Properties of liquid crystalline polymers in solution, 2. Viscosimetric investigations of polymethacrylates with mesogenic sidegroups

Dilute solutions of poly(methacrylic acid) derivatives with mesogenic side groups of the benzoic acid phenyl ester type are structurally viscous. This property due to aggregation appears during storage and is attributed to the geometric shape of the dissolved particles. While solutions of freshly polymerized samples show no, or only a slight temperature dependence of the intrinsic viscosity between 298 and 368 K, the intrinsic viscosity of the stored samples decreases suddenly at temperatures above 348 K.     

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.     

Spacer length and molecular weight variation on ferroelectric liquid crystalline polymers

The influence of the spacer length (n = 2, 6, 8 and 11 methylene groups) on the polymorphism and the ferroelectric properties of liquid crystalline side-group polyacrylates with a 4-[(R)-4-(1-methylheptyloxy)-4′-biphenylyloxycarbonyl]phenoxy moiety was studied. Various molecular weights were obtained from the polymers with n = 8 and n = 11. All the polymers show SmC^* phases. A SmC^* phase polymorphism was even found for most of the polymers. In some of these SmC^* phases, no ferroelectric properties could be measured. The structural differences between the SmC^* phases and their influence on the appearance of the ferroelectric properties are not yet clear. However, a SmC?^* phase was put in evidence, revealing a tendency to a chevron-like ordering which may explain the complex SmC^* polymorphism of these systems.     

Mobility of mesogenic side-chains in liquid crystalline polymers based on the Baylis-Hillman reaction

The hydrodynamic and electro-optical (Kerr effect) properties of two side-chain liquid crystalline (SCLC) polymers have been investigated in benzene and chloroform solution. The polymers are unique in structure and have been prepared using the Baylis-Hillman methodology. The polymers contain (a) a polar group (CN or COCH₃) along the polymer backbone and (b) a hydroxyl group on the spacer group which is adjacent to the point of attachment of the spacer/mesogenic side groups to the polymer backbone. Molecular dimensions and the Kerr constants were determined for two SCLC polymers and for three low molar mass materials, the n-alkoxycyanobiphenyls (nOCB), which are structurally similar to the side chain used in the SCLC polymers. These results are discussed in relation to the chemical structure and dipole moment of the monomer unit. Dielectric and electro-optical properties of the SCLC polymer containing the —CN group along the polymer backbone were found to be similar to the properties of the nOCB compounds. The difference in the mesogenic side-chain mobility of the polymers was related to the dipole-dipole interactions of the polar groups along the polymer backbone.     

γ-ray polymerization of a mesogenic methacrylate in the crystalline,liquid crystalline and liquid state: kinetics,molecular weight distribution and conversion-dependent phase behaviour

The γ-ray polymerization of mesogenic 4-methoxy-4′-(6-methacryloyloxyhexyloxy)biphenyl ( 1 ) was investigated. 1 can be polymerized in the crystalline, in the liquid crystalline and in the molten state. The rate of polymerization increases from the crystalline to the melt phase. In order to reach 90% conversion to polymer in melt, smectic and crystalline phase, radiation doses of 10, 40 and more than 120 kGy are necessary. Melt polymerization leads to a weight-average molecular weight of about 1 × 10⁶ independent of the conversion to polymer. In the smectic and crystalline phase the corresponding molecular weights are about 3.8 × 10⁵ and 4.1 × 10⁴ at low conversion, and shifted to 2.6 × 10⁵ and 3.9 × 10⁴ at high conversion. The polydispersity generally increases with the γ-ray dose and the reaction temperature. The origin of the different reaction rates and molecular weight obtained in the various phases is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length. Differential scanning calorimetry (DSC) of partially polymerized samples was used to describe the conversion-dependent phase behaviour. Optical microscopy showed that the melt polymerization at 78.5°C proceeds under phase separation into a liquid, monomer-rich and a solid, polymer-rich phase. As a consequence, liquid monomer samples solidify upon γ-irradiation once a higher conversion to polymer is reached.     

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.     

γ-Ray-induced post-polymerization of a mesogenic methacrylate in the crystalline,liquid crystalline and melt phase: Reaction kinetics and molecular weight distribution

The γ-ray-induced post-polymerization of mesogenic 4-methoxy-4′-(6-methacryloyloxyhexyloxy)biphenyl (1) was investigated. Reaction kinetics and molecular weight distribution of the polymer were studied. Molecular weights and weight distributions were analyzed using size exclusion chromatography. Especially, effects of radiation dose, polymerization time and temperature were investigated. Post-polymerization was found in the crystalline, liquid crystalline and melt phase. Rate of polymerization and limiting conversion to polymer increase with radiation dose, polymerization time and temperature. Samples exposed to a low γ-ray dose of 5 kGy and subsequently polymerized in the crystalline state at 20°C, for example, only reach a limiting conversion of about 7% within 100 h, while samples exposed to a high dose of 20 kGy and polymerized in the melt at 79°C reach a limiting conversion of 85.1% within 90 s. Molecular weights increase with polymerization temperature and reach values comparable with in-source polymerization. Different from in-source polymerization, they are additionally affected by the parameters radiation dose and polymerization time. High molecular weights are obtained, if samples are exposed to a low radiation dose and isothermally polymerized at high temperature for a long time, while low molecular weights result from high radiation dose and short time of polymerization at low temperature, e.g., at 20°C. By carefully adjusting the reaction parameters, molecular weights can be reliably tailored in the range from 2 × 10⁴ to 6 × 10⁵. Polymerization at room temperature leads to a normal molecular weight distribution, while distributions are broader when the polymerization is carried out in the liquid crystalline or melt phase. The origin of the different reaction rates and molecular weights obtained under the various conditions is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length.     

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.     

Side‐chain functionalized liquid crystalline polymers and blends, 5. Order parameter for hydrogen‐bonded blends of the functionalized liquid crystalline polymer with a low molecular weight dopant

A hydrogen‐bonded blend based on comb‐shaped functionalized LC copolymers containing alkyloxy‐4‐hydroxybenzoic acid fragments (proton donor) and a low molecular weight dopant 4‐(4‐pyridyloyl)methoxybenzoate‐d₄ (proton acceptor) was obtained. The temperature dependence of the order parameter S, the birefringence Δn and the bend elastic constant K₃ of the LC copolymer and blend were studied by the ²H NMR spectroscopy and the Freedericksz method of threshold transitions in a magnetic field. The formation of the blend leads to a decrease of the order parameter S of the functionalized LC copolymer. The essential difference in the order parameter S of the mesogenic groups of the LC copolymers and the dopant molecules was established.     

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.     

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.     

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 polymers containing heterocycloalkanediyl groups as mesogens, 1. Liquid crystalline polymethacrylates and polyacrylates containing 1,3-dioxane-2,5-diyl groups as mesogens in the side chain

Polymethacrylates and polyacrylates containing 4-[5-(4-methoxyphenyl)-1,3-dioxan-2-yl]-phenol moieties as mesogenic units and an aliphatic spacer containing eleven methylene units were synthesized. Their phase behavior was studied by differential scanning calorimetry and optical polarizing microscopy, and compared with the phase behavior of the corresponding polymethacrylates and polyacrylates containing 4′-methoxy-4-biphenylol as mesogens. All polymers present smectic mesomorphism. The substituted 2,5-diphenyl-1,3-dioxane mesogen resembles a terphenyl mesogen in which the internal phenyl ring is hydrogenated. Even so, when compared with polymers containing biphenyl moieties, the glass transition temperatures of the polymers containing 1,3-dioxane moieties are lower, although their isotropization occurs at higher temperatures.     

Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 4. Synthesis of side-chain liquid crystalline polycarbonates with mesogenic groups having tails of different lengths

Side-chain liquid crystalline polycarbonates with alkoxyphenylbenzoate side groups, having a short spacer and tails ranging from 1 to 8 C-atoms, were synthesized. The polymers were prepared by an organo-zinc catalysed copolymerization of carbon dioxide and mesogenic 4-alkoxyphenyl 4-(2,3-epoxypropoxy)benzoates. Model polycarbonates were prepared by copolymerization of glycidyl phenyl ether (GPE) and carbon dioxide and by terpolymerization of GPE, propylene oxide and CO₂. The copolymerizations of LC monomers were carried out at 70°C in an autoclave at 60 atm CO₂ pressure in dioxane as the solvent. Catalysts based on diethylzinc and water or diethylzinc and resorcinol were used. High catalyst concentrations (up to 20 mol-% on the basis of diethylzinc with respect to epoxide) and long reaction times (72 h) were found to be necessary for acceptable yields and reasonable molar masses. The resulting polymers had a high polydispersity and the maximum yield was about 30%. A major side product, formed directly from CO₂ and epoxide, as well as indirectly by depolymerization, was the corresponding five-membered cyclic carbonate. The polymers on average contained about 30 mol-% of ether units and 70 mol-% of carbonate units due to a weak tendency of the epoxides to undergo homopolymerization. ¹H and ¹³C NMR analysis revealed detailed information about the overall ether content and the distribution of the ether and carbonate groups in the chain, as well as the tacticity of the polymers. Surprisingly, the monomer distribution and the tacticity were molecular weight dependent. Some possible explanations for these phenomena are suggested.     

Characterization of cellulose ethers by 13C NMR, 1. Studies of high molecular weight polymers in solution and in the solid state

Carbon-13 NMR was used to investigate the structure of a cellulose derivative, ethyl-O-(hydroxyethyl)cellulose (EHEC). Analysis of high molecular weight samples in solution showed that the line widths of the main chain carbon resonances are very sensitive to the concentration of the solution. Above a certain concentration, excessive broadening occurs due to gel formation, which makes a quantitative evaluation of the degree of substitution difficult. If the concentration is decreased below the critical value for gel formation, the use of NMR parameters suitable for quantitative measurements (no NOE and long pulse intervals) leads to excessively long accumulation times. Spectra of solid samples obtained by cross-polarization magic angle spinning (CP-MAS) NMR exhibit better defined resonances of the main-chain carbons than do the spectra of solutions with concentrations above the critical value for gel formation. Variable contact time experiments with a specific sample indicated that a suitable contact time should be 400 μs. The quantitative evaluation of the total degree of substitution by CP-MAS NMR yielded values in agreement with those obtained by GC-analysis.     

Phase distribution chromatography (PDC) as a method for the investigation of polymers in solution, 3. Determination of narrow molecular weight distributions

Phase distribution chromatography (PDC) is demonstrated to be a powerful method for the correct determination of narrow molecular weight distributions. This is shown by comparing the molecular weight distributions (MWD) of polystyrenes obtained from PDC measurements with those obtained by Baker-Williams fractionation and by gel permeation chromatography. Four problems are formulated and analyzed which are closely connected with the calculation of the MWD from a measured PDC-elution curve at temperatures below the theta point of the system. Thus, the theory of PDC outlined previously is tested.     

γ-ray polymerization of mesogenic diacrylates in the crystalline,liquid crystalline and liquid state — a kinetic study

A kinetic study of the γ-ray polymerization of three mesogenic diacrylates, 4,4′-bis(3-acryloyloxypropoxy)biphenyl (DAPB), 4,4′-bis(6-acryloyloxyhexyloxy)biphenyl (DAHB) and 4,4′-bis(11-acryloyloxyundecyloxy)biphenyl (DAUB), is presented. All compounds were studied on their polymerizability in the crystalline state. It was found that they can be completely polymerized though the reaction rates are very different and increase with increasing length of the alkylene spacer unit. Up to moderate conversion, solid-state polymerization is comparable with that of acrylamide and follows a t² rate law. DAPB was chosen for a detailed kinetic study of the in-source and post-polymerization in the crystalline, liquid crystalline and liquid phase. In-source polymerization is complete in the various phases though reaction rates and curve shapes of dose vs. conversion curves are very different. Possible origins such as enhanced molecular diffusion at elevated temperatures are discussed. X-ray studies indicate that a well-ordered polymer can be obtained from polymerization in a smectic phase, while polymers obtained from a crystaline or melt phase are considerably less ordered. Post-polymerization was studied as a function of the radiation dose and the temperature of the subsequent annealing process. It was found that the limiting conversion increases with increasing radiation dose and temperature of the annealing process, but the polymerization is always incomplete. Even post-polymerization in the melt phase does not exceed 55 percent conversion. The post-polymerization behaviour is comparable with that of acrylamide and can be described by a linear relation between conversion and annealing time t.     

Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 2. Polycondensation of mesogenic diols and diphosgene

A series of new side-chain liquid crystalline polymers was prepared with a polycarbonate backbone, bearing (E)-4′-nitrostilbene mesogenic groups, connected to the backbone by spacers of different length. The polymers were synthesized in 1,4-dioxane by polycondensation of diphosgene and diol monomers with the general structure 2-[ω-(4′-nitrostilben-4-ylcxy)alkyloxy]-1,3-propanediol in the presence of pyridine as a proton trap and catalyst. The mesomorphic properties of all monomers, polymers and intermediates were studied by polarized optical microscopy and differential scanning calorimetry (DSC). The polymers were also studied by X-ray diffraction of non-oriented and oriented samples (fibres). All monomers were liquid crystalline with nematic and/or smectic phases. For the polymers a clear relation was found between spacer length and glass transition temperature (T_g), which decreases with increasing spacer length and between spacer length and clearing temperature, which increases with increasing spacer length. X-ray diffraction measurements indicated the presence of a smectic A double layer structure (S_Ad) for all polymers at room temperature, with an antiparallel of overlapping mesogenic groups. A distinct difference in the orientation of the mesogens was found when fibres were drawn from the smectic phase or from the nematic or isotropic phase. In the first case the mesogenic groups were oriented perpendicular to the fibre axis and in the second case the mesogenic groups were oriented parallel to the fibre axis. By gel-permeation chromatography (GPC) measurements of heated samples and by thermogravimetric analysis (TGA) and DSC the polymers were found to be thermally stable up to temperatures well above their clearing point.