Synthesis and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers, containing the flexible non-mesogenic crosslinking agent M-1 and the cholesteric monomer M-2, is described by a one-step hydrosilication reaction. The chemical structures of the monomers and network polymers obtained were confirmed by FT-IR spectroscopy. Their mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and x-ray diffraction measurements. The influence of the crosslinking units on the phase behavior is discussed. The network polymers showed elasticity, reversible phase transitions, and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition temperatures and isotropization temperatures of network elastomers decreased as the concentration of crosslinking units was increased, but the cholesteric phase was not disturbed.     

Synthesis,properties and photopolymerization of liquid crystalline dioxetanes

The synthesis and photopolymerization of various liquid crystalline dioxetanes is described. The effects of the spacer length, structure of the mesogenic group and oxetane group on the liquid crystalline properties, polymerization behaviour and optical properties (birefringence) of the oriented and crosslinked network formed in photo-polymerization are discussed. Thermally stable films with birefringence values up to 0.13 can be formed from these materials. The dioxetanes show significantly lower polymerization shrinkage than do structurally related diacrylates.     

Synthesis, properties and photopolymerization of liquid crystalline dioxetanes

The synthesis and photopolymerization of various liquid crystalline dioxetanes is described. The effects of the spacer length, structure of the mesogenic group and oxetane group on the liquid crystalline properties, polymerization behaviour and optical properties (birefringence) of the oriented and crosslinked network formed in photo-polymerization are discussed. Thermally stable films with birefringence values up to 0.13 can be formed from these materials. The dioxetanes show significantly lower polymerization shrinkage than do structurally related diacrylates.     

Synthesis of UV-curable liquid crystalline diacrylates for the application of polarized electroluminescence

Two series of UV-curable rod-like diacrylates containing bis-tolane (M1-M5) and bis-stilbene (M6-M10) mesogens were synthesized. All of the diacrylates were prepared through convergent syntheses involving palladium-catalysed coupling reactions. The thermal transitions and mesomorphic properties of the diacrylates were characterized by differential scanning calorimetry and optical microscopy. All the bis-tolane-based monomers reveal nematic phases, except M5 which shows no mesomorphic behaviour. For the bis-stilbene based monomers, most show no mesomorphic behavior and only M7 and M9 exhibit a SmA phase.     

Synthesis and characterization of fluorine-containing cholesteric liquid crystalline polysiloxanes bearing trifluoromethyl-substituted mesogens

A series of side-chain liquid crystal (LC) polysiloxanes were synthesised with Poly(methylhydrogeno)siloxane, 4?-(undec-10-enoyloxy) biphenyl – 4 – yl 4- (trifluoromethyl) benzoate (M_th) and a chiral nematic (N*) LC monomer 1-allyl 10-(cholesteryl)-decanedioate (M_ch). The chemical structures and LC properties of the monomers and polymers were characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analysis, POM and X-ray diffractometer. M_ch is monotropic N* LC. The homopolymer derived from monomer M_ch is enantiotropic N* LC. Monomer M_th is a smectic A liquid crystal. The copolymers derived from M_ch and M_th are N* LCs. The temperatures at which 5% weight loss occurred are greater than 300°C for all the fluoro-containing polymers, and the residue weights of the samples at 600°C increased slightly as the content of trifluoromethyl mesogens increased in the polymers. The glass transition temperatures of the polymers increased as trifluoromethyl mesogens increased, too. The N*–I phase transition temperatures show a negative deviate from ideal or linear behaviour. The values of the enthalpy changes for the cholesteryl containing polymers are rather low and this is attributed to the biaxiality of cholesteryl moiety which tends to reduce the change in the orientational order at the N*–I transition. Compared to the monomers, the polymers show wider mesophase region.     

Synthesis, structure and characterization of side chain cholesteric liquid crystalline polysiloxanes

A series of liquid crystalline homopolysiloxanes and copolysiloxanes were synthesized. The chemical structures of the monomers M₁-M₇ were confirmed by FTIR and ¹H NMR spectroscopy. The structure-property relationships of the monomers and polymers are discussed; their phase behaviour and optical properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. All the monomers, except M₂ and M₇ showed smectic and nematic phases; the copolymers P₈-P₁₅ displayed cholesteric phases. The homopolymers P₁-P₇ exhibited smectic phases. The selective reflection of cholesteric monomers and copolymers shifted to longer wavelengths with increasing length of the rigid mesogenic core, with decreasing length of the flexible spacer, or with increasing content of nematic units. Experimental results demonstrated that a flexible polymer backbone, a rigid mesogenic core and a long flexible spacer tended to produce a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.     

Synthesis of UV‐curable liquid crystalline diacrylates for the application of polarized electroluminescence

Two series of UV‐curable rod‐like diacrylates containing bis‐tolane (M1–M5) and bis‐stilbene (M6–M10) mesogens were synthesized. All of the diacrylates were prepared through convergent syntheses involving palladium‐catalysed coupling reactions. The thermal transitions and mesomorphic properties of the diacrylates were characterized by differential scanning calorimetry and optical microscopy. All the bis‐tolane‐based monomers reveal nematic phases, except M5 which shows no mesomorphic behaviour. For the bis‐stilbene based monomers, most show no mesomorphic behavior and only M7 and M9 exhibit a SmA phase.     

Photoinduced hyper-reflective cholesteric liquid crystals enabled via surface initiated photopolymerization

A structured polymer was synthesized by surface initiated photopolymerization in the presence of a cholesteric liquid crystal (CLC). The templated helical polymer (traversing 2/3 the cell thickness) was backfilled with an opposite handedness, photoresponsive CLC mixture yielding a photo-induced, large contrast, hyper-reflective (R > 99%) CLC film.     

Synthesis and characterization of side chain liquid crystalline polymers exhibiting cholesteric and blue phases

A series of cyclosiloxane-based cholesteric liquid crystalline (LC) polymers were synthesized from a cholesteric LC monomer cholest-5-en-3-yl(3β) 4-(2-propenyloxy)benzoate and a nematic LC monomer butyl 4-[4-(2-propenyloxy)benzoxy]benzoate. All the polymers exhibit thermotropic LC properties and show cholesteric phases. Most of the polymers display four types of phase transition behaviour corresponding to glass transition, melting point, cholesteric phase-blue phase transition and clearing point. The mesophase temperature range of the blue phases are as broad as 20°C. The blue phase was confirmed by the apperance of planar textures and cubic packings. With an increase of non-chiral component in the polymers, the clearing point decreases slightly, while the glass transition and melting temperatures change little. In the reflection spectra of the polymer series the reflected wavelength broadens and shifts to longer wavelength with increase of the non-chiral component in the polymer systems, suggesting that the helical pitch P lengthens.     

Rotational reorganization of doped cholesteric liquid crystalline films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.     

近红外电致变色胆甾相液晶高分子的设计、合成及旋光开关性质

通过单体4′-甲氧基苯基-4-烯丙氧基苯甲酸酯(M1)、(S)-(-)-N-(5-己烯基)-6-(4′-(2-甲基丁氧基)苯基)-蒽醌-2,3-二羧酸酰亚胺(M2)与聚甲基氢硅氧烷间的硅氢加成反应,制得了3个新的具有近红外电致变色性质的侧链型液晶共聚物(PC5A10,PC5A20CB,PC5A30CB),并对其液晶性,电化学,光谱电化学和旋光开关性质进行了表征.当M2单体的含量为10mol%,20mol%时,共聚物可形成胆甾相液晶,而当其含量为30mol%时,所能形成的液晶相为近晶A相.3个共聚物的循环伏安曲线均出现了两对可逆的氧化还原峰,分别对应于蒽醌酰亚胺基团得电子而形成自由基阴离子和二价阴离子.中性态时,共聚物在420nm处有较强吸收,而当被还原为自由基阴离子后,在近红外区域840nm出现了新的强烈的吸收.以聚合物为阴极电致变色层,普鲁士蓝为离子储存层的全固态电致变色器件在800nm有较好的光学调制性.此外,本文还就该器件的电化学调控手性光开关性质进行了初步研究.     

Synthesis of diaza-crown-ether cholesteric liquid crystals

Five diaza-crown-ethers were synthesized and four of them shown to have smectic liquid crystalline properties. Their properties were determined by DSC and polarized microscopy. This type of smectogenic diazacrown ether has not been reported so far. A novel lyotropic crown ether liquid crystal was obtained from the thermotropic crown ether liquid crystal 7.     

Synthesis and characterization of side chain cholesteric liquid crystalline polymers containing isosorbide as a chiral centre

A series of new side chain cholesteric liquid crystalline polysiloxanes was synthesized by grafting copolymerization of a mesogenic monomer (M₁) and a chiral monomer (M₂). The chemical structures of the monomers and polymers obtained were confirmed by FTIR, and ¹H and ¹³C NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the content of the chiral unit on phase behaviour of the polymers is discussed. Monomer M₁ showed nematic and smectic phases on cooling. The polymers P₁ and P₂ showed a nematic phase, P₃-P₅ showed cholesteric Grandjean texture, and P₆ and P₇ exhibited smectic short-rod texture. The polymers containing more than 7.2 mol % and less than 28.6 mol % of the chrial unit showed an induced cholesteric phase. Experimental results demonstrated that the glass transition, melting and clearing temperatures decreased with increasing content of the chiral unit.     

Synthesis and characterization of side chain cholesteric liquid crystalline elastomers derived from chiral bisolefinic crosslinking units

In this work we prepared a nematic monomer (4′‐allyloxybiphenyl 4′‐ethoxybenzoate, M₁ ), a chiral crosslinking agent (isosorbide 4‐allyloxybenzoyl bisate, M₂ ) and a series of new side chain cholesteric liquid crystalline elastomers derived from M₁ and M₂ . The chemical structures of the monomers and polymers were confirmed by FTIR and ¹H NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X‐ray diffraction. The effect of the content of the crosslinking unit on phase behaviour of the elastomers is discussed. Polymer P₁ showed a nematic phase, P₂ –P₇ showed a cholesteric phase; P₆ formed a blue Grandjean texture over a broad temperature range 145–209.6°C, with no changed on the cooling. Polymers P₄ –P₇ , with more than 6?mol?% of chiral crosslinking agent, gave rise to selective reflection. Elastomers containing less than 15?mol?% of the crosslinking units displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. Experimental results demonstrated that, with increasing content of crosslinking agent, the glass transition temperatures first fell and then increased; the isotropization temperatures and mesophase temperature ranges decreased.     

Synthesis and characterization of side chain cholesteric liquid crystalline elastomers derived from chiral bisolefinic crosslinking units

In this work we prepared a nematic monomer (4'-allyloxybiphenyl 4'-ethoxybenzoate, M₁), a chiral crosslinking agent (isosorbide 4-allyloxybenzoyl bisate, M₂) and a series of new side chain cholesteric liquid crystalline elastomers derived from M₁ and M₂. The chemical structures of the monomers and polymers were confirmed by FTIR and ¹H NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X-ray diffraction. The effect of the content of the crosslinking unit on phase behaviour of the elastomers is discussed. Polymer P₁ showed a nematic phase, P₂-P₇ showed a cholesteric phase; P₆ formed a blue Grandjean texture over a broad temperature range 145-209.6°C, with no changed on the cooling. Polymers P₄-P₇, with more than 6 mol % of chiral crosslinking agent, gave rise to selective reflection. Elastomers containing less than 15 mol % of the crosslinking units displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. Experimental results demonstrated that, with increasing content of crosslinking agent, the glass transition temperatures first fell and then increased; the isotropization temperatures and mesophase temperature ranges decreased.     

Patterned retarders prepared by photoisomerization and photopolymerization of liquid crystalline films

Isomerizable diacrylates derived from cinnamic acid are designed, synthesized and mixed with liquid crystalline diacrylates with the aim of making films with alternating birefringent and isotropic domains by applying the E-Z isomerization process at room temperature. The effects of the structure of the isomerizable-mesogenic group on the isotropization efficacy, the efficiency of the E-Z isomerization reaction, and film formation are discussed. Compounds derived from cyclohexyl cinnamate are proved to be good candidates that meet a whole set of parameters related to processing and application. These compounds exhibit a low nematic-to-isotropic transition temperature. In addition, they show no yellowing upon irradiation, unlike similar compounds derived from phenyl cinnamate. To elucidate the origin of isotropization of the film by irradiation, the pure Z-isomer is prepared by photolysis of the E-isomer and subsequent chromatographic separation of both isomers. Analysis of reference samples containing the pure isomers reveals that the decrease in transition temperature can be attributed exclusively to the E-Z photoisomerization process. Finally, thin films with alternating birefringent and isotropic parts of 100×100 µm² are obtained by using a combination of photoisomerization in air and photopolymerization in a nitrogen atmosphere, which is referred to as photo-patterning.     

Patterned retarders prepared by photoisomerization and photopolymerization of liquid crystalline films

Isomerizable diacrylates derived from cinnamic acid are designed, synthesized and mixed with liquid crystalline diacrylates with the aim of making films with alternating birefringent and isotropic domains by applying the E–Z isomerization process at room temperature. The effects of the structure of the isomerizable‐mesogenic group on the isotropization efficacy, the efficiency of the E–Z isomerization reaction, and film formation are discussed. Compounds derived from cyclohexyl cinnamate are proved to be good candidates that meet a whole set of parameters related to processing and application. These compounds exhibit a low nematic‐to‐isotropic transition temperature. In addition, they show no yellowing upon irradiation, unlike similar compounds derived from phenyl cinnamate. To elucidate the origin of isotropization of the film by irradiation, the pure Z‐isomer is prepared by photolysis of the E‐isomer and subsequent chromatographic separation of both isomers. Analysis of reference samples containing the pure isomers reveals that the decrease in transition temperature can be attributed exclusively to the E–Z photoisomerization process. Finally, thin films with alternating birefringent and isotropic parts of 100×100 µm² are obtained by using a combination of photoisomerization in air and photopolymerization in a nitrogen atmosphere, which is referred to as photo‐patterning.     

Optical activity of a side group cholesteric liquid crystalline silicone

We measured, using a phase modulation technique, the optical rotation in the isotropic and blue phases of a side group cholesteric liquid crystalline silicone polymer. For comparison, similar measurements were performed on cholesteryl nonanoate (CN) and CE6. The polymer has a selective reflection close to that of CE6 and a chiral mesogenic side group chemically similar to CN. The optical activity data for the polymer, unlike those for low molecular mass liquid crystals, does not follow a simple Landau-de Gennes temperature dependence.     

Broadband reflective cholesteric liquid crystalline gels: volume distribution of reflection properties and polymer network in relation with the geometry of the cell photopolymerization

The ultraviolet (UV) light‐absorbing properties of the liquid crystal (LC) constituent during the photo‐induced elaboration of a cholesteric LC (CLC) gel may induce the broadening of the reflection bandwidth of the material, a situation that is promoted by asymmetrical irradiation conditions (only one side of the cell is irradiated). The in situ structure of the polymer network, included in the LC, was investigated by transmission electron microscopy and the temperature dependence of the reflection properties examined; it is shown that the network has a structure gradient that is at the origin of the broadening phenomenon. The smallest reflection wavelength is related to the cell side from which the UV light beam came in. A priori, this situation was unexpected since it is shown that this part of the gel is enriched with nematic (infinite‐pitch CLC) network‐forming material. The result is discussed in relation to the variation of the reflection band characteristics with polymer concentration, which offers the opportunity for indirect access to the volume distribution of the cholesteric periodicities. For applications, broadband reflective cholesteric gels may be of interest for reflective polarizer‐free displays or for the light management with smart electrically‐switchable reflective windows.     

Broadband reflective cholesteric liquid crystalline gels: volume distribution of reflection properties and polymer network in relation with the geometry of the cell photopolymerization

The ultraviolet (UV) light-absorbing properties of the liquid crystal (LC) constituent during the photo-induced elaboration of a cholesteric LC (CLC) gel may induce the broadening of the reflection bandwidth of the material, a situation that is promoted by asymmetrical irradiation conditions (only one side of the cell is irradiated). The in situ structure of the polymer network, included in the LC, was investigated by transmission electron microscopy and the temperature dependence of the reflection properties examined; it is shown that the network has a structure gradient that is at the origin of the broadening phenomenon. The smallest reflection wavelength is related to the cell side from which the UV light beam came in. A priori, this situation was unexpected since it is shown that this part of the gel is enriched with nematic (infinite-pitch CLC) network-forming material. The result is discussed in relation to the variation of the reflection band characteristics with polymer concentration, which offers the opportunity for indirect access to the volume distribution of the cholesteric periodicities. For applications, broadband reflective cholesteric gels may be of interest for reflective polarizer-free displays or for the light management with smart electrically-switchable reflective windows.