Cholesteric Mesophase Research Articles

Optical-activity anisotropy of ordered phases in cellulose acetates

The optical activity of anisotropic solutions of cellulose diacetate in nitromethane and dimethyl sulfoxide and of diand triacetate films with vitrified ordered structure has been studied. The systems under study are characterized by high specific optical rotation [α], suggesting formation of a cholesteric mesophase. The value of [α] is found to depend on the angle of rotation of the samples relative to the direction of the polarization vector of an incident light beam in the plane perpendicular to this beam (the anisotropy of optical activity). This dependence (indicatrix) shows an irregular pattern and, when plotted in Cartesian coordinates, can be described with a distorted sinusoid. The data on the resolution of indicatrices into harmonic constituents and isolation of contributions due to isotropic components and anisotropic components, each of which is determined by the structural element with the corresponding asymmetry, are analyzed.

Synthesis and characterization of non-symmetrical liquid crystals containing a cholesteryl ester moiety

In this paper, a series of chiral non-symmetrical liquid crystals (nBA-chol) consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core with different terminal alkyl chain has been synthesized and investigated for their liquid crystalline properties. Effects of numbers of methylene units in the terminal alkyl chain on the phase transition temperatures and on the temperature-dependent pitch lengths of the chiral liquid crystals have been studied. The long terminal alkyl chain tends to exhibit smectic A mesophases. The pitch lengths of the cholesteric mesophases of the chiral liquid crystals decrease with increasing temperature and with increasing numbers of methylene units in the terminal alkyl chain, respectively.

Molecular Structure and Liquid-Crystalline Characteristics of Chitosan Phenylcarbamate

Chitosan phenylcarbamate (CtsPC) samples were synthesized to have different degrees of substitution (DS) ranging from approximately 2.7 to approximately 3.7, and the lyotropic liquid crystallinity was mainly characterized by spectrophotometry. The products of DS>2.8 formed a cholesteric type of mesophase in concentrated solutions of >44 wt % with polar aprotic solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO), and some of the solutions imparted vivid colorations because of selective visible light reflection. The cholesteric helical pitch increased with increasing temperature and with decreasing polymer concentration, but the helical sense remained left-handed under the adopted measurement conditions. It was also found that the helical pitch increased in proportion to the increment of DS, whereas an average degree of phenylcarbamoyl polyaddition in the side chains (DPs) rather affected the pitch distribution measuring an orderliness in the cholesteric structure. With a small increase in DPs (e.g., from 1.01 to 1.04), the cholesteric orderliness decreased and the temperature sensitivity of the pitch turned sluggish. Wide-angle X-ray diffractometry was also used for evaluating the distance and azimuth difference between adjacent nematic thin layers in the cholesteric mesophase.

Synthesis and characterization of side‐chain cholesteric liquid‐crystalline polysiloxanes containing different space groups

AbstractA series of liquid‐crystalline (LC) polysiloxanes were synthesized by two different cholesteric monomers, cholest‐5‐en‐3‐ol(3β)‐10‐undecenoate and cholesteryloxycarbonylmethyl 4‐allyloxybenzoate. The chemical structures and LC properties of the monomers and polymers were characterized by various experimental techniques, including Fourier transform infrared spectroscopy, 1H‐NMR, elemental analysis, differential scanning calorimetry, and polarized optical microscopy. The specific rotation absolute values increased with increasing rigid spacers between the main chain and the mesogens. All of the polymers exhibited thermotropic LC properties and revealed cholesteric phases with very wide mesophase temperature ranges. With a reduction in the soft‐space groups in the series of polymers, the glass‐transition temperature and the isotropic temperature increased slightly on heating cycles. Reflection spectra of the cholesteric mesophase of the series of polymers showed that the reflected wavelength shifted to short wavelengths with decreasing soft‐space groups in the polymers systems, which suggested that the helical pitch became shorter with increasing rigid‐space groups. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Mesogenic amido ester with cholesteryl moiety

A new series of mesogenic compounds having a cholesteryl moiety has been synthesized. Thirteen homologues were synthesized by condensing 4-(4′-n-alkoxy benzoyl) amido benzoyl chloride with cholesterol. The methyl to n-hexyl derivatives exhibit only cholesteric mesophases. The n-heptyl to n-dodecyl derivatives exhibit SC and cholesteric phases. The n-tetradecyl to n-octadecyl derivatives exhibit SC, SA and cholesteric phases. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exibit odd–even effect and falling tendency for cholesteric–isotropic transition temperatures. The mesomorphic and thermal stability is compared with other related homologous series. The results indicate that amide linkage enhances smectic as well as cholesteric thermal stability, but enhancement in smectic phase is much higher.

Synthesis and characterization of chiral liquid‐crystalline polysiloxanes containing fluorinated units and sulfonic acid groups

AbstractChiral side‐chain liquid‐crystalline polysiloxanes (PS‐1, PS‐2, PS‐3, PS‐4, PS‐5, PS‐6) bearing fluorinated units and sulfonic acid groups were synthesized with poly(methylhydrogeno)siloxane, cholest‐5‐en‐3‐ol(3β)‐4‐(2‐propenyloxy)benzoate, and 3‐trifluoromethyl‐phenyl 3‐sulfo‐4‐undec‐10‐ enoyloxy‐benzoate. The effects of fluorinated units and sulfonic acid groups on characteristic of liquid‐crystalline properties were studied. PS‐1, PS‐2, and PS‐3 exhibited both smectic and cholesteric mesophases, while PS‐4, PS‐5, and PS‐6 exhibited only cholesteric mesophase. As the polymers contained more fluorinated units and sulfonic acid groups, segregation of the fluorinated segment to the surface and aggregation of hydrogen bonding should occur. Therefore, the highly ordered lamellar mesogen–siloxane matrix systems should be disturbed severely, suggesting that PS‐4, PS‐5, and PS‐6 show no smectic phase. The maximum reflection bands become broad and shifted slightly to long wavelength from PS‐1 to PS‐6. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of core group substituents on the monomer mesophase, photocuring, and film viscoelastic properties of mesogenic diacrylates

AbstractIn this paper, the influence of molecular structure variations on the mesophase behavior, photocurability, and film properties of polymerizable mesogenic diacrylates is reported. The effect of methoxy or chloro substituent on the mesophase behavior of the monomers, present at the 2‐position of the phenylene core, in comparison to the unsubstituted core, has been studied using thermal and photo‐DSC techniques. These monomers were used in combination with a chiral dopant to form cholesteric mesophases, which was subsequently crosslinked through photopolymerization. Optical microscopy and polarized optical microscopy (POM) were used to study the morphology of monomers as well as the photopolymerized films. Optical images show the influence of chiral dopant on the texture present in the films, both before and after the crosslinking. POM images show the mesophase present in the monomers and the free films. Evaluation of viscoelastic properties of the photocured films demonstrates the influence of mesogenic core substituents on the network transition temperature, modulus, and damping characteristics. Copyright © 2008 John Wiley & Sons, Ltd.

Cholesteric Liquid-Crystalline Thermosets Derived from Side-Chain Liquid-Crystalline Epoxy Oligomers

A series of side-chain liquid crystalline (LC) oligomers bearing glycidyl ether groups were synthesized with cholest-5-en-3-ol(3 β)-4-(2-propenyloxy) benzoate, octyl 4-(4-allyloxy-benzoyloxy)-benzoate and epoxy monomer allyl glycidyl ether. The chemical structures and LC properties of the monomers and oligomers were characterized by use of various experimental techniques. All the chiral LC oligomers showed cholesteric mesophase with very wide mesophase temperature ranges. All the synthesized LC oligomers were cured with 4,4'-diaminodiphenylmethane at the same curing conditions. The thermal behaviors, mesomorphic phase and optical properties of the thermosets were characterized. The synthesized thermosets have high thermal stability. The cholesteric phases of LC oligomers were frozen in the polymer networks. Compared with those of corresponding oligomers, the reflection located at wide angles of thermosets became more diffuse, and the peak intensity decreased in X-ray diffraction analysis. The selective light reflection maximum λmax of the thermosets at room temperature shifted slightly to short wavelengths as the LC monomer bearing cholesteryl component was increased, indicating that the helical pitch P becomes shorter due to more chiral groups in the polymer network systems. The temperature dependences of both reflection wavelength and optical rotation under the influence of the crosslinking reaction were also investigated.

Chiral Smectic *C Mesogens Having Ester and Amide Central Linkages

Ten members of a chiral homologous series 4-(4′-n-alkoxy benzoyloxy benzoyl)4″- S- (+) 2-methyl butoxy anilides were synthesized in multiple step synthesis. Series exhibits chiral smectic *C and cholesteric mesophases. In this series, n-propyl to n-dodecyl derivatives exhibit smectic *C and cholesteric mesophases. n-Tetradecyl and n-hexadecyl derivatives exhibit only smectic *C phases. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits odd–even effect for cholestric-isotropic transition temperatures. The effect of amide linkage and chirality is discussed in detail. High-temperature cholestric phases exhibit vivid colors. Two compounds were analyzed by calorimetric study (C12 and C14) to confirm the transition temperatures of which was observed under polarizing microscope. The mesogenic behavior of the series compared with other series where amide central linkage differs in its position. The mesogenic thermal stability is compared with other related homologous series. The result indicates that amide linkage enhances smectic as well as cholesteric thermal stability but enhancement in smectic phase is much higher. Compounds are characterized by elemental analysis, IR spectral data, Mass spectral data, DSC, NMR, and microscopic study.

Photo-optical properties of photopolymerizable cholesteric compositions

Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the long-wavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated.

Electron microscopic studies on planar texture and disclination of cholesteric mesophases in acyloyl chitosan/acrylic acid composite films

A novel amphiphilic chitosan derivative, acryloyl chitosan (AcCs), was synthesized by a homogeneous reaction of chitosan and acryloyl chloride using methanesulfonic acid as solvent and catalyst. The concentrated solutions of AcCs/acrylic acid were investigated by polarized optical microscopy (POM) and found demonstrate cholesteric mesophase above the critical concentration of 45 wt%. AcCs/polyacrylic acid (PAA) composite films were prepared via the photopolymerization of 52 wt% AcCs/acrylic acid solution, and subsequently the permanganic etching technique was employed to expose the textures and defects inside the composite films. The Grandjean planar textures were observed by POM, but a series of dark and bright striation lines, which were attributed to the periodic structure of the cholesteric helix, were detected by scanning electron microscopy (SEM). The disclination pairs, τ −λ + and τ +λ −, were found, however λ +λ − and τ +τ − were absent. Effect of etching time on morphology of AcCs/PAA composite films was also investigated, comparing with 30 wt% free-acetyl chitosan (CS)/PAA composite films. The results showed that AcCs/PAA composite films had better anticorrosion ability than (CS)/PAA composite films, due to the crosslinking network between AcCs and PAA. FTIR confirmed that double bonds of AcCs involved in photo-solidification. Therefore the system studied takes the advantage of easy controlling for permanganic etching.

Preparation and cholesteric mesophase properties of (Butyl-co-pentyl) propylcellulose

Butyl and pentyl ether derivatives of (2-hydroxypropyl) cellulose (HPC) and butyl/pentyl mixed ethers of HPC (BPPC) with different alkyl compositions were prepared in nonaqueous solution and their thermotropic cholesteric properties examined. The temperature dependence and the composition dependence of the optical pitch, nP, were then determined for all of the ether derivatives. The molecular conformation and chirality of BPPC appeared to be to be variably smooth with the side chain composition of the polymer. The response rate of cholesteric configurational change of the thermotropic mesophases arising from a temperature jump from 45 to 75 °C, was also determined. The transformation of the cholesteric mesophase formed by an equimolar ether derivative (BP-50) was faster than that of the cholesteric one formed by a single-alkyl (pentyl) ether derivative (BP-0). This seemed to be a general result reflecting a pseudo-copolymer effect of cellulose derivatives on the properties of their thermotropic cholesteric mesophases.

Side-chain liquid-crystalline polysiloxanes containing ionic mesogens and cholesterol ester groups

A series of ionic liquid-crystalline polysiloxanes were synthesized in a one-step reaction by using cholest-5-en-3-ol(3β)-10-undecenoate and potassium 2-allyloxy-5-cholesteryloxycarbonyl-benzenesulfonate, which contain potassium ion between 0 and 1.05 wt%. The content of potassium ion was characterized by spectrophotometric analysis using sodium tetraphenylboron as titrant. All the polymers showed thermotropic liquid crystal properties. All the samples showed cholesteric mesophase, while the sample with more ionic mesogens displayed smectic mesophase and cholesteric mesophase. With the increase of ionic groups in the polymers, the temperatures of glass transition and clear point change a little, and the enthalpy changes of melt increase with increase of ionic content of the polymers. Reflection spectra of cholesteric mesophase of the series of polymers showed that the reflected wavelength becomes broad and shifts to long wavelength with increase of the ionic mesogens in the polymer systems, suggesting that helical pitch P become long.

2H-NMR of the Induced Chiral Phases of Acrylate-Metacrylate Liquid Crystalline Copolymers

ABSTRACT It is shown that the addition of 2 to 6 wt.% of a chiral dopant (CD) to a series of nematic copolymers (Pn) induces blue and cholesteric mesophases. The mesomorphic behaviour of various CD+Pn mixtures is investigated by polarized optical microscopy and the phase transition temperatures are determined by DSC measurements. Deuterium NMR spectra are recorded as a function of temperature within the mesophases and the spectral line shapes characteristic of different local orientational order parameters and orientational ordering processes of molecules in a magnetic field are observed depending on temperature, heating or cooling process, chiral dopant concentration, copolymer composition, and magnetic field strength.

Cholesteric Liquid-Crystalline Elastomers Prepared from Ionic Bis–Olefinic Crosslinking Units

A series of cyclosiloxane-based cholesteric LCEs were synthesized by using a LC monomer cholest-5-en-3-ol(3β)-4-(2-propenyloxy)benzoate and an ionic divinyl monomer 2,2'-(1,2-ethenediyl)-bis[5-[(4-undecenoyl-oxy)phenyl]-azo]-benzenesulfonic acid. The polymers were prepared in a one-step reaction with ionic crosslinking contents ranging between mass content 0 and 11.8%. Their chemical structures and liquid-crystalline properties were characterized by FT IR, 'H NMR, DSC, POM and X-ray measurement. The effective crosslink density (M c ) was determined by swelling experiments in mixed buffer/organic solvent mixtures, using Brannon-Peppas models. All the polymers exhibit thermotropic LC properties and reveal cholesteric phase. With increase of ionic crosslinking component in the polymers, the melting behavior disappears and the temperature of clear point decreases. Reflection spectra of cholesteric mesophase of the series of polymers showed that the reflected wavelength becomes broad and shifts to long wavelength with increase of the ionic crosslinking component in the polymer systems.

Effect of nonmesogenic crosslinking units on the mesogenic properties of side‐chain cholesteric liquid crystalline elastomers

AbstractA series of cyclosiloxane‐based cholesteric liquid crystalline elastomers were synthesized by using cholest‐5‐en‐3‐ol(3β)‐4‐(2‐propenyloxy)benzoate and a soft nonmesogenic crosslinking agent, acryloyl‐hexyl acrylate. The polymers were prepared in a one‐step reaction with the crosslinking contents ranging between 0 and 5.6 weight %. The effective crosslink density (Mc) was determined by swelling experiments employing Flory–Rehner models. All the polymers exhibited thermotropic LC properties and revealed a cholesteric phase. With increase of the crosslinking component in the polymers, the melting behavior disappeared and the enthalpy of transition decreased. But the temperature of glass transition and clear point changed little and did not show uptrend or downtrend. Reflection spectra of the cholesteric mesophase of the polymers showed that the reflected wavelength became broad and shifted to long wavelength with increase of the soft crosslinking component in the polymer systems. All these results originate from the effect of the soft nonmesogenic chemical crosslinking. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 625–631, 2005

Molecular conformation effects on mesomorphism and twisting ability of chiral cyclohexanones in mesophases

The molecular conformations of 2,6-bis-(4-phenyl)benzylidene-3R-methyl-cyclohexanone and its mono-(4-phenyl)benzylidene methylcyclohexanone isomers were investigated by molecular simulation using the semiempirical AM1 and PM3 methods and by analyzing the experimental spin-spin coupling constants in the PMR spectra. Mesomorphism and the twisting ability of the induced cholesteric mesophases of the title compounds are analyzed using the ratios between conformers with the axial and equatorial methyl groups and other peculiarities of the spatial structure of molecules (differences in anisometricity and in the degree of flattening of the cinnamoyl fragment). The equatorial orientation of the methyl group in the dominant conformations generally favors the formation of mesophases. The twisting ability is higher for chiral compounds with an axial methyl substituent and with the chiral center directly bonded to the enone and arylidene groups.

Synthesis and mesomorphic behaviour of novel chiral nematic side chain liquid crystalline polysiloxanes

A series of novel thermotropic side chain liquid crystalline polymers was synthesized by grafting copolymerization of a mesogenic monomer, 4-allyloxybenzoyl-4′ -(4-n-alkylbenzoyl)–p-benzenediol bisate and a chiral monomer, menthyl undecylenate. The mesogenic monomers exhibited nematic threadlike textures during heating and cooling. The polymers showed thermotropic liquid crystalline properties with a broad mesomorphic region over a range of 100°C. The polymers exhibited a cholesteric mesophase with a colourful Grand-Jean texture when the content of chiral units was greater than 15 mol %; the others exhibited nematic threadlike textures. All of the polymers were thermally stable over 300°C, and most were laevorotatory as the chiral monomer.

Liquid-crystalline elastomers produced by chemical crosslinking agents containing sulfonic acid groups

A series of siloxane-based liquid-crystalline (LC) elastomers were synthesized by using chemical crosslinking agents containing sulfonic acid groups. The crosslink densities of the elastomers were determined by swelling experiments. Two kinds of melting temperature corresponding to crystallization of siloxane matrix and crystallization of LC segments were clearly detected. Some of the polymers exhibited smectic mesophase textures or cholesteric mesophase textures. A proposed model containing LC segment structure and ionic crosslinking lamellar structure separated by siloxane chains was given. The ion aggregated in domains forces the siloxane chains to fold and form an irregular lamellar structure. Ionic aggregates and LC segments may be dispersed each other to form multiple blocks with increasing ionic crosslinking content.

Electrooptical Behavior of Liquid-Crystalline (Hydroxypropyl)cellulose/Inorganic Salt Aqueous Solutions

The selective light reflection and turbidimetric property of concentrated aqueous solutions of (hydroxypropyl)cellulose (HPC), originating from their cholesteric mesophase structure and LCST-type phase behavior, respectively, are much affected by coexistence of inorganic salts therein. This is ascribed to a strong dependence of both cholesteric helical pitch (P) and cloud point (Tc) of the aqueous cellulosic solutions on the sort and concentration of the additive salts. By virtue of electrophoretic motility of the coexistent salt ions as P and/or Tc shifter, the dynamic variation in cholesteric coloration and/or optical turbidity of the HPC solutions was realized under the action of a relatively weak electromotive force to electrodes in contact with their layered sample. It was reasonably taken that the electrical stimulation allowed the ion particles to migrate and localize in this lyotropic system, resulting in occurrence of a certain degree of gradient in magnitude of P and/or Tc in response to the uneven ionic distribution. To make clearer the mechanism of the novel electrooptical function, discussion took into consideration a storage effect of electric charge in the solution samples, detectable as a remaining potential difference after cessation of the application of the electric field.