Piezoelectric fiber mats containing polar rod-shaped pigment particles

We demonstrate converse piezoelectric behavior of electrospun polylactic acid (PLA) fiber mats containing anisometric pigment nanoparticles (Novoperm Carmine HF3C). The effective piezoelectric constant of the fiber mats was estimated to be 2 nm V-1. These findings suggest polarity of the pigment particles obtained by a milling process. Transient electric current measurements were conducted to test the residual polarization of the particles. Piezoelectric fiber mats may be utilized to make colorful electro-active fiber mats and wearable smart clothing

Collective dynamics in dispersions of anisometric pigment particles

© 2018 Elsevier B.V. Dynamics of suspensions of solid rodlike pigment particles in a non-polar solvent were studied in a concentration range from the isotropic up to the orientationally ordered nematic-like phase. Using dynamic light scattering and gradient recovery measurements, we studied the rotational and translational diffusion coefficients. We demonstrate that the translational diffusion coefficient in this system is increasing with increasing concentration of the pigment particles in the vicinity of the transition into an ordered phase. This unexpected behaviour can be attributed to the collective interactions between the particles and the alignment effects

Multiple ferroelectric nematic phases of a highly polar liquid crystal compound

Ferroelectric nematic liquid crystals represent not only interesting fundamental science, but they also hold promise for storage capacitors with high power density or new information display technology having sub-millisecond switching. In this work we describe the synthesis and measurements of the physical properties of a new highly polar ferroelectric nematic compound, 4-nitrophenyl 4-[(2,4-dimethoxylbenzoyl)oxy]-2-fluorobenzoate (RT11001). The dipole moment of this material (along the long molecular axis) is calculated to exceed 11.5 Debye. We employ a wide range of physical characterization methods including differential scanning calorimetry (DSC), mass density measurement, optical birefringence, polarizing optical microscopy (POM), electric current analysis, and electro-optical switching, to show that RT11001 has three distinct ferroelectric states, F1, F2 and F3. F1 is purely orientationally ordered ferroelectric nematic phase (NF), F2 has a ferroelectric nematic with possibly short-range hexagonal order normal to the director (NhF), and we conjecture that F3 has a long-range hexagonal order normal to the director (ColhF).Comment: 31 pages, 10 figure

The interplay between spatial and heliconical orientational order in twist-bend nematic materials

The helical pitch formed by organic molecules, such as the α-helix of proteins, usually requires hydrogen bonding between chiral units and long-range positional order. It was recently found that certain liquid crystal oligomers can have a twist-bend nematic (NTB) phase with nanoscale heliconical structure without hydrogen bonding, molecular chirality or positional order. To understand the nature of this unique structure, here we present hard and resonant tender X-ray scattering studies of two novel sulfur containing dimer materials. We simultaneously measure the temperature dependences of the helical pitch and the correlation length of both the helical and positional order. In addition to an unexpected strong variation of the pitch with the length of the spacer connecting the monomer units, we find that at the transition to the NTBphase the positional correlation length drops. The helical structure was found not only in the NTBphase but observed even in the upper range of a smectic phase that forms just below the NTBstate. The coexistence of smectic layering and the heliconical order indicates a layered (SmATB) phase wherein the rigid units of the dimers are tilted with respect to the smectic layer normal in order to accommodate the bent conformation of the dimers and the tilt direction rotates along the heliconical axis

Ferroelectric Nematic Droplets in their Isotropic Melt

The isotropic to ferroelectric nematic liquid transition had been theoretically studied over one hundred years ago, but its experimental studies are rare. Here we present polarizing optical microscopy studies and theoretical considerations of ferroelectric nematic liquid crystal droplets coexisting with the isotropic melt. We find that the droplets have flat pancake-like shapes that are thinner than the sample thickness as long as there is a room to increase the lateral droplet size. In the center of the droplets a wing shaped defect with low birefringence is present that moves perpendicular to a weak in-plane electric field, and then extends and splits in two at higher fields. Parallel to the defect motion and extension, the entire droplet drifts along the electric field with speed that is independent of the size of the droplet and is proportional to the amplitude of the electric field. After the field is increased above 1V/mm the entire droplet gets deformed and oscillates with the field. These observations led us to determine the polarization field and revealed the presence of a pair of positive and negative bound electric charge due to divergences of polarization around the defect volume

Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible: N -mers

© 2019 the Owner Societies. We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer exhibiting a uniaxial nematic (N) to twist-bend nematic (NTB) phase transition. The elastic constants for director splay (K11), twist (K22) and bend (K33) exhibit the relations K11 > K22 > K33 and K11/K22 > 2 over the bulk of the N phase. Their behavior near the N-NTB transition shows dependency on the parity of the number (n) of the rigid mesomorphic units in the flexible n-mers. Namely, the bend constant K33 in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast, K33 for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K22 increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K11 increases steadily on cooling without evidence of pretransitional behavior in any n-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-NTB transition. The N-NTB pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the NTB phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A∗ phase

Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films : a new design space for organic light-emitting diodes

B.L. acknowledges financial support from the Binational Science Foundation under grant No 2014396 and from the National Science Foundation under grant No 1639073. C.K. acknowledges funding from the Kent State University Internal Post-Doctoral Competition. M.C.K. acknowledges funding from the Belgian Agentschap voor innovatie door wetenschap en techniek under grant No IWT 131498.Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.Publisher PDFPeer reviewe