Abstract

1. Time Dependence of Liquid Crystal Orientational Memory Effect at a Polymer SurfaceFrederick Davey, Joel Pendery, Sameh FerjaniPrincipal Investigator: Professor Charles RosenblattCWRU Department of Physics Abstract Surface interactions of liquid crystals are fundamental to our ability to exploit them for technological purposes.  In this experiment, we observed the anchoring strength dynamics of a previously observed effect, the Surface Memory Effect (SME), wherein an otherwise orientationally-degenerate surface is rendered anisotropic upon exposure to an orientationally ordered liquid crystal. The strength of the SME was measured on a PMMA surface by utilizing a periodically applied torque to the liquid crystal, with the resulting rotation monitored optically. The anchoring strength was found to reach a maximum value after approximately five minutes. More work is currently underway to generate quantitative values for the anchoring strength coefficient of the PMMA surface based on the observed optical data. Results Data We qualitatively determined the development time of the SME. Below is an example intensity response waveform achieved when the electric field is applied, along with a plot of the peak heights detected when the pulses are applied over the course of six minutes. Example Intensity Response Waveform Introduction Background Interactions between a liquid crystal and are surface are fundamental to our ability to exploit them. This research characterizes one lesser-understood surface interaction behavior of liquid crystals, the Surface Memory Effect, first reported by Clark in 19851. The SME describes behavior where a polymer surface that initially has negligible anchoring develops stronger anchoring over time upon exposure to an ordered phase of liquid crystal. The presence of a SME can be established qualitatively by observing the Schlieren texture of a sample over a phase change, as shown below. Methods Alignment Control Methods The PMMA surface was paired with a unidirectionally rubbed polyimide surface, which provides a steady state alignment for the director’s azimuthal angle. A short (100 ms) electric field was applied once every 30 seconds to perturb this alignment. Molecules at the rubbed surface are approximately fixed in place with the field applied, while molecules in the bulk and at the PMMA surface rotate; the extent of which depends on the strength of the memory effect (and thus on the anchoring strength). These director distributions can be seen in the plots below. Intensity detected upon applying a torque to the sample for 100ms. Total relaxation time once the torque is turned off is approximately two seconds Measured Peak Intensities Alignment due to Rubbing and Applied Electric Field Alignment due to Rubbing, Electric Field, and Strong SME Alignment due to Rubbing Peak intensities detected when applying a periodic torque over six minutes Optical Observation Methods The optics were set up as shown below, with the sample placed between two crossed polarizers. Laser light polarized in the rubbing direction was incident on the rubbed surface, resulting in extinction of the light with no field applied. Any rotation of molecules at the PMMA surface would correspond to a higher intensity of light transmitted. At each pulse, the maximum intensity detected corresponds to the final rotation of the molecules. Results from Clark’s research1 on the SME showing a negligible anchoring surface (top) and a surface with SME anchoring (bottom). The letter ‘N’ designates a sample in the nematic phase, ‘SA’ designates the smectic-A phase which has a more crystalline liquid crystal structure, and ‘I’ refers to the isotropic or liquid phase Analysis The SME fully sets in after approximately five minutes, with the anchoring strength reaching a maximum value. At present, unknown secondary effects are interfering with attempts to achieve a steady magnitude of rotation for the molecules. This steady state value is necessary to be able to achieve quantitative values for the anchoring strength from this experimental setup. Objectives The goal of this research was to establish an experiment that could determine the onset time of the SME for a surface, and obtain quantitative results for the anchoring strength of a PolymethylMethacrylate (PMMA) surface in the presence of a nematic liquid crystal. References [1] N. Clark, Phys. Rev. Lett. 55, 292 1985.