Order of gel formation

The NMR measurement was made on PVA solutions at low temperature in order to clarify the mechanism of gel formation by freeze-thaw cycles [16]. The C CP/MAS NMR spectrum for the PVA solution was measured at -50°C by going from room temperature to -50°C (Fig. 20.6). The signals for the CH group shows that instead of the three peaks, which arise from the splitting by tacticity, peaks I, II and III appear. The result shows that in the PVA solution at -50°C, some hydroxyl groups form hydrogen bonds as in the PVA gel and so the crosslinked structure is formed in the PVA solution. This means that the gel formation is induced in going from room temperature to -50°C. 

Gonsideiing the reactions in formulas (1) and (2) and the foregoing discussion of the possible reaction mechanisms, it seems likely that the photoinitiated crosslinking is a second order reaction, i.e. the rate should be a linear function of the UV intensity in square. Assuming that the rate of crosslinking (which is difficult to measure) is proportional to the rate of gel formation (which is easy to measure), the data in Fig. 6 support thds conclusion at low UV intensities, i.e. at... 

Finally, an entirely different approach to simulating gelation is the Dynamic Monte Carlo (DMC) method, in which chemical reactions are modeled by stochastic integration of phenomenological kinetic rate laws [23]. This has been used successfully to understand the onset of gel formation, first-shell substitution effects, and the influence of cyclization in silicon alkoxide systems [24—26]. However, this approach has not so far been extended to include the instantaneous positions and diffusion of each oUgomer, which would be necessary in order for the calculation to generate an actual model of an aerogel that could be used in subsequent simulations. 

In order to estimate the effect of adiabatic conditions on the evolution of reaction, we performed a series of laboratory tests on polymerization in a cuvette placed into a foam-plastic case with organic glass windows for observation and photoinitiation. The reaction mixture - cuvette mass ratio was 1 20. The experiments showed that in this condition the process of gel formation accelerated. However, the increasing cuvette mass allowed us to reduce the maximal increase of the cuvette temperature to 10 °C (in the earlier series of orbital experiments the overheating measured several tens of degrees). This mass-to-mass ratio was kept unchanged in our orbital experiment. 

The rate of gel formation and the conformational order depend strongly on the polymer concentration. This indicates that the conformational order proceeds with the aggregation of molecular segments. In order to elucidate its mechanism, the dynamic mechanical behavior of conformational order is analyzed. As a model, we will consider the process of... 

After irradiation by a dose of 100 Mrad, the value of increases up to 200% for polyurethane-sulfosemicarbazide added with Co(AcAc)3 and A takes its minimal value of 78%. Processes of gel formation evidence that Fe(AcAc)3 and Cr(AcAc)3 have little effect on this process. Upon irradiation of the poly-urethane-sulfosemicarbazides modified by Zn(AcAc)2 with a doze of 50 Mrad, the coefficients A and A are 49.2 and 48.5%, respectively. The inhibitory activity of M(AcAc)n in fiin process of radiolysis of the elastomers studies obeys the order Co(ni)>Cr(III)>Fe(III) Zn(II). The stabilizing effect of M(AcAc) upon radiolysis of polyurethane-sulfosemicarbazides is possibly related to redox processes and to a decrease in the oxidation number of the metal upon the destruction of hydroperoxide. 

This concept may be invoked to account for electrolyte formation in microcracks in a metal surface or in the re-entrant angle formed by a dust particle and the metal surface. More importantly, it can also explain electrolyte formation in the pores of corrosion product and hence the secondary critical humidity discussed earlier. Ferric oxide gel is known to exhibit capillary condensation characteristic and pore sizes deduced from measurements of its adsorptive capacity are of the right order of magnitude to explain a secondary critical relative humidity as70 7o for rusted steel . 

An unusual method has been used to prepare a hyperbranched polyimide starting from two monomers a difunctional A2 and a trifunctional B3. The gel formation can be avoided with careful control of the polycondensation conditions (molar ratio, order of the monomer addition, and low concentration). The A2 and B3 monomers were respectively 6FDA and tris(4-aminophenyl)... 

Recently the polymeric network (gel) has become a very attractive research area combining at the same time fundamental and applied topics of great interest. Since the physical properties of polymeric networks strongly depend on the polymerization kinetics, an understanding of the kinetics of network formation is indispensable for designing network structure. Various models have been proposed for the kinetics of network formation since the pioneering work of Flory (1 ) and Stockmayer (2), but their predictions are, quite often unsatisfactory, especially for a free radical polymerization system. These systems are of significant conmercial interest. In order to account for the specific reaction scheme of free radical polymerization, it will be necessary to consider all of the important elementary reactions. 

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