Critical Exponents of Photoinitiated Gelation at Different Light Intensities

A photo-differential scanning calorimetric (Photo-DSC) technique was used to study the photoinitiated radical polymerization of a 75% epoxy diacrylate (EA) and 25% tripropyleneglycoldiacrylate (TPGDA) mixture with 2-mercaptothioxanthone (TX-SH) as photoinitiator by using different light intensities. Photopolymerization reactions were carried out under identical conditions of temperature and initiator concentration. It was observed that all conversion curves during gelation at various UV light intensities present good sigmoidal behavior as predicted by the percolation model. Observations around the critical time, called the glass transition point (t_g), taken for polymerization to reach the maximum rate (Rp _max) show that the gel fraction exponents β obeyed the universal percolation picture. On the other hand, Rp _max, t _g, and final conversion values were found to be dependent on the UV light intensity.