Dynamic light scattering in polymer-dispersed liquid crystals

A polymer dispersed liquid crystal was studied by dynamic light scattering. In the experiments, where no electric field is applied, the system exhibits additional slow dynamics to the ones observed in bulk nematic liquid crystals. This slow dynamics gradually disappears with an increasing electric field. While the measured time intensity correlation function does not depend on the scattering vector in the system where no field is applied due to the multiple scattering, its dependence on the electric field shows expected size effects. In confined systems, with typical size $d$, there are no eigenmodes of the orientational fluctuations in droplets with the wave vectors less than the minimal wave vector $q_{\min }\sim \pi /d$. We observed a quadratic dependence of the inverse relaxation time on the scattering vector down to a certain scattering vector, below which the relaxation time remains constant. The size calculated from this minimal scattering vector is in agreement with the average droplet size obtained from scanning electron microscope photographs and the atomic force microscope images of our sample. The electric field changes the temperature behavior of the inverse relaxation time near the nematic-isotropic phase transition, which increases with the temperature when the electric field is applied, but decreases when the field is absent.