New side-group liquid-crystalline polymethacrylates

A new route of synthesis of liquid-crystalline polymethacrylates was investigated. The synthetic route involved side-group liquid-crystalline polymethacrylates (LCPMA) with a turned ester group between the aromatic rings in the central unit of mesogens ( 2 ) as compared with the usual LCPMA ( 1 ). For a series of compounds of this type the liquid-crystalline properties were investigated using differential scanning calorimetry, polarizing microscopy and X-ray diffraction techniques. Generally stable smectic phases were found for these isomeric LCPMA. The described synthetic route opens possibilities to form new polymers from a wide field of educts.     

Orientational behaviour of polymer blends containing a liquid-crystalline component

This paper describes the orientational behaviour of polymer blends containing a side-group liquid-crystalline polymer (LCP). The macroscopic orientation of the LC phase in these blends, and the possibility to freeze in this state are of technical interest. The orientation of LCP-containing blends through mechanical forces depends on a variety of structural, thermal, energetical and also morphological properties, which are strongly dependent on the kinetical and thermal history. This report describes the optical- and scanning electron microscopy (SEM), X-ray diffraction and interfacial tension studies of the liquid-crystalline polyacrylate 1a and the poly(methacrylate) 1b in poly(methyl methacrylate) (PMMA) and polycarbonate matrices. 1a droplets in thermoplastic matrices can be oriented macroscopically during fibre-drawing, but, as a result of higher interfacial tension between the components, 1b -containing blends can not.     

X-ray diffraction study on the phase behaviour of a liquid-crystalline side-group polymethacrylate with biphenylyl mesogen moieties

X-ray investigations at different temperatures of a polymethacrylate 1 with biphenylyl moieties as side-group mesogens reveal structural changes at 100°C for a quenched sample of the polymer which cannot be detected by differential scanning calorimetry (DSC). A low-order smectic phase exists between 136 and 157°C, and an isotropic phase above 157°C. The side groups are much shorter than the observed thickness of the smectic layers. Models are discussed which can account for these spacings. The phase sequence for normally cooled and at 120°C annealed samples is represented by crystalline or higher-order smectic, low-order smectic, isotropic. Different structures in the solid state are detected for the two differently prepared samples.     

Photoreactive chiral liquid-crystalline side-group copolymers containing azobenzene mesogens

A series of liquid-crystalline copolymers 1 with a chiral azobenzene moiety as photoreactive mesogenic unit was prepared. The polymers were fractionated and the mesophase behaviour of the high- and low-molecular weight fractions was examined. The copolymers display smectic A and cholesteric phases. For the cholesteric phases the pitch of the helix was determined, which ranges from 3 to 20 μm. Stable monolayers at the air-water interface were obtained from two monomers and one of the homopolymers. Multilayer assemblies of the azobenzene-containing polymers can be used to study photo-induced order/disorder transitions.     

Stress-induced orientation in lightly crosslinked liquid-crystalline side-group polymers

X-ray measurements of stretched crosslinked liquid-crystalline side group polymers show that two types of orientation of the mesogenic groups relative to the polymer chains (parallel or perpendicular) are possible, depending on the structure of the polymer chain. For two liquid-crystalline polymethacrylates with long spacer groups (6 methylene units) the mesogenic groups orient preferably perpendicularly to the polymer chains (axis of strain). For three liquid-crystalline polyacrylates with long or short spacer groups (6 or 2 methylene units), the mesogenic groups orient preferably parallel to the polymer chains (axis of strain). The reason for this difference is not yet clear. These results are compared with earlier results of melt-drawn fibres of uncrosslinked polymers.     

Crosslinkable liquid-crystalline combined main-chain/side-group polymers with low glass transition temperatures

Crosslinkable and crosslinked liquid-crystalline polymers with elastic properties were prepared from main-chain and combined main-chain/side-group polymers. These polymers exhibit nematic, smectic A, and smectic C phases. Polymers of these types with azoxybenzene as mesogen do not crystallize upon cooling. As a consequence, the liquid-crystalline phases are frozen in glassy around room temperature.     

Rheological properties of liquid-crystalline side-group polymers in the isotropic,nematic, and smectic states

The melt viscosity of seven liquid-crystalline side-group polymers with polyacrylate and polysiloxane main chains is measured as a function of shear rate and temperature. In the isotropic and also in the nematic phase none or only a very small dependence of the melt viscosity on shear rate is observed for the shear rates investigated. The melt viscosity in the nematic phase is higher than that in the isotropic phase. This is contrary to the results for liquid crystalline mainchain polymers and points to the fact, that there is no resulting orientation in shear flow of liquid-crystalline side-group polymers in the nematic phase. In the smectic phase the melt viscosity is very high and strongly shear-rate dependent. These results are compared with dielectric relaxation measurements on polymethacrylates and a polychloroacrylate.     

Kerr effect in a laterally substituted side-chain liquid-crystalline polymer

The static and dynamic Kerr effects in a liquid-crystalline (LC) side-end polymer with a laterally substituted mesogenic fragment are studied. The characteristic relaxation times of the two different relaxation processes observed diverge at the same temperature. The main feature of the dynamic behavior of the polymer, e.g., the type of the electro-optical response is similar to that of side-on LC polymers.     

Dielectric properties of a combined main-chain/side-chain liquid-crystalline polymer

The dielectric relaxation properties of a combined main-chain/side-chain liquid-crystalline polymer were investigated. It was found that the rotation of the side chain about the main chain (δ-process) is not as strongly restricted as in side-chain liquid-crystalline polymers. This is attributed to the facts that the side chain is attached to the flexible spacer within the chain backbone and that the concentration of the side chains is comparatively small. Two low-temperature relaxation processes were observed to occur in the glassy smectic and the crystalline state. They are attributed to intramolecular motions with in the mesogenic groups.     

Surface tensions of liquid-crystalline and isotropic polymer melts

The surface tensions (γ) of a branched polyethylene and two side-group liquid-crystalline (lc) polyacrylates were measured in the isotropic, nematic and smectic states by the pendant-drop method. The surface tension vs. Temperature characteristics of the lc polyacrylates studied show anomalous behaviour. The slope in the isotropic state is initially negative but changes its sign near the isotropic-nematic transition. The increase of γ with decreasing temperature is regained in the nematic phase, until at the nematic-smectic transition the surface tension of the lc polymers discontinuously jumps to higher values and shows a very low temperature coefficient. The observed features have been found to be partly in agreement with the results of surface tension measurements on low-molar-mass liquid crystals, which indicates that the surface properties of the lc polymers are governed by the mesogenic side groups.     

Mechanical behaviour of liquid-crystalline polymers and their networks

Liquid-crystalline polymers with mesogenic groups in the main chain and side chains as well as their networks are studied by the dynamic mechanical method. The homopolymers are compared with chiral copolymers of a similar chemical structure. Mechanical relaxation transitions are correlated with transitions observed by differential scanning calorimetry. In the nematic phase a flow-thinning effect has been observed both for uncrosslinked polymers and for crosslinked systems. A possible molecular interpretation of the observed mechanical behaviour is discussed.     

Permeation and separation of binary organic mixtures through a liquid-crystalline polymer membrane

When benzene/cyclohexane, toluene/cyclohexane and o-xylene/cyclohexane mixtures are subjected to pervaporation through a side-chain liquid-crystalline polymer (LCP) membrane in the liquid-crystalline state, the permeation rate increases with increasing temperature and the LCP membrane exhibits permselectivity for the aromatic hydrocarbon. The permeation rate and permselectivity of the LCP membrane for each mixture decreases with increasing molecular size of the aromatic hydrocarbon in the binary feed mixture.     

NMR investigations on a liquid-crystalline side-chain siloxane polymer with a wide biphasic region

A polysiloxane bearing two different kinds of side chains has been studied by nuclear magnetic resonance and relaxation investigations. Both side chains contain the same mesogenic core but differ in their connection to the main chain: one of them is bound directly to the backbone, the other is linked to it via a ? (CH₂)₃? spacer. In this sample, the isotropic and mesomorphic phases coexist over a relatively wide range of temperatures. We have tried to explain this phenomenon by microphase separation. For the s_A –s_B transition a similar biphasic behaviour could not be observed with our methods. The order parameter has been estimated selectively for carbons in different parts of the side chain (spacer, wing group, mesogenic core). The behaviour of the order parameter in the mesomorphic regions of the biphase was observed, and a possible explanation is given.     

Liquid-crystalline polymer gels, 4. Photocontrol of dye orientation in a polymer gel having a cholesteric liquid-crystalline order

A dichroic dye, 1,4-diaminoanthraquinone, was covalently linked to an azobenzene derivative carrying a long alkyl group. The photochromic dye showed a trans-to-cis photoisomerism with somewhat reduced quantum yield, probably due to an energy transfer from the azobenzene unit to the dye unit. The cis-to-trans reverse photoisomerization was induced not only by irradiating the absorption band of the cis-azobenzene unit, but also by irradiating the red-edge of the absorption band of the dye unit. The photochromic dye was doped in a polymer gel having a cholesteric liquid-crystalline order (CLC gel). Circular dichroism was induced at the absorption bands of both the azobenzene and the dye unit. The induced circular dichroism (ICD) of the dye absorption band was reversibly changed when the azobenzene unit was photoisomerized reversibly, indicating that the orientation of the dye group is affected by the structural change of the azobenzene unit.     

Lyotropic and thermotropic liquid-crystalline phase formation from fractions of a semiflexible cellulosic polymer

The benzoic acid ester of (2-hydroxypropyl)cellulose (BzPC) was prepared and fractionated from acetone solution by precipitation with methanol-water (85:15, by volume). The fractions were characterized by low-angle laser light scattering in tetrahydrofuran, and by size exclusion chromatography with a low-angle laser light scattering detector. Dilute solution viscosities were measured in acetone and benzene, and the results were interpreted on the basis of a worm-like chain model for BzPC. The critical concentrations of polymer necessary to form lyotropic cholesteric phases in acetone and benzene were found to decrease as the molar mass of the polymer increased, reaching a limiting volume fraction of ≈ 0,35 for high molar mass fractions. The experimental results were compared with the prediction of recent theories for the formation of liquid crystalline phases from semiflexible polymers; the chain parameters required by the theories were estimated from dilute solution viscosities. The fractions of BzPC also showed a thermotropic transition from liquid crystalline phase to isotropic melt. The transition temperature increased sharply with increasing molar mass to reach a limiting value of 176°C.     

Electric field effects on a liquid-crystalline side chain polymer with a negative dielectric anisotropy

The behavior of methacrylic side chain polymers in an electric field was observed for a homopolymer ( 1 ) exhibiting a negative dielectric anisotropy compared with a copolymer ( 2 ) of positive dielectric anisotropy. Homopolymer 1 orients spontaneously homeotropic and thus enables the determination of the threshold voltage, rise time and decay time. Based on these experimental results the bend elastic constant and viscosity coefficient were estimated using literature values for the dielectric anisotropy. With respect to the bend elastic constant and electrohydrodynamic effects (e.g. Williams Domains) the homopolymer behaves similarly to low-molar-mass liquid crystals, while owing to the high viscosity coefficient response times are large in magnitude.     

Influence of the chemical constitution of the polymer backbone on the phase behaviour of liquid-crystalline side-chain polymers with side-on fixed mesogenic groups

The synthesis and phase behaviour of liquid-crystalline acrylates and chloroacrylates with side-on fixed mesogenic groups are described. Polymers which have similar mesogenic groups but whose polymer backbones differ in chemical constitution exhibit identical liquid-crystalline to isotropic phase transformation temperatures (T_c). The glass transition temperature (T_g) decreases with increasing flexibility of the main chain and with increasing length of the terminal groups of the mesogenic side groups. All polymers with side-on fixed mesogenic groups exhibit a nematic phase. The domain texture indicates biaxial nematic phase behaviour. In copolymers consisting of mesogenic and non-mesogenic monomers, nematic phase behaviour is still observed, T_c is strongly depressed, and T_g is similar to that of the non-mesogenic homopolymer.     

Optical investigations on a liquid-crystalline side-chain polymer with biaxial nematic and biaxial smectic A phase

A liquid-crystalline side-chain polymer with laterally incorporated naphthalene groups in the mesogenic moiety is investigated in binary mixtures with an optical uniaxial low-molar-mass liquid crystal. Mixtures with up to 50 mol-% content of the monomeric liquid crystal show a biaxial nematic phase. At lower temperatures a new biaxial smectic phase is observed, which corresponds to the smectic A phase. The angles between the optical axes are measured in dependence of temperature and mixture composition. Starting from the neat polymer, the angles decrease in the nematic as well as in the smectic phase with increasing mole fraction of the uniaxial liquid crystal. From the results a phase transformation uniaxial nematic to biaxial nematic can be inferred.     

In-situ photopolymerization of oriented liquid-crystalline acrylates, 3. Oriented polymer networks from a mesogenic diacrylate

Synthesis, mesomorphism, orientation and photo-initiated chain crosslinking of the liquid-crystalline diacrylate 1,4-phenylene bis{4-[6-(acryloyloxy)hexyloxy]benzoate} ( 1 ) are studied. Monomer 1 exhibits a broad nematic phase between 108 and 155°C and a monotropic smectic phase below 88°C. The monomer is uniaxially oriented in its nematic phase at a substrate which has been coated with polyimide and unidirectionally rubbed with tissue. At the transition temperature to the smectic phase the order parameter is measured to be 0,7. During polymerization, the ordering of the mesogens is frozen-in, yielding a uniaxially crosslinked network. The clear films of oriented poly( 1 ) exhibit a birefringence Δn between 0,12 and 0,15, depending on the polymerization temperature. In the highest oriented state of 1 a small reduction of the degree of order is observed during the crosslinking reaction, whereas at higher temperatures and lower ordering of 1 , the uniaxially orientation increases upon reaction. A special feature of the oriented networks is that the ordering is maintained while heating at high temperatures. The polymerization of the acrylate groups in the mesomorphic phases proceeds fast and to high conversion. Below 90°C the polymerization behaviour is similar to that of conventional isotropic diacrylates. Above 90°C the polymerization reaction of the liquid-crystalline diacrylate proceeds faster than that of an isotropic diacrylate.     

Thermotropic liquid-crystalline aromatic-aliphatic polyimides, 2. Effect of aliphatic spacer lengths and polymer compositions on the liquid-crystalline properties of poly(imide-carbonate)s

Novel thermotropic liquid-crystalline (LC) aromatic-aliphatic poly(imide-carbonate)s were prepared by melt polycondensation of N,N′-bis(ω-hydroxyalkyl)pyromellitdiimide and/or 6,6′-(4,4′-biphenylylenedioxy)dihexanol with hexamethylene bis(phenylcarbonate). The monomers, dihydroxyalkyl derivatives of pyromellitdiimide, were obtained by deacetylation of the intermediate diacetylated compounds with p-toluenesulfonic acid. The intermediates were synthesized by condensation of pyromellitic dianhydride (PMDA) with ω-aminoalcohols followed by cyclization with acetic anhydride. Most of the poly(imide-carbonate)s obtained in high yields have high molecular weights and show very good solubilities in organic solvents. The structures of the resulting polymers were confirmed by FT/IR and NMR spectroscopy measurements and elemental analyses. Thermotropic LC properties of the polymers were evaluated by differential scanning calorimetry (DSC), polarizing microscopy and powder X-ray analyses. These measurements suggested that only biphenyl unit-rich copolymers and polymers with shorter spacers next to the imide ring, which favor the orientation of the imide ring and the polymer chains, form nematic LC mesophases. The glass transition temperatures (T_g) and the LC phase transition temperatures tend to decrease with increasing imide-neighboring aliphatic spacer lengths.