Rheological behavior and gel-point determination for a model Lewis acid-initiated chain growth epoxy resin

The sol-gel transition, or gelation, in a polymer network-forming system is defined as the critical point in the polymerization at which an infinite macromolecule is formed. There are many studies in the literature concerning the exact and accurate determination of the gel point and its interpretation in terms of the critical conversions of reactant functional groups. However, these have mainly involved step-growth polymerizations with few studies of chain-growth polymerizations such as that described in this study. Chemorheological studies on a cationically polymerized epoxy network-forming system are used to determine accurate gel times and critical conversions, which are interpreted in terms of classical branching and chemical bond percolation models for nonlinear polymerizations. Rheological studies during the polymerization of a trifunctional epoxide monomer, using BF3 as the Lewis acid initiator, yielded a gel conversion of 0.17 +/- 0.02 and a power law dependence for the development of shear modulus, with a critical exponent n = 0.64 +/- 0.03, which is shown to be independent of temperature and BF3 concentration. The viscosity at conversions close to the gel point was shown to vary according to a power law with a critical exponent k = 1.33 +/- 0.03 in the pregel region. The critical behavior of this complex network-forming system is shown to be closely predicted by percolation theory.