Fourier Transform Infrared Imaging of Nematic Liquid Crystals

Fourier transform infrared spectroscopy is often used to study liquid crystals and liquid crystal systems. In this work, the factors that influence the IR absorbance of nematic liquid crystals and their consequences on FT-IR imaging are examined. In the IR spectrum of E7, ordering in the nematic phase causes a 28.7% reduction in the nitrile band height relative to the isotropic phase, and the combination of ordering and liquid crystal director alignment causes a 65.0% decrease in the nitrile band height. For a phase-separated blend of E7 and 10% NOA65 polymer, the FT-IR images were found to be concentration-based for the polymer-specific carbonyl band and phase/alignment-based, which masked the concentration dependence, for the liquid crystal-specific nitrile band. These findings were exploited to produce an FT-IR image of the isotropic-to-nematic phase transition of neat E7. This transition image emphasizes the dependence of liquid crystal bands on ordering and alignment and the need for caution in the interpretation of FT-IR images of liquid crystals and liquid crystal systems.