Side Chain Liquid Crystalline Polyacrylates Research Articles

Synthesis of well-defined easily crosslinkable azobenzene side-chain liquid crystalline polymers via reversible addition–fragmentation chain transfer polymerization and photomechanical properties of their post-crosslinked fibers

The synthesis of a series of well-defined easily crosslinkable azobenzene (azo) side-chain liquid crystalline polyacrylates (including both homopolymers with an N-hydroxysuccinimide carboxylate-substituted azo mesogen and copolymers with both the above crosslinkable azo mesogen and a non-crosslinkable azo mesogen) via reversible addition–fragmentation chain transfer (RAFT) polymerization and photomechanical properties of their post-crosslinked fibers are described. The RAFT polymerizations of the azo monomers proved to be living and well-controlled, as revealed by their linear kinetic plots as well as the linear increase of the molecular weights of the azo polymers with monomer conversions and their low molar-mass dispersities (Đ⩽1.17). All the obtained azo polymers exhibited high thermal stability and relatively low glass transition temperatures, but only the azo homopolymers with longer flexible spacers and azo copolymers showed liquid crystallinity. In addition, the crosslinked azo polymer fibers with easily controlled diameters and high alignment order were obtained by the first fabrication of the uncrosslinked azo polymer fibers by the simple melt spinning method and their subsequent crosslinking with a difunctional primary amine under mild conditions. The contents of the crosslinkable azo mesogen in the polymers had significant influence on the photomechanical properties of their post-crosslinked fibers, and those with appropriate crosslinking densities could exhibit fast and reversible photoinduced bending and unbending behaviors and excellent photodeformation fatigue resistance even at close to room temperature.

Thermal recordable novel cholesteric liquid crystalline polyacrylates containing various chiral moieties

AbstractTo investigate the effect of spacer length and linkages between the rigid mesogenic core and the terminal group on the molecular interaction and physical properties of polymers, two series of novel side chain liquid crystalline polyacrylates were synthesized. These were composed of liquid crystalline monomers with six or eleven methylene segments as spacers, and chiral monomers end capped with menthyl or cholesteryl groups. Liquid crystalline phases of the polymers were investigated using differential scanning calorimetry and polarized optical microscopy, and confirmed with X‐ray diffractometry. Color image recording of the synthesized polymer films was achieved using a thermal treatment, and then fixed by quenching. This investigation demonstrates that the introduction of carbonate linking groups between the rigid mesogenic core and terminal group decreases both the lateral molecular interaction and thermal stability of the liquid crystalline polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6214–6228, 2008

Synthesis and characterization of optically active liquid crystalline polyacrylates containing mesogenic phenylbenzoate groups

A series of novel side chain liquid crystalline polyacrylates with pendant chiral groups were synthesized. It was found that monomers with electron releasing –OC4H9 terminal groups seem beneficial for the formation of liquid crystalline phases. Copolymerization of the monomers was carried out and the physical properties of the copolymers were investigated. All synthesized polymers revealed liquid crystalline phases and appeared highly thermally stable with decomposition temperatures (Td) at 10% weight loss greater than 384°C and about 50% weight loss occurred beyond 442°C under nitrogen atmosphere. Two miscible chiral compounds were also synthesized and used as chiral dopants to induce cholesteric liquid crystalline phases of polymers. Liquid crystalline phases of the polymers were investigated using DSC and XRD, and confirmed with POM technique. The optical properties of the induced cholesteric liquid crystalline polymers were investigated using UV–vis spectrometer. The appearance and the color variation of the polymer films before and after UV irradiation were also investigated. Typical helical morphology of the cholesteric liquid crystalline film was analyzed by SEM technique.

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P4M and P11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A1 phases were observed for the polyacrylates with four (P4M) and eleven (P11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P4M exhibits an injected smectic phase, whereas P11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.

Synthesis and characterization of new chiral liquid crystalline polyacrylates from L-isoleucine

The synthesis of chiral side chain liquid crystalline polyacrylates with a two-stereogenic centre from L-α-aminoacid is described. The chiral tail is 2-chloroalcohol obtained from L-isoleucine and the spacer group has either four or eleven methylene units. The mesogenic moiety is derived from phenyl benzoate. The stereochemistry of the key intermediate (2S,3S)-(+)-4- [1-(2-chloro-3-methyl)pentyloxy]phenyl benzoate (6) obtained by a Mitsunobu reaction was established by single crystal X-ray analysis. This result indicates that the nucleophilic displacement of chiral diazonium salts proceeds with overall retention of configuration. The liquid crystalline behaviour of polyacrylates P13 and P14 was investigated by DSC, optical microscopy, small angle X-ray scattering and depolarized light scattering. The polyacrylate P13, with eleven methylene units in the spacer, exhibits a chiral smectic A phase whereas the polyacrylate P14, with a spacer containing four methylene units, displays a chiral nematic phase.

Polarized Blue Emission Based on a Side Chain Liquid Crystalline Polyacrylate Containing Bis-tolane Side Groups

A novel side chain liquid crystalline (LC) polyacrylate (1P) containing highly conjugated bis-tolane side groups was synthesized and used for polymer light emitting diodes application. Polymer 1P shows a nematic phase at room temperature. It exhibits a UV absorption peak at 330 nm and emits a blue light at 425 nm. Polyimide (PI), polyaniline (PA) and poly(3,4-diethylenedioxythiophene) (PEDOT) were used as alignment materials for polymer 1P. The photoluminescence dichroic ratios of 1P on rubbed PI, PA and PEDOT films are 10.4, 8.3 and 7.3, respectively. The electroluminescence dichroic ratio of 1P aligned on a PEDOT film is 6.1.

Synthesis and Electroluminescence of Side-chain Liquid Crystalline Polyacrylates and Polyoxiranes Containing Bistolane Side Groups

The synthesis and characterization of side-chain liquid crystalline (LC) polyacrylates and polyoxiranes containing bistolane side-groups are presented. The phase behavior of the prepared monomers and polymers was characterized by differential scanning calorimetry and optical polarizing microscopy. All of the obtained monomers and polymers reveal an enantiotropic nematic phase. The birefringences of the LC monomers are in the range from 0.35–0.6 depending on the measuring wavelength. The photoluminescence (PL) and electroluminescence (EL) properties of the obtained monomers and polymers are also reported.

Frustrated Behavior of a Side-Chain Liquid Crystalline Polyacrylate

Polyacrylate with phenyl benzoate mesogenic side groups was synthesized and fractionated into narrow fractions. Structure and phase behavior of the narrow-molecular-weight samples have been studied by X-ray diffraction. Depending on the degree of polymerization the polymer shows nematic, smectic A d , re-entrant nematic and new phase with two-dimensional (2D) periodicity. Such behaviour is implicit to so-called frustated systems. However, the mesogenic side chains are nonpolar and thus the origin of frustation is different from that in conventional polar liquid crystals. We found that this phenomenon is steric (entropic) in nature and depends on the length of the polymer backbone. The phase transformations of mesogen side chain polyacrylates are discussed in terms of coupling between the smectic ordering of the side groups and the polymer backbone conformations.

Thermotropic side chain polyacrylates with azobenzene mesogenic groups: Synthesis and thermal behaviour

Four side chain liquid crystalline (LC) polyacrylates containing substituted (methyl/long alkyl) azobenzene mesogens attached directly to the polymer backbone were synthesized. The polyacrylates were synthesized by free radical polymerization of the respective monomers, 4-[(4-acryloylexyphenyl)azo]benzoic acid, 4-[(4-acryloyloxy-3-methylphenyl)azo]benzoic acid, 4-[(4-acryloxy-2-methyl]phenyl)azo]benzoic acid and 4-[(4-acryloyloxy-2-pentadecylphenyl)azo[benzoic acid. The monomers were obtained by acryloylation of the azo derivatives prepared by the diazo coupling reaction between p-aminobenzoic acid and the respective phenolic derivatives with varying hydrocarbon substituents. All polymers were soluble in polar solvents. The inherent viscosities of the polymers in DMSO at 28°C were in the range of 0.55-0.62 dL/g. The monomers and their polymers were characterised by elemental analysis, IR, UV-vis, 1 H and 13 C NMR spectroscopies and WAXD. The thermal and phase transition behaviour of the polymers were investigated using DSC and a hot stage polarized light microscope (PLM). The polyacrylates exhibited stir opalescence and low viscosity characteristic of nematic phase on melting. However, the polymer with the long alkyl side chain (C 15 H 31 ) was polymorphic with a smectic phase besides a nematic phase. The methyl substituted polymers showed only nematic phases. The effect of regioisomers on transition temperatures was significant. The carboxylic acid groups of the azophenyl mesogen are found to exist as hydrogen bonded dimers as indicated by IR, NMR and finally confirmed by WAXD. It is proposed that the long alkyl substituent might be involved in the stabilisation of the mesophase through its alignment paralled to the long axis of the molecule and might probably be supporting the formation of a smectic phase by reducing the transition temperature enough to maintain the layered arrangement of hydrogen bonded dimers.

Thermotropic side chain polyacrylates with azobenzene mesogenic groups: Synthesis and thermal behaviour

Four side chain liquid crystalline (LC) polyacrylates containing substituted (methyl/long alkyl) azobenzene mesogens attached directly to the polymer backbone were synthesised. The polyacrylates were synthesised by free radical polymerization of the respective monomers, 4-[(4-acryloyloxyphenyl)azo]benzoic acid, 4-[(4-acryloyloxy-3-methylphenyl)azo]benzoic acid, 4-[(4-acryloyloxy-2-methylphenyl)azo]benzoic acid and 4-[(4-acryloyloxy-2-pentadecylphenyl)azo]benzoic acid. The monomers were obtained by acryloylation of the azo derivative prepared by the diazo coupling reaction between p-aminobenzoic acid and the respective phenolic derivatives with varying hydrocarbon substituents. All polymers were soluble in polar solvents. The inherent viscosities of the polymers in DMSO at 28°C were in the range of 0.55–0.62 dL/g. The monomers and their polymers were characterised by elemental analysis, IR, UV-vis, 1H and 13C NMR spectroscopies and WAXD. The thermal and phase transition behaviour of the polymers were investigated using DSC and a hot stage polarized light microscope (PLM). The polyacrylates exhibited stir opalescence and low viscosity characteristic of nematic phase on melting. However, the polymer with the long alkyl side chain (C15H31) was polymorphic with a smectic phase besides a nematic phase. The methyl substituted polymers showed only nematic phases. The effect of regioisomers on transition temperatures was significant. The carboxylic acid groups of the azophenyl mesogen are found to exist as hydrogen bonded dimers as indicated by IR, NMR and finally confirmed by WAXD. It is proposed that the long alkyl substituent might be involved in the stabilisation of the mesophase through its alignment parallel to the long axis of the molecule and might probably be supporting the formation of a smectic phase by reducing the transition temperature enough to maintain the layered arrangement of hydrogen bonded dimers.

Side-chain liquid crystalline polyacrylates containing 3,4-disubstituted pyrrole moiety

Polymerization of liquid crystalline bifunctional monomer N-{{ω-{4-[4-(11-acryloyloxy)undecanoxybenzoyl]biphenyleneoxy} alkyl}}-3,4-dimethylpyrrole gave a side-chain liquid crystalline polyacrylate containing 3,4-dimethylpyrrole group. Liquid crystallinity was determined by DSC and optical polarizing microscope measurements. The monomers having pentylene, hexylene, and decylene as the alkylene spacer group between the mesogenic unit and the 3,4-dimethylpyrrole group exhibited smectic and nematic phases on the heating and cooling stages. The radically polymerized polyacrylate derivatives containing the 3,4-dimethylpyrrole group showed nematic phases on the heating and cooling stages. These polymers are the first example of side-chain liquid crystalline polymers containing 3,4-dimethylpyrrole as functional group for further reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1214–1221, 2000

The glass transition relaxation in a side-chain liquid crystalline polymer studied by modulated temperature differential scanning calorimetry

A side-chain liquid crystalline polyacrylate was investigated by modulated temperature differential scanning calorimetry (MTDSC) in the temperature region of the glass to liquid-crystal transformation. The technique provides the possibility of analysing the temperature dependence of the complex heat capacity, and of its real and imaginary components. This allows determination of the kinetic parameters associated with the glass transition, as well as the index of fragility of the substance. The obtained results show very good agreement with those recently obtained on the same sample by thermally stimulated depolarisation currents.

Synthesis and characterization of liquid crystalline polyacrylates and non-liquid crystalline polypyrroles from bifunctional monomers

Two kinds of polymerizations of liquid-crystalline N-{{W-{4-[4-(11-acryloyloxy)undecanoxybenzoyl]biphenyleneoxy}alkyl}}pyrrole gave a side-chain liquid-crystalline polyacrylate containing pyrrole group and a non-liquid crystalline polypyrrole containing acrylate group. Liquid crystallinity was determined by DSC and optical polarizing microscope measurements. The monomers having hexyl or decyl group as the alkyl group exhibited an undefined smectic, smectic B, and smectic A phases on the heating stage. The radically polymerized polyacrylate derivatives containing the pyrrole group showed smectic A, smectic C, and undefined smectic phases on the cooling stage. On the other hand, soluble N-substituted polypyrrole derivatives containing the terminal acrylate group which were prepared by chemical oxidative polymerization by ferric chloride catalyst did not show liquid crystallinity. Structure analysis of the polypyrroles performed by 1H and 13C-NMR and FT-IR spectroscopies demonstrated that the polymerization occurred at the pyrrole ring. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3877–3887, 1999

Mesomorphic and dielectric properties of a side-chain liquid crystalline polyacrylate and copper(II) crosslinked derivatives

A side-chain liquid crystalline polyacrylate containing a salicylaldimine group has been synthesized and complexed with copper(II) ions to generate metallomesogenic crosslinking of polymeric chains. The mesomorphic properties of the parent and complexed polymers have been studied by optical microscopy, DSC and X-ray diffraction. In addition, these materials have been investigated in depth by dielectric spectroscopy. Dielectric measurements of the parent polymer reveal the presence of two sub-glass relaxations (β and γ). The α relaxation associated with the T g is masked by a conductive process, but at higher temperatures a δ relaxation is observed. These relaxation processes have been characterized and their activation energies evaluated. The influence of the mesogenic order and metal crosslinking on some relaxation processes has also been determined.

Effect of the composition on the structure and ferroelectric properties of side chain liquid crystalline copolyacrylates

The structure and physical properties of two side chain liquid crystalline polyacrylates are discussed, one bearing biphenyl core and chiral carbon atom in the flexible spacer, and the other a phenyl benzoate core. The biphenyl version displays two smectic phases of S*F and S*F types and no ferroelectric switching. The phenyl benzoate variant displays a single SC phase. The copolymerisation of the two monomers led to polymeric materials exhibiting a S*c polymorphism and composition dependent structural features. Only the copolymer of equimolar composition displayed a strong piezoelectric response in the S*F phase and ferroelectric behaviour in the lower temperature S*c modification.

Influence of spacer structure on the molecular dynamics of side-chain liquid-crystalline polyacrylates as studied by dielectric relaxation spectroscopy

The dynamic behaviour of side-chain polyacrylates, whose polymer backbones were connected to a mesogenic core composed of cyanophenyl benzoate or cyanobiphenylyl benzoate via an alkylene (C11) chain or that containing a siloxane bond as a flexible spacer chain, have been studied by dielectric relaxation spectroscopy (102–106 Hz) at different temperatures in the smectic phase. Analyses using the Fuoss–Kirkwood function, WLF equation and activation parameters on the obtained dielectric loss revealed that the siloxane-spacer apparently enhanced the mobility of a mesogenic group owing to a decrease of Tg, and that the introduction of as small a mesogen as possible might be one of the most important factors in responding rapidly to changes of an external field.

A new synthetic approach to side-chain liquid crystalline polyacrylates containing siloxane spacers and their thermal behaviours

The syntheses of novel side-chain liquid crystalline polyacrylates (LCPAs) containing siloxane spacers were carried out according to a convenient route by preparing alcohol compounds having a mesogenic group linked with a siloxane bond as key intermediates of the monomers. A relatively large mesogenic group was necessary to exhibit a mesophase in this system. The introduction of a siloxane spacer lowered the temperature range of mesophase of LCPAs resulting from the decrease of T, as compared with the corresponding LCPAs containing an alkylene spacer. It was also revealed that the chemical structure of spacer component played an important role for the assembly structure in the mesophase. g

Temperature Dependence of the Surface Behaviour of a Side-Chain Liquid Crystalline Polymer Probed by Contact Angle Measurements

In this study the temperature dependence of the surface properties of a side-chain liquid crystalline polyacrylate is investigated through contact angle measurements. In particular, the effect of the glass transition (Tv=46.7°C) on the surface tension of the liquid crystalline polymer is analysed. The total surface tension as well as its dispersive and polar components are determined as a function of temperature, between 24°C and 73°C, from contact angle values of water, diiodomethane and glycerol. For water and diiodomethane the proximity of the glass transition temperature induces an abrupt change in the temperature dependence of the contact angle; the same is not observed for glycerol. The calculations were performed using the most common techniques: the Neumann equation of state for interfacial tensions, the Owens-Wendt approach (also known as the geometric mean approach) and Lifshitz-van der Waals donor-acceptor approach (also known as the acid-base approach). The results obtained for the total surface tension of the polymer surface were compared and discussed. The temperature dependence of the surface tension exibits the behaviour characteristic of a second-order transition: the surface tension is continuous in the vicinity of the glass transition temperature but its first derivative is discontinuous. Furthermore, the decomposition of the surface tension into its components shows that the polar component, γp s, has a small contribution throughout the entire temperature range, while the dispersive component, γd s, largely predominates. The temperature dependence of both components γd s and γp s is clearly different in the two phases (glassy and liquid crystalline), despite the fact that the total surface tension does not present any appreciable discontinuity across the glass transition.

Dielectric relaxation of liquid crystalline polymers used in non-linear optical materials

Dielectric permittivity and loss have been measured over the frequency range 0.1–1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of smectic monomers. The polyacrylates were based on mixtures of chiral monofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and nonchiral bifunctional monomers, either with an ester group in the mesogen or with an ester group in the mesogen and a lateral nitro group, creating networks with different crosslink densities. Poled copolymers based on these monomers exhibit second-order nonlinear optical properties. Polymers based only on monomers with the chiral and nitro groups exhibited a pronounced γ process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temperature subglass process (β) associated with reorientation of the mesogen. The polymers with the bifunctional monomer without the nitro group exhibited clearly both γ and β processes. The dielectric data indicate that the ester groups in the bifunctional units undergo conventional reorientation according to the β mechanism and that the β process is suppressed in the pendant chains with the lateral nitro groups. The β process involves a coordinated torsion about two bonds in the mesogen and the swept-out volume involved in the motion becomes extensive when a lateral nitro group is present. © 1997 Elsevier Science Ltd.

Influence of chirality on phase behaviour of side chain liquid crystalline polymers

Two kinds of chiral side chain liquid crystalline polyacrylates were synthesized and characterized by DSC, POM and x-ray diffraction. The results indicated that chirality of side chain end group and spacer length strongly influence the phase behaviour of the polymers. The polymer P-I with one asymmetric carbon atom of end group exhibits enantitropic Smectic SA and SC phases. The polymer P-III having two asymmetric carbon atoms displays enantitropic Smectic SA and SB phases. Formation of liquid crystalline polymorphism of polymers depends on spacer length of side chains.