Synthesis and properties of side chain liquid crystalline ionomers containing quaternary ammonium salt groups

New thermotropic side chain liquid crystalline ionomers (LCIs), containing 4-(4-allyloxybenzyloxy)-4'-alkoxybiphenyl (IM) as mesogenic unit and allyltriethylammonium bromide (ATAB) as non-mesogenic unit, were synthesized by graft copolymerization upon polymethylhydrosiloxane. The chemical structures of the polymers were confirmed by IR spectroscopy. Differential scanning calorimetry (DSC) was used to measure the thermal properties of these polymers; the mesogenic properties were characterized by polarizing optical micrography, DSC and X-ray diffraction. The influence of the alkoxy chain length on the clearing temperatures of the ionomers is clearly shown in an odd-even effect, similar to other side chain liquid crystal polymers. The mesomorphic behaviour of the ionomers is compared with that of isomeric ionomers synthesized in previous work. The results demonstrate that the phase behaviour of the two series of isomeric ionomers is similar, but with the difference that the melting temperature of ionomers with biphenyl located at the end of the mesogen is higher than for ionomers with biphenyl located at the middle of the mesogen. The latter are more useful for smectic orientational order than the former.     

Theory of main chain nematic polymers with spacers of varying degree of flexibility

Main chain liquid crystal polymers are modelled as either worms or jointed rods. In reality they are composed of mesogenic units (rods) linked by spacers with varying degrees of flexibility. We present a molecular model to describe non-homogeneous nematic polymers. The model takes account of molecular parameters, such as the lengths of the mesogenic group and the spacer units, and the interactions between them. The spacers are found to have an order differing from the mesogenic units. If the spacer is not very long and thus in effect is inflexible, one end of the spacer can retain to some extent the orientation of the other end, allowing orientational correlation between spacers mediated by the intermediate mesogenic unit. This is important in giving the chain a global rod-like behaviour as the nematic field becomes strong or the temperature low. The nematic order of the two components (mesogens and spacers), the nematic-isotropic transition as well as the latent entropy are examined. Furthermore, the anisotropic conformations of the polymers are investigated, which show either rod-like or random walk behaviour. Comparison of our results with experiment is found to be satisfactory.     

Synthesis of side chain liquid-crystalline polysiloxane containing trans-cyclohexane-based mesogenic side groups

The synthesis and characterization of nine new side chain liquid-crystalline polysiloxanes containing one cyclohexyl ring and another 2-4 aromatic rings in their mesogenic side groups are described. All synthesized polymers displayed nematic mesomorphism. Most of the polymers showed a very wide mesomorphic temperature range. The mesogenic core length has profound influence on the phase transitions of the polymers. The mesomorphic temperature range increased with increasing mesogenic core length.     

Orientational behaviour of a liquid crystalline side chain polymer in applied electric and magnetic fields as detected by NMR

The molecular orientational order of a side chain thermotropic nematic polymer has been measured by proton NMR. The alignment of the side chains is reflected in the splitting of the NMR signal into a doublet. Simultaneous orientational effects of magnetic and electric fields on the mesogenic groups were studied in magnetically preoriented layers. Temperature, twist angle and thickness dependences of the spectra were measured. The order parameter S and the parameter of orientational order S' of the samples were estimated.     

Ordering in a nematic side-chain polymer. A proton and deuterium nuclear magnetic resonance study

 The aim of this work is to study in detail the orientational structure of liquid-crystalline polymers with different molecular weight. The advantage of our approach is the use of broad-band ¹H and ²H NMR spectroscopy for the analysis of the orientational order of the main-chain and mesogenic groups in conjunction with small-angle neutron scattering results. Investigations of a series of partially deuterated side-chain polymers with methoxybenzylbenzoate mesogenic groups have been carried out. The deuteration of the main chain enables separate access to order and orientation effects of the side chain and the main chain. The molecular orientation of the substances investigated can be frozen in its nematic phase below the glass-transition temperature. It is independent of the molecular weight, whereas the dynamics of the director reorientation (rotational viscosity) in the nematic phase strongly depends on the molecular weight. The main-chain deuterons provide powder spectra after orientation of the polymer in magnetic fields, indicating a very weak or lacking orientational anisotropy. A complete decoupling of the main and the side group can be deduced. Received: 15 September 1999 Accepted: 7 February 2000     

Coupling order and conductivity. Liquid crystalline conjugated polymers

Acetylenic and diacetylenic liquid crystalline monomers have been prepared in order to obtain conjugated polymers with an orientational character. Unlike the polydiacetylene derivatives obtained and which do not exhibit any mesomorphic behaviour, a smectic phase, stable over a large temperature range, occurs in the case of all the polyacetylenes. This mesophase appears to be stabilized by an in situ polymerization of the monomers oriented in the nematic state. After iodine doping, a nematic phase appears in the polymer in addition to the smectic phase. AC complex conductivity measurements, realized over a large frequency range. are reported for some doped and undoped polyacetylenes. Preliminary results concern both non-oriented polymers and an undoped polymer in which the mesogenic groups have been oriented after polymerization.     

Twin nematic phenylbenzoates in a.c. electric fields

The orientational behaviour of nematic compounds having twin phenylbenzoate mesogens was examined under a wide range of a.c. electric fields (0-2 V mum-1 and 10Hz-50 kHz). For this study, crossed polarizing optical microscopy (POM) and real-time X-ray diffraction (RTXRD) measurements were employed to investigate optical and orientational response. These nematic compounds have a positive dielectric anisotropy and a relatively low epsilon// relaxation frequency which allowed study in both homeotropic and planar orientations over a controllable frequency range. The optical behaviour and X-ray results corresponded well, providing a tool for understanding the orientational behaviour of these liquid crystals. For homeotropic alignment, an electric field of over 1 V mum-1 was required in order to obtain good orientation. However, homeotropic orientation depended on a delicate balance between thermal fluctuations and dielectric torque imposed by the electric field, which are both strongly related to the elasticity of the LC domains. Due to this effect, the highest orientation parameter achieved for homeotropic orientation was only 0.48, which indicated that this state was still non-equilibrium. On the other hand, for planar orientation, a uniform texture with orientation parameter of 0.65 was easily obtained even at electric fields as low as 0.2 V mum-1. The application of an electric field stronger than 1 V mum-1 induced a distortion in the texture, and reduced the orientation parameter to 0.45 for planar alignment.     

The effect of a lateral aromatic branch on the orientational ordering of laterally alkoxy substituted nematics

Two new mesogenic series have been synthesized. They have the same main core which contains four aromatic rings, with two lateral substituents on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The positions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is obtained for most of the compounds. The orientational ordering behaviour of these new compounds has been investigated by C-13 NMR, and two isomeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C-13 NMR technique with variable angle spinning. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sample parallel to the magnetic field. The first methylene fragment in each lateral substituent of each compound has a positive C-H bond order parameter, implying that this fragment adopts a cis conformation in the nematic phase. The two lateral substituents are folded back along the mesogenic core in opposite directions. The lateral chain is found to be roughly aligned along the molecular long axis imposed by the core, whereas the para -axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with the molecular long axis. It was also found that the different patterns of substitution do not affect the position of the molecular long axis to any large extent.     

Synthesis of side chain liquid-crystalline polysiloxane containing trans-cyclohexane-based mesogenic side groups

Abstract

The synthesis and characterization of nine new side chain liquid-crystalline polysiloxanes containing one cyclohexyl ring and another 2-4 aromatic rings in their mesogenic side groups are described. All synthesized polymers displayed nematic mesomorphism. Most of the polymers showed a very wide mesomorphic temperature range. The mesogenic core length has profound influence on the phase transitions of the polymers. The mesomorphic temperature range increased with increasing mesogenic core length.     

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.     

Side group functionalized liquid crystalline polymers and blends VII. Phase behaviour and elastic constants K1 and K3 for hydrogen bonded blends of a functionalized LC copolymer with a low molecular mass non-mesogenic dopant

New hydrogen bonded blends of LC copolymers containing functional carboxyl groups with a low molecular mass pyridine-containing dopant were obtained and the orientational, optical and elastic properties of the blends were measured using the Fréedericksz method of threshold transitions in a magnetic field. The averaged order parameter S of the hydrogen bonded blends is found to be lower than that of the initial functionalized LC polymers. Furthermore, a considerable increase in the K₃/K₁ ratio is observed caused by an increment in the average 'effective' length of the hydrogen bonded mesogenic group. For the first time it is proven that LC blends with hydrogen bonded mesogenic groups obey the same main relationship of orientational elastic deformations as common nematic LC polymers with covalent bonding of mesogenic side groups.     

Polysiloxane side-chain liquid crystalline polymers prepared by alkyne hydrosilylation

We describe in this work an alkyne-hydrosilylation approach to synthesize a series of novel polysiloxane sidechain LCPs attaching end-on or side-on mesogenic side groups. Their properties are characterized by NMR, FTIR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy and small-angle X-ray scattering. These obtained alkenylsilane linked novel LCPs exhibit higher glass transition temperatures and clearing points, and especially display the tendency of forming smectic phases, in strong contrast with the nematic phases of their comparative conventional alkylsilane linked analogues, which manifests more rigid features of the alkenylsilane linkages and their significant influence on the mesomorphic properties.     

Anisotropic thermoset networks based on ring-forming monomers (cyanate esters and isocyanates)

Cyanate esters and isostructural isocyanates have been synthesized and investigated in terms of their thermal and phase behaviour. Model cyclotrimers of both were synthesized to study the influence of the triazine rings on the mesophase formation. Liquid crystalline properties (calamitic mesophases) were found only for cyanurates. Polycyclotrimerization of 4-cyanatophenyl 4-cyanatobenzoate starts in an isotropic phase with a transition to a mesophase after reaching the gel point. The gel point observed by rheological measurements is temperature-dependent and probably caused by association of the reactive oligomers as no gel particles are found below the chemical gel point. The gel point of the commercial 2,2-bis(4-cyanatophenyl)propane was found at 50 % conversion by the same method. Due to the higher rigidity of the mesogenic cyanate the final conversion is below 100 %, as opposed to the conventional material. Under appropriate conditions isotropic networks can be obtained from the mesogenic monomer, which sets the basis for a quantitative investigation of the influence of liquid crystalline order on physical properties of the thermosets.     

Liquid-crystalline copolysiloxane system including side-on and side-end fixed mesogenic groups

The work described here concerns new side-chain liquid-crystalline copolymers with different proportions of mesogenic groups attached laterally (side-on fixed) and terminally (side-end fixed) to a polysiloxane backbone. The mesomorphic properties, as well as structural and diamagnetic investigations, were studied as a function of the terminal-lateral ratio for each copolymer. In particular we show that equivalent proportions of both fixation types reduce significantly the orientational order of the nematic phase.     

New bent core mesogens with exceptionally high clearing points

A homologous series of novel two-ring alkoxythiophene-containing liquid crystals has been synthesized using a recently developed cyclization methodology to construct the heterocyclic core. The new heterocyclic unit is expected to confer high negative dielectric anisotropy and high dielectric biaxiality when incorporated into suitable mesogenic structures. The materials reported here possess nematic phases, with the longer chain homologues displaying smectic mesomorphism. In all cases the clearing points are significantly enhanced when compared with other two-ring thiophene-based mesogens. The synthesis, mesomorphic behaviour and stability are discussed, and the mesomorphic behaviour is compared with the analogous 4-alkoxy-4'-cyanobiphenyl derivatives synthesized by Gray.     

New bent core mesogens with exceptionally high clearing points

A homologous series of novel two-ring alkoxythiophene-containing liquid crystals has been synthesized using a recently developed cyclization methodology to construct the heterocyclic core. The new heterocyclic unit is expected to confer high negative dielectric anisotropy and high dielectric biaxiality when incorporated into suitable mesogenic structures. The materials reported here possess nematic phases, with the longer chain homologues displaying smectic mesomorphism. In all cases the clearing points are significantly enhanced when compared with other two-ring thiophene-based mesogens. The synthesis, mesomorphic behaviour and stability are discussed, and the mesomorphic behaviour is compared with the analogous 4-alkoxy-4′-cyanobiphenyl derivatives synthesized by Gray.     

Synthesis and mesomorphic properties of a novel ladder-like 1,4-phenylene-bridged liquid crystalline polysiloxane containing ester-based mesogenic side groups

A new ladder-like 1,4-phenylene-bridged liquid crystalline polysiloxane bearing ester-based mesogenic side groups, LPLCPS, has been synthesized by hydrosilylation of a SiH-terminated ester-based mesogen compound (1) with a ladder-like 1,4-phenylene-bridged polyvinylsiloxane (LPPVS) containing vinyl-terminated side groups in the presence of dicyclopentadienylplatinum(II) chloride (Cp₂PtCl₂) catalyst. Compound 1 was synthesized by hydrosilylation of 1,1,3,3-tetramethyldisiloxane (HMM) with the vinyl-terminated ester-based mesogenic compounds. Various characterization methods including FTIR, ¹H NMR, ¹³C NMR, ²⁹Si NMR, polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction were used and the results combined to demonstrate that LPLCPS shows thermotropic smectic mesomorphic behaviour with a high clearing point and a wide mesophase range.     

Synthesis and phase behaviour of liquid-crystalline side group polyesters

Several series of liquid-crystalline side group polymers with a polyester backbone structure have been synthesized using mesogenic diethylmalonate and 1,3-propanediol derivatives. The structures of the polymer backbone and the mesogenic units have been varied systematically. As shown by differential scanning calorimetry, polarizing microscopy and X-ray diffraction, the side group polyesters exhibit nematic and/or smectic mesophases. Polyesters synthesized from mesogenic diethylmalonate derivatives can tolerate long non-mesogenic segments in the polymer main chain without losing their liquid crystal properties. X-ray studies suggest that some of the smectic polymers exhibit interdigitated bilayer S_A phases.     

Laterally attached liquid-crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. III. Effect of the local anisotropic order on the separation of polycyclic aromatic hydrocarbons

Specific stationary phases based upon non-liquid-crystalline polymers, liquid-crystalline molecules and side-on fixed liquid-crystalline polymers (SO-LCP) have been synthesized for use as silica modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SO-LCP was composed of three phenyl ring benzoate type with terminal alkoxy chains and was laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. This study demonstrated that the shape recognition of stationary phases based upon SO-LCP towards the length-to-breath ratio (L/B) was strongly connected to the existence of a local liquid-crystalline order into the pores of silica gel, warranting the interest of the collective organization of mesomorphic materials in liquid chromatography. Furthermore, the chromatographic performances depended on the kind of anisotropic order and it was more advantageous to use smectic side-on liquid-crystalline polymer than nematic and obviously non-liquid-crystalline ones. Finally, for a series of polymers having the same mesomorphism, the larger the temperature stability range of the mesophase, the more pronounced the local order effect and the higher the shape recognition.     

Synthesis and characterization of side‐chain liquid‐crystalline poly(ethyleneimine)s with cyanobiphenyl groups

A series of polyethyleneimine‐based side‐chain liquid‐crystalline polymers substituted with different ratios of cyanobiphenyl as pendent mesogenic groups has been synthesized in which the spacer length varies between two and six methylene units. The structures of the synthesized polymers are confirmed by infrared and ¹H nuclear magnetic resonance spectroscopy. The thermal properties of these polymers have been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. The results indicate that the thermal behaviour of the polymers is strongly dependent on the degree of substitution. Polymers containing more than 69% of mesogenic groups exhibit nematic‐type thermotropic liquid‐crystalline behaviour with schlieren textures. Below this limit, the polymers are amorphous. Polymers with a higher degree of substitution present the crystalline states. The phase transition temperatures increase and the mesomorphic temperature ranges widen with increasing degree of substitution. The clearing temperatures decrease as the spacer length increases. An odd–even effect in the clearing temperatures is observed and the odd members display the higher values.