纤维素胆甾型液晶研究进展

主要介绍了近年来在纤维素胆甾型液晶领域的研究进展,包括其制备：形成机理、织构特性及其影响因素等方面。     

Synthesis, structure and mesomorphism of new cholesteric monomers and smectic comblike polymers

Seven new cholesteric monomers (M-1−M-7) and the corresponding smectic comblike polymers containing cholesteryl groups (P-1−P-7) were synthesized. The chemical structures and purity were characterized by FT-IR, ¹H NMR, and elemental analyses. The specific optical rotations were evaluated with a polarimeter. The mesomorphism was investigated by polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction. The specific optical rotation values of these monomers and polymers with the same numbers of phenyl ring and terminal groups were nearly equal, however, they decreased with increasing the aryl numbers in the mesogenic core. M-1−M-7 showed oily streak texture and focal conic texture, or fingerprint texture, or spiral texture of cholesteric phase. P-1−P-7 showed the smectic A phase. The melting, clearing, and glass transition temperatures increased, and the mesophase temperature ranges widened with increasing the aryl number in the mesogenic core. Surprisingly, although the molecular structures of M-6 and M-7 were similar to those of M-4, namely the mesogenic cores contained three phenyl rings, their phase behavior had a considerable difference, and T_m and T_i of M-6 and M-7 were less than those of M-4. In addition, M-6 and M-7 also showed an obvious glass transition. TGA showed that all the polymers had good thermal stabilities.     

CONFORMATIONAL ANALYSIS AND MOLECULAR MODELING OF CHOLESTERIC LIQUID CRYSTAL POLYESTERS BASED ON XRD,RAMAN, AND TRANSITION THERMAL ANALYSIS

Molecular modeling of the cholesteric liquid crystal polyester poly[oxy(1,2 - dodecane)oxycarbonyl-1,4-phenyleneoxycarbonyl-1,4-phenylenecarbonyloxy-1,4-phenylenecarbonyl] (PTOBDME), [C₃₄H₃₆O₈] _n , synthesized in our laboratory and thermally characterized by differential scanning calorimetry (DSC), was performed to explain both its cholesteric mesophase and 3D crystalline structure. Conformational analysis (CA) was run for the monomer both by systematic search and with molecular dynamics (MD) simulations. Minima energy conformers were “polymerized” with Cerius² and helical, cholesteric molecules were obtained in all cases. Our models agree with the chiral behavior observed by X-ray diffraction (XRD), thermooptical analysis (TOA) and circular dichroism (CD) experiments. Crystal packing of the polymer molecules were simulated in cells with parameters a and b obtained from experimental powder X-ray diffraction patterns and c calculated from the translational repetitive unit during the theoretical polymerization. Recalculated X-ray powder diffraction patterns of our models matched the observed ones. Morphology simulation from those crystal models is in good agreement with the crystals observed by optical microscopy. We have also modeled the self-associating nature of those polyesters when dispersed in aqueous media. Simulation of our models surrounded by different solvents, such as water and chloroform, were performed by calculating their interaction energies, coordination numbers, and mixing energies, applying Monte Carlo simulation techniques based on the Flory-Huggins theory. These results were compared with their experimental vibrational Fourier transform (FT)–Raman spectra in the regions in which structural marker bands of the polymer appear.     

Optical and thermal properties of Fe3O4 nanoparticle-doped cholesteric liquid crystals

A composite system of Fe₃O₄ nanoparticle-doped cholesteric liquid crystals with properties of broadband reflection and controllable temperature under high-frequency electric field is proposed. The broadband reflection can shield the near-infrared light in summer and the electromagnetic-thermal effect by the Fe₃O₄ nanoparticles can deice or defrost in winter for high transmittance and good safety. Furthermore, the thermal effect may be precisely controlled and significantly enhanced by adjusting the factors of the doped concentrations and the applied electric field parameters (duration time, magnitude and frequency). This composite system may have potential applications for multifunctional windows of architectures and vehicles.     

A Photo- and Thermo-Driven Azoarene-Based Circularly Polarized Luminescence Molecular Switch in a Liquid Crystal Host

The development of chiral optical active materials with switchable circularly polarized luminescence (CPL) signals remains a challenge. Here an azoarene-based circularly polarized luminescence molecular switch, (S, R, S)-switch 1 and (R, R, R)-switch 2 , are designed and prepared with an (R)-binaphthyl azo group as a chiral photosensitive moiety and two (S)- or (R)-binaphthyl fluorescent molecules with opposite or the same handedness as chiral fluorescent moieties. Both switches exhibit reversible trans/cis isomerization when irradiated by 365 nm UV light and 520 nm green light in solvent and liquid crystal (LC) media. In contrast with the control (R, R, R)-switch 2 , when switch 1 is doped into nematic LCs, polarization inversion and switching-off of the CPL signals are achieved in the resultant helical superstructure upon irradiation with 365 nm UV and 520 nm green light, respectively. Meanwhile, the fluorescence intensity of the system is basically unchanged during this switching process. In particular, these variations of the CPL signals could be recovered after heating, realizing the true sense of CPL reversible switching. Taking advantage of the unique CPL switching, the proof-of-concept for “a dual-optical information encryption system” based on the above CPL active material is demonstrated.     

Chiral mirror and optical resonator designs for circularly polarized light: suppression of cross-polarized reflectances and transmittances

A left-handed chiral sculptured thin film (STF) that reflects strongly at the wavelength of the circular Bragg resonance tends to partially convert the handedness of incident LCP (left-circularly-polarized) light to RCP (right-circularly-polarized). We show that the cross-polarized component of the reflected RCP beam can be eliminated by interference with an additional RCP beam that is reflected at the interface of an isotropic cover and an AR (antireflecting) layer. For best results the refractive index and thickness of the AR layer need to accommodate a phase change on reflection that occurs at the chiral film. Effective suppression of the reflectances R_RR, R_RL, R_LR and the transmittances T_RL, T_LR can be achieved by sandwiching the chiral reflector between such amplitude and phase-matched AR coatings. Co-polarized chiral reflectors of this type may form efficient handed optical resonators. For LCP light the optical properties of such a handed resonator are formally the same as the properties of the isotropic passive or active Fabry–Perot resonators, but the handed resonator is transparent to RCP light.     

Laser emission at the second-order photonic band gap in an electric-field-distorted cholesteric liquid crystal

ABSTRACT

We study the optical properties of a cholesteric liquid crystal doped with a fluorescent dye in the regime of highly distorted helix without full helix unwinding. The distortion was achieved by applying a pulsed AC electric field, perpendicular to the helix axis. If the pulse is in the millisecond range, the helix is deformed but keeps its original pitch even for electric fields higher than the theoretical critical field for helix unwinding. In this field regime, very pronounced high-order photonic band gaps are observed, in agreement with our calculations. We theoretically explore the possibility of obtaining viable laser emission at the second-order photonic band gap, and experimentally find that lasing is not only possible but has a figure of merit similar to that of the usual laser at the main-gap region. Therefore, electric-field-induced high-order photonic band gaps are potentially useful for multiline laser applications.     

Voltage-induced pseudo-dielectric heating and its application for color tuning in a thermally sensitive cholesteric liquid crystal

ABSTRACT

We previously proposed an electrical approach enabling the tuning of the center wavelength λ_c of the cholesteric liquid crystal (CLC) bandgap in the full-color visible spectrum based on the electro-thermal effect. The idea involved the design of a negative CLC with a thermally sensitive bandgap feature and the use of a sandwich-type cell with finite electrode conductivity, allowing the control of cell temperature by applied voltage via pseudo-dielectric heating. It has been demonstrated experimentally that the induced pseudo-dielectric heating is predominated by the pseudo-dielectric relaxation originating from the designated cell geometry. On the basis of this technique, key factors determining the tuning efficacy of the temperature and thus λ_c are primarily investigated in this study. Our results suggest that lowering the electrode resistivity and the specific heat conductivity of the cell can promote the maximum tunable temperature range. Expectedly, optimizing the electrode area, cell gap and dielectric permittivity of the CLC favors a decreased relaxation frequency and, in turn, reducing the voltage as well as the frequency required for λ_c tuning.     

Towards Cholesteric Absorbers for Microwave Frequencies

Metal-backed cholesteric absorbers for microwave frequencies are presented, and their circular-polarization-sensitive absorption properties are numerically illustrated.