Liquid crystalline polyesters with 1,4-cyclohexylene units

A new series of liquid crystalline polyesters ( 5a – f ) containing 1,4-cyclohexylene dibenzoate moieties as mesogenic units in the main chain was prepared and characterized. The rigid segments in the polymers are connected by α,ω-alkanedioyl units containing 4–8 or 10 methylene groups.     

Thermotropic polyesters with flexible spacers in the main chain and oligo(oxyethylene) substituents

An expanded series of alkoxy-and oligo(oxyethylene)-substituted thermotropic polyesters with flexible decamethylene spacers in the main chain, which have higher molecular weights than those described in an earlier report, were prepared and characterized. All of the polymers in the series are soluble in common organic solvents, and have low melting temperatures (T_m) and isotropization transition temperatures (T_i). The members of the series with oligo(oxyethylene) substituents form both smectic and nematic phases.     

Thermotropic polyesters, 5. Liquid-crystalline segmented polyesters containing non-rigid mesogenic units with variable geometry

Thermotropic liquid-crystalline polyesters, containing the 3,3′-biphenylylene unit as part of a conformationally mobile mesogenic group, were synthesized. Preliminary investigation of polymers as to their ability for manifesting liquid-crystalline properties was carried out by polarizing microscopy and DSC.     

Liquid crystalline main chain polymers with highly flexible siloxane spacers

The synthesis and the phase behaviour of liquid crystalline main chain polymers are described. In these poly- and copolyesters the mesogenic units are linked by highly flexible oligodimethylsiloxane spacers. The comparison of their phase behaviour with that of polyesters known from the literature shows that dimethylsiloxane spacers as a replacement for alkyl spacers are leading to a drastic decrease of the transition temperatures. Depending on the length of the spacer glass transitions as low as ?111°C can be achieved. The widths of the liquid crystalline phases thereby are retained up to a spacerlength of 13 oxydimethylsilanediyl (dimethylsiloxane) units. The homopolyesters mainly show smectic phases. In contrast to this nematic phases are found for copolyesters in which either the spacerlength or the mesogenic unit was varied. In one case a nematic phase was found over a temperature range of 311°C.     

Liquid crystalline polymers, 8. Structurally ordered thermotropic polyesters of glycol ethers

A series of polysters based on a triad aromatic ester mesogenic unit but containing different poly(alkylene oxide) flexible spacers in the main chain was prepared and its properties examined. The flexible spacers consisted of poly(alkylene oxide)s of varying lengths based on either oligomers of ethylene oxide or propylene oxide. The spacer types, lengths and distributions were found to strongly affect the mesophase and melting behaviors of the polymers. Polymers with spacers having more than 10 units were not liquid crystalline, whereas those of shorter length exhibited mesophase properties. The polymers containing spacers with two, three, or four oxyethylene units showed two mesophases, and the textures of their mesophases suggested that both smectic and nematic phases were formed.     

Liquid crystalline polysiloxanes with thioether type spacers and chiral biphenyl mesogens

Two chiral liquid-crystalline polymers with polysiloxane main chains, thioether type spacers, and biphenyl derivatives bearing one or two chiral atoms as mesogens were prepared by polyhydrosilylation of polymethylhydrogenosiloxane with α-thioolefins carrying biphenyl-based mesogens in the presence of hexachloroplatinic acid. The polymers exhibit two mesophases as a function of temperature. The nature of the first mesophase depends upon the nature of the chiral substituent of the biphenyl, but the second one is always of the SmC^* type and covers a large range of temperature.     

Thermotropic liquid-crystalline main-chain polyesters containing cyclooctylene units, 6. Cholesteric copolymers

A series of chiral nematic copolymers were prepared based on the new mesogenic unit cis-1,5-cyclooctylene bis(p-hydroxybenzoate) (COHB). The copolymers were prepared by linking COHB coupled with (+)-3-methyladipic acid to COHB units combined with terephthalic acid and its bromo and nitro derivatives. A biphasic region was observed in most of the samples which made phase diagrams more difficult to establish. Mesophase stability could normally be sustained up to 50 mol-% content of the methyladipic acid group in the copolymer. They were also sensitive to both temperature changes and shearing forces.     

Liquid-crystalline side-group polyesters with definite lengths of the main-chain spacers

New liquid-crystalline polyesters 1 with 4-(phenylazo)azobenzene moities as mesogenic side groups and a polyester backbone structure characterized by a systematic variation of the length of the flexible non-mesogenic main-chain segments were synthesized. The polyesters were investigated by differential scanning calorimetry, polarizing microscopy, miscibility studies and X-ray diffraction. With increasing number of methylene groups in the backbone spacer, the thermal stability of the nematic and/or smectic phases first decreases and then increases.     

Thermotropic polyesters, 7.. Liquid-crystalline polymers with meta-phenylene units in the mesogenic moiety

Polyesters containing 3,3′-substituted azobenzene and benzanilide moieties as a part of conformationally mobile aromatic blocks were synthesized. Polymers based on 3,3′-substituted azobenzene residues exhibit liquid-crystalline properties and unusual thermal properties such as opposite odd-even effect and unexpected cis-trans isomeric ratio of azo group. Similar polymers with an amide group between the 1,3-substituted benzene rings cannot form liquid crystals.     

Thermotropic liquid crystalline polymers, 4. Study of crystalline and liquid-crystalline comb-like polymers with mesogenic side groups by IR spectroscopy

The IR spectra (in the region of 400–3600 cm^?1) of comb-like polymers in crystalline, liquid-crystalline, and fluid isotropic phases were studied, and the effect of the chemical structure of the long side chains on the crystalline and liquid-crystalline states were examined. The spectroscopic criteria of phase transitions in comb-like polymers were established and the fragments of the side chains involved in the formation of the liquid-crystalline phase, were determined.     

Liquid crystalline side group polyesters with a systematic variation of main chain and side group spacer length

To investigate the influence of a systematic variation of the length of main chain and side group spacers on the thermal phase behaviour four homologous series of liquid crystalline side group polyesters with 4-(phenylazo)azobenzene moieties as mesogenic side groups have been prepared and characterized. The polyesters exhibit smectic and/or nematic phases. The mesomorphic properties were studied by using optical polarizing microscopy, differential scanning calorimetry (DSC) and X-ray diffraction. Packing models of the low ordered smectic phases have been derived from X-ray data.     

Thermotropic polyesters, 2. Synthesis of regular polyesters from aromatic dicarboxylic acids and phenols or aliphatic diols,and study of their mesomorphic properties

Several series of thermotropic liquid-crystalline polyesters, based on 4-chlorocarbonyl phenyl esters of aromatic dicarboxylic acids ( 1–4 ) and phenols or aliphatic diols, were synthesized and studied by polarizing microscopy and differential scanning calorimetry (DSC). A relationship between geometry of mesogenic moiety and stability of mesophase formed by the polymers was established. The effect of both the length and chemical structure of the flexible fragments on the mesomorphic properties was revealed for polyesters obtained from aliphatic diols. Several polymers were obtained whose temperatures of transition into the liquid crystalline state were found to be below 100°C. Two series of low-molecular-weight liquid-crystalline compounds were synthesized to compare their properties with those of the polymers of similar structure. Most of the low-molecular-weight analogs have not been described so far. They predominantly form a smectic type mesophase.     

Characterization and kinetics of mesophase transition for naphthalene-based thermotropic polyesters with polymethylene spacers

Main-chain thermotropic polyesters containing naphthalene rings and flexible spacers with 2 ≤ n ≤ 10 methylene groups were prepared in this study. Their thermotropic properties were investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The nematic mesophase dominates the anisotropy of the prepared polymers except for the polymers with n = 7 and n = 9. Moreover, the nematic domain size depends upon the number n. On the other hand, the naphthalene ring in the mesogen has a strong effect on the mesomorphic properties of the prepared polyesters. The polymers with n = 7 and n = 9 exhibit cooling rate-dependent anisotropic behavior of nematic and smectic phases. The crystallization for the investigated polymers is strongly influenced by the anisotropic behavior. The kinetics of crystallization at higher temperatures does not satisfy the Ozawa theory, while it does at lower temperatures. Nucleation begins to occur in the nematic shreads and the crystal growth proceeds toward liquid-crystalline domains from the observation by POM. Thus, the Avrami exponent a is dependent upon the polymethylene spacer with n being even or odd and high or low. In addition, the parameters of crystallization obtained from DSC and X-ray diffraction were consistent with the results of non-isothermal crystallization kinetics and the observation by POM.     

New polymer syntheses, 30. Thermotropic polyesters of 2-(2-naphthylthio)terephthalic acid

2-(2-Naphthylthio)terephthalic acid (3 b) was synthesized from 2-naphthalenethiol and dimethyl 2-nitroterephthalate. Homopolyesters of this diacid were prepared by thermal condensation with the diacetates of hydroquinone or 4,4′-biphenyldiol. Copolyesters were obtained by concondensation with 4-acetoxybenzoic acid. For comparison a homopolyester was prepared from 2-(2-naphthyloxy)terephthalic acid and hydroquinone. DSC measurements and microscopic observation under polarized light revealed that the homopolyesters are semicrystalline and form a nematic mesophase above their melting points. The polyester from 3 b and hydroquinone crystallizes significantly more rapidly than the polyester of 2-(2-naphthyloxy)terephthalic acid. All copolyesters are amorphous and form a nematic phase between the glass-transition temperature (130 –140°C) and the temperature at which rapid thermal decomposition takes place (470 ± 10°C). The polyesters were also characterized by elemental analyses, viscosity measurements, thermomechanical analyses and thermogravimetry.     

New polymer syntheses, 29. Thermotropic (co)polyesters of 2-phenylthioterephthalic acid and various diphenols

2-Phenylthioterephthalic acid (1a) was prepared from thiophenol and dimethyl-2-nitroterephthalate. Homopolyesters of this diacid were prepared by transesterification with the diacetates of hydroquinone, methylhydroquinone, or 2,5-biphenyldiol. Copolyesters were prepared from equimolar amounts of hydroquinone and 4,4′-biphenyldiol or from hydroquinone and 4-hydroxybenzoic acid in various mole ratios. Inherent viscosities in the range of 0,4–2,0 dl/g were found. DSC-measurements indicate that in the case of (co)polyesters from hydroquinone the homopolyester has the highest glass-transition temperature (T_g = 181°C) and the highest melting point (T_m = 332°C). Also the homopolyesters from 1a and methyl- or phenylhydroquinone are crystalline. Furthermore, the homopolyesters from hydroquinone and 2-(p-tolylthio)terephthalic acid or 2-(4-chlorophenylthio)terephthalic acid were prepared. Microscopic observation under polarized light and WAXS-measurements confirmed that all polyesters form a nematic melt above T_g or T_m. In most cases the mesophase extents up to ca. 460°C where thermal decomposition prevents further observation. The homo- and copolyesters from 1 a and hydroquinone were also subjected to thermomechanical analyses by means of the penetration method.     

Syntheses of thermotropic liquid crystalline polymers, 1. Azoxy and azo type polyesters

Azoxy and azo type polyesters were obtained by combination of an azoxybenzene or an azobenzene group as a mesogenic core with different flexible chain lengths of ethylene glycol groups. The thermal properties of these mesomorphic polymers are discussed related to their chemical structures. The mesomorphic temperature region becomes narrower with increasing flexible chain length. Further, the transition temperature from the mesomorphic phase to the isotropic liquid is higher for the azoxy type polyester than for the azo type polyester.     

Liquid-crystalline side group polyesters with definite lengths of flexible spacers in the main chain

Liquid-crystalline polymalonates with a definite distance between the contact points of two neighbouring side groups were synthesized. The mesogenic group is azobenzene with a terminal nitro-substituent acting as an indicator for dielectric measurements. The influence of the number of methylene groups in the main chain on the polymorphism and the dielectric behaviour has been investigated. Generally, the thermal stability of the mesophases decreases when the main chain spacer becomes longer. The relaxation frequency for the reorientation of the mesogenic side group around the short axis increases with increasing number of methylene groups in the main chain in both, the isotropic and liquid-crystalline phases.     

Thermotropic polyesters, 8, comparison of two methods of synthesis of regular liquid-crystalline multiblock copolymers

Two methods of synthesis of regular liquid-crystalline (LC) multiblock copolyesters are compared. A new method for the synthesis of polyesters with increased length of mesogenic units is given. This method is based on the preparation of intermediate oxalate-groups-containing polymers which are hydrolyzed in the next stage. The difference consists in the structure of the initial complex monomer. The new complex monomer makes it possible to obtain LC polyesters with seven 1,4-phenylene fragments in the mesogenic unit by a simpler route.     

Thermotropic liquid crystalline polymers, 3. Comb-like polymers with side chains simulating the smectic type of liquid crystals

The syntheses of a number of new comb-like polymers are described, which contain mesogenic groups as side branches presenting models of low-molecular liquid crystals of smectic type. The resulting polymers are capable to form an enantiotropic liquid crystalline phase, which may be defined as smectic, according to the terminology used for low-molecular liquid crystalline substances. The thermodynamic limits of the liquid crystalline state in these polymers were determined, which made it possible to define the liquid crystalline state as a thermodynamically stable phase with spontaneously appearing anisotropy of properties (in particular, the optical anisotropy). The characteristic feature of the structure of these polymers was found to be the layer order of side groups providing — in combination with the packing of the mesogenic groups — the possibility of liquid crystalline properties. Only the mesogenic groups take part in the formation of the crystalline packing in these polymers, whereas the methylene chains are in the amorphous phase together with the backbone chain.     

Liquid crystalline polymers, 1. Synthesis and structure-properties relationship in some liquid crystalline polyetheresters

Liquid crystalline aromatic-aliphatic polyetheresters with mesogenic groups of different isomeric structures and flexible oligo(oxyethylene) spacers in the backbone were synthesized and their properties were studied. The influence of the mesogenic group structure, spacer length and nature of bonds linking the mesogenic groups and the spacers, on the phase transition temperatures was elucidated. On the basis of optical textures, observed in the polarizing microscope, smectic or nematic melts were found to be present in the polymers studied, depending on the mesogenic group structure.