Development, Kinetic Analysis and Applications of 2-D Nanostructured Layered Metal Hydroxides


Nanodimensional layered metal hydroxides which include layered hydroxy salts (LHSs) and hydroxy double salts (HDSs) have the ability to accommodate species between the layers. The structural composition of these materials can be tuned so as to create materials with targeted physico-chemical properties for applications where it is advantageous to intercalate or release molecules. Some of the applications include ion-exchange, fire retardation, catalysis, and controlled release delivery. Anion exchange reactions are among methods used to optimize these materials for targeted applications, making the characterization of exchange kinetics of practical interest. In addition, understanding fundamental factors that control retention and release of functional anions is important in designing hosts for storage and triggered release. Isomers of hydroxycinnamate were used as model compounds to systematically explore the effects of anion structure on controlled release delivery in layered metal hydroxides. Following intercalation and subsequent release of the isomers, it has been demonstrated that the nature and position of substituent groups on interlayer anions have considerable effects on the rate and extent of release. The extent of release was correlated to the magnitude of dipole moments of the anions while the reaction rate showed strong dependence on the level of hydrogen bond network within the layers. Anion exchange kinetic analyses of this class of compounds have traditionally been carried out using model fitting methods. Isoconversional (model-free) approach can be utilized to identify when fitting to a single model is not appropriate, particularly for characterizing the temperature dependence of the reaction kinetics. We established a systematic analysis for identifying cases when model based approaches are not appropriate in modeling anion exchange kinetics in these compounds. Results obtained demonstrate the utility of the isoconversional approach for identifying when fitting kinetic data to a single model is not appropriate. In another study, nanocomposites prepared by compounding poly (methyl methacrylate) with boron containing LHSs showed enhanced thermal stability and reduced flammability (up to 48 %) as evaluated by thermogravimetry analysis and cone calorimetry. Effective activation energies for the first step of the degradation process (evaluated using Flynn-Wall-Ozawa, Friedman, and Kissinger methods) were shown to be higher in the nanocomposites

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