Liquid crystal side chain polysiloxanes containing various proportions of non-mesogenic units

Abstract

Recent studies have established that side chain polymeric liquid crystals composed of mesogenic and non-mesogenic side groups keep their liquid-crystalline properties even for a low proportion of mesogens. We show that the detailed structures of three kinds of new diluted liquid crystal polysiloxanes depend on the nature of the co-substituent as well as on the proportion of the silicon sites occupied by the mesogenic groups. Mixtures of these systems with low molar mass liquid crystals were also investigated in terms of compatibility and/or stabilization of smectic A phases.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Analogy between liquid crystal side chain polysiloxanes and liquid crystal twins with siloxane spacers

The goal of this present work was to compare the properties of dimers with a siloxane spacer to those of side chain polysiloxanes including the same mesogenic groups and the same dilution with dimethylsiloxane segments. A systematic study of the influence of the dilution parameter on the mesomorphic and structural properties of dimers and diluted side chain polysiloxanes, as well as on the behaviour of their solutions with low molar mass liquid crystals is reported.     

Analogy between liquid crystal side chain polysiloxanes and liquid crystal twins with siloxane spacers

Abstract

The goal of this present work was to compare the properties of dimers with a siloxane spacer to those of side chain polysiloxanes including the same mesogenic groups and the same dilution with dimethylsiloxane segments. A systematic study of the influence of the dilution parameter on the mesomorphic and structural properties of dimers and diluted side chain polysiloxanes, as well as on the behaviour of their solutions with low molar mass liquid crystals is reported.     

Side chain polysiloxanes with fluorinated mesogenic groups. The role of the fluorine content on the properties of the smectic A state

A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.     

Molecular dynamics of a liquid-crystalline polymer with laterally fixed mesogenic side groups

Abstract

The long range molecular dynamical behaviour of liquid-crystalline side chain polymers with the mesogenic groups linked laterally to the backbone have been studied by using dielectric relaxation spectroscopy over a broad temperature and frequency range. The samples were oriented homeotropically and homogenously by electric and magnetic fields and the relaxations were recorded during alignment and with the fully aligned samples. By fitting the data to theoretical relaxation curves, accurate relaxation parameters could be determined, allowing us to perform a comparison with end-fixed liquid-crystalline side chain polymers on the one hand and with low molecular weight liquid crystals on the other. The relaxation in homeotropic alignment for the laterally fixed compound has more analogies in some aspects, for example, the relaxation time distribution, with low molecular weight liquid crystals than with the corresponding end-fixed compounds, though the activation energy is very large (241 kJ/mol). We relate this to the length of the rigid mesogenic unit and the resulting stronger repulsion by the neighbouring side chains during reorientation. In homogeneous alignment the relaxation is very broad and also has a large activation energy. Different molecular processes are related to this relaxation regime. The relationship between the different relaxation processes and the molecular structure is discussed.     

Liquid crystal dimers and oligomers

A liquid crystal dimer is composed of molecules containing two mesogenic groups linked via a flexible spacer. Initial interest in these materials stemmed from their ability to act as model compounds for semi-flexible main chain liquid crystal polymers but are now of fundamental interest in their own right because their behaviour is significantly different to that of conventional low molar mass liquid crystals. Recently research has begun to focus also on higher monodisperse oligomers such as trimers and tetramers consisting of molecules containing either three or four mesogenic units, respectively, linked via flexible spacers. In this review the most recent developments in our understanding of structure–property relationships in liquid crystal dimers and higher oligomers is discussed.     

Dimer liquid crystals with bent mesogenic units

Three series of dimer liquid crystals containing an aromatic and a cholesteryl mesogenic unit were synthesized and investigated The compounds within the series differ in the length of the spacer connecting the mesogenic groups. Two of the three series contain an aromatic mesogenic group that is connected to the spacer at a meta-position instead of the para-position. Due to the presence of this bent mesogenic group, the odd-even effect in the transitional properties that is normally observed for dimer liquid crystals is reversed as compared with that of the third series, which contains a para-substituted aromatic mesogenic group. The reversed odd-even effect is found for the isotropization temperatures, the associated enthalpy changes and also for the optical properties.     

Investigations of comblike polysiloxanes. I. The influence of spacer length on dielectric relaxation studies of aligned samples

Dielectric relaxation studies on aligned liquid-crystalline comblike polysiloxanes are presented. The polymers differ only in the length of the flexible spacer separating the mesogenic side groups from the polysiloxane backbone. The characteristic features of the observed relaxation process as a function of frequency and temperature, in both the liquid-crystalline and isotropic phases, are described, and the effect of the spacer length is discussed. Within the liquid-crystalline phase a narrow single loss process is observed, due to the relaxation of the mesogenic side groups around the polymer backbone, whereas in the isotropic phase a much broader loss curve is seen. The transition from an aligned polymer mesophase to the randomly aligned isotropic phase is also studied in detial, and comparison with earlier work on comblike polyacrylate and methacrylate liquid crystals is presented.     

Optical activity of a side group cholesteric liquid crystalline silicone

We measured, using a phase modulation technique, the optical rotation in the isotropic and blue phases of a side group cholesteric liquid crystalline silicone polymer. For comparison, similar measurements were performed on cholesteryl nonanoate (CN) and CE6. The polymer has a selective reflection close to that of CE6 and a chiral mesogenic side group chemically similar to CN. The optical activity data for the polymer, unlike those for low molecular mass liquid crystals, does not follow a simple Landau-de Gennes temperature dependence.     

Specific features of liquid-crystalline comb-like polymers with mesogenic groups

The results of structural investigation of the new type of the liquid-crystalline thermotropic methacrylic polymers are discussed. These polymers contain mesogenic groups as models for cholesteric, nematic and smectic types of low-molecular liquid crystals. The groups are attached to the backbone through methylene bridges of various lengths. The polymers of the first group have amorphous structure; the polymers of the second group are characterized by liquid crystalline structure and those of the third group can exist in both crystalline and liquid-crystalline states, manifesting properties of enantiotropic liquid crystals. The temperatures and heats of phase transitions have been determined. It is shown that the ability to realize the liquid-crystalline state of comb-like polymers with mesogenic groups depends on and is determined by the proceess of ordering of these groups.     

Relaxation of a liquid-crystalline order induced by an electric field

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with mesogenic groups in its side chains is investigated. It is established that the electric field induces the isotropic liquid-nematic liquid crystal phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was established that, for polymer liquid crystals, the time of transition from the ordered state to the isotropic phase is several orders longer then that for low-molecular liquid crystal.     

Dimer liquid crystals with bent mesogenic units

Three series of dimer liquid crystals containing an aromatic and a cholesteryl mesogenic unit were synthesized and investigated The compounds within the series differ in the length of the spacer connecting the mesogenic groups. Two of the three series contain an aromatic mesogenic group that is connected to the spacer at a meta-position instead of the para-position. Due to the presence of this bent mesogenic group, the odd-even effect in the transitional properties that is normally observed for dimer liquid crystals is reversed as compared with that of the third series, which contains a para-substituted aromatic mesogenic group. The reversed odd-even effect is found for the isotropization temperatures, the associated enthalpy changes and also for the optical properties.     

在电场和磁场下手性侧链丙烯酸酯液晶聚合物的行为

在电场和磁场下手性侧链丙烯酸酯液晶聚合物的行为江雪平金顺子漆宗能张树范（中国科学院化学研究所北京１０００８０）关键词手性侧链丙烯酸酯液晶聚合物，红外二向色性，液晶分子排列含有手性液晶基团的聚合物，分子中含有不对称碳原子，在一定条件下使液晶聚合物...     

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.     

Liquid Crystalline Polymers

History of Liquid Crystalline PolymersThe liquid crystalline(LC)state was first observed by Austrian botanist and chemist F.Reinitzer more than a century ago,and it was then confirmed in 1888 by German physicist O.Lehmann who named such a state of matter as"liquid crystal"in 1900.While low molecular mass(LMM)liquid crystals were successfully used in LC displays(LCDs),the development of LC polymers(LCPs)followed an independent path.Conceptually,LCPs are prepared with the incorporation of mesogenic groups that are responsible for the formation of LC mesophases,such as rod-like(calamitic)and discotic ones,into polymer chains.Depending on where the mesogens are attached,traditionally there are three major categories of LCPs.Main-chain LCPs(MCLCPs)have mesogens in the polymer backbone,while mesogens of side-chain LCPs(SCLCPs)are incorporated as side groups in a polymer with a relatively flexible main chain.In main-chain/side-chain combined LCPs(MCSCLCPs),mesogens are in both the backbone and side chains.Other classes of LCPs include mesogenjacketed LCPs(MJLCPs),dendronized LCPs,and LC networks(LCNs).     

胆甾型侧链液晶共聚物的研究

合成并表征了两个胆甾型液晶烯类单位以及它们分别与辛烯－１，二氧化硫进行自由基共聚合得到的一系列不同组成的胆甾型侧链液晶三元共聚砜。研究结果表明：该共聚砜的两个液晶热转变温度均随其中辛烯－１单体单元组份含量的增加而降低。     

Liquid-crystalline side chain polymers by chemical modification of poly(chloroalkylvinylether)s

The synthesis of polyalkylvinylethers with pendant 4-cyano-4'-oxybiphenyl groups gives thermotropic liquid-crystalline polymers. The new method developed here consists of the living cationic polymerization of chloroalkylvinylethers and the subsequent modification of the polymer by the mesogenic groups. The liquid-crystalline polymers have a controlled degree of polymerisation and narrow molecular weight distributions. The influence on the mesomorphic properties of various parameters such as the degree of polymerization, the spacer length and the proportion of the mesogenic side chain content has been investigated. Binary phase diagrams with low molar mass analogues are also reported and the properties of both neat materials and binary mixtures are compared.