Giant Flexoelectricity of a Bent-core Liquid Crystal Elastomer

Samuel N. Sprunt, Kent State University, DMR 0606160

Along with fascinating basic physics arising from coupling liquid crystal (LC) order to a polymer network, elastomers containing LC molecules can stabilize useful properties associated with the fluid phases of these molecules, within a durable rubbery film, for new materials’ technology. Using bent-core liquid crystals (BCLCs), we developed the first LC elastomer that exhibits the giant flexoelectricity (electric polarization induced by mechanical flexing) that we previously discovered in the fluid nematic phase of certain BCLCs.

The large flexoelectric effect, combined with the processing advantages, extended temperature range, and robustness of an elastomer matrix, appears promising as a new approach for compact, low-cost electromechanical energy conversion.

(a) and (b) show the structure and a sample film of a bent-core liquid crystal elastomer exhibiting the large flexoelectric response shown in (c). The small film shown next to the 18 mm dime generates a 20 nA polarization current when flexed at 1.7 Hz.

Nanostructure of Bent-core Nematic Liquid Crystals

Samuel N. Sprunt, Kent State University, DMR 0606160

Bent-core nematic liquid crystals (BCNs) are orientationally ordered fluids formed by molecules shaped like boomerangs. Small angle X-ray diffraction conducted by our team at the National Synchrotron Light Source reveals a developing pattern of diffuse peaks as the liquid crystal is cooled from the isotropic into the nematic state. The pattern and its evolution can be reasonably modeled by assuming that the basic units of the fluid are not single molecules but rather short-range layered domains or “smectic clusters” of ~100 molecules. Nanoscale smectic domains, and features of them that are specific to bent-shaped molecules, can explain a range of the remarkable properties observed in bent-core molecular fluids, including an unusually large flexoelectric effect, a spectacular flow birefringence, and non-Newtonian rheology.

Two-dimensional X-ray diffraction images taken on a bent-core liquid crystal when cooling from the isotropic to nematic phases (top to bottom). Left column (below): Raw images. Middle column: After processing into simple contour plots. Right column: Simulated results from a model based on smectic nanodomains.

Mentoring Students for Success

Samuel N. Sprunt, Kent State University, DMR 0606160

In 2009-2010, six undergraduate students and two high school students pursued research projects under our supervision. At right, Laura, a senior at Theodore Roosevelt HS in Kent, OH is filming popcorn kernels exploding at 1000 fps.

Above: Extraordinary opportunities for student interaction with renowned scientists across fields are a key component of our education program. Tanya Ostapenko, a Ph.D. student, confers with Theodor Haensch at the 2010 meeting of Nobel Laureates in Lindau, Austria.

Above: Jeremy Stromer and Bobbi Arnold, visiting undergraduate physics and chemistry majors, respectively, from the University of Nebraska, Kearney, performed research at KSU in May-June 2010 supported by our grant, and made important contributions to project goals.