Effects of eliminating the chain extender and varying the grating periodicity on the morphology of holographically written Bragg gratings

Experimental variables affecting the morphology of holographically written Bragg gratings in polymer dispersed liquid crystalline composite systems are discussed. The spatial anisotropic photo-crosslinking of a multifunctional acrylate monomer results in periodic regions of phase separated LC droplets. These films have many electro-optic applications owing to the fact their refractive index profiles can be modulated. Low-voltage high resolution scanning electron microscopy (LVHRSEM) was used to investigate the morphology of these films due to the very small LC domain sizes formed. Specifically, the morphology is discussed in terms of the rates of LC diffusion parallel to the grating vector, and the relative rates of nucleation and subsequent gelation. Using micrographs, the effect of writing intensity, LC content, and chain extender concentration is examined first in uniformly illuminated (flood lit) samples and then in transmission gratings. Elimination of the chain extender increases the gelation time relative to the nucleation time resulting in larger LC domains. The effect of increasing the Bragg spacing on the phase separation behavior is also examined.