Liquid Crystalline Copolyester Research Articles

Physical ageing in a thermotropic liquid-crystalline polymer

The effects of physical ageing on the mechanical properties of a liquid-crystalline copolyester (LCP) were determined. Creep measurements in torsion as a function of ageing time were performed on macroscopically unoriented samples in the temperature range of −60 to 180°C. Below the glass transition temperature ( T g ≅ 100°C), the results indicate physical ageing effects analogous to those of flexible polymers. However, the curvature exponent of the compliance as a function of creep time of the LCP is distinctly smaller. Above T g, physical ageing remains present while additional annealing effects arise. The molecular interpretation of the time dependence of mechanical properties in LCPs cannot rely upon free-volume concepts as in flexible polymers, but might be based on new structural models that were recently introduced.

Structure and molecular mobility in liquid crystalline copolyesters

Copolyesters of poly(ethyleneterephthalate) (PET), poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) and poly(p-hydroxybenzoate) (PHB) form liquid crystalline (LC) phases if the fraction of PHB is larger than about 30%. In some of these copolyesters the liquid crystalline state as well as the isotropic state can be frozen in by quenching the material from different temperatures. This makes it possible to compare both states with each other. By means of dynamic mechanical analysis it is demonstrated that the glass transition temperature Tg of the LC-state is about 30°C lower than that of the isotropic state. Neutron scattering measurements show that the radius of gyration of the molecules in the LC-state is larger than for molecules in the isotropic state. The molecular mobility below and above Tg in both the LC and the isotropic state is investigated by means of deuteron NMR.

The morphology and crystallization kinetics of a rigid-rod, fully aromatic, liquid-crystalline copolyester

The kinetics of the isothermal crystallization of a phenylethylhydroquinone, phenylhydroquinone, terephthalic acid nematic liquid crystalline copolyester have been studied by DSC in the temperature interval 319-325°C. The polymer is characterized by being highly crystalline in spite of its copolymer nature. The crystallization process is describable by an Avrami equation. This polymer gives the opportunity of studying a crystallization process for chains in which the crystalline morphology is preformed, due to nematic state ordering, thereby giving a well-defined chain morphology from which to initiate crystallization

Orientational changes induced by axial stress in a highly aligned liquid crystalline copolyester

The non-linear mechanical behaviour exhibited by oriented thermotropic liquid crystalline polymers has been attributed to an increase in molecular orientation caused by the applied tensile stress. In this work we have detected the postulated change in orientation by polarized infra-red spectroscopy. It is shown that there is reasonable quantitative agreement between the infra-red measurement of change in molecular orientation and the change predicted by modelling the change in tensile modulus with stress.

Reversible changes in the solid state of HBA/HNA liquid crystalline copolyesters studied by X-ray diffraction

Real time X-ray diffraction patterns were recorded as a function of heating and cooling cycles using a synchrotron radiation source for copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with monomer ratios 75 25 , 58 42 and 30 70 . The angular positions of the two main interchain X-ray diffraction reflections 110 and 200 were obtained at temperatures below the melting point with a powder diffractometer, and the unit cell dimensions of the copolymers were measured. For the 30 70 and 58 42 compositions the cell dimensions vary linearly and reversibly with temperature. For the 75 25 copolymer the cell dimensions show a clear bend with temperature, which can be associated with a transformation from an orthorhombic to a quasi-hexagonal phase. It is further shown that at higher temperature the dimensions of the D 110 and D 200 coherently diffracting domains increase while the scattering intensity of the diffraction peaks decreases with temperature. These changes are reversible with temperature. Results are discussed in terms of partial melting of smaller crystallites contributing in a reversible manner to the occurrence of a mobile high temperature quasi-hexagonal phase.

Rheological properties of carbon black-filled blends of liquid-crystalline copolyester with thermoplastic polysulfone

Rheological properties of a two-phase polymeric blend containing LCCPE of poly(ethylene terephthalate) and p-hydroxybenzoic acid and thermoplastic polysulfone with varying concentrations of polymeric components and particulate filler have been studied. The theological behavior of such blends at different temperatures is governed by variation of the degree of ordering of LC-CPE macromolecules associated with the phase transition in the CPE at 260°C. Experimental results are discussed on the basis of concepts of compatibility of polymeric components in the melt or, if the system is incompatible, of the degree of interphase interaction between the components, as well as the impact of the filler and of the shear straining conditions on structurization in the system and compatibility. The filler exerts a compatibilizing effect on blend components, while the shear stress encourages the phase separation in the system. An extremal variation of viscosity of the LC-CPE/carbon black, silica and talk blends with the filler concentration on both at the flow in a uniform shear stress field and at the capillary flow has been found. Normalization of the filler concentration with respect to its specific surface yields a unified concentration dependence of the relative viscosity of LC-CPE filled with solid particles of various natures and specific surfaces.

Rheology of thermotropic liquid crystalline copolyester Vectra E

The melt rheology of a new liquid crystalline copolyester Vectra E from Hoechst-Celanese Corp. was investigated. A Rheometrics mechanical spectrometer was used to characterize the change in melt rheology produced by annealing below the melting temperature with dynamic, step shear rate, and step strain tests. The coupled relaxation model was employed in analyzing the step strain test data. Typically, thermotropic liquid crystalline copolyesters exhibit a reversible annealing effect which is erased by heating above the melting transition followed by rapid cooling. The unique feature of Vectra E is an irreversible annealing phenomenon. The unannealed copolyester melts near 330 °C, above which the viscosity decreases by at least two orders of magnitude. After annealing at 300 °C for 16 h, the copolyester exhibits a large increase in the melting transition temperature to 380 °C. Furthermore, upon melting the annealed sample the shear relaxation modulus and viscosity of the melt remain 2–3 orders of magnitude greater than those for the unannealed sample. These observations are consistent with a significant increase in the molecular weight of the Vectra E copolyester during annealing in the solid state at 300 °C.

Thermotropic liquid crystalline copolyester based on 8-(3-hydroxyphenyl) octanoic acid and p-hydroxybenzoic acid

A thermotropic liquid crystalline copolyester formed by polycondensation of 8-(3-hydroxyphenyl) octanoic acid (3-HPOA) and p-hydroxybenzoic acid has been synthesized and characterized. 3-HPOA having both a flexible segment and a rigid kink in its structure has been synthesized by phase transfer catalysed oxidation of cardanol. The copolyester was anisotropic under crossed polarized light between 200°C and 409°C. D.s.c. showed two mesophase transitions with maxima at 256°C and 342°C, respectively.

Blends of poly(aryl ether ether ketone) with thermotropic liquid-crystalline copolyesters: 2. Crystallization kinetics

The crystallization kinetics of blends of poly(aryl ether ether ketone) (PEEK) and thermotropic liquid-crystalline copolyesters have been studied by differential scanning calorimetry and optical microscopy. The overall crystallization rates of the blends were found to be dependent on the blend composition. With increasing mesomorphic content, the rate increased to pass through a maximum and then decreased to lower values. The linear spherulite growth rate of PEEK in the blends during isothermal crystallization was found to be smaller than that of pure PEEK. On the other hand, the nucleation density determined in the blends was about two orders of magnitude higher than that observed in a pure PEEK specimen, which is shown to be a more important factor in influencing the overall crystallization rate of these systems. On the basis of experimental results and theoretical treatments, it was believed that the crystallization process of the blends was initiated by heterogeneous nuclei, which were formed by the existing microdomains of the liquid-crystalline polymer, and that the number of heterogeneous nuclei was mainly determined by the size of the microdomains instead of the amount of liquid-crystal component present in the blends.

Blends of a chlorinated poly(vinyl chloride) compound and a thermotropic liquid crystalline copolyester: Mechanical properties of squeezed flow films

AbstractThe mechanical properties of squeezed flow films were measured 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. CPVC is immiscible with LCC. The most serious and unique problem of liquid crystalline polymers is their tendency to fibrillate when fabricated into films and injection molded parts, primarily because of a high degree of mechanical anisotropy. It has been found that the mechanical anisotropy of LCC and blends could be lessened using nonisothermal squeezing equibiaxial extension flow. The maximum tensile properties of LCC are achieved when processed in the vicinity of 260°C. For blends of CPVC containing LCC, the mechanical properties are dependent on the processing temperature and compositions. Blends with no more than 20 wt% of LCC exhibit significant increases in tensile properties. This is due to the possibility of frozen‐in macroscopic biaxial orientation of LCC in the blends during the nonisothermal squeezing flow. Within the range of processable temperatures, the reinforcement of CPVC due to the incorporation of LCC can be achieved at a temperature below the optimum processing temperature of LCC, although the thermal history of blends never reaches that temperature.

Phase transformations in a thermotropic liquid crystalline copolyester and its blends with polyether sulfone

AbstractMesophase transitions in a thermotropic liquid crystalline polymer comprising p‐hydroxybenzoic acid (PHB) and polyethylene terephthalate (PET) in a copolymer ratio of 60/40 were studied by means of optical microscopy, light scattring, and wide‐angle X‐ray diffraction (WAXD). According to time‐resolved light scattering and optical microscopic studies, at about 250°C, a mesophase transition was evident in which Schlieren texture developed with elapsed time. This transition has been identified as a solid crystal‐nematic mesophase transition associated with the melting of PHB‐rich crystals. It is concluded that the copolymer sequence may not be random. An isotropic, amorphous blend of PHB‐PET/polyether sulfone (PES) was prepared by solvent casting from hexafluoro‐2‐propanol solution. However, this homogeneous blend appears to be kinetically entrapped during solvent removal. Thermally induced phase separation takes place upon heating slightly above the glass transition temperature of PES. Morphology development within phase‐separated domains has been qualitatively examined by light scattering and optical microscopy.

Rheology and morphology of some thermoplastic blends with a liquid crystalline copolyester

AbstractLiquid crystalline polymers (LCPs) are known for their high performance properties. However, owing to their high cost, research efforts are much oriented to their use as reinforcements for different thermoplastics. In this study, we investigated the morphology, mechanical and dynamic rheological properties of blends of a 60/40 para hydroxybenzoic acid–ethylene terephthalate copolyester LCP (PHB/PET) with poly(butylene terephthalate) (PBT), poly(hexamethylene terphthalate) (PHMT), and polycarbonate (PC). Addition of up to 30 wt% of LCP to the different thermoplastics was performed in a Haake Rheomix mixer at 300°C. The dynamic rheological properties of the blends showed significant changes upon the addition of LCP, but no improvement in the mechanical properties was observed. The rheological properties of the blends below the nematic transition temperature of the LCP (210°C) were similar to those of solid filled thermoplastics. At 270°C, at which the LCP is in the nematic phase, the viscosity of LCP blends with PC blends decreased, whereas that obtained with PBT blends was increased. This is interpreted as being due to the differences in viscosity and interfacial tension between the components and to a possible reaction between the LCP and the thermoplastics.

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.

Liquid Crystalline Copolyesters as High Temperature Adhesives for Aluminum

Abstract A liquid crystalline copolyester consisting of 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, with a composition (73:27) was studied as a melt processible, high temperature adhesive. The polymer was used as received without any additional treatment and was applied on an aluminum substrate as a film. The adhesive joints were prepared using different processing conditions and good bonding between the two polymer surfaces was observed under most of these conditions. The reliability of the adhesive bond was estimated by measuring the lap shear strength (1.4-1.6Ksi) and the crack propagation in the wedge test. Improved adhesion (2 times) over epoxy adhesives, applied on the same substrate, was observed when tested at room temperature and there was no significant decrease in the lap shear strength in the range RT-180[ddot]C. The post-failure surfaces were studied with SEM and it was observed that separation occurred within the adhesive.

Mechanical and structural properties of extruded strands of blends containing a liquid-crystalline polyester with poly(ethylene terephthalate)

Mechanical and structural properties of extruded strands of blends of a liquid-crystalline copolyester (LCP), containing p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid, with poly(ethylene terephthalate) (PET) were investigated using tensile testing, X-ray diffraction measurements and scanning electron microscopic observation. The extruded strands of the blends consist of a crystalline and oriented LCP phase and an amorphous and unoriented PET phase. The tensile modulus increases almost linearly with increasing LCP content for the strands of the blends of more than 10% LCP content. The LCP component hardly contributes to the improvement of the mechanical tensile properties for the strands of the blends of less than 5% LCP content. The LCP phase tends to form fibrous structures, which orient almost parallel to the direction of extrusion in the blend strands. The fibrils of the LCP component become longer and thinner with increasing extension draw ratio of the blend strands. A relationship between the mechanical tensile properties and the morphology of the LCP phase in the blend strands was discussed.

Shear-induced crystallization in a liquid crystalline random copolyester

X-ray observations have been made during high temperature shear of a liquid crystalline copolyester. Whilst being sheared above the polymer melting point, a sharp crystallite diffraction peak develops. This peak grows in intensity as the shear rate increases and disappears on cessation of shear. It is associated with the shear stabilization of crystallites, presumably of the non-periodic-layer type.

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 monomer composition on the rheological properties of poly(p-oxybenzoate-co-ethylene terephthalate)s

Effect of monomer composition on the dynamic viscoelastic properties and the steady flow properties of thermotropic liquid crystalline poly (p-oxybenzoate-co-ethylene terephthalate)s in the fluid state have been investigated as compared with the isotropic melt of the copolyester including 28 mol% p-oxybenzoate unit (POB). The liquid crystalline copolyesters show the rheological transition temperature, which increases with increasing the content of POB unit. Above the transition temperature, all the thermotropic copolyesters exhibit much higher compliance and much more shear-thinning behavior of the apparent viscosity than the isotropic melt of the non-liquid crystalline copolyester.

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.