Photoactive silica nanoparticles: Influence of surface functionalization on migration and kinetics of radical-induced photopolymerization reactions

The present work provides a comparative study on the photoreactivity of nano-sized silica particles bearing different types and amounts of photocleavable species as organic shell. Newly synthesized trialkoxysilyl-functionalized bis(acyl)phosphane oxide and aromatic alpha-hydroxyketone derivatives, which efficiently undergo Norrish type I photofragmentation reactions, are immobilized on the surface of nanosilica by using convenient silanization techniques. Qualitative and quantitative characterization of the immobilized species is carried out by spectroscopic measurements and thermal gravimetric analysis. In dependence on the reaction conditions, particles with varying amount of photoactive groups are obtained. The influence of the degree of modification on the initiation efficiency in the radical mediated thiol-ene photopolymerization is determined. It is shown that in thin films the curing rates are influenced by the employed grafting method. As an approach towards low extractable photoinitiators, the migration of the immobilized photoinitiators was evaluated by Soxhlet extraction. Elemental analysis of the extracts evidenced that the extractable content of particles with a high degree of modification is below the detection limit of the method.