Influence of microwave heating on liquid-liquid phase inversion and temperature rates for immiscible mixtures.

Time dependencies of component temperatures for mixtures of immiscible liquids during microwave heating were studied for acetonitrile-cyclohexane and water-toluene. For the first time, we report microwave induced liquid-liquid phase inversion for acetonitrile-cyclohexane mixture: acetonitrile layer was initially at the bottom of the mixture, after 10 sec of microwave heating its density decreased and it inverted to the top of the mixture for the remainder of the microwave heating. This phase inversion could not be achieved by conventional radiant heating. The maximum rate of temperature growth for the polar component of the mixtures was 2 - 5 times larger than for the non-polar component. This suggests that microwave energy is absorbed by polar liquids (water or acetonitrile) and heat is transferred into the non-polar liquid (toluene or cyclohexane) in the mixture by conduction (in case of cyclohexane) or conduction and convection (in case of toluene). Comparison between experimental data and semi-empirical mathematical models, proposed in [Kennedy et at., 2009] showed good correlation. Average relative error between theoretical and experimental results did not exceed 7%. These results can be used to model the temperature kinetics of components for other multiphase mixtures.

Time dependence of component temperatures in microwave heated immiscible liquid mixture.

Measured influence of microwave heating on time dependencies of component temperatures for two immiscible liquids in a mixture shows differences for polar (water) and non-polar (cyclohexane or carbon tetrachloride) liquids. The rate of increase for the temperature of water with time of microwave heating is larger than other liquids in the mixture (maximum rate of temperature growth for water is 8 times larger than corresponding rate for carbon tetrachloride and 2 times larger than cyclohexane). This leads to creating, for a considerable time period, a unique environment where there is a significant temperature difference between two liquids in a mixture. The maximum value of the difference between water and carbon tetrachloride temperatures in the mixture was 107 degrees C at 300 sec of microwave heating. While the maximum value of the difference between water and cyclohexane temperatures in the mixture was 57 degrees C at 135 sec microwave heating. This suggests that electromagnetic waves lose most of their energy to polar liquids (water), while the difference in rates of temperature growth for carbon tetrachloride and cyclohexane can be explained by different mechanisms of heat transfer from water to cyclohexane (conduction and convection) and to carbon tetrachloride (conduction only). Semi-empirical mathematical models for the time dependencies of temperature growth for components of the mixtures gave good correlation with experimental data (relative error less than 9%). These results can be used to model the temperature kinetics of components for other multi-phase immiscible liquid mixtures.

Dielectric in situ monitoring of microgravity polymerizations.

The objective of the research described here is to develop dielectric spectroscopy to monitor polymerization reactions and processes in microgravity. Ground based measurements have been made on both neat thermoset resins and solution polymerization reactions. Polymerization of diglycidyl ether of bisphenol A (DGEBA) using two curing agents, m-phenylenediamine (m-PDA), and 4,4'-diamino-diphenyl-methane (DDM) were investigated to determine the relationship between the dielectric spectra and the thermoset polymer network and morphology. The results for the epoxy thermosets indicate that this technique can be used to monitor the extent of polymerization and the qualitative nature of polymer network. The photopolymerization of 6-(2-methyl-4-nitroanilino)-2,4-hexadiyn-1-ol (DAMNA) in solution was also investigated to determine if gravitational effects could be observed using dielectric spectroscopy. The technique is able to distinguish between surface and solution polymerizations during formation of the polydiacetylene thin films. In addition, it is capable of monitoring convective effects during solution polymerization.