Gel conversion

TS-l/MCM-41 catalysts synttiesized by the dry gel conversion method are shown to have hexagonal mesopores. The catalytic activity of synthesized TS-l/MCM-41 catalysts was tested with qroxidation reaction of olefins to reved that both the conversion of olefins and selectivity to epoxide are higher than those of H-MCM-41,... 

In this work, highly active epoxidation catalysts, which have hydrophobic surface of TS-1, were synthesized by the dry gel conversion (DGC) method. Ti-MCM-41 was synthesized first by a modifed method and the TS-l/MCM-41 catalysts were subsequently synthesized by the DGC method. The catalysts were characterized by the XRD, BET, FT-IR, and UV-VIS spectroscopy. TS-l/MCM-41 catalysts were applied to the epoxidation of 1-hexene and cyclohexene with aqueous H202to evaluate their activities for the epoxidation reaction. ... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

The mathematical model of network formation in the pregel stage will focus on the prediction of the gel conversion and the evolution of number-and mass-average molar masses, Mn and Mw, respectively. For chainwise polymerizations, calculations will be restricted to the limit of a very low concentration of the polyfunctional monomer (A4 in the previous example). Thus, homogeneous systems will always be considered. 

The gel condition may be obtained by making Mw —> oo. This gives Xgei = 0.5. The gel conversion could have also been obtained by making W —> oo in Eq. (3.13). 

This value is close to experimental values of gel conversion reported for several diepoxide - diamine systems. 

Figure 3.13 shows the variation of the gel conversion of the limiting reactant as a function of the stoichiometric ratio. For r > 3, no gel is formed and the polymer remains in the liquid state after complete reaction of epoxy groups. If the amount of epoxy monomer necessary to obtain a stoichiometric system is added in a second step, polymerization restarts, leading to gelation and the formation of a network. The two-step polymerization is the basis of several commercial thermosetting polymers. 

Figure 3.13 Gel conversion of the limiting reactant as a function of the stoichiometric ratio.

Substitution effects produce only a slight shift in the gel conversion from 0.577 to 0.618 in the limit of R21 -> 0. Most of the reported experimental values of gel conversion in stoichiometric epoxy-amine systems are close to 0.60. Within the experimental error of the measurement of gel conversion, it is difficult to quantify the value of the reactivity ratio from this single determination. 

Because of the effective generation of branched fragments (F4) from the very beginning of polymerization, the gel conversion is advanced with respect to the value for the ideal case. 

Low values of the gel conversion are expected for the usual case of long primary chains. 

As in most chainwise polymerizations, q -> 1, the first observation arising from Eq. (3.180) is the very low value of the gel conversion. In actual systems, intramolecular cyclization and microgel formation produce an increase in the gel conversion. But reported values of xgei for the free-radical polymerization of systems containing multifunctional monomers are usually below 0.10. This is the case for the crosslinking of unsaturated polyesters (Af) with styrene (A2 ). 

Then, when termination takes place by combination (2, = 1), the gel conversion decreases by a factor 2/3 with respect to the case where termination occurs by chain transfer or disproportionation. 

Apart from the condition defining an ideal polymerization, the equations derived in this section assume that both q and E, remain constant in the pregel stage e.g., no drift occurs. This is usually a good assumption in the pregel period due to the extremely low value of the gel conversion. However, if drift occurs, integral equations must be used (Dotson et al., 1988). 

TEOS is a tetrafunctional monomer that homopolymerizes, forming Si-O-Si bonds by a stepwise mechanism. According to the statistics of stepwise polymerizations discussed in Sec. 3.1, the theoretical gel conversion is... 

However, 29Si nuclear magnetic resonance shows experimental gel conversions,... 

To provide a rough estimation of the gel conversion arising from such a mechanism, let us assume a very simple model of network formation in which a first stage involves the hydrolytic condensation of the monomer to produce octahedra as shown in Fig. 3.23. In the formation of octahedra,... 

This simple model provides an explanation of the large values of gel conversion found in practice. 

The polymerization temperature, often called the cure temperature, affects both transitions in different ways. In Chapter 3 it was shown that the gel conversion does not depend on temperature for ideal stepwise polymerizations but may show a small dependence on temperature for the case where unequal reactivity of functional groups or substitution effects vary... 

The reaction is often described as a living polymerization (Matejka et al, 1983), but neither the distribution of molar masses nor the observed gel conversion corresponds to a pure living mechanism (Mauri et al., 1997). Experimental observations may be explained by assuming the presence of a chain transfer step that regenerates the active initiator. This step deter-... 

The time and conversion corresponding to the inflection point of log a(t) curves are lower than those of the gel point (e.g., the inflection point is found at x 0.4 for epoxy-diamine systems while the gel conversion is found at x 0.6). 

Phase separation begins when the cloud-point curve reaches the point a at (4>mo>T ). This is defined as the cloud-point conversion, xCp, which is usually lower than the gel conversion, xgei. 

Figure 9.17 shows the gel conversion, estimated from a statistical analysis of the complex cure reaction, as a function of the amount of hexa used in the formulation. As a result of the sharp conversion profile in the part, the influence of the gel conversion value on the predicted thickness is rather small. 

Figure 9.17 Gel conversion as a function of the amount of hexa in the formulation. (Aranguren et al, 1984. Reprinted with permission from SAMPE)...

As the polymeric network extends throughout the total volume, the sol thickens to a gel.6 The sol-to-gel conversion is a gradual process, which is easily observed qualitatively, but difficult to measure analytically. The gelation point tge, is defined as the point where elastic stress is supported.13 The t is not an intrinsic property of a sol. It is influenced by the size of the container, the solution pH, the nature of the salt concentration, the anion and solvent, the type of initial alkoxy group and the amount of water. 

As water is withdrawn from a membrane during entry into anhydrobiosis, the strengths of interactions among acyl chains increase, and a shift from the fluid to the gel state is favored. This shift is noticeable as water content is reduced below about 20% (Crowe et al., 1997). Transition temperatures (Tm), the temperatures at which the fluid (liquid crystalline) to gel conversion occurs, increase significantly during dehydration. A membrane composed of palmitoyloleoylphosphatidyl choline has a Tm of —7°C when fully hydrated, but the Tm rises to approximately 60°C in the dry lipid (Crowe et al., 1997). Thus, a phospholipid membrane that would be in a fluid state at normal cell temperatures when hydrated acquires a rigid gel structure when dehydrated. 

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