Influence of microencapsulation on stability and reactivity of 2,4,6-triphenylpyrylium gallate as cationic photoinitiator

The viability of 2,4,6-triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa) as a visible light photoinitiator was tested in several epoxy resins. TPPGa possesses significantly high reactivity as an initiator for the photopolymerization of epoxy resins but its poor thermal stability is a major disadvantage. TPPGa was microencapsulated in polystyrene by an in-liquid drying technique in order to increase its stability. The microcapsules obtained were characterized with scanning electron microscopy (SEM) and optically sectioned with laser scanning confocal microscopy (LSCM). 3D images of the microcapsules were reconstructed. Encapsulated TPPGa showed excellent thermal and photostability in the siloxane resin containing epoxy pendant groups. The wall protects TPPGa from decomposition. Several methods were developed to release TPPGa from the microcapsules.; Sorption of iodide anions by TPPGa microparticles was discovered and studied. The driving force and the kinetics of the sorption process was investigated.; Photoinduced electron transfer between polystyrene and TPPGa has been studied in dichloromethane solution and in polyvinylchloride film by steady state and time-resolved fluorescence quenching techniques. The dependence of the observed rate constant of the fluorescence quenching on the molecular weight of polystyrene was explained by the influence of the viscosity change in dilute solutions with the increase in the molecular weight of the polymer. The theoretical dependence of the diffusion constant on the molecular weight of the polystyrene was elucidated. Positive curvature in Stern-Volmer plots for quenching with polystyrene and toluene in solution and negative curvature in the Perrin plot for polystyrene in film was assigned to ground state charge transfer complex formation. Though no new absorption, as well as fluorescence, bands appear upon addition of the donor to the dichloromethane solution of TPPGa, the association constant of the complex between TPPGa and donors (polystyrene and toluene) in dichloromethane has been calculated by two methods. Nanosecond laser flash photolysis was performed. The major peak was observed at 550 nm in the presence of quenchers. This peak was assigned to pyranyl radical, the reduced form of pyrylium cation.; Grafting of the epoxy monomer onto polystyrene chain confirmed radical cation formation and, therefore, an electron transfer mechanism for fluorescence quenching. Forward and back electron transfer was proposed as a possible pathway for stabilization of TPPGa in polystyrene microparticles since 100% of the encapsulated compound can be recovered after even prolonged irradiation.