Abstract
Series of metal based polyurethanes (PUs) have been synthesized from hydroxyl terminated polybutadiene (HTPB) with combination of a potential energetic material, 2,4-dinitrobenzene (DNB) and burn rate (BR) enhancer, ferrocene with two objectives: (1) exerting flexibility into otherwise brittle PU film and (2) increasing BR of composite solid propellant (CSP). HTPB was first functionalized with DNB at the terminal carbons and then ferrocene was grafted radically as poly (vinyl ferrocene) (PVF) chain onto the pendant vinyl bond of HTPB. The degree of PVF grafting was altered by appropriate reaction recipes to find out the effect of Fe content on various physical properties including fluidity of the HTPB. Density function theory (DFT) calculation showed that the dominating intra-chain interactions over inter-chain owing to the strong interactions between NO2 of DNB and cyclopentadiene of ferrocene are the driving force for improvement in various physical properties of PVF-grafted-HTPB-DNB. Cyclic voltammetry (CV) measurement showed one electron reversible redox behavior of the grafted PVF polymer chain with slow electron transfer process. Further the mechanical stability, thermal stability and properties of PUs have been studied thoroughly; the results indicated a strong influence of DNB and ferrocene in the chains on the physical properties. All the DNB modified PU membranes displayed exceptionally enhanced flexibility along with much lower Tg value compared to neat PVF-g-HTPB-PU. The presence of DNB at the chain end of soft segment (SS) causes strong segmental mixing between SS and hard segment (HS) domains which helps in enhancing the elasticity of SS chain by increasing the inter polymer chain distance. Morphology of the hard segment domains formation has been probed by small angle X-ray scattering (SAXS) and further confirmed by FESEM. Burn rate of composite solid propellant made from the HTPB-DNB-g-PVF binder is found to be ~18% larger than the HTPB-DNB.
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