光致变色液晶高分子研究(Ⅳ)─含苯甲酸－4－甲氧基苯酯介晶基元和偶氮苯光色基元侧链共聚硅氧烷的液晶行为

用热台偏光显微镜和DSC法研究了含苯甲酸-4-甲氧基苯酯介晶基元和偶氮苯光色基元侧链共聚硅氧烷(PSⅡ)的液晶性。将非介晶基元并入液晶均聚物PSⅡ-1,共聚物PSⅡ-2、PSⅡ-3的液晶态类型不变,PSⅡ-4仅存在近晶相,PSⅡ-5～8无双折射现象,保持共聚物液晶性的最低含介晶基元单体的极限摩尔含量为80%.在液晶共聚物中,非介晶基组分含量增加时,其T_m、ΔH_m和ΔS_m降低;非介晶组分在10mol%时,T_i、ΔH_i和ΔS_i具有最小值。     

Structure-property relationships in dopant-induced ferroelectric liquid crystals

Measurements are reported of the ferroelectric polarization P_s induced in a non-chiral smectic C phase by a variety of chiral dopants having different molecular structural features. Using molecular calculations of contributing dipole moments, ferroelectric order parameters are determined from the experimental results. The relationships between the P_s and various other molecular properties are discussed, and it is shown that restricted rotation of the molecule due to its shape and internal energy barriers to rotation can result in relatively high values of P_s. In contrast dipolar groups flexibly attached to a chiral centre may have their contribution to P_s greatly reduced through internal rotation.     

Design of calamitic self-assembling reactive mesogenic units: mesomorphic behaviour and rheological characterisation

Several calamitic reactive mesogens containing only two benzene rings in the molecular core and with or without lateral substitution by the methyl/methoxy groups have been designed and their mesomorphic behaviour was characterised. Depending on the molecular structure, some of the materials exhibit the nematic and the orthogonal smectic mesophases. The reactive mesogens are aimed for further design of the macromolecular materials like polysiloxane-based polymers and elastomers. Beyond the mesomorphic and structural properties, the electrorheological properties within the temperature range of the nematic and smectic A mesophases were studied with and without applied electric field for the selected reactive mesogen. The increase of viscosity was found not to be higher than three times under applied electric field strength of 2 kV/mm. The mesomorphic, structural and rheological properties of the newly designed reactive mesogens are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation properties relationship of such mesogenic materials.     

Ferroelectric properties of liquid-crystalline SC* phases induced by optically active cyanocyclohexylcyclohexanone derivatives

Derivatives of optically active cyanocyclohexylcyclohexanone have been synthesized and used as dipolar chiral dopants to induce ferroelectric S_C* phases in an achiral host phase. The dopant molecules are the first examples in which the chiral centres are incorporated into a rigid core with transverse dipoles directly attached. The spontaneous polarization P_s and the tilt angle θ of the induced S_C* phases have been measured. P_s is strongly influenced by the relatively small changes of the molecular structure of the cyclohexanones, for example a change of the sign of P_s or a vanishing value of P_s. These effects are discussed in terms of a sterically hindered rotation of the dopant molecules around their long axes and explained by the assumption that the transverse dipole must not be necessarily parallel to P_s in the equilibrium state of rotation.     

Molecular rotational potentials in the SC* phase of binary ferroelectric liquid crystal mixtures

The spontaneous polarization and tilt angle have been measured as a function of temperature and concentration in the S_C* phase of four binary ferroelectric liquid crystal mixtures. The substances have been chosen in a way that different situations could be studied, for example mixtures of a chiral substance (P_s non-zero) with a non-chiral one (P_s = 0) and mixtures of chiral compounds with the same sign of P_s as well as with opposite signs. The results have been interpreted by means of a molecular rotational model and potentials associated with the tilt as well as with the chirality have been determined as a function of concentration.     

Ferroelectricity of induced S*C phases with novel chiral dopants

Induced S*_C phases can be obtained by dissolving chiral dopants in achiral S_C host phases. If the chiral guest molecules bear a transverse dipole, ferroelectricity will occur. The novel dopants under discussion are characterized by chiral centres and the transverse dipole situated not in the alkyl end groups of the mesogenic molecules, but directly in their rigid cores. As a rigid core, analogues of decalin were used. In those dopants, rotation around the molecular long axis is sterically restricted. According to the microscopic model of Zeks, this leads to enhanced values of the spontaneous polarization P_s. The magnitude as well as the sign of the spontaneous polarization P_s of the S*_C phases induced by the novel dopants in different host phases has been investigated. It has been found for the first time that for a given dopant, the polarization as well as the sign of P_s depends on the structure of the host phase. The results are discussed in terms of two microscopic models. They can be understood taking into account the situation that the potential of the restricted long axial rotation is determined by the hard core interactions of the molecules involved or that an orientation of the host dipoles by a guest/host interaction takes place.     

The influence of double bonds in the terminal chain of 2-ring compounds on the physical properties of S*c mixtures

We present the properties of S*_c mixtures containing new 2-ring 5-n-alkyl-2-(4-n-alkenyloxyphenyl)pyridines and pyrimidines with systematically varying positions and configurations of the double bond Trans configurations at odd positions (counting the number of atoms from the core including the oxygen and the first carbon atom of the double bond) suppress the S_A phase, increase the S*_c tilt angle, Θ, and the spontaneous polarization, P_s, and lead to long switching times τ. Cis configurations at even positions suppress the nematic phase in favour of smectic phases, decrease Θ and P_s, and shorten τ. Other positional configurational combinations strongly reduce the clearing point. Furthermore, our results indicate that the preferred conformation of the alkenyloxy chain consists of alternating cis and trans units.     

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     

Differences between smectic homo- and co-polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the 'de Vries' type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the 'de Vries' type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC-elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Effect of molecular rotational hindrance on the spontaneous polarization in the S*C phase of a ferroelectric liquid crystal

The spontaneous polarization, P_s, and the tilt angle, θ, have been measured simultaneously in the S*_C phase of binary mixtures of two ferroelectric liquid crystals as a function of temperature and concentration. A molecular rotational model was used in order to calculate the temperature dependence of the P_s/θ ratio. A two parameter fit of the experimental data was carried out which showed good agreement. The molecular rotational potentials have been determined from the fit parameters and are presented as a function of concentration. It is shown that the chiral part of the potential depends very weakly on the composition in contrast to the non-chiral part which shows a strong negative non-linear deviation with a minimum corresponding to the concentration at which the smectic C* phase width is maximal.     

Liquid crystalline polymers containing heterocycloalkane mesogens. 2. Side-chain liquid crystalline polysiloxanes containing 2,5-disubstituted-1,3-dioxane mesogens

Polysiloxanes and copolysiloxanes containing 2-(p-hydroxyphenyl)-5-(p-methoxyphenyl)-1,3-dioxane and 5-(p-methoxyphenyl)-1,3-dioxan-2-yl as mesogenic units and an aliphatic spacer containing 11 and 10 methylene units, respectively, were synthesized. Their phase behavior was studied by differential scanning calorimetry and optical polarization microscopy, and compared with the phase behavior of the polysiloxanes and copolysiloxanes containing 4-methoxy-4′-hydroxybiphenyl and 4-cyano-4′-hydroxybiphenyl mesogens attached to the polymer backbone through an aliphatic spacer containing 11 methylene units. All synthesized polymers present smectic mesomorphism. The polymers containing 4-methoxy-4′-hydroxybiphenyl and 4-cyano-4′-hydroxybiphenyl are also crystalline, while the polymers containing 1,3-dioxane based mesogens do not crystallize.     

Laterally linked mesogens in main chain polymers

The syntheses of liquid-crystalline main chain polyesters with laterally linked mesogens are presented. The terminal groups of the mesogens and the spacers fixation of mesogens, the polyesters with 1,4-di(p-alkoxybenzoyloxy)benzenes 1,4-di(p-alkoxybenzolyloxy)benzenes, 2,5-di(p-alkoxyphenyl)pyrimidines or a 4,4'-bis(p-ethoxyphenylazo)biphenyl derivative. Despite the unusual bilateral fixation of mesogens, the polyesters with 1,4-di(p-alkoxybenzoyloxy)benzenes show mainly monotropic phases which were found to be nematic in investigations with the polarizing microscope, in DSC measurements, temperature-dependent measurements of the Kerr constant and X-ray diffraction experiments. The monotropic nematic character remains when varying the terminal groups of the mesogen as well as the length of the n-alkanedioic acid spacer. However, when the spacer is branched, crystallization can be disturbed effectively and a stable nematic phase can be obtained. All of the polyesters with only 2,5-di(p-alkoxyphenyl)-pyrimidines as mesogens are amorphous. The reason probably lies in the unsymmetric 4,6-substitution of the pyrimidine ring. In contrast to this, oligomers with a 4,4'-bis(p-ethoxyphenylazo)biphenyl derivative show broad, predominantly enantiotropic nematic phases with relatively high clearing temperatures. With the knowledge gained about relations between structure and properties, especially of polyesters with 1,4-(p-alkoxybenzoyloxy)benzenes as mesogens, polyesters with this mesogenic group but with various architectures have been compared. Speculations about possible arrangements of the structural elements have been included to explain the phase behaviour of the polyesters with laterally fixed mesogens.     

Differences between smectic homo‐ and co‐polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC‐homo‐ and LC‐co‐polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo‐ and co‐polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC‐elastomers formed from homo‐ and co‐polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

Dye-containing S*c side-on/end-on copolymers

Mesomorphic copolysiloxanes, which combine 'normal' end-on linked chiral mesogenic units inducing S*_c phases and side-on fixed chromophores were prepared. The interplay of the different orientational tendencies of these moieties, which are perpendicular (end-on linked mesogenic groups) or parallel (side-on fixed chromophores) to the polymer chain, lead to a strong destabilization of the S*_c phase. However, copolymers with up to about 10 mol % of chromophores still show a smectic C* phase. FTIR measurements show that both moieties orient parallel to each other and perpendicular to the polymer chains. These copolymers are interesting to consider as coloured guest-host S*_c materials for coloured displays or as pyro-electric detectors.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

From monomeric to polymeric ferroelectric liquid crystals A comparative study of ferroelectric properties

Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization P_s. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer.     

Liquid Crystalline Side Chain Polysiloxanes with 4-Cyano- and 4-Alkoxy-Stilbene Mesogenic Units

Abstract

Side chain homo- and copolysiloxanes with 4-cyano- and 4-alkoxy-4′-stilbene mesogens, spaced apart from the backbone by oligomethylene segments of variable lengths, were synthesized via a hydrosilylation coupling reaction of five stilbene-containing α-olefins with four commercial poly(methylhydrosiloxane)s and poly(methyl-hydro-∞-dimethylsiloxane)s. Broad smectic phases were observed for the polysiloxanes with cyanostilbene mesogens, whereas the homologous with alkoxy-terminated stilbenes displayed only narrow mesophases in the high temperature range. Preliminary room tem-perature X-ray diffraction studies on mechanically oriented samples evidenced the occurrence of side chain crystallization, microphase separation and indicated the interdigitated smectic A nature of the mesophases.     

Modular assembly of elliptical mesogens

Discotic mesogens featuring a pyridine ring were synthesized, and were found either to form ordered hexagonal columnar liquid crystalline phases or melt directly from a crystal to an isotropic liquid, depending on the position of the pyridyl nitrogen atom. Binary mixtures of the mesogenic pyridine derivatives with a similar discotic mesogen having a carboxylic acid group resulted in the formation of modular elliptical complexes through hydrogen bonding. The binary mixtures were found to exhibit ordered hexagonal columnar or ordered rectangular columnar and nematic mesophases, depending on the length of the alkyl chains, and displayed dramatically different properties from their constituent components. Binary mixtures of the non-mesogenic pyridine derivatives with carboxylic acid-functionalized discotic mesogens did not result in the formation of hydrogen-bonded complexes.     

On the relationship between electro-optic and dielectric parameters of a smectic C* ferroelectric liquid crystal

A relationship between the electro-optic switching time and dielectric parameters of a S*_c ferroelectric liquid crystal (FLC) is obtained. This relationship is derived in terms of spontaneous polarization P_s, relaxation time τ_G and dielectric strength Δε_G of the Goldstone mode. It shows clearly that the switching phenomenon in FLCs is governed by the dielectric behaviour of the Goldstone mode. Based on the Landau model, the switching time has also been related to the material parameters of the FLC.