Holographically Directed Assembly of Polymer Nanocomposites

Layered polymer/nanoparticle composites have been created through the one-step two-beam interference lithographic exposure of a dispersion of 25 and 50 nm silica particles within a photopolymerizable mixture at a wavelength of 532 nm. The polymerizable mixture is composed of pentaerythritol triacrylate (monomer), 1-vinyl-2-pyrrolidinone (monomer), and photoinitiator. In the areas of constructive interference, the monomer begins to polymerize via a free-radical process and concurrently the nanoparticles move into the regions of destructive interference. The effects of exposure time, power density, nanoparticle size, and periodicity on the final nanocomposite structure were measured with transmission electron microscopy to determine the mechanism for particle segregation. Diffraction from the sample was monitored as well, though its magnitude was not a good predictor of nanostructure in this relatively low index contrast system. Exposure time did not have a strong effect on the final structure. The best nanoparticle sequestration was observed at reduced laser power density, smaller interferogram periodicity, and decreased nanoparticle size, indicating that particle segregation is dominated by diffusion-limited nanoparticle transport directed by a matrix containing a gradient of polymerization kinetics.