Gel effect index

We now define a quantitative measure of the magnitude of the gel effect the gel effect index, y ... 

Experimental conversion-time data, obtained from the literature, on the bulk free radical polymerization of MMA initiated by AIBN at several temperatures and initiator concentrations, were described by the model. However, the expressions for the rate of conversion and gel effect index were first simplified and rearranged. ... 

Assuming that no chain transfer reaction was sufficiently important to be considered, useful simplifications result for equations (la), (3) and (4), so that the gel effect index, y, is given by the expression ... 

If equations (7) and (9) are then introduced into equation (6), as well as the correct expressions for n and the variation of with conversion, the expression for tfie gel effect index may be written as ... 

The general equation for the gel effect index, equation (la) which incorporates chain transfer, was used in those cases where there was not a good agreement between model predictions and experimental data. The same values of and (derived from the values of and C2 found at high rates) were used in the integration of equation (1) and the value of the constant of chain transfer to monomer, C, was taken as an adjustable parameter and used to minimize tfie error of fitting the time-conversion data by the model. 

The Tromsdorf effect, also called the Norrish effect or gel effect, is associated with exothermic reactions during bulk polymerization. Autoacceleration of the polymerization rate can occur with medium to high polymerization conversions. This phenomenon inhibits termination. Strength of the Tromsdorf effect is calculated as the gel effect index [12]... 

With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the gel effect is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become normal. Ultimately, the stationary-state conditions may eventually apply. 

The initial overshoot in number-average molecular weight (Fig. 2.3.6) and poly-dispersity index (Fig. 2.3.7) at low conversions seems to be consistent with the initial OA line segment of the system trajectory as depicted in Fig. 2.1.11, wherein the system follows a solution polymerization process during the first 10 min (based on Figs. 2.3.5-2.3.7). Subsequently, when the polymer-rich phase was being formed as the system traversed the OB line segment of the reaction trajectory in Fig. 2.2.5, the tendency for this phase was to go into gel effect, as seen from the sharper rise... 

Recently, Enami and co-workers [118] presented a hybrid cross-linked EO polymer/sol-gel Mach-Zehnder waveguide modulator with of 0.65 V at 1,550 nm. Such a low value of half wave voltage, which is one of the best results obtained in literature, is related to the combination of a very high electro-optic effect and to the device design that combines physical vertical tapers in the sol-gel core, index tapers in the EO polymer, a suitable electrode structure and a top buffer layer designed for cross-linked EO polymer systems. In such a device also the optical losses result to be reduced with respect to a more traditional configurations. 

These data can then be used to calculate the multiple parameter values by a best-fit approach. For example (Gartner, 2003) used a Bruggemann effective index approximation to fit several parameters to spectroscopic elUpsometer measurements of multi-layer FcgOg sol-gel films. By this method they were able to extract an additional characteristic, by dispensing with the assumption ofisotropy found in the simplified model of Eqs. (18-3)-(18-5),... 

The three modes of FP have proven to offer advantages for different applications. Photofrontal polymerization is driven by a continuous flux of energy and has been applied to the preparation of microfluidic chips. It can be applied to any photopolymerization. IFP relies on the gel effect to create a slowly moving localized polymerization through monomers like methyl methacrylate. This method can be used to prepare gradient refractive index materials. 

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

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