Rheological gel point

The Winter and Chambon criterion was adopted to define the critical rheological gel point (i.e. the percolation threshold at the formation of an incipient continuous network of infinite molecular weight). This condition is marked where a power-law variation of G and G viscoelastic moduli with a is obeyed, hence G (co) G (co) cd, and irrespectively of o, tan 5 (=G"/G )... 

Table 1. Critical power-law parameters at the rheological gel point for incipient i-carrageenan and t-carrageenan /chitosan NPEC gels in 0.25 mol-L NaCl.

The extent of the long-chain branching of a series of ethyl xanthogen disulfide modified polymers carried to increasing conversion is shown in Table 1. No branches are found until 56% conversion. The gel point is a Htfle over 82%. Polymer rheology deteriorates between 56 and 82% conversion. 

The pulse mode experiments were conducted by placing the sample between the quartz plates and exposing it to 30 second bursts of UV radiation. Dynamic rheological tests were then applied to the sample and this process was repeated until the sample had passed its gel point and became highly cross-linked. 

Dynamic rheological measurements have recently been used to accurately determine the gel point (79). Winter and Chambon (20) have determined that at the gel point, where a macromolecule spans the entire sample size, the elastic modulus (G ) and the viscous modulus (G") both exhibit the same power law dependence with respect to the frequency of oscillation. These expressions for the dynamic moduli at the gel point are as follows ... 

We also correlated the elastic modulus (G ) with the extent of reaction by combining data obtained from rheology and FTIR. We found that the elastic modulus did not show an appreciable increase until a short time before the gel point of the sample had been reached. However, in the vicinity of the gel point and beyond, the elastic modulus increased significantly with conversion. Information relating modulus to extent of reaction, which plays a critical role in the application of these systems, can therefore be readily obtained using our approach. 

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. 

Note 3 The gel point is often detected using rheological methods. Different methods can give different gel points because viscosity is tending to infinity at the gel point and a unique value cannot be measured directly. 

Thermosetting polymers represented in Figs 6.4, 6.5, and also 6.6 were cured at T > Tgw In many cases and particularly for high-Tg networks, a precure step is performed at Tj < Tgco. In this case the two transformations, gelation and vitrification, occur and the rheology in the vicinity of the critical gel point could be affected by vitrification. 

Rheological analyses give information about the viscosity and gel point. Such analyses have been coupled with thermogravimetry and have been used to determine a good way to prepare thermostable foams [32]. 

The low shear rheology measurements also show a rapid increase in the viscoelastic properties (modulus and zero shear viscosity) with increase of bentonite concentration above the gel point (> 30 g dm bentonite). Several models have been proposed to account for the elastic properties of concentrated dispersions, of which that originally proposed by van den Tempel (25) and later developed by Papenhuizen (26) seems to be the most appropriate for the present system. According to this model, if the interaction energy minimum between adjacent particles is sufficiently negative, a three-dimensional network structure may ensue, giving an elastic component. Various models can be used to represent the three dimensional structure, the simplest of which would be either an ideal network where all particles are... 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

---