Conversion at gel point

Table 21.4 Conversion at gel point as function of oligo-polyol functionality and isocyanate functionality ...

Table 1. Conversion of Carboxyl Group at Gel Point when mPDA was used as Cure Agent.

Table 11.1 Times of liquid crystalline phase appearance (tj, gel point and conversion at phase transition (o, ) and at gel point Reproduced from reference 38.

A recent series of papers [18, 24, 32-34] substantially clears up the three-dimensional polymerization mechanism in the AAm-MBAA system. Direct observation of the various types of acrylamide group consumption using NMR technique, analysis of conversion at the gel-point, and correlation of the elastic modulus with swelling indicate a considerable deviation of the system from the ideal model and a low efficiency of MBAA as a crosslinker. Most of these experimental data, however, refer to the range of heterogeneous hydrogels where swelling is not more than 80 ml ml-1 [24]. 

For each data set examined, the onset of the gel effect (which is the initial value for the integration of the differential equations) was taken at the point where there is a departure from linearity in the conversion-time plot. While a good argument can be made ( ) for using another definition of the onset of the gel effect, the data available did not allow for a more detailed approach. 

Gelation occurs at relatively low conversions of monomer to polymer hence the number-average molar mass at the gel point is low. By contrast, however, the weight-average molar mass becomes infinite at the gel point. 

Figures 2 and 5 show the increase of the mass average molar mass, R i, with conversion for the systems with branched and linear prepolymers, respectively. These results indicate that addition of the branching monomer in the first stage yields much higher values of R, and the gel point is reached at lower conversion than addition in the third stage. Translated into practical properties this means that the processing and application qualities (e.g. flow) of a paint based on formulation F40 will be inferior to those of one on the basis of for-...

As might be expected, these differences in the pre-gel properties are also reflected in the post-gel regime. The sol fraction varies more smoothly with conversion for the branched prepolymer compositions, cf. Figures 3 and 6. But because the gel point is at (so... 

First of all it is necessary to determine the branching coefficient a, w hich is defined as the probability that a given functional group of a branch unit leads via a chain of bifunctional units to another branch unit. In a polymer of the type shown in Fig. 61, a is the probability that an A group selected at random from one of the trifunctional units is connected to a chain the far end of which connects to another trifunctional unit. As will be shown later, both the location of the gel point and the course of the subsequent conversion of sol to gel are directly related to a. 

Despite the importance of initiators, synthesis conditions, and diluents on the properties of a gel, composition is, of course, the most important variable. When growing polymeric chains are first initiated, they tend to grow independently. As the reaction proceeds, different chains become connected through cross-links. At a critical conversion threshold, called the gel point or the sol-gel transition, enough growing chains become interconnected to form a macroscopic network. In other words, the solution gels. The reaction is typically far... 

This critical value is, however, only an approximation leading often to an overestimation of the critical conversion value. The main reason to this failure is that gelation actually occurs, at least for some molecules, at a finite degree of polymerisation. Equations based on a statistical approach can provide better results in the estimation of gel point. 

Storey [32] observed some anomalies in the dependence of the gel point at higher concentrations of DVB which suggested some inhomogeneity and a tendency to microgel formation which explained the shift of the gel point towards higher conversions. 

---