聚吡咯衍生物的合成及液晶性能

系统论述了新型导电功能性液晶聚合物3-和N-液晶基元取代聚吡咯的合成和液晶行为。指出通过化学氧化聚合、电化学氧化聚合和脱卤缩合聚合可以获得液晶性聚吡咯衍生物。它们均显示热致液晶行为，且多数呈现近晶液晶相，少数呈现向列液晶相，有些具有2种近晶相，有些具有单变液晶性。N-液晶基元取代聚吡咯比3-位取代聚吡咯具有较高的液晶稳定性。较长的亚甲基间隔和极性的介晶基团能够使N-取代聚吡咯具有较大的液晶微区和稳定的液晶相。N-取代液晶聚吡咯在摩擦力的作用下还可以诱发单轴取向。这种热致液晶性聚吡咯衍生物的研究成功有希望克服聚吡咯难以成型加工的巨大障碍。     

Conformational structure of liquid-crystalline polyesters with mesogens and spacers in the main chain

The conformations of thermotropic polyesters with alternating mesogens and poly(methylene) spacers in the main chain were investigated by conformational calculations using the data of linear-dichroism IR-spectroscopy, X-ray analysis and differential scanning calorimetry. The conformations of monomeric units and of the macromolecules as a whole in different phase-aggregated states were estimated. It is shown that in the liquid-crystalline (LC) state the polymer chains are strongly extended. The results are compared with the data of various theories of LC ordering.     

Conformation,ordering and mobility of macromolecules in aromatic polyesters as studied by IR and solid-state NMR spectroscopy

Thermotropic main-chain LC polyesters have been studied with the aid of IR and NMR spectroscopy. Rigid mesogens and mesogens with a possibility of conformational isomerism were examined. The dependence of the order parameters of the mesogens and spacers on the chemical structure of the polymer has been determined. The IR spectroscopy data and the data of the calculation of the possible conformations of the mesogen and the spacer confirm the theoretical predictions about the straightening of macromolecules in the anisotropic melt. Solid-state ²H NMR spectra show that p-oxybenzoic rings adjacent to a flexible spacer are more mobile than inner terphthalic rings. In decamethylene spacer the contrary holds true: the mobility of inner methylene groups is more marked than that of the α-methylene groups bound to the mesogen.     

X-ray diffraction study of smectic A layering in terminally fluorinated liquid crystal materials

We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate proportional chambers. It was found that in contrast to -CN or -NO₂ terminated smectics, the polar -OCF₃ compounds form only a monolayer smectic A₁ phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorinated chains was found to be two orders of magnitude higher than is commonly reported for low molar mass thermotropic mesogens, indicating deviations of the density distribution function p(z) from a pure sinusoidal form. The layer periodicity d for these mesogens exceeds the molecular length L: d/L ≃ 1·1, which corresponds to a smectic A_d phase consisting of parallel or antiparallel dimers. We have observed that lateral fluorine substitution in the benzene ring adjacent to the perfluorinated chain leads to the disturbance of uniform smectic A layering and to the formation of a defect-modulated smectic A structure of a chequer-board type. For polyphilic compounds containing both hydrogenous and perfluorinated units in the terminal chain, the in-plane structure factor displays double-peaked liquid-like profiles indicating the existence of nearest-neighbour molecular stacking at different distances. The peculiarities of smectic A layering in fluorinated mesogens are discussed in terms of steric coupling and enhanced conformational rigidity of fluorine containing moieties.     

On the liquid-crystalline properties of methacrylic polymers containing 4′-(4-alkyloxyphenyl)azobenzene mesogens

In this article, we report on the synthesis and thermotropic behaviour of methacrylic polymers containing 4′-(4-alkyloxyphenyl)azobenzene mesogens attached to the backbone through n-alkyloxy spacers of 6 or 10 methylene groups. Polymerisations were carried out via free radicals using azobisisobutyronitrile (AIBN) as initiator. Chemical structures of polymers and their precursors were characterised by ¹H NMR spectroscopy. Thermogravimetric analysis showed that azopolymers are thermally stable up to temperatures around 300°C. The thermotropic liquid-crystalline (LC) behaviour was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). Results indicate that all synthesised mesogens, monomers and polymers developed two or more orthogonal LC phases in wide temperature ranges. Mesogens and monomers developed nematic and smectic phases, whereas polymers exhibited only smectic phase. In polymers, the arrangement of mesogens depends on the relative length of the spacer and the terminal flexible chain; two distinct structural models were proposed based on chemical interactions and steric constrains. The trans–cis photoisomerisation of monomers and polymers in solution was also studied. High cis-isomer contents (>80%) were reached in relative short irradiation times despite the steric constrains imposed by the polymer backbone.     

Influence of a liquid crystalline block on the microdomain structure of block copolymers

We report on the phase behavior and microdomain structure of two types of diblock copolymers containing a liquid crystal (LC) block joined to a flexible coil block. Consideration of the symmetry groups of the liquid crystalline phases and of the block copolymer microdomain structures provides a rationale for predicting the possible types of liquid crystalline block copolymer morphologies. Both previously reported and newly discovered structural types are identified. Possible organizational schemes are developed for the mesogens and periodic disclination defects with respect to the intermaterial dividing surfaces separating the liquid crystalline and flexible coil domains. The first type of copolymer investigated has a rod-like LC block whereas the second type copolymer has a side chain LC block. Five different rod-coil diblocks based on poly(hexyl isocyanate-b-styrene) P(HIC-b-S) were synthesized by anionic polymerization. Wavy lamellae, zig-zag and arrowhead microdomain morphologies corresponding to smectic-C and smectic-O structures were observed depending on the composition. These layered phases have the director (PHIC chain axis) tilted at various orientations with respect to the layer normal. Side-chain LC diblocks based on functionalized poly(isoprene-b-styrene) P(I-b-S) were also investigated. These polymers were synthesized using polymer analogous chemistry from P(I-b-S) precursors. Three different mesogenic groups were attached to the PI blocks: one based on biphenyl benzoate and two based on azobenzene. The microdomain structures found for the functionalized poly(isoprene side-chain LC-b-styrene) P(ILC-b-S) diblocks are typical of traditional coil-coil diblocks (lamellae and cylinders). However, these morphologies possess an additional smectic layering of the mesogens within the microdomains of the LC block. In the case of the rod-coil diblocks, the transformation from an initially isotropic state to the final microphase separated solid state occurs via nematic and then smectic liquid crystalline states, whereas for the side-chain LC-coil cases, the microphase separation transition occurs prior to development of orientational order. The long-range microdomain order of LC block-coil block copolymers can extend over very large distances due to the influence of the orientational ordering of the LC block.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

SYNTHESIS AND CHARACTERIZATION OF β-DIKETONE BASED SIDE CHAIN LIQUID CRYSTALLINE POLYSILOXANES

A new type of β-diketone based side chain liquid crystalline polysiloxanes (DKLCP) with different length of flexible spacers and end groups have been synthesized by hydrosilation reaction. This is liquid crystal polymers (LCP) using coordinating β-diketone ligand as mesogens. The phase behaviour of DKLCP polymers was studied by differential scanning calorimetry and polarizing microscopy. X-ray diffraction investigations demonstrated that the polysiloxanes with sufficiently long flexible spacers were smectic liquid crystal polymers, while those with much shorter spacers were nematic ones.     

Effect of the chemical constitution of the side-chain on the formation and the structure of mesophases in some polysiloxanes

The synthesis and properties are described for a series of new liquid crystal side-chain polymethylsiloxanes with the same mesogenic rigid core but different substitutents or flexible spacers. The effect of the chemical structure of these polymers upon the polymorphism is studied by means of optical, DSC and X-ray experiments. By comparison with conventional mesogens, the partially bilayered structure of the smectic A phases (i.e. when the layer spacing is between once and twice the length of the side-chain) could be explained, taking into account the asymmetric character of the side-chains.     

Thermal properties of non-symmetric bibenzoate liquid crystalline dimers

The liquid crystal behaviour of a family of non-symmetric liquid crystalline dimers is reported. These systems contain two bibenzoate rigid units that are linked to distinct terminal groups at one end, and to a flexible interconnecting spacer at the other. Several systems having different terminal and central chains are studied using calorimetric, microscopic and diffraction techniques. All the samples form phases with variable degrees of order (from low ordered smectic to crystalline phases) depending on the chemical constitution of the different segments. The influence of the length, parity and lateral substitution of the spacers on the transitional properties and the symmetry of the mesophases that are formed is analysed. It is found that a decrease in the transition temperatures and enthalpies occurs when the length of the flexible spacers increases, when lateral methyl substituents are introduced, or when the parity of the central spacer changes from an even to odd number of carbon atoms or ether groups. The arrangement of the mesogens and dissimilar flexible groups within the ordered structure is discussed with respect to the observed L/d ratios. Different values were obtained depending on the parity of the central spacer and on the degree of order. Interpenetrated structures, in which the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when constructing crystalline phases.     

Chiral side-chain liquid crystalline polysiloxanes bearing fluorinated mesogens and cholesteryl groups

A series of chiral side-chain liquid crystalline (LC) polysiloxanes bearing fluorinated mesogens were synthesized with a cholesteric LC monomer and a fluorinated nematic LC monomer. They were characterized by use of various experimental techniques, and effect of fluorinated mesogens on characteristic of LC polysiloxanes was studied as well. In photoluminescence spectra, a narrow and a broad peak occur at around 270-317 nm, originated, respectively, from fluorinated phenyl groups and the conjugated xenene structure. The specific rotation analysis of all polymers showed negative values, but absolute values were lower than those of the chiral monomers. All polymers showed smectic LC phase with very wide temperature ranges on heating and cooling cycles. Especially, only polymers bearing more fluorinated component exhibited smectic-cholesteric phase transition when they were heated. As the polymers contained more fluorinated mesogens, segregation of the fluorinated segment to the surface should occur at mesomorphic temperature. The highly ordered lamellar mesogen-siloxane matrix systems should be disturbed severely by separation of fluorinated mesogens, suggesting mesogenic orders transition from lamellar smectic to cholesteric phase.     

New side chain cholesterol-functionalised aliphatic polycarbonate copolymer: synthesis and phase behaviour

A new side cholesterol-functionalised liquid crystal (LC) copolymer based on aliphatic polycarbonate backbone was synthesised. The chemical structures of the block copolymers obtained in this study were characterised with Fourier Transform Infrared Spectroscopy (FT-IR) and Proton Nuclear Magnetic Resonance (¹H NMR) spectra. Their thermal stability and phase behaviours were investigated with thermogravimetric analysis (TGA) measurements, differential scanning calorimetry, and polarising optical microscopy. The molecular organisation in the mesophase was studied by temperature-dependent X-ray diffraction (XRD). As a result, the block copolymer bearing side cholesteryl groups showed a glass transition at 15.8°C and a smectic A (SmA) to isotropic phase transition at 151.3°C on heating cycle. XRD indicated that the block LC copolymer showed an interdigitated molecular arrangement of the mesogenic units within the smectic layers. This partial bilayer structure was similar to the SmA phase formed by polar mesogens.     

Synthesis and liquid crystalline properties of novel laterally connected twins

Several laterally connected twin mesogens incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy and differential scanning calorimetry. If one compares the twins with their respective halves, the mesophase stabilizing influence of the lateral fixation becomes evident. It obviously pre-organizes the calamitic units and therefore stabilizes smectic phases. The mesophase behaviour of these twins depends strongly on the length and the structure of the spacer. Short as well as highly polar and rigid spacer units give rise to smectic phases with high transition temperatures. By increasing the spacer length the mesophase stability is drastically diminished. Dimesogenic 4,4'-didecyloxyterphenyl derivatives containing spacers consisting of two connecting atoms exhibit smectic C phases exclusively. Elongation of the spacer to three connecting atoms gives an S_A-S_c dimorphism, and in the case of compounds with longer spacers mostly the S_A phase and in one case the nematic phase were found.     

热致型液晶聚合物的近晶相结构研究

本文借助于不同热处理样品的WAXD研究和变温FTIR分析,从液晶态与结晶态面间距的变化,晶态、液晶态和各向同性液态下结构构象的差异出发,研究了聚合物液晶态下的近晶相结构模型。     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.     

In situ photochemical conversion from cinnamoyl-functionalized liquid-crystalline monomers to liquid-crystalline dimers

Liquid-crystalline (LC) monomers, which were functionalized with a cinnamoyl group on their extremity, were synthesized and irradiated with UV light in their LC phases. In the presence of a triplet sensitizer, most LC monomers were converted into the corresponding dimers, which were produced by the cycloaddition reaction of the cinnamoyl group. The photodimerization reaction could proceed while the LC phases were maintained, because the dimers showed LC phases whose temperature ranges were wider than those of the corresponding monomers. A ¹H NMR study of the LC dimers indicated that the cyclobutane unit dominantly had an anti-head-to-head configuration, that is, δ-truxinate. As the LC monomers, which had a phenyl biphenyl-4-carboxylate moiety as a mesogen, showed smectic A phases and the corresponding dimers also exhibited smectic A phases, we estimated the smectic layer distances by X-ray diffraction analysis and found that the dimers adopted the structure in which the two mesogens aligned laterally and existed in the same smectic layer in the LC phases.     

Investigation of molecular order and phase transitions of smectic poly(ester imide)s by means of small-angle X-ray scattering

The molecular order and phase transitions of two smectic poly(ester imide)s based on aminobenzoic acid trimellitimide (PEI 1) or aminocinnamic acid trimellitimide (PEI 2) and α,ω-dihydroxydodecane were investigated by X-ray scattering. During cooling, the polymers pass through monotropic smectic liquid-crystalline (LC) phases (S_A, S_C), which transform into higher-ordered smectic-crystalline phases (S_E, S_H). The smectic layer structure of about 3 nm gives rise to a sharp reflection at 2θ ≅ 3°. Peak shape analysis and analysis of the interface distribution function revealed long-range longitudinal correlation among the mesogens in the LC phase but short-range lateral correlation. The development of a broad reflection in the small-angle X-ray scattering (SAXS, 2θ < 1°) indicates the formation of a lamellar two-phase system. The long-period changes reversibly between 10 and 30 nm with increasing temperature. The crystalline lamellae comprise a number of smectic-crystalline layers with packed mesogens, while the noncrystalline interlamellar regions keep their smectic-LC order. In the metastable S_B phase, formed during annealing of quenched PEI 1, the diffuse SAXS indicates a random distribution of small, probably fringed, crystals with hexagonal-packed mesogens. In the lamellar S_E and S_H phases, tie molecules play an important role, but chain folding cannot be excluded. Received: 16 July 1999/Accepted: 28 April 2000     

Liquid crystalline compounds having a tribranched structure: substituted malonic esters consisting of two Schiff's base units and a cholesteryloxyalkyl substituent

New tribranched thermotropic liquid crystal compounds were synthesized and their liquid crystalline properties studied by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The compounds are the bis-{10-[4-(4-alkylphenyliminomethynyl)phenoxy]decyl} 2-[6-(cholesteryloxy)hexyl]malonates and the corresponding alkoxy derivatives. These compounds contain three mesogenic units, two identical Schiff's base type mesogens and one cholesteryl either moiety, interconnected in a tribranched structure via spacers. The cholesterly moiety is attached to the malonic acid core through an oxyhexamethylene spacer while the two Schiff's-base moieties are attached through oxydecamethylene spacers. The terminal alkyl group of the Schiff's base unit is either a butyl or decyl group, and the alkoxy terminal group is either a butoxy or decyloxy chain. All the compounds form only an enantiotropic smectic phase, most probably of the smectic C type. The larger spacings determined by small angle X-ray diffraction range from 3.3 to 4.1 nm, which are much shorter than the end-to-end distance (5.9-7.4 nm) of the molecules estimated using molecular models assuming an all trans extended conformation for all the alkyl spacers.     

Synthesis and characterization of a range of heterocyclic liquid crystalline materials incorporating the novel thiophene-pyrimidine moiety

In this structure/property study six 2-(4-alkyloxyphenyl)-5-thiophen-2-ylpyrimidines and thirty-six 2-(4-alkyloxyphenyl)-5-(5-alkylthiophen-2-yl) pyrimidines were synthesized to evaluate the effect of the introduction of heterocyclic rings on the liquid crystalline (LC) behaviour of mesogens. The 'bent' nature of the structure of the 2-(4-alkyloxyphenyl)-5-thiophen-2-yl-pyrimidines, due to the presence of the thiophene ring, was further exaggerated in the 2-(4-alkyloxyphenyl)-5-(5-alkylthiophen-2-yl) pyrimidines by the incorporation of an alkyl group at the 5-position in the thiophene ring. This simple change in structure led to a remarkable change in the LC behaviour of the mesogens. From a series of mesogens having high melting points and exhibiting monotropic nematic phases, the incorporation of the alkyl group gave a series of three-ring mesogens with relatively low melting points (60-80°C), and exhibiting a range of tilted smectic phases, including SmI, SmC and SmC alt phases.