Thermal stability of modified clays and nanocomposites

As mentioned in the earlier section, ammonium-based modifications of the surfaces of montmorillonites have been commonly employed. Thermogravimetric analysis has also been used to quantify the extent of surface modification, that is, the amount of 

Awad et al. reported extensive studies on the various imidazolium salts and montmo-rillonites modified with these salts [37]. The imidazolium salts included l,2-dimethyl-3-propylimidazolium, l-butyl-2,3-dimethylimidazolium, 1 -decyl-2,3-dimethylimidazolium, l,2-dimethyl-3-hexadecylimidazolium, l,2-dimethyl-3-eicosylimidazolium, 1,2-dimethyl-3-ethylbenzene imidazolium, and l-ethyl-3-methylimidazolium. The authors investigated the effect of counterion, alkyl chain length, and structural isomerism on the thermal stability of the imidazolium salts and compared their behavior with that of the conventional quaternary ammonium ions using a number of characterization techniques. It was reported that 

Note Reproduced from [37] with permission from Elsevier. 

In another study, pyridine- and quinoline-containing salts were used to modify the mont-morillonite organically. It was observed in thermogravimetric analysis that the quinolinium 

Commercial PS PS + 3%QC16 clay, Bulk PS + 5%QC16 clay, Bulk PS + 3%QC16, MB PS + 5%QC16 clay, MB 

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Slade et al. [38] used hexadecylpyridinium IL to modify vermiculite for the adsorption of nonpolar molecules. The adsorption of pollutants such as chlorobenzene or phenanthrene by dodecylpyridinium-modified bentonite was also studied [39, 40]. But thermal stability of modified nanoclays and their uses in the preparation of PLS nanocomposites have been scarcely studied. Nevertheless, the importance of the chemical nature of the anion on the thermal stability of the modified clays has been shown. Lee and Kim [41] have demonstrated that the combination of pyridinium cation with fluorinated anion such as BF " leads to an increase in the thermal stability of the exchanged clays. 

Styryltropylium (a carbo-cation) was used to modify clay for producing PS nanocomposites by in situ emulsion polymerization (Table 3.4) [60]. The resulting nanocomposites exhibited a mixture of intercalated and exfoliated structures. The nanocomposites exhibited improved thermal stability and fire retardancy. Halloysite nanotubes (HNT) are a kind of aluminosilicate clay with a hollow nanotubular structure (about 20-50 nm in diameter and several hundred nanometers in length) [61]. HNT was modified with y-methacryloxypropyltrimethoxysilane to produce a nanoclay (Table 3.4). HNT/PS nanocomposites were prepared by in situ bulk polymerization. The thermal stability of the HNT/PS nanocomposites was better than that of the pure polystyrene. 

The development of a complete formulation and processing system for the production of nanocomposites or thermally stable clays and nanocomposites must resolve a number of important issues, beyond the thermal stability of the clay modifier. We will describe some aspects of the approach followed by our group to achieve this goal. The main objective of the case study to be described was to evaluate a number of phosphonium compounds for the production of thermally stable organoclays that would be suitable for preparing satisfactory PS nanocomposites by melt-compounding. 

S. Su, D. D. Jiang, and C. A. Wilkie, Study on the thermal stability of polystyryl surfactants and their modified clay nanocomposites. Polymer Degradation and Stability, 84 (2004), 269-77. 

Several successful strategies are available in the literature [14] that increase the thermal stability of organic molecules. Full utilization of these strategies for the preparation of surface treatments of layered silicates with enhanced thermal stability for the development of polymer-clay nanocomposites has yet to be realized. An example of an effective strategy is the utilization of quaternary ammonium and phosphonium functional polystyrene as a surface treatment for montmorillonite that is employed to prepare polymer-clay nanocomposites [15]. TGA indicated a significant increase in the thermal stability of the organoclay and the polymer-clay nanocomposite. Imidazolium functional surface modifier for montmorillonite demonstrated a significant increase in the thermal stability of ABS terpolymer-clay nanocomposite when compared to the pure polymer and polymer-clay nanocomposites where the surface modification of the montmorillonite was produced with traditional quats [16]. These experiments were via TGA measurements. 

Incorporation of modified clays into thermosetting resins, and particularly in epoxy35 or unsaturated polyester resins, in order to improve thermal stability or flame retardancy, has been reported.36 A thermogravimetric study of polyester-clay nanocomposites has shown that addition of nanoclays lowers the decomposition temperature and thermal stability of a standard resin up to 600°C. But, above this temperature, the trend is reversed in a region where a charring residue is formed. Char formation seems not as important as compared with other polymer-clay nanocomposite structures. Nazare et al.37 have studied the combination of APP and ammonium-modified MMT (Cloisite 10A, 15A, 25A, and 30B). The diluent used for polyester resin was methyl methacrylate (MMA). The... 

The incorporation of 5 % organically modified sepiolite, which is a microcrystalline-hydrated magnesium silicate, in a bisphenol A-based epoxy resin has no significant effect over the thermal stability of the epoxy resin, due to the poor dispersion of the clay and poor diffusion of the resin between fibres [69]. The effect of attapulgite (magnesium aluminium phyllosilicate) over the thermal properties of hyperbranched polyimides was studied. The presence of this silicate in the nanocomposites significantly improved the thermal stability of the neat polyimide [70]. 

Ollier et al. reported the effects of modified and unmodified bentonite loading on the decomposition behavior of unsaturated polyester thermosets [71]. It was shown that the addition of bentonite increased the thermal stability of the resin. Furthermore, the clay modification did not significantly influence the degradation temperatures of the nanocomposites. 

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