Dependence of the electro-optical properties of polymer dispersed liquid crystals on the photopolymerization process
We have studied the dependence of the electro-optical properties of polymer dispersed liquid crystals (PDLC) on the ultraviolet (UV) cure of the solution of monomer and liquid crystal. The kinetics of UV polymerization and its effect on the morphology of the phase separated droplets of liquid crystal determine the switching voltage, response time, and luminance of the PDLC. Using a series of statistically designed experiments, we have mapped the dependence of these responses on the weight fraction of liquid crystal, the temperature of the cell during cure, and light intensity. Temperature and composition are strongly coupled parameters that influence switching voltage, luminance, and response times. Switching voltages are minimized at 4–5 V for an 8 μm cell gap over a large region of temperature-composition space. An abrupt transition line occurs through that space. On one side of the transition line, voltage increases linearly either as temperature increases or composition decreases, and on the other sid...
Molecular alignment of rigid rods in nonrigid spherical pores.
We have investigated the orientation ordering of two shish-kebab chains confined by spherically harmonic potentials through Monte Carlo simulations and asymptotic analysis. The rigid rod is modeled as shish-kebab chains consisting of tangent hard spheres aligned in the same axis, and the harmonic potential is chosen to model nonrigid cavities. We first show that the interactions between a rod and the spherically harmonic potential are independent of chain orientation, indicating that the alignment of two confined rods arises from the excluded volume interactions alone. In the strong fields, the order parameter of two confined rods converges to different values, depending on the parity of chain length. From asymptotic order parameters, we find that the rods of odd-number beads rotate more freely even under the limiting strong confinement. However, the two rods of even-number beads are essentially trapped in a configuration of perpendicular alignment through intercalation of their central grooves. We attribute the dependence of the parity of chain length to the different locations of the center-of-mass in a rod for these two cases. Furthermore, we compare the shish-kebab chains with different rod models in the simulations, and utilize these models to explore the effect of the local rod smoothness on molecular alignment. Our findings suggest that increasing local rod smoothness enhances the rotational degree of freedom for confined rods, and the effect of local rod roughness emerges under strong enough applied potentials.
Anomalous Annealing Behavior of Arsenic Implanted Silicon as a Function of Dose and Energy
Anomalous annealing behavior of damage in As implanted Si has been investigated by channeling effect measurements. Room temperature As implants with doses from 1014 to 2 × 1016cm-2 at energies ranging from 50 to 250 keV into Si were annealed over the range of 600°C to 1100°C. Anneal temperatures ≥ 950°C reduce the disorder for ion doses of 0.5 to 1 × 1016 cm-2 at energies between 50 and 250 keV. Anomalously high residual disorder was found for the intermediate doses of 2 × 1014 to 2 × 10/cm2 implanted above 100 keV. For this intermediate dose, annealing of implants performed at liquid nitrogen temperatures did not show significant reduction in this damage. However implants at 200° to 300°C indicate a lower amount of damage compared to the room temperature implants.