Polyimide aligned ferroelectric smectic C liquid crystals

Abstract

27 alkyl-aromatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectric liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained and the stability of the alignment is discussed.     

Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells

Eight aliphatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectic liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained is discussed. It is shown that the structure of the polymer alignment layer strongly influences the cone angle found between the relaxed states in a surface stabilized ferroelectric liquid crystal cell. Highly crystalline polymers give a larger cone angle than less crystalline homologues.     

Organosoluble and transparent polyimides derived from alicyclic dianhydride and aromatic diamines

Organosoluble polyimides were synthesized with the alicyclic dianhydride 1,8‐dimethylbicyclo[2,2,2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride and aromatic diamines. The polyimides possessed good solubility both in strong dipolar solvents and in common solvents; the thermal decomposition temperature of the polyimides exceeded 420 °C. Strong and flexible films of the polyimides, with the cutoff of ultraviolet–visible absorption lower than 310–320 nm, exhibited good features as the alignment layers for nematic liquid crystals with pretilt angles of 1.5–2.9°. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 110–119, 2002     

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.     

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.     

The relationship between liquid crystal alignment in the nematic and smectic C* phases

The pretilt angle in the nematic phase is related to two different surface dependent factors, while the quality of surface stabilized ferroelectric liquid crystal (SSFLC) devices is dependent on only one of them. We have synthesized two series of polyalkyleneimides and found that the pretilt angles for thick nematic cells are not related to the cone angles in SSFLC cells.     

Electro-optic characteristics of surface-stabilized ferroelectric liquid crystal devices with the stripe-shaped domain structure

The stripe-shaped domain (SSD) structure has been obtained in the initial ferroelectric liquid crystal (FLC) alignment after the doping of a naphthalocyanine compound into rubbed polyimide films. Pre-tilt angles on the aligning films have been measured. The memory capability of aligned SSFLC cells has been enhanced nearly to completion, and contrast ratios reached 86:1 with the appearance of the SSD structure. Thus fabricated, 64 64 electrically controlled FLC spatial light modulator has better electro-optic characteristics and stability than a non-doped device.     

Synthesis and characterization of novel polyimides bearing a side chain composed of stilbene mesogen and undecyl spacer and the alignment ability of nematic liquid-crystal molecules

Four different polyimides with side chains containing undecyl spacers and 4-fluorostilbene mesogen end groups were prepared, and their structures and the controllability of the liquid-crystal (LC) alignment with rubbing were investigated. From X-ray scattering and differential scanning calorimetry data, pyromellitic dianhydride (PMDA)-, benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA)-, and 4,4′-oxydi(phthalic anhydride) (ODPA)-based polyimides were shown to have layered structures and short-range ordering. Pretilt angles of LCs on the rubbed surfaces were 1–9°, depending on the rubbing density and backbone. In particular, the long undecyl spacers caused the mesogenic stilbene end groups to lie next to the main chain, resulting in a reduction in the pretilt angles. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1800–1809, 2001     

脂环族聚酰亚胺及其液晶取向膜材料

综述了近年来在脂环族聚酰亚胺领域内的研究进展,从单体的合成方法和分类以及脂环族聚酰亚胺的合成方法和实际应用等几方面系统阐述了该领域的研究进展,并着重讨论了其作为液晶显示器取向膜材料中的应用。     

A novel photoalignment film for ferroelectric liquid crystal based on self-assembled monolayer method

A novel strategy based on self-assembly technology was devised for design of photosensitive material as a ferroelectric liquid crystal (FLC) alignment layer. This development offers new tools for the study and control at the molecular level of the interaction of FLCs with solid surfaces. The photoreactive material was self-assembled to the substrate by covalent bond linkage due to a special chemical adsorption reaction. Through ester bond linkage, a cyano group with strong polarity was introduced to be terminus of the film. Under irradiation of linearly polarised ultraviolet light, an optically anisotropic self-assembled film was easily obtained. The irradiated film was demonstrated to result in homogenous alignment of FLC by optical transmittance measurements and polarising optical microscopy images of a FLC cell at different rotation angles. The alignment quality of the FLC on this self-assembled monolayer film is comparable to that of commercial rubbed polyimide film. Furthermore, it was also found that the fine alignment of the FLC may be related to the smoothness of the self-assembled film surface owing to its polar end.     

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.     

可溶性聚酰亚胺液晶取向膜的制备与性能

液晶取向膜是液晶显示器中的关键材料,为了制得性能优良的液晶取向膜,我们通过铃木偶联反应制备了3种含有不同末端基团的3,5-二氨基联苯,将制得的3种二胺单体分别与环丁烷四甲酸二酐聚合得到3种新型的聚酰亚胺。 利用核磁氢谱对单体和聚合物的结构进行了表征,测试了聚合物的溶解性、热稳定性以及用作液晶取向膜的性能。 结果表明,所得的聚酰亚胺具有良好的溶解性和热稳定性,经过机械摩擦后,末端带有甲氧基的聚酰亚胺能使液晶分子平行于摩擦方向取向,末端带有苯氧羰基和联苯氧羰基的聚酰亚胺能使液晶分子垂直于摩擦方向取向。 此类可溶性的聚酰亚胺液晶取向膜将简化制备工艺并在柔性显示器件中具有较大的应用价值。     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

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.     

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.     

Synthesis of novel hyperbranched polyimide for liquid crystal alignment

A novel hyperbranched polyimide (HPPI) was synthesized from a new four‐functional amine and 1,2,3,4‐cyclobutanetetracarboxylic dianhydride. The polyimide with hyperbranched structure exhibited excellent alignment for nematic liquid crystal (LC) compared with traditional linear polyimides. The solution viscosities of the poly(amic acids) (PAAs) and the surface morphology of PI films were investigated using an Ubbelhode‐type viscometer and atomic force microscopy, respectively. The viscosities of the hyperbranched PAAs were much lower than that of the linear PAA solution at any monomer ratio (amine:dianhydride). Pretilt angles of LCs above 2.8° were achieved with HPPI.     

Synthesis of novel polypyromellitimides with n‐alkyloxy side chains and their liquid‐crystal aligning property

New well‐defined brush polypyromellitimides with n‐octyloxy and n‐dodecyloxy side chains were prepared by two‐step polycondensations of 3,6‐di(n‐alkyloxy)pyromellitic dianhydrides with various conventional aromatic diamines. Their structures and properties were investigated and compared with those of polyimides without the side chains. The alkyloxylated poly(amic acid)s had inherent viscosities of 0.45–1.09 dL/g. The polyimides showed enhanced solubility in organic solvents and had layered structures in the solid state. As the side‐chain length increased from n‐octyloxy to n‐dodecyloxy, the extent of layered structure formation increased, whereas the glass‐transition temperature and thermal resistance decreased. As for the liquid‐crystal (LC) aligning ability measured with 4‐n‐pentyl‐4′‐cyanobiphenyl on rubbed thin‐film surfaces, all the side‐chain polyimides revealed homogeneous LC alignment parallel to the rubbing direction with distinctively higher pretilt angles than those of the polyimides without the side chains. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3130–3142, 2004     

Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Alignment of polymer network-stabilized ferroelectric liquid crystal using a magnetic field

Polymer network-stabilized ferroelectric liquid crystals with homogeneous alignment have been produced in cells without a surface alignment layer. In this technique, a crosslinkable monomer is mixed into a ferroelectric liquid crystal and polymerized in a magnetic field to form a polymer network that will stabilize the alignment of the ferroelectric liquid crystal. The concentration of the monomer is an important factor in achieving alignment of the ferroelectric liquid crystal. Both the morphology of the final composite layer and the molecular alignment of the host FLC are affected by the curing temperature at which the UV curing of the sample is started.