Gelation/gels kinetics

Fig. 15. TTT cure diagram TJT vs. times to gelation and vitrification. Theoretical (solid lines) First-order kinetics using the following parameters T = —19 °C T, = 166 °C Ej/Em = 0.34 FJPft = 0-19 E, = 12.6 kcal/mole A = 4.5x 10 min" pgy, = 0.75 g,Tg = 49 °C. Experimental , pregel (TBA) , gelation (TBA) Q, vitrification (TBA) , diffusion control (infra spectroscopy) A, gelation (gel fraction). The system studied was Epon 828/PACM-20 (see Fig. 4 caption)...

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

Kinetic gelation simulations seek to follow the reaction kinetics of monomers and growing chains in space and time using lattice models [43]. In one example, Bowen and Peppas [155] considered homopolymerization of tetrafunctional monomers, decay of initiator molecules, and motion of monomers in the lattice network. Extensive kinetic simulations such as this can provide information on how the structure of the gel and the conversion of monomer change during the course of gelation. Application of this type of model to polyacrylamide gels and comparison to experimental data has not been reported. 

Torkelson and coworkers [274,275] have developed kinetic models to describe the formation of gels in free-radical pol5nnerization. They have incorporated diffusion limitations into the kinetic coefficient for radical termination and have compared their simulations to experimental results on methyl methacrylate polymerization. A basic kinetic model with initiation, propagation, and termination steps, including the diffusion hmitations, was found to describe the gelation effect, or time for gel formation, of several samples sets of experimental data. 

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

Decrease of the lowfield line amplitude during gelation is chosen as the spectrum feature it) used for the kinetic analysis (Figure 9). This variation is related to the growth of the gel solid-like component. The big difference in iinewidth between the sharp lines of the fluid component and the broad Lorentzian of the solid-like component allows a sufficient sensitivity for the method. 

When the helix amount increases the medium changes from a viscous liquid (sol) to an elastic solid (gel). The kinetics of gelation depends strongly on the quenching temperature. The rheological measurements that we performed are particularly focused on the sol-gel transition and on the definition of the "gel point". The greatest difficulty encountered is due to the weakness of the bonds which can easily be destroyed by the mechanical stress. 

The helix amount appears as the "key" parameter for the gel formation. As for chemical gelation, the temperature influences only the kinetics of the reaction, but not the structure. However, in our case, a temperature increase slows down the process. 

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