Free radical photocuring is used extensively due to the inherent advantages of temporal and spatial control over polymerization. Unlike ionic polymerizations, where direct termination reactions between polymer chains are restricted, radical polymerizations are prone to various biradical reactions leading to efficient termination. As a result, radical photocuring typically requires illumination throughout the polymerization procedure to achieve full conversion. Objective: To evaluate the potential of three-component photoinitator systems to promote visible light-activated methacrylate reactions with extensive dark cure potential.
Methods: 2-Hydroxyethyl methacrylate (HEMA) was combined with a photosensitizer (methylene blue; MB), a reductant (N-ethyldiisopropylamine; EDIPA) and an oxidant (diphenyliodonium chloride; DPI). A control formulation of HEMA with MB/EDIPA was also evaluated. Photocuring was activated by broad-band white light illumination (400-800 nm) with a variety of exposure times used to produce different conversion values at the point of light source shuttering. The progress of the polymerization process was monitored continuously for 60 min by near-infrared (NIR) spectroscopy.
Results: The high mobility of HEMA at early stages of polymerization produces an almost immediate cessation of polymerization in the control when the curing light is extinguished. A separate negative control involving no curing light exposure (0 s) for the experimental system demonstrates that NIR monitoring does not induce any polymerization. Visible light exposure of increasing duration lead to monotonically increasing levels if conversion at the point of light shuttering (initial conversion). The experimental three-component initiators induces dramatic additional dark cure in HEMA.
Illumination Time (s) | Initial Conversion (%) | Final Conversion (%) |
76 | 43.3 | 80.0 |
65 | 23.8 | 74.4 |
50 | 17.2 | 67.4 |
30 | 8.6 | 52.5 |
20 | 6.1 | 48.1 |
10 | 1.3 | 44.6 |
0 | 0.0 | 0.0 |
30 (control) | 9.2 | 9.9 |
Conclusion: The novel dark curing behavior demonstrated here indicates the potential for materials which reach high conversion and greater depths of cure with limited light exposure.
Support: NIH/NIDCR-2R01DE14227