Hiroya Nishikawa

Abstract An anomalously large dielectric permittivity of ≈10 4 is found in the mesophase temperature range (MP phase) wherein high fluidity is observed for a liquid‐crystal compound having a 1,3‐dioxane unit in the mesogenic core (DIO). In this temperature range, no sharp X‐ray diffraction peak is observed at both small and wide Bragg angles, similar to that for a nematic phase; however, an inhomogeneous sandy texture or broken Schlieren one is observed via polarizing optical microscopy, unlike that for a conventional nematic phase. DIO exhibits polarization switching with a large polarization value, i.e., P = 4.4 µC cm −2 , and a parallelogram‐shaped polarization–electric field hysteresis loop in the MP phase. The inhomogeneously aligned DIO in the absence of an electric field adopts a uniform orientation along an applied electric field when field‐induced polarization switching occurs. Furthermore, sufficiently larger second‐harmonic generation is observed for DIO in the MP phase. Second‐harmonic‐generation interferometry clearly shows that the sense of polarization is inverted when the +/− sign of the applied electric field in MP is reversed. These results suggest that a unidirectional, ferroelectric‐like parallel polar arrangement of the molecules is generated along the director in the MP phase.

A novel chiral nematic phase with a polar helical order is realized via the introduction of helical twisting power into a polar nematogen. The properties of the induced polar nematic (polar cholesteric: Np*) phase differ from those of the conventional cholesteric (N*) phases existing thus far. Np*, which is a new class of N* structures, is characterized not only by its helically twisted nematic director, but also by a continuously twisted polarization. Transmission spectroscopy and helical pitch measurements in a wedge cell revealed that the half-helical pitch in the Np* phase vanished because of the polar response in the Np* helix. The inner polar director in the Np* phase is confirmed in dielectric and second-harmonic-generation studies. Furthermore, this unique Np*LC, which corresponds to a half-/full-pitch helix, enables ultrafast electro-optic switching (τ < 20 µs), and proposes new potential applications for electrically interchangeable photonic bandgaps.

Dielectric permittivity, a measure of polarisability, is a fundamental parameter that dominates various physical phenomena and properties of materials. However, it remains a challenge to control the dielectric permittivity of materials reversibly over a large range. Herein, we report an anisotropic fluid with photoresponsive dielectric permittivity (200 < ε < 18,000) consisting of a fluorinated liquid-crystalline molecule (96 wt%) and an azobenzene-tethered phototrigger (4 wt%). The reversible trans-cis isomerisation of the phototrigger under blue and green light irradiation causes a switch between two liquid-crystalline phases that exhibit different dielectric permittivities, with a rapid response time (<30 s) and excellent reversibility (~100 cycles). This anisotropic fluid can be used as a flexible photovariable capacitor that, for example, allows the reversible modulation of the sound frequency over a wide range (100 < f < 8500 Hz) in a remote manner using blue and green wavelengths.

Abstract In recent years, ferroelectric nematic liquid crystals have attracted considerable attention owing to their unique properties such as a colossal polarization, high electro-optic activity, and high fluidity. However, despite large efforts in designing and developing new ferrofluid molecules based on molecular parameters, the control and stabilization of ferroelectric nematic phase transitions remain challenging. Here, we discuss the impact of mixing 1,3-dioxane-tethered fluorinated (DIO) diastereomer molecules, namely trans DIO and cis DIO, in controlling the ferroelectric nematic phase transition, using X-ray diffraction to investigate the effect of smectic cybotactic cluster formation. Our results show that the ferroelectric nematic phase transition can be tuned by a smooth exchange of the ferroelectric nematic trans DIO and non-liquid crystal cis DIO components, where the similar dipole and molecular backbone of the two components ensures a consistent macroscopic polarization of the diastereomeric-controlled ferroelectric nematic phase.

The widespread electro-optical applications of polymer dispersed liquid crystals (PDLCs) are hampered by their high-driving voltage. Attempts to fabricate PDLC devices with low driving voltage sacrifice other desirable features of PDLCs. There is thus a clear need to develop a method to reduce the driving voltage without diminishing other revolutionary features of PDLCs. Herein, we report a low-voltage driven PDLC system achieved through an elegantly simple and uniquely designed acrylate monomer (A3DA) featuring a benzene moiety with a dodecyl terminal chain. The PDLC films were fabricated by the photopolymerization of mono- and di-functional acrylate monomers (19.2 wt%) mixed in a nematic liquid crystal E7 (80 wt%). The PDLC film with A3DA exhibited an abrupt decline of driving voltage by 75% (0.55 V/μm) with a high contrast ratio (16.82) while maintaining other electro-optical properties almost the same as the reference cell. The response time was adjusted to satisfactory by tuning the monomer concentration while maintaining the voltage significantly low (3 ms for a voltage of 0.98 V/μm). Confocal laser scanning microscopy confirmed the polyhedral foam texture morphology with an average mesh size of approximately 2.6 μm, which is less in comparison with the mesh size of reference PDLC (3.4 μm), yet the A3DA-PDLC showed low switching voltage. Thus, the promoted electro-optical properties are believed to be originated from the unique polymer networks formed by A3DA and its weak anchoring behavior on LCs. The present system with such a huge reduction in driving voltage and enhanced electro-optical performance opens up an excellent way for abundant perspective applications of PDLCs.