Side-chain Liquid Crystalline Polymethacrylates Research Articles

Spontaneous homeotropic alignment of nematic liquid crystals induced by a double-armed side chain liquid crystalline polymethacrylate

ABSTRACTA liquid crystalline polymethacrylate having two cyclohexylphenyl mesogens on its side-chain (PMG0) was synthesized by the radical polymerization of the corresponding methacrylate monomer (MG0). PMG0 exhibited a smectic A phase around room temperature showing a focalconic fan texture in POM observations and a broad diffusion in a wide angle range of its WAXD profile. Upon slow cooling at −1 °C min−1 from the isotropic melt, PMG0 exhibited spontaneous homeotropic alignment between two native glass surfaces. A mixture of the commercial nematic liquid crystal (ZLI-4792) containing 1.0 wt% PMG0 also exhibited a homeotropic alignment, which easily covered hydrophilic surfaces such as glass or polyacrylamide.

Influence of alkyl tail length on self-organisation of side-chain liquid crystalline polymers with biphenyl hemiphasmidic mesogens

ABSTRACTA new and simple series of side-chain liquid crystalline polymethacrylates containing biphenyl hemiphasmidic mesogens based on 4-[(3,4,5-trialkan)-1-carbonylbenzoate]biphenyl (denoted as PTmBMA, m = 6, 8, 10, 12, 14, 16, 18, m refers to the carbon number of alkoxy tails) groups was designed and successfully synthesised via free radical polymerisation. The phase structures and transitions of the polymers were investigated by the combination of techniques including differential scanning calorimetry, polarizing microscope, one-dimensional/two-dimensional wide-angle X-ray diffraction, and small-angle X-ray scattering. The experimental results revealed that the liquid crystalline phase structures and behaviours of the polymers were found to be strongly dependent on the alkyl tail length (m). When m = 6, the PTmBMA formed a stable smectic phase above Tg. When m = 8, 10, 12, 14, 16, 18, the sample presented a hexagonal columnar mesophase (Φh), suggesting that the alkyl length has played an important role in the phase behaviours of hemiphasmidic side-chain liquid crystalline polymers.

Photoresponsive side-chain liquid crystalline polymers with amide group-substituted azobenzene mesogens: effects of hydrogen bonding, flexible spacers, and terminal tails

The synthesis of a series of new photoresponsive side-chain liquid crystalline polymethacrylates with amide group-substituted azobenzene (azo) mesogens and different length of flexible spacers and terminal tails via conventional free radical polymerization is described. The resulting azo polymers proved to have high thermal stability and good solubility in common organic solvents (e.g., tetrahydrofuran and chloroform). Differential scanning calorimetry, polarizing optical microscopy, and small angle X-ray scattering studies confirmed the presence of obvious enantiotropic smectic C liquid crystalline phases (with a bilayer lamellar structure) for all these polymers. The introduction of an amide group into the azo mesogen led to the formation of strong hydrogen bonding among the side chains of the polymers (as revealed by variable temperature FT-IR), which played a decisive role in forming and stabilizing the liquid crystalline mesophases of the polymers. In addition, the length of the flexible spacers and terminal tails also significantly influenced their phase transition behaviors. Furthermore, the photoresponsivity of the polymer solutions was verified and the effects of the molecular structures of the polymers on their photoresponsive properties were also studied.

Pendant triazole ring assisted mesogen containing side chain liquid crystalline polymethacrylates: Synthesis and characterization

Two series of click chemistry assisted alkoxymethyl-1H-[1,2,3]-triazol-1-yl containing side-chain liquid-crystalline polymethacrylates were synthesized by free radical polymerization technique. Mesogen was linked to backbone through various spacer units. Monomers and polymers were characterized by FT-IR, 1H and 13C-NMR spectral techniques. Thermal stability of polymers was confirmed by thermogravimetric analysis. Mesomorphic property and phase transition temperature of polymers were analysed by differential scanning calorimetry and polarized optical microscopy. Phase transition temperature and mesomorphic property of polymers with respect to insertion of polar alkoxy group on terminal triazole ring and spacer length between backbone and mesogen were investigated. Polymers exhibited grainy like textures under polarized optical microscopy. Spacer length between mesogen and backbone alters phase transition temperature of the polymers. Two series of click chemistry assisted alkoxymethyl-1H-[1, 2, 3]-triazol-1-yl containing side-chain liquid-crystalline polymethacrylates were synthesized by free radical polymerization technique.

Molecular packing and phase transitions of side-chain liquid crystalline polymethacrylates based on p-methoxyazobenzene

The phase structures and transition behaviors of a series of side-chain liquid crystalline (LC) polymethacrylates based on p-methoxyazobenzene (PMnAzs, n=6, 8, 10, 12) were studied using differential scanning calorimetry, one- and two-dimensional (1D and 2D) wide-angle X-ray diffraction, and Fourier transform infrared spectroscopic experiments. The LC phase transition of PMnAz follows the sequence of smectic A (SmA)↔nematic (N)↔isotropic (I). For PM10Az and PM12Az, the transition of SmA-to-N is not complete upon heating. In the low-temperature SmA phase, the polymers adopt a fully interdigitated side-chain packing with the smectic layer period almost identical to the side-chain length. For all the four samples, the first-order diffraction of the SmA structure only renders when the temperature approaches the transition of SmA ↔ N, with the intensity much lower than that of the second-order diffraction. The absence of the first-order diffraction at low temperatures is ascribed to the possible matching of the electron densities between the center portion of the side-chain sublayer and the main-chain sublayer of the SmA structure. Since only the mesogens from the same main-chain sublayer can stack parallel together, the distribution of the azobenzene domains may cause some sort of density undulation within the smectic layer. Among the samples, PM6Az presents the strongest undulation with some additional orders. We also examined the annealing effect on the H-aggregation of PMnAzs. It is found that isothermal annealing at a temperature slightly higher than the Tg of PM8Az and PM10Az can significantly enhance the UV absorption at 326nm, indicating a further development of H-aggregation. However, for PM6Az and PM12Az, the UV–vis spectrum of the annealed sample is nearly identical to that without annealing.

Preparation and properties of azobenzene-containing amphiphilic miktoarm star polymers

A series of novel miktoarm star polymers composed of a poly(ethylene oxide) (PEO) and two side-chain liquid crystalline azobenzene-containing polymethacrylate (PEO-(PMMAZO)2) were prepared using atom transfer radical polymerization (ATRP). Bifunctional macroinitiator PEO-Br2 was synthesized by condensation reaction in two steps and characterized by 1H NMR, 13C NMR and IR. Kinetic study showed that it was a first order reaction referred to the monomer MMAZO, namely, 6-(4-methoxy-4’-oxy-azobenzene)hexyl methacrylate. The liquid crystalline behaviors of the miktoarm star polymers were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM). They exhibited smectic and nematic mesophases when Mn was beyond 9.4×103 g/mol. The phase transition temperatures of the smectic and nematic phases increased while the melting temperature of PEO decreased with increasing molecular weight of the LC block. Compared with diblock polymer PEO-PMMAZO, the melting temperature of PEO in miktoarm star polymer decreased more rapidly.

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.

Dielectric Relaxation and Thermally Stimulated Discharge Currents in Liquid-Crystalline Side-Chain Polymethacrylates with Phenylbenzoate Mesogens Having Tail Groups of Different Length

Molecular dynamics of six side-chain liquid-crystalline polymethacrylates with phenylbenzoate mesogenic groups having tail groups of different lengths was investigated by dielectric spectroscopy (D...

Switchable Liquid-Crystalline Polymers Based on the Ferrocene-Ferrocenium Redox System

A ferrocene-containing side-chain liquid-crystalline polymethacrylate was oxidized with iodine into the corresponding ferrocenium polymeric species. The reduced polymer showed enantiotropic smectic C and smectic A phases. The oxidized polymer exhibited an enantiotropic nematic phase. The results described in this report confirm that ferrocene is a valuable redox-active unit for the design of switchable liquid-crystalline polymers.

Synthesis and mesomorphic behavior of a new series of side-chain liquid crystalline polymethacrylates bearing 2,6-naphthylene-based mesogen

A new homologous series of liquid crystalline (LC) polymethacrylates, the poly[ω-(2-phenoxycarbonyl-naphthalene-6-yloxy)alkyl methacrylate]s, in which the length of the alkyl spacer is varied between 3 and 12 methylene units, have been synthesized by free-radical polymerization. The prepared polymers were studied by IR and 1H-NMR spectroscopy, viscosity measurements, differential scanning calorimetry (DSC), and polarized microscopy. Polymers with 3 or 4 methylene units spacer were glassy in nature, whereas those with 5–12 methylene units spacer exhibited smectic behavior. The glass transition temperatures decrease on increasing spacer length. The isotropization temperatures and the corresponding transition entropies showed an odd–even effect, with the odd members exhibiting the higher values. This effect attenuates on increasing spacer length. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2391–2399, 1999

Synthesis and characterization of novel photochromic side-chain liquid crystalline polymethacrylates containing para-nitroazobenzene group

Methacrylate monomers containing a mesogenic group and a photochromic para-nitroazobenzene group were prepared and polymerized in tetrahydrofuran (THF) using 2,2′-azobis(isobutyronitrile) (AIBN) as initiator at 67°C. The polymers were characterized by 1H NMR, IR, UV-Vis spectra and elemental analyses. The molecular weights M w and M n of the polymers were determined by gel permeation chromatography (GPC). The thermal properties of the polymers such as thermal stability, glass transition temperatures and liquid crystallinity were studied by thermogravimetric analysis, differential thermal analysis, polarizing optical microscopy and X-ray diffractometry. The results show that all the polymers exhibit nematic phases and the increasing of the azo group content broadens the temperature range of the liquid crystalline state. The UV-induced trans/ cis photoisomerization and the thermal back-isomerization were also investigated for the azo monomer MAAZO6 and the copolymer P-2 in chloroform.

Ferrocene-Containing Thermotropic Side-Chain Liquid-Crystalline Polymethacrylate from a Mesomorphic Trisubstituted Ferrocene Monomer

The synthesis and mesomorphic properties of a methacrylate-containing ferrocene monomer bearing two flexible chains at the 1,1‘-positions and the corresponding side-chain homopolymer are described. Both polymer and monomer showed enantiotropic smectic C and smectic A phases. The liquid-crystalline phases were fully characterized by means of X-ray diffraction experiments. Dilatometric measurements pointed to a head-to-tail molecular arrangement of the monomeric units within the smectic A phase. The results described in this report confirm that ferrocene-containing thermotropic side-chain liquid-crystalline polymethacrylates respresent a valuable family of metallomesogenic polymers.

Synthesis and Characterization of Side-Chain Liquid Crystalline Polymethacrylates Containing Fluorinated Diphenyldiacetylene Side-Groups

Abstract The synthesis of four side-chain liquid crystalline polymethacrylates containing either 4-alkanyloxy-4′-flourodiphenyldiacetylenes or 4-alkanyloxy-3′,4′-difluorodiphenyldiacetylenes side groups is presented. The phase behavior of both monomeric and polymeric liquid crystals was characterized by differential scanning calorimetry and optical polarizing miocroscopy. All synthesized polymethacrylates present nematic mesomorphism. The glass transition temperature of the obtained polymers decreases as the spacer length increases. The mesomorphic behavior of the prepared polymers depends very much on the degree of substitution of the mesogenic side groups on the polymer backbones.

The temperature dependence of nematic liquid crystalline polymer melt diffusion

Abstract Polymer chain diffusion in the nematic mesophase was studied using a model main chain liquid crystalline (LC) polyether based on 2,2′-dimethyl-4,4′-dihydroxyazoxybenzene and mixed alkane spacers. A side chain LC polymethacrylate containing an azobenzene mesogenic group was also investigated. Tracer diffusion coefficients were determined as a function of temperature by an ion-beam depth profiling technique, forward recoil spectrometry. The results confirm that main chain LC polymer chain dynamics are dramatically affected by phase transitions and sample geometry. This behaviour is in marked contrast to the side chain LC polymer which exhibited no phase dependence on the part of the tracer diffusion coefficient.

Side chain liquid crystalline polymethacrylates with three‐ring azobenzene groups

AbstractThe influence of three aromatic rings in the azobenzene mesogenic moities on the LC character of side chain polymethacrylates is described. The three‐ring azobenzene groups exhibit some retardation and inhibition effect during the radical methacrylate polymerization. Therefore the polymerization conditions had to be modified. Dimethylformamide was found to be a solvent realizing high polymer yields and satisfactory molar masses. The phase behaviour of the obtained SCLC polymethacrylates and of their monomers is characterized by X‐ray, DSC and polarizing microscopy in dependence of spacer length. The polymethacrylates with three‐ring azobenzene mesogens show significant higher glass transition and clearing temperatures than two‐ring analogues. The change in the liquid crystalline phase arrangement from smectic C to smectic B with increasing spacer length is explained by the better movability of the mesogens.

Synthesis of side-chain liquid crystalline polyacrylates, polymethacrylates and polysiloxanes containing 4-cyano-biphenyl 4-alkanyloxybenzyl ether side groups

The synthesis and characterization of side-chain liquid crystalline polyacrylates, polymethacrylates and polysiloxanes containing 4-cyanobiphenyl 4-alkanyloxybenzyl ether side groups are presented. All polymers display respectively a smectic A mesophase. The influence of the polymer backbone flexibility on the phase transition temperatures of the synthesized side-chain liquid crystalline polymers is discussed. The results demonstrate that flexible backbones enhance the decoupling of the motions of the side chain and main chain. Therefore, among three kinds of polymer backbones which contain the same mesogenic side groups, the most flexible one, i.e., polysiloxane, reveals a highest isotropization temperature and a widest temperature range of mesophase.

Liquid crystal polymers containing macroheterocyclic ligands. III. Side chain liquid crystalline polymethacrylates containing mesogenic units based on diarylacetylenes and benzo‐15‐crown‐5

AbstractThe synthesis and polymerization of 4′‐[4‐ethynyl‐1‐(11‐methacryloylundecan‐1‐yloxy) benzene] benzo‐15‐crown‐5 (8), 4′‐[4‐ethynyl‐4′‐(11‐methacryloylundecan‐1‐yloxy) biphenyl] benzo‐15‐crown‐5 (15), 4′‐[2‐ethynyl‐6‐(11‐methacryloylundecan‐1‐yloxy) naphthalene] benzo‐15‐crown‐5 (24), and 4′‐[2‐ethynyl‐6‐(11‐methacryloylundecan‐1‐yloxy)naphthalene]‐5′‐ethylbenzo‐15‐crown‐5 (35) is described. The synthesis and characterization of 4′‐[4‐(4‐ethynyl‐1‐(2‐ethynyl‐6‐(6‐hydroxyhexan‐1‐yloxy)naphthathalene)benzene]) benzo‐15‐crown‐5 (29) is also presented. Both monomers and polymers were characterized for their mesomorphic behavior. 8, poly(8), and 15 are crystalline. Due to the insolubility of 15, poly(15) could not be synthesized. 24 is crystalline, while poly(24) displays a monotropic nematic mesophase. 29 exhibits also a monotropic nematic mesophase. 35 is crystalline, while poly(35) displays an enantiotropic nematic mesophase which is kinetically controlled due to its close proximity to the glass transition temperature.

The influence of the polymer backbone flexibility on the phase transitions of side chain liquid crystal polymers containing 6-[4-(4-methoxy-?-methylstyryl)phenoxy]hexyl side groups

The influence of the polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane, and polyphosphazene containing 6-[4-(4-methoxy- β- methylstyryl)phenoxy]hexyl side groups is discussed. Flexible backbones enhance the decoupling of the motions of the side chain and main chain and therefore, increase the rate of side-chain crystallization. Subsequently, the kinetically controlled crystallization process may transform the thermodynamically controlled mesomorphic phases from enantiotropic into monotropic. However, the highest degree of order in the mesophase is exhibited by the polymers based on the most rigid polymer backbone.