Synthesis and thermotropic properties of novel side‐chain dimer liquid crystalline polymers

A new class of side‐chain liquid crystalline polymers with polymethacrylate or polyether backbones and two different mesogenic moieties in one side‐chain, i.e. a biphenyl group and a cholesterol group, have been synthesized. The thermotropic behavior of both, the monomeric precursors and the polymers, has been investigated. The monomers exhibit smectic A and chiral nematic phases and the polymers exhibit smectic A mesophases. For both, monomers and polymers, the isotropization temperatures and the corresponding dimensionless entropy changes ΔS/R depend on the parity of the spacer between the two mesogens. Replacement of the biphenyl group in the polymethacrylates by a phenyl group lowers the isotropization temperatures, but the smectic A phase is retained.     

Syntheses of thermotropic liquid crystalline polymers, 2. polyurethanes

Polyurethanes were synthesized by reaction of a rigid molecule, 3,3′-dimethyl-4,4′-biphenyldiyl diisocyanate (DBD), with flexible molecules, oligoethylene glycols (di-, tri-, and tetraethylene glycol) or α,ω-alkanediols, and some of their thermal properties were studied. Poly(DBD-oligoethylene glycol)s are amorphous polymers which exhibit no mesomorphic phase. Poly(DBD-alkanediol)s are found to show a mesomorphic phase depending on the number of methylene units in the alkanediol. The polymers having methylene units from five to twelve are crystalline and show a mesomorphic phase. On the contrary, polymers having less than four methylene units do not show a mesomorphic phase since thermal decomposition of the polymers occurs before melting. The transition temperature from the mesomorphic phase to the isotropic liquid decreases with increasing length of the flexible chain of alkanediol. Ternary polymers are obtained by reaction of DBD with a mixture of hexanediol (HD) and dodecanediol (DD). The ternary polymers containing a small amount of HD and DD give a clear mesomorphic phase.     

Synthesis and liquid-crystalline properties of thermotropic homopolycarbonates

Six series of polycarbonates with flexible spacers have been prepared by melt condensation of aromatic diols such as 4,4′-bis[(2-hydroxyethyl)oxy]biphenyl, 4,4′-bis [(6-hydroxyhexyl)oxy]biphenyl, 1,5-bis[(2-hydroxyethyl)oxy]naphthalene, 1,5-bis[(6-hydroxyhexyl)oxy]naphthalene, 2,6-bis[(2-hydroxyethyl)oxy]naphthalene, 2,6-bis[(6-hydroxyhexyl)oxy]naphthalene, 4,4′-bis[(6-hydroxyhexyl)oxy]stilbene, 3,3′-dimethyl-4,4′-bis[(2-hydroxyethyl)oxy]stilbene, 3,3′-dimethyl-4,4′-bis[(6-hydroxyhexyl)oxy]stilbene, 4,4′-bis[(2-hydroxyethyl)oxy]-α-methylstilbene and 4,4′-bis[(6-hydroxyhexyl)oxy]-α-methylstilbene

1 For systematic IUPAC nomenclature see Experimental part.

with several alkylene diphenyl dicarbonates. Polycarbonates with biphenyl and stilbene mesogenic groups show liquid-crystalline behavior with the formation of a nematic phase over a temperature range of 46°C and 21°C, respectively. Amorphous polycarbonates were obtained when 1,5-naphthalene or α-methylstilbene groups are incorporated. Polymers with 2,6-naphthalene and 3,3′-dimethylstilbene groups show mesophase formation up to a certain spacer length, a transition range of 15°C and 28°C, respectively, being observed.     

Synthesis and mesomorphic properties of fishbone-like liquid crystalline polysilsesquioxanes, 3. Fishbone-like,azo-based liquid crystalline polysilsesquioxane

Homopolymeric H-FP (FP: fishbone-like, azo-based liquid crystalline polysilsesquioxanes) and copolymeric C-FP were synthesized by hydrosilylation reaction of a vinylterminated azo compound with a ladder-like poly(hydrosilsesquioxane) (H-T) and random poly(methylhydrosilsesquioxane) (MH-T), respectively, in the presence of Cp₂PtCl₂ (Cp: cyclopentadienyl) used as the catalyst. This new type of liquid crystalline polymers is thermotropic. With increasing temperature FP's first enter a mesophase showing a normal texture, and then some peculiar textures different from the ordinary mesophase appear at an elevated temperature showing some bright belts, lines and spots on the black background, which can be supposed to be a self-organized structure originating from ladder-like main chain aggregation or association via interaction between the mesomorphic side chains.     

Synthesis and properties of liquid crystalline aromatic copolyesters with lactide moieties

High molecular weight liquid crystalline copolyesters were obtained by copolycondensation of aromatic diols and diacyl chlorides with oligolactides. The molecular structure of these copolyesters was verified by NMR studies. The copolyesters form nematic melts, which can be frozen in into a nematic glass. Unexpectedly, a fibrillar structure was observed exclusively on the surface of solution cast films. In spite of a significant content of lactide moieties of the copolyesters their films and fibers are characterized by exceptional mechanical properties. Initial experiments indicated excellent biocompatibility based on cell seeding experiments and microscopic evidence.     

Synthesis and mesomorphic properties of side chain liquid crystalline polymers having thiadiazole ring

New liquid crystalline monomers having 1,3,4-thiadiazole ring as mesogen were synthesized by using Lawesson's reagents to prepare the thiadiazole ring and acryloyl chloride and methacryloyl chloride to introduce the vinyl group. These monomers were polymerized in the presence of 2,2′-azoisobutyronitrile as initiator in THF. All monomers and polymers show cholesteric behavior and a broad chiral smectic C^* phase. As the spacer length increases, the phase transition temperature is lowered and the polymers show higher phase transition temperatures and improved mesophase stabilities compared to the corresponding monomers.     

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.     

Synthesis and thermotropic liquid crystalline behaviour of novel poly(aryl ether ketone)s with a lateral methoxy group

Novel poly(aryl ether ketone)s containing a lateral methoxy group were synthesized by nucleophilic substitution reactions of 4,4′‐biphenol and methoxyhydroquinone with 1,4‐bis(4‐fluorobenzoyl)benzene in a sulfolane solvent in the presence of anhydrous potassium carbonate. Their thermotropic liquid crystalline properties were characterized by a variety of experimental techniques, e. g. differential scanning calorimetry (DSC), polarized light microscopy and temperature‐dependent FTIR. Thermotropic liquid crystalline behaviour was observed in the copolymers containing 30–80 mol‐% mexthoxyhydroquinone. Both melting (T_m) and isotropization (T_i) transitions appeared in the DSC curves. The polarized light microscopy study of the liquid crystalline copolymers suggested their ordered smectic structures. As expected, the copolymers had lower melting transitions than the biphenol‐based homopoly(aryl ether ketone)s because of the copolymerization effect of the crystal‐disrupting monomer methoxyhydroquinone.     

Synthesis and thermotropic properties of new mesogenic diacrylate monomers

A new class of mesogenic α,ω-alkylene bis[4-(4-acryloyloxybenzoyloxy)benzoate]s 1a — 1g were synthesized and studied for their mesophase behaviour. The thermodynamic parameters of the isotropization transition T_i, ΔH_i and ΔS_i show distinct odd-even alternations and demonstrate that the thermotropic liquid crystalline properties in homologous series of mesogens can not be a continuous function of the length of the inner spacer segment. It is anticipated that the investigated diacrylates are of value as precursors of thermotropic semiflexible polymers, crosslinking agents for mesomorphic polymer networks and low molecular weight twin model compounds.     

Synthesis and thermotropic liquid-crystalline properties of polyurethane-polystyrene graft copolymers

New polyurethane-graft-polystyrenes (4) of well-defined structures and compositions were synthesized by macromonomer technique and the structure-liquid-crystalline (LC) property relationship is discussed. The expected graft copolymers with polystyrene grafts were prepared by melt polycondensation of a mixture of a dihydroxy derivative of biphenyl as a mesogenic moiety (1) and a dihydroxy-terminated styrene macromonomer (2) with diphenyl N,N′-hexamethylenedi-carbamate (3) . Of the resulting polymers mesogenic unit-rich graft copolymers retain the LC mesophases in spite of introduction of the polystyrene segments onto the polyurethane backbones and possess microphase-separated structures. Polystyrene-rich copolymers reveal no well-defined LC mesophases and analogous thermal properties to the polystyrene segments.     

Synthesis and thermal properties of liquid crystalline side chain polymers with pendant cellobiose residues

Two series of thermotropic polymethacrylate (PM) samples with pendant cellobiose residues and an alkyl spacer (number of carbon atoms n = 4 and 10) were synthesized, and their mesomorphic properties were investigated to elucidate the function of acylated cellobiose moieties as discotic mesogens by differential scanning calorimetry (DSC), polarization microscopy and X-ray diffraction. The PM-4 samples with a short spacer (n = 4) showed two kinds of mesophases in a wide temperature region up to the degradation temperature of the sample, i. e., about 230°C, whereas the PM-10 samples with a long spacer (n = 10) showed a single mesophase between 45 and 135°C. X-Ray diffraction data suggested that both the mesophases formed by PM-4 and PM-10 belong to a kind of discotic columnar phases in which the side chain mesogens form the discotic columns around the polymer backbone.     

Molecular orientation of paramagnetic metal complexes in melt-drawn fibers of thermotropic liquid crystalline polymers

Liquid crystalline polyesters as host polymers and paramagnetic metal complexes as guest molecules were prepared, and melt-drawn fibers were made from their blend at mesophase temperature. The wide-angle X-ray diffraction patterns of the fibers indicated that the nematic or smectic phase is frozen during melt-drawing. The molecular orientation of the metal complexes in the fibers was investigated by means of ESR. The order parameter obtained from the ESR spectra of the fibers suggests that the base of the square planar Cu(II) and pyramidal V(IV)?O complex is almost completely oriented parallel to the fiber axis.     

Synthesis and phase behaviour of liquid crystalline polyacrylates

The synthesis and phase behaviour of a series of polymers having liquid crystalline sidechains and a poly(acrylic acid) backbone are described. These polymers show a dependence of the phase transition temperatures on molecular weight. In copolymers of two liquid crystalline acrylic esters, induced smectic phases occur. A comparison of the phase transitions of the different polyacrylates, with those of liquid crystalline polymethacrylates and polysiloxanes, published elsewhere, establishes that the spacer group achieves partial “decoupling” of liquid crystalline and polymer properties.     

Oligosaccharide-based thermotropic liquid crystals, 4. Synthesis of cellobiose-based twin and triplet derivatives and their mesophase properties

Previous work from this laboratory has shown that cellobiose octaalkanoate (COA) functions as a thermotropic discotic mesogen, forming a hexagonal ordered columnar phase (D_ho). In this work, we prepared the COA-based discotic twin and triplet derivatives and examined the mesomorphic properties of the derivatives by DSC, polarization microscopy and X-ray diffraction. In the former derivative, two cellobiose heptadecanoate (CHD) molecules (monomers) are combined through an alkyl spacer of varying length by ether and ester linkages at the C-1 position of the reducing end units of the monomers. Three CHD molecules having an alkyl spacer are connected with a coupling agent for the latter derivative. The results revealed that (i) all the twin derivatives form a distinct discotic columnar phase, (ii) the thermal stability of the mesophases of the twins is enhanced, compared with that of the monomer, when the length of the flexible spacer is appropriate, (iii) the twins with a relatively short spacer form a discotic rectangular ordered (D_ro) phase, while those with a longer spacer form a pseudo-D_ho phase, (iv) the triplets also form a discotic columnar mesophase, but not a hexagonal ordered columnar (D_ho) phase, (v) the thermal stability of the mesophases of the triplets strongly depends on the chemical nature of coupling cores, and (vi) the packing structures of the cellobiose cores within the columns of both derivatives are markedly different from that of the monomer.     

Synthesis,characterization and properties of liquid crystalline bismaleimides containing phenylbenzoate mesogens and polymethylenic spacers

This paper describes the synthesis and characterization of four bismaleimides containing a phenylbenzoate mesogenic group and differing by the length of the polymethylenic spacers. They were prepared by reacting 4‐maleimidobenzoyl chloride with diphenols containing methylene bridges. The cured resins are stable up to 380°C in air. Smectic liquid crystalline behavior was observed in those materials. Two of them could be crosslinked in either smectic or isotropic phase. We have also found that the degree of cure can affect the formation of a liquid crystalline phase. Isothermal cure of initially isotropic material can form a material that exhibits a liquid crystalline phase.     

Synthesis and properties of thermotropic liquid-crystalline polymers containing transition metals

New thermotropic, liquid-crystalline polyesters ( 5a, b, 6a, b ) with mesogenic bipyridinediyl units were synthesized. They form complexes with Fe(II) and Cu(II) salts. Homopolyesters 5a, b , containing no metal or less than a certain amount of metal, are smectic. A nematic mesophase was observed in the case of copolyester 6a, b . The structural properties of a polyester complex 5a with Cu, as crystal, liquid-crystal and isotropic liquid, were examined by ESR.     

Synthesis and physical properties of novel side-chain type liquid crystalline poly(aryleneethynylene)s

Novel side-chain type liquid crystalline poly(aryleneethynylene)s with a biphenyl or azobenzene moiety in the side chain were obtained by coupling reaction between a dihalo-aromatic compound and a diethynyl-aromatic compound using a Pd complex as a catalyst. All polymers exhibit a stable enantiotropic liquid crystalline phase. The degree of orientation of the polymer backbone was confirmed by UV-vis measurements with polarized light. High quantum yield of fluorescence was obtained for these polymers.     

13C NMR studies of thermotropic liquid crystalline poly(γ-alkylaryl L-glutamate)s

The molecular motions of three kinds of poly(γ-alkylaryl L -glutamate)s ( 1 – 3 ) having a phenylene or a biphenylylene group and alkyl groups with various length, containing 6 to 22 carbon atoms, were investigated by means of ¹³C NMR spectroscopy as a function of temperature. The observed range of temperature covers the “crystal-to-liquid-crystal” phase transition temperature, T_m. It was found that these phase transitions are caused by an increase of the motions of aromatic or alkyl groups in the side chain, depending on the samples, and that the motion of main chains of polymers 1 – 3 with α-helical conformation does not occur on the NMR time scale at least up to 160°C. The degree of the mobility of the alkyl chain in the side chain of 1 – 3 depends on their length above T_m.     

Synthesis of simple main-chain type polyimides derived from aliphatic diamines and 4,4″-terphenyltetracarboxylic acid,and their thermotropic liquid crystalline behavior

We have synthesized a series of aliphatic-aromatic polyimides by high pressure polycondensation of monomer salts derived from diaminoalkanes and diethyl 4,4″-terphenyl-tetracarboxylates. The polyimides, designated as P-nTPE, were obtained with n, the number of methylene units in the spacer, ranging from 6 to 12. Among these, the P-nTPE polyimides with 8?n?12 from mesophases. The mesophases of P-8TPE and P-11TPE are enantiotropic, whereas those of the other polyimides are monotropic. The nematic phase was identified for all polyimides from X-ray and microscopic observations; only P-11TPE exhibits polymorphism, in which a nematic phase is followed by a smectic phase upon cooling. The ability for liquid crystal formation in main-chain polyimides is discussed.     

Liquid crystalline coumarin polymers, 1. Synthesis and properties of side‐group liquid crystalline polymers with coumarin moieties

This paper deals with two series of novel side‐group liquid crystalline polymers containing coumarin (2H‐1‐benzopyran‐2‐one) moieties. Several kinds of polymethacrylates of Series I (Fig. 1) containing a 7‐benzoyloxycoumarin unit in the side group with or without substituent at 3 or 4 position of coumarin ring were prepared, in order to study the influence of spacers, tail groups and the positions of the tail groups on their liquid crystalline properties. Three polymethacrylates (Series II, Fig. 1) containing a phenyl 7‐octyloxycoumarin‐3‐carboxylate unit, which consisted of a turned ester group between the aromatic rings in the central unit of side group as compared to Series I, were also prepared. The investigation of the thermal behavior of the two series indicated that the structure of the side‐group significantly affected the liquid crystalline properties of the polymers. It was found that the polymers in Series I with substituents at the 3 position of the coumarin ring exhibited mesophase with a wide temperature range, whereas the polymers with substituents at the 4 position showed no mesophase. In addition, each polymer of Series I tended to exhibit a glassy state, whereas each polymer of Series II tended to exhibit a crystalline state. This would be due to the different interactions of side groups of the polymers in Series I and II. It was also revealed that the UV‐Vis absorption and fluorescence spectra were influenced by their mesogenic structures and chromophore aggregations. Furthermore, the preliminary results about the emission spectra suggested that the annealing of the polymer film had a great influence on its fluorescence property.