Douglas M. Fox

Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Abstract Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF6−, N(SO2CF3)2− and BF4− anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.

Flammability, thermal stability, and phase change characteristics of several trialkylimidazolium salts

Room temperature ionic liquids (RTILs) have emerged as tunable and potentially “greener” solvents for a multitude of applications. To investigate the solvent properties and potential use as a thermal fluid, a study was initiated to determine the effects of anion type, C-2 hydrogen substitution, and alkyl chain length on the flammability, thermal stability, and phase change characteristics of 1,2,3-trialkylimidazolium room temperature ionic liquids. A Setaflash flashpoint apparatus was used to determine the flammabilities of the RTILs. No flashpoints were detected for any of the imidazolium based RTILs below 200 °C, the maximum temperature of the instrument. The thermal stabilities of the RTILs were measured using the technique of thermogravimetric analysis. The 1,2,3-trialkylimidazolium compounds exhibit slightly higher thermal stabilities than the comparable 1,3-dialkylimidazolium compounds; RTILs with nucleophilic anions decompose about 150 °C lower than RTILs with bulky fluoride containing anions; the alkyl chain length does not have a large effect on the thermal stability of the RTILs; and the pyrolysis decomposition exhibits higher thermal stabilities via a different mechanism than the oxidative decomposition. In addition, it was found that although the calculated onset temperatures were above 350 °C, significant decomposition does occur 100 °C or more below these temperatures. The phase change behaviors of several imidazolium based RTILs were characterized by differential scanning calorimetry. The melting points of the RTILs increased with increasing alkyl chain length. Most of the salts studied exhibited significant undercooling, which decreased as the length of the alkyl chain was increased. The hexafluorophosphate and bromide RTILs exhibited polymorphic and liquid crystalline behaviors as the alkyl chain length was increased above C10. The clearing point temperatures increased more rapidly with alkyl chain length than the melting point temperatures.

Vibrational spectroscopy and dynamics of small anions in ionic liquid solutions

Fourier-transform infrared (FTIR) and time-resolved IR spectroscopies have been used to study vibrational band positions, vibrational energy relaxation (VER) rates, and reorientation times of anions in several ionic liquid (IL) solutions. The ILs primarily investigated are based on the 1-butyl-2,3-dimethylimidazolium ([BM(2)IM]) cation with thiocyanate (NCS-), dicyanamide (N(CN)2-), and tetrafluoroborate (BF4-) anions. Spectroscopic studies are carried out near 2000 cm-1 for the C[Triple Bond]N stretching bands of NCS- and N(CN)2- as the IL anion as well as for NCS-, N(CN)2-, and azide (N3-) anions dissolved in [BM2IM][BF4]. The VER studies of N(CN)2- are reported for the first time. VER of N3-, NCS-, and N(CN)2- is measured in normal solvents, such as N-methylformamide, to compare with the IL solutions. The spectral shifts and VER rates of the anions in IL solution are quite similar to those in polar aprotic, conventional organic solvents, i.e., dimethylsulfoxide, and significantly different than those in methanol, in which there is hydrogen bonding. Similar studies were also carried out for the anions in another IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), in which the C2 hydrogen is present. The results for the anions are similar to those in the [BM2IM] containing ILs, in which the C2 hydrogen is methyl substituted. This suggests that substituting this hydrogen has, at most, a minor effect on the degree of hydrogen bonding in the anion-IL solvation interaction based on the infrared spectra and dynamics.

Epoxy Composites Using Wood Pulp Components as Fillers

The components of wood, especially lignin and cellulose, have great potential for improving the properties of polymer composites. In this chapter, we discuss some of the latest developments from our lab on incorporating wood based materials into epoxy composites. Lignosulfonate was used as a flame retardant and cellulose nanocrystals were used as reinforcing materials. Lignosulfonate will disperse well in epoxy, but phase separates during curing. An epoxidation reaction was developed to immobilize the lignosulfonate during curing. The lignosulfonate – epoxy composites are characterized using microcombustion and cone calorimetry tests. Cellulose also has poor interfacial adhesion to hydrophobic polymer matrixes. Cellulose fibers and nanocrystals aggregate when placed in epoxy resin, resulting in very poor dispersion. The cellulose nanocrystal surface was modified with phenyl containing materials to disrupt cellulose interchain hydrogen bonding and improve dispersion in the epoxy resin. The cellulose nanocrystal – epoxy composites were characterized using tensile tests and microscopic techniques.