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.     

(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.

     

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.     

Liquid‐Crystalline Complexes of Polyurethane Containing an Isonicotinamide Moiety with 4‐Dodecyloxybenzoic Acid

A series of supramolecular side‐chain liquid crystalline polyurethanes (SCLCPUs) was prepared by utilizing the selective hydrogen bonding interaction between polyurethane containing pyridine moieties ( 1 ) and 4‐dodecyloxybenzoic acid ( 2 ). Hydrogen bonding was confirmed by FTIR spectroscopy and supported by medium high level ab‐initio MO calculations. The components are miscible up to 0.7 mole of 2 versus the polyurethane repeat unit and the resulting complexes behave as uniform liquid‐crystalline polymers, which exhibit stable, reversible mesophases. The liquid crystalline behavior of these supramolecular polymeric complexes was established by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X‐ray diffractometry (XRD). The complexes show a highly ordered smectic phase, which does not appear for either of components 1 and 2 separately , and a nematic phase. Above the concentration limit of 0.7 mole of the acid 2 , the excess low molar mass molecules emerge to form a two phase system consisting of a polyurethane supramolecular complex and 4‐dodecyloxybenzoic acid ( 2 ). For complexes derived from polyurethane 1 and 4‐octyloxybenzoic acid ( 3 ) or 4‐butyloxybenzoic acid ( 4 ) with alkyl tails shorter than 2 , the phase separation occurs at lower molar ratios of 3 and 4 versus 1 . This means that the length of the alkyl tail in 4‐(alkoxy)benzoic acids plays an important role in the stabilization of supramolecular polyurethane complexes.     

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.

     

Ultrasonics Sonochemistry Assisted Preparation of Polysiloxane Main‐Chain Liquid‐Crystalline Elastomers

For the first time, an ultrasonics sonochemistry method is developed to promote the one‐pot hydrosilylation polyaddition polymerization and crosslinking reaction in the preparation of polysiloxane main‐chain liquid‐crystalline elastomers (MC‐LCEs). Due to the extraordinary effect of acoustic cavitation, the polyaddition polymerization and crosslinking reaction can be successfully carried out in an ordinary laboratory ultrasonic cleaner at room temperature, and generates the LCE matrix network in about 30 min. The prepared MC‐LCEs demonstrate good quality, good properties, and stimuli‐actuation performances. Compared to the traditional thermal processing methods for preparing polysiloxane MC‐LCEs, this method exhibits superior properties rapidly, with high convenience and efficiency, and can be a path for batch fabrication at low cost. The work also demonstrates that the ultrasonics sonochemistry method is effective in generating linear main‐chain liquid crystal polymers through hydrosilylation polyaddition and polysiloxane side‐chain LCE matrix through hydrosilylation crosslinking reaction, thus confirming the high availability of ultrasonics sonochemistry in the processes of hydrosilylation polymerization, crosslinking reactions, or synchronous polymerization and crosslinking reactions.     

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.

     

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.     

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.

     

Liquid‐Crystalline Polymethacrylates by Atom‐Transfer Radical Polymerization at Ambient Temperature

Side‐on and side‐end liquid‐crystalline (LC) polymethacrylates were synthesized by atom‐transfer radical polymerization at 20°C using monofunctional and difunctional initiators. The polymers have narrow molecular‐weight distributions (M̄_w /M̄_n = 1.15–1.45). The polymerization kinetics were determined for a side‐on LC methacrylate, and appear to be first‐order, whatever the initiator used. However, the measured molecular weight is much larger than expected, probably because the initiation step is slow. The thermotropic properties of the LC polymers were studied by thermal optical polarizing microscopy, differential scanning calorimetry, and X‐ray diffraction. For a side‐end LC polymethacrylate, two LC phases were detected although only one has been reported in the literature. The phase sequence was shown to be Cr–SmA–N–Iso.     

Two‐Step Smectic CA Phase Formation from Isotropic Liquid upon Supercooling in Main‐Chain Liquid‐Crystalline BB‐5(1‐Me) Polyester

The mesophase formation process of a main‐chain BB‐5(1‐Me) polyester in the supercooled isotropic‐liquid state has been studied using DSC, WAXD, and optical microscopy. The supercooled isotropic liquid follows two different SmCA formation processes. Above 85 °C, it forms the expected SmCA phase. Below, it yields a metastable SmA phase that displays a layer spacing almost twice the one of the SmCA phase. The SmA phase develops from a homogeneous nucleation followed by 3D growth and thereafter transforms to the SmCA phase. This two‐step SmCA formation involves a change in polymer chain configuration: first, the chains are folded in each two units to form the SmA phase and then they stretch to achieve the characteristic zigzag arrangement of the mesogens in the SmCA phase.

     

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.     

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.

     

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.

     

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.     

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.

     

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.     

Functional Photochromic Methylhydrosiloxane‐Based Side‐Chain Liquid‐Crystalline Polymers

A new azobenzene‐group‐containing monomer and several respective functional side‐chain polymers grafted on a methylhydrosiloxane backbone (with two different degrees of poly­merization; with and without the addition of a photoreactive benzophenone derivative) are designed, synthesized, and characterized. The resulting materials clearly show self‐assembly behavior and possess a nematic liquid‐crystal phase over a broad temperature range, which extends below 0 °C. The optical properties of these new photo­chromic liquid‐crystalline materials are determined from the absorbance spectra of oriented samples and by photo­induced birefringence studies. The results indicate a considerable dichroism of the side‐chain liquid‐crystalline polymers (SCLCPs), and hence demonstrate their potential applicability for optical storage.

     

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.     

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.