Holographic composites with gold nanoparticles: Nanoparticles promote polymer segregation

New nanocomposites containing functionalized acrylate monomers and An nanoparticles (NPs, 1.5-3 nm core diameter, 1-2 wt %) have been developed for an all-optical fabrication of periodic bulk structures by holographic photopolymerization. The Au NPs were coated with ethyl I I -mercaptoundecanoate to ensure good solubility in low-polarity organic media. The materials with only 1.5 wt % An NPs show unusually high amplitude of the refractive index modulation in the diffraction volume gratings (0.0073). Both volume and surface relief gratings are formed during the holographic exposure. The proposed mechanism of the refractive index contrast amplification (as compared to monomer mixture without NPs) includes interception of free radicals by Au nanoparticles. This slows down the free-radical photopolymerization, thus prornoting the increase in lateral periodic redistribution of the components in the interference pattern. The redistribution of both NP and monomers provides high efficient grating formation with the spatial period of 0.5-4.5 mu m. The formation of the surface relief coincident with the interference pattern is also observed. The introduction of 1.5 wt % An NPs into the prepolymer mixture more than doubles the surface relief depth (from ca. 150 to 400 nm for the period 4.2 mu m), thus proving the NP influence on the structure and, consequently, on the shrinkage of the polymer matrix.