Preparation and alignment properties of photosensitive polyimide containing benzophenone in main chain

ABSTRACT

In order to study the effect of benzophenone (BP) group in the main chain on the photo-alignment behaviour of cinnamoyl groups, a novel photosensitive polyimide (BTDA-PI-CA) is synthesised by 3,3?,4,4?-benzophenonetetracarboxylic dianhydride (BTDA), 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (AHHFP) and cinnamoyl chloride. The BTDA-PI-CA films exhibit good alignment performance after the non-polarised ultraviolet ageing lamp exposure, which is due to the polyimide with BP group can be used to improve the photosensitivity. Moreover, the thermal stability of the LC cells is pretty good and the pretilt angle of the LC cells is 0.5, which can be used in parallel mode LCD.     

Polyimide liquid crystal alignment layers prepared by soft-lithography

Elastomeric replicas of surface relief structures were prepared by molding poly(dimethylsiloxane) (PDMS) precursors against photo-inscribed surface-relief-gratings on azo polymer films. The PI solutions were micro-contact printed with the elastomeric PDMS replica on quartz slides. Good surface-relief-grating structures were formed on the quartz slides. The quartz slides covered with surface-relief-grating polymide (PI) films were then assembled into liquid crystal (LC) cells. The transmittance passing through the cell between crossed polarizers changed periodically with a regular 90° separation of the rotational angle. The pretilt angle of the cell was found to be 2.8°. Results showed that the PI films with surface-relief-grating structures by this micro-contact printing process have good liquid crystal alignment ability. This preparation method of alignment layers can be considered a potentially useful technique in the LC display (LCD) industry in the future. Translated from Acta Polymerica Sinica, 2006, (7): 908–911 [译自: 高分子学报]     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

Abstract

The homogeneous alignment of a liquid crystal material, 4′-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.     

Liquid crystal alignment properties of n-alkylsulphonylmethyl-substituted polyoxyethylenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of n-alkylsulphonylmethyl-substituted polyoxyethylenes (#S-PEO, #?=?4, 6, 7, 8 and 10), where # is the number of carbon atoms in the n-alkyl side groups having different n-alkyl chain length, were investigated as a function of the rubbing density. The LC cells made from unrubbed #S-PEO (# ≥8) films having more than eight carbon atoms in the n-alkyl side groups showed homeotropic LC alignment. The homeotropic LC alignment behaviour correlated well with the surface energy values of the unrubbed #S-PEO films; homeotropic LC alignment was observed when the surface energy values of the unrubbed #S-PEO films were smaller than about 21.62 mJ m^?2. The LC cells made from rubbed #S-PEO (# ≥7) films having more than seven carbon atoms with a rubbing density of 150 showed homeotropic LC alignment. It was also found that the tilt angle of the LCs on the rubbed #S-PEO films was affected not only by the n-alkyl chain length of the polymers, but also by the rubbing density, regardless of the surface energy value of the #S-PEO film.     

Synthesis of soluble polyimides for vertical alignment of liquid crystal via one-step method

A series of polyimides (PIs) were copolymerized from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (DBPDA), 3,3′-dimethyl-4,4′-methylene-dianiline (DMMDA) and 4,4′-oxydi(phthalic anhydride) (ODPA) via one-step method. The PIs possessed excellent solubility in polar aprotic solvents and easily formed thin flexible films by solution casting. The glass-transition temperatures (T_gs) of the PIs were in the range of 219-242 °C and thermal decomposition temperatures in nitrogen occurred above 350 °C. The resultant PI films exhibited high transparency at wavelengths greater than 400 nm and induced excellent uniform vertical alignment of liquid crystal (LC). Even after the rubbing process, the pretilt angles of LC were still above 89°. The PI seems to be a prospective material for alignment layers in flexible displays.     

Vertical alignment of liquid crystal on tocopherol-substituted polystyrene films

We synthesised a series of vitamin-based and renewable tocopherol-substituted polystyrene (PTOC#, # = 20, 40, 60, 80 and 100), where # is the molar content of tocopherol moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a higher molar content of tocopherol side group showed vertical LC alignment behaviour. The vertical alignment (VA) behaviour was well correlated with the surface energy value of these polymer films. For example, VA was observed when the surface energy values of the polymer were smaller than about 35.22 mJ/m² generated by the nonpolar tocopherol moiety having long and bulky carbon groups. Good electro-optical characteristics, such as voltage holding ratio and residual DC voltage, and aligning stabilities at 200°C and ultraviolet irradiation of 10 J/cm² were observed for the LC cells fabricated using PTOC100 as a LC alignment layer. Therefore, it was first found that the renewable tocopherol-based materials can produce an eco-friendly vertical LC alignment system.     

Novel alignment mechanism of liquid crystal on a hydrogenated amorphous silicon oxide

The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB.     

Thermal stability of alignment of a nematic liquid crystal induced by polyimides exposed to linearly polarized light

The thermal stability of alignment of a nematic liquid crystal (LC) on three polyimide (PI) films exposed to linearly polarized light at 366 nm was investigated. Polarizing optical microscopy analysis indicates that the thermal stability of the LC alignment on the PI film without significant structural change was higher than that with obvious structural change.     

Thermal stability of alignment of a nematic liquid crystal induced by polyimides exposed to linearly polarized light

The thermal stability of alignment of a nematic liquid crystal (LC) on three polyimide (PI) films exposed to linearly polarized light at 366 nm was investigated. Polarizing optical microscopy analysis indicates that the thermal stability of the LC alignment on the PI film without significant structural change was higher than that with obvious structural change.     

Homogeneous alignment of liquid crystals on low-temperature solution-derived gallium oxide films via IB irradiation method

In this paper, solution-derived gallium oxide (GaO) films are fabricated for the homogeneous alignment of liquid crystals (LCs) after an ion-beam (IB) irradiation process. GaO thin films are prepared under a variety of temperatures and different IB irradiation intensities, and the physicochemical performances of the fabricated GaO thin films are analysed using a UV-vis spectrometer, an atomic force microscope, and X-ray photoelectron spectroscopy. A higher transmittance of 85.40% from GaO thin film is obtained compared with that of polyimide (PI) film (83.52%), which indicates the feasibility for a GaO thin layer to substitute for a conventional PI layer as an alignment layer. LCs are found to align on the GaO thin film after pre-baking at 100°C and homogeneous and uniform low-IB intensity irradiation. We also determined the electro-optical (EO) characteristics of the twisted nematic (TN) cells fabricated with GaO thin layers and found them to be similar to those of cells fabricated with PI layers. Overall, GaO films achieved via the IB irradiation method are promising LC alignment layers due to the method’s low-temperature solution-derived process.     

Influence of molecular mass on the liquid crystal alignment of photosensitive fluorinated polyester films

Photosensitive fluorinated polyesters (polymer-n) of varying molecular mass M _n (number-average molecular mass) were synthesized. The thin films formed from polymer-n samples could induce liquid crystal (LC) alignment after irradiation by linearly polarized ultraviolet light. The LC alignment direction on the irradiated films was investigated in detail by linearly polarized infrared spectroscopy and polarizing optical microscopy. It was found that LC alignment behaviour changed with change in the molecular mass of polymer-n: irradiated films with lower or higher M _n induced homeotropic or homogenous alignment, respectively. There was no clear morphological anisotropy in these aligned films, as observed by atomic force microscopy. The surface energies of the irradiated films were also measured using the indirect contact angle method, where both surface energy and its polar component increased with increasing M _n. The variation in M _n could be considered as a main reason for varying alignment behaviour.     

Preliminary communication Investigation of pretilt angle generation in nematic liquid crystal with oblique non-polarized UV light irradiation on polyimide films

We investigated new rubbing-free techniques for liquid crystal (LC) alignment with non- polarized ultraviolet (UV) light irradiation on plates coated with two kinds of the polyimide (PI) films. It was found that monodomain alignment of nematic (N) LC is obtained in the cell having a PI surface without a side chain. We successfully observed that the generated pretilt angle of the NLC is about 3 with an angle of incidence of 70 on the PI surface without side chains. This pretilt angle generation is attributed to interaction between the LC molecules and the polymer surfaces; the uniform alignment of NLC is attributed to anisotropic dispersion force effects due to photo-depolymerization of polymer on PI surfaces.     

Preparation and characterization of photo-decomposable ester-containing semi-alicyclic polyimide alignment layers for potential applications in in-plane switching thin-film transistor liquid crystal display devices

Poly(amic acid)s (PAAs) alignment agents have been prepared from the alicyclic dianhydrides, including 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA, I), 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride (DMCBDA, II), and ester-linked aromatic diamines, including 4-aminophenyl-4′-aminobenzoate (APAB, a) and bis(4-aminophenyl)terephthalate (BPTP, b), respectively. The derived PAAs, including PAA-I_a (CBDA-APAB), PAA-I_b (CBDA-BPTP), and PAA-II_a (DMCBDA-APAB) exhibited the preferable molecular weights, while the PAA-II_b (DMCBDA-BPTP) showed the low one due to the low polymerization reactivity both for the DMCBDA dianhydride and the BPTP diamine. All the PAA solutions except PAA-II_b were adopted as the alignment components to fabricate liquid crystal (LC) minicells with a mode of in-plane switching (IPS). The polyimides (PI) alignment layers derived from the thermal dehydration reaction of the PAA precursors at 230°C for 30 min showed good alignments effects to the LC molecules, which exhibited the pretilt angles (θ_p) from 0.09 to 0.15° after being exposed by the linear polarized ultraviolet light sources with the wavelength of 254 nm. In addition, the PI alignment layers afforded good optoelectronic features to the minicells, including the voltage holding ratio values over 97% at room temperature, and the residual direct circuit voltages lower than 1.0 V. The anchoring energy results indicated that the PI-II_a (DMCBDA-APAB) alignment layer showed the highest interaction with the LC molecules, and thus exhibited the highest threshold voltage (V_th) in the voltage-transmittance (V-T) measurements for the minicells.     

Bent-core liquid crystal-functionalised flexible polymer substrates for liquid crystal alignment

ABSTRACT

A transparent flexible polymer film is chemically functionalised with a bent-core liquid crystal (BCLC) compound for effective alignment of the bulk BCLC sample at the substrate–LC interface. The surface attachment was achieved via a simple procedure which involved pre-treatment of the polymer film (commercial name: over head projector film) using piranha solution followed by chemically attaching the BCLC compound through silane condensation reaction. Surface characterisation of the unmodified and BC-modified flexible films was carried out through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, contact angle (CA) and atomic force microscopy (AFM) techniques. The BC-modified flexible substrates are analysed for their efficiency to orient the bulk LC sample. Remarkably, the chemically modified polymer substrates are highly efficient in vertically aligning both the BC and rod-like LC samples at the substrate–LC interface, in comparison to their unmodified counterparts. The described method is simple, reproducible, surface modified substrates are highly stable and more importantly reusable. The demonstrated method for the alignment of BCLCs advances a step forward towards the realisation of applications proposed for these fascinating compounds.     

Liquid crystal alignment properties of 2-naphthoxymethyl-substituted polystyrenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of 2-naphthoxymethyl-substituted polystyrenes with different contents of naphthoxymethyl side groups were investigated. The polymer films exhibited good optical transparency in the visible light region (400–700 nm). The LC cells made from the unrubbed films of polymers having more than 57 mol%?of 2-naphthoxymethyl containing monomeric units showed homeotropic LC alignment with a high pretilt angle of about 90^o. Good electro-optical characteristics, such as the threshold voltage, response time, voltage holding ratio and residual DC voltage were observed for the LC cells fabricated with the polymer having 100 mol%?of 2-naphthoxymethyl containing monomeric units as an LC alignment layer.     

Effects of plasma treatments on correlation between chemical structures of DLC films and liquid crystal alignment

In an effort to obtain an improved liquid crystal (LC) alignment layer for liquid crystal display device applications, amorphous diamond‐like carbon thin films were deposited on ITO‐coated glass substrates by an rf magnetron sputtering technique at room temperature and then treated with plasma in various atmospheres. The polarized images and pretilt angles of the LC cells showed that LC alignment was enhanced by post‐plasma treatments of the films. In Raman and X‐ray photoelectron spectroscopy spectra of the films, an increase in the fraction of sp²‐bonding was observed after post‐plasma treatments of the films. In particular, H₂ plasma‐treated film had the largest fraction of sp²‐bonding at the film surface and showed much improved alignment capabilities. These results suggest that π‐bondings of the sp²‐structure at the surface rather than the bulk play an important role in LC alignment.     

Hyperbranched polyimide application in liquid crystal alignment layers

Several hyperbranched polyimides (HBPIs) were applied in liquid crystal (LC) alignment layers and exhibited outstanding performance for LC alignment. The maximum pretilt angle was above 8°, and the minimum pretilt angle was 4.2°. The results of atomic force microscope measurement disclosed that a lot of grooves were aligned parallel to the rubbing direction and found that the grooves were not main factor for LC alignment. The LC alignment and pretilt angles are unambiguously associated with the intrinsic HBPI chemical structures. The results of thermal gravimetric analysis and ultraviolet–visible spectra showed that the HBPIs had good thermal stability and excellent transmittance. T₅ and T₁₀ were higher than 360°C and 400°C, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Molecular orientation and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Isotropic thin films of three original phenyl substituted cinnamate-based polymers, here-after referred to as 'Para', 'Meta' and 'Metamet' have been exposed to linearly polarized UV light and their photoinduced molecular orientations have been studied. The resulting photocrosslinked anisotropic polymer films were characterized using UV, conventional and polarization modulation (PM) FTIR spectroscopies. From UV and PM-IR linear dichroism measurements, at least two simultaneous orientation processes appear to play a key role in these phenyl substituted cinnamate-based systems. On the one hand, isomerization reactions deplete chromophores along the polarization direction (P) of the UV light and induce a preferential orientation of remaining 'trans'-isomers perpendicular to P; on the other hand, cycloaddition reactions lead to the formation of either head to head or head to tail photodimers aligned preferentially along P in the 'Para' and to a lesser extent in the 'Meta' and 'Metamet' systems. These last results are related to the different liquid crystal alignment properties of the films, and the influences of the chemical structure of the chromophores are discussed.     

Nano P(VDF-TrFE) doped polyimide alignment layers for twisted nematic liquid crystal devices

ABSTRACT

The rapid development of consumer electronics and portable devices imposes a great demand for energy efficient information display systems. Among the information display devices, liquid crystal display (LCD) devices stands in the front. The fabrication of energy-efficient LCD systems demands new material and techniques. In this work, the conventional polyimide alignment layer of twisted nematic liquid crystal device (TNLCD) was replaced with ferroelectric polymer nanoparticle doped alignment layer. Morphology of the alignment layer was analysed using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The ferroelectric nature of the polymer alignment layer was studied using dynamic contact electrostatic force microscopy (DC-EFM). TNLCD cells are fabricated with this modified alignment layer and the switching characteristics are compared with the conventional TNLCD devices. The TNLCD with modified alignment layer has shown a reduction of 50% in threshold (V_thr) and 47% reduction in saturation voltage (V_sat).     

Preparation and characterization of a novel polyimide liquid crystal vertical alignment layer

A series of diamines with a side chain containing rigid biphenyl unit and nonpolar alkoxy side end group [4-alkoxy-biphenol-3′,5′-diaminobenzoate] (Cm-BBDA, m = 4, 6, 12) were synthesized and their chemical structures were confirmed by Fourier Transform Infrared Spectroscopy (FT-IR) and nuclear magetic resonance spectroscopy (¹H NMR). Then three polyimides (PIs) were prepared by copolymerization of pyromellitic dianhydride (PMDA), 4,4′-methylenedianiline (MDA) and Cm-BBDA in N-methyl-2-pyrrolodone (NMP), and chemical structures of all PIs were confirmed by FT-IR. Structural identification of all poly(amic acid)s (PAA) was performed by ¹H NMR. Liquid crystal (LC) cells were fabricated using these PIs as the alignment layer for characterization of the alignment properties of LCs. It was found that the planar alignment was obtained when PI with side chain containing alkoxy side end group of 4 carbon atoms was employed and the vertical alignment was observed when alkoxy side end groups of 6 or 12 carbon atoms were included. A uniform vertical alignment was validated by polarizing microscopy. It was testified that LC vertical alignment possessed high thermal stability.