ABSTRACT
The geopolymerization process necessitates the activation of an aluminosilicate source by an alkaline solution. Its kinetics is followed by rheology. The storage modulus (G') and loss modulus (G'') are monitored through oscillatory rheological measurements from the early stage, geopolymer paste, to the gel point with the formation of a geopolymer network. The results show that the temperature increase shortens the reaction time. The principle of Reaction Time-Temperature Superposition (RTTS) is introduced to predict this phenomenon. Furthermore, it is pointed out that using metakaolin blends with different reactivities allows modifying and controlling the reaction time. A critical weight fraction of reactive metakaolin is identified as necessary for the formation of the geopolymer network. The reaction times of the different formulations are linked to the temperature and the weight fraction of metakaolin by the Arrhenius law. A model is established to predict the reaction time according to the temperature and the weight fraction between the two metakaolins used.
ABSTRACT
In this work, thermoreversible poly(trimethylene carbonate) (PTMC) based networks with different crosslinking densities were obtained by Diels-Alder (DA) reaction between furan-functionalized PTMC precursors and a bismaleimide. Furan-grafted PTMC with various functionalities determined by 1H-NMR analyses were prepared from telechelic PTMC oligomer, glycerol, 4,4'-methylenebis(cyclohexyl isocyanate) (H12MDI) and furfuryl alcohol. The formation of network structures by DA reaction between furan and maleimide groups were proved by Fourier-transform infrared spectroscopy (FT-IR). Although both exo and endo DA adduct forms exist, the thermally more stable exo form dominates. The thermoreversibility of networks was evidenced by FT-IR, solubility, differential scanning calorimetry (DSC) and rheology experiments at different temperatures. By increasing furan functionality or node concentration, denser and stiffer networks could be formed with higher Young's modulus and true stress at break in tensile tests, as well as higher crossover temperature, which indicates a nominal transition from elastic behavior to viscous state. The disruption of networks was found to occur in high temperature ranges from 130 to 160 °C, depending on their crosslinking density.
ABSTRACT
Currently, there is very strong interest to replace synthetic polymers with biological macromolecules of natural source for applications that interact with humans or the environment. This research describes the development of drug delivery hydrogels from natural polymers, starch, lignin and hemicelluloses by means of reactive extrusion. The hydrogels show a strong swelling ability dependent on pH which may be used to control diffusion rates of water and small molecules in and out of the gel. Also the hydrogels degradation rates were studied in a physiological solution (pH 7.4) for 15days. The results indicated that for all three macromolecules, lower molecular weight and higher level of plasticizer both increase the rate of weight loss of the hydrogels. The degradation was extremely reduced when the polymers were extruded in the presence of a catalyst. Finally the dynamic mechanical analysis revealed that the degradation of the hydrogels induce a significant reduction in the compressive modulus. This study demonstrates the characteristics and potential of natural polymers as a drug release system.
