Physico-chemical role of CdSe/ZnS quantum dots in the photo-polymerization process of acrylate composite materials

Abstract

The work presented in this paper is devoted to the physico-chemical role of quantum dots (QDs) in the photopolymerization of composite materials. We show first that the photoexcitation of CdSe/ZnS semiconductor quantum dots can initiate the polymerization of an acrylate monomer in association with a tertiary amine and in the absence of a photoinitiator. But by using a holographic technique, among the others, the gratings recorded in such composite materials are not stable and relax once the light interference pattern is switched off. This behaviour is attributed to a very low degree of conversion of monomers, which does not prevent the post-exposure diffusion of monomers and nanoparticles. The comparison with gratings obtained in the presence of a photosensitizer dye (higher hologram quality and stability) highlights the key importance of using such dye in the polymerization process. The gratings recorded in the complete syrup, containing the dye and QDs which differ by their size and UV-visible absorption spectra, exhibit an additional refractive index modulation. The latter results from the spatial distribution of QDs and the change in the kinetics of polymerization. We further focus on understanding the fall of the polymerization rate owing to the presence of QDs. This decrease may result from the trapping of the tertiary amine (co-initiator) at the surface of those QDs, which reduces the rate of decay of the dye and, consequently, the polymerization rate. Under such conditions, the diffusion of QDs and monomers is favoured and their spatial redistribution is enhanced. These features contribute to modify the physical properties of the gratings.