Design of new super-high birefringent isothiocyanato bistolanes – synthesis and properties

We report the synthesis of new laterally ethyl-substituted high birefringent phenylethynyltolane liquid crystals with alkylsulphanyl chain and isothiocyanate terminal group. The thermal and optical properties such as birefringence are measured and discussed based on their molecular structures. The compounds exhibit birefringence (Δn) in the range of 0.63–0.75 and are expected to be excellent components of high birefringent nematic mixtures for various applications.     

Chain-like [Sx] (x=2–6) Units Realizing Giant Birefringence with Transparency in the Near-Infrared for Optoelectronic Materials

Birefringent crystals are requisite optical devices in laser and modern opto-electronic fields. Development of excellent birefringent materials is still challenging. Herein, the linear or chain-like [S_x] (x=2–6) species were theoretically proved to be the origin of the large birefringence, and could be regarded as birefringent genes. Besides, the metal polysulfide family was first proposed to be rich birefringent materials source, among which Cs₂S₆ realizes giant birefringence 0.58@1064 nm together with a wide band gap of 1.70 eV (based on the generalized gradient approximation). Moreover, the first dual-anion group polysulfide Na₄Ba₃(S₂)₄S₃ was obtained, showing wide infrared transmission range (0.5–6.2 μm), wide band gap (2.3 eV), and large birefringence (0.37 at 1064 nm). This work provides a new guiding thought for exploring large birefringence crystals in the future.     

Refractometry of lyotropic mesophase systems: Ethyl cellulose/m-cresol and ethyl cellulose/acetic acid

An Abbé refractometer with a rotatable polarizer mounted on the eyepiece is used for determining the two principal refractive indices of birefringent concentrated solutions of ethyl cellulose in m-cresol and in acetic acid. At a certain concentration, birefringence appears for both systems, and the concentration agrees with that for the onset of iridescence (here defined as the critical concentration C_a for formation of the liquid-crystalline phase). The birefringence of concentrated solutions depends strongly on concentration and temperature and can be generalized by a master curve. The refractive index increments for both systems are almost independent of temperature at high concentrations.     

A new approach for controlling birefringent property of cyclic olefin copolymers

This article reports an original method to control the birefringent property of the cyclic olefin copolymer, which has been recently commercialized as a new type of optical resins, via introduction of a third monomer that possesses a negative birefringence into the cyclic olefin copolymer that exhibits an inherently positive birefringence. The mutual compensation between these opposite‐sign birefringences effectively reduced the birefringent magnitude of the corresponding terpolymer. In fact, terpolymerization of norbornene (N), ethylene (E), and styrene (S), in which S exhibits a negative birefringence regarding to the positive birefringence of the NE copolymer was successfully prepared using fluorenylamidodimethyltitanium‐based catalyst, yielding NES terpolymers with controllable birefringent property. Especially, when the S content in the NES terpolymer was controlled at optimum values, it is possible to synthesize a new type of the cyclic olefin copolymer that exhibits an extremely low birefringent magnitude close to zero regardless of high degrees of chain orientation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7395–7400, 2008     

Electron microscopic and light scattering observation on a system with two iridescent phases

Electron microscopic observations and classical light-scattering measurements have been carried out for dodecyldimethylaminoxide/hexanol/water mixtures in the concentration range where iridescent colors occur. This system has two different iridescent phases. The iridescent phase with more hexanol forms quickly, and the phase with less hexanol forms very slowly. Three different isotropic phases which show strong flow birefringence are found near both iridescent phases. The electron microscopic pictures show clearly that only one of these isotropic phases with strong flow birefringence is a bicontinuous sponge phase (L_3h -phase). This is the phase which comes out by adding some alkanol to the upper lamellar phase. The flow birefringent phase below the lower lamellar phase forms unilamellar vesicles. The flow birefringent phase which occurs between both iridescent phases contains multilamellar vesicles and is shown to be a precursor of a lamellar phase.     

Zero-Birefringence Optical Polymer by Birefringent Crystal and Analysis of the Compensation Mechanism

Summary: We reported a method for compensating the birefringence of optical polymers by doping them with inorganic birefringent crystals. In this method, an inorganic birefringent material is chosen that has the opposite birefringence to the polymer and needle-like shape crystals which are oriented when the polymer chains are oriented. The birefringence of the polymer is thus compensated by the opposing birefringence of the crystal. Orientation behavior of the needle-like crystals and polymers was investigated. The orientation function of the needle-like crystal was increased with an increase in the aspect ratio of the needle-like crystal.     

Acetylene-containing highly birefringent rod-type reactive liquid crystals based on 2-methylhydroquinone

New highly birefringent reactive liquid crystal materials based on the 2-methylhydroquinone core were designed and synthesised. Rod-type liquid crystal compounds bearing photo-crosslinkable reactive group of acryloyl, methacryloyl, cinnamoyl, furylacryloyl group were synthesised by introducing acetylene groups via Sonogashira coupling to obtain high birefringence, and lateral groups such as fluoro and methyl to adjust the temperature of the liquid crystal phase. The synthesised compounds were characterised using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. In addition, their thermal behaviour was investigated using differential scanning calorimetry and polarised optical microscopy. After aligning the synthesised compounds, liquid crystal films were prepared by photo-irradiation. Photo-elastic modulator results showed that the obtained liquid crystal films had high birefringence (Δn) values of 0.32–0.40.     

Recent Development of SnII,SbIII-based Birefringent Material: Crystal Chemistry and Investigation of Birefringence

The combination of Sn^II or Sb^III with π, non-π-conjugated units has produced birefringent crystals with birefringence ranging from 0.005 to 0.468@1064 nm. It is proven that introducing Sn^II or Sb^III into crystals is a feasible strategy to enlarge the birefringence, which not only promotes the miniaturization of fabricated devices, but also effectively modulates polarized light. Herein, recently discovered Sn^II, Sb^III-based birefringent crystals with birefringence investigated are summarized, including their crystal structure and optical properties, especially birefringence. This review also presents the influence of Sn^II, Sb^III with stereochemically active lone pair on the optical anisotropy. We hope that this work provides a clear perspective on the crystal chemistry of Sn^II, Sb^III-based optical functional crystals and promotes the development of new birefringent crystals with large optical anisotropy.     

Effect of trifluoromethyl substituents on birefringence of polystyrene

Transparent thermoplastic polymers that exhibit no birefringence are ideal for optical components such as optical films for liquid crystal displays and various lenses. Copolymerization of a positive birefringent monomer with a negative monomer is an effective technique for obtaining low birefringent polymers, especially zero‐photoelastic birefringence polymers that exhibit no photoelastic birefringence even during elastic deformation. We prepared four types of trifluoromethyl‐substituted polystyrenes. By substituting hydrogens at the ortho or meta positions of the benzene ring of polystyrene, we demonstrated that poly(2‐(trifluoromethyl)styrene), poly(3‐(trifluoromethyl)styrene), and poly(3,5‐bis(trifluoromethyl)styrene) had negative photoelastic coefficients. However, poly(4‐(trifluoromethyl)styrene) had a positive photoelastic coefficient similar to that of polystyrene. Based on these results, we synthesized a zero‐photoelastic birefringence polymer of poly(2‐(trifluoromethyl)styrene‐co‐4‐(trifluoromethyl)styrene) (55/45 wt.) exhibiting no photoelastic birefringence in elastic deformation, in which the positive photoelastic birefringence of the poly(4‐(trifluoromethyl)styrene) unit was compensated for by the negative photoelastic birefringence of the poly(2‐(trifluoromethyl)styrene) unit. The discovery of polymers having negative photoelastic coefficients is valuable for the design and synthesis of zero‐photoelastic birefringence polymers. The four types of trifluoromethyl‐substituted polystyrenes are promising optical materials because they have high transparency (transmittance > 89–92% for 27–34‐µm thickness films) in the visible and near‐infrared regions and a high decomposition temperature of approximately 400°C. Copyright © 2016 John Wiley & Sons, Ltd.     

A Hydrogen Bonded Supramolecular Framework Birefringent Crystal

Birefringent crystals could modulate the polarization of light and are widely used as polarizers, waveplates, optical isolators, etc. To date, commercial birefringent crystals have been exclusively limited to purely inorganic compounds such as α-BaB₂O₄ with birefringence of about 0.12. Herein, we report a new hydrogen bonded supramolecular framework, namely, Cd(H₂C₆N₇O₃)₂⋅8 H₂O, which exhibits exceptionally large birefringence up to about 0.60. To the best of our knowledge, the birefringence of Cd(H₂C₆N₇O₃)₂⋅8 H₂O is significantly larger than those of all commercial birefringent crystals and is the largest among hydrogen bonded supramolecular framework crystals. First-principles calculations and structural analyses reveal that the exceptional birefringence is mainly ascribed to strong covalent interactions within (H₂C₆N₇O₃)⁻ organic ligands and the perfect coplanarity between them. Given the rich structural diversity and tunability, hydrogen bonded supramolecular frameworks would offer unprecedented opportunities beyond the traditional purely inorganic oxides for birefringent crystals.     

Hierarchical Modulation of Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates MIIB4O6F2 (MII=Be,Mg, Pb,Zn, Cd)

The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d¹⁰ electron configuration, and 6s² lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB₄O₆F₂ (AE=Ca, Sr) is adopted for UV/deep-UV birefringent structures, namely, M^IIB₄O₆F₂ (M^II=Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6 % with the cation substitution from Ca, Sr to Be, Mg (route-I), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB₄O₆F₂ structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm, and the PbB₄O₆F₂ structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.     

A rod‐like fluorinated polyimide as an in‐plane birefringent optical material 2: Control of optical retardation using spontaneous molecular orientation

Following previous work, a fluorinated polyimide with a rod‐like structure has been investigated as an in‐plane birefringent optical material whose birefringence and thickness can be precisely controlled. Poly(amic acid) films fixed in a metal frame by two sides and thermally cured without any drawing resulted in a polyimide film with an in‐plane birefringence (Δn) larger than 0.1 at 1543 nm. The optical retardation, which is defined as the product of Δn and the film thickness, was controlled by varying the curing and post‐annealing temperatures and by using reactive ion etching. In situ measurements of the tensile stress and the generated retardation showed that the initial orientation at below 200°C was due to the large tensile stress caused by the film shrinkage during imidization and that the increased Δn at higher temperatures was caused by the spontaneous orientation of the polyimide molecules. The curing temperature dependence of refractive indices, optical transmittance in the visible and near‐infrared region, and the wavelength dispersion of retardation of the in‐plane birefringent polyimide films are also reported. Copyright © 1999 John Wiley & Sons, Ltd.     

Sn2B5O9Cl: A Material with Large Birefringence Enhancement Activated Prepared via Alkaline‐Earth‐Metal Substitution by Tin

The excellent birefringent materials are needed for optical systems. Herein, we reported a new compound, the first tin borate chloride, Sn₂B₅O₉Cl (SBOC) with a large birefringence (0.168 at 546 nm) measured by the polarizing microscope. Its birefringence is 16 times that of the isostructural Ba₂B₅O₉Cl (BBOC) compound (0.010@ at 546 nm). The results show that the birefringence enhancement originates mainly from the Sn²⁺ polyhedra. We propose that the birefringence can be enlarged by substituting the alkaline‐earth metal cation by the Sn²⁺ cation in the isostructural borate with small birefringence. This strategy will guide the discovery of large birefringent materials in the future.     

Supermolecular structures and flow birefringence in polymer solutions

Experimental studies of supermolecular structures and localized flow birefringence in solutions of high-molecular weight polymer are described. Advantage is taken of poly(ethylene oxide) and polyisobutylene. Supermolecular structures are examined with the aid of optical microscopy using freeze-dried samples of the polymer solutions. Birefringence is investigated that arises in planar elongational flow in a cross-slot cell. Flow velocities at which the onset of the localized birefringence occurs are determined. Then these velocities are correlated with viscoelastic characteristics of the solutions. The presence of a liquid-crystalline fibrillar network in the polymer solutions exhibiting flow birefringence is ascertained. The fibrils are birefringent objects. The fibrils are birefringent objects. The localized birefringence phenomenon is explained in term of the orientation of the fibrils in elongational flow. It has been shown that the onset of localized birefringence occurs at a critical Weissenberg number, the value of which is close to unity.     

Formation of ordered phases of (hydroxypropyl)cellulose in miscible and immiscible isobutyric acid-water mixtures

(Hydroxypropyl)cellulose (HPC) is known to form birefringent liquid-crystalline phases at elevated polymer concentrations in either water or isobutyric acid (IBA). The HPC concentration at which the polymeric phase exhibits birefringence decreases as the IBA content in mixed H₂O/IBA solvents decreases, even though the concentration ?ⁱ_c for the formation of an ordered phase of HPC in water is greater than that in IBA. Water is a spectator component and apparently does not participate in the formation of a birefringent phase when IBA is present. A birefringent phase forms once the concentration of HPC in the solution omitting the H₂O equals the ?ⁱ_c of binary HPC/IBA solutions for temperatures from 23 to 95°C. The strong preferential affinity of HPC for IBA is visually evident as an HPC coagulate separates from dilute solution when the solvent mixture contains as little as 5% IBA. The coagulate dissolves to give a monophasic isotropic solution as the IBA content in the solvent is increased. A heterogeneous system in which a clear supernatant fluid covers a pearly white polymeric phase forms when the solvent mixture is immiscible and the HPC content is less than 50%. At high HPC content, the classical appearance associated with concentrated HPC solutions is seen. The optical and rheological properties of the heterogeneous systems are compared with those of homogeneous solutions at several HPC concentrations.     

Some optical and mechanical properties of poly(vinyl chloride)

The optical and mechanical properties of poly(vinyl chloride) film were examined by observing both the stress and birefringence during stretching at constant rate, during relaxation at constant length and during a dynamic birefringence experiment. Experiments were also done by varying the temperature at constant length. The changes in birefringence are interpreted in terms of changes in negative distortional birefringence, changes in positive orientation birefringence, and possible reversible changes in birefringence with temperature arising from conformational changes in the polymer chain and changes in the contribution of birefringent crystals.     

An Exceptional Peroxide Birefringent Material Resulting from d–π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂VO(O₂)₂F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂VO(O₂)₂F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂²⁻ anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂VO(O₂)₂F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides.     

An Exceptional Peroxide Birefringent Material Resulting from d–π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂VO(O₂)₂F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂VO(O₂)₂F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first‐principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂^2? anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂VO(O₂)₂F. These findings distinguish peroxides as a brand‐new class of birefringent materials that may possess birefringence superior to the traditional oxides.     

An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂C₂O₄F₅, by a combination of delocalized π-conjugated [C₂O₄]²⁻ groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂O₄]²⁻ groups are not in an optimal arrangement in the crystal structure of KSb₂C₂O₄F₅; nonetheless, KSb₂C₂O₄F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂O₄, even though the latter features an optimal arrangement of π-conjugated [B₃O₆]³⁻ groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂O₄]²⁻ anions, highly distorted SbO₂F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.     

Birefringence and dichroism of oriented epoxy thermoset films

A series of oriented liquid–crystalline epoxy thin films were prepared by the in situ polymerization of a liquid–crystalline diepoxide, 1,4‐phenylene bis[4‐(2,3‐epoxypropoxy)benzoate], with an aromatic diamine, 4,4′‐diaminobiphenyl, in a 7.0‐T magnetic field. The birefringent measurements of the oriented films were made from 543.5 to 830 nm. In this range, the values of birefringence (Δn) range from 0.155 to 0.130. When they are extrapolated to the microwave region, Δn = 0.105. The dichroism of a guest azo dye, 4‐(4‐nitrophenylazo)‐3‐hexyloxyaniline, in the oriented thin films was examined in the visible region. From the results, the order parameter of the polymer was calculated to 0.65 by extrapolating the concentration of the guest azo dye to zero. The guest azo dye compound does not affect the birefringence. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 915–919, 2001