diamines and the Effect on Melt Viscosity and Thermal Stability of PMR-15/ Silicate
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Orientation of Aromatic Ion Exchange Diamines and the Effect on Melt Viscosity and Thermal Stability of PMR-15/ Silicate Nanocomposites Sandi Campbell and Daniel Scheiman1 NASA Glenn Research Center, Cleveland, Ohio, 44135 1 NASA contract via QSS Abstract Nanocomposites of PMR-15 polyimide and a diamine modified silicate were prepared by addition of the silicate to the PMR-15 resin. The orientation of the ion exchange diamine within the silicate gallery was determined by x-ray diffraction and found to depend on the clay cation exchange capacity. The oligomer melt viscosity exhibited a dependence on the orientation of the diamine in the silicate interlayer, and in some cases, on the length of the diamine. A correlation was observed between the oligomer melt viscosity and the crosslinking enthalpy, where nanocomposites with an increased melt viscosity exhibited a decrease in enthalpy on crosslinking. After crosslinking, those nanocomposites with a high melt viscosity had poorer thermal oxidative stability compared to the less viscous systems. The melt viscosity was tailored by co-exchange of an aromatic diamine and an aliphatic amine into the silicate. Nanocomposites prepared with this silicate exhibited an increase in thermal oxidative stability compared to the neat resin. Introduction The durability and reliability of materials used in aerospace components is a critical concern. Many of these applications, in particular those in propulsion components, require a high glass transition temperature (Tg), high temperature stability in a variety of environments, and good mechanical properties over a wide range of temperatures.1 PMR (Polymerization of Monomer Reactants)- type polyimides are thermosetting polymers which combine excellent processability, mechanical properties, and thermal oxidative stability (TOS). PMR-15 is commercially available and prepared in two stages from three monomer reactants: 2carbomethoxy-3-carboxy-5-norbornene (nadic ester, NE), 4,4’-methylenedianiline (MDA), and the dimethyl ester of 3,3’,4,4’-benzophenonetetracarboxylic acid (BTDE). Curing under heat and pressure results in a highly crosslinked network structure.2 There has been a significant amount of research aimed at increasing the TOS of PMR-15 by altering the structure of the diamine,2 the dianhydride,3 or the end-cap4. An alternative to modification of the polymer, as a means of increasing TOS, is the dispersion of a layered silicate in the polymer matrix. Layered silicates have quickly become recognized as useful fillers in polymer matrix composites. Their platelet morphology and high aspect ratio results in greatly improved thermal,5 mechanical,6 and barrier properties.7 A number of papers have reported improvements in the physical properties of thermoplastic polyimide nanocomposites.8 However, the majority of work in thermosetting polymers has focused on epoxy systems.9 The purpose of the work described in this paper was to investigate the TOS of PMR-15 nanocomposites. Specifically this research was focused on the relationship between
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