Universality in gelation of epoxy acrylate with various photoinitiators: a photo differential scanning calorimetric study

Photo-differential scanning calorimetric (Photo-DSC) technique was used to study the gelation of P-3038 (epoxy acrylate (EA) 75% and tripropyleneglycoldiacrylate (TPGDA) 25%) in the presence of various thioxanthone-based initiators, namely, thioxanthone (TX), 5-thia-naphtacene-12-one (TX-NP), 2-(carboxymethoxy) thioxanthone (TX-OCH₂COOH), 2-thioxanthone-thioacetic acid (TX-SCH₂COOH), and 2-mercaptothioxanthone (TX-SH). Photopolymerization reactions were performed under identical conditions of temperature, initiator concentration, and UV light intensity. Photo-DSC technique allowed us to monitor the gelation, without disturbing the system mechanically, and to test the universality of the gelation as a function of some kinetic parameters like initiator concentration. During gelation, it was observed that all conversion curves present a good sigmoidal behavior by predicting to employ percolation model. Observations around the glass transition point, t _g shows that the gel fraction exponents β obeyed the percolation picture. On the other hand, t _g was found to be much higher for the crosslinked networks obtained with TX-OCH₂COOH and TX-SCH₂COOH initiators than those with the other initiators.