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40536-B7
Photo-Induced Reorientation of Dyed Nematic Liquid Crystals
The objective of this research has been to explore the impact of dye dopants on the optically induced reorientation of liquid crystals. This research continues to involve collaborations with Dr. Istvan Janossy (Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences). Undergraduate students at Allegheny College have also continued participate in this research as part of a summer internship, the senior project required of all Allegheny students, or as an independent study project. This year's students include Valerie Basore, who also worked on the project last year (Allegheny College '07) and was supported by the grant, as well as Yusufu Sulai (Allegheny College '08), and Barbara Dunlap (Allegheny College '09), who were not supported by the grant.
During the third year of the project, we continued studies on the impact of azo dye Disperse Orange 3 on photo-induced reorientation of nematic liquid crystals. Mixtures of the dyes with the liquid crystals 5CB, E7 and E63 were prepared at concentrations of about 0.4% by volume. In this third year, however, we focused our studies on surface interactions between the liquid crystal, the dyes and the surface of the sample. We found that a significant contribution to optically induced realignment of the bulk liquid crystal involved azimuthal gliding of the easy axis at the liquid crystal surface. This gliding is facilitated by photo-excitation of the dye dissolved in the liquid crystal. In these studies, liquid crystal cells were prepared with one surface coated with rubbed polyimide and the other surface coated with unrubbed polyethylmethacrylate (PEMA). Gliding on the surface was measured using pump-probe techniques, where the pump beam from an argon ion laser was incident on the sample with an azimuthal polarization at some angle, theta, to the director. The probe beam consisted of a white light source with an orange filter. The probe was polarized parallel to the rubbing direction and passed through the sample. Polarization direction of the exiting probe was determined by passing the signal through a Hinds photoelastic modulator (PEM) and then a polarizer oriented 45 degrees to the director. The second harmonic signal from the PEM was calibrated to be directly proportional to the polarization angle of the exiting beam, which is the direction of the surface director and therefore corresponds to the gliding angle.
The photoinduced gliding was found to be characterized by two dynamic regimes; a fast regime and a slow regime. Our results indicate that the fast regime was described by director anchoring and surface viscosity, while the latter was described by a stretched exponential. Stretched exponentials are sometimes associated with adsorption and desorption processes. This photoinduced gliding was also found to involve a sensitizing effect. In particular, for a series of measurements, the amount of gliding increased with each subsequent measurement. In addition, illumination of the sample with circularly polarized light increased the amount of gliding by several times. In our planar aligned samples, we also observed some irreversibility of photoinduced reorientation, both on the PEMA surface and the polyimide surface. It is clear that this photoinduced gliding is the result of photophysical/ photochemical interactions between the dye molecules, the liquid crystal molecules, and the surface polymer. Our results suggest that the dye is adsorbed onto the polymer surface upon irradiation.
Work described in this report has been submitted for publication to the journal Liquid Crystals, and relevant computational work related to research described in previous reports has been submitted for publication to the journal Molecular Crystals Liquid Crystals, both with appropriate acknowledgement to ACS-PRF.
As part of this study, the PI spent two and a half weeks of the summer of 2007 at the Research Institute in Budapest. During the summer, a student, Yusufu Sulai and Barbara Dunlap, spent time at the Research Institute. This time was used to conduct experiments investigating azimuthal gliding with soft polymer coating (PEMA) for planar aligned systems, as well as to consult with Dr. Istvan Janossy. The College provided funds for both students. The work completed over the summer is currently being written up for publication.
Finally, work done with Istvan Janossy in Budapest during the summer of 2005 has been published in the journal Molecular Crystals Liquid Crystals (I. Janossy, A. Vajda, and D. Statman, "Influence of the Molecular Weight of a Polymer on the Gliding of Nematic Liquid Crystals," Mol. Cryst. Liq. Cryst., 466, 77 - 82 (2007)), and work described in the previous report has been published in in the journal Physical Review E (D. Statman, E. Page, V. Werner, and J. C. Lombardi, "Photo-induced reorientation of nematic liquid crystals doped with azo-dye: A dynamic and steady-state study of reorientation and loss of liquid crystal order." Phys. Rev. E. 75, 021703 (2007)). Appropriate acknowledgement has been given to ACS-PRF.
