Effect of photoreactivity of polyimide on the molecular orientation of liquid crystals on photoreactive polymer/polyimide blends

We prepared blend alignment layers from polymethacrylate with coumarin side chains (PMA-g-coumarin) and polyimides for the orientation of liquid crystals (LCs) using linearly polarized ultraviolet (UV) irradiation. We used two different polyimides, namely 4,4′-(hexafluoro-isopropylidene) diphthalic anhydride-3,5-diamino-benzoic acid (6FDA-DBA) and pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA). It was found that the molecular orientation of the LC depended on the type of polyimide in the blend alignment layer. The thermal stability of the LC orientation was enhanced regardless of the type of polyimide, while the direction of LC orientation was different for each type of polyimide. The photoreactivity of the polyimide was a very important factor in determining the molecular orientation of the LC on the blend alignment layer. This may be attributed to the different mechanisms of LC orientation on PMA-g-coumarin and polyimide induced by the polarized UV irradiation. The direction of the LC orientation could be changed by controlling the photoreaction of the polyimides using the appropriate UV filter for the polarized UV irradiation.     

Effect of photoreactivity of polyimide on the molecular orientation of liquid crystals on photoreactive polymer/polyimide blends

We prepared blend alignment layers from polymethacrylate with coumarin side chains (PMA-g-coumarin) and polyimides for the orientation of liquid crystals (LCs) using linearly polarized ultraviolet (UV) irradiation. We used two different polyimides, namely 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride-3,5-diamino-benzoic acid (6FDA-DBA) and pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA). It was found that the molecular orientation of the LC depended on the type of polyimide in the blend alignment layer. The thermal stability of the LC orientation was enhanced regardless of the type of polyimide, while the direction of LC orientation was different for each type of polyimide. The photoreactivity of the polyimide was a very important factor in determining the molecular orientation of the LC on the blend alignment layer. This may be attributed to the different mechanisms of LC orientation on PMA-g-coumarin and polyimide induced by the polarized UV irradiation. The direction of the LC orientation could be changed by controlling the photoreaction of the polyimides using the appropriate UV filter for the polarized UV irradiation.     

Liquid crystalline comb-like polyimides with biphenyl-based side groups

We have synthesized series of comb-like polyimides with mesogenic units in their side groups. Such comb-like polyimides were obtained by polycondensation of aromatic diamines bearing biphenyl-based mesogenic moieties with bicyclo[2,2,2]oct-7-ene 2,3,5,6-tetracarboxylic dianhydride (BCDA). The different diamines, with two lengths of spacer (with 6 and 11 methylene groups), were synthesized in three steps using as mesogenic groups: biphenyl, 4-cyanobiphenyl and 4-(2-methyl-1-butoxy)biphenyl. The synthesis of the polyimides was performed in two steps: polycondensation of a dianhydride with a diamine in N-methyl-2-pyrrolidone at room temperature giving the corresponding polyamic acid, followed by thermal cyclization into the corresponding polyimide. The comb-like polyimides were studied by X-ray diffraction between room temperature and 250°C. Two types of smectic structure were established: SmA₁ for the long spacer and SmC₁ for the short spacer.     

Unusual electro-optical behaviour of the nematic polyacrylate

ABSTRACT

High sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties.

The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated.     

Synthesis of novel aromatic polyimides containing bulky side chain for vertical alignment liquid crystals

In this study, a novel 4-（4-octyloxybenzoyloxy）biphenyl-3′,5′-diaminobenzoate and polyimides based on it were synthesized. The polyimide with mesogenic unit side chain exhibited excellent vertical alignment for nematic liquid crystal （LC）. The pretilt angles of LCs above 89° were kept after the rubbing process with 220 mm rubbing strength. The polyimide films as the alignment layer were baked at 120℃ for 12 h, the vertical alignment of LCs was still uniform and stable. Meanwhile, the UV-vis spectra of the noyel polyimide films showed the high transparency in a visible wave length.     

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.     

Liquid crystalline comb-like polyimides with biphenyl-based side groups

We have synthesized series of comb-like polyimides with mesogenic units in their side groups. Such comb-like polyimides were obtained by polycondensation of aromatic diamines bearing biphenyl-based mesogenic moieties with bicyclo[2,2,2]oct-7-ene 2,3,5,6-tetracarboxylic dianhydride (BCDA). The different diamines, with two lengths of spacer (with 6 and 11 methylene groups), were synthesized in three steps using as mesogenic groups: biphenyl, 4-cyanobiphenyl and 4-(2-methyl-1-butoxy)biphenyl. The synthesis of the polyimides was performed in two steps: polycondensation of a dianhydride with a diamine in N-methyl-2-pyrrolidone at room temperature giving the corresponding polyamic acid, followed by thermal cyclization into the corresponding polyimide. The comb-like polyimides were studied by X-ray diffraction between room temperature and 250°C. Two types of smectic structure were established: SmA₁ for the long spacer and SmC₁ for the short spacer.     

Synthesis and properties of fluorine tail-terminated cyanobiphenyls and terphenyls for chemoresponsive liquid crystals

ABSTRACT

A series of fluorine tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesised and mesogenic properties described. Comparison with the non-fluorinated K series and M series indicates that the terminal fluorine atom generally decreases the transition temperatures and, more interestingly, depresses the formation of a smectic phase. Several binary LC mixtures formed by the fluorine tail-terminated compounds were found exhibiting promising room temperature nematic phases with wide ranges. The mixture F7OCB and F8OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which may have potential application in designing a more sensitive and faster LC system to targeted analytes.     

Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal

We have explored the change in alignment of a nematic liquid crystal, 4'-pentyl-4-cyanobiphenyl (5CB) with three types of photosensitive polyimide as the alignment layer by photoirradiation at 366 nm. The photosensitive polyimide alignment layer induced a reversible change in alignment of 5CB. It was observed that the 5CB molecules became aligned from homogeneous alignment to homeotropic on photoirradiation with a d.c. electric field as a bias, and reversed to the homogeneous state when photoirradiation was ceased. This result indicates that optical switching could be repeated by on and off switching of the excitation light at 366 nm. The optical switching of the nematic liquid crystal might be mainly due to a photophysical change in the polyimide surface which is affected by the chemical structures of the polyimides at the temperature at which 5CB exhibits a nematic phase. The optical switching of nematic liquid crystals with photosensitive polyimides as the alignment layer is a novel driving method for nematic liquid crystals.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

Synthesis and properties of polyimides derived from bis-(Aminophenoxy) containing naphthalene, [Phenyl] propane and [Methyl] cyclohexane segment and 4, 4′-carbonyldiphthalic anhydride

ABSTRACT

Three wholly, semi aromatic and aliphatic-aromatic polyimides containing bis(phenoxy) naphthalene, bis[(phenoxy) phenyl] propane and bis(phenoxy-methyl) cyclohexane segments by the two-step procedure from 2, 7-bis(4-aminophenoxy) naphthalene (BAPON), 2, 2-bis[4-(4-aminophenoxy)phenyl]propane (BAPOP), 1, 4-bis (4-aminophenoxy methyl) cyclohexane (BAPMC) as a diamine and 4,4′-carbonyldiphthalic anhydride (CDPA) were prepared. The first step of this procedure including ring-opening polyaddition in a polar solvent to give poly(amic-acid)s, second step containing cyclodehydration reaction to form polyimides. Synthesized monomer and polyimides were characterized by FT-IR, ¹H NMR spectroscopy and elemental analyses (CHN) that obtained results gave the most powerful evidence. The polyimide synthesized from BAPON was characterized as semi-crystalline, whereas the other polyimides showed amorphous patterns by the x-ray diffraction studies. The inherent viscosity was ranging between 0.87–1.01 dL/g. Tensile strength, initial moduli, and elongation at break of the polyimide films ranged from 88–117 MPa, 1.98–2.32 GPa, and 5–8%, respectively. Thermogravimetric analysis in nitrogen atmosphere shows that these polymers having good stability, so 10% weight will be lost in the range of 500–630°C. The point of polyimide with BAPMC segment, is “adding of good thermal stability and processability” lower moisture absorption and dielectric constant (0.75% and 2.90).     

Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal

We have explored the change in alignment of a nematic liquid crystal, 4'-pentyl-4-cyanobiphenyl (5CB) with three types of photosensitive polyimide as the alignment layer by photoirradiation at 366 nm. The photosensitive polyimide alignment layer induced a reversible change in alignment of 5CB. It was observed that the 5CB molecules became aligned from homogeneous alignment to homeotropic on photoirradiation with a d.c. electric field as a bias, and reversed to the homogeneous state when photoirradiation was ceased. This result indicates that optical switching could be repeated by on and off switching of the excitation light at 366 nm. The optical switching of the nematic liquid crystal might be mainly due to a photophysical change in the polyimide surface which is affected by the chemical structures of the polyimides at the temperature at which 5CB exhibits a nematic phase. The optical switching of nematic liquid crystals with photosensitive polyimides as the alignment layer is a novel driving method for nematic liquid crystals.     

Polyimides with main-chain photosensitive groups: Synthesis, characterization and their properties as liquid crystal alignment layers

In this work, a series of polyimides containing main-chain chalcone groups and side-chain cinnamate or acetate groups were synthesized. In the synthetic route, three dianhydride monomers were prepared by the reactions of 1,3-dihydro-1,3-dioxoisobenzofuran-5-carbonyl chloride with 1,3-bis(4-hydroxylphenyl)prop-2-en-1-one, 1,3-bis(4-(2-hydroxyethyl)phenyl)prop-2-en-1-one, and 1,3-bis(4-hydroxyphenyl)-pent-1-en-3-one-4-en, respectively. The precursor polyimides were obtained by the polycondensation of the dianhydride monomers with 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and the in situ imidization. After the polymerization, the final products were obtained by introducing the side groups through the reactions of the polyimide precursors with cinnamoyl chloride or acetic anhydride. The chemical structures and properties of the polyimides were characterized by elemental analysis, ¹H NMR, ¹³C NMR, FTIR, GPC, and thermal analysis. The polyimides show high thermal stability and good solubility in aprotic polar organic solvents. The polyimides can undergo sensitive [2+2] photocycloaddition reaction upon the UV light irradiation. After irradiation with linearly polarized UV light, the polyimide thin films can induce 5CB liquid crystal molecules to homogeneously align in the liquid crystal cells. Both the alignment ability and pretilt angles of the molecular orientation depend on the chemical structures of the polyimides.     

Preparation of ordered and crosslinked films from liquid crystalline vinyl ether monomers

Thermally stable ordered films were prepared by in-situ photopolymerization of an oriented monomer mixture, consisting of mesogenic monofunctional and bifunctional vinyl ethers. Orientation was achieved by a simple surface treatment, using an unidirectionally rubbed polyimide film. The films restored their orientation when cooled down from temperatures of 200°C. Highly ordered densely crosslinked films have been prepared by polymerization of bifunctional mesogenic vinyl ether monomers. Polymerization from various monomer phases resulted in LC polymer network films with different molecular organizations. It was shown that films with nematic, smectic A and smectic B structures were obtained, the latter having a very high degree of orientation. The films were analyzed with small-angle X-ray scattering, polarized light microscopy and infrared- dichroism measurements.     

Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety

ABSTRACT

We designed and synthesised a reactive mesogen containing diphenylacetylene moiety in the mesogenic core and two polymerisable acrylate groups at both ends. By irradiating linearly polarised UV light on the conventional host LC mixture containing a small amount of the synthesised reactive mesogen in a sandwiched cell without an alignment layer, we demonstrated an in-situ photo-induced homogeneous alignment of liquid crystals without a pre-treated alignment layer, which was achieved by an irreversible polarisation-selective [2 + 2] photodimerization of diphenylacetylene moiety with linearly polarised UV irradiation at above the isotropic temperature of LC mixture. The resulting homogeneous alignment showed a superior initial dark state, negligible pretilt angle and excellent stabilities. Furthermore, the in-plane switching (IPS) LC cell prepared by this method exhibited a better dark state and electro-optic performance compared to that with conventional-rubbed polyimide alignment layer. The single photoirradiation process automatically resulted in a perfect alignment matching of optical axes between the top and bottom substrates in the LC cell, giving rise to an excellent dark state overcoming an intrinsic misalignment issue and complex fabrication process. The proposed in-situ alignment method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment technique in the manufacturing of high-resolution liquid crystal displays.     

New room temperature nematogens by cyano tail termination of alkoxy and alkylcyanobiphenyls and their anchoring behavior on metal salt-decorated surface

ABSTRACT

A series of cyano tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesized and mesogenic properties described. Comparison with the K series and M series indicates that the terminal cyano group generally enhances the supercooling of the molecules. Furthermore, several binary LC mixtures formed by the cyano tail-terminated compounds were found exhibiting promising room temperature nematic phases ranges comparable to the commercial quaternary mixture E7. The equimolar binary mixture of CN5OCB and CN7OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which is consistent with G_BE values we calculated. As such, these materials are promising candidates for sensor devices which display a rapid response to a variety of analytes.     

Controlling domain orientation of liquid crystalline block copolymer in thin films through tuning mesogenic chemical structures

Controlling the macroscopic orientation of nanoscale periodic structures of amphiphilic liquid crystalline block copolymers (LC BCPs) is important to a variety of technical applications (e.g., lithium conducting polymer electrolytes). To study LC BCP domain orientation, a series of LC BCPs containing a poly(ethylene oxide) (PEO) block as a conventional hydrophilic coil block and LC blocks containing azobenzene mesogens is designed and synthesized. LC ordering in thin films of the BCP leads to the formation of highly ordered, microphase‐separated nanostructures, with hexagonally arranged PEO cylinders. Substitution on the tail of the azobenzene mesogen is shown to control the orientation of the PEO cylinders. When the substitution on the mesogenic tails is an alkyl chain, the PEO cylinders have a perpendicular orientation to the substrate surface, provided the thin film is above a critical thickness value. In contrast, when the substitution on the mesogenic tails has an ether group the PEO cylinders assemble parallel to the substrate surface regardless of the film thickness value. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 532–541     

Hydrogen-bonded LC nanocomposites: characterisation of nanoparticle-LC interactions by solid-state NMR and FTIR spectroscopies

Although the molecular-level nanoparticle-liquid crystal (NP-LC) interactions are key for forming stable NP dispersions in LC, characterisation of these interactions is scarce in the literature. Recently, we have developed hydrogen-bonded LC nanocomposites based on mesogenic carboxylic acids and NPs functionalised with hydrogen-bond acceptor/donor groups. Here, we apply solid-state ¹H and ¹³C NMR and Fourier transform infrared spectroscopies to model systems consisting of mixtures of trans-4-n-butylcyclohexanecarboxylic acid (4-BCHA), 4-hexylbenzoic acid (6BA) and 4,4?-bipyridine. The binary mixture, 4-BCHA:6BA, was also studied. The results were compared to monolayer quantities of 6BA and 4-BCHA adsorbed on pyridine functionalised zirconia NPs (ZrO₂-n₃-pyridine) to understand the different effects of the functionalised NPs on the LC properties.     

UV-induced homeotropic orientation of nematic liquid crystals dissolving two species of monomers carrying liquid-crystalline side-chain group and anthracene group

We propose the technique for formation of the polymer orientation film (POF) by polymerisation of the monomers being dissolved in the liquid crystal (LC) material for controlling the orientation of the LC molecules. For obtaining the homeotropic orientation, combination of the two monomers, 4-(4?-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-anthracene (DMAAnth), was found to be useful. The monomer DMAAnth initiates the polymerisation under ultraviolet (UV) light exposure, and the AOBBE unit induces the homeotropic orientation. The monomer DMAAnth is useful for maintaining the high voltage holding ratio and low residual direct-current voltage after UV light exposure because the molecules of DMAAnth do not remain in the LC layer.     

Stabilization of the Columnar Mesophase of Perylenediimide by Racemic Triple Tails

Whereas perylene tetracarboxdiimides derived from amino‐n‐alkanes if at all only show monotropic (thermodynamically unstable) mesogenic self‐assembly, the hexagonal columnar liquid crystalline state can be stabilized over a broad temperature range with doubly branched, doubly racemic alkyl residues. An improved tendency to homeotropic surface orientation is observed, and the orientation of the liquid crystalline domains is maintained upon cycling through the crystalline state at room temperature.