Liquid Crystalline Polymethacrylates Research Articles

The role of lateral substituent size in azobenzene chromophores on photo- and chiro-optical properties of photoactive polymethacrylates

Properties of functional liquid crystalline (LC) macromolecular materials, especially those showing self-organizing behaviour and targeted for specific purposes, strongly rely on the molecular structure of the mesogens used as side-chains. In order to get inside this relationship, new series of chiral LC polymethacrylates (PMA6Azo-R) with azobenzene groups having different lateral substituents (R), namely the methyl (-Me) and ethyl (-Et) groups on the central benzene ring, were synthesized and their comparative study was carried out. The synthesized polymers consist of photochromic azobenzene fragments that differ in the size of the lateral substituent (R = -H, -Me, -Et). All polymers exhibit the cholesteric phase, whereas the polymer without lateral alkyl group in addition forms the low-temperature SmĈ* phase. Investigations of the photoorientation process under linearly polarised visible light irradiation reveal that an increase in the size of side substituent considerably reduces the values of maximum achievable photoinduced dichroism (from 0.89 for PMA6Azo-H to 0.33 for PMA6Azo-Et that is associated with a decrease in the degree of mesogen anisometry). The effect of the lateral substituent size in azobenzene photochrome on the chiro-optical properties and mesomorphic behaviour of studied polymers was established. The presence of the lateral substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature. Chiro-optical properties of the polymers and their photoswitching were studied, and it was found, that the introduction of the lateral substituent in the azobenzene fragment prevents the formation of exciton couples, that leads to a decrease in the circular dichroism (CD) values. UV irradiation and E-Z isomerisation of the azobenzene chromophores results in complete disappearance of CD signal; subsequent visible light irradiation does not lead to its full recovery. On the other hand, annealing of the films at temperatures higher than the glass transition almost fully recovers the initial CD values. The designed polymethacrylates can be considered as promising materials for various photonic applications.

Photoinduced Reorientation and Polarized Fluorescence of a Photoalignable Liquid Crystalline Polymer

Thermally stimulated photoinduced reorientation of liquid crystalline (LC) polymethacrylate composed of a phenyl benzoate mesogen connected with N-benzylideneaniline (NBA2) end moiety exhibits a significant molecular reorientation (D > 0.7) when the film is exposed to linearly polarized 313 nm light and subsequently annealed in the LC temperature range of the material. Hydrolysis of the NBA2 end moieties yields an oriented mesogen with phenylamine moieties without distorting the oriented structure. In situ condensation of 2-hydroxybenzaldehyde derivatives and phenylamine moieties yields oriented N-salicylideneaniline side groups. The resultant film displays a polarized fluorescence with a polarization ratio up to 3.4.

Photo-orientation Processes in Liquid Crystalline Polymethacrylates with Side Azobenzene Groups Having Lateral Methyl Substituents

Design of new photoactive materials exhibiting nano-organization aiming for smart practical applications remains an actual and highlighted task. The synthesis and comparative study of chiral liquid...

Photoinduced Reorientation and Photofunctional Control of Liquid Crystalline Copolymers with in Situ-Formed N-Benzylideneaniline Derivative Side Groups.

The influence of the annealing conditions on the thermally stimulated photoinduced molecular reorientation of a photoinactive liquid crystalline polymethacrylate with phenyl aldehyde and benzoic acid side groups (P1) doped with 4-methoxyaniline, which forms photoalignable 4-methoxy-N-benzylideneaniline (MNBA) side groups in situ, was investigated. Light exposure and subsequent thermal stimulation under a N2 atmosphere realized sufficient cooperative molecular reorientation (D > 0.7), but the simultaneous thermal hydrolysis of the MNBA groups under humid air lowered the molecular reorientation performance. By contrast, subsequent thermal hydrolysis of MNBA after molecular reorientation introduced different aromatic amines into the reoriented P1 film, which regulated the birefringence and photofunctionality of the oriented film.

The effect of spacer and alkyl tail lengths on the photoorientation processes in amorphousized films of azobenzene-containing liquid crystalline polymethacrylates

ABSTRACTIn our days, photochromic polymers possessing the liquid crystalline behaviour attract considerable attention of the scientific community due to their functionality and highlighted effects which can be used for opto-electronics and photonics. Herein, the photoorientation processes under the action of the polarised UV (355 nm) and visible (457 nm) light are studied for the amorphousized films of new azobenzene-containing polymethacrylates with different lengths of alkyl spacers and tails (5, 6 and 10 methylene units). The prominent effects of the chemical structure, namely length of the alkyl spacers and tails, as well as wavelength of the excitation light, on the peculiarities of the photoorientation process are demonstrated. Those effects can lead to quite important conclusions concerning possible influence of the microphase separation in polymer films.

Collective Competition between Two Mesogens showing Opposing Orientational Nature in Side Chain Liquid Crystalline Polymers

Thin films of liquid crystalline polymethacrylates possessing an azobenzene and a cyanobiphenyl side chain exhibit a homeotropic and planar mesogen orientation, respectively. Here, we report a syst...

Influence of alkylene spacer length on the photoinduced reorientation of polymethacrylate with benzoic acid side groups and N-benzylideneaniline carboxylic acid composite films

Photoinduced reorientation of composite films of liquid crystalline polymethacrylate comprised of benzoic acid (BA) side groups with different length alkylene spacer (PnBAM; n = 2, 6, 10) and N-benzylideneaniline carboxylic acid (NBA) is investigated using linearly polarized (LP) 365 nm light. Significant thermally stimulated in-plane molecular orientation is attained when P6BAM (n = 6) is used, but higher amount of NBA is required for the in-plane reorientation of P2BAM (n = 2) films and out-of-plane motion of the BA side groups is observed for P10BAM (n = 10) films.

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.

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.image

Molecular Mobility in Liquid Crystalline Side-Chain Polyacrylates and Polymethacrylates with Cyanoazobenzene Side Groups: Dielectric Spectroscopy and Thermally Stimulated Depolarization Currents

Molecular mobility in liquid crystalline (LC) side-chain polyacrylates and polymethacrylates with cyanoazobenzene side mesogens and methylene spacers of different lengths was investigated by dielectric spectroscopy and thermally stimulated depolarization currents methods. For both series of LC polymers, near glass transition temperature (Tg), two relaxations of cooperative character related to segmental motion (α process) and reorientation of mesogens around their short axis (δ process) are observed. Cooperativity of these processes decreases with increasing length of side chains being higher for the α process. Good correlation between dielectric spectroscopy and thermally stimulated depolarization currents methods is obtained for the temperature transitions of the α and δ processes.

Photoinduced orientation of photoresponsive copolymers with N-benzylideneaniline and nonphotoreactive mesogenic side groups

The photoinduced reorientation behaviors of liquid crystalline (co)polymethacrylates (LCPs) comprised of N-benzylideneaniline (NBA) and 4-cyanobiphenyl or 4-methoxybiphenyl side groups are compared using linearly polarized (LP) 313 nm and LP 365 nm light. LCP films realize effective cooperative reorientation based on trans-cis-trans photoisomerization of the NBA moieties using LP 365 nm light and stabilization of the orientation structure due to copolymerization. In contrast, photochemical hydrolysis lowers the reorientation efficiency of LCP films when using 313 nm light. Changing the polarization direction of the LP 365 nm light controls the reorientation direction of the LCP films for several cycles, but the in-plane orientation order gradually decreases. Finally, LP 365 nm exposure and subsequent solvent annealing using a photo-cross-linked film achieve a repeatable write-erase-write of the orientation structure of the copolymer film.

Influence of End Groups in Photoinduced Reorientation of Liquid Crystalline Polymer Films Based on Axis-Selective Photo-Fries Rearrangement

Photoreactive liquid crystalline polymethacrylates with phenylbenzoate side groups with different molecular terminal were synthesized and their photoresponsive behavior was investigated. All polymer films underwent axis-selective photo-Fries rearrangement and generated photoinduced optical anisotropy upon exposure to linearly polarized ultraviolet (LPUV) light. In addition, annealing the photoreacted films enhanced molecular reorientation perpendicular to polarization direction of LPUV light, and the molecular terminal groups strongly affected the reorientation behavior.

Directed self-assembly of nematic liquid crystalline polymers on a rubbed polyimide alignment layer

Amphiphilic diblock copolymers PEO-b-PMA(4OPB) comprising hydrophilic poly(ethylene oxide) (PEO) and hydrophobic nematic liquid crystalline polymethacrylate [PMA(4OPB)] subunits were synthesized by atom transfer radical polymerization. The liquid crystalline property and microphase-separated structure were investigated by differential scanning calorimetry, polarized optical microscopy, atomic force microscopy, and X-ray diffraction analysis. Unimodal PEO-b-PMA(4OPB) with 114 ethylene oxide repeating units and phenylbenzoate moieties as methacrylate side chain mesogens showed a nematic phase when the methacrylate repeating unit was around 57. Hexagonal cylinders of the PEO subunits were formed in nematic phase and supercooling states. The thin film on a rubbed polyimide alignment layer showed a microphase-separated structure independent of the rubbing direction, while horizontally aligned cylinders were observed in the presence of small nematic liquid crystalline molecules of 4-cyano-4′-pentyloxybiphenyl (5OCB). The assistance of nematic liquid crystalline molecules was necessary for directed self-assembly of the nematic liquid crystalline diblock copolymer in accordance with the rubbing direction of the polyimide alignment layer.

Photomobile Properties of Interpenetrating Polymer Network Films Composed of Azobenzene Liquid Crystalline Polymer and Polymethacrylates

Various fabrication methods of photomobile polymer materials containing photochromic moieties have been developed to improve their photoresponsive and mechanical properties. Here, we newly prepared photomobile polymer materials with interpenetrating polymer network (IPN) structures. The IPN films, which are composed of azobenzene liquid crystalline polymer (LCP) and polymethacrylates, showed reversible bending behavior upon exposure to UV and visible light. Bending speed of the IPN films became higher with decreasing glass-transition temperature of polymethacrylate components. The IPN film containing poly(dodecyl methacrylate) showed faster bending than the pristine azobenzene LCP film.

Highly birefringent side-chain LC polymethacrylate with a dinaphthyl-acetylene mesogenic unit

We synthesised a highly birefringent liquid crystalline (LC) side-chain polymer based on polymethacrylate with a dinaphthyl-acetylene mesogenic unit in the side chain. Three polymer types with different carbon numbers in the terminal alkoxy chain (m = 1, 3 and 6) were prepared, and their optical properties as well as phase sequential behaviours were investigated. All polymers formed a nematic phase with a wide temperature range, and a smectic C phase was detected between the nematic and crystal phases when m was extended to 6. Notably, for homologous m = 1, we successfully obtained a nematic glassy-state LC film with high optical performance under ambient temperature conditions. The optical properties of this LC film were shown to be consistent with those of a highly birefringent, low molar mass LC material, with birefringence, extraordinary and ordinary refractive indices of 0.36, 1.82 and 1.46, respectively at 550 nm.

Photoinduced Orientation of Photoresponsive Polymers with N-Benzylideneaniline Derivative Side Groups

Axis-selective photoreaction, photoinduced orientation, and formation of surface relief (SR) gratings of liquid crystalline polymethacrylates comprised of N-benzylideneaniline (NBA) derivative side groups are explored. Irradiating with linearly polarized (LP) 313 nm light generates molecular reorientation of the NBA side groups based on the axis-selective trans–cis–trans photoisomerization and photocleavage of the NBA groups. The in-plane order parameter (S) and birefringence (Δn) are ∼0.53 and ∼0.16, respectively. Furthermore, thermally stimulated amplification of the photoinduced optical anisotropy occurs upon exposing films to LP 313 nm light and subsequently annealing in the liquid crystalline temperature range of the material. The amplified S and Δn values are ∼0.71 and ∼0.21, respectively. Finally, polarization holography using 325 nm He–Cd laser beams with various interferometric polarization conditions demonstrates the formation of SR gratings with a molecularly oriented structure based on the per...

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.

Axis-Selective Photo-Fries Rearrangement and Photoinduced Molecular Reorientation in Liquid Crystalline Polymer Films

The thermally enhanced photoinduced molecular reorientation based on an axis-selective photo-Fries rearrangement was explored for the first time using three liquid crystalline polymethacrylates with 4-(2′-(4″-methoxyphenyl)ethynyl)phenyl benzoate side groups (1a), 4-methoxybiphenyl benzoate side groups (1b), and 4-methoxyphenyl benzoate side groups (1c). All the polymeric films exhibited an axis-selective formation of the photo-Fries products at the phenyl benzoate derivatives side groups by irradiating with linearly polarized (LP) UV light. Annealing the films in the liquid crystalline temperature range of the material greatly amplified the photoinduced small optical anisotropy of the irradiated films. For 1a and 1b, the photoinduced small anisotropy was amplified in a direction parallel to the polarization (E) of LPUV light and the reorientational order (S) was greater than 0.7. In contrast, the orientation direction was perpendicular to E for 1c (S ∼ 0.57). Finally, a simple double-exposure method usin...

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.