Lightly crosslinked,mesomorphic networks obtained through the reaction of dimeric,liquid‐crystalline epoxy–imine monomers and heptanedioic acid

We reacted various dimeric, liquid‐crystalline epoxy–imine monomers, differing in the length of the central aliphatic spacer or the dipolar moments, with heptanedioic acid. The resulting systems showed a liquid‐crystalline phase in some cases, depending on the dimer and on the reaction conditions. The systems were characterized with respect to their mesomorphic properties and then were submitted to dynamic mechanical thermal analysis in both fixed‐frequency and frequency‐sweep modes in the shear sandwich configuration. The arrangement in the liquid‐crystalline phase seemed to be mainly affected both by the polarization of the mesogen and by the reaction temperature, which favored the liquid‐crystalline arrangement when it was lying in the range of stability of the dimer mesophase. In agreement with other recent literature data, dynamic mechanical thermal analysis results suggested that the presence of the mesogen directly incorporated into the main chain increased the lifetimes of the elastic modes both in the isotropic phase and in the liquid‐crystalline phase with respect to side‐chain liquid‐crystalline elastomers and that the time–temperature superposition principle did not hold through the liquid‐crystalline‐to‐isotropic transition. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:6270–6286, 2006     

甲壳型液晶高分子的化学结构及其与性能间关系的研究

设计并合成了7种新型甲壳型液晶高分子,研究了液晶基元的化学结构和立体效应对单体及其聚合物液晶性的影响.发现在液晶基元的末端引入极性或可极化的原子基团提高了单体的熔点和液晶相的热稳定性;液晶基元的长径比越大,单体的熔点和清亮点越高;聚合使单体的液晶稳定性增加、液晶相温度范围变宽;侧链液晶基元的极性、刚性和空阻越大,聚合物的玻璃化温度越高;酰胺基团无论是在分子的末端还是在连接部位,都使单体的熔点和聚合物的玻璃化温度提高,但在分子末端时液晶相较稳定,作为中心桥键时不利于液晶相的稳定形成.     

4,5‐Pyrenocyanine—Just Another Phthalocyanine? A STM and 2D WAXS Study

Pyrene‐fused tetraazaporphyrins were synthesized from pyrene‐4,5‐dicarbonitrile precursors using a recently reported procedure as the key step for the asymmetric substitution of pyrene. Metal‐free, zinc‐ and lead‐centered pyrenocyanines were obtained and their optical properties as well as their molecular assembly in the solution and bulk phases and at the liquid/solid interface were studied. The characteristic Q‐band appears broadened, most likely owing to distortion of the molecule introduced by the steric demand of the angularly extended aromatic residue. The angular annulation does not bathochromically shift the Q‐band as far as would have been expected for the linear case. Peripheral substitution with linear and branched alkoxy chains affords solubility of the compounds in organic solvents. The influence of the distinct steric demand of the substituents on aggregation was investigated for metal‐centered pyrenocyanines by using temperature‐dependent ¹H NMR and UV/Vis spectroscopy. The self‐assembly at the liquid/solid interface was studied using scanning tunneling microscopy. The alkoxy substituents facilitate the anchoring of these slightly non‐planar molecules on the surface of graphite. Pyrenocyanine molecules form well‐ordered 2D arrays in which the molecules are arranged in rows. The angular annulation of the pyrenocyanine residue leads to characteristic adsorption behavior at the liquid/solid interface, in which the molecules adsorb in two different adsorption geometries. The alkoxy side‐chains give rise to a discotic columnar superstructure and induce distinct thermotropic behavior. Dependent on the steric demand of the branched chains and the central metal atom, the molecules are rotated with respect to each other to form helical organization.     

Side chain polysiloxanes with fluorinated mesogenic groups. The role of the fluorine content on the properties of the smectic A state

A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.     

Thermotropic properties of ferrocene derivatives bearing a cholesteryl unit: structure–properties correlations

The synthesis and structural characterization of new liquid‐crystalline compounds containing ferrocene, azo‐aromatic and cholesteryl groups are reported. Taking into account the advantage brought by chirality, ferrocene and azo units, these structures could be good precursors for obtaining materials capable of responding to magnetic and electric fields or to UV‐light exposure. The influence of each structural unit (ferrocene, cholesterol, azo aromatic core and flexible chain length) has been studied by comparing analogous compounds possessing the same structure but without the element being analyzed. Ferrocene is a three‐dimensional bulky unit, so that, regardless of the substituents' nature, this unit could cause steric repulsions with neighboring molecules. These interactions could lead to a decrease of the transition temperature domain. Surprisingly, a decrease in the clearing point was not observed for the compounds discussed. This behavior was possible because ferrocene is connected to the mesogen via a flexible unit. As a consequence, both phenyl analogues and ferrocene derivatives presented liquid‐crystalline properties with similarly high clearing points, but above the thermal stability of derivatives with azo groups. Their melting points depend on the way the molecules are packed, with different crystalline states being detected in the case of ferrocene derivatives. In order to explain the liquid‐crystalline behavior of the compounds synthesized, molecular simulations were performed using the Hyperchem program. Copyright © 2005 John Wiley & Sons, Ltd.     

First liquid crystalline cuneane‐caged derivatives: a structure–property relationship study

The synthesis and phase behaviour of the first cuneane derivatives having liquid crystalline properties are reported. This new class of liquid crystalline materials is the homologous series of bis[4‐(n‐alkoxyphenyl)]cuneane‐2,6‐dicarboxylates. They were synthesized by two methods: either by isomerization of the previously prepared bis[4‐(n‐alkoxyphenyl)]cubane‐1,4‐dicarboxylates or by direct esterification of the cuneane‐2,6‐dicarboxylic acid. Enantiotropic nematic phases were observed for the first four homologues. The higher homologues exhibited enantiotropic smectic A phases. The bis[4‐(n‐octylphenyl]cuneane‐2,6‐dicarboxylate exhibited the SmA phase at a lower temperature than its octyloxy analogue. The mesophases were investigated and established by polarizing optical microscopy, differential scanning calorimetry and X‐ray diffraction. Quantum chemical calculations suggest elongated structure for these new liquid crystalline compounds.     

Influence of branched chains on the mesomorphic properties of symmetrical and unsymmetrical triphenylene discotic liquid crystals

A series of novel symmetrical and unsymmetrical triphenylene‐based discotic liquid crystalline materials with one or six branched peripheral alkoxy chains have been prepared. These materials have been compared with analogous known symmetrical and unsymmetrical compounds to reveal a balance between steric and space‐filling effects of the peripheral branched chains, which significantly affects intermolecular forces of attraction and packing, and hence affects melting and isotropisation temperatures of the liquid crystalline materials. The desired result of reduction of melting points and enhancement of isotropisation temperatures has been accomplished by use of branched alkoxy chains in both symmetrical and unsymmetrical materials.     

Synthesis and tailoring phase behavior of mesogen jacketed liquid crystalline copolymers based on 2, 5‐bis[(4‐alkoxyphenyl)oxycarbonyl]styrenes

Based on 2, 5‐bis[(4‐alkoxyphenyl)oxycarbonyl]styrenes (M‐OCm, m is the number of the carbons of alkyl tails, m = 1, 4, and 18), three series of binary copolymers with high‐molecular weights, {poly(M‐OC1‐co‐M‐OC4), poly(M‐OC1‐co‐M‐OC18), and poly(M‐OC4‐co‐M‐OC18)} have been prepared via free‐radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities because of the analogous monomers. The phase behaviors of copolymers were studied by DSC, POM, and one‐dimensional wide‐angle X‐ray diffraction. The results showed that liquid crystalline (LC) phase structures of copolymers, containing smectic phase, reentrant isotropic phase, columnar phase. and isotropic phase, were strongly depended on the composition and the alkyl length due to the competing among the steric effect, the microphase separation and the driving force of the entropy. When one of them occupied a dominant position, the LC phase structure can be presented for the copolymers. Otherwise, the LC phase structure is lost despite the pair of corresponding homopolymers forming mesogenic structure. Therefore, through copolymerization, LC behavior of the mesogen‐jacketed liquid crystalline polymers can be greatly varied. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2804–2816     

Studies on the synthesis and properties of thermotropic liquid crystalline polycarbonates. VII. Liquid crystalline polycarbonates and poly(ester‐carbonate)s derived from various mesogenic groups

Three series of thermotropic liquid crystalline polycarbonates and poly(ester‐carbonate)s were prepared by solution polycondensation of 4,4′‐biphenyldiol (BP), 4′‐hydroxybiphenyl‐4‐hydroxybenzoate (HHB), or 4‐hydroxyphenyl‐4″‐hydroxybiphenyl‐4′‐carboxylate (HHBP), as mesogenic unit, with 1,10‐bis(p‐hydroxybiphenoxy)decane (N10), bisphenol A (BPA), 4,4′‐dihydroxy‐diphenyl ether (BPO), 4,4′‐[phenylbis(oxy)]bisphenol (BPOO), methylhydroquinone (MeHQ), or phenylhydroquinone (PhHQ). One series of cholesteric poly(ester‐carbonate)s were also prepared by using HHBP, the aromatic diols mentioned above and isosorbide as the chiral moiety. All polycondensations were implemented in pyridine by using triphosgene as the condensation agent. The synthesized polycarbonates were characterized by viscometer, FTIR, DSC, TGA measurements, polarizing microscopy equipped with a heating stage, and WAXD powder pattern. In this study, it was found that the liquid crystalline properties of polycarbonates strongly rely on the mesogenic unit applied. HHBP‐series exhibits a wide temperature region of liquid crystalline (LC) phase even with 50% of bisphenol A (BPA), which is a V‐shaped structure and usually destroys liquid crystalline properties. In addition, homopolycarbonate with HHBP structure possesses extraordinarily low phase‐transition temperature and wide liquid crystalline phase range, due to its asymmetric structure. This asymmetric structure results in head‐to‐tail, head‐to‐head, and tail‐to‐tail random conformation of polymer chain. The isosorbide containing poly(ester‐carbonate)s formed cholesteric phase, which showed homogeneous blue, green, or red Grandjean texture upon shearing in molten state and the Grandjean texture could be frozen easily while quenching the sample to the room temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1852–1860, 2000     

A concept for bifocal contact‐ or intraocular lenses: liquid single crystal hydrogels (“LSCH”)

The synthesis of liquid single crystal hydrogels (“LSCH”) in suitable molds offers an innovative concept to realize bifocal contact‐ or intraocular‐lenses. LSCH combine the properties required for applications as bifocal ophthalmic lenses: the soft and water‐containing hydrogel enables oxygen permeation and exhibits high birefringence due to the liquid crystalline phase structure built up by rigid rod‐like amphiphiles. Via a photo‐initiated crosslinking reaction of aqueous solutions of monomeric lyotropic liquid crystalline amphiphiles in the macroscopically ordered liquid crystalline state, we obtain optically uniaxially ordered and transparent LSCH. The orientation process and the phase structure of the anisotropic hydrogel is analyzed by deuterium NMR‐spectroscopy. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and properties of photochromic liquid‐crystalline copolymers containing both spironaphthoxazine and cholesteryl groups

Photochromic liquid‐crystalline copolymers consisting of a photochromic monomeric unit containing both a spironaphthoxazine group and an undecamethylene spacer, and a liquid‐crystalline monomeric unit containing both a cholesteryl group and a decamethylene spacer were prepared to investigate the effect of the thermal properties of the photochromic monomeric unit on the mesomorphic order of the side chain of the related copolymers. The photochromic liquid‐crystalline copolymers containing a photochromic liquid‐crystalline monomeric unit showed only a smectic phase. On the other hand, the photochromic liquid‐crystalline copolymers containing a photochromic non‐liquid‐crystalline monomeric unit showed a chiral nematic phase (cholesteric phase). The photochromic chiral nematic liquid‐crystalline copolymer containing 14 mol % photochromic monomeric unit reflected visible light around 104 °C. To lower the temperature range of reflection of visible light, cholesteryl oleyl carbonate was used as a chiral nematic plasticizer for the photochromic chiral liquid‐crystalline polymer systems. Photo‐induced pitch change of the mixture by means of UV irradiation was investigated and it was concluded that the pitch change observed under UV irradiation was mainly induced by thermal effect in the case of our system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 887–894, 2000     

Steric hindrance effect on thermo‐responsive behaviors of well‐defined water‐soluble semi‐rigid polymers

Three series of water‐soluble semi‐rigid thermo‐responsive polymers with well‐defined molecular weights based on mesogen‐jacketed liquid crystal polymers, poly[bis(N‐(2‐hydroxypropyl) pyrrolidone) 2‐vinylterephthalate] [P(2‐HPPVTA)], poly[bis(N‐(1‐methyl‐2‐hydroxyethyl) pyrrolidone) 2‐vinylterephthalate] [P(1‐M‐2‐HEPVTA)] and poly[bis(N‐hydroxypropyl pyrrolidone) 2‐vinylterephthalate] (PHPPVTA) have been synthesized via reversible addition‐fragmentation chain transfer polymerization. The steric hindrance effects on liquid crystalline property and thermo‐responsive behaviors of semi‐rigid water‐soluble polymers (P(2‐HPPVTA), P(1‐M‐2‐HEPVTA), and PHPPVTA) were carefully investigated. From molecular structure, the steric hindrance of P(1‐M‐2‐HEPVTA) is stronger than that of P(2‐HPPVTA). Polarized light microscope and one‐dimensional wide‐angle X‐ray diffraction revealed that both the P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) display a columnar nematic phase, indicating that the steric hindrance effect do not affect liquid crystalline behavior of the polymers. The dynamic light scattering results demonstrated that P(1‐M‐2‐HEPVTA) exhibited lower cloud point compared with that of P(2‐HPPVTA) at the same mass concentration and the same molecular weight. The more significant molecular weight and concentration dependence on cloud point have been observed in P(2‐HPPVTA) solution than in P(1‐M‐2‐HEPVTA) solution. We also discovered that the cloud points of both P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) solution are lower in D₂O than in H₂O. It is noted that the cloud point of PM‐2 is 9.9 °C lower in D₂O than in H₂O, much less pronounced than the cloud point difference of PH‐2. The differences of thermo‐responsive behaviors between P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) were resulted from the steric hindrance effect existed in their side groups. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3429–3438     

Synthesis of comb‐type polycarbosilanes via nucleophilic substitution reactions on the main‐chain silicon atoms

A series of comb‐type polycarbosilanes of the type [Si(CH₃)(OR)CH₂]_n {where R = (CH₂)_mR′, R′ = ? O‐p‐biphenyl? X [X = H (m = 3, 6, 8, or 11) or CN (m = 11)], and R′ = (CF₂)₇CF₃ (m = 4)} were prepared from poly(chloromethylsilylenemethylene) by reactions with the respective hydroxy‐terminated side chains in the presence of triethylamine. The product side‐chain polymers were typically greater than 90% substituted and, for R′ = ? O‐p‐biphenyl? X derivatives, they exhibited phase transitions between 27 and 150 °C involving both crystalline and liquid‐crystalline phases. The introduction of the polar p‐CN substituent to the biphenyl mesogen resulted in a substantial increase in both the isotropization temperature and the liquid‐crystalline phase range with respect to the corresponding unsubstituted biphenyl derivative. For R = (CH₂)₁₁? O‐biphenyl side chains, an analogous side‐chain liquid‐crystalline (SCLC) polysiloxane derivative of the type [Si(CH₃)(O(CH₂)₁₁? O‐biphenyl)O]_n was prepared by means of a catalytic dehydrogenation reaction. In contrast to the polycarbosilane bearing the same side chain, this polymer did not exhibit any liquid‐crystalline phases but melted directly from a crystalline phase to an isotropic liquid at 94 °C. Similar behavior was observed for the polycarbosilane with a fluorocarbon chain, for which a single transition from a crystalline phase to an isotropic liquid was observed at ?0.7 °C. The molecular structures of these polymers were characterized by means of gel permeation chromatography and high‐resolution NMR studies, and the crystalline and liquid‐crystalline phases of the SCLC polymers were identified by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 984–997, 2003     

Virtual isotropic‐nematic transitions in alkyl cyanobiphenyls

Pretransitional fluctuations in the isotropic phase of liquid crystalline and non‐liquid crystalline alkyl cyanobiphenyls have been investigated using light scattering and magnetic birefringence measurements. We find evidence for a virtual isotropic‐nematic phase transition in short‐chain alkyl cyanobiphenyls with no observable nematic phase. The measured temperature dependence of fluctuations is well‐described by mean‐field theory. Virtual phase transition temperatures extrapolated from separate light scattering and magnetic birefringence experiments are in good agreement. Landau–de Gennes model parameters for the compounds investigated are calculated from the experimental results.     

Metal‐Ion‐Induced Switch of Liquid‐Crystalline Orientation of Metallomacrocycles

A new series of shape‐persistent imine‐bridged macrocycles were synthesized based on dynamic covalent chemistry. The macrocycles had an alternating sequence of dibenzothiophene and N,N′‐bis(salicylidene)‐ethylenediamine (salen) tethering branched alkyl chains. The macrocycles and tetranuclear metallomacrocycles bearing long and branched alkyl chains exhibited thermotropic columnar liquid‐crystalline phases over a wide temperature range and the metallomacrocycles greatly depended on the characteristics of the coordinated metal ions. The metal‐free macrocycle showed a liquid‐crystalline phase with a lamellar structure and poor birefringence. In sharp contrast, the macrocyclic Ni complex showed a columnar oblique liquid‐crystalline phase, whereas the Pd and Cu complexes showed columnar liquid‐crystalline phases with a lamellar structure. The macroscopic organization and thermal properties of the corresponding liquid‐crystalline metallomacrocycles were significantly dependent on the subtle structural differences among the planar macrocycles, which were revealed by single‐crystal X‐ray crystallographic analysis of the macrocycles with shorter alkyl chains.     

On the Uniaxial–Biaxial Nematic Phase Transition in Liquid‐Crystalline Polymers and Elastomers

We study how the uniaxial–biaxial nematic phase transition changes its nature when going from a low‐molecular‐weight liquid crystal to a liquid‐crystalline elastomer or polymer (the latter above the Maxwell frequency) and find a qualitative change due to the presence of a coupling to the strain field in these materials. While this phase transition can be of second‐order in low‐molecular‐weight materials, as is also experimentally observed, we show here that the order of this phase transition is changed generically to no phase transition at all or to a first‐order phase transition in mean‐field approximation. We analyze the influence of an external mechanical stress field above the uniaxial–biaxial nematic phase transition and find that either biaxial nematic order is induced, which is linear or quadratic in the stress intensity, or no response to an external stress results at all, depending on the relative orientation of the applied shear with respect to the director of the uniaxial nematic phase.     

Effect of cholesteric liquid crystalline elastomer with binaphthalene crosslinkings on thermal and optical properties of a liquid crystal that show smectic A‐cholesteric phase transition

A side‐chain polysiloxane cholesteric liquid crystalline elastomer (ChLCE) with binaphthalene derivate as crosslinkings and cholesterol derivate as liquid crystalline monomers was designed and synthesized. A binaphthyl chiral dopant (CD) was synthesized as well. The chemical structures and liquid crystalline properties of the ChLCE and the CD were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, element analyses, differential scanning calorimetry and polarizing optical microscopy measurements. The helical twisting power of the ChLCE exhibited a turning point with changing temperature and was smaller than that of the CD. In addition, the effect of the ChLCE on phase transition temperatures and thermal‐optical properties of a liquid crystal that show smectic A (SmA)‐cholesteric (Ch) phase transition was studied. Worthily, the testing of the reflection wavelength with changing temperature suggested that the adding of the ChLCE in liquid crystals that show SmA‐Ch phase transition can expedite their SmA‐Ch transition. In addition, the network structure of the ChLCE may play a significant role in the accelerating of the transition. These properties provided theoretical and experimental foundations for applying ChLCE in thermally sensitive liquid crystal devices requiring fast response, such as thermally controllable windows, materials and displays. Copyright © 2012 John Wiley & Sons, Ltd.     

Liquid crystalline and dielectric properties of homologous series of substituted 1,3,4-oxadiazoles

Three homologous series of 2-alkylthio-5-phenyl-1,3,4-oxadiazole derivatives were synthesized [1, 2] and their liquid crystalline and dielectric properties investigated. Depending on the chain length, nematic, smectic A and smectic C phases were observed [1] . Dielectric investigations indicate a cross over of epsilon o and epsilon o at the N/SmA phase transition. This is explained by a transition from a more statistical into an antiparallel correlation of the dipoles, mainly caused by steric effects.     

Phase Transfer Catalysis in the Polycondensation Processes. XXV. The Relationship Between Structure and Supramolecular Ordering of Some Aromatic Polyethers Containing Flexible Spacer

Abstract

The paper presents a study concerning the influence of the chemical structure on the supramolecular ordering [crystalline, liquid crystalline (LC), or amorphous] of some aromatic polyethers. The polymers were synthesized using a phase transfer catalysis technique, in a liquid/liquid system, starting from 1,4‐dichlorobutane or 1,5‐dichloropentane and various bisphenols, as follows: 4,4′‐dihydroxyazobenzene (DHAB), 4,4′‐dihydroxydiphenyl (DHD), bisphenol A (BPA), and 2,7‐dihydroxynaphtyl (DN). All polymers were characterized by using ¹H‐NMR, DSC, and optical microscopy in polarized light. Some samples containing azobenzene moieties showed LC properties, but the majority of them resulted as semi‐crystalline compounds. If a bent bisphenol was used (BPA) to obtain homopolymers or copolymers, only amorphous products were obtained. The low thermal stability of the LC products generated some difficulties regarding the characterization of the mesophase. Generally, the copolymerization reaction influenced only the crystalline melting points, the isotropization temperature of the LC polymers being situated near their thermal degradation limit.     

Solvent‐free Liquid Crystals and Liquids from DNA

As DNA exhibits persistent structures with dimensions that exceed the range of their intermolecular forces, solid‐state DNA undergoes thermal degradation at elevated temperatures. Therefore, the realization of solvent‐free DNA fluids, including liquid crystals and liquids, still remains a significant challenge. To address this intriguing issue, we demonstrate that combining DNA with suitable cationic surfactants, followed by dehydration, can be a simple generic scheme for producing these solvent‐free DNA fluid systems. In the anhydrous smectic liquid crystalline phase, DNA sublayers are intercalated between aliphatic hydrocarbon sublayers. The lengths of the DNA and surfactant are found to be extremely important in tuning the physical properties of the fluids. Stable liquid‐crystalline and liquid phases are obtained in the ?20 °C to 200 °C temperature range without thermal degradation of the DNA. Thus, a new type of DNA‐based soft biomaterial has been achieved, which will promote the study and application of DNA in a much broader context.