Liquid Crystalline Polyacrylates Research Articles

Synthesis of Side-Chain Liquid Crystalline Polyacrylates with Bridged Stilbene Mesogens.

In recent years, π-conjugated liquid crystalline molecules with optoelectronic functionalities have garnered considerable attention, and integrating these molecules into side-chain liquid crystalline polymers (SCLCPs) holds potential for developing devices that are operational near room temperature. However, it is difficult to design SCLCPs with excellent processability because liquid crystalline mesogens are rigid rods, have low solubility in organic solvents, and have a high isotropization temperature. Recently, we developed near-room-temperature π-conjugated nematic liquid crystals based on "bridged stilbene". In this work, we synthesized a polyacrylate SCLCP incorporating a bridged stilbene that exhibited a nematic phase near room temperature and could maintain liquid crystallinity for more than three months. We conducted a thorough phase structure analysis and evaluated the optical properties. The birefringence values of the resulting polymers were higher than those of the corresponding monomers because of the enhanced order parameters due to the polymer effect. In addition, the synthesized polymers inherited mesogen-derived AIE properties, with high quantum yields (Φfl = 0.14-0.35) in the solid state. It is noteworthy that the maximum fluorescence wavelength exhibited a redshift of greater than 27 nm as a consequence of film formation. Thus, several unique characteristics of the SCLCPs are unattainable with small molecular systems.

Synthesis, structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

ABSTRACTA series of novel liquid crystalline monomers (M1−M8) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and 1H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (Tm) and isotropic temperature (Ti) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, Tm and Ti of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.

Well-Organized Columnar Superlattices via Positive Coupling between Polymer Backbone and Discotic Side Groups

Very little is known about some fundamental issues such as the spacer length influence and molecular weight (MW) effect of discotic liquid crystalline polymers (DLCPs), despite the elucidation of such aspects are of crucial importance for their structure tuning and device performance. Here in this article, a systematic comparative study has been conducted to investigate the MW effect and especially gain a deeper insight into the spacer length influence of side-chain DLCPs based on a homologous series of well-defined discotic liquid crystalline polyacrylates with triphenylene (TP) side groups of variant spacer lengths. The series DLCPs of shorter spacers display various well-organized columnar superlattices based on multicolumn bundles organization with “coordination number” from two to six through individual discogens or discrete columnar stack (DCS)-based intracolumnar stacking modes. It is disclosed for the first time that the positive coupling effect (PCE) prevails in side-chain DLCPs, and proper coupling between discotic side groups and polymer backbone is desirable and required for achieving well-organized ordered columnar mesophases, in striking contrast with the renowned classical spacer decoupling principle directing the fruitful exploration of their side-chain calamitic counterparts for several decades. These findings are inspiring for in-depth understanding of self-assembly of aromatic interactions involved complex functional chemical and biological systems and especially opening an avenue for rational design and synthesis of well-controlled side-chain DLCPs for low-cost solution processable optoelectronic device applications.

Synthesis of well-defined easily crosslinkable azobenzene side-chain liquid crystalline polymers via reversible addition–fragmentation chain transfer polymerization and photomechanical properties of their post-crosslinked fibers

The synthesis of a series of well-defined easily crosslinkable azobenzene (azo) side-chain liquid crystalline polyacrylates (including both homopolymers with an N-hydroxysuccinimide carboxylate-substituted azo mesogen and copolymers with both the above crosslinkable azo mesogen and a non-crosslinkable azo mesogen) via reversible addition–fragmentation chain transfer (RAFT) polymerization and photomechanical properties of their post-crosslinked fibers are described. The RAFT polymerizations of the azo monomers proved to be living and well-controlled, as revealed by their linear kinetic plots as well as the linear increase of the molecular weights of the azo polymers with monomer conversions and their low molar-mass dispersities (Đ⩽1.17). All the obtained azo polymers exhibited high thermal stability and relatively low glass transition temperatures, but only the azo homopolymers with longer flexible spacers and azo copolymers showed liquid crystallinity. In addition, the crosslinked azo polymer fibers with easily controlled diameters and high alignment order were obtained by the first fabrication of the uncrosslinked azo polymer fibers by the simple melt spinning method and their subsequent crosslinking with a difunctional primary amine under mild conditions. The contents of the crosslinkable azo mesogen in the polymers had significant influence on the photomechanical properties of their post-crosslinked fibers, and those with appropriate crosslinking densities could exhibit fast and reversible photoinduced bending and unbending behaviors and excellent photodeformation fatigue resistance even at close to room temperature.

Orientational Behavior of Liquid-Crystalline Polymers with Amide Groups

Liquid-crystalline polyacrylamides with mesogenic side-chains were synthesized by the radical polymerization of a mesogenic acrylamide derivative. Their thermal properties and orientational behavior were examined by polarizing microscopy, differential scanning calorimetry, temperature-variable IR, and X-ray diffraction measurements. The liquid-crystalline polyacrylamides containing secondary amide groups formed smectic A and smectic B phases during heating and cooling processes. The glass-smectic B, smectic B-A, and smectic A-isotropic phase transition temperatures increased with increasing molecular weight. The liquid-crystalline polyacrylamide showed higher phase transition temperatures than a liquid-crystalline polyacrylate, in which the secondary amide group was replaced with an ester group. The X-ray diffraction pattern of a smectic B-oriented sample of the liquid-crystalline polyacrylamide consisted of sharp inner and very sharp outer reflections. The very sharp reflection in the wide-angle region of the X-ray diffraction pattern indicated the formation of hexatic packing within the layer. The relationship between the layer distance and the extended mesogenic side-chain length suggested that the liquid-crystalline polyacrylamide formed an interdigitated bilayer structure. The IR spectra of the liquid-crystalline polyacrylamide exhibited that the number of hydrogen bonds between the secondary amide groups decreased with increasing temperature. In the liquid-crystalline polyacrylamide, smectic A and smectic B phases, with enhanced thermal stability, were formed through the formation of hydrogen bonds between the secondary amide groups.

Synthesis, characterisation and functionalisation of ZnO and TiO2 nanostructures: used as dopants in liquid crystal polymers

Uniform ZnO nanorods (NRs), TiO2 and ZnO spherical nanoparticles (NPs) were synthesised and their surface modified using polyvinylpyrolidine (PVP), 1-decanethiol (SH∼), trioctylphosphine oxide (TOPO) and TOPO-ethylenediamine derivative as a capping agent molecule. The pre-functionalised nanomaterials were doped into liquid crystalline (LC) polyurethane and polyacrylate in order to obtain nanocomposites. The morphological studies of pure NPs, functionalised NPs, pure polymer and doped polymers were accomplished by transmission electron microscopy (TEM), scanning electron microscopy (SEM) methods as well as energy-dispersive X-ray spectroscopy (EDX)-Point analysis and mapping. It has been observed that after doping, the morphology of the polymer was changed to a highly porous one.

Crown-ether and azobenzene-containing liquid crystalline polymers: An influence of macromolecular architecture on optical properties and photo-orientation processes

A series of novel crown ether-containing photochromic comb-shaped liquid crystalline polyacrylates with different macromolecular structure of side groups were synthesized and investigated. Phase behavior, optical and photo-optical properties of thin spin-coated films of these polymers were studied. A special attention was paid to a comparative study of the photo-orientation phenomena occurring in the polymer films under a polarized light action. It was shown that complex formation with the potassium ions results in the decrease in degree of the photoinduced order that can be used for the creation of new materials for sensor devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Mesophase structure of low-wetting liquid crystalline polyacrylates with new perfluoroalkyl benzoate side groups

The synthesis, thermal behavior, bulk microstructure, and wettability of new polyacrylates carrying spaced 4-perfluorohexylpropyl benzoate and 4-perfluorooctylpropyl benzoate units in the side groups were investigated. X-ray diffraction analysis proved the formation of different smectic mesophases (SmI 2 , SmF 2 , and SmC 2 ) and the evolution of their structures and lattice parameters with temperature. The mesophase polymorphic behavior depended on the length of the perfluorinated chain segment in the repeat unit. The electron density profiles along the smectic layer normal were drawn and provided a deeper insight into the packing of the side chains in a tilted, double layer structure. Thin polymer films were cast from solution, and their low wettability was established by measurements of contact angles with different probing liquids. We suggest that the hydrophobicity and lipophobicity of the films are enhanced by the mesophase surface structure which is mediated by the high-order, mesophase bulk structure.

Thermal recordable novel cholesteric liquid crystalline polyacrylates containing various chiral moieties

AbstractTo investigate the effect of spacer length and linkages between the rigid mesogenic core and the terminal group on the molecular interaction and physical properties of polymers, two series of novel side chain liquid crystalline polyacrylates were synthesized. These were composed of liquid crystalline monomers with six or eleven methylene segments as spacers, and chiral monomers end capped with menthyl or cholesteryl groups. Liquid crystalline phases of the polymers were investigated using differential scanning calorimetry and polarized optical microscopy, and confirmed with X‐ray diffractometry. Color image recording of the synthesized polymer films was achieved using a thermal treatment, and then fixed by quenching. This investigation demonstrates that the introduction of carbonate linking groups between the rigid mesogenic core and terminal group decreases both the lateral molecular interaction and thermal stability of the liquid crystalline polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6214–6228, 2008

Synthesis and characterization of optically active liquid crystalline polyacrylates containing mesogenic phenylbenzoate groups

A series of novel side chain liquid crystalline polyacrylates with pendant chiral groups were synthesized. It was found that monomers with electron releasing –OC4H9 terminal groups seem beneficial for the formation of liquid crystalline phases. Copolymerization of the monomers was carried out and the physical properties of the copolymers were investigated. All synthesized polymers revealed liquid crystalline phases and appeared highly thermally stable with decomposition temperatures (Td) at 10% weight loss greater than 384°C and about 50% weight loss occurred beyond 442°C under nitrogen atmosphere. Two miscible chiral compounds were also synthesized and used as chiral dopants to induce cholesteric liquid crystalline phases of polymers. Liquid crystalline phases of the polymers were investigated using DSC and XRD, and confirmed with POM technique. The optical properties of the induced cholesteric liquid crystalline polymers were investigated using UV–vis spectrometer. The appearance and the color variation of the polymer films before and after UV irradiation were also investigated. Typical helical morphology of the cholesteric liquid crystalline film was analyzed by SEM technique.

Smectic ordering in side‐chain liquid crystalline polymers (LCPs) and in LCP–silica nanocomposites

The mesophase behaviour of a side‐chain liquid crystalline polyacrylate (LCP) grown by drying a solution has been investigated. This LCP, characterised by a short spacer (four carbon atoms) and a long tail (10 carbon atoms), displays, at increasing temperatures, SmC and SmAd phases. The effect of the mean molecular weight, i.e. the mean number of side chains per polyacrylate main chain (18 and 51) on the lamellar width, was studied. LCP–silica nanocomposites have been synthesised by a sol‐gel process in the presence of LCP in the solution, followed by subcritical drying. The mesophase behaviour of these nanocomposites was compared to that of the corresponding bulk LCP. The experimental methods were polarised optical microscopy, differential scanning calorimetry and synchrotron X‐ray scattering.

Structure-property relationships of smectic liquid crystalline polyacrylates as revealed by SAXS

A influencia da estrutura quimica dos grupos mesogenicos e do tamanho dos grupos espacadores, no comportamento de fase de uma serie de cristais liquidos polimericos de cadeia lateral (SCLCP), foram estudados utilizando-se espalhamento de raios-X a Baixo Ângulo (SAXS) e Microscopia Otica de Luz Polarizada (POM). Analises do arranjo das mesofases em amostras nao orientadas e orientadas por acao do campo magnetico sao descritas. O papel do tamanho do espacador lateral no empacotamento local e na largura da camada esmetica determinados nas mesofases SmA e SmC e elucidado. Os ângulos θ formados entre os grupos mesogenicos e a normal as camadas nas mesofases SmC foram determinados. Um estudo a respeito do grau de ordem em funcao da temperatura, para os polimeros esmeticos foi possivel atraves de medidas de SAXS. Uma ordenacao particular em um dos SCLCPs estudados e relacionada com a coexistencia de duas fases distintas. The influence of the chemical structure of the mesogenic groups and the length of the spacer groups on the phase behavior in a series of side-chain liquid crystalline polyacrylates (SCLCP) have been studied using Small Angle X-ray Scattering (SAXS) and Polarized Optical Microscopy (POM). Analyses of the mesophase arrangement in unaligned and aligned samples by magnetic field are reported. The role of the spacer length on the local packing and on the thickness of the layers encountered in the SmA and SmC mesophases is elucidated. The tilt angles θ of the mesogenic cores related to the normal of the layers in the SmC mesophases are measured. A study about the degree of order as a function of temperature for the smectic polymers was possible using SAXS measurements. A particular arrangement in one of the studied SCLCPs is related to the coexistence of two different phases.

Synthesis of Benzothiadiazole-Based Liquid Crystalline Polyacrylates for Polarized Light Emitting Diodes

Two benzothiadiazole-based liquid crystalline polyacrylates were synthesized. These polymers revealed a nematic liquid crystal phase and exhibited photoluminescence as well as polarized electroluminescence when incorporated into light-emitting diode applications. The polymers showed dichroic ratios of about 8.3–8.8 in UV-vis absorption and photoluminescence emission. The polymer with vinylene linkages (P2) showed better electroluminescence device performance than that with acetylene linkages (P1). The P2 device emitted red light at 604 nm with a turn-on voltage at 6 V, and a maximum polarized luminance of 235 cd/m2 at 12 V, with an efficiency of 0.09 cd/A and a polarization ratio of 6.5.

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P4M and P11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A1 phases were observed for the polyacrylates with four (P4M) and eleven (P11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P4M exhibits an injected smectic phase, whereas P11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.

Synthesis and characterization of new chiral liquid crystalline polyacrylates from L-isoleucine

The synthesis of chiral side chain liquid crystalline polyacrylates with a two-stereogenic centre from L-α-aminoacid is described. The chiral tail is 2-chloroalcohol obtained from L-isoleucine and the spacer group has either four or eleven methylene units. The mesogenic moiety is derived from phenyl benzoate. The stereochemistry of the key intermediate (2S,3S)-(+)-4- [1-(2-chloro-3-methyl)pentyloxy]phenyl benzoate (6) obtained by a Mitsunobu reaction was established by single crystal X-ray analysis. This result indicates that the nucleophilic displacement of chiral diazonium salts proceeds with overall retention of configuration. The liquid crystalline behaviour of polyacrylates P13 and P14 was investigated by DSC, optical microscopy, small angle X-ray scattering and depolarized light scattering. The polyacrylate P13, with eleven methylene units in the spacer, exhibits a chiral smectic A phase whereas the polyacrylate P14, with a spacer containing four methylene units, displays a chiral nematic phase.

Polarized Blue Emission Based on a Side Chain Liquid Crystalline Polyacrylate Containing Bis-tolane Side Groups

A novel side chain liquid crystalline (LC) polyacrylate (1P) containing highly conjugated bis-tolane side groups was synthesized and used for polymer light emitting diodes application. Polymer 1P shows a nematic phase at room temperature. It exhibits a UV absorption peak at 330 nm and emits a blue light at 425 nm. Polyimide (PI), polyaniline (PA) and poly(3,4-diethylenedioxythiophene) (PEDOT) were used as alignment materials for polymer 1P. The photoluminescence dichroic ratios of 1P on rubbed PI, PA and PEDOT films are 10.4, 8.3 and 7.3, respectively. The electroluminescence dichroic ratio of 1P aligned on a PEDOT film is 6.1.

Synthesis and Electroluminescence of Side-chain Liquid Crystalline Polyacrylates and Polyoxiranes Containing Bistolane Side Groups

The synthesis and characterization of side-chain liquid crystalline (LC) polyacrylates and polyoxiranes containing bistolane side-groups are presented. The phase behavior of the prepared monomers and polymers was characterized by differential scanning calorimetry and optical polarizing microscopy. All of the obtained monomers and polymers reveal an enantiotropic nematic phase. The birefringences of the LC monomers are in the range from 0.35–0.6 depending on the measuring wavelength. The photoluminescence (PL) and electroluminescence (EL) properties of the obtained monomers and polymers are also reported.

Structure and frustration in liquid crystalline polyacrylates I. Bulk behaviour

The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers show a nematic, a smectic-Ad, a re-entrant nematic and a C phase, for shorter chains only a nematic and a C phase. This constitutes a new example of nematic re-entrance for which the driving field is the length of the polymer chain. The smectic-Ad layers consist of partially overlapped side groups while in the C phase the side chains are rearranged into chevron-like blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing length scales and their coupling to the backbone conformations.

Synthesis and Structure Characterization of Liquid Crystalline Polyacrylates with Unconventional Fluoroalkylphenyl Mesogens

The synthesis, thermal behavior, and bulk microstructure of fluorinated new liquid-crystalline polyacrylates containing spaced fluoroalkylphenyl units in the side chains were studied. While in polyacrylates poly(1) and poly(2) there was a 4-(perfluorooctylethoxy)phenyl benzoate unit, i.e., a two-phenyl mesogenic core, poly(3) contained a 4-perfluorooctylethyl benzoate, i.e., a one-phenyl aromatic core that per se would not be able to promote a thermotropic behavior. X-ray diffraction studies allowed identification of the mesophases (hexatic smectic, disordered smectic, nematic) and study of the evolution of their structures and lattice parameters with temperature. It was found that each polymer presented a rich mesophase polymorphism, including the rather unusual smectic F2- (or I2-) to-nematic transition in poly(3). The electron density profiles along the smectic layer normal were drawn and provided a deeper insight into the packing of the side chains in tilted double layer structures. The strong mesogenic character of the polymers was attributed to the presence of the fluorinated tail on the aromatic core owing to its capability of adopting a rigid-rodlike conformation. A phase separation occurring at the molecular level of the different incompatible aliphatic−aromatic−fluoroaliphatic sections of the polymer repeat unit should efficiently cooperate to the onset of thermotropic mesomorphism.

A Series of Novel Liquid Crystalline Polymers Showing a Nematic Discotic and/or a Nematic Columnar Phase

A series of novel liquid crystalline side chain polyacrylates was prepared, bearing a disk-shaped pentakis(4-methylphenylethynyl)benzene mesogen via an undecanoxy spacer. By using the concept of the polymer analogous reaction, a variety of copolymers with different alkyl acrylate “co-monomers” and increasing mesogen contents were synthesized. The thermal properties of the polymers were investigated by optical polarization microscopy and differential scanning calorimetry. The results indicated that the liquid crystalline characteristics of the polymers are not determined by nature of the co-monomers, but mainly by the weight fraction of rigid mesogen present in the material. All polymers exhibit a nematic columnar phase at lower temperatures. Polymers with a high mesogen content (>61%-wt) exhibit an additional nematic discotic phase at high temperatures, which is unique for polymeric materials.