Producing critical exponents from gelation for various photoinitiator concentrations; a photo differential scanning calorimetric study

Photoinitiated radical polymerization of an 80 wt% epoxy diacrylate (EA) and 20 wt% tripropyleneglycoldiacrylate (TPGDA) mixture with various 2-Mercaptothioxanthone (TX-SH) photoinitiator concentrations was studied by using photo-differential scanning calorimetric (Photo-DSC) technique. Photopolymerization reactions were carried out under the same conditions of temperature and light intensity. It was observed that all conversion curves during gelation at various photoinitiator concentration present nice sigmoidal behavior which suggests application of the percolation model. The critical time, where polymerization reaches the maximum rate (Rp(max)) is called the glass transition point (t(g)). The gel fraction exponents, beta were produced from the conversion curves around t(g). The observed critical exponents were found to be around 0.55, predicting that the gel system obeys the percolation model. Rp(max) and final conversion (C-5) values were found to be increased as the photoinitiator concentration was increased. On the other hand t(g) values decreased as photoinitiator concentration was increased, indicating higher TX-SH concentration causes early glass transition during radical polymerization. (C) 2011 Elsevier B.V. All rights reserved.