Liquid Crystalline Elastomers

Today, material science is directed towards the development of multifunctional and oriented structures. One example of such supramolecular systems are liquid crystalline (LC) elastomers which combine the properties of LC phase (the combination of order and mobility) with rubber elasticity, one of the most typical polymer properties. Their most outstanding characteristic is their mechanical orientability; strains as small as 20% are enough to obtain a perfectly oriented LC monodomain. This orientability, if LC elastromers with chiral phases are used, leads, for example, to elastomers with chiral smectic C* phases which are likely to show piezo-electric behavior.     

Polymeric liquid crystals: structural basis for ferroelectric and nonlinear optical properties

This paper describes the rational design and structure–property relations in three different types of polar LC polymers with interesting material properties, as follows. (i) Chiral LC polymers, which are functionalized with crosslinkable groups, can be converted into LC elastomers with chiral smectic C^* phases. The mechanical orientability of these elastomers leads to new piezoelectric materials. (ii) The curing (dense crosslinking) of a polymer matrix provides one possibility of stabilizing the polar order of dye molecules, which is necessary for frequency doubling. Additionally, LC phases can help to stabilize this polar structure, which leads to large and stable nonlinear optical coefficients. (iii) Polymer analogous esterifications offer a convenient method for the synthesis of chiral smectic C^* polymers with large ferrolectric polarizations.     

Orientability of the Minor Director of Homeotropically Aligned Smectic‐A Elastomers in External Mechanical Fields

We present studies on bulk smectic‐A copolymer networks with end‐on attached homeotropically oriented mesogens that show spontaneous optical biaxiality at room temperature. Orthoscopic and conoscopic investigations under uniaxial extension in the layer planes give first evidence of the orientability of the minor director in mechanical fields yielding biaxial monodomains with 3‐d orientational long‐range order of all three principle axes. This is an important step towards the synthesis of permanently oriented biaxial monodomain elastomers for which highly interesting mechanical and optical properties are expected.     

Liquid crystalline thermosets based on anisotropic phases of cellulose nanocrystals

A new class of liquid crystalline thermosets (LCTs) was successfully produced containing lyotropic cellulose nanocrystals (CNCs) as the primary mesogenic component (up to 72 wt%) by the addition of non-mesogenic epoxy monomers. Cellulose-based LCTs were produced by totally aqueous processing methods and ultimately cured at elevated temperatures to produce ordered networks of ‘frozen’ liquid crystalline (LC) phases. Various degrees of birefringence were obtained via self-assembly of CNCs into oriented phases as observed by polarized optical microscopy and transmission electron microscopy. X-ray diffraction measurements highlighted the effects of texture of CNCs within LCT films compared to lyophilized CNCs. Cellulose-based LCT films uniquely exhibited thermo-mechanical properties of both traditional LCTs and LC elastomers, such as high elastic modulus (~1 GPa) under ambient conditions and low glass transition temperature (~?25 °C), respectively. The development of LCTs based on CNCs and aqueous processing methods provides a renewable pathway for designing high performance composites with ordered network structures and unique optical properties.     

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.     

Differences between smectic homo‐ and co‐polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC‐homo‐ and LC‐co‐polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo‐ and co‐polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC‐elastomers formed from homo‐ and co‐polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Anisotropic networks, elastomers and gels

Anisotropic networks, elastomers and gels exhibit piezoelectric, pyroelectric, ferroelectric and NLO properties of potential interest for use communication and processing technologies. The formation, properties and applications of such anisotropic, mainly liquid crystalline, networks are described. If some of the molecules in a liquid mixture contain at least two reactive groups which can be either photochemically or thermally polymerized, then crosslinked, anisotropic networks, elastomers and gels can be produced. Solid macroscopically aligned elastomers or networks can be formed as required beforehand or simultaneously by orientation of the sample. Anisotropic gels consist of a solid anisotropic network and non-covalently bonded, but strongly oriented domains of low molar mass liquid crystals. Anisotropic networks, elastomers preformed amorphous or liquid crystalline polymers incorporating additional reactive groups, which can be macroscopically oriented in the additional crosslinking reactions. Reversible networks, elastomers and gels can be prepared either non-covalently or covalently by thermally side group polymers and low molar mass molecules, liquid crystalline properties in the pure state. in many electro-optic devices for optical and gels can be prepared from liquid crystalline state and then fixed by reversible linkages between, for example, neither of which necessarily exhibit     

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.     

Infrared Spectra and Structure of Polyurethane Elastomers from Polytetrahydrofuran,Diphenylmethane-4, 4′-diisocyanate,and Ethylenediamine

Abstract

The infrared spectra (4000–650 cm^?1) of a series of polyurethane elastomers synthesized from polytetrahydrofuran, diphenylmethane-4,4′-diisocyanate, and ethylenediamine were examined. By comparing the spectra with those of the constituent polytetrahydrofuran and of the model compounds for the hard segments of the elastomers containing the urea, urethane, and diphenylmethane groups, most of the stronger bands could be assigned with reasonable assurance to the vibrations of each constituent part. The frequencies of the urea and the urethane characteristic bands correspond to complete association of the polar groups in the polymers. The spectral changes produced by uniaxially stretching polyurethane films can be interpreted as due to the induced crystallization of the polyether blocks in the elastomer, which remain amorphous in the undistorted state. Polarization measurements on stretched samples confirmed that the polyether chains are oriented parallel to the direction of the stretch.     

A Novel Side-Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli-controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)-isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side-chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC-disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side-group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

Photoswitchable Smectic Liquid‐Crystalline Elastomers

Summary: We succeeded in the synthesis of azo side chain containing polysiloxanes with broad smectic C* and A phases. In these polymers the phase transition temperatures can be shifted reversibly by up to 17 °C by irradiation with UV (cis) or VIS (trans) light. Thin films of these polymers in the smectic phase (both on substrates and as free‐standing films) orient perfectly in a homeotropic manner. As a consequence, the azo chromophores do no longer absorb during a perpendicular illumination with light (dichroism). It is thus possible to crosslink these films photochemically to prepare “photoswitchable smectic LC elastomers”.

The trans‐cis isomerization in homeotropically oriented LC elastomers.     

Self‐Assembled Fibers Containing Stable Organic Radical Moieties: Alignment and Magnetic Properties in Liquid Crystals

Macroscopically oriented stable organic radicals have been obtained by using a liquid–crystalline (LC) gel composed of an l ‐isoleucine‐based low molecular weight gelator containing a 2,2,6,6‐tetramethylpiperidine 1‐oxyl moiety. The LC gel has allowed magnetic measurements of the oriented organic radical. The gelator has formed fibrous aggregates in liquid crystals via intermolecular hydrogen bonds. The fibrous aggregates of the radical gelator are formed and oriented on cooling by applying a magnetic field to the mixture of liquid crystals and the gelator. Superconducting quantum interference device (SQUID) measurements have revealed that both oriented and nonoriented fibrous aggregates exhibited antiferromagnetic interactions, in which super‐exchange interaction constant J is estimated as ?0.89 cm^?1.     

A Novel Side‐Chain Liquid Crystal Elastomer Exhibiting Anomalous Reversible Shape Change

Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli‐controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)‐isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side‐chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC‐disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side‐group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit.     

Effect of chiral isosorbide groups on mesomorphic properties of side-chain liquid-crystalline polysiloxanes

Chiral side-chain liquid-crystalline (LC) polysiloxanes containing isosorbide groups were graft copolymerised with poly(methylhydrogeno)siloxane, a chiral LC monomer 6-(4-methoxy-benzoyloxy)-hexahydro-furo[3,2-b]furan-3-yl 4'-(4-undec-10-enoyloxy-benzoyloxy)-biphenyl-4-yl adipate and a nematic LC monomer 4'-(4-methoxy-benzoyloxy)-biphenyl-4-yl 4-(2-undec-10-enoyloxy-ethoxy)-benzoate. The chemical structures and LC properties of the monomers and polymers were characterised by use of various experimental techniques including Fourier transform infrared spectroscopy (FTIR), ¹H-nuclear magnetic resonance (NMR), element analyses (EA), differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (XRD). All the chiral LC polymers showed LC properties with very wide mesophase temperature ranges and the chiral component in the LC polymer systems lead to the appearance of a cholesteric phase. The polymers bearing most chiral LC monomer component showed smectic phases by reason of regular structures in the polymer systems. With the increase of another nematic LC monomer in the polymers, the regular polymer structures were destroyed because of different chemical structures between the two kinds of LC monomers, leading to the disappearance of the smectic arrangement.     

Liquid crystal phases in confined geometries

ABSTRACT

The orientation control of liquid crystal (LC) phases is essential for fundamental studies as well as practical applications. Surface treatment and topographic confinement have emerged as two of the most effective tools to control ordering and orientation of various types of LC phases. This review is intended to give an overview of the LC phases controlled in confined geometries at micro- and nanometre scales, in which the orientation control methods and the effective analytical techniques will be covered. Finally, the review closes with the applications using such confined LC phases.     

Model liquid crystalline dimers in a slit pore: Monte Carlo simulation study

ABSTRACT

In this work, we present results from (isobaric–isothermal) Monte Carlo Simulation studies of liquid crystalline dimer systems confined in a slit pore. Liquid crystalline dimer systems of various spacer numbers have been considered. Surface-induced conformational and alignment properties of these systems at different pressures under homeotropic anchoring condition have been investigated. We have used easily manageable coarse grained force fields to model both monomer–monomer and monomer–substrate interaction potentials. According to the simulated result, the anchoring of dimers to the surface and orientation of mesogenic units with respect to the surface normal seem to depend on the spacer number for messogen attractive confinement. Dimers with lower spacer number are able be adsorbed to the surface and most of their mesogens are oriented along the surface normal even at lower pressure. Those with larger spacer number are distributed throughout the volume at lower pressure. In the case of mesogen repulsive confinement, most of the dimers are adsorbed to the surface and most mesogens are randomly oriented at low pressure. As the pressure gets higher, the adsorption and orientability increase depending on the type of confinement and spacer number. As a result, clear submolecular partitioning and smectic A like structure have been identified.     

Stationary phases with special structural properties for high-throughput separation techniques: preparation, characterization and applications

Stationary phases with specific structural properties for high-throughput liquid chromatographic (LC) techniques are described. Special attention was paid to phases with special structural properties, mainly containing internal functional group (e.g. amide). Such materials are generally called "embedded phases". There are phases created in amidation process of aminopropylated silica gel, especially phases based on biological compounds, like phospholipids and cholesterol, which are called immobilized artificial membranes (IAM's). The synthesis and applications of polar embedded amide LC stationary phases were also reviewed. Methods of characterization of synthesized packing materials were presented, with general focusing on spectroscopic measurements like (13C and 29Si CP/MAS NMR and FT-IR), elemental and thermal analysis as well as chromatographic quantitative structure-retention relationships (QSRR) and extended chemometric tests. The potential applications of various dedicated stationary phases in a high-throughput LC screening procedures were also presented.     

From Mesomorphic Phosphine Oxide to Clustomesogens Containing Molybdenum and Tungsten Octahedral Cluster Cores

New clustomesogens (i.e., metal atom clusters containing liquid crystalline (LC) materials) have been obtained by grafting neutral cyanobiphenyl (CB)‐ or cholesteryl‐containing tailor‐made dendritic mesomorphic triphenylphosphine oxide ligands on luminescent (M₆Clⁱ₈)⁴⁺ octahedral cluster cores (M=Mo, W). The LC properties were studied by a combination of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X‐ray powder diffraction analyses. While the organic ligands showed various mesophase types ranging from nematic, SmA columnar (SmA_Col), SmA, and SmC phases, it turned out that the corresponding clustomesogens formed layered phases (SmA) over a wide range of temperatures that depend on the nature and density of mesogenic groups employed. Intrinsic luminescence properties of the cluster precursors are preserved over the entire range of LC phase existence.     

Preparation and phase behavior of side‐chain cholesteric liquid‐crystalline elastomers

A series of new side‐chain cholesteric elastomers derived from cholesteryl 4‐(10‐undecylen‐1‐yloxy)‐4′‐ethoxybenzoate and phenyl 4,4′‐bis(10‐undecylen‐1‐yloxybenzoyloxy‐p‐ethoxybenzoate) was synthesized. The chemical structures of the monomers were confirmed by elemental analyses, Fourier transform infrared, and ¹H NMR and ¹³C NMR spectra. The mesomorphic properties of elastomers were investigated with differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. Monomer M₁ showed a cholesteric phase, and M₂ displayed smectic and nematic phases. The elastomers containing <15 mol % of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3315–3323, 2005     

A novel biaxial smectic liquid crystalline phase formed by rodlike molecules with a 1,3-diazaazulene skeleton

Novel rodlike liquid crystalline (LC) materials incorporating the nonbenzenoid aromatic 1,3-diazaazulene unit have been synthesized and investigated by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction of oriented samples. Some representatives form, in addition to conventional SmC phases, a novel biaxial smectic LC phase, in which the rotation of the molecules around their long axes is greatly restricted. Attractive face-to-face interactions of the flat aromatic units lead to columnar aggregates which are organized in layers. As there is no positional correlation between adjacent layers, these mesophases are related to lamellar columnar phases formed by some disklike and boardlike molecules. Such LC materials may be promising candidates in the field of organic semiconductors, photoconduction, and other applications.