Synthesis,characterization and photoreaction of side-chain liquid-crystalline polymers and copolymers comprising alkenyloxy-biphenyl mesogen

Photoreactive (meth)acrylate monomers having mesogenic groups based on 4-hexyloxy-4′-(3-methyl-2-butenyloxy)biphenyl and 4-hexyloxy-4′-(2-butenyloxy)biphenyl were synthesized. Methacrylate monomers ( 3c, d ) and acrylate monomer having 3-methyl-2-butenyloxy group ( 3b ) were polymerized by free radical mechanism; the acrylate monomer having 2-butenyloxy group ( 3a ) caused gelation during the polymerization. To obtain photoreactivity, 4-benzoylphenyl (meth)acrylates were radically copolymerized with monomers 3b and 3d . Thermal behavior of (co)polymers was determined by differential scanning calorimetry, polarized optical microscopy and temperature-controlled X-ray analysis. (Co)polymers showed liquid-crystalline behaviour with nematic and smectic phases. UV-irradiation was used to photochemically crosslink copolymer films, and the resultant films were insoluble in organic solvents. The uniaxially oriented copolymer thin film was photo-crosslinked and showed thermally stable orientation.     

Synthesis,characterization and photoreaction of photoreactive side-chain liquid-crystalline copolymers comprising 2-benzoylethoxybiphenyl and 3-methyl-2-butenyloxybiphenyl side groups

Photo-crosslinkable side-chain liquid-crystalline copolymers comprising 2-benzoylethoxybiphenyl and 3-methyl-2-butenyloxybiphenyl groups were synthesized and characterized. Thermal behavior of the copolymers was determined by differential scanning calorimetry and polarized optical microscopy. All synthesized copolymers were liquid-crystalline having nematic and/or smectic phases, and their mesomorphic transition temperatures hardly varied with composition of the copolymers. Photochemically crosslinked copolymer films obtained by UV-irradiation were insoluble in ordinary organic solvents.     

Synthesis and characterization of some new photochromic side-chain liquid-crystalline polymers

The synthesis and characterization of a series of liquid-crystalline copolymethacrylates containing side chains with both azobenzene and benzanilide moieties is described. The phase behaviour of the polymers is discussed in dependence of their copolymer composition. The copolymers are suitable for optical investigations on the photochemical trans-cis isomerization of the azo group.     

Phase transition of liquid-crystalline polyesters with biphenyl mesogen under high pressures

The structural and thermal behavior of liquid-crystalline polyesters at hydrostatic pressure was investigated by using a high-pressure DTA and a wide-angle X-ray scattering apparatus equipped with a high-pressure sample vessel. The thermotropic polyesters (two homopolymers and the corresponding copolymer) are based upon 4,4′-biphenyldiol as the mesogen and aliphatic dibasic acids containing 7 and 8 methylene groups as flexible spacer, which are labeled as PB-7, PB-8, and PB-7,8 (30/70 mol-%), respectively. The phase diagrams of the PB-7, PB-8 and PB-7,8 samples were constructed for pressures up to 300 MPa. At room temperature both the PB-7 and the PB-8 crystals are compressed hydrostatically in the molecular chain and lateral packing directions. The d spacing of the low-angle reflection of the PB-8 crystal, corresponding to the layer spacing, decreases by about 2,6 ± 0,2 Å at the crystal (K)-smectic H(S_H) transition under all pressures, while the crystal reflections of PB-7 disappear at the K-nematic (N) transition. The d spacings of the wide-angle reflections of both crystals, corresponding to the lateral packing in the crystals, expand gradually with increasing temperature up to the K-mesophase transition. The increase in lateral spacing of the PB-8 crystal at the K-S_H transition changes slightly from ca. 0,29 Å at 0,1 MPa to 0,22 Å at 300 MPa. It is concluded that the transition mechanisms of the K-S_H transition of PB-8 and the K-N transition of PB-7 are maintained at pressures up to 300 MPa.     

Side-chain liquid-crystalline polymers containing a siloxane spacer, 1. Synthesis of side-chain liquid-crystalline polymers containing siloxane bond in the spacer unit

Side-chain liquid-crystalline polymers containing siloxane bond in the spacer units were prepared. For the synthesis of the desired monomers, novel silanol compounds carrying a mesogenic group were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, p- or p′-cyano-substituted biphenyl group, phenyl and biphenylyl benzoates were chosen, and the structural effects on the thermal properties were investigated. Introduction of a disiloxane group in the side chain resulted in a decrease of T_g which was more marked for longer siloxane units. Furthermore, only when a terphenylene-type mesogen was introduced, a stable smectic mesophase was observed with wide range of phase stability.     

Preparation of side-chain liquid-crystalline polymers via pyridylphenol hydrogen bonding

A series of side-chain liquid-crystalline polymers via pyridyl-phenol hydrogen bonding complexation was prepared by using non-liquid-crystalline precursors, such as poly[(4-vinylpyridine)-co-(butyl acrylate)] and 4-(4′-nitrophenylazo)phenol, 4-(4′-methoxyphenylazo)phenol or 4-(phenylazo)phenol. The association by hydrogen bonding was confirmed by means of FTIR. While the hydrogen bonds are weakened, and some dissociation occurs at the isotropization temperature, complete dissociation needs much higher temperature. The hydrogen bonds re-form as the complex is cooled to room temperature. The nematic liquid crystallinity of the polymer complexes was shown by means of wide-angle X-ray diffraction (WXRD) and polarized optical microscopy (POM). The different substitution groups of azophenol give isotropization temperatures of the polymer complexes in the following sequence:  NO₂ >  H >  OCH₃.     

Asymmetrically substituted liquid-crystalline phthalocyanines and side-chain polymers derived from them

Asymmetrically substituted, liquid-crystalline phthalocyanines were synthesized. They contain one methoxy, six dodecyloxy and one acryloyloxy- or methacryloyloxydodecyloxy groups. The latter functions were polymerized under free radical conditions to give polymers with molecular weights between 9000 and 47000. The aggregation behaviour of the phthalocyanines in the polymer side chains was studied in solution by absorption and fluorescence spectroscopy. An enhanced intramolecular interaction between the phthalocyanines was observed. In the solid state this interaction is reflected in the spontaneous formation of a columnar structure. The polymers, however, do not display liquid-crystalline behaviour. Temperature-dependent luminescence measurements (at 4,2 – 300 K) were performed and the results are explained by the presence of an efficient energy migration process through the stacked phthalocyanine units.     

Synthesis and properties of liquid-crystalline side-chain polymers containing β-diketonato transition metal complexes

Polymers ( 5-7 ) containing β-diketonato metal complexes of Cu²⁺, Ni²⁺ and Co³⁺ were synthesized from new thermotropic liquid-crystalline poly(acrylate ester)s ( 4 ) with a mesogenic bidentate β-diketone in the side chain, and the liquid-crystalline properties were examined by differential scanning calorimetry (DSC), X-ray diffraction and polarizing microscopy. The mesophase of 4 is an enantiotropic smectic A phase showing a batônnet and focal conic fan texture in polarizing microscopy. Polymers 5a-c containing a bis(β-diketonato) copper(II) complex also exhibit an enantiotropic smectic A phase, and polymer 5d prepared from the vinyl monomer 8 shows a nematic phase. Concerning the complexes of polymers 6 and 7 with nickel(II) and cobalt(III), only 6a and 7a containing a small amount of metal-chelating β-diketonate units exhibit liquid-crystalline behavior like the parent polymer 4 . It was found that the fourcoordinated square planar bis(β-diketonate)copper(II) acts as the mesogen; both nickel(II) and cobalt(III) complexes do not work as mesogens and rather hinder the liquid crystallinity of 4 . The electron spin resonance (ESR) spectra of 5a and 5b change markedly along with the transition from the smectic A phase to the isotropic one. The broadening and simplification of the spectra for the isotropic phase indicates increased interaction between neighboring bis(β-diketonato)copper(II) complexes and substantial electron spin-spin interaction. The ESR spectra of 5c and 5d also imply this increased interaction between neighbouring complexes and spin-spin interaction regardless of the phase transition.     

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.     

Synthesis and characterization of a homologous series of chiral smectic C side-chain polymers

The synthesis and characterization of a homologous series of liquid-crystalline side-chain polymers with a chiral smectic C phase are described. The polymer backbone consists of methacrylate monomeric units which link the mesogenic moieties of the chiral cinnamic acid 2S- (?)-4-(2-chloro-4-methylpentanoyloxy)phenyl ester via a flexible spacer of six to eleven CH₂ groups. The influence of this variation on this variation on transition temperatures, phase structure and range of the l. c. phases is studied. In contrast to the monomers, the chiral smectic C phase of the polymers is significantly broadened with respect to the temperature regime and can be vitrified in the glassy state.     

Nematic-nematic immiscibility and induced smectic a phases in mixtures with liquid-crystalline side-chain polymers

Two non-ideal kinds of behaviour for solutions of liquid-crystalline side-chain polymer (LC polymer) solutes in nematic solvents of low molecular weight (LMW) are investigated, viz. immiscibility and induced smectic A phases. The former property already observed in the nematic state of some polysiloxanes is extended to polymethacrylates (or polyacrylates) leading to the conclusion that immiscibility is strongly influenced by the chemical structure of the whole macromolecule. The latter property responds in the same way as the former to molecular modifications, suggesting that they can be connected somehow.     

Synthesis of side-chain liquid-crystalline polyethers by chemical modification of polyepichlorohydrin

Copolymers derived from polyepichlorohydrin (PECH), bearing pendant mesogenic units are obtained by chemical modification of atactic samples of PECH with sodium 4-cyano-4′-biphenoxide. Rate and yield of substitution are strongly dependent on the molecular weight of the used PECH. A substantial rise (ΔT ～ 110°C) of glass transition temperature with the percentage of incorporated cyanobiphenyl groups is observed. Copolymers with an amount of substitution higher than 60% present thermotropic liquid-crystalline behaviour and form nematic phases. Chemical modification of PECH offers a simple method for the synthesis of new liquid-crystalline polyethers whose transition temperatures can be adjusted by varying the amount of substitution.     

Synthesis of a liquid-crystalline polysiloxane with a diol mesogen in the side chain

The synthesis and thermal behaviour of a cis,cis-(3,5-dihydroxycyclohexyl) 3,4,5-tris(alkoxy)-benzoate, functionalized by one terminal C-C double bond is described. Its mesophase of the hexagonal columnar type is formed by a “supramolecular” unit, consisting of six single molecules aggregated via hydrogen bonding. Subsequently, a liquid-crystalline side-chain polymer with a siloxane backbone was synthesized. Its mesophase behaviour, investigated by means of differential scanning calorimetry (DSC) measurements and polarizing microscopy, is discussed.     

Synthesis of novel sulfonyl-containing liquid-crystalline side-chain poly(vinyl ethers)

The synthesis of some new liquid-crystalline polymers with sulfonyl-containing mesogenic groups is described. 4-[(S)-(-)-2-MethylbutyIsulfonyl]-4′-[(11-vinyloxy)undecyloxy]biphenyl ( 10 -11), 4-[(S)-(-)-2-methylbutylsulfonyl]-4′-[8-(vinyloxy)octyloxy]biphenyl ( 10 -8), 4-[(S)-(-)-2- methylbutylsulfonyl]biphenyl 4-[11-(vinyloxy)undecyloxy]benzoate ( 12 -11), 4-[(S)-(-)-2-methylbutylsulfonyl]biphenyl 4-[8-(vinyloxy)octyloxy]benzoate ( 12 -8), 4-[(S)-(-)-2-methylbutyloxy]-4′- [11-(vinyloxy)undecylsulfonyl]biphenyl ( 18 -11) and 2-[11-(vinyloxy)undecyloxy]-6-{4-[(S)-(-)-2- methylbutylsulfonyl]phenyl}naphthalene ( 23 -11) were all synthesized, and polymerized with the initiating system CF₃SO₃H/S(CH₃)₂ in CH₃Cl₃ at 0°C. Monomers 10 -11, 10 -8, 18 -11 and 23 -11 are crystalline, while both 12 -11 and 12 -8 show an enantiotropic smectic A phase. All polymers exhibit the same thermotropic behaviour as their corresponding monomers, except poly( 23 -11) which exhibits an enantiotropic smectic A (s_A) and a monotropic chiral smectic C phase (S*_C).     

Thermally irreversible photoresponsive polymers: Synthesis and preliminary characterization of copolymers containing side-chain trans-cinnamamide chromophores

A novel photoresponsive polymer containing organic chromophores which possess thermal stability as well as substantial and reversible responsiveness upon external photostimulus has been developed. trans-2{4-[2-(Dimethylcarbamoyl)vinyl]phenoxy}ethyl methacrylate ( 1 ) was synthesized and copolymerized by free-radical methods with methyl methacrylate giving a copolymer poly[(methyl methacrylate)-co- 1 ] (poly(MMA-co- 1 )) ( 2 ) with cinnamamide moiety chromophores randomly attached as side groups. Photoisomerization and ultraviolet absorbance were used to examine the photostationary states and solubility changes upon photoirradiation. Poly[(methyl methacrylate)-co- 1 ] (90,7 : 9,3 mol%) with 9,3 mol-% cinnamamide content has better solubility in a polar solvent such as acetonitrile (theta temperature Θ_PMMA = 30°C) than in a nonpolar solvent such as carbon tetrachloride (Θ_PMMA = 27°C). Photoisomerization of the trans-cinnamamide to cis-cinnamamide units significantly shifts the cloud point of poly[(methyl methacrylate)-co- 1 ] (90,7 :9,3 mol-%) in carbon tetrachloride; this cloud point shift is reversible by photoisomerization back to the trans state. However, in a hydrogen-bonding solvent such as tert-butyl alcohol, while photoirradiation of poly[(methyl methacrylate)-co- 1 ] (90,7 :9,3 mol%) yields a significant shift in cloud point, this is not reversible by photoirradiation as hydrogen-bond formation between the tertiary amide and the hydroxylic solvent reduces the isomerization efficiency. Fatigue of the photoreversibility of the copolymer, which is not present in dilute solutions of the cinnamamide chromophore, is noted.     

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.     

Synthesis of side-chain liquid-crystalline polysalts with a poly(methylsiloxane) backbone

The synthesis and thermal properties of side-chain liquid-crystalline polysiloxanes, incorporating quaternized pyridyl groups, are described. The polymers are obtained by hydrosilylation of poly(hydrogenmethylsiloxane) with a protected alcohol. Reaction of the deprotected polysiloxanes with 4-(hydroxymethyl)pyridine and subsequent reaction with mesogenic alkyl bromides of different structure yielded the desired poly[(6-{N-[ω-(4′-alkoxy-4-biphenylyloxy)alkyl]-4-pyridiniomethoxy}hexyl)methylsiloxane bromide]. Four different types of polymers are prepared with either octyl or dodecyl alkyl chain, and with either methoxy or S-2-methyl-1-butoxy as an alkoxy group. Degrees of quaternization up to 80% are obtained. First results of their liquid-crystalline behaviour investigated by optical polarizing microscopy and differential scanning calorimetry are reported.     

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 thermal behavior of liquid-crystalline block copolymers containing both main-chain and side-chain mesomorphic blocks

A novel type of block copolymers comprising both side-chain and main-chain liquid-crystalline (LC) blocks in the same macromolecular structure was synthesized and studied. The former block was either one of two LC polymethacrylates containing an azobenzene mesogen with different substituents (block A), and the latter was a semiflexible LC polyester block (block B). Thermal, dynamic-mechanical, and X-ray diffraction data indicated that the two structurally different blocks were at least partly phase-separated within the glassy and LC states. The thermodynamic phase transition parameters of block A were not affected by copolymer composition. However, significant deviations of the thermodynamic parameters of block B were observed relative to those of the corresponding homopolymers. In particular, the normalized transition enthalpies of block B were much lower, suggesting the occurrence of a more or less diffuse interphase. An increase in the nematic-isotropic temperature was found at variance with previous results on most of the LC block copolymers, in which only one block was an LC component.     

Local mobility and liquid-crystalline state in side-chain polymers with mesogenic groups directly bonded to the main chain

The intramolecular mobility of copolymers 4 was investigated by means of dielectrospectroscopy. The copolymers exhibit a convenient model for the study of local mobility in side-chain thermotropic polymers without spacer. The dependencies of the dielectric losses on frequency and temperature point to the presence of relaxation processes related to the intramolecular mobility of fragments of the mesogenic group. For the formation of the liquid-crystalline state two factors are important: different forms of local mobility inside the mesogenic groups and a flexible connection between side chain and main chain.