Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

Summary: A new series of side‐chain liquid‐crystal polymers, the poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane], has been synthesized in which the spacer length is varied from 2 to 8 methylene units. The synthesis used for the chemical modification of polyepichlorohydrin (PECH) involved the phase transfer catalyzed etherification of the chloromethyl groups with sodium 4‐cyano‐4′‐biphenoxide and lithium n‐(4‐cyano‐4′‐oxybiphenyl)‐alkanoates. All the resulting polymers (except polymers 7 and 8), including that without spacer, characterized by ¹H NMR, and IR, exhibit thermotropic liquid crystalline mesomophism. The thermal behavior of the polymers has been characterized using differential scanning calorimetry (DSC) and polarized light microscopy (POM) equipped with hot‐stage. A more pronounced odd‐even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display slightly higher values, which were also dependent on the substitution degree of PECH. The flexible PECH chain assists nematic LC formation compared with other more rigid backbone polymers where a smectic phase is often encountered with the same mesogen. A comparison of the thermal properties of the cyanobiphenyl based series reported here. Polyacrylate (PA) and poly(methacrylate)‐based (PMA) SCLCPs containing 4‐cyanobiphenyl as the mesogenic unit are consistent with the general rule that increasing backbone flexibility for a given mesogenic unit and spacer length enhance the clearing temperature. This was not found from the PECH‐based series, which show the lower clearing temperature than the PMA and PA series, even though they have more flexible backbone than PA and PMA series. So the clearing temperature is not solely determined by the flexibility of the backbone.

Schlieren textures of PECHOC2‐B taken at 110 °C (cooling from isotropic phase after annealing 1 h).     

Self‐Assembly of a Hydrogen‐Bonded Association Chain Liquid Crystalline Polymer (LCP)

A hydrogen‐bonded association chain liquid crystalline polymer (LCP) based on tetraethyleneglycoxy‐bis(2,6‐dimethyl‐4‐benzoic acid) and 4,4′‐(p‐phenylenedi‐1,2‐ethenediyl)bipyridyl in a 1 : 1 stoichiometric ratio was synthesized using melting mixing method. X‐ray scattering from fiber drawn from its nematic phase exhibits preferred orientation, indicating that hydrogen bonding association chain polymer can also be strong enough to withstand the shear or tensile force and could be macroscopically oriented under shear or tensile force. Two complimentary components are likely to form a co‐crystallized structure to form a monoclinic crystal.     

State of Order of the Crosslinker in Main‐Chain Liquid Crystalline Elastomers

Rigid anisotropic crosslinkers have been shown to decrease the nematic order and the transition temperatures of main‐chain liquid crystalline elastomers (MCLCEs). In order to look into this phenomenon, the state of order of an anisotropic crosslinker in an MCLCE was investigated separately from that of the matrix. For this purpose, multifunctional perylene derivatives were synthesized and used as a crosslinker and as a reference mesogen probe. Their states of order were measured by their dichroism, and were compared to that of the MCLCE matrix. A systematically lower degree of order was observed for the crosslinker in comparison to the matrix, both when attached to and dissolved in the network.

     

Synthesis and Curing of Liquid‐Crystalline Epoxy Resins Containing a Biphenyl Mesogen

Summary: Liquid crystalline epoxies with a biphenyl mesogenic group have been widely studied although it is still in contention as to whether the diglycidyl ether of DGEBP shows a thermotropic phase. DEPBP was synthesized by partial oxidation and the liquid crystalline properties were investigated. Despite their similar structures their molecular weights are quite different. DEPBP has a distinct molecular weight, while DGEBP shows a molecular weight distribution. DEPEBP was also synthesized and characterized to investigate the influence of the flexible spacer. By incorporation of a flexible ethoxy group, the monomer shows no thermotropic property even in its curing reaction with an aromatic diamine, DDM. The curing rate was decelerated and the cured network showed a T_g even lower than that of cured DGEBA/DDM. The curing kinetics of DEPBP and DEPEBP were also tentatively investigated.

Arrhenius plot of DEPBP/DDM at α = 0.15.     

Liquid Crystals Embedded in Polymeric Electrolytes for Quasi‐Solid State Dye‐Sensitized Solar Cell Applications

A new series of liquid crystal embedded in polymeric electrolytes was developed for obtaining high efficiency in quasi‐solid state dye‐sensitized solar cells (DSSCs). The polymeric electrolytes were composed of iodide and tri‐iodide redox species in polyacrylonitrile (PAN) as a polymer matrix and liquid crystals (E7 or ML‐0249) for increasing the order parameter of electrolyte components with easy transport of redox species. The highest efficiency (6.21 and 6.29% at 1 sun) was obtained for the quasi‐solid state DSSCs using E7 to PAN and ML‐0249 to PAN, respectively, under AM 1.5 G illumination and an aperture mask condition. The high efficiencies of the quasi‐solid state DSSCs are due to the effective formation of pathways through liquid crystal orientation for the transport of redox species.

     

(Photo)crosslinkable Smectic LC Main‐Chain Polymers

This paper presents the synthetic route to SmA LC main‐chain polymers, that can be (photo)crosslinked without solvent in the bulk phase. They are based on soluble polymalonates, in which higher ordered phases can be suppressed by copolymerization with a laterally brominated biphenyl. Two routes were developed to incorporate the crosslinkable groups into the polyester backbone. The first consists in the incorporation of phenols into the polyester. These phenols are not reactive enough to participate in the transesterification reaction used to build up the polymer, but they can be esterified afterwards with acrylates. Thermally or photochemically created radicals then start the crosslinking. The second route is based on the incorporation of benzophenone as side group. It allows a photochemical crosslinking. Crosslinked fibers (monodomains) show the potential of the smectic LC main‐chain elastomers as actuators.

     

Synthesis of Liquid‐Crystalline Colloids in Nonpolar Media and their Manipulation in Electric Fields

The first synthesis of anisotropic liquid‐crystalline colloids in silicone oil by a direct (radical) polymerization of a monomer in THF/silicone oil mixtures with the help of siloxane containing stabilizers is described. The size of the colloids is in the lower µm range and can be adjusted by varying the mixture. The resulting colloids show a bipolar director configuration if they are small (<1.5 µm) and a radial configuration if they are larger. The colloids are sterically stabilized, and, due to the nonpolarity of the solvent, the disturbing effects of migrating ions are excluded and experiments in the electric field can be conducted. Both line formation in DC fields and a periodic switching of the bipolar colloids in AC fields is observed.

     

Directional Mechanochromic Luminescent Behavior in Liquid Crystalline Composite Polymeric Films

The doping of a liquid crystalline (LC) photoemissive compound into a LC polymethacrylate with photoemissive side groups induces repeatable changes in the photoemission wavelength upon mechanical grinding and thermal annealing. The color changes observed in a handwritten pattern demonstrate the successful induction of anisotropic photoemission behavior in a LC composite polymeric film parallel to the grinding direction. The LC characteristics of the composite films play an important role in the directional selectivity of the polarized mechanoinduced color change.

     

Directing the Smectic Layer Orientation by Shear Flow in Hierarchical Lamellar‐within‐lamellar Liquid Crystalline Diblock Copolymers

A self‐assembled lamellar‐within‐lamellar structure of a side chain liquid crystalline diblock copolymer was shear aligned to induce overall alignment and to direct the smectic layer orientation within the copolymer lamellae. The copolymer consisted of a polystyrene block and a poly(methyl methacrylate) block bearing cholesteryl mesogens with only short oxycarbonyloxyethyl spacers separating the mesogens from the backbone. Upon shearing, the copolymer lamellae exhibited uniaxial alignment whereas the smectic layers of the mesogens showed coexisting perpendicular and parallel orientations with respect to the copolymer lamellae. The fraction of the parallel oriented domains could be systematically increased by tuning the oscillation frequency and strain amplitude.

     

A Liquid‐Crystalline Co‐Polysiloxane with Asymmetric Bent Side Chains

A co‐polysiloxane has been prepared with an asymmetric bent side group based on a liquid‐crystalline monomer showing a B₂ (SmCP) phase. In order to dilute the side chain mesogens along the polymer backbone, octyl side chains were also incorporated. X‐ray studies confirmed that the resulting co‐polysiloxane exhibits a liquid‐crystalline layered phase, the siloxane polymer chains being microphase segregated from the incompatible side chain mesogens. The mesophase of polymer exhibits a homeotropic structure that can be successfully aligned by unidirectional shear. This phase does not exhibit polar switching under an applied electric field unlike the bent‐core monomer used as the side chains of this polymer. This can be caused by the homeotropic alignment of the polymer samples.

     

Molecular Shapes of Monosubstituted Polyacetylenes in their Liquid Crystalline Phases

In this short review, we discuss the molecular shapes of monosubstituted polyacetylenes (MSPAs) in their liquid crystalline (LC) phases. Their rigid backbone with high stereoregularity can endow MSPAs with significant shape persistency, which largely determines their LC structures. Helical cis‐cisoidal and cis‐transoidal MSPAs render rod‐like shapes and can form columnar LC (Φ) phases. In the smectic LC (Sm) phases of MSPAs with mesogenic side‐chains, the main chain, with trans‐cisoidal, trans‐transoidal or extended cis‐transoidal conformation, is in fact largely coplanar with the side‐chains, resulting in a sheet‐like molecular shape. From a simple geometric point of view, the rod‐ or sheet‐like MSPA molecules serve as the building block of their LC phases.

     

A Liquid Crystalline Elastomer with a p‐Pentaphenyl Transverse Rod Laterally Attached to the Main Chain

An LCE with a p‐pentaphenyl transverse rod in the main chain was synthesized, in which the rod can be oriented parallel or normal to the main chain under uniaxial tension. DSC and WAXD studies indicate a highly ordered lamellar structure typical of a smectic A phase. Stress‐strain curves showed a large Young modulus at small strains, followed by a yield point, at which necking occurred and the specimen became transparent. An interesting rigid‐soft‐rigid phenomenon was observed in the yield section, which is likely to indicate a self‐assembly‐driven reconstruction process. Two possible arrangements of transverse rods and chain extenders are proposed for the network structure.

     

X‐Ray Diffraction Studies on the Phase‐Transformational Behavior of a Smectic Liquid‐Crystalline Elastomer Composed of Chiral Mesogens

Structural changes through successive phase transformations of a chiral smectic liquid‐crystalline elastomer are investigated by X‐ray scattering technique. In uniaxially deformed elastomers, the smectic layer seemingly tilts even in the SmA phase, in which an in‐plane chevron structure formed in the tilted smectic phase. On the basis of an analysis of the layer reflection peaks, the layer correlation length in the tilted smectic phases is shorter than that in the non‐tilted SmA phase, though smectic layers in the tilted smectic phases are better ordered than those in SmA.

Experimental arrangement for X‐ray measurements of the uniaxially deformed elastomer in the tilted smectic phase at room temperature.     

Structure and Properties of Phosphorous‐Containing Thermotropic Liquid‐Crystalline Aliphatic–Aromatic Copolyesters

A series of thermotropic liquid crystalline aliphatic–aromatic copolyesters derived from various ratios of dodecane‐1,12‐diol ( 1 ), terephthaloyl bis‐(4‐oxybenzoyl‐chloride) ( 2 ), and 2‐(6‐oxido‐6H‐dibenz〈c,e〉〈1,2〉 oxaphosphorin‐6‐yl)‐1,4‐naphthalene diol ( 3 ), has been synthesized. The chemical structures of the monomers and polymers have been confirmed by elemental analyses, and FT‐IR and ¹H NMR spectroscopy. The mesomorphic properties of polymers have been investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the content of the aliphatic unit on the phase behavior of the polymers has been examined. The polymers that contain >30 mol‐% aliphatic diol showed smectic phases while the polymers that contained <30 mol‐% aliphatic diol displayed nematic phases. The polymers revealed a reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability. The degree of crystallinity increased upon increasing the content of aliphatic moieties. The char yield at high temperature increased by increasing the content of phosphorous‐containing bisphenol.

     

Cholesteric Liquid‐Crystalline Side‐on Polysiloxanes: Effects of Biaxiality on the Cholesteric Structure

Summary: Biaxial side‐on cholesteric copolysiloxanes with laterally attached mesogenic groups were investigated in mixtures with a low molar mass liquid crystal. While mixtures with a low concentration of the polymer exhibit the conventional fingerprint texture, for concentrations above about 60 mol‐% characteristic irregularities appear in the cholesteric structure that are obviously due to phase biaxiality. Besides the absence of pseudo‐isotropic lines, irregular patterns perpendicular to the helix axis emerge and the periodic distance of regions with similar optical properties along the helix axis is strongly disturbed. This suggests that the photonic band gap width of a uniaxial cholesteric phase becomes strongly affected when a phase transformation into a biaxial cholesteric phase occurs.

Cholesteric fingerprint texture of a mixture of uniaxial nematic monomer and biaxial cholesteric polymer.     

Synthesis and Liquid Crystalline Properties of Poly(ethylene oxide) Brushes with Amphiphilic Side Groups

Summary: Poly(ethylene oxide)s containing decylthiomethyl (DTP), decylsulfonylmethyl (DSP), methyl‐oligo(ethylene oxide) thiomethyl (MnOTP), methyl‐oligo(ethylene oxide) sulfonylmethyl (MnOSP), decyl‐oligo(ethylene oxide) thiomethyl (DnOTP), and decyl‐oligo(ethylene oxide) sulfonylmethyl (DnOSP) groups in the side‐chains, where n (the number of ethylene oxide units) = 1, 2, 3, or 4 for DnOTP and DnOSP, and n = 2, 3, or 7 for MnOTP and MnOSP, were synthesized to study the effect of the structure of the side‐chains on the ordered phases of the polymers. All of the MnOSPs, as well as M2OTP and M3OTP, were found to be amorphous polymers. DTP, M7OTP, D1OTP, D2OTP, and D3OTP showed crystalline phases below their isotropic temperatures. DSP, D1OSP, D2OSP, D3OSP, D4OSP, and D4OTP were found to be liquid crystalline. Therefore, the phase separation facilitated by the introduction of the sulfone group and/or by the addition of the longer ethylene oxide units between the ethylene oxide backbones and the hydrophobic decyl tails was found to be a key factor in the formation of the liquid crystalline phases. These ordered phases were characterized using differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction.

Schematic illustration of the structures of poly(ethylene oxide) brushes showing liquid crystalline phases.     

An H‐Bonded Main‐Chain Liquid‐Crystalline Polymer Obtained by In Situ Photochemical Conversion from an H‐Bonded LC Dimer

An H‐bonded main‐chain liquid‐crystalline (LC) polymer was obtained by in situ photochemical conversion from an H‐bonded LC dimer. A bifunctional compound, 1 , having a cinnamoyl group at one end and a carboxylic acid group at the other, was synthesized as the H‐bonded LC dimer. UV irradiation of 1 in the LC phase in the presence of a sensitizer resulted in its conversion to a photodimer, with a carboxylic acid at both ends, through photocycloaddition of the cinnamoyl moiety. The LC phase was maintained during irradiation, because that of the photodimer was more thermally stable than that of 1 . FT‐IR analysis revealed that the carboxylic acids of the photodimer dominantly formed hydrogen bonds in the LC phase, which suggests that the photodimers assembled into an H‐bonded main‐chain LC polymer.

     

Synthesis of New Side‐Chain Liquid‐Crystalline Polyoxetanes with Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain

The synthesis of a different geometry of side‐chain liquid‐crystalline polymers is reported where two mesogenic units connected by a flexible spacer are attached as pendent groups of a polymeric chain. The resulting structure is the following, where R₁, R₂ and R₃ are methylenic or oxymethylenic segments: The monomers were obtained by linking the dimer structure to an oxetane ring. These substituted oxetanes were polymerised by a boron trifluoride‐initiated cationic ring‐opening reaction to obtain the corresponding polymers. The polymerisation proceeds with high yields, in spite of the steric hindrance derived from the bulkiness of the side chain, and the presence of some functional groups in the molecule that compete against the cyclic ether for the nucleophilic attack at the propagating centre. A fraction of cyclic oligomer is obtained together with high molecular‐weight polyoxetane, as a result of back‐biting reactions during the polymerisation. Several systems with spacers of different length and parity were synthesised in order to analyse the influence of the nature of the spacer on the phase behaviour. The formation of mesophases was proved by means of differential scanning calorimetry, X‐ray diffraction and microscopic analysis. Most of the monomers form smectic structures at subambient temperatures. The polymers display a similar phase behaviour, although shifted to higher temperatures.     

Orientation of the Main Chain in Liquid‐Crystalline Side‐Chain Polyesters with Variation of Main‐Chain Spacer Length

Previous X‐ray investigations on liquid‐crystalline side chain polyesters with variable spacer length in the side chain as well as in the main chain led to the assumption that the structure will change qualitatively if the main chain spacer length exceeds a certain value. We obtained indications supporting this assumption by applying two‐dimensional ¹³C NMR. It was shown that for a sample with main chain spacer shorter than the critical length, the main chain segments align perpendicular to the side chains (〈P₂〉 = ?0.46), and for spacer length larger than critical length, the main chain segments preferentially orient themselves parallel to the side chain (〈P₂〉 = 0.28).

Visualization of the main‐chain order as revealed by the values of Table 2 . The opening angle of the cones corresponds to the average angle of deviation as defined in the text.     

Photocontrol of Birefringence and In‐Plane Molecular Orientation in Copolymer Liquid Crystal Films with 4‐Methoxyazobenzene and Photo‐Cross‐Linkable Side Groups

Control of the in‐plane orientation direction in a polymethacrylate copolymer liquid crystal film substituted with 4‐methoxyazobenzene (4MAz) and 4‐methoxycinnamoyloxybiphenyl (4MCB) side groups, and control of the reversible birefringence of the resultant oriented film are described. High‐order in‐plane molecular orientation was generated by irradiating with linearly polarized (LP) 365 nm light and subsequent annealing. By adjusting the exposure dose, the reorientation direction was controlled parallel and perpendicular to the electric vector ( E ) of the LP light. Cooperative reversion of the orientation direction for both the 4MAz and 4MCB groups was observed, and the orientational order, S, varied from ?0.79 to +0.75, without the side groups aggregating. The degree of the photoreaction and the distribution of the photoproducts from the 4MAz and 4MCB side groups through the axis‐selective photoreaction explain the reversion of the orientation direction. Optical recording, erasing, and birefringent control of the resultant oriented films were demonstrated by employing the photoisomerization of the 4MAz side groups using nonpolarised (NP) 405 nm light.

Order parameters of a P1 film after irradiating with LP 365 nm light (circles) and that following subsequent annealing (squares), as a function of exposure energy.