The two measure different phenomena. As the previous response mentions, Tg is the temperature at which the polymer becomes glassy, due to the chains no longer having the cooperative movement necessary for strain relaxation. The gel point on the other hand is a measure of the degree of monomer conversion at which the polymer become solid and no longer flows. This is a result of crosslinking, not lowering the temperature as with Tg.
Tg at the gel point ,the poymer has lost almost 100 % of its mobility, in this point Tg is maximum, since the polymer is completely crosslinked. Remmeber that Tg is associated with a cooperative movement of the molecules, when it increases, the mobility decrease.
The two measure different phenomena. As the previous response mentions, Tg is the temperature at which the polymer becomes glassy, due to the chains no longer having the cooperative movement necessary for strain relaxation. The gel point on the other hand is a measure of the degree of monomer conversion at which the polymer become solid and no longer flows. This is a result of crosslinking, not lowering the temperature as with Tg.
.With increasing the degree of reaction the evolution of the glass transition temperature can be modeled according to a widely accepted DiBenedetto equation:
Tg(DR)=[(1-DR)*Tgi+ b*DR*Tgf.]/[(1-DR)+ b*DR]
DR=degree of reaction
Tg(DR)= Tg as function of the degree of reaction, DR
Tgi=Tg of the liquid (unreacted) resin
Tgf=Tg of fully cured rresin
b=adjustable parameter.
Please see also
- A.T. DiBenedetto, in Journal of Applied Polymer and Science Part B Polym. Phys., 25, 1949, (1987)
Gel point is the formation of an infinite network that prevents flow to occur. Normally it is measured with dynamic mechanical analysis. In a temperature ramp test the crossing of G' and G" is taken as the gel point. The degree of curing where the gel point occurs can be found from the same temperature ramp tests in a DSC.
The gel point is not a unique function of the degree of curing and the problem is rather involved. For instance: the molecular weight, Mw, increases due to both the propagation and the cross-linking reactions. The two phenomena depend on the temperature and resin formulation.
Please, see the pioneering work by Gillham:
Polymer Engineering and Science, Mid November 1986, Vol. 26, N. 20, p. 1492-1433
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