Shear strain-induced changes in liquid crystal alignment monitored by planar array infrared spectroscopy

Planar array infrared (PAIR) spectroscopy was applied to the study of the orientational motions of liquid crystals (LCs) under the influence of shear strain. Due to the ability to simultaneously monitor spectral bands corresponding to transition moments oriented along different directions along the molecular axes, it is possible to unambiguously determine the motion of the nematic director under the influence of external stimuli. Unlike previous attempts at spectroscopic studies of shear-induced liquid crystal orientation, PAIR is able to study the samples in real time, without the need to continuously subject the samples to cyclic perturbation. Of the three classical geometries employed in the rheological study of LCs, it was found that only one produced a measurable spectroscopic signal. This geometry was further explored by constant shear rate experiments over a range of shear rates and by exposing the samples to electric fields of varying strengths during shearing. The results obtained were seen to agree qualitatively with theoretical predictions.