Hot-stage Polarizing Optical Microscopy Research Articles

Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid: Synthesis and mesophase characterization

A series of star mesogens based on 1,3,5-benzenetricarboxylic acid central core, side arms containing two phenyl rings, linked through azomethine groups and hexamethylene spacers, which connect the central core and side arms were synthesised via divergent approach. The mesogens synthesised and their intermediates were structurally well characterized, using FT-IR, 1H NMR, 13C NMR spectroscopy and elemental analysis techniques. The mesophase characterization was carried out using a hot stage-polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. Enantiotropic mesophases were observed for all the mesogens synthesised. An increase in the terminal chain length stabilized the smectic phase. Nematic phase was observed for the mesogens with short terminal chain length, whereas smectic polymorphism was observed on increasing the terminal chain length. XRD results suggested a tripod-like assembly of molecules in the mesophase.

Star mesogens — Synthesis and structural characterization using 1D and 2D solution NMR techniques and mesophase characterization

Novel star mesogens based on trimesic acid and symmetrical side arm cores with terminal alkoxy groups were synthesized via a divergent approach. The central core and side arms were connected through alkyl spacers. All the synthesized mesogens and their intermediates were characterized thoroughly using Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), 13C NMR, and mass spectrometers. One representative mesogen was subjected to the two-dimensional (2D) NMR experiments to ascertain the structure of the mesogens. The mesophase characterization was carried out using hot-stage optical polarizing microscopy (HOPM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) techniques. Many of the molecules with an ethyloxy spacer were found to be nonmesogenic, whereas all the molecules with a butyloxy spacer showed liquid crystalline phases. The increase of terminal chain length decreased the transition temperatures. The nematic phase was observed for the mesogens with short terminal chain length, whereas smectic polymorphism was observed on increasing the terminal chain length. The results of a variable temperature powder X-ray diffraction of the representative sample support the smectic layer ordering.

Synthesis, characterization and liquid crystalline behavior of poly(monomethyl itaconate)s with new pendant cholesterol moieties

In order to investigate the effects of cholesteryl pendant groups on inducing liquid crystalline (LC) phase formation, a new series of cholesteryl-modified poly(monomethyl itaconate) (PMMI-Chol-C6) have been synthesized from poly(monomethyl itaconate) (PMMI) and 6-(cholesteryloxycarbonyloxy) hexanol (Chol-C6) with different degrees of substitution (DSchol). All the obtained compounds were characterized by conventional spectroscopic methods. The DSchol values of the modified PMMI were obtained by 1H NMR spectroscopy and conductometric titration. The inherent viscosities of polymers were determined by an Ubbelohde viscometer at 35 °C in DMSO solution. The resulting products of modified PMMA polymers were soluble in a variety of organic solvents, and the solubility improved by increasing the DSchol. Thermal behavior and optical properties of Chol-C6 and PMMI-Chol-C6 polymers were investigated by thermal gravimetric analysis, differential scanning calorimetry and hot-stage polarizing optical microscopy. The glass transitions of the modified PMMI polymers occurred at lower temperatures than the parent PMMI. It was found that among the synthesized polymers with different molar ratios of the substitution, only the PMMI-Chol-C6 (1) polymer bearing 77 mol% of the side chains, with reduced melting point and increased thermal stability, formed thermotropic liquid crystalline smectic phases. This polymer exhibited wider mesophase than the mesogenic side chain Chol-C6 and amorphous morphology. The results of a comparative study on the structure and properties of the polymers showed that liquid crystalline polymers may be achieved through the attachment of mesogenic cholesterol side chain onto the main chain via methylene chains of side branches.

Isophthalic acid based mesogenic dimers: Synthesis and structural effects on mesophase properties

Two series of mesogens based on isophthalic acid and side arms containing two and three phenyl rings linked through azomethine or ester groups were synthesized via divergent approach. Another type of structurally similar mesogen was synthesized via convergent approach in which the azomethine groups were replaced by ester groups. All the synthesized mesogens and their intermediates were structurally well characterized using FT-IR, 1H NMR, 13C NMR and EI-MASS spectrometry. The mesophase characterization was carried out using hot stage-polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The structural changes made in the mesogens i.e. increase of terminal chain length and replacement of azomethine groups with ester groups decreased the transition temperatures whereas the increase of side arm length improves the phase stability to a wide temperature range. Nematic phase was observed for the mesogens with short terminal chain length whereas the smectic polymorphism was observed on increasing the terminal chain length. The structural modification (three ring) on the side arm core stabilizes the mesophases enantiotropically.

Synthesis, Characterization, and Properties of Thermotropic Novel Liquid Crystal Photosensitive Polymers Containing Mesogen and Photocrosslinkable Group

A series of side chain liquid crystalline photosensitive polymers containing methyl methacrylate followed by the spacer and chalcone moiety were synthesized and characterized. Monomers, namely 4-[4{biphenyl oxy carbonyl phenyl oxy carbonyl} phenyl carbonyl ethylene phenyl] oxy butyl methacrylate (BOCPOCPCEPOBMA) and 4-[4{1-methoxy phenyl oxy carbonyl phenyl oxy carbonyl} phenyl carbonyl ethylene phenyl] oxy octyl methacrylate (MPOCPOCPCEPOOMA) was synthesized and subsequently polymerized using benzoyl peroxide as an initiator. The chemical structures and purity were characterized by FT-IR, 1H- NMR, and 13C-NMR analyses. The photocrosslinking studies of the polymers were investigated by UV spectroscopy. The rate of crosslinking increased with increase in the length of the methylene chain. The results of differential scanning calorimeter (DSC) analysis confirmed the formation of mesophase in the polymers. Hot stage optical polarized microscopy (HOPM) showed presence of nematic phase for poly (BOCPOCPCEPOBMA) and poly (MPOCPOCPCEPOOMA).

Synthesis and characterization of certain photocrosslinkable thermotropic liquid crystalline random copolyesters containing arylidene moiety

A new series of 12 thermotropic liquid crystalline and photocrosslinkable random copolyesters were synthesized from terephthalic acid, isophthalic acid and 4,4'-oxybis(benzoic acid) with four different arylidene diols and a common diol 1,2-bis(4-hydroxybenzoyloxy)ethane by direct polycondensation with diphenylchlorophosphate. The resulting polymers had inherent viscosities in the range 1.20–1.74 dl/g. Their structural features were investigated by Fourier transform infrared (FTIR), 1H, and 13C nuclear magnetic resonance (NMR) spectroscopy. The thermal behaviour of the copolyesters was investigated with differential scanning calorimetry and hot stage optical polarized microscopy. Our investigation on the effect of monomer structure on liquid crystalline behaviour shows that the copolymers containing bis(benzylidene)cycloalkanone exhibited higher melting and isotropization temperatures and extended liquid crystalline range. All of the copolymers showed some degree of crystallinity as evidenced by wide-angle X-ray diffraction and scanning electron microscopy. The photocrosslinkability of these polymers was established with UV irradiation/UV spectroscopy and scanning electron microscopy.

Synthesis, Characterization, and Properties of Thermotropic Novel Liquid Crystalline Photosensitive Polymer

Side-chain liquid crystalline photosensitive polymer was synthesized using pendant chalcone moiety. The pendant chalcone moiety was reacted with methylene spacer followed by incorporating methacryloyl chloride which is then reacted with 4-hydroxy phenyl oxycarbonyl acetophenone which is then polymerized using BPO as an initiator. The structures of the synthesized liquid crystalline photosensitive polymer have been confirmed by spectroscopic techniques like IR, 1H and 13C NMR, and UV visible spectrometer. The thermal stability of the polymer was investigated by thermo gravimetric analysis (TGA). The thermal stability of the polymer decreased with increase in flexible methylene chain. The photochemical properties of this polymer were investigated by UV spectroscopy. Crosslinking proceeds via 2π–2π cycloaddition of the –CH=CH– of the chalcone moieties. Liquid crystalline phase was confirmed by the differential scanning calorimetry (DSC) and hot stage optical polarized microscopy (HOPM).

TEMPO Containing Esters and Their Magnetic and Liquid Crystal Properties

Six new magnetic liquid crystals have been prepared, in which trans-bicyclohexyl or trans-cyclohexyl phenyl and biphenyl carboxylic acid phenyl bis-ester mesogenic cores with n-propyl and n-pentyl substituents were terminated by 4-amino-TEMPO (TEMPO=2, 2, 6, 6-tetramethyl piperidine-1-oxyl). All the compounds have been characterized on the basis of their spectral datum. Determined by superconducting quantum interference device (SQUID), electron paramagnetic resonance (EPR), differential scanning calorimeter (DSC), and hot stage polarizing optical microscope (HS-POM), the compounds have both magnetic and liquid crystalline properties. Their temperature ranges of mesophase are about 3.0°C–6.0°C and the susceptibility is 0.137 emu K mol–1 and θ is –5.54 K for 7a between 135 K and 185 K.

Phase Characterization and Study of Molecular Order of a Three-Ring Mesogen by 13C NMR in Smectic C and Nematic Phases

Molecules exhibiting a thermotropic liquid-crystalline property have acquired significant importance due to their sensitivity to external stimuli such as temperature, mechanical forces, and electric and magnetic fields. As a result, several novel mesogens have been synthesized by the introduction of various functional groups in the vicinity of the aromatic core as well as in the side chains and their properties have been studied. In the present study, we report three-ring mesogens with hydroxyl groups at one terminal. These mesogens were synthesized by a multistep route, and structural characterization was accomplished by spectral techniques. The mesophase properties were studied by hot-stage optical polarizing microscopy, differential scanning calorimetry, and small-angle X-ray scattering. An enantiotropic nematic phase was noticed for lower homologues, while an additional smectic C phase was found for higher homologues. Solid-state high-resolution natural abundance (13)C NMR studies of a typical mesogen in the solid phase and in the mesophases have been carried out. The (13)C NMR spectrum of the mesogen in the smectic C and nematic phases indicated spontaneous alignment of the molecule in the magnetic field. By utilizing the two-dimensional separated local field (SLF) NMR experiment known as SAMPI4, (13)C-(1)H dipolar couplings have been obtained, which were utilized to determine the orientational order parameters of the mesogen.

Dysprosium-Based Ionic Liquid Crystals: Thermal, Structural, Photo- and Magnetophysical Properties

[C12mim]3[DyBr6] (C12mim = 1-dodecyl-3-methylimidazolium) represents a new material with interesting luminescent behavior as well as mesomorphic and magnetic properties. The compound was found to show thermotropic liquid crystalline behavior and forms smectic mesophases which were investigated by hot-stage polarizing optical microscopy and differential scanning calorimetry. The emission color of [C12mim]3[DyBr6] can be tuned from white to orange-yellow by the choice of the excitation wavelength. Sample excitation with λex = 366 nm leads to the blue-whitish luminescence from the imidazolium cation itself. With λex = 254 nm the common Dy(III) emission is observed which mainly arises from the 4F9/2 → 6H13/2 transition and, in consequence, the sample appears orange. Magnetic measurements show for Dy(III) in [C12mim]3[DyBr6] an effective magnetic moment of μeff = 9.6 μB at room temperature. The sample shows superparamagnetism and can be manipulated by an external magnetic field. In addition, the crystal struct...

Synthesis and Characterization of Benzylidene and Azobenzene Based Photochromic Liquid-Crystalline Polymers

Two series of novel thermotropic main chain liquid crystalline polymers containing photocrosslinkable chalcone and photoisomerizable azobenzene moieties were synthesized from bis[4-( m-hydroxyalkoxy)benzylidene]cycloalkanones [ m = 6,8,10] with 4,4'-azobenzene dicarbonylchloride. The chemical structure of the monomers and polymers were confirmed by spectral techniques. The mesophase properties were investigated using hot stage optical polarized microscopy and differential scanning calorimetry. All the polymers exhibited liquid crystalline properties. The thermogravimetric analysis data revealed that the polymers were stable up to 220 °C and underwent two-stage decomposition. Photocrosslinking property of the polymers was investigated using UV spectroscopy. The cyclopentanone containing polymers showed faster photocrosslinkability than that of their cyclohexanone counterpart. In addition to the photocrosslinkability of cycloalkanone units, the azo moiety displayed E-Z (trans-cis) isomerization in the polymer backbone.

Synthesis and Mesomorphic Behavior of Some Novel Compounds Containing 1,3,4-Thiadiazole and 1,2,4-Triazole Rings

Two new mesogenic homologous series of liquid crystalline compounds containing 1,2,4-triazole and 1,3,4-thiadiazole in the molecule viz. 1,4-bis{3-[2-(4-alkoxybenzelideneamino)-1,3,4-thiadiazole-5-ethylenethio]1,2,4-triazole-5-yl}phenyl and 5-(4-methoxyphenyl)-3-[2-(4-n-xybenzelidenamino)-1,3,4-thiadiazole-5-methylenethio]-1,2,4-triazole were synthesized by many step procedures. Both series have been characterized by elemental analysis, FT-IR, mass spectrometry, 1H-NMR, and 13C-NMR. Phase transition temperature and the thermal parameters were obtained from differential scanning calorimetery (DSC). The texture observations were performed under hot-stage polarizing optical microscopy (POM). All compounds of the first series exhibited an enantiotropic nematic mesophase. The first compound of the second series exhibited an enantiotropic nematic mesophase, and that with propyloxy terminal substituent shows one enantiotropic nematic phase on heating and a monotropic smectic A (SmA) phase on cooling, while the last two homologues exhibit a monotropic SmA phase. In contrast, the compound with an ethoxy terminal substituent does not show any liquid crystalline properties. The mesomorphic behavior has been analyzed in terms of structural property relationship.

Diglycidylphenylphosphate based fire retardant liquid crystalline thermosets

Organophosphorus liquid crystalline thermosets (LCTs) were developed using diglycidylphenylphosphate (DGPP) and various aromatic diamines. DGPP was prepared by the esterification of phenylphosphorodichloridate with glycidol. 2,5-Bis( p-aminophenyl)-1,3,4-oxadiazole (BPOD) was synthesized and three different commercial diamines, 4,4′-diaminodiphenyl sulphone (DDS), 4,4′-diaminodiphenyl methane (DDM) and biphenyl diamine (BPD) were used as curing agents. The mesomorphic behaviour of DGPP/diamine mixtures and their curing kinetics were monitored by differential scanning calorimetry and hot stage optical polarized microscopy. The thermogravimetric analysis data showed that the LCTs are stable in the range 261–292 °C and afford 36–48% char yield. The limiting oxygen index values are between 35 and 47 proving fire retardance. The mechanical properties of the cured LCTs were characterized by dynamic mechanical analysis.

Investigation on thermotropic liquid crystalline and photocrosslinkable polyarylidene arylphosphate esters containing cyclohexanone units

Two series of novel liquid crystalline-cum-photocrosslinkable divanillylidene cyclohexanone containing polymers have been synthesized from 2,6-bis[ m-hydroxyalkyloxy(vanillylidene)]cyclohexanone [ m=6, 8, 10] with different arylphosphorodichloridates (naphthyl, biphenyl) by solution polycondensation method at ambient temperature. Their chemical structures were confirmed by FT-IR, 1H, 13C and 31P NMR spectroscopy. The intrinsic viscosity values were measured to find out molecular weight of the synthesized polymers. The mesogenic properties and phase behaviors were investigated with differential scanning calorimetry and hot stage optical polarized microscopy. The experimental results demonstrated that the mesogenic transition and isotropization temperature gradually decreases with increase in even number of methylene spacer of the polymer chain. All the polymers showed anisotropic behavior under hot stage optical polarized microscope (HOPM). The thermal behaviors of the polymers were studied by thermogravimetric analysis and stable between 292 and 330 °C. The photocrosslinking of the polymers was investigated in thin film by UV light/UV spectroscopy and the cyclobutane ring formation via 2π–2π cycloaddition reactions of the divanillylidene exo-cyclic double bond of the polymer backbone. The pendant naphthyloxy containing polymers show faster crosslinking than the pendant biphenyloxy containing polymers.

Synthesis and characterization of thermotropic liquid crystalline poly(azomethine ether)s

Two series of monomers, namely 4,4′-diformyl-α,ω-diphenoxydecane and 4,4′-diformyl-3,3-methoxy-α,ω-diphenoxydecane, were prepared from 1,10-dibromodecane with p-hydroxybenzaldehyde and 4-hydroxy-3-methoxybenzaldehyde (vanillin), respectively. The poly(azomethine ether)s were prepared by solution polycondensation using 4,4′-diformyl-α,ω-diphenoxydecane, 4,4′-diformyl-3,3-methoxy-α,ω-diphenoxydecane with various diamines. The monomers and polymers were characterized by intrinsic viscosity, FT-IR, 1H, and 13C NMR spectroscopy. The thermogravimetric analysis reveals that the polymers are stable up to 320–500°C and decomposed with good char yield. The thermotropic liquid crystalline properties of the polymers were examined by differential scanning calorimetry (DSC) and their textures observed under hot stage optical polarized microscopy (HOPM). All the polymers were exhibited thermotropic liquid crystalline properties except tetramethylene diamines-based polymers.

Effect of ethylphosphate on azobenzene-containing thermotropic liquid-crystalline polymers

A new series of liquid-crystalline polymers with azobenzene mesogens and an even number of methylene spacers along with phosphorus heterogeneity were synthesized. The structures of the monomers and polymers were characterized with various spectral techniques. The thermal analyses were performed with thermogravimetric analysis and differential scanning calorimetry (DSC). The formation of the liquid-crystalline phase was confirmed with hot-stage optical polarizing microscopy. All the polymers exhibited grainy and nematic textures during melting; this was confirmed by DSC analysis. The char yield of the polymers was quite high and was related to the formation of phosphoric acid. The glass-transition temperature, melting temperature, and isotropization temperature were quite low. The reduction in the transition temperature could be attributed to the incorporation of the phosphorus heterogeneity and pendant ethyl group. Molecular modeling studies supported the experimental data obtained. The suitability of these polymers for optical data storage was studied with ultraviolet spectroscopy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 455–462, 2004

Liquid-crystalline azines formed by the rare-earth promoted decomposition of hydrazide “habbe” ligands: structural and thermal properties

In an attempt to synthesise liquid-crystalline rare-earth complexes of some habbe-type ligands (habbe = N-(2-{(2E)-2-[2-hydroxy-4-(alkoxy)benzylidene]hydrazino}-2-oxoethyl)benzamide), decomposition of these ligands was observed. A synthetic pathway is proposed, in which the rare-earth ion promotes the decomposition of the habbe ligand into a symmetrical azine, which shows mesomorphism. The thermal behaviour of four azines obtained in this way was studied by hot-stage polarizing optical microscopy, differential scanning calorimetry (DSC) and high-temperature powder diffraction methods and shows that all four compounds are liquid-crystalline. The crystal structure of one of the azines has been determined by single crystal X-ray diffraction and shows an elongated orientation of the molecules and a layered crystal packing. Both starting and end products were further characterized by 1H-NMR.

Thermal behaviour of lanthanum(III) alkanoates

The mesophase behaviour of the lanthanum(III) alkanoates [La(CxH2x+1COO)3] (x =3-19) has been investigated by hot-stage polarizing optical microscopy, differential scanning calorimetry and high-temperature X-ray diffraction. Lanthanum(III) butyrate monohydrate shows no mesomorphism, whereas for the remaining short chain homologues (x = 4-9) a highly viscous mesophase M and a smectic A phase were observed. The longer chain lanthanum(III) soaps (x = 10-19) exhibit only a smectic A phase. However, the chain length has a pronounced effect on the transition temperatures. The thermal behaviour of lanthanum(III) alkanoates is compared with that of other lanthanide(III) alkanoates.

Mesomorphic behaviour of praseodymium(III) alkanoates

In this paper, we describe the synthesis, characterization and thermal behaviour of praseodymium(III) alkanoates. The compounds have the stoichiometry [Pr(C x H2x+1COO)3], where x = 5-19, and were characterized by elemental analysis and infrared spectroscopy. The thermal behaviour was investigated by hot-stage polarizing optical microscopy, DSC and high temperature X-ray diffraction. A highly viscous mesophase M and a smectic A phase were observed for the shorter chain compounds (x = 5-8), whereas only a smectic A phase was observed for the longer chain compounds of this type of metallomesogens. The chain length has a pronounced effect on the transition temperatures.

Rare-earth complexes of mesomorphic Schiff's base ligands

Rare-earth complexes of mesomorphic Schiff 's bases, 4-[(alkylimino)methyl]-3-hydroxyphenyl 4-alkyloxybenzoates, were synthesized. Whereas the ligands LH display a nematic and/or a smectic C phase, the metal complexes show a viscous smectic A phase and decompose at the clearing point. The mesophase was investigated by hot-stage polarizing optical microscopy, by differential scanning calorimetry and by high temperature X-ray diffraction. Two types of complex were found, [Ln(LH)3 (NO3)3] and [Ln(LH)2L(NO3)2], depending on the ligand or the central metal ion. The first coordination sphere of the rare-earth ion in these metallomesogens is comparable to that in the structure of complexes with 4-alkoxy-N-alkyl2-hydroxybenzaldimine ligands.