Temperature profiles, frontal

Figure 11.7 shows the temperature history at a fixed point in the reaction tube as a front passes. The temperature at this point is ambient when the front is far away and rises rapidly as the front approaches. Hence, a polymerization front has a very sharp temperature profile (Pojman et al., 1995b). Figure 11.7 shows five temperature profiles measured during frontal free-radical polymerization of methacrylic acid with various concentrations of BPO initiator. Temperature maxima increase with increasing initiator concentration. For an adiabatic system, the conversion is directly proportional to the difference between the initial temperature of the unreacted medium and the maximum temperature attained by the front. The conversion depends not only on the type of initiator and its concentration but also on the thermodynamic characteristics of the polymer (Pojman et al., 1996b). 

The defining features of frontal polymerization are the sharp temperature and concentration gradients present in the front. Figure 1 shows a temperature profile for benzyl acrylate polymerization. Notice that the temperature jumps about 100 °C over 1 cm. The concentration gradient is two orders of magnitude larger. 

Figure 12 Temperature profiles for the frontal polymerization of undiluted acrylamide, of acrylamide diluted with commercial polyacrylamide, of acrylamide diluted with barium carbonate, and of acrylamide diluted with frontally polymerized acrylamide. Adapted from Fortenberry, D. I. Pojman, J. A. J. Polym. Sci. Part A Polym. Chem. 2000, 38,1129-1135. ...

Figure 15 The temperature profile of frontal polymerization of acrylamide immersed in liquid nitrogen. Adapted from Pojman, J. A. Ilyashenko, V. M. Khan, A. M. J. Chem. Soc. Faraday Trans. 1996, 92, 2825-2837. ...

Fig. 5.13. Example of overloaded elution profiles from the frontal analysis runs described in Fig. 5.8 and Fig. 5.12. D,L-PA was injected at sample volumes of 240, 160 and 80 p at a sample concentration of 1 g/L and a temperature of 40°C. Experimental data dotted lines. Numerical calculations solid lines. (A) L-PA, main figure = 117.3 C " "/min, upper...

Finally we pay attention to the ideal frontal TC (cf. Fig. 4.1). The high temperature front of the zone profile is obviously proportional to the adsorption isobar and so, at least for the localized adsorption model, to the adsorption constant. As such, it would obey Eq. 5.14. It holds for the activities which do not appreciably decay in the course of run. As for the shorter-lived nuclides, both the elution and the formally frontal TC result in non-ideal frontal chromatograms. Their shapes are close to what would arise from ideal processing during t . but they are smeared due to the random lifetimes of nuclei. Still the initial part of the thermochromatogram might be useful for evaluation of the required quantity, provided that the statistics of detected decay events is good. 

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

IGC measurements can be carried out using a pulse or continuous technique. The pulse of probe molecule is introduced into the carrier gas stream. This pulse is transported by the carrier gas through the system to the column with the solid sample. On the stationary phase, adsorption and desorption occur and the result is a peak in the chromatogram. The ratio of adsorption/desorption is governed by the partition coefficient. At fixed conditions of temperature and flow rate, the time of retention of a compound is characteristic of the system. An alternative is the fi ontal technique. This is carried out by injection into the carrier gas stream of a continuous stream of the probe molecule. When the sample enters into the column, there is a distribution between phases, and the concentration profiles takes the shape of a plateau, preceded by a breakthrough curve. The shape of this curve is characteristic of each system [3]. The benefit of the frontal technique is that equilibrium can be always established due to its continuous nature while pulse chromatography requires the assumption of a fast equilibration of the probe molecule adsorption on the surface. Between both techniques, the main part of publications describes pulse experiences, since they are faster, easier to control and more accurate, especially if interactions between probe molecules and the adsorbent are weak. 

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