Benny K. George

Stabilization of thermal degradation of poly(methyl methacrylate) by polysulfide polymers

The thermal degradation of poly(methyl methacrylate) (PMMA) in the presence of polysulfide polymers, namely, poly( styrene disulfide) (PSD) and poly(styrene tetrasulfide) (PST) was studied using thermogravimetry (TG) and direct pyrolysis-mass spectrometric (DP-MS) analysis. Both PSD and PST were found to stabilizethe PMMA degradation, which was explained by both radical recombination and a chain-transfer mechanism

Graphite oxide–iron oxide nanocomposites as a new class of catalyst for the thermal decomposition of ammonium perchlorate

Graphite oxide (GO) is receiving increased attention due to its special surface properties and layered structure for the synthesis of GO containing nanocomposites. It is possible that integration of GO sheets and iron oxide nanoparticles may result in enhanced properties and enlarge the application range. Herein, we report the effect of Fe2O3–GO nanocomposite as a new class of catalyst for the decomposition of ammonium perchlorate (AP), a rocket propellant oxidizer, and study the effect of Fe2O3 : GO ratio on the catalytic activity. The material was characterized by X-ray diffraction and Raman spectroscopy and the formation of Fe2O3 and GO were confirmed. FESEM analysis showed that the Fe2O3 nanoparticles are highly dispersed between and on the graphene layers. With the addition of 3% of the composite with 1 : 1 Fe2O3–GO ratio, the decomposition temperature of AP was reduced by 45 °C, showing a high catalytic activity for the new composite. The high catalytic activity of the in situ synthesized Fe2O3–GO composite may be attributed to the uniform distribution of iron oxide nanoparticles which in turn provide a number of active sites on the surface due to the presence of GO.

Mechanistic outlook on thermal degradation of 1,3-dialkyl imidazolium ionic liquids and organoclays

The thermal degradation mechanisms of ionic liquids (ILs) 1-butyl-3-methylimidazolium chloride (BMImCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) have been established using pyrolysis-GC-MS (Py-GC-MS) and B3LYP/6-311+G(d,p) level of density functional theory (DFT). BMImCl decompose through a bimolecular nucleophilic substitution (SN2) while BMImBF4 exhibit SN2 along with a competitive E2 elimination pathway. Activation energy parameters obtained using Kissinger–Akahira–Sunose method and Ozawa–Flynn–Wall method is compared with the computed activation barriers. The montmorillonite based organoclay prepared using these ionic liquids exchange only the cation part ([BMIm]+) into the clay gallery leading to an expansion of d-spacing from 12.08 to 13.64 A. The organoclay showed the maximum decomposition at 462 °C in the TGA experiment and the decomposition products were identified as methyl imidazole and 1-butene using Py-GC-MS. DFT studies employing a model compound Si(OH)3O− suggested a mechanism involving an imidazole-2-ylidine (carbene) intermediate for the decomposition of [BMIm]+ in the clay. Theoretical results were further supported by 13C NMR analysis of IL in presence of colloidal silica which showed a characteristic carbene NMR signal at 187.6 ppm.

Imidazolium based energetic ionic liquids for monopropellant applications: a theoretical study

A large variety of 1-ethyl-3-methylimidazolium ([EMIm]+) based energetic ionic liquids (ILs) have been studied via their ion pair ([EMIm]+[X]−) formation using high accuracy G3MP2 method and density functional theory (DFT) methods M06L, M05-2X, M06-2X and B3LYP. The selected X− includes nitrogen rich derivatives of tetrazolate and triazolate, dinitramine, dicyanamide as well as conventional anions BF4− and PF6−. The nitrogen enrichment in the system produces energetic ionic liquids (EILs) which showed comparable and in some cases superior thermochemical, fluid and specific impulse (Isp) properties than conventional ionic liquids. The binding energy values for [EMIm]+[X]− are in the range 336–400 kJ mol−1 at DFT levels while the atomization procedure used to compute their heat of formation (ΔfH°) at the G3MP2 level produced results in very close agreement with available experimental data (maximum deviation < 5%). The ΔfH° of conventional ILs is negative whereas that of EILs (167–559 kJ mol−1) confirmed their high energy state. The predicted Isp of all EILs are slightly lower compared to hydrazine in monopropellant systems whereas a significant increase in Isp is observed with the addition of hydroxyl ammonium nitrate (HAN). A good linear correlation between Isp and the wt% of (N + O) content of the EIL is also observed. Our results suggest that imidazolium based energetic ionic liquids have attractive thermochemical properties for use as green substitutes to hazardous hydrazine for monopropellant application in spacecraft technology.

Functionalized white graphene – Copper oxide nanocomposite: Synthesis, characterization and application as catalyst for thermal decomposition of ammonium perchlorate

Reactivity is of great importance for metal oxide nanoparticles (MONP) used as catalysts and advanced materials, but seeking for higher reactivity seems to be conflict with high chemical stability required for MONP. There is direct balance between reactivity and stability of these MONP. This could be acheived for metal oxide by dispersing them in a substrate. Here, we report a simple, efficient and high-yield process for the production of copper oxide (CuO) nanoparticles dispersed on a chemically inert material, few-layer hexagonal boron nitride (h-BN) with a thickness around 1.7nm and lateral dimensions mostly below 200nm. The mechano-chemical reaction which take place at atmospheric pressure and room temperature involves a urea assisted exfoliation of pristine boron nitride. Copper oxide nanoparticles dispersed on the surface of these few layered h-BN reduced its tendency for aggregation. The optimum concentration of CuO:h-BN was found to be 2:1 which shows highest catalytic activity for the thermal decomposition of ammonium perchlorate. The high catalytic activity of the in situ synthesized CuO-h-BN composite may be attributed to uniform distribution of CuO nanoparticles on the few layered h-BN which in turn provide a number of active sites on the surface due to non aggregation.

1,3-Dialkylimidazolium modified clay sorbents for perchlorate removal from water

Sodium montmorillonite clays modified using 1-alkyl-3-methylimidazolium based ionic liquids with varying alkyl chain length viz. C4, C6, C8, C10 and C16 were used for perchlorate adsorption from water. Pristine MMT showed negligible adsorption whereas ionic liquid modified clays showed an increase in adsorption with increase in chain length of the exchanged cation. 1-Hexadecyl-3-methylimidazolium modified clay (C16-clay) with a d-spacing of 18.55 A showed a maximum adsorption of 0.16 mmol g−1 of clay. The d-spacing of the C16-clay decreased on adsorption of perchlorate to 13.70 A without a change in the composition of the modified clay, as confirmed by CHN analysis. Raman spectroscopic studies substantiated the conformational change from gauche to trans for the imidazolium cations on perchlorate adsorption. The adsorption followed the Freundlich isotherm with pseudo second order kinetics. The modified clays were thermally stable (<200 °C) and regenerated by heating to 175 ± 5 °C in air and 95% regenerability was observed.

Effective SERS detection using a flexible wiping substrate based on electrospun polystyrene nanofibers

The progress of surface-enhanced Raman scattering (SERS) substrates, which offers flexibility, improved sample collection efficiency and high SERS activity, has provided results in numerous applications. The present study demonstrates the development of a flexible substrate based on electrospun polystyrene (PS) nanofibers decorated with silver nanoparticles for in situ sampling and detection using SERS. The surface of the PS nanofibers contains uniformly distributed nanopores in the range of 70–100 nm due to the phenomenon called ‘breathe figure’. The nanoparticles are strongly adsorbed over the nanofiber surface due to the presence of these nanopores. The present substrate offers effective in situ sampling by wiping directly from the surface of luggage, fruits or any surface of interest. We demonstrated the SERS activity of the substrate by choosing highly energetic materials such as, RDX, DNT and pesticides, namely carbofuran. The results demonstrated that the nanoparticle coated electrospun PS nanofiber mats are promising for in situ sampling and SERS-based detection.

TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine chromate, a new precursor for copper chromite catalyst

Abstract Copper chromite is a well-known burn rate modifier for the combustion of composite solid propellants. In this study, basic copper ethylamine chromate (CEC), a new precursor for copper chromite catalyst, was synthesized by precipitation method. The thermal decomposition of the precursor was followed by thermogravimetry–mass spectroscopy (TG–MS) and X-ray diffraction techniques and compared with that of copper ammonium chromate, a conventional precursor for copper chromite catalyst. TG–MS analysis for the decomposition of CEC revealed that the decomposition starts with the liberation of ethylamine. The change in enthalpy for the decomposition reaction of copper ethylamine chromate was higher than that of copper ammonium chromate due to the oxidation of ethyl group. The reducing atmosphere created by the presence of carbon during the decomposition of CEC produced a mixture of Cu, CuCr2O4, CuCrO2 and CuO, while the oxidizing atmosphere of copper ammonium chromate produced a mixture of CuCr2O4 and CuO. Mechanistic study based on Criado and Coats–Redfern methods showed that CEC follows random nucleation (F1) mechanism as the rate-determining step for the thermal decomposition process.

Inter molecular azide–diisocyanate coupling: new insights for energetic solid propellants

Hydroxyl terminated azide binders can undergo a spurious reaction with diisocyanates to form tetrazoline-5-one via an inter molecular 1,3-dipolar cycloaddition reaction apart from urethane/allophanate groups which has been overlooked. This has serious implications on solid propellants. The computed activation barrier using density functional theory (DFT) for urethane formation reaction is 28.4 kJ mol−1 and that for tetrazoline-5-one formation reaction is 108.0 kJ mol−1. DFT studies reveal that the rate limiting step of the reaction is 1,3-dipolar cycloaddition between azide and isocyanate. A dual cure was observed in the temperature ranges 42–77 °C and 78–146 °C by differential scanning calorimetry (DSC) and rheological studies, confirming multiple reactions. Tetrazoline-5-one formation was confirmed by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectroscopy (NMR).