Solvent Manipulation of Liquid Crystal Order and Other Properties in Azo‐Containing Surfactomesogen/Poly(styrene sulfonate) Complexes

Stoichiometric ionically bonded side‐chain polymer complexes of poly(styrene sulfonate) and quaternary ammonium‐functionalised azo‐containing surfactomesogens were exposed to controlled humidity atmosphere, to organic non‐solvents (acetone, THF, ethyl acetate) and to a good solvent (DMF) for the complex. This modifies or increases the packing order, and includes induction of a crystal‐like lamellar structure that is amenable to photoisotropisation. The molecular packing order influences photoinduced birefringence relaxation. Conditions for frozen‐in morphologies are discussed in the light of higher temperature data. The observed diversity of phase changes upon solvent exposure is comparable to what is observed in surfactant–polyelectrolyte complexes.

     

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.     

Synthesis and Characterization of Supramolecular Polymeric Materials Containing Azopyridine Units

Summary: The synthesis and characterization of a series of supramolecular polymeric complexes formed by H‐bonding interactions between benzoic acid and azopyridine derivatives are described. A series of polymeric networks have been synthesized using a polymethacrylate bearing benzoic acid units as side groups, and several polymers with azopyridine as H‐acceptor side groups. Furthermore, low‐molecular‐weight pyridine derivatives have been used to prepare an homologous side‐chain polymer, and a network with azopyridine as a non‐covalent crosslinker. Special attention was paid to the thermal and mesomorphic properties of these materials, which were studied by DSC, POM, and XRD.

Micrograph of the mesomorphic melt of a sample taken at 130 °C on cooling from the isotropic state.     

Tailoring Linear and Nonlinear Optical Properties of a Side‐Chain Liquid Crystalline Azo‐Polymethacrylate

The molecular orientation in a side‐chain liquid crystalline azopolymer with push–pull chromophores ( Pol‐PZ‐CN ) is studied by UV‐vis spectroscopy, refractive index and second harmonic generation measurements. Fresh films show homeotropic arrangement, but irradiation with polarized light induces “in‐plane” birefringence, which is enhanced by subsequent thermal annealing up to values as high as 0.34. The nonlinear response of Corona poled films with “in‐plane” anisotropy is governed by three independent NLO d_ij coefficients, evidencing symmetry different from the usual (C_∞v). For irradiated films, stable values d₃₁ = 1.4, d₃₂ = 0.9 and d₃₃ = 11 pm·V^?1 are measured at 1.9 µm. A noticeable higher d₃₁/d₃₂ ratio is obtained for preirradiated and annealed films. The proposed two‐step method consisting of light irradiation and heating is an effective strategy to tailor the polar molecular orientation of nonlinear liquid crystalline azopolymers.

     

Effect of Molecular Structure and Thermal Treatment on Photo‐optical Properties of Photochromic Azobenzene‐containing Polymer Films

Three novel photochromic azobenzene‐containing comb‐shaped polyacrylates are synthesized and their phase behavior and photo‐optical properties are studied. The influence of the side photochromic group structure, thermal treatment, and light irradiation on aggregation of azobenzene chromophores in thin spin‐coated films of the polymers is investigated in detail. Special attention is paid to studying the photo‐orientation processes in polymers films induced by polarized blue light (473 nm). The relationship between the photochromic group architecture, phase behavior, thermal treatment of films, and kinetics of chromophore photo‐orientation is established and discussed. It is found that the position of the N?N bond in chromophores plays an important role in the kinetics of the process, but does not affect the maximum value of dichroism.

     

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.

     

Coatable Photomobile Polymer Films Using Spring‐Like Photochromic Compounds

Photoinduced macroscopic deformation behavior of crosslinked liquid‐crystalline polymer is induced in crosslinked N‐benzylideneaniline polymer films. The film is prepared by in situ polymer reaction using polymethacrylate‐containing phenylaldehyde side chains and o‐tolidine crosslinker. The film exhibits macroscopic bending upon exposure to UV light and then immediately reverts to the initial shape when the light is turned off. The bending behavior is strongly affected by the fabrication conditions; the changes in the macroscopic shape and photomechanical properties in response to UV light for the film prepared from a mixture solution are small, while those for the film prepared by top coating the crosslinker solution onto the prepolymer substrate are large.     

Electromechanics of a Chiral Smectic C Elastomer: Measurement of Complex Piezoelectric Constant through Successive Phase Transformations

Summary: Piezoelectric properties of a chiral smectic liquid crystalline elastomer are investigated by means of dynamic electromechanical measurements. We have confirmed that the piezoelectric constant is fairly large in the chiral smectic C (SmC*) phase. A relaxation of the piezoelectric constant is observed in the frequency dispersion curve of a director orientated elastomer obtained by a uniaxial deformation. The temperature dependence of the relaxation frequency in the smectic phases is nicely fitted to the Vogel‐Fulcher‐Tammann law, because the dynamics observed here is closely related to the glass transition phenomena. A resonance instead of a relaxation is recognized in the case of a biaxial deformed elastomer in which both layer and director are oriented to their preferred directions. In addition, the piezoelectric constant of the biaxially deformed elastomer possessing a macroscopically spontaneous polarization is smaller than that of the uniaxially deformed elastomer with no net spontaneous polarization. We discuss the mechanism of polarization generation in the electromechanical response of the SmC* elastomers, and also consider the emergence of the resonance in the biaxial deformed elastomer.

Geometry of sample deformation.     

New Siloxane‐Organic Polyesters with Azobenzene Side Chains. Synthesis,Thermotropic Behavior and Surface Properties

Summary: New siloxane‐organic polyesters, bearing azo‐type units in the side chain, were synthesized by direct solution polycondensation at room temperature, starting from 2,2′‐[4‐[(4‐nitrophenyl)azo]phenyl]iminobisethanol (Disperse Red 19) and different siloxane diacids, using dicyclohexylcarbodiimide as an activator and 4‐(N,N‐dimethylamino)pyridinium‐p‐toluenesulfonate as the catalyst. The structure of the obtained polyesters was confirmed by ¹H and ¹³C NMR as well as IR spectroscopy. The molecular weight and polydispersity were estimated by GPC. The thermotropic behavior was investigated by POM, DSC and TOA. The surface composition was determined by ESCA (XPS) analysis and proved enrichment in silicon for all samples. All compounds show good solubility in common organic solvents.

An example of a nitroazobenzene‐containing side‐chain polyester with various siloxane‐organic backbones.     

Synthesis of Laser‐Alignable Liquid Crystalline Conducting Polymers

Summary: The synthetic routes to three series of liquid crystal conducting polymers (based on pyrrole, thiophene, and aniline monomers) are reported and the optimum conditions for polymer preparation are described. These polymers show increased conductivity when laser‐aligned, the greatest effect being shown by the N‐substituted pyrrole‐based system. Information on their liquid crystal and spectroscopic properties and other characteristics are also included.

Structure of the monomers M1, M2, and M3.     

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.     

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.

     

Self‐Assembly Behavior and Optical Properties of Poly(3‐thiopheneacetate)/Dialkyldimethylammonium Complexes

Poly(3‐thiopheneacetate)/dialkyldimethylammonium complexes (PTA‐Cn) were prepared by mixing regiorandom poly(3‐thiopheneacetic acid) with dialkyldimethylammonium bromide to study the effect of their self‐assembled structures on the optical properties. The lengths of the alkyl groups in the ammonium salts were varied from decyl (C10) to octadecyl (C18) groups. These complexes showed fully developed layer structures with ordered mesophases. PTA‐C16 and PTA‐C18 with longer alkyl groups showed side‐chain crystalline phases, and PTA‐C10, PTA‐C12, and PTA‐C14 with shorter alkyl groups showed side‐chain crystalline and liquid crystalline phases at room temperature. When PTA‐C18 was heated, a melting transition from side‐chain crystalline to smectic liquid crystalline phase was observed at 41.5 °C. The maximum absorption and emission wavelengths of these complexes increased with increasing alkyl chain length indicating that the π‐conjugated structure of the regiorandom thiophene backbone changes according to the alkyl groups.

     

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).     

Investigation of the Effect of an Ether Moiety on the Liquid Crystallinity by Thin Film Polymerization

The effect of an ether moiety on liquid crystallinity was determined by means of in situ thin film polymerization under a polarizing microscope with a heating stage. The polymerization system consists of p‐acetoxybenzoic acid (ABA)/acetoxyacetanilide (AAA)/4,4′‐oxybis(benzoic acid) (OBA). Because of the swivel structure introduced by the ether group in the OBA monomeric unit, there is a critical composition to form the mesogenic phase during the polymerization. Liquid crystallinity forms when the concentration of the OBA is lower than 25 mol‐%. Surprisingly, both LC phase and crystalline phase appear simultaneously in the isotropic phase if the reaction system is at the critical composition. In addition, we have observed the disclination strengths change from ±1 to ±1/2 with an increase in ether linkages in the polymerization systems where LC phases appear, which is quite different from the homopolymerization system containing rigid rod‐like p‐acetoxybenzoic acid (ABA) and other systems without ether linkages where their disclinations have strengths of ±1. Clearly, the chain structure not only determines the formation of liquid crystallinity but also affects the strength of disclinations and the annihilation process.     

(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.

     

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.

     

Molecular dynamics of a series of nematic polyesters

The molecular dynamics of a family of nematic main chain liquid crystalline polyesters with methylenic flexible spacers of different lengths has been studied by means of dielectric and dynamic mechanical spectroscopies. All polymers show three different relaxational processes: the dynamic glass transition (α‐relaxation) and two secondary relaxations attributed to reorientations of the dipoles located at the mesogenic group (β‐relaxation) and at the ends of the methylenic spacer (γ‐relaxation). The influence of the number of methylenic units in the spacer on the parameters of the relaxations was analyzed. The α and β processes appear at lower temperatures when length of the spacer increases, whereas the γ process seems to be independent on the spacer length. The possible cooperative character of the β process was studied based on the values of its Arrhenius prefactor, the activation energy and the dielectric relaxation strength. The cooperativity tends to increase when increasing the number of methylenic units in the spacer.