From Synthesis to Piezoelectric Studies of Central-Ring-Substituted Bent-Core Liquid Crystals and Their Composites

We have presented the results of various experiments on several central ring substituted bent-core liquid crystals. We have synthesized a bent-core molecule that exhibits a nematic phase and is functionalized for future use in a liquid crystalline elastomer. Various experimental techniques were used to characterize properties such as smectic-like clusters in the nematic phases and piezoelectricity of polymer composites. We provide the detailed results and analysis of the SAXS measurements. The SAXS measurements were useful in analyzing the behavior of the smectic-like clusters that exist in the nematic phase of many bent-core molecules. The clusters have been shown to exist over a wide range of temperatures. We showed that the SAXS measurements agree with previous Cryo-TEM experiments on the three-ring bent-core compounds. We also compared the cluster behavior in two different materials - one with a smectic phase below the nematic and one without. We have found that, as expected, the smectic clusters show more pre-transitional behavior in the material with a smectic phase. Additionally, the clusters, just like the smectic phases of bent-core molecules, likely exhibit polar order. This could be the origin of the piezoelectric and flexoelectric responses found in some bent-core nematics. Addtionally, some of the bent-core molecules form a relatively unstudied phase called the X phase. It is shown to be optically isotropic and ferroelectric, and therefore, piezoelectric. A polymer composite was made from one of these materials. The material showed a piezoelectric response that was in agreement with previous converse piezoelectric measurements under static (DC) fields. It was shown to be comparable to commercially available piezoelectric ceramics. However, under oscillating forces of more than a few hertz, the piezoelectric response dropped off significantly likely due to stiffening of the polymer under oscillating strain with increasing frequency.