Thermotropic Liquid Crystalline Copolyester Research Articles

The effect of molecular weight on the structure and temperature behavior of a thermotropic main-chain liquid crystalline copolyester

The structure of a fully aromatic thermotropic liquid crystalline (LC) copolyester poly-[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenylhydroquinone, andp-hydroxybenzoic acid at a molar ratio of 45/45/10, respectively, was studied at ambient and elevated temperatures by means of x-ray diffraction and differential scanning calorimetry as a function of molecular weight. On heating of PES fibers with fixed ends an irreversible phase separation process takes place above the glass transition point and two different crystalline phases are formed. A model is proposed where the phases are assumed to contain the constituents of the statistical copolymer in different amounts. The relative volume fraction of the two crystalline modifications depends on the molecular weight of the investigated fibers. At higher temperatures the melting of the two crystalline phases and their transition to a LC nematic mesophase is observed.

Synthesis and characterization of potentially biodegradable thermotropic polyesters based on p‐hydroxybenzoic acid and glycolic acid

AbstractThe synthesis of novel thermotropic liquid crystalline copolyesters derived from aliphatic hydroxy acid (glycolic acid, GA) and aromatic hydroxy acid (p‐hydroxybenzoic acid, PHBA) via a melt‐copolycondensation process in the presence of various catalysts was explored. The following three possible routes were checked: PHBA and GA in different feed ratios with or without a catalyst; PHBA and GA in different feed ratios with or without a catalyst in the presence of acetic anhydride as a condensation agent; and different PHBA derivatives were used to examine the reactivity of aromatic hydroxy acid. The copolycondensability, chemical structure, liquid crystallinity, textures and morphology, phase transition behaviors and thermal stability, and solubility were investigated by FTIR, NMR, DSC, TGA, and polarized‐light microscope. It has been found that only the 60–70 mol % PHBA‐containing copolyesters could exhibit a nematic liquid crystallinity. The as‐prepared polymers were brittle due to relatively lower molecular weights. © 1994 John Wiley & Sons, Inc.

Rheology and physical properties of ternary blends containing a thermotropic liquid crystalline copolyester

AbstractTernary blends of polyarylate (PAR) U‐Polymer 100, thermotropic liquid crystalline copolyester (LCP) Vectra A950, and a block copolyesterether Hytrel 7246 were investigated in terms of rheological properties, morphology, and mechanical properties. The PAR/Hytrel blend exhibited melting point depression and gave a unique single Tg over the entire range of blend compositions. Addition of Hytrel to the PAR/LCP blend decreased both dynamic viscosity and storage modulus over the normal processing temperature range. Further, it notably reduced the voids between the LCP domains and the matrix, and improved the mechanical properties. The optimum usage level of Hytrel proved to be 2 phr.

The potential for using alkaline hydrolysis to study the fine structure of fibres manufactured from a thermotropic liquid crystalline copolyester

The possibility of utilizing alkaline hydrolysis for studying changes in the properties of thermotropic liquid crystalline copolyester fibres is evaluated. The copolyester fibre manufactured from 4-hydroxybenzoic acid (HBA) and 2-hydroxy 6-naphthoic acid (HNA) in a molar ratio of 73 27 HBA/HNA was hydrolysed at 21°C with 2.5 M NaOH, 0.1% cetyltrimethylammonium bromide solution. After hydrolysis the shape of the HBA/HNA fibre cross-section changed from round to polygonal, unlike the cross-section of poly(ethylene terephthalate) fibre which does not change in shape after a similar treatment. Although heat treatment markedly increased the tenacity of HBA/HNA fibre, the tenacity fell steadily with increasing weight loss by hydrolysis. In contrast, the tenacity of the non-heat-treated fibre remained constant up to a weight loss of 58%, indicating that hydrolysis was occurring at the fibre periphery and leaving its core unaffected. For both the heat-treated and non-heat-treated products the initial modulus of the fibres fell progressively with increasing weight loss. Significant change in brittleness did not appear to occur when either form of HBA/HNA fibre was hydrolysed to a weight loss of 38%.

Processing and Mechanical Properties of Strands of Liquid Crystalline Copolyester and Copolyesteramide

Extrusion moulding behaviour, mechanical and morphological properties of thermotropic liquid crystalline copolyester (LCP-A) and copolyesteramide (LCP-B) strands were investigated using a co-rotating twin-screw extruder, tensile testing, x-ray diffraction measurements, and scanning electron microscopic observation. Extrusion pressure and torque of LCP-B strands were larger than those of LCP-A strands under the same extrusion conditions, reflecting a difference in the apparent melt viscosity of the two polymers. The LCP-B strands obtained in this study had more homogeneous diameters, as compared with the LCP-A strands. The irregularity in diameter of the LCP-A strands tended to become smaller with increasing the length-to-diameter ratio (l/D) and output flow rate. The molecular orientation of LCP-B strands rose rapidly with draw ratio and reached almost a plateau at a much lower one, whilst that of the LCP-A strands became slowly higher with draw ratio. The tensile modulus and strength of LCP-B strands increased gradually with a draw ratio, even after the molecular orientation of the strands levelled off. The effect of the draw ratio on the molecular orientation and the mechanical properties of the strands was discussed from a morphological point of view.

Processing and Mechanical Properties of Strands of Liquid Crystalline Copolyester and Copolyesteramide

Extrusion moulding behaviour, mechanical and morphological properties of thermotropic liquid crystalline copolyester (LCP-A) and copolyesteramide (LCP-B) strands were investigated using a co-rotating twin-screw extruder, tensile testing, x-ray diffraction measurements, and scanning electron microscopic observation. Extrusion pressure and torque of LCP-B strands were larger than those of LCP-A strands under the same extrusion conditions, reflecting a difference in the apparent melt viscosity of the two polymers. The LCP-B strands obtained in this study had more homogeneous diameters, as compared with the LCP-A strands. The irregularity in diameter of the LCP-A strands tended to become smaller with increasing the length-to-diameter ratio (l/D) and output flow rate. The molecular orientation of LCP-B strands rose rapidly with draw ratio and reached almost a plateau at a much lower one, whilst that of the LCP-A strands became slowly higher with draw ratio. The tensile modulus and strength of LCP-B strands increased gradually with a draw ratio, even after the molecular orientation of the strands levelled off. The effect of the draw ratio on the molecular orientation and the mechanical properties of the strands was discussed from a morphological point of view.

X-ray analysis of the structure of a thermotropic main-chain liquid-crystalline copolyester

The structure and phase behaviour of the thermotropic main-chain liquid-crystalline copolyester poly[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenylhydroquinon andp-hydrobenzoic acid (molar ration 45/45/10) is determined by X-ray diffraction and differential scanning calorimetry. This random copolyester shows up to 340°C periodicc-axis order in its X-ray diffraction pattern as well as sharp and diffuse reflections at the equator and other crystallographic directions, indicating well ordered three-dimensional crystalline structure. Two crystalline modifications are observed for quenched and annealed fibres, which are both orthorhombic with slightly different unit cell sizes. Indications of other phases at still higher temperatures are reported. They are believed to be a novel mesophase followed by a nematic liquid-crystalline phase and the isotropic melt.

Rheological behaviors of bisphenol series thermotropic liquid crystalline wholly aromatic copolyesters

AbstractRheological experiments were performed on a series of thermotropic liquid crystalline aromatic copolyesters based on terephthalic acid (TPA), isophthalic acid (IPA), p‐hydroxybenzoic acid (HBA), and a family of bisphenols. The anisotropic melts behaved as non‐Newtonian fluids in a shear elongation flow. The non‐Newtonian exponents of the melts decreased with increasing the linearity of the bisphenol bridge group, and decreased with increasing the optical anisotropy in the liquid crystalline phase. The cross‐section morphologies along the longitudinal direction of the extrudates were surveyed by scanning electronic microscopy (SEM) and discussed with melt‐flow rate distributions. The extrudates exhibited skin–core microstructures and oriented fibril structures. The die swells of the extrudates decreased sharply to negligible levels when shear stress was above 1000 Pa. © 1993 John Wiley & Sons, Inc.

On the solution characterization of an all‐aromatic liquid‐crystalline copolyester

AbstractSolution characterization of the thermotropic liquid–crystalline copolyester synthesized from terephthalic acid, phenyl hydroquinone, and (1‐phenylethyl) hydroquinone (2 : 1 : 1) has been performed. Viscometry, size exclusion chromatography, and light scattering have been carried out under the optimal conditions found for measurement: 85°C in a 50/50 mixture by weight of phenol/1,2,4‐trichlorobenzene. The absolute weight‐average molecular weight from light‐scattering measurements served for calibration of indirect methods of charac‐terization (e.g., the limiting viscosity number [η] is related to the molecular weight by [η] = 5.10 × 10−4 Mw0.72), and the molecular weight per unit chain length, \documentclass{article}\pagestyle{empty}\begin{document}$ \bar M_L * $\end{document}, from light scattering and size exclusion chromatography (SEC) is found to be 28 Å−1, consistent with theoretical expectations. The calculated persistence length q is 28 Å. Moreover, the meth‐odology of SEC characterization enables the kinetics of solid‐state postpolymerization of this liquid‐crystalline copolyester to be studied. © 1993 John Wiley & Sons, Inc.

Synthesis and characterization of a family of wholly aromatic thermotropic liquid crystalline copolyesters

A family of melt processable, wholly-aromatic thermotropic liquid crystal copolyesters are prepared, which are characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry and polarized optical microscopy. The copolyesters exhibit broad nematic-mesophase temperature ranges without isotropic transitions and excellent thermal stability. The kinetics of melt polycondensation of the copolyesters is also investigated.

Thermotropic liquid crystalline copolyesters. I. Synthesis and thermal characterization of the copolyesters of terephthaloyl chloride, hydroquinone, and 1,4-butane diol

A new series of random thermotropic liquid crystalline polyesters (LCPs) was synthesized from terephthaloyl chloride, hydroquinone, and 1,4-butane diol. The copolyester composition was varied by changing the relative mole ratio of hydroquinone and 1, 4-butane diol in the feed. All the copolyesters displayed thermotropic liquid crystallinity at relatively low temperatures. Liquid crystalline behavior of the copolyesters has been characterized by polarizing light microscopy and by differential scanning calorimetry (DSC). The effect of co-polyester composition on the temperature and thermodynamic parameters related to liquid crystalline transition is discussed. © 1993 John Wiley & Sons, Inc.

Modelling of microstructure in mesophases

Abstract Small-molecule liquid crystals show textures which are readily studied at low magnification in the optical polarizing microscope. In polymeric liquid crystals, however, the textures are often much finer, taxing the microscope's resolution. Nevertheless, studies of microstructure in such polymers have been made and it is apparent that they can differ widely both from small-molecule liquid crystals and, indeed, from polymer to polymer. This paper sets out to account for these variations by exploring the effect on microstructure of the marked differences between the magnitudes of the splay, twist and bend elastic constants which are a characteristic of many liquid crystalline polymers. We report a computer model which simulates the development of microstructure for different ratios of the elastic constants. When these are approximately equal, textures characteristic of small-molecule liquid crystals result, such as those involving escape into the third dimension with the degeneration of line defects into points. When the splay energy is high in comparison with bend and twist, as is the case for many thermotropic liquid crystalline copolyesters, escape does not occur and half integral disclination lines predominate. For simulations involving planar boundary conditions, layered microstructures result, frequently with little matching of the orientation from layer to layer. Within the layers the trajectory of the orienting units is sinuous. This simulated microstructure resembles textures observed in thermotropic copolyesters, studied both in this laboratory and elsewhere. The computer model uses a lattice approach which is similar in some respects to that developed by Lebwohl and Lasher. It should not be thought of as a molecular scale model, however, but rather as one based on assemblies of molecules which share a common director.

Orientation distribution and layerlike morphology in extrusion‐molded sheets of a liquid crystalline copolyester amide

AbstractFracture surface morphology and orientation distribution in the extrusion‐molded sheets of a thermotropic liquid crystalline copolyester amide were investigated by scanning electron microscopy (SEM), wide‐angle X‐ray diffraction (WAXD), and polarized Fourier transform infrared (FTIR) microspectroscopy. The layerlike morphology consisting of two outer layers and a central layer between them was observed on the fracture surface. The microscopic orientation functions in the extrusion‐molded sheets were evaluated from the polarized FTIR microspectra that were measured in the microscopic domain 40 μm wide using a redundantly apertured infrared microscope. The microscopic orientation function in the outer layer was shown to be higher than that in the central layer. The effects of the draw‐down ratio and extrusion temperature on the orientation distribution and the layerlike morphology are discussed. © 1993 John Wiley & Sons, Inc.

Thermal and Viscoelastic Properties of Blends of Polypropylene with Thermotropic Liquid Crystalline Copolyesters

The thermal and viscoelastic properties of blends of polypropylene (PP) with thermotropic liquid crystalline copolyesters (LCP) have been investigated. (1) Thermal characteristics, change of tan δ peaks, and SEM observation on the fracture surfaces of PP-LCP blends indicate behaviour characteristic of incompatible blend systems. (2) Ternary blend systems containing chlorinated polypropylene (Cl-PP) showed increased interfacial interaction between the LCP and the matrix in comparison with the binary blend. (3) Blends containing dimethylglyoxine (DMG) showed further improvement in interfacial interaction between the LCP and matrix. DMG is believed to act as a crosslinking agent.

Thermal and Viscoelastic Properties of Blends of Polypropylene with Thermotropic Liquid Crystalline Copolyesters

The thermal and viscoelastic properties of blends of polypropylene (PP) with thermotropic liquid crystalline copolyesters (LCP) have been investigated. (1) Thermal characteristics, change of tan δ peaks, and SEM observation on the fracture surfaces of PP-LCP blends indicate behaviour characteristic of incompatible blend systems. (2) Ternary blend systems containing chlorinated polypropylene (Cl-PP) showed increased interfacial interaction between the LCP and the matrix in comparison with the binary blend. (3) Blends containing dimethylglyoxine (DMG) showed further improvement in interfacial interaction between the LCP and matrix. DMG is believed to act as a crosslinking agent.

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.