ABSTRACT
In-depth understanding of the thermal stability of polymer-clay nanocomposites requires the use of advanced time-resolved techniques combined with multivariate data analysis, as well as the preparation of layered nanofillers with well-defined composition. The layered double hydroxide (LDH) compounds Zn2Al(OH)6·nH2O, Zn2Al0.75Fe0.25(OH)6·nH2O, ZnCuAl(OH)6·nH2O, and ZnCuAl0.5Fe0.5(OH)6·nH2O were prepared, each designed to specifically identify the physical barrier, radical trapping, and char formation contributions to the thermal stability of the PMMA-LDH nanocomposites. The unique combination of conventional methods (TG, DSC, and Raman spectroscopy) and synchrotron radiation techniques (XAS and WAXS), applied during PMMA-LDH heating, revealed the synergetic (of iron) and antagonist (of copper) effects of the LDH layers transformations on the three main endothermic steps of mass loss of the polymer. The diffusion barrier effect was proved by the downshift of the PMMA thermal decomposition temperature caused by the decrease of the LDH edifice thermostability when divalent cations were substituted in the LDH (passing from PMMA-Zn2Al(OH)6·nH2O to PMMA-ZnCuAl(OH)6·nH2O). For PMMA-Zn2Al0.75Fe0.25(OH)6·nH2O, a cooperative contribution of iron reduction, stabilisation of layered edifice, and radical trapping effects was observed for the thermal stability of the nanocomposite. LDH also acted as a diffusion barrier to the efflux and evaporation of depolymerized species, favouring the charring which exerts an additional contribution to thermal stability of the PMMA-LDH nanocomposites.
ABSTRACT
We evaluated the effects of crude extracts from the plantain Plantago lanceolata and the bitter gourd Momordica charantia on the oviposition preference and development of the coffee leaf miner Leucoptera coffeella Guérin-Mèneville & Perrottet under laboratory and/or greenhouse conditions. The ovicidal effects of these extracts were also studied in a greenhouse. Plantago lanceolata and M. charantia extracts also underwent fractionation directed by oviposition tests with the coffee leaf miner. The extracts of both plants reduced L. coffeella oviposition and egg hatching, apparently as a result of action of plant metabolites on the embryo. Adults originating from eggs treated with the extracts exhibited similar survival rates, but a higher female/male ratio. Fecundity was reduced for females obtained from eggs treated with the M. charantia extract. Partial chemical analysis indicated that both extracts produced polar fractions that reduced the oviposition of L. coffeella on coffee leaves under laboratory conditions. The extracts of P. lanceolata and M. charantia have potential for use in the development of new products to control the coffee leaf miner.
