Comparison of Network Structure Photopolymerized under UV-LED and High-Pressure Mercury Lamp with Different Photoinitiators

UV-curable resin of multifunctional monomers is believed to form a cross-linked network structure. However, the nature of this network is difficult to analyze. In this study, a kinetic approach was applied to compare the network structures of photopolymerized diurethane dimethacrylate cured by different light sources and photoinitiators. The conversion after UV irradiation was monitored to calculate the rate coefficients of propagation and termination (k(p) and k(t), respectively) under varying irradiation times and at constant UV intensity and temperature. The values of k(P) and k(t) were compared for different UV light sources (a 365-nm peak UV-LED and a high-pressure mercury lamp) and different photoinitiators (1-hydroxycyclohexylphenylketone (HCAP) and 2,4,6-trimethylbenzoylphenyl phosphinate (TPO)). The results indicate that the network structure formed under different light sources with the same photoinitiator could be scaled, namely that the formed cross-linked network structures could be superimposed on top of each other by linear expansion or reduction. The reason is that the light sources only differ in the rate of photon delivery, while the photons are identical to each other in terms of chemical reactivity. In contrast, the network structures formed with different photoinitiators could not be scaled, since the photoinitiators differ in their quantum yield, kinetic constants, affinity, and molecular size. These factors could affect the network structure. Although the kinetic analysis in this study only provides very limited information about the network structure, we experimentally confirmed that the network structure depends on both the UV light source and photoinitiator.