Gelation conductivity

The strain temperature sweep measurement is conducted with a preselected amplitude for the applied strain (y) and a constant frequency (f). The changing parameter is the temperature T, which is given in a temperature-time profile [T = T(t)]. This test method serves to illuminate the structural build-up, the softening, the melting and the gelation of pectins influenced when the temperature changes. 

Functional property tests were conducted in duplicate. AACC (21) methods were used for the determination of water hydration capacity (Method 88-04) and nitrogen solubility index (NSI) (Method 46-23). Oil absorption capacity was measured by the procedures of Lin et al. (22) and oil emulsification by a modification (22) of the Inklaar and Fortuin (23) method. Pasting characteristics of 12.0% (w/v, db) slurries of the flours and processed products were determined on a Brabender Visco/Amylograph (Method 22-10). The slurries were heated from 30 to 95°C before cooling to 50°C to obtain the cold paste viscosity value. Gelation experiments were conducted by heating 15% (w/v db) slurries in sealed stainless steel containers to 90°C for 45 min in a water bath C3). 

Figure 20. Spectroelectrochemical analysis of thin films of V2O5 ambigels supported on conductive glass (indium—tin oxide). The current response is given by the continuous line, and the change in absorbance monitored at 400 and 800 nm as a function of potential (and time) is shown as individual data points. The V2O5 ambigel was prepared by gelation of aqueous metavanadate, dried from cyclohexane, and calcined in air at 170 °C. (Printed with permission from ref 232.)...

Due to the fact that there is no accurate technique valid for the entire curing reaction, two techniques should be used to follow cure one applied to before gelation (DSC) and another one applied after gelation (TBA, DMA, TMA, d.c. conductivity [50,128,174,175]). 

Casting of composite propellants is usually conducted under reduced pressure to eliminate air or other gases from the liquid propellant mix. Any voids caused by expanded gases will remain fixed in the propellant grain if gelation occurs before the vacuum is released. Hence, the vacuum must be released after volatiles are removed but before gelation... 

Experimental e"(t) curves exhibit an inflection point that corresponds to the maximum rate of change of conductivity, associated to the maximum decrease of mobility. For this reason this inflection point has been attributed to gelation (Wasylyshyn and Johari, 1997). In some cases good agreement between the inflection time and the gel time obtained by rheological methods was observed. A very complex explanation taking into account inter-molecular H bonds, protonic conduction, and a bond percolation law has been proposed to explain this mobility change (Johari and Wasylyshyn, 2000). 

Dielectric measurements are insensitive to gelation. This important point is mainly based on experiments with epoxy-amine reactions for which the dielectric parameters are controlled by ionic conductivity. More experiments with other chemistries are needed to reach a more universal conclusion. 

Measurements of dielectric properties have been used to monitor chemical reactions in organic materials for more than fifty years. In 1934, Kienle and Race 11 reported the use of dielectric measurements to study polyesterification reactions. Remarkably, many of the major issues that are the subject of this review were identified in that early paper the fact that ionic conductivity often dominates the observed dielectric properties the equivalence between the conductivity measured with both DC and AC methods the correlation between viscosity and conductivity early in cure the fact that conductivity does not show an abrupt change at gelation the possible contribution of orientable dipoles and sample heterogeneities to measured dielectric properties and the importance of electrode polarization at low frequencies. 

This same idea helps explain the knee in the conductivity-versus-time data noted by many workers, but specifically interpreted as gelation by Acitelli, et al.60). The data are shown in Fig. 30 a, including Acitelli s observation of the coincidence between the time to reach gelation and the time at which the extrapolated linear regions intersect. We now understand this coincidence it is not related to gelation, but results from the autocatalyzed reaction kinetics coupled with the specific properties of the WLF Equation, as explained below. 

In Section 4, we have examined, from a fundamental point of view, how temperature and cure affect the dielectric properties of thermosetting resins. The principal conclusions of that study were (1) that conductivity (or its reciprocal, resistivity) is perhaps the most useful overall probe of cure state, (2) that dipolar relaxations are associated with the glass transition (i.e., with vitrification), (3) that correlations between viscosity and both resistivity and dipole relaxation time are expected early in cure, but will disappear as gelation is approached, and (4) that the relaxed permittivity follows chemical changes during cure but is cumbersome to use quantitatively. 

Similar studies with dilute and semidilute suspensions of luminescent libers in organogels should be possible, but none to our knowledge has been conducted w ith the same level of detail. It is most probable that interchain hopping between gelator fibers will be much less efficient than transfer within and along the long axis of a fiber. 

Discoveries of new luminescent gelators or thickeners remain mainly fortuitous for the same reason that the selection a priori of any low-mass molecule as a gelator is not possible. Only organogelators for which at least preliminary emission studies have been conducted are included, though many others contain structural units that should be luminescent. Correlations, when possible, are made between the molecular structures and electronic properties of the gelators. and some properties of their gels. 

To study the mobility of Li" " on the gel matrix, we measured the Li diffusion coefficient before and after gelation of the 2-Li (with 5 wt% PAMPSLi) using pfg-NMR. The Li diffusion coefficient in the gel was almost unchanged from the value before gelation, despite the decrease in the ionic conductivity with gelation. Because, as this result suggests, the mobility of Li is not suppressed by gelation, this gel system is favorable for Li" " conductive materials. 

Figure 26.4 Dependence of ionic iiquid gei conductivity on concentration of gelator 1 at room temperature (A)C4mim/TFSi, (W) C4mim/BF4, (9) C4mim/PFe, (A) T, (Q) C4Py/BF4, ( ) Cemim/BF4, (x) B, and (U) CePy/PFe-...

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