Side chain liquid crystalline composites,occurrence of interdigitated bilayer smectic C phases

We report on the results of X-ray investigations in two series of polymer monomer composites, PM6Rm-33 and PMnR12-33, which consist of mixtures of achiral liquid crystalline side chain polymers and their monomers. These mixtures present a unique integration of monomer in the polymeric base which assists in modifying their properties and forming homogenous composites. X-ray measurements for all the investigated composites indicate the existence of bilayered smectic C phases (SmC₂). In several composites, the interlayer distance of the SmC₂ phase abnormally increases with cooling; this is associated with the aliphatic interdigitation at the tail-to-tail interface being more prominent when longer aliphatic tails are present.     

Synthesis and characterization of liquid crystalline polyacrylates and non-liquid crystalline polypyrroles from bifunctional monomers

Two kinds of polymerizations of liquid-crystalline N-{{W-{4-[4-(11-acryloyloxy)undecanoxybenzoyl]biphenyleneoxy}alkyl}}pyrrole gave a side-chain liquid-crystalline polyacrylate containing pyrrole group and a non-liquid crystalline polypyrrole containing acrylate group. Liquid crystallinity was determined by DSC and optical polarizing microscope measurements. The monomers having hexyl or decyl group as the alkyl group exhibited an undefined smectic, smectic B, and smectic A phases on the heating stage. The radically polymerized polyacrylate derivatives containing the pyrrole group showed smectic A, smectic C, and undefined smectic phases on the cooling stage. On the other hand, soluble N-substituted polypyrrole derivatives containing the terminal acrylate group which were prepared by chemical oxidative polymerization by ferric chloride catalyst did not show liquid crystallinity. Structure analysis of the polypyrroles performed by ¹H and ¹³C-NMR and FT-IR spectroscopies demonstrated that the polymerization occurred at the pyrrole ring. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3877–3887, 1999     

Influencing factors on liquid crystalline properties of cholesterol side-chain liquid crystalline polymers without spacer: molecular weight and copolymerisation

ABSTRACT

It is a challenge to tailor the phase behavior and phase structure of side-chain liquid crystalline polymers carrying targeted ordered structures and functional properties. In this work, liquid crystalline (LC) properties of cholesterol side-chain polymers without spacer were controlled by molecular weight (M_n) and copolymerization. On the one hand, two series of homopolymers without the spacer, poly (methacrylic acid) cholesterol esters (PCholMC_n) and poly (acrylic acid) cholesterol esters (PCholAC_n) with different M_n and low polydispersity, were achieved by reversible addition-fragmentation chain transfer polymerization. The experiment results indicated that the M_n had little effect on the LC properties of PCholMC_n and all homopolymers formed the smectic A phase. However, the phase structures of PCholAC were found to be strongly M_n dependent. The polymers PCholAC_n were amorphous when the M_n was lower than a critical value of approximately 1210₃ g/mol. But when the M_n exceeded the critical value, the polymers exhibited smectic A phase. On the other hand, two kinds of random copolymers, poly(cholest-5-en-3-methacrylate)-co-polymethyl acrylate (PCholMC-co-MA) and poly(cholest-5-en-3-acrylate)-co-polymethyl acrylate (PCholAC-co-MA) were synthesized with various composition. The findings suggested that the steric effect of main-chain and the interaction of mesogens would promote the formation of LC phase.     

Synthesis and thermal properties of side-chain liquid crystalline polyethers with racemic and chiral backbone

Racemic and chiral [(4-cyano-4′-biphenyl)oxy] and [(4-methoxy-4′-biphenyl)oxy]methyloxiranes were prepared from racemic epichlorohydrin or racemic and chiral glycidols and polymerized in dimethylsulfoxide (DMSO) with Bu^tOK as the initiator system. Initial phase identifications were made by differential scanning calorimetry (DSC) and optical microscopy techniques and confirmed by X-ray diffraction measurements. Upon heating, all the monomers show only a crystal–isotropic phase transition. The racemic and chiral [(4-cyano-4′-biphenyl)oxy]methyloxiranes exhibit a nematic and a cholesteric monotropic phase, respectively. Methoxybiphenyl substituted polyethers are crystalline and insoluble in virtually all common solvents. Cyanobiphenyl substituted polyethers are soluble under the same experimental conditions and show enantiotropic liquid crystalline properties. The racemic polymer exhibits a nematic phase, while the optically active polymer forms a cholesteric phase.     

Synthesis and mesomorphic behavior of a new series of side-chain liquid crystalline polymethacrylates bearing 2,6-naphthylene-based mesogen

A new homologous series of liquid crystalline (LC) polymethacrylates, the poly[ω-(2-phenoxycarbonyl-naphthalene-6-yloxy)alkyl methacrylate]s, in which the length of the alkyl spacer is varied between 3 and 12 methylene units, have been synthesized by free-radical polymerization. The prepared polymers were studied by IR and ¹H-NMR spectroscopy, viscosity measurements, differential scanning calorimetry (DSC), and polarized microscopy. Polymers with 3 or 4 methylene units spacer were glassy in nature, whereas those with 5–12 methylene units spacer exhibited smectic behavior. The glass transition temperatures decrease on increasing spacer length. The isotropization temperatures and the corresponding transition entropies showed an odd–even effect, with the odd members exhibiting the higher values. This effect attenuates on increasing spacer length. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2391–2399, 1999     

Monotropic smectic A to double layer smectic C transition of biphenyl containing liquid crystal acetylene

5-[（4’-Heptoxy-4-biphenylyl）carbonyloxy]-1-pentyne（A-3,7） was synthesized and the phase structures and transitions were investigated by differential scanning calorimetry（DSC）,wide angle X-ray diffraction（WAXD）,polarized light microscopy（PLM） and the molecular packing in the crystal and liquid crystalline phases were simulated by molecular dynamic simulation.The results showed that the sample formed thermodynamically metastable SmA and SmC₂ phases before crystallized during cooling and the crystal phase directly transformed into isotropic phase during heating.     

Synthesis and properties of new chiral mesogenic monomers and side-chain smectic homopolymers containing (−)-menthyl groups

The synthesis of new chiral monomers (M₁−M₅) and the corresponding smectic homopolymers (P₁−P₅) containing menthyl groups is described. The chemical structures and purity were characterized by FT-IR, ¹H NMR and elemental analyses. The specific optical rotations were evaluated with a polarimeter. The phase behavior and mesomorphism were 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 a chiral smectic C , and cholesteric or blue phase when a flexible linkage chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. M₁ showed no mesomorphism, while M₂−M₅ showed enantiotropic and cholesteric phases. Moreover, M₅ also exhibited a cubic blue phase on cooling. With increasing temperature, the selective reflection of light shifted to the long wavelength region at the phase range, and to the short wavelength region at the cholesteric phase range, respectively. The homopolymers P₁−P₅ all exhibited the batonnet textures of a smectic A phase. The melting, clearing, and glass transition temperatures increased, and the mesophase temperature ranges widened with increasing the aryl number in the mesogenic core.     

New hydrocarbon side chain liquid crystalline polysiloxane polymers: Synthesis,characterization, and thermotropic behavior

A series of new and high-purity hydrocarbon liquid crystal monomers were synthesized through the acylation reaction, deoxygenation reaction, and Grignard reaction. ¹H-NMR spectra and elemental analyses were used to examine their purity. The liquid crystalline polysiloxane polymers were obtained by grafting the monomers onto poly(methylhydrosiloxane). The thermal transition temperature, mesomorphic properties, and mesophase textures of the monomers and the polymers were determined by differential scanning calorimetry (dsc), polarized optical microscopy, and X-ray diffraction analysis. Moreover, we observed the even–odd effect of the smectic/isotropic transition temperature with the length variation of the substituents. In this study, we found by X-ray diffraction that the liquid crystalline polysiloxane polymers undergo a transition from smectic B to smectic E mesophase. However, dsc has difficulty detecting the phase transition process. By considering the spin–lattice relaxation time (T₁), we can systematically explain the relation between the flexibility of the substituent with the smectic/isotropic transition temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2849–2863, 1998     

The effect of ethanol on the phase behaviour and micro-rheology of liquid crystals

The effect of ethanol on the phase behaviour and micro-rheology of lyotropic liquid crystals (LC) has been studied using a binary mixture of monoglyceride (MG) and aqueous ethanol. The phase behaviour study reveals the structural modulation of surfactant aggregates with increasing ethanol concentration, namely a bicontinuous cubic phase (Ia3d) transitions to the lamellar phase (L_α), at a fixed MG concentration. This behaviour is explained by considering the critical packing parameter (CPP) of the surfactant molecule. Because ethanol dehydrates the surfactant head group (a _s), the CPP values increase (decreasing a _s) and thus the formation of larger CPP aggregates is favoured (i.e., the Ia3d–L_α transition occurs). Cross-polarised images and X-ray scattering data support this conclusion. The structural modulation of the LC has further been investigated using a diffusing wave spectroscopy technique. The correlation and relaxation times, determined from the intersection point at short and long time scales of the mean square displacement (MSD), decrease with increasing concentrations of ethanol, indicating structural modulation of the LC. The micro-viscoelastic moduli (G′ and G′′) derived from the Laplace transformation of the MSD decrease with increasing ethanol concentrations, due to the LC modulation. The thermal effects on the micro-rheology of the LC have also been studied.     

Optical and thermal properties of unsymmetrical liquid crystalline compounds based on isoxazole

The thermal and optical properties of two series of unsymmetrical liquid crystalline compounds based on an isoxazole ring are described. Of these materials, 3–7 and 8–12 displayed strong absorption at 285 nm and a weak blue fluorescence in solution at around 377 nm, even with the presence of a high conjugated core. The fluorescence quantum yields observed varied from low to moderate (Φ _F ?=?1–62%), with large Stokes shifts (60–178 nm). All compounds exhibited liquid crystalline behaviour with a wide mesomorphism temperature range and characteristic phases of calamitic compounds, among them smectic A (SmA), smectic C (SmC) and nematic (N) phases. Their phases were characterised using polarising optical microscopy and differential scanning calorimetry. In addition, with X-ray diffraction experiments, layer spacing of 33.3–40.0 Å were observed for the smectic phases.     

Inversion of the phase sequence between the cubic and smectic C phases under pressure

The phase behaviour of a thermotropic cubic mesogen of 1,2-bis(4′-n-tetradecyloxybenzoyl)hydrazine BABH-14 was studied under hydrostatic pressure using a polarising optical microscope equipped with a high-pressure optical cell, and the P–T phase diagram was constructed. BABH-14 shows the Cr–Cub–I transition sequence under atmospheric and lower pressures, but the Cub phase is replaced completely by the high-pressure SmC, SmC(hp), phase under higher pressures. There is a narrow intermediate-pressure region between the low- and high-pressure regions, in which the Cr–SmC(hp)–Cub–I phase sequence is recognised. The SmC(hp)–Cub transition line has a positive slope with pressure and there are two triple points: one is for the Cr, Cub and SmC(hp) phases and the other is for the I, Cub and SmC(hp) phases. Comparing the phase sequence of BABH-14 with those for BABH-8 and -10, the pressure-induced inversion of the phase sequence between the cubic and SmC phases occurs in the BABH-n homologous compounds. Another new phenomenon is the formation of the monotropic cubic phase on cooling in the intermediate- and high-pressure regions, and an intriguing phenomenon of the cubic phase appearing twice, i.e. I–Cub–SmC(hp)– Cub–Cr phase transition, occurs in the intermediate-pressure region.     

From smectic to columnar phase of polypedal liquid crystals based on tetrathiafulvalene/1,3-dithiol-2-thione and cholesterol

Bipedal 1,3-dithiole-2-thiones attached two cholesteryl through a ω-thioalkanoyloxy spacer of varying length were synthesized. The bipedals were easily transformed to the appropriate tetrapedal tetrathiafulvalene derivatives by self-coupling reaction in net triethyl phosphite. All the synthesized compounds exhibit mesogenic phases in a wide temperature region, no crystallization but vitrifying to form glassy mesogens during cooling from the isotropic melt. The liquid crystals with shorter spacer (n=2-6) exhibited only a smectic A phase and those with the longest spacer (n=7) exhibited only a hexagonal columnar. In these series, the molecular packing depended on the length of spacers.     

Substrate-induced phases: transition from a liquid-crystalline to a plastic crystalline phase via nucleation initiated by the substrate

The presence of substrate-induced phases in thin films is an intriguing phenomenon with the physical and chemical factors responsible for their formation are not yet clearly understood. In this article, we present the structure and morphological changes associated with a substrate-induced phase in a discotic liquid crystal. The thin films of the discotic molecule are characterised by a combination of various X-ray diffraction methods to investigate the structural properties. Atomic force microscopy and polarised optical microscopy are used to determine the thin film morphologies. This is the first experimental proof of the presence of a substrate-induced phase in discotic liquid crystal, although they are known in thin films of molecular crystals. Moreover, we sought to decipher how the substrate-induced phase behaves with the evolution of time, temperature and changes at the interfaces. This work gives a unique example where the two-dimensional liquid-crystalline phase converts to a three-dimensional crystal plastic phase because of nucleation caused by the solid substrate over a time scale of a month or longer.     

Study of the hydrogen bonding of 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene in crystalline and liquid crystalline phases using infrared,Raman, and two-dimensional correlation spectroscopy

The vibrational bands of a dihydrazide derivative, 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene (TC6), observed in the Raman and infrared spectra were assigned. The intermolecular hydrogen bonding vibrational bands due to CO and NH groups in the low-frequency Raman spectra were observed at 111 and 94 cm⁻¹ in the crystalline and liquid crystalline (LC) phases, respectively. The sequential order of changes in the hydrogen bonding and alkyl chains was opposite in the crystalline and LC phases. The modifications in the hydrogen bonding occurred prior to conformational changes in the hydrocarbon chains in the crystalline phase; however, a reverse trend was observed in the LC phase. Simultaneously, the two-dimensional (2D) IR and Raman correlation spectroscopic analysis showed that the amide I band of TC6 in the LC phase comprised at least five distinct bands. In addition, the hetero 2D correlation between the NH and CO groups confirmed that no free NH and CO groups existed in the LC phase.     

Comparative structural analysis of a histone-like protein from Spiroplasma melliferum in the crystalline state and in solution

A solution of a histone-like protein from Spiroplasma melliferum (HUSpm) was examined by small-angle X-ray scattering (SAXS). The experimental SAXS curve was compared with those calculated for the HUSpm structures from the PDB databank obtained by both X-ray diffraction analysis and nuclear magnetic resonance spectroscopy. The model of the HUSpm structure in solution, which best agrees with the experimental SAXS data, has a shorter distance between the centers of mass of the HUSpm monomers compared to the crystal structure, indicating that the HUSpm monomers can be located closer to each other in solution than in the crystalline state.     

The effect of CoPt-coated reduced-graphene oxide nanosheets upon the Smectic-A to Smectic-C* phase transition of a chiral liquid crystal

ABSTRACT

We report on high-resolution calorimetry and small-angle X-ray scattering measurements along the Smectic-A to chiral Smectic-C* phase transition of the liquid crystal 4-(2-methyl butyl) phenyl 4-n-octylbiphenyl-4-carboxylate with dispersed, CoPt nanoparticle-coated reduced-graphene oxide nanosheets. The temperature dependence of heat capacity and smectic layer spacing are obtained in the vicinity of Smectic-A to chiral Smectic-C* phase transition. Though no remarkable pretransitional effects are present, the critical fits show a crossover from mean-field near a tricritical point to classical mean-field compared to pure liquid crystal. The X-ray data yield a dilation of smectic layer thickness, indicating the assembly of graphene oxide nanosheets between the smectic layers.     

Polymorphism of racemic 1,2-diols with aliphatic chains: isostructural series of phases in dry and hydrated samples

The different phases occurring in homologous dried n-alkane-1,2-diols (1,2-diols) and n-alkane-1,2,1′,2′-tetraols (1,2-boladiols), as well as in hydrated n-alkane-1,2-diols were characterised according to their X-ray diffraction patterns. The analysis of the X-ray data is mainly based on the small-angle region. The scattering in the wide-angle region, which may be regarded as fingerprint, is at present used for a qualitative discrimination of the phases. A homologous series of compounds is considered isostructural if the data show a linear relationship between the repeat distance d _L and the number of C atoms (Cm) in a chain. Assuming that a given lateral packing remains constant, two characteristic values can be obtained from the plot of d _L versus Cm. The slope b contains information about the tilt angle of the chains for the respective layer model and the intercept a represents the thickness of the hydrophilic part of the aggregate. A great variety of headgroup thicknesses and tiltings of the chains were found. Bilayers or double bilayers are the repeat unit in 1,2-diols, whereas in 1,2-boladiols the molecules are packed in monolayers. Water changes the packing of the chains and new packing modes with crossed chain axes were obtained. The ability to form hydrogen-bridged networks on both ends of the molecules forces the chains to occupy larger areas in dried 1,2-boladiols compared to the corresponding 1,2-diols. Received: 24 June 1998 Accepted in revised form: 15 February 1999     

Side-chain cholesteric liquid crystalline elastomers derived from a mesogenic crosslinking agent: I. Synthesis and mesomorphic properties

The synthesis of new side-chain cholesteric elastomers derived from a cholesteric monomer and mesogenic crosslinking agent is presented. The chemical structures of the monomers obtained were confirmed by elemental analyses, FT-IR, ¹H NMR, and ¹³C NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurements. M₁ showed cholesteric phase, and M₂ displayed enantiotropic nematic phase and monotropic smectic 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.     

Ionic azo-codendrimers: Influence of the acids contents in the liquid crystalline properties and the photoinduced optical anisotropy

A series of codendrimers has been prepared by the grafting of poly(propylene imine) PPI-(NH₂)₁₆ (G = 3) with two different carboxylic acids: the aromatic-based 5-[4′-(4-cyanophenylazo)phenyloxy]pentanoic acid (AZO) and the aliphatic 2,2-bis(undecanoyloxymethylene)propionic acid (BUPA) in different proportions. Their thermal properties, liquid crystalline behavior and photoinduced optical anisotropy have been investigated. The formation of the ionic compounds was confirmed by nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. The influence of the acid percentage in the liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffractometry (XRD). All the resulting co-dendrimers present liquid crystalline properties displaying an evolution from nematic mesophase for the azo-richer dendrimers to smectic A mesophase for the BUPA-richer dendrimers. Only materials with low azobenzene contents up to four AZO units per dendrimer on average were processable by casting from dichloromethane solutions and resulted in transparent thin films appropriate for optical studies. The photoinduced dichroism reached was higher in the dendrimer with higher azo content (four units per dendrimer) being this material a good compromise between processability and optical response.