Liquid Crystalline Polyesters Research Articles

Banded texture in sheared a semiflexible thermotropic liquid-crystalline polyester and its graft copolymers with polyolefins

The formation of banded texture both in a semiflexible thermotropic liquid-crystalline polyester (SBH 112, synthesized from sebacic acid, 4,4′-dihydroxybiphenyl and 4-hydroxybenzoic acid, from Eniricerche, Milan) and in a blend of SBH with graft copolymers, consisting of a polyolefin (polyethylene or polypropylene) backbone and SBH grafts (COPPE, COPPP) has been studied by polarization microscopy and by scanning electron microscopy. The influence of the preparation conditions upon the perfection, characteristics and thermal stability of the banded texture has been investigated. The results show that the banded texture formed in SBH sheared films possesses greater perfection, smaller band width and larger angle of deviation, and higher thermal stability than the banded texture formed in COPPE and COPPP prepared under similar conditions. It has been assumed that the chain rigidity of the macromolecules of the system is mainly responsible for the formation and perfection of the banded texture.

Synthesis and structure of wholly aromatic liquid–crystalline polyesters containing meta‐ and ortholinkages

AbstractWholly aromatic main‐chain rigid‐chain polymers containing meta‐ and orthokink linkages were synthesized. The thermal property, liquid crystallinity, and crystalline structure were studied using DSC, polarized light microscopy, and wide‐angle X‐ray scattering. These polymers exhibited liquid crystallinity up to 50 mol % of meta‐ and orthokink linkages. The existence of liquid crystallinity in these polymers may be attributed to the adoption of cis conformation in kink units in these polymers because the energy penalty for doing so can be compensated by the formation of the liquid–crystalline phase. The crystallinity of the polymers was low, and the crystal structure was quite similar to that of the pure polymers (without kink units). This can be explained by that fact that the crystal region mainly consists of the nonkink units, and the kink units disrupt the crystallization of the polymers and form defects. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1242–1248, 2001

Rheological and Morphological Properties of Various Blends Containing Liquid Crystalline Polyesters

This contribution is concerned with the unique rheological properties of blends of liquid crystalline polymers (LCP) dispersed in thermoplastic matrices. The materials under investigation were rigid rod polyesters solution blended with poly(ε-caprolactone), polypropene, polystyrene or poly(methyl methacrylate) as matrix polymers. Oscillatory measurements of these melted blends revealed that LCP contents of only 1 wt.-% cause the dynamic shear moduli to increase at low frequencies by a factor of several magnitudes compared to the neat matrices. The extraordinary effect is primarily governed by a spatial arrangement of the LCP molecules forming a network as shown by microscopy. The different strength of this effect in different matrices is explained by the interaction between matrix and LCP.

Hyperbranched Thermotropic Liquid Crystalline Polyesters Composed of Aromatic Ester Type Mesogens and Polymethylene Spacers

Two pairs of new isomeric hyperbranched polyesters composed of aromatic ester type mesogenic units and penta- or decamethylene spacers were prepared and their liquid crystallinity was studied. Branch points in these polymers are connected to each other through the nonmesogenic spacers. All the polyesters were found to be amorphous, but thermotropic, regardless whether the terminal groups are esters or free carboxylic acid or phenolic groups. Although these polymers form nematic phases, their apparent melt viscosity appears to be much higher than those of conventional nematic liquid crystalline polyesters (LCPs), indicating that linear alignment of the mesogens in these polymers may not be as readily achieved as in the common LCPs that are known to have very low melt viscosities in nematic state. Thermal and LC properties were studied by differential scanning calorimetry and X-ray diffractometry and by the observation of the optical textures on a polarizing microscope.

The Azobenzene Optical Storage Puzzle – Demands on the Polymer Scaffold?

The basic mechanism of optical information storage utilizing the azobenzene photoaddressable moiety will briefly be introduced. A synthetically flexible polyester matrix covalently integrating cyanoazobenzene in regularly spaced side chains is particularly well suited for holographic storage. Notable figures of merits of thin film materials of liquid crystalline polyesters are: response time to laser light in the order of ns, storage capacity of 5000 lines/mm, high permanent (more than eight years) diffraction efficiency in the order of 50% or higher, and erasability. The implications of the nature of the main chain on polyester morphology and on the permanency of the induced anisotropy are discussed. Arguments for the design and methods of preparation of other very different polymer scaffolds supporting the cyanoazobenzene are elucidated. Whereas oligopeptides invariably form amorphous materials, both copolymethacrylates and dendritic or hyperbranched polyesters provide some materials that exhibit liquid crystallinity. However, none of these other scaffolds offer materials that allow long-lasting anisotropy to be laser light induced.

Specific rheology - morphology relationships for some blends containing LCPs

For the blend melts of isotropic polysulfone (PSF) and LC polyester (PES), differing in viscosity more than 10 times, the flow curves with maxima were observed in cone and plate geometry. The low shear rate branch is located near the PSF flow curve, and the high shear rate branch is close to the PES flow curve. At high strains, the formation of the ring-like morphology of the blend sample, accompanied by appearance of maximum on flow curve, was registered. The scaling analysis of the reasons for the ring morphology formation was based on stretching of the large, low-viscous LC droplet, embedded to the high-viscous polymer matrix, in a homogeneous shear field. It was shown that, if the critical Taylor radius is not exceeded, the droplet may form the closed torus. Under strong flows, the PSF melt manifests the “spurt effect”, consisting of a drastic increase of the shear rate when the critical value of the shear stress is reached. The pattern of the blend flow curves with maxima may be explained by a vanishing PSF input to the total shear stress, inherent for blends, while the PES melt continues to be in a liquid state and, consequently, is responsible for the blend viscosity at the high shear rates. The presence of regular heterogeneities in the blend in the form of LC rings may initiate the rupture of the entanglements network of the matrix PSF (close to LC rings) under strong shear flows. The appearance of the low-viscous “cracks” at the critical shear stress will diminish the contribution of the PSF to the blends rheological response.

Crystallization and nematic-isotropic transition activation energies measured using the Kissinger method

The activation energies for crystallization in poly(3-hydroxybutyrate) and for nematic–isotropic transition in liquid–crystalline polyesters have been calculated based on data obtained with a dielectric method and differential scanning calorimetry (DSC), respectively. This study explores the feasibility of using a modified Kissinger method to calculate the activation energies of these transitions. It has been found that the activation energy for crystallization in poly(3-hydroxybutyrate) is 109 kJ/mol. The activation energy for nematic–isotropic transition in the liquid crystals is 487 kJ/mol. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2535–2537, 2001

Change in the Mechanical Properties of Liquid-Crystalline Thermoplastic Polymers at High Temperatures

The change in the modulus of elasticity and tensile strength of initial and heat-treated liquid-crystalline polyester fibres after annealing at a temperature 20-40°C higher than the melting point of the crystallites in the polymers was investigated. It was shown that from 50 to 90% of the initial values of the strength characteristics were preserved. In heating above the melting point of the crystallites, LC polyester fibres can melt into a monolith with preservation of elevated mechanical properties, which permits production of self-reinforced composites. The fibres melt as a result of cooperative molecular movement, and preservation of the strength indexes is ensured by the cross-links formed at high temperatures. Selection of the temperature—time conditions of heat treatment and molding is especially important for production of high-strength self-reinforced composites by melting of LC polyester fibres into a monolith.

Synthesis and characterization of liquid-crystalline polyesters with chiral and achiral twin spacers

A series of novel thermotropic main-chain chiral liquid–crystalline random copolyesters consisting of spacers of two different types—chiral and achiral—was synthesized. Polyesters (BmTa) with tartaric acid as the chiral spacer (Ta), aliphatic diols (with ‘m’ = 2–10 methylene groups) as the achiral spacers, and 4,4′-dihydroxy biphenyl (B) as the mesogen were synthesized via condensation polymerization in solution after duly protecting the 2,3-dihydroxy groups of tartaric acid by acetylation. The copolymers were characterized by Fourier transform infrared spectroscopy, 1H and 13C NMR spectra, gel permeation chromatography, and thermogravimetric analysis. Transition temperatures for phase changes recorded by DSC were corroborated with the textures observed by a hot-stage optical polarizing microscope. The wide-angle X-ray diffraction (WAXD) profiles indicated a SmE phase at room temperature. The lower angle region at 2θ = 0.5–2.45 covered by WAXD indicated a layer of thickness of 161 Å, less than the molecular length for B0Ta. The [αD] values were recorded on a digital polarimeter. The birefringence was lost at higher mesophase temperatures in lower members with m < 5, a behavior found in certain chiral systems, and the higher members with m > 5 showed a lesser number of phase transitions. On cooling, the polyesters produced a texture with the formation of transition bars. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1743–1752, 2001

Molecular Structure in the Crystalline Phase of a Segmented Liquid Crystalline Polyester

Molecular structure refinement of a liquid crystalline segmented polymer using X-ray diffraction methods is reported. The investigated polymer contains the 3-allylbiphenyl unit as nematogenic unit. The space group is P21/a, with cell parameters a = 12.82 Å, b = 8.44 Å, c = 19.72 Å, and β = 153.2°, with two chains per unit cell. The main feature of this molecular packing is the layered organization of chains with a transversal intercalation of the flexible and the rigid units in the layer. Repetition of layers in space leads to an herringbone packing of biphenyl units along [001]. Transversal register between chains is such that the two phenyl rings of biphenyl unit are involved in “edge to face” contact.

Synthesis, Characterization, and Molecular Structure of Cross-Linkable Rigid-Rod Liquid Crystalline Polyesters Containing Side Allyl Groups.

The synthesis and characterization of a new class of rigid-rod liquid crystalline polyesters [R(n)] bearing side groups is reported. R(n) were prepared by interfacial polycondensation of n-alkoxyterephthaloyl chloride and 3,3‘-diallyl-4,4‘-dihydoxybiphenol exploiting the good solubility in chloroform promoted by the side flexible groups. R(n) show a mesophase at room temperature, and crystallize in some cases (n = 4−7) upon annealing. The crystalline phase has been resolved and refined (space group P21, 2 chains per unit cell). The molecular packing is characterized by a layered organization of chains, with pendant groups placed between layers. At high temperature, R(n) show the nematic phase. Polymers can be processed to form fibers, with the exception of R(1) which cross-links very quickly in the molten state. Heating at temperature over 200 °C or annealing in the solid-state induces cross-linking of allyl groups. In the latter case, the macro-orientation of fibrous samples can be quenched. R(5) and R(10) each show a constant tensile modulus of 1 and 0.5 GPa, respectively, in the range of temperature from about 150 to 300 °C.

Thermotropic liquid-crystalline polymers having five-membered heterocycles as mesogens, 6. Synthesis, thermotropic liquid-crystalline and photoluminescent properties of novel semi-rigid polyesters based on a terphenyl analogue of 1,3,4-thiadiazole

Novel semi-rigid thermotropic liquid-crystalline (LC) polyesters containing terphenyl analogue of 1,3,4-thiadiazole in the main chain were synthesized by melt polycondensation of a dioxydiundecanol derivative of 2,5-diphenyl-1,3,4-thiadiazole with four aromatic diesters, and their structures were confirmed from FTIR, 13C NMR spectra and elemental analyses. Differential scanning calorimetry (DSC) measurements and polarizing microscope observations of textures showed that all the polymers form enantiotropic LC (smectic) phases. UV-vis and photoluminescence (PL) spectra indicated that polyesters without nitro groups display maximum absorbances arising from 1,3,4-thiadiazole moiety and blue-emission maxima, the Stokes shifts being 69.5–79 nm in solution and 54–97 nm in the solid state.

Thermotropic liquid-crystalline polymers having five-membered heterocycles as mesogens, 6. Synthesis, thermotropic liquid-crystalline and photoluminescent properties of novel semi-rigid polyesters based on a terphenyl analogue of 1,3,4-thiadiazole

Novel semi-rigid thermotropic liquid-crystalline (LC) polyesters containing terphenyl analogue of 1,3,4-thiadiazole in the main chain were synthesized by melt polycondensation of a dioxydiundecanol derivative of 2,5-diphenyl-1,3,4-thiadiazole with four aromatic diesters, and their structures were confirmed from FTIR, 13C NMR spectra and elemental analyses. Differential scanning calorimetry (DSC) measurements and polarizing microscope observations of textures showed that all the polymers form enantiotropic LC (smectic) phases. UV-vis and photoluminescence (PL) spectra indicated that polyesters without nitro groups display maximum absorbances arising from 1,3,4-thiadiazole moiety and blue-emission maxima, the Stokes shifts being 69.5–79 nm in solution and 54–97 nm in the solid state.

Dynamic Responses of a Thermotropic Main-Chain Liquid Crystalline Polyester during Polymerization under Electric Fields

The dynamic responses of a main-chain liquid crystalline polymer (LCP) during the polycondensation reaction under electric fields have been studied for the first time by a modified thin-film polymerization technique using a polarizing microscope. The evolution of the in situ liquid crystal (LC) texture during polymerization under electric fields is found to be quite different from that without electric fields. Compared to the low-molecular-weight LC systems, the polymerized thermotropic poly(p-oxybenzoate/2,6-oxynaphthoate) (P(OBA/ONA)) system does not show distinct morphological change from the fluctuating Williams domains to the dynamic scattering mode. The dynamic response of the LC phase during the polymerization is strongly dependent on the frequency and strength of the external field. In a low-frequency range, molecular orientation is found to be in the low-voltage range. In a medium-frequency range, electric field induced flow (electrohydrodynamic instability) is observed. In a high-frequency range...

Aromatic Polyesters with Flexible Side Chains. 8. Studies on Long Periodical Structure Observed in Layered Crystalline Phase

Long periodical structure was studied for the crystal of a rigid-rod liquid crystalline BC-6 polyester composed of the 1,4-dihexyl ester of pyromellitic acid and 4,4‘-biphenol with small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM) measurements. The highly oriented crystalline fibers which were prepared by spinning a nematic melt of the sample and succeeding annealing showed well-defined scattering maxima with a spacing in the range of 250−300 Å in the SAXS pattern and the corresponding clear stripes in the TEM photograph. The long spacing is attributed to the crystalline lamellae stacked along the chain axis. This is the first example of long periodical structure in the fully aromatic polymer systems. The origin of the long periodic structure is discussed.

Biphotonic holographic gratings in azobenzene polyesters: Surface relief phenomena and polarization effects

Biphotonic holographic gratings have been recorded in a side-chain azobenzene liquid crystalline polyester using a blue incoherent source and a He–Ne laser. Intensity gratings and the appearance of surface relief have been observed when two linearly polarized beams from a He–Ne laser are made to interfere on a film illuminated with blue light. Polarized holographic gratings are also created with two orthogonally circularly polarized He–Ne beams. All these gratings are stable in darkness but can be erased with blue light.

Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals

A main-chain non-racemic chiral liquid crystalline polymer has been synthesized from (R)-(−)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-nonyloxy}-4-biphenyl carboxylic acid, abbreviated PET(R∗)-9. Based on differential scanning calorimetry, wide angle X-ray diffraction (WAXD) and polarized light microscopy experiments, this polymer undergoes at least three liquid crystalline (LC) transitions in addition to crystallization. The LC transition sequence isI↔3.21kJ/mol185°CTGBA∗↔1.27kJ/mol175°CSA∗↔0.35kJ/mol130°CSC∗↔37°CTgThe transition sequence is reversible since they are LC phases, which are close to thermodynamic equilibrium. Among these phases, the twisted grain boundary smectic A (TGBA∗) phase is, for the first time, found in a main-chain LC polymer. The TGBA∗ is only stable within a temperature region of 10°C between the Smectic A∗ (S∗A) phase and the isotropic melt. Crystallization that takes place in the (S∗A) phase can form both flat-elongated and double-twist helical single lamellar crystals, as observed by transmission electron microscopy (TEM). Analysis of the WAXD fiber patterns of this polymer indicates that the crystal structure of PET(R∗)-9 in the bulk is orthorhombic, identical with that determined using selective area electron diffraction in TEM [Macromolecules 32 (1999) 524; Phys Rev B 60 (1999) 12 675].

Synthesis of Liquid Crystalline Polyesters of Various Types by Acyclic Diene Metathesis Polymerization

A series of liquid crystalline polyesters of different types were prepared by the acyclic diene metathesis polymerization method. These polymers are of main-chain, side-chain, and combined types. Polymerizations were conducted at 25−50 °C either in solution or in bulk using the so-called Grubbs' catalyst, bis(tricyclohexylphosphine)benzylideneruthenium dichloride. The monomers utilized in this investigation are all isolated dienes containing a built-in aromatic type mesogenic unit or an aromatic ester type mesogen bearing the cholesterol pendant through oxydodecyloxy spacer or monomesogenic terephthalate ester unit bearing the cholesterol pendant through a spacer. Bulk polymerization produced higher MW polymers than solution polymerization as expected from the known polymerization mechanism. Liquid crystallinity and phase transition properties of obtained polymers were studied by differential scanning calorimetry, optical microscopy, and X-ray diffraction.

Crosslinking liquid crystalline polyesters with allyl group as lateral substituent: thermal and UV activation

The synthesis of networks from liquid crystalline polyesters bearing allyl groups as lateral substituents is reported. Crosslinking is performed both by thermally activated radical reaction and by UV irradiation of film samples containing the suitable activator. Moreover, fibrous samples have been crosslinked in the solid state by UV irradiation, “quenching” the nematic organisation on a macroscopic scale and resulting in a macro-oriented network. Crosslinking does not hamper molecular mobility. As a consequence, the degree of orientation and the length of crosslinked fibres change continuously and reversibly upon heating and cooling through the isotropisation transition.

Studies on the ternary blends of liquid crystalline polymer and polyesters

Thermotropic liquid crystalline polymer made up of poly(p-hydroxybenzoate) (PHB)-poly(ethylene terephthalate) (PET) 8/2 copolyester, poly(ethylene 2,6-naphthalate) (PEN) and PET were mechanically blended to pursue the liquid crystalline phase of ternary blends. Complex viscosities of blends decreased with increasing temperature and PHB content. DSC thermal analysis indicated that glass transition temperature (Tg) and melting temperature (Tm) of blends increased with increasing PHB content. Both tensile strength and initial modulus increased with raising PHB content and take-up speed of monofilaments. In the WAXS diagram, only PEN crystal reflection at 2Θ=15.5o appeared but PET crystal reflection was not shown in all compositions. The degree of transesterification and randomness of blends increased with blending time but sequential length of both PEN and PET segment decreased.