Thermotropic Polyesters Research Articles

Direct observation of director fields of disclinations in the nematic mesophase of a main chain thermotropic aromatic polyester by surface microcrack decoration

For a main chain thermotropic aromatic polyester with a flexible spacer, Cr 194 N 245 I (°C), the director fields around disclinations in the nematic mesophase can be decorated by both the solidification-induced band texture and surface microcracks. Director fields of various types of disclinations, including inversion walls, in the nematic mesophase of this semi-rigid polyester have been observed directly by polarizing optical microscopy. It was found that when the polymer was pre-sheared in the nematic state and then quenched to room temperature, a shear-induced band texture was observed, which relaxed slowly during annealing at 200°C, and then on quenching the solidification-induced band texture and surface microcracks appeared, displaying the pattern of the disclination fields. Pre-shearing is a necessary condition for the appearance of microcracks. On annealing, disclinations of various types were generated, quite often connected by inversion walls in the direction of pre-shearing. In some areas of the specimen where the shear-induced bands had not been completely relaxed during annealing, the shear-induced band texture was shown by the birefringence and surface microcracks to have a supermolecular structure of sinusoidal chain fibrils.

On the molecular orientation and viscoelastic behaviour of liquid crystalline polymers: the influence of macromolecular architecture

The influence of macromolecular architecture on the flow‐induced orientation of main‐chain thermotropic liquid crystalline polymers (LCPs) is investigated using in situ wide‐angle X‐ray scattering. In order to get more insight into the interrelationship between microscopic and rheological behaviour, the viscoelastic properties of the nematic melts were also studied. The LCPs studied are wholly aromatics , composed only of mesogenic units, and semiflexibles , which consist of mesogenic units separated by alkyl spacers. Rheo‐X‐ray scattering evidenced the detrimental influence of the flexible spacers on the orientation process. The wholly aromatic LCPs readily orient along the flow direction, even under modest shear flows ( γ = 0.1 s‐1). The semiflexible LCPs, however, display lower levels of orientation than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterization shows that, like conventional flexible‐chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measurements, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behaviour is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associated with a molecular network formed by the less‐than‐rigid molecular chains. Furthermore, the steady shear viscosity is always smaller than the dynamic viscosity, i.e. the Cox‐Merz rule does not hold. This is due to the fact that steady shear induces molecular orientation, whereas oscillatory shear does not. The relaxation of orientation after steady shear showed that the molecular orientation relaxes after hundreds of seconds. The shear stress, however, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.

Thermotropic Polyesters with Flexible Spacers Bearing Ether Bonds in Asymmetric Position

The synthesis of several polyesters from p,p‘-bibenzoic acid and diols bearing ether bonds in asymmetric positions is described. The polyesters develop smectic mesophases, the type of which depends on the chemical structure of the spacer. The ether groups and the asymmetry of the spacers stabilize the liquid crystalline order in these polymers. Glass transition and isotropization temperatures, enthalpies, and entropies of the thermotropic polyesters are reported. Comparison of the melting entropies with the conformational entropies of the chains suggests a great disorder in the mesophases of thermotropic polyesters. For some polyesters, the stretched polymer chains do not follow the direction of the fiber axis, an unusual phenomenon that has already been described for some polybibenzoates with aliphatic separators. The influence of the strain rate on the anomalous flow is discussed.

Phase Behavior of Crystal Polymorphs of Thermotropic Poly(hexamethylene 4,4‘-biphenyldicarboxylate) under Hydrostatic Pressure

Structural phase behavior of a thermotropic polyester, poly(hexamethylene 4,4‘-biphenyldicarboxylate), abbreviated as BB-6, was studied under hydrostatic pressures up to 200 MPa by wide-angle X-ray diffraction apparatus equipped with a high-pressure vessel. The Cα form of BB-6, cooled slowly from the melt under atmospheric pressure, is the most stable in the low-pressure region below 70 MPa, and this sample exhibits the reversible phase transition of Cα ↔ SA ↔ I. An application of high pressure above about 100 MPa induces another crystal polymorph (Cδ) at high temperature: the Cα → Cδ → SA → I transition process is observed on heating under high pressure. Since the Cδ ↔ SA ↔ I transition occurs reversibly at high pressure, the Cδ form can be regarded as the stable form in the high-pressure region. But this form becomes metastable in the low-pressure region because the Cδ → Cα → SA → I transition is observed at pressures below 50 MPa. The shrinkage of the fiber repeat of the Cδ form by about 2 Å would suggest the conformational change from all-trans to an introduction of gauche (g+ or g−) conformation in the aliphatic sequence. Since the X-ray fiber photograph of the Cδ form shows intense fiber reflections along the direction tilted by 25° from the equatorial line, the molecular chains in the unit cell would be arranged in a herringbone pattern. The phase stability between the crystal modifications of BB-6 was analyzed as s function of temperature and pressure.

Rheological behavior and prediction for blending conditions of a thermotropic liquid crystalline polyester with nylon

The dynamic rheological behavior of a liquid crystalline polymer (LCP), Vectra™ A, and nylons was investigated. The viscosities of nylon 66 and nylon 6 decrease slowly with an increase in temperature, while the viscosity of Vectra A drops dramatically at 280 °C, but remains slightly changed above 300 °C. At constant frequency and above 300 °C, the mean value of the activation energy of Vectra A is about 87.0 kJ/mole, but jumps to a much higher value of about 407.0 kJ/mole if the melt temperature is below 300 °C. The activation energy of Vectra A above 300 °C is lower than nylon 66, which shows that the viscosity of nylon 66 has a greater temperature dependence than Vectra A. The viscosity ratio of Vectra A to Nylon 66 is less than 1 at temperatures higher than 290 °C, which indicates that Vectra A can form the fibrils in the nylon 66 matrix and reinforce nylon 66 when blending them above this temperature. Experimental data confirm our prediction. Copyright © 2000 John Wiley & Sons, Ltd.

Thermotropic copolyesters containing spirobicromane moieties: synthesis and primary characterizations

Spirobicromane (SPI) are supposed to have a drastic influence on the mesomorphism due to their rigid, bent geometry and bulkiness; therefore, we attempted to build the SPI unit into conventional thermotropic polyester and examine its effect on mesomorphism. In this study, thermotropic copolyester series I and II, with their SPI moieties aggregated or evenly distributed in the polymer chain, can be prepared according to single- and two-step polycondensation reactions of different amounts of SPI, 1,7-heptanediol (HD), and terephthaloyl bis(4-oxybenzoyl chloride) (TOBC), respectively. Thermal transition temperatures (Tm and Ti) for both copolyester I and II series decreased with the increasing SPI moieties incorporated and no liquid crystalline properties were observed for copolyesters with SPI content more than 20mol%. With more aggregated SPI moieties in the chain, copolyester I series possess higher isotropization temperature and better smectic order as compared to copolyesters II of the same composition, in which the detrimental SPI moieties are assumed to be evenly distributed to efficiently prevent an ordered chain packing in the mesomorphic state.

Rheological and thermal properties of blends of modified poly(ethylene terephthalate) withp-acetoxybenzoic acid and poly(butylene terephthalate)

Blending of thermotropic liquid crystalline polyesters (LCPs) with conventional polymers could result in materials that can be used as an alternative for short fiber-reinforced thermoplastic composites, because of their low melt viscosity as well as their inherent high stiffness and strength, high use temperature, and excellent chemical resistance and low coefficient of expansion. In most of the blends was used LCP of 40 mol % of poly(ethylene terephthalate) (PET) and 60 mol % of p-acetoxybenzoic acid (PABA). In this work, blends of several copolyesters having various PABA compositions from 10 to 70 mol % and poly(butylene terephthalate) (PBT) were prepared and their rheological and thermal properties were investigated. For convenience, the copolyesters were designated as PETA-x, where x is the mol % of PABA. It was found that PET-60 and PET-70 copolyesters decreased the melt viscosity of PBT in the blends and those PBT/PETA-60 and PBT/PETA-70 blends showed different melt viscosity behaviors with the change in shear rate, while blends of PBT and PET-x having less than 50 mol % of PABA exhibited totally different rheological behaviors. The blends of PBT with PETA-50, PETA-60, and PETA-70 showed the morphology of multiple layers of fibers.

Effect on TSDC relaxation spectra of substitutions in the mesogenic unit and in the flexible spacer of poly(tetramethylene terephtaloyl-bis-4-oxybenzoate)

A family of four thermotropic polyesters starting with the poly(tetramethylene terephthaloyl bis-4-oxybenzoate), substituting either asymmetrically in the tetramethylene spacer or incorporating polar substituents onto the aromatic units of the mesogen, is studied by the Thermally Stimulated Depolarization Currents (TSDC) technique. The results are compared to the Dynamic-Mechanical Analysis at frequencies ranging from 0.1 to 1 Hz from 123 to 423 K. The unsubstituted polymer has a complex low-temperature TSDC spectrum corresponding to local reorientation modes due to the motion of the COO groups with different locations along the main chain. By comparing the effect of the Cl and CH3 substituents on the relative intensity and on the mean energies of the distribution of relaxation times determined by the DSA procedure, the lowest temperature mode is attributed to the COO peripheral groups and the modes located at higher temperatures, to the internal COO groups, which may be accompanied by adjacent segments. The mechanic and dielectric α-transitions are also very sensitive to the substituents, the addition of CH3 in the 1,4 flexible spacer shifting the glass transition temperature above room temperature. The existence of the three-dimensional order that is present in the materials with a linear spacer significantly broadens the α-relaxation. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3038–3049, 1999

Computer simulation of structure and mechanical properties of polymer liquid crystals

In this contribution, we report on a study of the self-organization and mechanical properties of polymer liquid crystals (PLCs). Both processes are computer simulated by the method of molecular dynamics. We investigated two real longitudinal PLCs (thermotropic polyesters) with macromolecules that consist of rigid and flexible parts arranged in a regular way. One rigid and one flexible part form a monomer containing 45 or 47 atoms. The total number of atoms in the macromolecules studied was 4700 (100 monomers) and 5400 (75 monomers). The self-organization was similar to that obtained earlier for a beads-on-a-string model, so compression calculations were done using this simpler model containing 1200 beads (100 monomers). Macroscopic characteristics such as the stress-strain relation, temperature change during deformation, as well as microscopic changes in structure, were investigated.

Hydrodynamic and Conformational Properties of Aromatic Polyesters Containing Flexible Polyoxyethylene Fragments

Conformational properties of molecules of the thermotropic aromatic polyesters containing mesogenic and flexible very long oxyethylene fragments are discussed on the basis of molecular hydrodynamic investigations. Changes in Conformational properties and equilibrium flexibility of molecules, caused by changes in the way of biphenylene fragments insertion into the chain are analyzed. Intramolecular orientational mobility of mesogenic fragments is characterized by a dimentionless parameter δ/A.

Elastic Constants and Order Parameters of Comblike Copolymers

Orientational elastic deformations of nematic phase of comblike copolymers in a magnetic field were studied. The temperature dependences of birefringence Δn, order parameter S and splay elastic constant K1 were determined. It is shown that with increasing of the molar fraction of nonmesogenic component from 0 to 0.4, the parameter S depends on copolymer composition x only near the phase transition point nematic — isotropic melt, at which S decreases with increasing x. For a copolymer with x = 0.51 at all temperatures, a decrease in the order parameter is detected. In the investigated temperature range the values of constant K1 do not depend on copolymer composition. The results are compared with characteristics of thermotropic linear aromatic polyesters.

Thermotropic liquid-crystalline polymers having five-membered heterocycles as mesogens, 3. New semi-rigid thermotropic liquid-crystalline polyesters based on a twin biphenyl analogue of 1,3,4-thiadiazole

New semi-rigid thermotropic liquid crystalline (LC) polyesters containing a twin biphenyl analogue of 1,3,4-thiadiazole in the main chain were prepared by melt polycondensation of the bismethyl ester derivative of twin 2-phenyl-1,3,4-thiadiazole having a decamethylene segment in the central part with three aliphatic diols. The polymer with an octamethylene segment forms a monotropic nematic phase and those with decamethylene and dodecamethylene segments form enantiotropic smectic phases, although their LC states are unstable. The melting and isotropization temperatures decrease with increasing length of alkylene spacers.

Transient Viscosity and Molecular Order of a Thermotropic Polyester LCP in Uniaxial Elongational Flow

The transient elongational viscosity and molecular orientation are measured for a TLCP under simple uniaxial elongational flow. Substantial strain hardening is observed for Hencky strains greater than one, with both the molecular order and transient elongational viscosity becoming independent of strain rate. The transient elongational viscosity is observed to follow a master curve in this regime, scaling as strain squared. X-ray scattering measurements of the orientational order parameter show the strong orienting ability of elongational flows. The quantitative results are compared to those of simple shear flow, as well as to molecular theories, showing important deviations from the behavior expected for model, rigid rod nematics.

New Wholly-Aromatic Thermotropic Polyesters with Controlled Flexibility

ABSTRACTIn this work, we present the synthesis and characterization of a new series of wholly-aromatic copolyesters derived from the condensation of various weight fractions of 4,4'-(o- phenylenedioxy)dibenzoic acid (OPDB) and substituted terephthalic acid (BTA) with 2-phenylhydroquinone (PHQ). The Higashi method, involving tosyl chloride and pyridine as solvent, was employed to yield polymer with significant molecular weight. These polymers are intended to enable accessible clearing transition and to control the balance of stiffness and toughness in melt-spun fibers systematically. We report the synthetic details along with characterization of quiescent phase behavior and morphology.

Non-isothermal crystallization kinetics of thermotropic polyesters with flexible spacers in the main chain

The non-isothermal crystallization kinetics of two thermotropic polyesters with flexible spacers in the main chain were studied. The influence of the spacer type on the kinetics has been considered. The Ozawa theory for non-isothermal crystallization was used to analyse the differential scanning calorimetry data. The polyester with an oxyethylenic group as the flexible spacer followed the Ozawa equation and allowed the calculation of the Avrami exponent under non-isothermal conditions, but in the case of the polyester with methylenic groups as the flexible spacer the Ozawa theory is not valid.

Fluorescence study on intermolecular complex formation between mesogenic 4,4'-biphenyldicarboxylate moieties of thermotropic liquid-crystalline polyesters with 5 and 6 methylene units

Fluorescence study on intermolecular complex formation between mesogenic 4,4'-biphenyldicarboxylate moieties of thermotropic liquid-crystalline polyesters with 5 and 6 methylene units

Fluorescence study on intermolecular complex formation between mesogenic 4,4′-biphenyldicarboxylate moieties of thermotropic liquid-crystalline polyesters with 5 and 6 methylene units

Fluorescence study on intermolecular complex formation between mesogenic 4,4′-biphenyldicarboxylate moieties of thermotropic liquid-crystalline polyesters with 5 and 6 methylene units

Pressure-Induced Crystal Polymorph of Thermotropic Polyesters Composed of p,p‘-Bibenzoic Acid and Penta- and Hexamethylene Diols

Pressure-Induced Crystal Polymorph of Thermotropic Polyesters Composed of <i>p,p</i>‘-Bibenzoic Acid and Penta- and Hexamethylene Diols

The structure of wholly aromatic polyesters with bulky side chains: Poly(phenylene phenyl-terephthalate)

The thermotropic polyester prepared from phenyl-terephthalic acid and hydro-quinone is highly crystalline, despite the probable random 2-and 3-disposition of the phenyl substituents. The x-ray pattern of melt-spun fibers contains 18 Bragg reflections that are indexed by a monoclinic unit cell with dimensions a = 28.0 Å. b = 4.89 Å, c = 12.48 Å (fiber axis), and γ = 114.8°, containing monomer units of four chains. In the ac-plane, the chains are arranged in pairs with the phenyl side chains interdigitated; successive pairs of chains are staggered by about c/2. We have used molecular mechanics modeling to simulate arrays of chains with random 2-and 3-disposition of the side chains on the terephthalic acid units and have compared the results with those for a similar structure in which all the substituents were at the 2-position. The refined model for random substitution is distorted, but the average separations of the monomer units are within experimental error of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the random substitution model is only about 1 kcal/mol of monomer higher than that for the model with all 2-substitution, indicating the random substitution is not a problem for the formation of an ordered structure.

Estudo cinético da cristalização dinâmica de um poliéster líquido-cristalino termotrópico com espaçadores flexíveis na cadeia principal

Foi realizado o estudo cinético da cristalização dinâmica de um poliéster líquido-cristalino termotrópico com espaçadores flexíveis na cadeia principal. O objetivo deste estudo foi verificar o mecanismo da cinética de cristalização do poliéster termotrópico semi-flexível através de um método não-isotérmico. O método empregado (método de Ozawa) permitiu descrever a cinética de cristalização dinâmica do poliéster e os valores de n (expoente de Avrami) encontrados correspondem a um crescimento de cristais em forma de feixe sem crescimento esferulítico posterior.