Liquid Crystalline Oligomers Research Articles

Self-Assembly of Amphiphilic Liquid-Crystalline Oligomers Possessing a Semiperfluorinated Alkyl Chain

Amphiphilic liquid-crystalline oligomers composed of a semiperfluorinated alkyl chain and cyanobiphenyl mesogenic moieties were prepared. Their physical properties were investigated using optical microscopy, differential scanning calorimetry, and X-ray diffraction measurements. The oligomers were found to show an unusual phase sequence of lamellar−columnar−bicontinuous cubic phases when the number of cyabobiphenyl moieties was increased. Results of the XRD study suggest that the smectic A phase of the oligomer with a SmA to Cub phase transition has an undulated layer structure. We discuss effects of the molecular structure on the appearance of the columnar and cubic phases of the novel liquid crystal oligomers.

Synthesis and Characterization of Aryl Ethynyl Terminated Liquid Crystalline Oligomers and Their Cured Polymers

In this paper the synthesis and characterization of two series wholly aromatic, main-chain, reactive liquid crystalline oligomers is reported. Phenylethynyl end-capped oligomers, based on 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA), were successfully synthesized using standard high-temperature melt-condensation techniques. Melt processable oligomers, with M n = 1000, 5000, 9000, and 13 000 g mol -1 , and oligomers with HBA or HNA concentrations as high as 95 mol % were prepared. All oligomers showed well-defined, homogeneous, nematic melt behavior over a broad temperature range. The phenylethynyl end-capped oligomers could be cured at 370 °C and formed films with excellent mechanical and thermal properties, and without losing the nematic order. Low molecular weight oligomers, i.e. M n 5000 g mol -1 , polymerize predominantly via chain extension chemistry. The fully cured nematic polymers exhibit glass-transition temperatures (Tg) up to 203 °C, as determined by DMTA measurements, and thermal stabilities (5 wt % loss) up to 500 °C in both air and nitrogen atmosphere. The cured 1000 and 5000 g mol -1 oligomers, with 95 mol % HBA or HNA compositions, show significant improvements in storage modulus at elevated temperatures as compared to their high-molecular weight counterparts. Rheology experiments showed that these reactive nematic oligomers are melt-stable for at least 30 min at 300 °C and exhibit complex melt viscosities |η*| as low as 1 Pa·s at 100 rad·s -1 . In all examples no loss of mesophase could be observed during chain extension or cross-linking, which indicates that phenylethynyl cross-link chemistry is completely compatible with mesophase formation.

Banana‐shaped side chain liquid crystalline siloxanes

Eight banana‐shaped side chain liquid crystalline oligomers and polymers have been synthesized by hydrosilylation of vinyl‐terminated bent‐core mesogens with trimethylsilyl‐terminated siloxanes. The synthesized oligomers and polymers, and their olefinic precursors, were investigated by polarizing optical microscopy (POM), differential scanning calorimetry, X‐ray diffraction (XRD), electro‐optical experiments and Maldi‐Tof. The short‐tailed olefins form a Colr mesophase, whereas those with longer chains exhibit the SmCPA mesophase. All the oligomers and polymers studied show liquid crystalline properties and do not crystallize upon cooling. Most oligomers with around four repeating siloxane units, show a lamellar (layer) structure and antiferroelectric switching properties, the SmCPA phase. XRD shows that the layer spacings are hardly influenced by the length of the terminal tails. The oligomer prepared from the smallest olefinic precursor, having the shortest alkyl tail, shows an XRD pattern reminiscent of a columnar phase, although POM displays domains of opposite chirality, and no switching behaviour could be detected. The polymers with around 35 repeating siloxane units are liquid crystalline, but due to their high viscosity a thorough characterization of the liquid crystalline phases was impossible.

A Main‐Chain Liquid‐Crystalline Oligomer Prepared by in situ Photopolymerization of an LC Monomer Having Cinnamate Moieties

AbstractSummary: A liquid‐crystalline (LC) compound, having a cinnamate moiety on each end of the molecule, was synthesized and irradiated with UV light in its LC phase in the presence of a triplet sensitizer. Various measurements of the irradiated sample revealed that the linearly structured LC oligomers were formed by [2+2] cycloaddition of the cinnamate moieties, and that the resultant cyclobutane units dominantly assumed an anti head‐to‐head configuration.Schematic structures of the LC oligomer obtained by photopolymerization.imageSchematic structures of the LC oligomer obtained by photopolymerization.

Packing and Uniaxial Alignment of Liquid Crystalline Oligofluorenes

Summary: Liquid crystalline oligomers of 9,9-bis(2-ethylhexyl)fluorene of defined degree of polymerization 4, 5, 6, and 7 were investigated by X-ray diffraction in the non-oriented and in the aligned state. The diffraction data give evidence for a smectic B type phase for all of the oligomers. Quenching below the glass transition does not change the structure of the liquid crystalline phase. This allows to align spin-coated films of these oligomers on rubbed polyimide substrates to give monodomain films. These are stable against thermal disordering below Tg, e.g. at room temperature. The degree of alignment is characterized by the dichroic spectra and polarized fluorescence spectra. Dichroic ratios and polarization ratios increase substantially with the chain length and values as high as D = 23 and P = 41 are obtained for the heptamer. The type of packing of the oligomers in the LC phase is discussed based on the X-ray single crystal structure of models. In one such model the packing of the 2-ethylhexyl side chains could be fully resolved, while the other model reveals the torsional angle between adjacent fluorene units in the same molecule as 144.2° which corroborates earlier work based on fiber diffraction of corresponding polyfluorenes.

Molecular theory of dendritic liquid crystals: self-organisation and phase transitions

We present tractable molecular theory descriptions of liquid crystalline dendrimers based on clearly defined approximations and in terms of the dominant interactions underlying the self-organisation of these large and complex supermolecular entities. We formulate the configurational partition function for dendrimers, taking explicit account of their conformations and segmental interactions. Two approximate schemes are presented: the first is based on the effective interactions of the dendrimers as a whole while the second scheme is based on the interactions among the mesogenic units contained in the dendrimers. Results of lattice calculations for phase transitions in the context of the first scheme are presented and they show that the minimal inclusion of shape anisotropy and of sub-molecular partitioning into chemically distinct parts is sufficient to reproduce the variety of phases and phase sequences observed experimentally and provides insights into the conformational aspect of these transitions. In the second scheme, the description of the dendromesogenic system reduces to that of an ensemble of mesogenic dimers. This scheme can be readily extended to the description liquid crystalline oligomers and polymers consisting of mesogenic units connected by flexible spacers in various architectures. It thus provides a unified approach for treating mesomorphic phase transitions of supermolecular and macromolecular systems that can be built by connecting the same submolecular units in topologically different ways.

액정올리고머에 있는 카보닐그룹들의 동적순서에 대한 2차원 라만상관분광법 연구

12-4라는 액정올리고머가 액정이 되는 과정에서 일어나는 동력학을 2차원 라만상관분광법을 이용하여 연구하였다. 분광법의 결과로부터 에틸그룹 옆에 붙어있는 자유 카보닐 그룹이 먼저 움직이고, 수소결합을 이룬 카보닐 그룹이 나중에 움직인다는 것을 알아내었다. 반면에 바이페닐그룹사이에 존재하는 자유 카보닐 그룹은 좀처럼 움직이지 않는 것으로 나타났다. 샘플-샘플상관분광법의 결과로부터는 액정이 되는 온도 135<TEX>$^{\circ}C$</TEX>보다 훨씬 낮은 온도인 50<TEX>$^{\circ}C$</TEX>에서 이미 카보닐 그룹의 움직임이 있음을 알 수 있었다. We investigated the order of local movement of functional groups (-C(=O)O) in a liquid crystalline (LC) oligomer (12-4) using 2D Raman correlation spectroscopy. The results suggest that the free carbonyl near the ethyl terminal group moved first, followed by the hydrogen-bonded group. The free carbonyl group between the biphenyl groups rarely moved. Interestingly, 2D sample-to-sample correlation spectroscopy (2D STSCS) revealed that some of the carbonyl modes started to move at far below (50<TEX>$^{\circ}C$</TEX>) the LC temperature (135<TEX>$^{\circ}C$</TEX>), countering conventional beliefs.

Synthesis and Characterization of Unsaturated Thermotropic Polyesters Prepared via Acyclic Diene Metathesis Polymerization

Acyclic diene metathesis (ADMET) polymerization was employed to prepare thermotropic liquid crystalline polymers (TLCPs) bearing double bonds within flexible spacers. Starting from a diene monomer, 2-tert-butyl-1,4-phenyl bis(4-pentenyloxybenzoate) (5tB), we successfully synthesized unsatur- ated liquid crystalline oligomers and polymers, P5tBn, via ADMET polymerization. Such unsaturated TLCPs were developed to allow the preparation of tailored, thermally cross-linkable, liquid crystalline thermosets as adhesives and composite matrix resins. High degrees of polymerization were achieved and the resulting polymers showed clear mesomorphic behavior with complete absence of crystallization. Thermal analysis of polymers prepared with varying synthetic conditions showed that TLCPs prepared using different ruthenium metathesis catalysts featured substantially different transition temperatures, implying an influence of the catalyst on the resulting TLCP chain microstructure. In light of these observations, NMR studies (1D 1 H and 13 C, and 2D COSY and HMQC) were carried out, allowing determination of the selectivity of the catalysts toward trans/cis linkage and coupling/isomerization of units in the polymer chain. Furthermore, by studying the polymerization and isomerization kinetics, we shed light on the cause of isomerization. It was found that the degradation product of Grubbs catalyst is implicated in isomerization catalysis, in agreement with reports on the metathesis of 1-octene. In contrast, second-generation Grubbs catalyst itself (not a degradation product) apparently causes isomerization. For a related catalyst, developed by Nolan, both the catalyst and its degradation product are concluded to be responsible for the isomerization reaction.

LIQUID CRYSTALLINE POLYMERS: 10: SYNTHESIS AND THERMAL BEHAVIOR OF SOME POLYETHERS CONTAINING A HEXAMETHYLENIC SPACER

The paper presents the synthesis and thermal behavior of some copolyethers containing a hexamethylenic spacer. The polymers were obtained using a phase transfer catalysis technique, starting from 1,6-dichlorohexane and various bisphenols as follows: 4,4′-dihydroxyazobenzene (DHAB), 4,4′-dihydroxydiphenyl (DHD), bisphenol A (BPA), 2,7-dihydroxynaphtyl (DN) and 4-hydroxydiphenyl sulfide (BPS). The synthesized polyethers have low molecular weights, being situated in the oligomeric domain. The polymers were characterized by 1H-NMR spectroscopy, DSC calorimetry and optical microscopy in polarized light. Only the polyethers containing DHAB, DHD and BPA units exhibited liquid crystalline phase. All liquid crystalline oligomers evidenced enantiotropic behavior.

Phase morphology and molecular dynamics of a polyurethane ionomer reinforced with a liquid crystalline filler

Solution-blended binary composites of ionic segmented polyurethane (SPU-I) and liquid crystalline oligomer (LCO) were characterized by wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and dielectric relaxation spectroscopy (DRS). Both components mutually influenced their states of aggregation in blends (most significantly, promoting smearing-out of interfaces between stiff and soft chain fragments of SPU-I into broad interfacial regions of intermediate composition). Apparently, the blend with w=0.10 happened to be most favorable for crystallization of the LCO, while the degree of microphase separation for SPU-I became lower and the distribution of stiff domains by sizes became broader, the higher the LCO content. The overall molecular mobility of SPU-I in blends was significantly reduced. This reduction included the intensity of the secondary and the primary relaxations, and of the interfacial Maxwell–Wagner–Sillars (MWS) relaxation, whereas the transition temperatures remained essentially composition-invariant. The Arrhenius-like behavior for the dc conductivity concomitant to the non-Arrhenius (i.e., Vogel–Tammann–Fulcher) frequency dependence for the α relaxation in blends suggested a decoupling of conductivity from the motion of the SPU-I soft chain segments.

Raman Study of the Intermolecular Interactions between Thermotropic Liquid-Crystalline Oligomers: Comparison of the Effect of Poly(Propylene Oxide) and Poly(Ethylene Oxide)

Polymer Science and Technology, Chonbuk National University, Chonju, Chonbuk 561-756, KoreaDepartment of Chemistry, Kunsan National University, Kunsan, Chonbuk 573-701, KoreaReceived November 27, 2001Keywords : Raman, Liquid crystal, Intermolecular interaction, Coil effect, Hydrogen bonding.IntroductionLiquid crystalline polymers (LCPs) have been intensivelystudied for their scientific and technological interests. AmongLCPs, rod-coil types have recently drawn attention, parti-cularly in relation to their likelihood to form various kinds ofsupramolecules depending on chain lengths. The LCPs basedon a biphenyl ester moiety can have different microscopicstructures. Most poly(ethylene oxide)s (PEO)-based LCPsresult in a layered smectic mesophase,

Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid and a Liquid Crystalline Oligomer on Different Metal Surfaces

Surface-enhanced Raman spectra of 4-biphenylcarboxylic acid (BPCA) and ethyl 4‘-[4‘-oxy-4-biphenylcarbonyloxy]-4-biphenylcarboxylate with a poly(propylene oxide) coil of 12 (hereafter named P-12-4) adsorbed on two different metal substrates, Ag and Au colloid monolayers, were examined, to determine the relationship between adsorption strength and geometry. A significant downshift of νCOO- by 60 cm-1 and lack of a C−H aromatic stretching vibration band indicated that BPCA lies flat on the Au surface, while a smaller shift of 40 cm-1 to a lower wavenumber and weak appearance of the C−H aromatic stretching mode implied adsorption through mainly oxygen lone pair electrons of carboxylate with the benzene ring tilted on the Ag surface. These orientations were confirmed by the desorption behavior of a liquid crystalline (LC) oligomer, P-12-4, which contains the BPCA moiety, on the Au and Ag colloid monolayers with increasing temperature.

Synthesis of Liquid Crystalline Epoxy Oligomers: Effect of Molecular Weight on the Phase Behavior

A series of epoxy-terminated liquid crystalline oligomers with different molecular weights having α-methylstilbene as the mesogenic unit were synthesized and characterized by means of 1H NMR and FT-IR spectroscopy, differential scanning calorimetry (DSC), polarized-light optical microscopy (POM) and X-ray diffraction. The effect of the initial ratio between epichlorohydrin and diol on the molecular weight was not significant enough to change the thermal behavior of the oligomers essentially. The copolymerization of epoxy- and hydroxyl-terminated comonomers yielded high-molecular weight oligomer, whose enantiotropic liquid crystalline character was proved by means of DSC, POM and X-ray diffraction.

Synthesis of Liquid Crystalline Epoxy Oligomers: Effect of Molecular Weight on the Phase Behavior

A series of epoxy-terminated liquid crystalline oligomers with different molecular weights having α-methylstilbene as the mesogenic unit were synthesized and characterized by means of 1H NMR and FT-IR spectroscopy, differential scanning calorimetry (DSC), polarized-light optical microscopy (POM) and X-ray diffraction. The effect of the initial ratio between epichlorohydrin and diol on the molecular weight was not significant enough to change the thermal behavior of the oligomers essentially. The copolymerization of epoxy- and hydroxyl-terminated comonomers yielded high-molecular weight oligomer, whose enantiotropic liquid crystalline character was proved by means of DSC, POM and X-ray diffraction.

FTIR spectroscopy and strengthening behavior of liquid crystalline oligomers of 2-alkoxy-4-hydroxybenzoic acids and 4-hydroxybenzoic acid

A series of liquid crystalline oligomers of 4-hydroxybenzoic acid (HBA) and 2-alkoxyHBAs were synthesized. The structural and conformational changes the oligomers were studied by FTIR spectroscopy. The strengthening abilities of the synthesized co-oligomers were investigated by blending the oligomers with polyamide 11 (PA 11).

FTIR spectroscopy and strengthening behavior of liquid crystalline oligomers of 2-alkoxy-4-hydroxybenzoic acids and 4-hydroxybenzoic acid

A series of liquid crystalline oligomers of 4-hydroxybenzoic acid (HBA) and 2-alkoxyHBAs were synthesized. The structural and conformational changes of the oligomers were studied by FTIR spectroscopy. The strengthening abilities of the synthesized co-oligomers were investigated by blending the oligomers with polyamide 11 (PA 11).

FTIR and strengthening studies on liquid crystalline oligomers of 2-thioalkoxy/2-alkoxyhydroquinones and terephthaloyl chloride

The properties of liquid crystalline (LC) oligomers produced from 2-thioalkoxyhydroquinones (2-thioalkoxyHQs) and 2-alkoxyhydroquinones (2-alkoxyHQs) with terephthaloyl chloride (1a–c, 2a–c) have been studied and their compatibility with polyamide 11 (PA 11) has been investigated. FTIR studies showed two conformations of the synthesized oligomers. A three-point bending test indicated that oligomeric chains could strengthen the final flexural properties of the polyamide. Strength of PA 11 was increased by addition of only 1% of LC oligomers to the matrix. Compatibility between oligomer chains and PA 11 was lower than the compatibility between corresponding oligomers of 2-alkoxy-4-hydroxybenzoic acids (2-alkoxyHBAs) and PA 11.

Physically crosslinked elastomers prepared from oligomeric polyolefins with mesogenic units

Two ABA-type liquid crystalline oligomers were newly synthesized, where A was a mesogenic group and B was polyolefin whose molecular mass was 2470. The A segment was prepared from p-hydroxyl benzoic acid and terephalic acid. The elastomeric films, whose moduli at 20% elongation were 0.4–1.0 MPa, were obtained by solution casting of the ABA-type oligomers. Dynamic mechanical analysis and differential scanning calorimetry measurement showed the glass transition of amorphous polyolefin segments, the melting of mesogenic groups, and the meso-to-isotropic transition of liquid crystalline phase. The formation of microphase-separated structures was confirmed by a small-angle X-ray scattering (SAXS) measurement. The presence of hexagonal cylinder domains, which were attributed to the aggregation of mesogenic groups in the polyolefin matrix, was also detected by SAXS. These liquid crystalline oligomers showed anisotropy under the crossed Nicoles, and the textures were observed to be nematic. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2247–2253, 2000

A novel calamitic liquid crystalline oligomer composed of three non-identical mesogenic entities: synthesis and characterization

The synthesis and evaluation of the mesomorphic properties of the first trimesogen consisting of three non-identical calamitic mesogenic entities have been described.

Unexpected Strengthening of Polyamide 11 with Liquid Crystalline Oligomers of 2-Alkoxy-4-hydroxybenzoic Acids. 1

Novel liquid crystalline (LC) oligomers of 2-alkoxy-4-hydroxybenzoic acids (5a−d) containing flexible side chains were produced, and their compatibilities with polyamide 11 (PA 11) were investigated. DSC and FTIR studies revealed variable interfacial adhesion between blended compounds. They indicated that oligomers with short or medium long alkoxy substituents (5a−c) have adhesion to PA 11 while the interactions between oligomers of 4-hydroxybenzoic acid (5e) or 2-octadecanyloxy-4-hydroxybenzoic acid (5d) and PA 11 were poor. FTIR spectra proved that oligomeric structures could self-associate and affect on the final mechanical properties of the polyamide. Strength of PA 11 in a three point bending test was increased by addition of only 1% of LC oligomers 5a−e to the matrix.