Ebru A. Buyuktanir

Ferroelectric Particles in Liquid Crystals: Recent Frontiers

In this article we describe electro-optical properties of recently discovered ferroelectric particles/liquid crystal colloids. We show that the presence of ferroelectric particles in a liquid crystal changes its birefringence and dielectric anisotropy. In contrast to the traditional time consuming and expensive chemical synthetic methods, this method to create liquid crystals with enhanced properties is relatively simple and has a great potential. We also demonstrate the performance of these new materials in various devices, including displays, light modulators, and beam steering devices.

Flexible Bistable Smectic-A Polymer Dispersed Liquid Crystal Display

We report a flexible bistable polymer dispersed smectic-A liquid crystal matrix display combining two advanced technologies developed by Kent State University and PolyDisplay ASA. The ion-doped smectic-A liquid crystal/polymer composite system produces lightweight, thin, and robust flexible displays with image memory, high contrast, wide viewing angle, and low power consumption.

Highly piezoelectric biocompatible and soft composite fibers

We report the fabrication of highly piezoelectric biocompatible soft fibers containing barium titanate ferroelectric ceramic particles dispersed in electrospun poly lactic acid (PLA). These fibers form mats that have two orders of magnitude larger piezoelectric constant per weight than single crystal barium titanate films. We propose that the observed apparent piezoelectricity results from the electrospinning induced polar alignment of the ferroelectric particles that pole the fibers similar to ferroelectret polymer foams that are poled by corona discharge. Due to the biocompatibility of PLA that encases the ferroelectric particles, these mats can be used in biological applications such as bio-sensors, artificial muscles, and energy harvesting devices.

Field-Induced Polymer Wall Formation in a Bistable Smectic-A Liquid Crystal Display

We developed a composite system to produce robust bistable smectic-A (SmA) liquid crystal based flexible displays by encapsulating the liquid crystal material in a polymer wall structure. While keeping all the intrinsic bistable properties of the SmA material, the field-induced polymer walls bridge the two display substrates and bring significant advantages over the polymer dispersed liquid crystal structure. Here we analyze the performance of an encapsulated pixel and demonstrate superior electro-optical characteristics, high contrast ratio, and excellent sunlight readability.

Direct piezoelectric responses of soft composite fiber mats

Recently soft fiber mats electrospun from solutions of Barium Titanate (BT) ferroelectric ceramics particles and polylactic acid (PLA) were found to have large (d33 ∼ 1 nm/V) converse piezoelectric signals offering a myriad of applications ranging from active implants to smart textiles. Here, we report direct piezoelectric measurements (electric signals due to mechanical stress) of the BT/PLA composite fiber mats at several BT concentrations. A homemade testing apparatus provided AC stresses in the 50 Hz-1.5 kHz-frequency range. The piezoelectric constant d33 ∼ 0.5 nC/N and the compression modulus Y ∼ 104–105 Pa found are in agreement with the prior converse piezoelectric and compressibility measurements. Importantly, the direct piezoelectric signal is large enough to power a small LCD by simple finger tapping of a 0.15 mm thick 2-cm2 area mat. We propose using these mats in active Braille cells and in liquid crystal writing tablets.

Raman Imaging of Nematic and Smectic Liquid Crystals

In this article, we demonstrate that confocal Raman microscopy can be utilized to image the director fields of liquid crystal molecules, revealing the spatial patterns of the molecular orientational order both in lateral and axial directions. Here we report on the polarized confocal Raman imaging of the electric field-aligned uniaxial nematic liquid crystal and the in-depth director profiling of the focal conic domains of smectic A liquid crystal. The C≡N stretching vibrational band [ν(CN) = 2228 cm−1] and the C–C stretch of aromatic rings [ν(CC) = 1606 cm−1] are used to probe the spatial director orientations of 4-pentyl-4′-cyanobiphenyl (5CB) and 4-octyl-4′-cyanobiphenyl (8CB).

62.4: Flexible Bistable Smectic‐A LCD Based on PDLC

The combination of two advanced technologies developed by Kent State University and TechnoDisplay ASA has resulted in the creation of high performance flexible bistable displays. While the bistable SmA (EASL DMD™) technology offers a more simplified display mode, long term stability, and an excellent contrast, the PDLC technology provides simplicity of manufacturing while maintaining the standard characteristics and strength of the display. Along with their lighter weight, the flexible display prototypes demonstrate high contrast ratio, excellent viewing angle characteristics, low power consumption, and a long lifetime.

Stressed liquid crystals and their application

Recently discovered stressed liquid crystals (SLCs) are of a great interest because they provide largest phase retardation shift achievable within shortest time interval. This effect was accomplished by decoupling the speed of a liquid crystal layer from its thickness. SLCs easily switch 5 microns of the phase retardation at sub-millisecond speeds while 50 microns requires only several milliseconds. SLCs are therefore able to modulate the IR light with the frequencies higher than 10 kHz. The SLCs are polymer/liquid crystal composites; however, their electro-optic properties differ significantly from previously developed polymer dispersed liquid crystals and polymer network/stabilized liquid crystals. The applied stressed force aligns the domains, eliminating scattering and hysteresis at the same time. The phase shift is highly linear with the applied voltage, greatly simplifying the drive electronics. The SLCs pose intriguing basic scientific questions and may be used in a lot of new electro-optical applications (micro-displays, diffractive optical elements, beam steering devices).

Liquid crystal microfibers lead to responsive optoelectronic textiles

Incorporating active materials within fibers holds great promise for tunable, non-woven, optoelectronic textiles that can respond to external stimuli.1–3 Previous studies have shown that light can be confined by infiltrating the microstructures with liquid crystal (LC) materials.4, 5 However, these fibers are mostly silica-based, and the LC material has generally been capillary-filled, limiting the length and flexibility of the photonic fibers. We produced and characterized thermally and electrooptically responsive microfibers endowed with LC properties. These include mesophase characteristics and birefringence, as well as molecular-level self-ordering. These LC microfibers are electrospun from a homogeneous solution of 4-pentyl4’-cyanobiphenyl (5CB) and polylactic acid (PLA) in chloroform/acetone solvent. In the electrospinning process,6, 7 the low molecular weight 5CB phase-separates and self-assembles to form a planarly aligned nematic core within a PLA shell. The optical birefringence and dielectric anisotropy of LCs are coupled with the polymer shell’s elongated structure to form the LC fibers’ light-modulating properties. Most importantly, the orientation of LC domains and, therefore, the optical properties of the 5CB/PLA fibers, can be tuned by applying external fields. We have shown that the LC content can be increased up to 70wt% while maintaining the core/shell fiber structure.2 Figure 1 shows a polarized optical microscope (POM) image of an LC fiber. The interference color shift determined the planar alignment of 5CB in the PLA core. The birefringence of the phase-separated LC phase ( n 0:2) in the core could be easily observed by the color changes as the fiber orientation (relative to the incident light polarization) rotated. Differential scanning calorimetry (DSC) analysis of the fibers also confirmed that 5CB Figure 1. Determination of the alignment of 5CB in the core of polylactic acid (PLA) by using a first-order retardation plate (RP) ( D 530nm). The fiber is placed at 45 with respect to polarizer (P) and analyzer (A). Blue represents the additive retardation effect indicated with a plus sign and yellow indicates the relative retardation decrease shown with a minus sign. N is the fiber’s long axis.

Properties of responsive liquid crystal/polymer fibers

The development of optically responsive liquid crystal (LC)/polymer composite fibers is presented. The morphology and electro-optical properties of LC/polymer fibers are studied with scanning electron microscopy and polarizing optical microscopy, respectively.