Deepthi Thomas

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.

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.

Silicone bridged iron metallocene butadiene composite solid propellant binder: aspects of thermal decomposition kinetics, pyrolysis and propellant burning rate

ABSTRACT Propellant binders are essential components of composite solid propellants (CSP’s) used in launch vehicles and missiles. Binders act as a fuel and contribute directly to the combustion in conjunction with oxidizer particles and metallic fuel apart from imparting structural integrity to the solid propellant grain .The performance of CSP’s are directly related to the burn rate of the propellant. The burn rates of the ammonium perchlorate (AP) propellants are generally moderated using various types of transition metal oxide (TMO) catalysts. However, TMO’s are associated with inherently large dispersions in propellant burn rates and compromise on energetics. One of the most suitable methods for achieving lower dispersion in burn rate is using binders wherein a burn rate catalyst is grafted to the polymer matrix. In the present paper, the thermal decomposition of ferrocene bound hydroxyl terminated polybutadiene (FC-Si-HTPB) grafted to butadiene backbone via hydrosilylation was investigated The thermal degradation mechanism, stability and its effectiveness as burn rate catalyst are the most important aspects for use in CSP’s. The mechanism of decomposition of the neat resin and in combination with AP has been elucidated using pyrolysis gas chromatography–mass spectrometric technique (GC-MS). FC-Si-HTPB exhibits single stage decomposition in the temperature range of 263–491°C. The decomposition of FC-Si-HTPB with AP oxidizer follows a two stage mechanism in the 195–490°C.The char residue was characterized using FTIR, Raman spectroscopy and FE-SEM analysis, which enables to vindicate the mechanism of reaction. The activation energy for the decomposition of HTPB is 283.6 kJ/mol, FC-Si-HTPB is 251.5 kJ/mol and for Fc-Si-HTPB-AP system is 67.1 kJ/mol. The major pyrolysis products of neat FC-Si-HTPB are ferrocenyl derivatives, silylated ferrocenyl derivatives and precursors emanating from polybutadiene backbone. The propellants based on the new binder exhibited an increase in burn rate with iron content and higher fine content. A comparison of propellant burn rate with conventional micron sized ferric oxide exhibited an improvement of 34%.Based on the thermal analysis studies, the thermal endurance of the system was computed to be FC-HTPB> HTPB> FC-HTPB-AP.

New insights into the spectral, thermal and morphological analysis of time dependent structural changes during open end functionalization of single walled carbon nanotubes

Even though several authors have investigated the chemical derivation of single walled carbon nanotubes (SWCNTs) through their treatment with concentrated nitric acid (conc. HNO3), this paper reports hitherto unaccounted for insights in this field of study that can be achieved through a spectral, thermal and morphological evaluation of SWCNTs, functionalized at a fixed temperature for different time intervals. In this study, multiple techniques were employed to systematically evaluate the structural changes on SWCNTs with refluxing time. Furthermore, the nanotubes, specifically functionalized for different extents of time, were characterized using a combination of analytical techniques including Raman spectroscopy, thermogravimetry-mass spectroscopy (TG-MS), X-ray photoelectron spectroscopy (XPS) and ultra violet near infrared spectroscopy (UV-Vis-NIR). To arrive at an optimum reflux point, an analysis of the changes in Radial Breathing Mode (RBM), D-band & G-band of Raman spectra, TG-MS profile and Zeta potential measurements was carried out. TG-MS (of the evolved carbon dioxide) and XPS were employed, respectively, to quantify the acid groups in the functionalised SWCNTs (F-SWCNTs) and the type of functional groups attached onto the SWCNTs. While the electronic nature of the SWCNTs was analysed through the line-shape of the G-band, the 2D band of the Raman spectra and the UV-Vis-NIR spectra, their morphological characterisation was done using a high resolution transmission electron microscope (HRTEM). Exploiting the advantages of each characterization techniques, the reflux time for the effective functionalization of SWCNTs, without any compromise of their quality, was converged down to 4 h. This paper proposes a new mechanism to account for the multistage oxidation with the extension of reflux time and explain the type of functional groups attached onto the surface of SWCNTs as functionalization proceeds with time. It is also claimed that the changes in the relative concentration of functionalized groups on the SWCNT surface are directly proportional to the increase in the extent of functionalization. Significantly, this paper reports that the introduction of any desired ratio of functional groups on the surface of carbon nanotubes can be achieved by tuning the extent of functionalization.