Macromolecules, Vol.53, No.13, 5034-5046, 2020
High-Efficiency Radical Photopolymerization Enhanced by Autonomous Dark Cure
Radical photopolymerization (RPP) has grown into a multibillion-dollar technology for reduced energy consumption and waste with increased productivity. However, radical-mediated polymerization ceases almost immediately following discontinuation of irradiation because of rapid termination of reactive centers. This restricts the wider use of RPP in applications that involve light attenuation or irregular surfaces because uniform polymerization is not guaranteed for these challenging exposure conditions and the resultant undercuring leads to compromised material properties and harmful leachable monomers. Herein, we developed a unique radical dark-curing photoinitiator (DCPI) that continues its polymerization beyond the cessation of irradiation. The DCPI achieved a remarkable 25-60% additional conversion over a 1 h period, when light was shuttered at 20% conversion, compared to the 1-3% additional conversion achieved by a Norrish type II control photoinitiator. We elucidated the origin of the high photon efficiency using computational studies and experiments, which suggest that the DCPI may be the most-photon-efficient photoinitiator to date. We also demonstrated that the mechanical properties of dark-cured polymer are similar to and even exceed those of the corresponding polymer obtained by extended photocuring. In particular, the initial 0.1 MPa storage modulus continuously developed to 4.3 MPa without further irradiation, while also exhibiting similar to 30% less shrinkage stress than the full exposure control. With its superior photo- and dark-polymerization efficiency, the DCPI enhances the performance of many existing RPP processes while extending RPP to heretofore unattainable applications. The DCPI accomplishes such a performance through an inherent automatic rectification of initially undercured regions and defies the RPP paradigm that light dosage directly correlates to conversion.