Photopolymerization behavior of di(meth)acrylate oligomers

Photo-initiated polymerizations of two series of di(meth)acrylate oligomers, one bisphenol-A based and the other ethylene glycol based, with a camphorquinone/tertiary amine radical photo-initiator system were studied using differential scanning calorimetry. Bisphenol-A based oligomers polymerized much faster than ethylene glycol based oligomers because the higher viscosity of E-bis-A oligomers induced an autoacceleration upon exposure to ultraviolet (UV) radiation; however, higher vinyl conversions were observed with ethylene glycol oligomers due to their greater segmental mobility and elastomeric character of the cured networks. The two series also showed very different temperature dependence for vinyl group conversion. The polymerization rate of E-bis-A DMA oligomers increased when the number of oxyethyl units increased from four to six, but the rate decreased when the spacer length further increased. The exponential factors of the rate dependence on the CQ concentration for all the oligomers were smaller than 0.5. Below 70°C, the maximum rate of polymerization increased with temperature, and yielded apparent activation energies between 10.3–11.6kJ/mol. At temperatures >80°C, the reduced viscosity and improved segmental mobility suppressed autoacceleration, causing the maximum rate to decrease with an increase of temperature.