Photo-Induced Deformations of Nematic Liquid Crystal Elastomers

Nathan J. Dawson

This dissertation is concerned with studying liquid crystal elastomers (LCEs), and investigating applications. The mechanisms that govern the photo-induced strain in the liquid crystal are studied using a series of experiments that decouple the proposed mechanisms and couple the results with the theoretical models. Then practical issues such as the fabrication of photomechanical optical devices (PODs) are investigated. The characterization experiments test LCEs made by The Liquid Crystal Institute at Kent State University. Once the samples are delivered, the materials are processed and cut into appropriate dimensions and placed between two partially reflecting mirrors to form a POD. The resulting Fabry-Perot device is used to measure the changes in elastomer length in response to an external pump beam. Afterward, a temperature probe is inserted into the sample and the experiments are repeated to measure changes in temperature in response to the light source. We observe that the temperature change and length change are correlated so that the dominant mechanism of the length change is photothermal heating for a disperse orange 3 (DO3), a photo-isomerizable dye, dye-doped LCE. Photo-isomerization, commonly believed to be the dominant mechanism of photo-induced deformations in LCEs, is found to be negligible in the POD geometry. This is confirmed by observations of the same temperature and length contraction trends in a disperse orange 11 (DO11) dye-doped LCE, where DO11 is a non-isomerizable dye. The response of a DO3-doped LCE to modulated functions of intensity was studied for various waveforms. A square waveform input intensity was used to study logic capabilities, and a triangle waveform was used to study phase lags due to the material response. The observed phase was consistent with predictions from the response function. The central result of this dissertation is the demonstration that mechanical information is transmitted over a beam of light. Furthermore, the time-dependent motion is shown to be transmitted over a beam of light through a series of cascaded devices. This explicitly shows that a POD displays photomechanical sensing, actuation, and transmission, which upon miniaturization and further cascading, makes the parallel beam POD configuration a candidate for photomechanical technology.

Photo-Induced Deformations of Nematic Liquid Crystal Elastomers

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