Thermotropic Liquid Crystalline Copolyester Research Articles

Rheology of phase separated blends of two thermotropic liquid crystalline copolyesters

AbstractThe melt rheology of phase separated blends of two thermotropic liquid crystalline polymers (LCPs) have been studied. The two components are a random copolyesters consisting of 73 mol% 4‐hydrobenzoic acid (HBA) and 27 mol% 6‐hydroxy‐2‐napthoic acid (Vectra A900 of Hoechst Celanese Corp.) and a poly(ethylene terephalate‐co‐4‐oxybenzoate) containing 60 mol% HBA units (PET/60HBA of Eastman Kodak Corp.). Most striking is the effect of adding 10% PET/60HBA to Vectra A900: The viscosity at 290°C drops by a factor of 4 and the terminal zone of the relaxation time spectrum is shifted to much shorter times. This is an interesting effect that could be used for LCP processing even if its origin is not yet understood. Differential scanning calorimetry measurements support the hypothesis that the blend is phase separated and that no transestification reaction occurs during the experiments.

Phase transition of thermotropic liquid crystalline copolyester film

4-Hydroxybenzoic acid/2-hydroxy-6-naphtoic acid (73:27) copolymer films were prepared by a solution casting method. Some films were considered to be amorphous. Phase transitions of the films during annealing processes were examined using DSC, polarized microscopy and X-ray diffraction methods.

Effect of Thermal History on Viscoelastic Properties of Thermotropic Liquid Crystalline Copolyesters

Abstract Dynamic viscoelastic properties of two samples of co-polyesters, PHB60 and PHB28, were measured over a wide range of temperatures. The copolyesters had been synthesized from polyethylene terephthalate (PET) and p-hydroxybenzoic acid (PHB). PHB60 and PHB28 contain 60 and 28 mol.-% PHB as the comonomer, respectively. PHB60 is well known as a thermotropic liquid crystalline copolyester. The effect of thermal histories on viscoelastic behavior of the copolyesters are discussed in terms of structure and liquid crystallinity of the copolyesters. A rheological transition in viscoelastic properties of the copolyester PHB60 was observed at about 270°C in the fluid state. The viscoelastic properties of the copolyester PHB60 strongly depend on thermal histories below the transition temperature, but not above the temperature. The transition behavior was attributed to a crystal-nematic transition by wide angle X-ray diffraction and differential scanning calorimetry. The attainable crystallinity was fairly low. Thermal history behavior found below the transition temperature is caused by slow crystallization and supercooling of the sample. PHB28 forms no liquid crystal phase, and is thermorheologically simple in the molten state: the sample showed no effect of thermal history in viscoelastic properties.

Molecular Orientation and Modulus of a Thermotropic Liquid Crystalline Copolyester

Abstract The relationship among processing parameters, molecular orientation, and modulus of a thermotropic liquid crystalline copolyester synthesized from poly(ethylene terephthalate) and 60 mol.-% p-hydroxybenzoic acid was systematically investigated so that the dominant processing factors to result in high orientation and high modulus were elucidated. Solid filaments were prepared by shear flow in capillaries and elongational flow by spinning from the liquid crystalline phase of the copolyester at various processing conditions. Superstructure, especially molecular orientation, of the filaments was analyzed by wide angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and polarizing optical microscope (POM). Also, tensile modulus of the filaments was determined. Although filaments prepared by means of shear flow in capillaries had the Hermans orientation function of about 0.3, tensile modulus and morphology by SEM and POM did not exhibit the evidence of high orientation of the filaments even at high ratios of length over diameter and high shear rates. In the elongational flow by spinning, the dominant processing parameters for molecular orientation and increasing of tensile modulus are a spin-draw ratio and an initial diameter of the filament. The small diameter results in high orientation and high modulus. Enhancement of modulus was independent of the processing conditions related to a relaxation process of molecular orientation, such as spinning temperature, strain rate, and cooling. These results imply that there exists a size effect for the orientation process.

Effect of flow history on rheological properties of a thermotropic liquid crystalline copolyester (abstract)

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Structural, dielectric, and rheological characterization of thermotropic liquid crystalline copolyesters based on 4-hydroxybenzoic acid, 4,4'-dihydroxybiphenyl, terephthalic acid, and isophthalic acid

Xydar SRT-300 (Dartco) comprising 4-hydroxybenzoic acid (HBA), 4,4'-dihydroxybiphenyl (BP), and terephthalic acid (TA) in a 68/16/16 molar ratio and POB S6635 (Sumitomo) comprising HBA, BP, TA, and isophthalic acid (IA) in a 62/19/15/4 ratio are investigated. X-ray diffraction measurements indicate the predominance of a high degree of three-dimensional order in the annealed Xydar polymer, with a substantial reduction in the degree of order upon the introduction of a small fraction of kinked comonomer units in the POB polymer. The dielectric relaxation results show that the rotational motions of HBA dipoles are as active in the highly (three-dimensionally) ordered annealed Xydar samples as they are in less ordered quenched material

Thermotropic liquid crystalline copolyesters. II. Synthesis and thermal characterization of copolyesters of terephthalic acid, methyl hydroquinone and 1,4‐butane diol

AbstractThe synthesis and thermodynamic evaluations of a series of random copolyesters based on terephthalic acid, methyl hydroquinone and 1,4‐butane diol are presented. The copolyester composition was varied by altering the relative concentrations of methyl hydroquinone and 1,4‐butane diol. The copolyesters display thermotropic transitions. The enthalpy changes during mesophasic transition display a minimum corresponding to equimolar composition. The copolyester with 75 mol% of spacer diol exhibited the lowest nematic transition temperature (190‐6°C). At this composition the nematic mesophase was most stable (δTN = 115‐6°C). ΔHN‐1 and ΔSN‐1 decrease with increasing rigid diol content. The isotropization temperatures were almost comparable since the largest mesogenic residues are similar in aspect ratio. These residues are required to melt prior to isotropization.

Polymer analysis: Molecular structure and properties of industrial polymers

Low density polyethylenes made by the known high pressure processes show significantly different molecular structures. The physical and technological properties are closely related to molecular structure and morphology. For example, the adhesive strength of a laminate consisting of an ozone treated low density polyethylene film and an aluminum foil depends strongly on the synthesis conditions. The molecular structures and dilute solution properties of many fully aromatic thermotropic liquid crystalline copolyesters can now be determined using the new solvent 3,5-bis(trifluoromethyl)phenol. A typical random copolyester was fractionated by precipitation from solution, and the fractions were studied in detail by viscometry, integrated and dynamic light scattering, and size exclusion chromatography. From the structure data thus obtained the molecular mass distribution and the persistence length were calculated. Polymer blends, block copolymers, and graft copolymers can be characterized by fractionation procedures using demixing solvents in an ultracentrifuge and subsequent analysis of the fractions.

Orientation kinetics of thermotropic main-chain liquid-crystalline polymers in a magnetic field

Abstract This work presents a quantitative analysis of the kinetics of orientation of main-chain thermotropic liquid-crystalline copolyesters in a magnetic field. Preferred chain orientation has been induced in samples with weight-average molecular weight up to 14 400. The kinetics of the orientation process have been explored as a function of the time in the field, sample temperature and field strength (up to 1.12 T), and in particular the influence of molecular weight has also been determined. It has proved possible to achieve a degree of preferred orientation up to 〈 P 2 〉 = 0.90. Curve fitting the data leads to the determination of maximum 〈 P 2 〉 values and characteristic time constants.

An optical study of phase transitions of poly (ethylene terephthalate‐co‐p‐hydroxybenzoic acid) liquid crystal

AbstractThe copolyester containing 40 mol % ethylene terephthalate (PET) and 60 mol % p‐hydroxybenzoate (HB) units has been reported by several investigators to be biphasic in the solid and the liquid states. The reported thermal transitions in the two phases, however, are in part contradictory, perhaps partly due to different polymerization conditions. The present work is a study of the transitions in each of the two phases of this copolyester by polarized light microscopy and by light transmission measurements. By light transmission measurements, the two phases were found to have two different glass transition temperatures for the onset of segmental motion, consistent with two assignable temperatures (T.). Cold crystallization and melting in each of the two different phases was also detected. The results help clarify the nature of transitions and agree with the results of dynamic mechanical analysis on the same thermotropic liquid crystalline copolyester.

Preparation and properties of a series of thermotropic liquid crystalline copolyesters

AbstractA series of thermotropic liquid crystalline copolyesters were prepared by melt polymerization using p‐hydroxybenzoic acid (HBA) and terephthalic acid (TPA) as mesogenic monomers, poly(ethylene terephthalate) (PET) to give a flexible linkage, and 1,4‐dihydroxynaphthalene (1,4‐DHN) as a dissymmetrical monomer. The composition of these monomers was varied as a means of manipulating processing temperature, morphology, and properties. Copolyesters with inherent viscosities near 0.7 that were melt processible in the temperature range of 200–300°C were obtained. The glass transition, thermal degradation, and melting temperatures, and crystal size all increased with increasing mesogenic unit content. The heterogeneous structure consisting of a PET‐rich phase and a phase rich in mesogenic units was observed by SEM after chemical etching and by X‐ray diffraction analysis. Highly anisotropic melts were observed indicating the presence of a nematic mesophase. The thermotropic LC copolyesters exhibit unusually well developed and highly oriented fibrillar structure at the neck area when injection molded. The orientation developed during processing is very similar to those of short‐fiber‐filled polymer composites. The LC copolyesters commonly have three relaxations in the temperature range of −100–200°C. By dynamic mechanical thermal analysis at 10 Hz, the relaxation temperatures were found to increase with increasing content of mesogenic units and of 1,4‐DHN. For the temperature range of 25–120°C, the storage modulus of one of the members of this series were compared with Vectra (Celanese) and with Eastman PHB80, which have almost the same mesogenic contents. This new copolyester maintained its modulus until 80°C, while the modulus of Vectra decreased in two steps; a small depression at 40°C and then a large reduction at 85°C. PHB80 shows a large single transition at 50°C.

Naphthalene-ring Containing Thermotropic Copolyester and its Blends with PET

Abstract A series of thermotropic liquid crystalline copolyesters based on p-acetoxybenzoic acid, 2,7-diacetoxynaphthalene, terephthalic acid and polyethylene 2,6-naphthalate were prepared via melt transesterification. Their thermotropic liquid crystalline behavior was studied by visual observation on the hot stage of a polarized light microscope. The dependence of the crystallizability and melting temperature of the copolyesters on the 2,7-diace-toxynaphthalene content was examined by x-ray diffraction and differential scanning calorimetry. A blend sample was prepared by physically mixing 40% thermotropic copolyester and 60% polyethylene terephthalate and the phase-separated charackter of the copolyester and PET in the blend was proved by TOT and dynamic mechanical measurements. The SEM micrographs of the as-spun fibers of the blend show separation of the two phases. The highly oriented macromolecules and the fibril alignment of the mesomorphic copolyester in the blend has also been revealed after the dissolution of the PET phase in a solvent.

Effects of annealing on the structure formation in a thermotropic liquid crystalline copolyester

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffects of annealing on the structure formation in a thermotropic liquid crystalline copolyesterAkira Kaito, M. Kyotani, and K. NakayamaCite this: Macromolecules 1990, 23, 4, 1035–1040Publication Date (Print):February 1, 1990Publication History Published online1 May 2002Published inissue 1 February 1990https://doi.org/10.1021/ma00206a021RIGHTS & PERMISSIONSArticle Views253Altmetric-Citations55LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Shear and time‐dependent rheology of a fully nematic thermotropic liquid crystalline copolymer

Rheological measurements are reported for a fully nematic thermotropic liquid crystalline copolyester composed of 80% p‐hydroxybenzoic acid/20% poly(ethylene terephthalate). The polymer displays shear thinning behavior with a constant power‐law index over eight decades of shear rate; no shear‐independent plateau (region II) is observed. Dynamic time sweeps indicate a high sensitivity of the rheological parameters to thermal history, apparently resulting from crystalline annealing. These annealing effects could be erased by appropriate thermal cycling. Torsional measurements (cone‐and‐plate) were limited at high shear rates by the occurrence of edge fracture. This instability was typically accompanied by an unusual sample texture composed of concentric rings, which was particularly distinct when offgassing had occurred in the polymer melt.

Comparison of the properties of thermotropic liquid crystalline copolyesters containing different disymmetrical units

AbstractThe effect of dissymetrical units on the properties of thermotropic liquid crystalline copolyesters based on hydroxybenzoic acid (HBA), terephthalate) acid (TPA), and poly(ethylene terephthalate) (PET) units was examined. The dissymmetrical units employed included 2,7‐dihydroxynaphthalene (2,7‐DHN),1,4‐dihydroxynaphthalene (1,4‐DHN), and bromo‐hydroquinone (Br‐HQ). All of these polymers exhibited turbid melts. The flexible PET units lower the melting and/or stick temperature, and between 10 to 20 mol % is a critical level to achieve values in the range of 250°C or lower. The copolyesters based on 2,7‐DHN have the highest glass transition temperatures and maintain stiffness to the highest temperatures. The properties of each series of copolyesters are discussed in terms of the efficiency of molecular packing. The effects of chemical and physical inhomogeneity are emphasized.

Effect of Flow History on Rheological Properties of a Thermotropic Liquid Crystalline Copolyester

Effect of Flow History on Rheological Properties of a Thermotropic Liquid Crystalline Copolyester

Morphological studies of blends containing liquid crystalline polymers with poly(ethylene terephthalate)

AbstractThe investigation involved the structure–property behavior of extruded cast films prepared from blends of thermotropic liquid crystalline copolyesters with poly(ethylene terephthalate) (PET). Data were obtained which showed not only the temperature dependence of the moduli and stress–strain behavior but also the orientation effects that must be prevalent in order to explain the differences between the moduli measured parallel and perpendicular to the extrusion direction. Only at high liquid crystal polymer (LCP) composition is the modulus particularly increased. The modulus enhancement with lower LCP content and utilization of process variables are discussed with respect to the induced morphological textures and nature of the process equipment. Specifically, the process variable extruder gear pump speed did not enhance Young's modulus at the same LCP content as extensively as did the process variable of extruder screw speed.

Blends of a chlorinated poly(vinyl chloride) compound and a thermotropic liquid crystalline copolyester: Some rheological behavior and spiral mold flow

AbstractRheological and spiral mold flow measurements were made on blends of a chlorinated poly (vinyl chloride) (CPVC) compound and a thermotropic liquid crystalline copolyester of p‐hydroxybenzoic acid/poly (ethylene terephthalate) (60/40), hereafter referred to as LCC, at 210°C. Several interesting flow phenomena have been observed. While the shape of the flow curve (i.e., G′ vs. ω; G″ vs. ω) of LCC is solidlike, those of the blends are pseudoplastic. The dynamic viscosity of the blends increases as the concentration of LCC increases. However, the shear viscosity of the blends is reduced with LCC and may be described using the relationship of additivity of fluidity. Although the melt elasticity of the blends is increased with the concentration of LCC, the extrudate swell of the blends after extrusion from a capillary is decreased. This phenomenon, however, is complicated. A Theological analysis based on an idealized runner system is used to describe the spiral flow length as a function of the Theological properties of the molten polymers and also the operating conditions. The relative improvement of the spiral flow length of the CPVC compound due to blending with LCC could be correlated with the power‐law fluid model at high shear rates.

X-ray Analysis of Chiral Thermotropic Liquid Crystal Copolyesters Based on Linear and Nonlinear Aromatic Ester Triads

Abstract A series of chiral thermotropic liquid crystalline copolyesters, based on linear mesogenic triads and nonlinear nonmesogenic triads consisting of two p-oxybenzoyl units coupled by a terephthaloyl or phthaloyl residue respectively, is studied by X-ray diffraction. Complementary data of differential calorimetry and polarizing microscopy are presented. The degree of crystallinity is found to depend significantly on the relative content of the nonlinear counit and to vary with thermal history. The occurrence of cholesteric phases is established, whose stability and breadth can be modulated on the basis of appropriate chemical compositions. The structural data of the investigated copolymer samples are compared with those previously obtained for an analogous series of copolymer samples based on the same linear aromatic triad and an isomeric triad containing a central isophthaloyl residue. Keywords: liquid crystalline polymers, chiral thermotropic polymers, cholesteric copolyesters, 4,4'-(terephthaloyld...

Drawing behaviour of polymer blends consisting of poly(ethylene terephthalate) and liquid crystalline copolyester

The drawing behaviour of rods (2 mm in diameter) of a polymer blend based on poly(ethylene terephthalate) (PET) and a thermotropic liquid crystalline copolyester, poly(ethylene terephthalate- co-p-oxybenzoate) (PET-POB), was investigated under conventional heating and also under microwave heating. With increasing PET-POB fraction, the modulus increased, while the drawing tension and draw ratio decreased. The microwave heating effect was found to be minimal. Sonic modulus, X-ray and birefringence measurements showed that orientation of PET-POB was induced as well as that of PET during drawing. The samples were also assessed by dynamic mechanical analysis, differential scanning calorimetry and scanning electron microscopy.