Control of Gelation and Network Properties of Cationically Copolymerized Mono- and Diglycidyl Ethers

The development of low temperature curing systems has become a major objective in thermoset technologies for both environmental and economic reasons. The use of protic and chelating additives have recently been underlined for the control of the cationic ring-opening polymerization of epoxies, a curing mode that is very efficient at temperatures close from the ambient but that can easily runaway. In this paper, we propose to use this strategy to control the kinetics of the cationic copolymerization of a diepoxy monomer (diglycidyl ether of bisphenol A, DGEBA) with a monoepoxy monomer (phenyl glycidyl ether, PGE). The purpose of the study is to tune the cross-link density (ν_<i>e</i>) in order to control the mechanical properties of the materials. The sol–gel transition was first investigated in details at several frequencies by using the Fourier transform mechanical spectroscopy method (FTMS). We found that the gel time (<i>t</i>_<i>gel</i>) and the critical conversion (α_<i>gel</i>) can be controlled to a great extent by promoting transfers and complexing cationic species involved in the polymerization mechanism. The FTMS method also gives some insight into the structure of the polymer clusters at the sol–gel transition. The results indicate that the various additives used to control the transition have mostly no influence on the clusters’ structure. The properties of the fully cured networks were then investigated via swelling and dynamic mechanical measurements. Both methods indicate that ν_<i>e</i> is strongly influenced by the cross-linker content (DGEBA) but also by the additive used to control the curing kinetics. Interestingly, the measurement of the tensile properties at large deformations demonstrates that the resulting system offers a series of materials with a wide range of mechanical properties