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.     

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.     

Thermotropic behavior of side-chain liquid-crystalline copolymers from maleic anhydride and mesogen-containing methacrylates

Side-chain liquid-crystalline copolymers (SCLCPs) were synthesized by copolymerization of maleic anhydride (MA) and mesogenic methacrylates. For copolymers with a hexyl spacer and methoxybiphenyl mesogens, smectic A1 and smectic E1 mesophases are observed. Furthermore, it has been found that the isotropization temperature increases and the width of the monotropic smectic A1 mesophase decreases with increasing MA content. For methoxybiphenyl-containing copolymers with about 25 mol-% MA, the glass transition temperature decreases with increasing spacer length, whereas the isotropization temperature shows little dependence on spacer length, although a small odd-even effect is observed. For octyl or shorter spacers, these polymers exhibit a smectic E mesophase, whereas for longer spacers smectic B mesophases were observed. These mesophases are succeeded by a smectic A1 mesophase for SCLCPs with heptyl or longer spacers. SCLCPs with cyanoazobenzene mesogens exhibit only a smectic Ad mesophase, whereas SCLCPs with cyanobiphenyl mesogens are not liquid crystalline.     

Synthesis of New Side‐Chain Liquid‐Crystalline Polyoxetanes with Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain

The synthesis of a different geometry of side‐chain liquid‐crystalline polymers is reported where two mesogenic units connected by a flexible spacer are attached as pendent groups of a polymeric chain. The resulting structure is the following, where R₁, R₂ and R₃ are methylenic or oxymethylenic segments: The monomers were obtained by linking the dimer structure to an oxetane ring. These substituted oxetanes were polymerised by a boron trifluoride‐initiated cationic ring‐opening reaction to obtain the corresponding polymers. The polymerisation proceeds with high yields, in spite of the steric hindrance derived from the bulkiness of the side chain, and the presence of some functional groups in the molecule that compete against the cyclic ether for the nucleophilic attack at the propagating centre. A fraction of cyclic oligomer is obtained together with high molecular‐weight polyoxetane, as a result of back‐biting reactions during the polymerisation. Several systems with spacers of different length and parity were synthesised in order to analyse the influence of the nature of the spacer on the phase behaviour. The formation of mesophases was proved by means of differential scanning calorimetry, X‐ray diffraction and microscopic analysis. Most of the monomers form smectic structures at subambient temperatures. The polymers display a similar phase behaviour, although shifted to higher temperatures.     

Highly ordered side‐chain liquid‐crystalline polymers from maleic anhydride and swallow‐tailed 1‐alkenes having two mesogens

Side‐chain liquid‐crystalline polymers (SCLCPs) were synthesized from the alternating copolymerization of maleic anhydride with swallow‐tailed 1‐alkenes having two mesogens. In contrast to similar polymers from 1‐alkenes with one mesogen per repeating unit, these novel polymers exhibited lower isotropization temperatures and higher glass transition temperatures. The degree of order in the mesophase strongly increased: these SCLCPs exhibited a highly ordered smectic H phase. The polymers with a hexyl spacer between the malonate junction and the mesogen exhibited a uniaxially limited‐correlation‐length smectic E mesophase (S_Eu) which has, according to our knowledge, not been observed before. The highest transition temperatures and entropy changes were observed for polymers with completely interdigitated side chains.     

Phase Behaviour of New Side‐Chain Liquid Crystalline Polyoxetanes

Summary: The synthesis and thermal behaviour of two new polyoxetanes with side chains having a dimer structure mesogen‐spacer‐mesogen and a long terminal alkyl group are reported. The two polymers differ in the chemical structure of the central spacer. The polyoxetanes have been obtained by cationic ring‐opening polymerisation of an appropriately substituted oxetane ring. Both polyoxetanes studied exhibit liquid crystalline properties showing a similar sequence of phases. DSC and synchrotron experiments indicate that a highly ordered phase with a probable orthorhombic packing is observed at low temperature. On heating, this phase melts into a mesophase with hexagonal order within the smectic layers, which, on further heating, is transformed into a disordered smectic mesophase.

Synchrotron X‐ray diffraction patterns of one of the poyoxetanes in a heating experiment.     

Side-Chain liquid-crystalline polymers containing a siloxane spacer, 2. Effects of mesogenic groups on the thermal properties of side-chain liquid-crystalline polymers containing a disiloxane unit in the spacer

A series of side-chain Iiquid-crystalline polymers was prepared containing disiloxane units in the spacer. The polymers consist of a polymethacrylate backbone and several kinds of mesogenic side groups. For the synthesis of the desired monomers, silanol compounds carrying a mesogenic groups were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, linear-type p- or p′-substituted phenyl and biphenylyl benzoates, p- or m-substituted benzoyloxybiphenyl groups and laterally attached biphenylyl benzoate-type mesogens were chosen, and the structural effects on the thermal properties of the polymers were investigated. In the case of linear-type mesogens, only when triple-core mesogens were introduced a stable smectic mesophase was observed with a wide range of phase stability, while on mesophase was observed for the linear double-core and the laterally attached mesogens. On the other hand, a highly ordered smectic phase was observed for polymers having linear-type triple-core mesogens with p-alkoxy and m-cyano end groups. Furthermore, it was revealed that the introduction of a disiloxane unit in the spacer lowered the temperature range of the mesophase, in comparison with polymers having an alkylene spacer with the same backbone component and mesogens.     

Phase behaviour of liquid crystalline side chain polymers

The phase behaviour of liquid crystalline side chain polymers is investigated by different experimental methods. PVT- and DSC-measurements as well as X-ray and optical investigations show that additionally to the known factors from low molecular weight liquid crystals the flexibility of the polymer backbone influences the liquid crystalline properties of the phase. Fixing a mesogenic side chain to a polymer backbone leads to a stabilisation or even a generation of a liquid crystalline phase in comparison to the corresponding low molecular weight liquid crystals. — The liquid crystalline phases which are known from low molecular weight liquid crystals are also found for liquid crystalline polymers (nematic, chiral-nematic, smectic). In contrast to low molecular weight liquid crystals the phase transformations smectic to crystalline and chiralnematic to smectic show strong time dependences for polymeric substances. — A crystallization of the liquid crystalline side chain polymers is observed, when the polymer main chain is so flexible, that the mesogenic groups can form a crystalline order. The crystallization process leads for a smectic sample to a layer lattice structure. From X-ray investigations one can conclude that the smectic arrangement of the mesogenic side chain exhibits a preorder for the crystalline structure. As known from other substances the crystallization rate depends on temperature and pressure. — The transformations in the liquid crystalline region are first-order transformations and obey the Clausius-Clapeyron equation. At lower temperatures all liquid crystalline polymers solidify to anisotropic glasses. Below the glass transition temperature nematic or smectic states of order are frozen-in and preserved in this way.     

Side‐chain liquid‐crystalline poly(ketone)s: effect of spacer length,mesogen type and mesogen density on mesomorphic behavior

Novel side‐chain liquid‐crystalline copolymers (SCLCPs) were synthesized via the Pd(II) catalyzed alternating copolymerization of mesogenic 1‐alkenes and carbon monoxide. For methoxybiphenyl mesogens, these copolymers exhibited highly ordered smectic E mesophases and high glass transition temperatures. The transition temperatures of these polymers were tuned by altering the spacer length or by dilution of mesogen‐containing alkenes with 1‐hexene. Polymers with cyanobiphenyl or methoxyazobenzene mesogens exhibited nematic mesophases. These polymers were the first ever nitrogen containing copolymers of this type. Increase of the spacer length or the 1‐hexene content of methoxybiphenyl‐containing copolymers resulted in a decrease of the surface polarity of spincoated films. The decomposition temperature of these SCLCPs started far above the isotropization temperature of the polymers.     

Liquid-crystalline polysiloxanes containing fluorene mesogens, 2. A new class of liquid-crystalline side-chain polymers with terminal 2-fluorenyl groups

The synthesis, characterization and liquid-crystalline (LC) properties of polysiloxanes containing side chains with terminal 2-fluorenyl groups and various lengths of spacers is reported. The polymers which have no flexible tail segment exhibit two smectic mesophases (S_A and S_E), as confirmed by polarizing microscopy, differential scanning calorimetry (DSC) and X-ray investigations; a nematic mesophase was not found. The clearing temperatures of the polymers (in the range between 120 and 170°C) are generally 70–80°C higher than the melting temperatures of the non-liquid-crystalline precursors with a terminal vinyl group. With increasing spacer length the less ordered S_A phase is favoured and has been found in a temperature range of up to 100 K width. For polymers with an S_E phase no glass transition was detectable with DSC. The synthesized polymers are examples of the strong potential of the fluorene core to form LC phases without further structural stabilization.     

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.     

Side chain liquid crystal polymers of 2,3-disubstituted norbornenes via ring-opening metathesis polymerization, 1 influence of flexible spacer length m = 2 to 12 on the thermotropic behaviour

Bis[(4′-cyanobiphenyl-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylates with alkyl chain lengths m = 2 – 12 were synthesized. Most of the monomers exhibit thermodynamically metastable liquid crystalline phases with respect to the crystalline phases. Polymers were prepared by ring-opening metathesis polymerization (ROMP) using the molybdenum carbene initiator [Mo(CH-t-Bu) (NAr) (t-BuO)₂] (t-Bu = tert-butyl) with NAr = 2,6-diisopropylaniline. All polymers show thermotropic liquid crystalline phases. Results on characteristic phases and the influence of the number of methylene units m in the flexible spacer on the thermal behaviour (glass transition temperature, isotropization temperature, odd-even effect, enthalpy change) of these new side chain liquid crystal polymers (SCLCP's) are given. They are compared to monosubstituted polynorbornene derivatives and to SCLCP's with other backbones.     

Synthesis and characterization of liquid crystalline (norbornene-block-acetylene) block copolymers

We report the synthesis and characterization of block copolymers of mesogenic acetylene and norbornene moieties. As monomers, a norbornene derivative with cyanostilbenyl side groups and an acetylene derivative with a cholesteryl group were used. The almost quantitative polymerization is initiated with the well-defined Schrock-type molybdenum carbene Mo(CH-t-Bu)(NAr)(O-t-Bu)₂ with NAr = 2,6-diisopropylaniline, t-Bu = tert-butyl. Polydispersity indices (PDIs) in the range of 2 for the block copolymers are obtained. The thermal and morphological behaviour of the polymers is characterized by means of differential scanning calorimetry (DSC) and X-ray diffraction. Both homo- and copolymers form thermotropic mesophases, the acetylene polymer exhibiting a smectic A phase, while the polynorbornene is nematic. The block copolymer shows microphase separation retaining the homopolymers' mesophases. To our knowledge, the materials presented here are the first example of phase-separated block copolymers of two side chain liquid crystalline polymers.     

Synthesis and Crosslinking of a Series of Dimeric Liquid Crystalline Epoxy Resins Containing Imine Mesogens

Eight aromatic imine liquid crystalline (LC) diepoxides with dimeric architecture from readily available commercial reagents were synthesized and characterized by spectroscopic techniques. We characterized their liquid crystalline mesophases by differential scanning calorimetry (DSC), hot‐stage polarized‐light optical microscopy (POM) and wide‐angle X‐ray diffraction (WAXS). The mesomorphism of the products was related to their structure, which varies in the length of the central spacer. Odd‐membered spacer mesogens lead to the formation of smectic C mesophases, whereas even‐membered samples of this series lead to the formation of smectic A and/or nematic mesophases. By relating the calculated molecular parameters to the layer spacings obtained by WAXS, we deduced the organization of the mesogens in the smectic phases, since the dimeric compounds present a variety of possible arrangements. Liquid crystal thermosets (LCTs) were obtained from these monomers by isothermal curing with equimolar amounts of 2,4‐diaminotoluene (DAT). The curing process was studied by calorimetry and their thermal stability was evaluated by thermogravimetry (TGA). Most monomers produced nematic‐like networks, but in one case a smectic C mesophase was also locked.     

Transitional properties of liquid-crystalline side-chain polymers derived from poly(p-hydroxystyrene)

A series of liquid-crystalline polystyrene derivatives with ω-(4-(4-fluorophenylazo)phenoxy)-alkoxy pendant groups have been synthesized and characterized. Homologues with four or more methylene units in the spacer chain exhibit smectic liquid-crystalline phases, while the homologue with three methylene units forms an amorphous glass. The smectic A-isotropic (S_A-I) transition temperatures show a marked odd-even effect as a function of the parity of the spacer chain. This odd-even behaviour is also found in the variation of ΔS/R at the S_A-I transition across the series. A marked hysteresis between the transition temperatures determined on heating and the transition temperatures determined on cooling is observed. A molecular interpretation of these phenomena is proposed.     

Self‐Assembly Behavior and Optical Properties of Poly(3‐thiopheneacetate)/Dialkyldimethylammonium Complexes

Poly(3‐thiopheneacetate)/dialkyldimethylammonium complexes (PTA‐Cn) were prepared by mixing regiorandom poly(3‐thiopheneacetic acid) with dialkyldimethylammonium bromide to study the effect of their self‐assembled structures on the optical properties. The lengths of the alkyl groups in the ammonium salts were varied from decyl (C10) to octadecyl (C18) groups. These complexes showed fully developed layer structures with ordered mesophases. PTA‐C16 and PTA‐C18 with longer alkyl groups showed side‐chain crystalline phases, and PTA‐C10, PTA‐C12, and PTA‐C14 with shorter alkyl groups showed side‐chain crystalline and liquid crystalline phases at room temperature. When PTA‐C18 was heated, a melting transition from side‐chain crystalline to smectic liquid crystalline phase was observed at 41.5 °C. The maximum absorption and emission wavelengths of these complexes increased with increasing alkyl chain length indicating that the π‐conjugated structure of the regiorandom thiophene backbone changes according to the alkyl groups.

     

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.     

Synthesis and structural characterization of chiral smectic C and smectic A poly(vinyl ether)s

A new series of chiral liquid-crystalline poly(vinyl ether)s was synthesized by cationic polymerization of the corresponding vinyl ether monomers. They consisted of a biphenyl mesogenic unit bearing an (S)-2-methylbutyl chiral substituent and spaced via an alkyl segment of varying number x of methylene groups. Different initiators and experimental conditions were used for the polymerization reactions. All the samples were semicrystalline, and smectic C and A sequential mesophases were formed in polymers comprising a sufficiently long spacer segment (x > 8). X-ray diffraction measurements were performed on unoriented specimens. In the smectic C phase, the occurrence of different orders of X-ray reflection on the smectic layers enabled drawing of the electron density profiles along the layer normal. By simple model considerations it was possible to choose the most physically acceptable one and to identify the low temperature mesophase as double layer smectic C.     

Synthesis and characterization of non‐covalent liquid crystalline diblock copolymers

Poly(dimethylaminoethylmethacrylate)‐b‐poly(sodium methacrylate) diblocks, (polyDMAEMA‐b‐polyMA), were synthesized as precursors of liquid crystalline (LC) copolymers. These LC copolymers were prepared by proton‐transfer between a carboxylic acid‐containing mesogen (A) and the dimethylamino substituent of the polyDMAEMA block and by electrostatic interactions between the polyMA subunits and an ammonium‐containing mesogen (B). When mesogen A is complexed with DMAEMA units, a dramatic enhancement of the mesophase stability is noted. The mesogenic properties of these LC copolymers were compared to those ones of the parent LC homopolymers, in relation to the copolymer composition. The supramolecular organization of the LC diblock copolymers was studied by small‐angle X‐ray scattering (SAXS), and smectic mesophases were observed in some LC (co)polymers. For samples containing a major LC block, a smectic mesophase coexists with microphases formed by the second amorphous block. Different organizations, including a body‐centered lattice of spheres and a hexagonal array of cylinders, were observed, depending on the investigated copolymers and their composition. When the LC block is the minor component, it forms microdomains too small for a liquid crystalline order to emerge. Moreover, no supramolecular organization of these dispersed microdomains was detected by SAXS. A homogeneous smectic mesophase was observed when the two blocks are bearing liquid crystalline moieties.     

Synthesis and phase behaviour of new carbazole containing liquid crystal side chain polysiloxanes

A series of chiral mesogenic side chain polymers containing a carbazole unit have been synthesized and investigated with regard to their liquid crystal (1c) phase behaviour. The carbazole units are linked either via the 2-position with a 1-oxycarbonyl-4-alkoxylphenyl moiety (end-on) or via the 9-position with an alkyl chain (side-on) to polymethylsiloxane. Only end-on polymers with long spacers or a ratio bigger one of spacer length and the free lateral alkyl chain length exhibit stable or metastable smectic phases. The side-on polymers are only isotropic or crystalline.