Preparation and evaluation of novel ferroelectric liquid crystalline cyclic siloxane tetramers

Two series of liquid crystalline cyclic siloxane tetramers, one containing the 2-methylbutyl chiral group and the other the 1-methylheptyl chiral group, were prepared to investigate, in a systematic manner, the role of molecular structure of (a) the spacer group, (b) the mesogenic side chain and (c) the chiral end group, on the liquid crystalline behaviour of these novel tetramers. The results from this systematic structure/property correlation study clearly showed the effect of the structure of both the chiral end group and the mesogenic side chain core on the thermal properties and temperature ranges of the SmC* phase (ferroelectric) exhibited by these novel materials. By the appropriate choice of spacer group, mesogenic side chain and chiral end group, a number of cyclic siloxane tetramers exhibiting wide SmC* ranges (ferroelectricity) around room temperature were synthesized.     

The effect of molecular structure on the thermal properties of ferroelectric liquid crystalline cyclic siloxane tetramers

A series of liquid crystalline cyclic siloxane tetramers was prepared to investigate, in a systematic manner, the role of molecular structure of (a) the spacer group, (b) the mesogenic side group and (c) the chiral end group, on the liquid crystalline behaviour of these novel tetramers. The results from this systematic structure/property correlation study clearly showed the effects of the structure of the chiral end group and the mesogenic side group on the thermal stability and temperature range of the SmC* phase (ferroelectric) exhibited by these materials. For a given chiral end group, the effect of the length of the spacer group on the thermal stability and temperature range of the SmC* phases depended greatly on the structure of the mesogenic side group. By appropriate choice of spacer group, mesogenic side group and chiral end group, a number of tetramers exhibiting wide SmC* ranges (ferroelectricity) from below room temperature were synthesized.     

Structural characterization of linear dimeric and cyclic tetrameric liquid crystalline siloxane derivatives

The phase behaviour and the structural characterization of a new series of linear dimeric and cyclic tetrameric molecules which contain three distinct parts, a cyanobiphenyl aromatic core (A), a paraffin chain (P) and a central siloxane group (B) with the A-P-B-P-A sequence, are described. Incompatible with one another, these parts tend to locate themselves in three separate sub-layers superposed in a partially bilayered smectic A structure. Each sub-layer adapts its internal structure in order to be appropriate for superposition, as in the case of organosiloxane molecules with the A-P-B sequence reported previously.     

Preparation and properties of siloxane liquid crystalline elastomers with a mesogenic crosslinking agent

A mesogenic crosslinking agent M-1 was synthesized to minimize the perturbations of non-mesogenic crosslinking agents in liquid crystalline elastomers. The synthesis of new side chain liquid crystalline elastomers containing the rigid mesogenic crosslinking agent M-1 and nematic monomer M-2 by a one-step hydrosilylation reaction is described. The chemical structures of the monomers and network polymers obtained were confirmed by FTIR and ¹H NMR spectroscopy. The mesomorphic properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the crosslinking units on phase behaviour is discussed. Liquid crystalline elastomers containing less than 15 mol % of the crosslinking units showed elasticity, reversible phase transitions and a threaded texture. The experimental results demonstrated that the glass transition temperature of polymers P-1-7 increased with increasing concentration of crosslinking agent M-1; but the isotropic temperature and liquid crystalline range decreased slightly.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

Electro-optical properties of ferroelectric liquid crystalline polymers

Ferroelectric liquid crystalline polymers (FLCPs) were synthesized. These ferroelectricities were confirmed directly by measuring polarization reversal currents and other electro-optical properties. The smectic layer structure of FLCP was also studied. A chevron structure similar to that of low molecular weight FLC was observed even in an FLCP cell; but the zigzag defects were not so sharp, which means that it is easy to obtain a good orientation even in large-area displays.     

Broad liquid crystalline temperature range of ferroelectric liquid crystals

Novel ferroelectric liquid crystalline compounds, containing the (S)-2-methyl-l -butyl (4-hydroxybiphenyl-4'-carbonyloxy)biphenyl-4-carboxylate mesogenic group and an oligooxyethylene spacer, were synthesized. The mesomorphic properties of these materials were investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurement. The results indicate that all members of this series exhibit a very broad temperature mesophase range (reaching a maximum around 210°C) including a blue phase (BP), cholesteric (Ch), twist grain boundary A (TGB_A), chiral smectic C (S*_c), and smectic X (S_x) phases. The mesomorphic properties are discussed and a comparison is made with three phenyl rings of ester core analogues.     

Electromechanical effects in ferroelectric liquid crystalline polymers

Electric fields can induce mechanical vibrations in planar aligned sandwich cells of ferroelectric liquid crystals. Measurements on a polysiloxane and a polyacrylate side chain polymer proved that the electromechanical effect also exists in ferroelectric liquid crystalline polymers. The main characteristics of the electromechanical responses of these polymers are described and compared to the response of low molecular weight ferroelectric liquid crystals. According to the proposed interpretation, the vibrations in the direction parallel to both the smectic layers and cell substrates are due to coupling between the director rotation and the flow, while the resonances in the vibrations normal to the plates are connected to layer deformations.     

Synthesis and properties of novel ferroelectric liquid crystalline polyacetylenes containing terphenyl mesogens with chiral groups

Abstract  

Novel ferroelectric liquid crystalline (FLC) polyacetylenes, namely, “side-end-fixed” type of PAM ₆ OTPhOR* and “side-on-fixed” type of PAM ₃ OCO(TPh)OR* were designed and synthesized in high yields (89.3 and 62.0%), respectively, where the terphenyl was linked at the different positions. The thermal stability of PAM ₃ OCO(TPh)OR* is better than PAM ₆ OTPhOR* owing to the “jacket effect” from terphenyl pendant linked at the waist position well protecting the main chain from the perturbations. The PAM ₃ OCO(TPh)OR* shows enantiotropic chiral smectic A phase (SmA*), but it is noteworthy that the PAM ₆ OTPhOR* exhibits enantiotropic chiral smectic C phase (SmC*) responsible for ferroelectric liquid crystallinity. Compared to “side-end-fixed” type of polymer, the “side-on-fixed” type shows better light emitting property, ascribed to the mesogen linked at the waist position has stronger tendency to enhance the main-chain coplanarity. Furthermore, the circular dichroism (CD) spectra demonstrate that the asymmetric force field generated by the chiral center affects the secondary structure of PAM ₃ OCO(TPh)OR*.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

A novel cyclic dithiocarbonate having siloxane moiety; Approach to curable siloxane oligomer

A novel 5‐membered cyclic dithiocarbonate (DTC) having a siloxane moiety, 5‐(3‐trimethoxysilylpropyloxymethyl)‐1,3‐oxathiolane‐2‐thione, was synthesized from the corresponding epoxide precursor by its cycloaddition with carbon disulfide. The siloxane group underwent condensation reaction by the treatment with water and a basic or acidic catalyst, to afford the corresponding oligomer having siloxane main chain and DTC pendant. The resulting oligomer was liquid and soluble in organic solvents such as THF and chloroform. Treatment of the oligomer with amines resulted in selective ring‐opening reaction of the DTC group, generating a thiol group, which underwent oxidative coupling reaction to make the oligomer successfully cured. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4422–4430, 2005     

Synthesis and electro-optical properties of some ferroelectric liquid crystalline polymers

The synthesis and characterization of several series of ferroelectric side-chain polymers were presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurement were used to determine their thermal transitions and analyze their anisotropic textures. The influence of polymer backbones, flexible spacers, mesogenic cores as well as chiral end groups on the mesomorphic properties of the polymers was discussed. Furthermore, the dielectric relaxation properties of the obtained polymers were also evaluated.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

The first examples of monodispersive liquid crystalline tetramers possessing four non-identical anisometric segments

The molecular design, synthesis and thermal behaviour of the first liquid crystalline tetramers composed of four non-identical mesogenic entities connected covalently through paraffinic spacers are presented. These new molecular structures consisting of a tolan (half disc-shaped), azobenzene (photoactive), biphenyl and cholesteryl ester (thermochromic) cores, connected through either an even-even-odd or an odd-even-odd paraffinic spacer, exhibit features of a columnar mesophase.     

Molecular and collective relaxations of ferroelectric side chain liquid crystalline polysiloxanes

The influences of mesogenic group chemical structures on dielectric relaxation behavior were investigated for ferroelectric side chain liquid crystalline polymers (FLCPs). The relaxation time and activation energies of the Goldstone mode, α‐, and β‐relaxations decrease with increasing spacer length because of the plasticizer effect of the spacer. Moreover, the relaxation intensity increases with increasing spacer length for FLCPs. An FLCP with a longer spacer length exhibits a higher mesogenic group mobility, and subsequently leads to easier reorientation toward the alternating electrical field. An increase in mesogenic core rigidity results in an increase in the relaxation time and activation energies, and a decrease in the relaxation intensities for the Goldstone mode, α‐, and β‐relaxations. Moreover, the β‐relaxation is suppressed and cannot be observed in the glassy state for FLCPs containing naphthyl biphenylcarboxylate as the mesogenic group. Shorter relaxation time, smaller activation energies, and higher intensity of the α‐, and β‐relaxations were obtained for FLCPs containing chiral moiety with a flexible heptyl alkyl chain. However, the relaxation intensity of the Goldstone mode for FLCPs containing this chiral moiety was smaller than that for FLCPs containing the chiral moiety with a butyl alkyl chain. For FLCPs containing a chiral moiety with two asymmetrical centers, their Goldstone mode relaxation showed larger amplitude. The α‐ and β‐relaxations are suppressed for these FLCPs because of the dense packing and memory effect of the smectic phase. The relationship between the chemical structure of the mesogenic group and dielectric relaxations is discussed in great detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2035–2049, 2006     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

A new series of siloxane liquid crystalline dimers exhibiting the antiferroelectric phase

In this paper, we report the study of a new series of symmetric chiral liquid crystalline siloxane dimers, their related monomers, and two of those monomers with heptamethyltrisiloxane attached. All the dimers coupled with a trisiloxane show the SmC_A phase, which in several cases has a large tilt angle greater than 40° over a wide temperature range, as well as high spontaneous polarisation in the field‐induced ferroelectric state. Spacer lengths of 3–6 and 11 carbons between the siloxane central unit and the mesogenic cores were used. Monomers with 3–6 carbons in the spacer showed only orthogonal phases while the monomer with an 11‐carbon spacer, as well as the monomers with siloxane attached, have a high tilt angle ferroelectric phase. The materials were characterized by means of DSC, NMR, X‐ray diffraction, electro‐optical methods (tilt angle and texture characterization) and polarization measurements.