Design and properties of novel photothermal initiators for photoinduced thermal frontal polymerization
Three photothermal initiators (2959-HDHP, 184-HDHP and 1173-HDHP) were designed and synthesized from three commercial photoinitiators (Darocur 2959, Irgacure 184 and Darocur 1173) and characterized by FTIR and 1H NMR. Their absorption spectra and photocleavage processes were investigated using a UV-vis spectrometer. They showed almost the same maximum absorption peaks and much lower photocleavage rates compared with the unmodified photoinitiators respectively. The synthesized photothermal initiators exhibited good deep initiation ability, but due to the photooxidation process of Darocur 2959 which decreased its photoinitiation efficiency, 2959-HDHP showed a relatively worse performance than 184-HDHP and 1173-HDHP. The photothermal initiator 1173-HDHP realized much deeper photoinduced polymerization than the mixture of the corresponding photo- and thermal initiators. This is because for the photothermal initiator, the heat transfer from photopolymerization to the thermal initiator occurs in situ, which has a much higher efficiency than the mixed initiator system that takes place at a longer distance during thermal diffusion.