Synthesis and characterization of side-chain cholesteric elastomers derived from an isosorbide crosslinking agent

New liquid crystalline monomer 4-(4-ethoxybenzoyloxy)biphenyl-4′-[(10-undecylen-1-yloxy)-4′-ethoxy]benzoate (M ₁ ), chiral crosslinking agent isosorbide di-(10-undecylen-1-yloxybenzoate) (M ₂ ), and the corresponding elastomers were prepared. The chemical structures of M ₁ and M ₂ were characterized by Fourier transform infrared and ¹H-nuclear magnetic resonance. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. M ₁ exhibited typical threaded texture and droplet texture of nematic phase. The use of chiral crosslinking agent in the polymer networks could induce cholesteric phase. The elastomers containing less than 10 mol% of the chiral crosslinking units showed elasticity, reversible phase transition, wide mesophase temperature ranges, and high thermal stability. For the elastomers P ₂ –P ₄ , the glass transition temperature (T _g) increased; clearing temperature (T _i) and mesophase temperature range (ΔT) decreased with increasing content of the crosslinking unit.     

Mesomorphic properties of side-chain cholesteric liquid-crystalline elastomers

New monomer cholesteryl 4-(10-undecylen-1-yloxybenzoyloxy)-4′-ethoxybenzoate (M₁), crosslinking agent biphenyl 4,4′-bis(10-undecylen-1-yloxybenzoyloxy-p-ethoxybenzoate) (M₂) and a series of side-chain cholesteric elastomers were prepared. The chemical structures of the monomers and elastomers obtained were confirmed by element analyses, FT-IR, and ¹H NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. M ₁ showed cholesteric phase, and M ₂ displayed nematic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.     

Synthesis and characterization of new cholesteric monomers and smectic polymers containing menthyl groups

New cholesteric monomers (M₂−M₅) and the corresponding smectic homopolymers (P₂−P₅) based on menthyl groups were synthesized. The chemical structures were characterized by Fourier transform infrared and ¹H NMR. The specific optical rotations were evaluated with a polarimeter. The structure–property relationships of the new compounds are discussed. The mesomorphism was investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The selective reflection property of light was studied with UV/Visible/NIR. The monomers M₂−M₅ formed the cholesteric or blue phase when a flexible link chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. M₁ showed no mesomorphism, while M₂−M₅ revealed enantiotropic cholesteric phase. In addition, M₂ and M₃ also showed a cubic blue phase on cooling. The selective reflection of light for M₂−M₅ shifted to the short reciprocal wavelength region with increasing the temperature or intramolecular spacer length. P₂−P₅ exhibited the smectic A phase. The melting, clearing, and glass transition temperatures increased when increasing the aryl number in the mesogenic core or decreasing the intramolecular spacer length.     

The effect of mesogenic and non-mesogenic crosslinking units on the phase behaviour of side-chain smectic and cholesteric elastomers

Chiral monomer (M₁ ), mesogenic and non-mesogenic crosslinking agents (C₁ and C₂ ), and the corresponding liquid crystalline elastomers (P₁ and P₂ series), have been synthesised. Their chemical structures have been characterised by Fourier transform infrared or ¹H nuclear magnetic resonance and their phase behaviour investigated by differential scanning calorimetry, polarising optical miscoscopy, thermo-gravimetric analysis (TGA) and X-ray diffraction. The effect of the crosslinking unit on the phase behaviour of the elastomers has been studied. M₁ showed a cholesteric oily streak and focal conic texture. C₂ exhibited a nematic enantiotropic thread-like and schlieren texture, and a monotropic fan-shaped texture in the S_A phase. Due to the introduction of the mesogenic crosslinking unit, elastomers, P_2-1 ?P_2-5 , exhibited a cholesteric phase, while elastomers, P_1-1 ?P_1-4 , derived from a non-mesogenic crosslinking unit, exhibit a S_A phase. As the content of the crosslinking unit increased, the T _g of the P₁ series initially decreased and then increased, and the T _i of the series decreased. In the P₂ series the T _g increased, but the T _i initially increased and then decreased. TGA confirmed that all the elastomers had improved thermal stability.     

Cyclosiloxane-based liquid-crystalline elastomers containing fluorinated mesogens and chiral crosslinking groups

A series of chiral fluorinated liquid-crystalline elastomers (LCEs) IP-VIP are prepared by 4-cyano-3-fluorophenyl 4′-(undec-10-enoyloxy)biphenyl-4-carboxylate, isosorbide bis(4-allyloxybenzoate) and 2,4,6,8-tetramethylcyclotetrasiloxane via Pt-catalyzed hydrosilylation. The chiral crosslinking moieties increase from IP to IVP. The elastomers IP, IIP, and IIIP containing low content of chiral crosslinking moieties display S_C ^* liquid-crystalline phase, but IVP, VP, and VIP do not show S_C ^* phase except for N^* mesophase. The mesophase is testified according to typical diffractogram measured by X-ray diffraction (XRD) analysis. The layer spacings of the LCEs decrease from IP (d-spacing of 34.2 Å) to VIP (d-spacing of 31.6 Å) with increase of chiral crosslinking groups in the polymers systems. Moreover, the optical properties are performed by ultraviolet–visible–near-infrared spectrophotometry. VP and VIP containing the most chiral crosslinking moieties display maximum reflection in near infrared spectra in the measurement of optical properties, while IP, IIP, IIIP, and IVP do not show obvious maximum reflection. It is interesting that a specific reflection of circularly polarized light appears along with a changing mesophase due to high enough concentration of chiral crosslinking dopants for VP and VIP. All these results suggest that the chiral crosslinking moieties exert influence on the structures of these kinds of LCEs.     

Photochemical tuning capability of cholesteric liquid crystal cells containing chiral dopants end capped with menthyl groups

In order to investigate the photochemical tuning capability of chiral monomers and polymers containing end-capped menthyl groups, a new series of chiral dopants was synthesized and added to commercially available nematic liquid crystals to induce cholesteric liquid crystal (LC) phases. The addition of chiral dopants with azo structure led to phototunability of the reflection colour of the LC cells. Photochromic variation of the LC cells due to photoisomerization of the azo compound was investigated. After photopolymerization of the monomers inside the cholesteric LC cells, the centre wavelength of the reflected band of the incident light was found to be fixed and the reflected bandwidth was broadened, resulting in a red shift. A schematic representation of both the photoisomerization of the azo dopants and its effect on variation of twisting pitches is proposed. Real image recording was performed using 365 nm UV through a mask with text. The top and side views of the morphological network structures of a fabricated cholesteric LC cell were investigated using scanning electron microscopy. The results of this investigation demonstrated that RGB reflected colours of LC cells can easily be achieved through the addition of the menthyl-containing synthesized chiral compounds to nematic LCs. The addition of synthesized AzoM helped further in recording the patterns onto cholesteric LC films using 365 nm UV exposure.     

Photochemical tuning capability of cholesteric liquid crystal cells containing chiral dopants end capped with menthyl groups

In order to investigate the photochemical tuning capability of chiral monomers and polymers containing end‐capped menthyl groups, a new series of chiral dopants was synthesized and added to commercially available nematic liquid crystals to induce cholesteric liquid crystal (LC) phases. The addition of chiral dopants with azo structure led to phototunability of the reflection colour of the LC cells. Photochromic variation of the LC cells due to photoisomerization of the azo compound was investigated. After photopolymerization of the monomers inside the cholesteric LC cells, the centre wavelength of the reflected band of the incident light was found to be fixed and the reflected bandwidth was broadened, resulting in a red shift. A schematic representation of both the photoisomerization of the azo dopants and its effect on variation of twisting pitches is proposed. Real image recording was performed using 365 nm UV through a mask with text. The top and side views of the morphological network structures of a fabricated cholesteric LC cell were investigated using scanning electron microscopy. The results of this investigation demonstrated that RGB reflected colours of LC cells can easily be achieved through the addition of the menthyl‐containing synthesized chiral compounds to nematic LCs. The addition of synthesized AzoM helped further in recording the patterns onto cholesteric LC films using 365 nm UV exposure.     

Comparative study of holographic recording in cholesteric and nematic azo-containing side-chain polymers

For the first time a comparative study of holographic recording in planarly oriented films of nematic and cholesteric azobenzene-containing polymers was performed. The influence of temperature and light intensity on the values of diffraction efficiencies of holographic gratings was investigated. The kinetics of grating relaxation at different temperatures was studied. It was shown that the helical supramolecular structure of cholesteric copolymer causes a significant decrease of the diffraction efficiency in comparison with the one observed for the nematic state of the homopolymer.     

Study on new chiral liquid crystalline monomers and polymers containing menthyl groups

A series of new chiral monomers (M1–M4) and the corresponding siloxane polymers (P1–P4) containing menthyl groups were synthesised to establish the relationship between their structure and liquid crystalline properties. The effect of the mesogenic core rigidity and the spacer length on the phase behaviour of the monomers and polymers obtained in this study was discussed. The selective reflection of light for the chiral monomers was studied with UV-Vis spectrometer. Polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and thermogravimetric analysis were used to characterise the phase behaviour and thermal stabilities. It was found that these chiral monomers and polymers were beneficial for the formation of the mesophases when a flexible spacer was inserted between the mesogenic core and terminal menthyl groups. M1–M3 showed enantiotropic chiral smectic C phase and cholesteric phase, and monotropic cubic blue phase on cooling cycle. M4 only showed cholesteric phase. P1–P4 showed a smectic A phase. With increasing the mesogenic core rigidity or decreasing the spacer length, the corresponding melting temperatures, glass transition temperatures and isotropic temperatures all increased.     

Radiation crosslinking of polymethylhydrogensiloxane containing methoxy groups

The bulk viscosity of polymethylhydrogensiloxane containing methoxy groups (PMHS-A) was measured before and after irradiation. The gel point was then determined from the variation of viscosity by irradiation by comparing it with methoxy-free polymethylhydrogensiloxane (PMHS-B) and polydimethylsiloxane blocked by methoxyldimethylsilyl (PDMS-C) or trimethylsilyl (PDMS-D). The following results were obtained: (1) The G value of crosslinking by irradiation is 167, 52, 25, and 3 for PMHS-A, PMHS-B, PDMS-C, and PDMS-D, respectively. (2) The scission of the SiO? CH₃ bond is easier than that of the Si? H, Si? CH₃, or SiCH₂? H bond. The SiO? CH₃ bond is PMHS-A is severed from SiO· and ·CH₃ to induce chain reactions at the initial stage by irradiation. (3) It was confirmed that the commercial PMHS is at present free of methoxy groups. To replace the hydrogen of Si? H in the commercial PMHS with methoxy groups heat treatment with methanol was performed by which the G value of crosslinking was increased.     

Synthesis and mesomorphism of new aliphatic polycarbonates containing side cholesteryl groups

New cholesterol side-functionalised polycarbonate polymers were synthesised by the ring-opening homo- and copolymerisation reaction of the cyclic monomer cholesteryl 5-methyl-2-oxo-1,3-dioxane-5-carboxylate and d,l-lactide using Sn(Oct)₂ as a catalyst. The chemical structures and average molecular weights of the cyclic monomer, homopolymer and block copolymers obtained in this study were characterised using FT-IR, ¹H NMR and gel permeation chromatographic measurement. The mesomorphism and mesophase structure were investigated with polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurement. As a result, the homopolymer and block copolymers showed an enantiotropic smectic A (SmA) phase. With the concentration of the lactide segment increasing, the glass transition temperature and isotropic temperature of the corresponding block copolymer all decreased. In addition, XRD suggested that the homopolymer and two block copolymers showed the SmA double-layer packing of side chains.     

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.     

On the reported mesomorphism of lanthanide complexes containing the hemicyanine structural unit

Close examination of the thermal behaviour of the (E)-N-hexadecyl-4-N', N'-dimethylaminostilbazolium tetrakis(1-phenyl-3-methyl-4-benzoyl-5-pyrazolonato)lanthanide(III) complexes has revealed that these compounds are not mesomorphic, in contradiction to an earlier report in the literature (K. Z. Wang et al., 1995, Solid State Commun. 95, 223). By replacing the bromide of the hemicyanine precursor (see the scheme) by a tetrakis(beta-diketonato)lanthanide(III) anion, new complexes were synthesized, but none shows a mesophase. Reasons for the absence of mesomorphism are discussed. Transition temperatures for the mesomorphic hemicyanine bromides are reported.     

Synthesis and characterisation of chiral liquid crystalline elastomers containing four-arm crosslinking agent

A new series of side-chain chiral liquid crystalline elastomers derived from M₁ (cholest-5-3-ol(3β)-4-(2-propen-yloxy)]benzoate) and M_C(2,5-[3,5-bis(4-(3-(4-(allyloxy)phenyl)propanoyloxy)benzoyloxy)benzoic acid]isosorbide diester). The structures of monomers and elastomers measured by using Proton Nuclear Magnetic Resonance Spectra (¹H-NMR) and Fourier transform infrared spectroscopy (FTIR) separately are consistent with our design. IIP~VIP all appeared blue Grandjean (GJ) texture on the heating cycle or cooling cycle. The glass sheets of IIP~VIP were made under 150°C and measured its ultraviolet–visible spectrophotometry by PerkinElmer Lambda 950 instrument (Shelton, CT, USA). IIP~VIP all have absorptions at about 481~483 and 561~562 nm. The optical activities were measured at different temperatures on heating and cooling cycles. And the blue selective reflection of IIP~VIP on the round glass sheet can be seen. The elastomers containing less than 6 mol% of the crosslinking units displayed elasticity, reversible phase transition and high thermal stability. The glass transition temperatures reduced first and then increased, the isotropisation temperatures and the mesophase temperature ranges increased first and then decreased with increasing content of crosslinking unit. The thermogravimetric analysis (TGA) results showed that the temperatures at which 5% weight loss occurred (T_d) were greater than 310°C for all the polymers.     

Synthesis and characterization of fluorinated liquid-crystalline elastomers containing chiral liquid-crystalline crosslinking units

Fluorinated chiral liquid-crystalline elastomers (LCEs) were graft copolymerized by a one-step hydrosilylation reaction with polymethylhydrogenosiloxane, a fluorinated LC monomer 4-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)phenyl 4-(undec-10-enoyloxy)benzoate (PPUB) and a chiral crosslinking LC monomer (3R,3aR,6S,6aR)-6-(undec-10-enoyloxy)hexahydrofuro[3,2-b]furan-3-yl 4′-(4-(allyloxy)benzoyloxy)biphenyl-4-carboxylate (UHAB). The chemical structure, liquid-crystalline behavior and polarization property were characterized by use of various experimental techniques. The effective crosslink density of the LCEs was characterized by swelling experiments. The thermal analysis results showed that the temperatures at which 5% weight loss occurred were greater than 250 °C for all the LCEs, and the residue weight nearby 600 °C increase with increasing chiral crosslinking components in the polymer systems. All the samples showed chiral smectic C mesophase when they were heated. The glass transition temperature and mesophase-isotropic phase transition temperature of fluorinated elastomers increased slightly with increase of chiral crosslinking mesogens in the polymer systems, but the enthalpy changes of mesophase-isotropic phase transition decreased slightly. In XRD curves, all the samples exhibited strong sharp reflections at small angles suggesting smectic layered packing arrangement. These fluorinated chiral LCEs showed 0.1–0.2 μC/cm² of spontaneous polarization with increasing chiral crosslinking component.     

Loading effect on swelling of nematic elastomers

Externally imposed loading has substantially different effects on the swelling of nematic elastomers in the high-temperature isotropic and low-temperature nematic states. In the isotropic state, the stretching drives a considerably large degree of further swelling, whereas the stretching-induced volume change in the nematic state is significantly suppressed. In the isotropic phase that favors the less anisotropic state, the further swelling occurs to reduce the shape anisotropy caused by the imposed elongation. In the nematic phase, no significant swelling is induced because further swelling decreases the nematic order enhanced by the applied stretching. These different loading effects in the isotropic and nematic states observed in the experiments are qualitatively described by a mean field theory.     

Ultrastructure studies of polystyrene-based side-chain liquid-crystalline copolymers containing charge transfer groups

A series of new side-chain liquid-crystalline copolymers has been prepared, and the thermal properties of the individual copolymers have been determined. These copolymers are derived from atactic polystyrene and contain both 4-methoxyazobenzene and 4-nitroazobenzene mesogens; these are linked through octyl spacers to the polystyrene backbone. All the copolymers exhibit a smectic phase that has been assigned smectic A on the basis of polarizing microscopy and x-ray diffraction studies. The glass transition temperatures of the polymers exhibit a linear dependence on composition, whereas the clearing temperatures and the associated entropies show significant deviations from such behavior. The smecticisotropic transition temperatures of the copolymers are higher than those of the composition-weighted averages for the corresponding homopolymers, whereas the entropies of transition are lower than expected. X-ray diffraction studies of fiber samples revealed that the director of the mesophase is oriented perpendicular to the fiber axis. The liquid-crystalline polystyrene containing 25 mol % nitro-substituted mesogen shows an unusual S_A-phase WAXS pattern. The copolymers were investigated further by ¹³C CP/MAS NMR spectroscopy, and the observed changes in the spectra are analyzed in terms of chemical composition and local electronic environment. The application of the interrupted decoupling technique revealed that the spacer contains a number of gauche defects. These observations lead us to suggest possible microstructural arrangements in the smectic phase. © 1993 John Wiley & Sons, Inc.     

Synthesis,structure and properties of new chiral liquid crystalline monomers and homopolysiloxanes containing menthyl groups

The synthesis is described of four new chiral liquid crystalline monomers (M₂–M₅ ) and their corresponding side‐chain homopolysiloxanes (P₂–P₅ ) containing menthyl groups. Chemical structures were characterised using FT‐IR or ¹H NMR spectra, and specific optical rotations were evaluated with a polarimeter. The phase behaviour and mesomorphic properties of the new compounds were investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy, UV/visible/NIR spectrocopy and X‐ray diffraction. The monomers and homopolymers with more aryl segments showed noticeably lower specific optical rotation value. The monomers M₂–M₅ formed a cholesteric or blue phase when a flexible spacer was inserted between the rigid mesogenic core and the terminal menthyl groups by reducing the steric effect. M₂–M₅ revealed enantiotropic cholesteric phase. Moreover, M₂ also exhibited a monotropic smectic A (SmA) phase, and M₄ also exhibited a cubic blue phase on cooling. The selective reflection of light shifted to the long wavelength region with increasing rigidity of the mesogenic core for M₂–M₅ . P₂–P₅ exhibited SmA phases, and the mesogenic moieties were ordered in smectic orientation with their centres of gravity in planes. Melting or glass transition temperature and the clearing temperature increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic core.