HBA ) And 2-hydroxy-6-naphthoic Acid Research Articles

Preparation of aromatic polyesters by direct polymerization in the presence of boronic anhydride under non-stoichiometric condition

Direct polymerizations of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) in the presence of boronic anhydride were examined by using crystallization under non-stoichiometric condition. The polymerizations were carried out with the addition of monofunctional compounds such as an aromatic carboxylic acid and a phenol. High molecular weight poly(p-oxybenzoyl) (POB) and poly(2-oxy-6-naphthoyl) (PON) were obtained in the form of precipitates, and their molecular weights were much higher than those calculated according to the stoichiometric imbalance. Oligomers were formed by the direct polymerization with eliminating water in the solution, and then they were precipitated to form the crystals at the initial stage of the polymerization. Oligomers end-capped by monofunctional compounds were also formed in the solution, but the polymerization continued with the elimination of monofunctional compounds by the ester exchange reaction. The end-capped oligomers were precipitated in the middle of the polymerization, and the polymerization proceeded efficiently even in the precipitated crystals by not only the direct polymerization but also the ester exchange polymerization, resulting in the increase in molecular weight.

“Foil spintrusion” of high‐performance polymer films

AbstractThis article describes the development of high‐performance polymer foils by direct orientation in the liquid‐crystalline phase of a thermotropic copolyester, based on p‐hydroxybenzoic acid (HBA) and 2‐hydroxy‐6‐naphthoic acid (HNA), better known under the trade name Vectra®. Application of a simple air‐gap‐filament‐fusion‐extrusion process (“foil spintrusion”) with the use of a newly developed die, results in highly oriented free‐standing foils, as thin as 3 μm. These foils have a mechanical performance that is comparable with those of commercially produced fibers of the same material. These uniaxially oriented foils could be laminated upon hot‐compacting without, or with as little as 5 wt % of an adhesive, yielding planar foils as thin as 18 μm, which feature excellent isotropic, in‐plane mechanical properties, that is, a Young's modulus E = 20 GPa and a tensile strength σ = 0.5 GPa. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Investigation of the Structural Deformation Mechanisms Induced by Microindentation in a Thermotropic Liquid Crystalline Copolyester Using Synchrotron X-ray Microdiffraction

The structural changes induced by microindentation on thermotropic liquid crystalline copolyester fibers of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) have been investigated by scanning X-ray microdiffraction techniques at the ESRF microfocus beamline (ID13). A phase transformation is observed upon deformation from a pseudohexagonal lattice (PH) to two distinct orthorhombic phases (OI, OII). In addition, a strong texture is observed in the plastically deformed region as revealed by changes in the local orientation of the crystalline domains in the vicinity of the indenter tip and the appearance of reflections in specific directions only. This allows quantifying the extent of the deformation both across and along the fiber and also suggests the presence of complex structural processes occurring during indentation tests.

Migration of monomers from liquid crystalline poly(p-hydroxybenzoic acid-co-2-hydroxy-6-naphthoic acid)

Liquid-crystalline co-polyesters (e.g. a random copolyester based on p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) known as Vectra A950) offer good barrier properties, but for food-contact use require overall and specific migration testing. For Vectra A950 films, the highest overall migration level obtained was 2.3 mg kg−1 in olive oil (10 days at 40°C) well below the EC limit of 60 mg kg−1. The highest specific migration for p-hydroxybenzoic acid was 15.2 µg dm−2 in olive oil (2h at 175°C). For 2-hydroxy-6-naphthoic acid, the highest value obtained was 4.3 µg dm−2 in 10% ethanol (4h at 100°C), although it was not on the EC positive and cannot yet be used for food-contact materials. At conditions considered as severe, the estimated daily intake for p-hydroxybenzoic acid was calculated as 11.9 µg/person day−1 and for 2-hydroxy-6-naphthoic acid it was 5.3 µg/person day−1. The results exceed the threshold of regulation of 1.5 µg/person day−1.

Structural studies on polymer whiskers by transmission electron microscopy: I. Morphological and high‐resolution observations

The structures of poly(p-hydroxybenzoic acid) (PHBA) whisker crystals and crystalline fine fibrils of copolymer prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA), namely poly(HBA/HNA) (monomer molar ratio: 97/3), were investigated by transmission electron microscopy (TEM). As for a PHBA whisker, microfibril-like bright regions were recognized in the 210 dark-field image. In the high-resolution TEM image of another PHBA whisker, however, lattice fringes were continuous through the whisker, showing that the whisker is highly crystalline and almost perfect. On the other hand, in the high-resolution images of poly(HBA/HNA) fine fibrils, granular crystallites (20– 40 nm in lateral dimension) were observed. The lattice fringes in the crystallites were not successfully indexed with the orthorhombic unit cells of the homopolymer crystal, suggesting the coexistence of another different structure as a minor component. Microsc. Res. Tech. 46:325–333, 1999. © 1999 Wiley-Liss, Inc.

Structural studies on polymer whiskers by transmission electron microscopy: I. Morphological and high-resolution observations.

The structures of poly(p-hydroxybenzoic acid) (PHBA) whisker crystals and crystalline fine fibrils of copolymer prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA), namely poly(HBA/HNA) (monomer molar ratio: 97/3), were investigated by transmission electron microscopy (TEM). As for a PHBA whisker, microfibril-like bright regions were recognized in the 210 dark-field image. In the high-resolution TEM image of another PHBA whisker, however, lattice fringes were continuous through the whisker, showing that the whisker is highly crystalline and almost perfect. On the other hand, in the high-resolution images of poly(HBA/HNA) fine fibrils, granular crystallites (20-40 nm in lateral dimension) were observed. The lattice fringes in the crystallites were not successfully indexed with the orthorhombic unit cells of the homopolymer crystal, suggesting the coexistence of another different structure as a minor component.

Effects of main chain rigidity on nonlinear dynamic viscoelasticity and fatigue performance for polymeric fibres

Fatigue performance of polymeric fibres with different molecular chain rigidity was investigated based on zone nonlinear dynamic viscoelastic analyses under tension-type cyclic straining. Poly(vinyl alcohol), Nylon 6(poly( ϵ-caprolactam)), poly(ethylene terephthalate), poly( p-phenylene terephtalamide) and Vectran (a copolyester of p-hydroxy benzoate (HBA) and 2-hydroxy-6-naphthoic acid (HNA)) fibres were used as specimens. Nonlinear dynamic viscoelasticity and fatigue performance of polymeric fibres strongly depended on the chain rigidity. The polymeric fibres with rigid molecular chains showed pronounced nonlinear viscoelastic characteristics compared with those with flexible ones. Also, based on the zone nonlinear dynamic viscoelastic analyses, it was found that the nonlinear viscoelastic behaviours of polymeric fibres was predominantly induced in the recovery process rather than in the tensile process during a single period of cyclic deformation, and the more remarkable nonlinear viscoelastic behaviour was exhibited in the zone with higher strain rate. The polymeric fibre which exhibited remarkable nonlinear dynamic viscoelasticity showed extremely poor fatigue performance.

Monte Carlo simulations of a liquid crystal copolymer in the solid state

The condensed phase of the alternating copolyester of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) is investigated by studying the room temperature packing arrangement of the copolymer chains. A molecular modeling methodology is employed with a Monte Carlo sampling of the configurational phase space. Realistic poly(HBA-alt-HNA) polymer chains are represented by an explicit atom representation of the HBA/HNA dimers. States are sampled from the NVT ensemble using a sampling scheme consisting of (1) valence and torsional variations, (2) rigid body rotations of the chain about the chain axis, and (3) rigid body translations of the chain. The effect of chain packing on the conformation of chains, as well as the relative intra- and intermolecular orientations of aromatic rings, is investigated. Correlation of chain positioning along the chain axis is dominated by aromatic rings maintaining a center-to-center plane of registry. These layers of aromatic units pack with a preference for edge-to-face orientations in a herringbone-type pattern and have an intermolecular ring angle between the pairs of aromatic rings in the unit cell that is ca. 68°. The aromatic rings, on average, are rotated 38° out from the b–c plane. The phenylene rings of these copolyesters are less restricted in their relative orientation in comparison to the naphthalene rings. Intramolecular orientational probability density distributions indicate a preference for staggering the successive aromatic rings along the chain, with a staggering angle of ca. 66°. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 727–741, 1998

Novel aspects of microstructure of liquid crystalline copolyesters as studied by microhardness: influence of composition and temperature

The microhardness of a series of random copolyesters of 4-hydroxybenzoic (HBA) and 2-hydroxy-6-naphthoic acid (HNA) has been investigated as a function of composition and temperature. The results reveal that, at room temperature, the microhardness of non-oriented materials deviates from the linear additivity of the hardness of single homopolymers. Such a deviation is shown to be mainly related to changes in the molecular packing of the rigid chains. This packing, and as a consequence microhardness, can be characterized by an average cross-sectional area which includes crystalline and non-crystalline regions. The fact that the microhardness behaviour and the temperature dependence of the cross-sectional area are closely related is emphasized. An analytical expression embracing the coefficient of thermal softening, β, and the thermal expansion coefficient of the cross-sectional area, α A, is proposed.

X-ray Analysis of the Kinetics of Transesterification in Blends of Wholly Aromatic Thermotropic Copolyesters

The kinetics of the transesterification reaction occurring in blends of wholly aromatic copolyesters have been investigated by simulation of the changes that occur in the X-ray scattering data as a result of the interchange. The X-ray fiber diagrams of copolyesters prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) contain nonperiodic diffraction maxima along the chain axis direction that vary in position (scattering angle) with the monomer composition and are due to the structural correlations in chains of completely random comonomer sequence. Melt blends of two different compositions initially show the scattering maxima of both components, but these shift as the specimen is held in the melt, until they merge to give the diffraction data characteristic of the intermediate composition. We have simulated the changes in these data by modeling the changes in the correlation function that occur as a result of the transesterification reaction. We find that the data are best reproduced using a completely random interchange model for a completely compatible blend. Simulations based on preferential reactivities of one or other of the monomers or partial segregation of one phase gave significantly inferior agreement with the positions of the observed diffraction maxima. The statistical modeling allows us to derive kinetic parameters for the reaction, for which we obtain an activation energy of 142 kJ/mol and a rate constant of 2.3 × 10-4 s-1 at 315 °C. Compared to the equivalent data for isotropic polyesters, the activation energy compares very well, but the rate constant is 1 order of magnitude lower, perhaps due to the ordering in the thermotropic melt.

Quasielastic neutron scattering study on rotational aromatic groups of a thermotropic copolyester (vectra)

The rotational motion of aromatic groups which constitute a thermotropic copolyester, vectra was investigated in terms of the quasielastic neutron scattering (QENS) originating from incoherent scattering of hydrogen. Vectra which is a random copolyester of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with a faction of 73/27 (mol/mol), gives rise to the nematic phase transition at Tm=558 K from semicrystalline phase, i.e., a mixture of crystalline and amorphous phases (Tg=393 K). In the region above Tm, the elastic incoherent scattering cross section due to hydrogen, obtained for optical geometry in which q vector is normal to the molecular axis of vectra, was successfully decomposed into (i) the Debye–Waller factor due to vibrational hydrogen, (ii) the elastic incoherent structure factor (EISF) for the rotational aromatic groups with hydrogen and (iii) the QENS for the rotational aromatic groups with hydrogen. The QENS spectra and EISF thus obtained were well reproduced by a free rotational model for the aromatic groups, in which residing sites for hydrogen on the rotational circle is infinite. On the other hand, in the solid (semicrystalline) phase below Tm, the QENS spectra and EISF became less q dependent with decreasing temperature; these observations agree qualitatively with a more restricted motion, e.g., a jump rotation having finite residing sites for hydrogen.

Structures of liquid crystalline wholly aromatic copolyester films with different copolymer compositions

Films of wholly aromatic copolyester composed of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) were prepared by a solution-casting method using a mixed solution of pentafluorophenol (PFP) and chloroform (weight ratio: PFP/chloroform = 3/7). Using five samples with different copolymer compositions (HBA/HNA [mol%] = 25/75, 40/60, 55/45, 62/38, 73/27), the effects of the copolymer composition on the fine structures of the films were investigated using thermal analyses, density measurements, X-ray diffraction methods, and tensile tests. The as-cast films obtained were shown to be trans-parent and highly amorphous in spite of changing the copolymer composition. When the films were heated above the Tg (100°C), cold crystallization first occurred during the heating process and they had melting points. The densities of the films increased with increasing annealing temperature throughout the cold crystallization. The elongation percentages of the as-cast films reached high values of 30–74% at room temperature, indicating their maximum of elongation at 55 mol% of HBA. © 1996 John Wiley & Sons, Inc.

The effect of branching on the mechanical properties of HBA/HNA copolymer

The mechanical properties of aligned fibres and monofilaments of random copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with and without the inclusion of a trifunctional branching monomer have been investigated using both macroscopic and X-ray techniques. Results for unbranched samples are in line with previous work while including the branching monomer reduces both the shear and tensile moduli as measured macroscopically. Annealing the samples increases the moduli. On the scale observed by X-ray diffraction the inclusion of branching has little effect whereas annealing increases the sinuosity of the chains and hence reduces the apparent chain modulus determined by X-ray measurements on samples under stress.

Crystallization of thermotropic liquid crystalline HBA/HNA copolymers

The crystallization behaviour of random copolymers based on p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) is reviewed. First a summary is given of what is known about the structure and thermal transitions of the constituent homopolymers. Next the crystallization behaviour of HBA/HNA copolymers is described starting with the glass transition, after which the two crystallization processes that have been found in these copolyesters are treated. The crystallization of the HBA/HNA copolymers will be compared to the crystallization of similar random copolymers that are not liquid crystalline. Two models have been proposed to describe the solid state of the HBA/HNA copolymers: the non-periodic layer (NPL) model and the paracrystalline lattice (PCL) model. In the NPL model, ordered domains are formed by the lateral register of similar but non-periodic sequences of neighbouring chains, whereas in the PCL model ordered domains are formed by the presence of conformational correlations between adjacent monomers without explicit sequence matching. The two models are compared in view of the experimental observations about the structure and the morphology of the crystallized material. This review ends with some conclusions.

Three-dimensional structure of a wholly aromatic copoly(ester carbonate)

The three-dimensional structure of the thermotropic copoly(ester carbonate) comprised of p-hydroxybenzoic acid (HBA), hydroquinone (HQ) and carbonate units has been investigated by X-ray methods, focusing on the copolymer containing a 75/25 HBA/HQ mole ratio. The fibre diagrams contain nonperiodic layer lines, and we have previously shown that these arise from a structure consisting of arrays of parallel chains with a completely random comonomer sequence. The chains have highly extended conformations, in which the carbonate groups are distorted from the minimum-energy planar structure. such that the 1,4-axes of the phenylene groups are all approximately parallel to the fibre axis. The X-ray data also contain a number of sharp equatorial and off-equatorial Bragg maxima due to the presence of three-dimensional order. We have first addressed the determination of the structure by calculation of the cylindrically averaged intensity transforms of single chains averaged overall possible sequences. We then considered the interference effects due to packing of the copolymer chains. The results show that the planes of successive phenylene rings, linked either by ester or carbonate groups, are mutually inclined by ∼60°. This leads to a pseudo-dimer repeat unit, which explains the existence of the layer line at Z = 1/13.1 Å, between the equator and the first observed meridional maximum. The off-equatorial maxima are predicted when we require that short, random segments form ‘crystallites’ by registration of the central monomer units. This registration is relatively minimal, in that there is a distribution of the centre of the sequences about the registry plane, with a standard deviation of ≈ 1 Å. It is seen that copoly(ester carbonate) chains with distorted carbonate groups can pack in a more ordered structure than is possible in the wholly aromatic copolyesters prepared from HBA and 2-hydroxy-6-naphthoic acid (HNA).

Transesterification in Blends of Wholly Aromatic Thermotropic Copolyesters

The X-ray diffraction data for the copolyesters prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) give nonperiodic layerlines that indicate a completely random monomer sequence distribution and also provide analytical information on the monomer proportions. Hence, these data provide a unique method to study transesterification between copolyesters with different monomer ratios. Immediately after melt blending, mixtures of the 30/10 and 15/25 copolymers exhibit a single solid-nematic transition at a temperature well below either the transitions of the starting copolymers or those of intermediate compositions. However, the meridional minima for both starting copolymers can be resolved in the fiber diffraction patterns, which indicates the formation of a compatible blend

Impedance behavior of liquid crystalline copolymers in the solid phase

The measurements were carried out as a function of temperature and frequency on nematic random liquid crystalline copolymers of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). The temperature was varied from 20 to 95°C while the frequency could vary from 100 Hz up to 100 kHz. It was found that the dispersion relation of ac-impedance shows a strong dependence on frequency and temperature. It falls from about 56 MΩ at 100 Hz to a value of about 13 MΩ at 100 kHz. The Cole-Cole plot shows a semicircle indicating that a certain polarization mechanism with a single relaxation time is dominant in the liquid crystalline copolymer material.

Diffraction measurements of crystalline morphology in a thermotropic random copolymer

The crystal structure and morphology has been examined in a series of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) thermotropic random copolymers having different compositions and molecular weights. For rapidly cooled HBA-rich samples, the structure is pseudo-hexagonal, while for the more HNA-rich it is slightly distorted to orthorhombic, a phase designated O″. On annealing, the HBA-rich samples undergo a partial phase transformation to a denser orthorhombic phase with a greater distortion from the hexagonal than the O″. This new phase is designated O′. Annealing of the samples richer in HNA leads to a greater distortion of the O″ cell away from hexagonal, but no increase in density. Discrete small-angle diffraction maxima corresponding to a long period of ∼ 300 A ̊ appear only in those samples containing the denser O′ phase, with the implication that both pseudo-hexagonal and O″ crystals have densities very close to that of the non-crystalline nematic matrix. Anneals above 230°C lead to a marked increase in long period and there was some evidence of a specific doubling mechanism. Precise measurement of crystallinity gave values between 17.5% and 25.5%, the lower values being in the samples closer to the 1:1 composition and in those annealed.

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.

A study of molecular motion in a liquid‐crystalline copolyester by broad‐line nuclear magnetic resonance

AbstractBroad‐line nuclear magnetic resonance (NMR) measurements have been carried out for the proton resonance in an oriented liquid‐crystalline copolyster prepared from 4‐hydroxybenzoic acid (HBA) and 2‐hydroxy‐6‐naphthoic acid (HNA). The Second moments were determined as a function of the angle γ between the specimen orientation direction and the static magnetic field. The low‐temperature NMR results are consistent with a rigid structure of random copolymer chains arranged on a hexagonal lattice. As the temperature is raised the NMR signal changes and can be modeled very satisfactorily by considering that there is free rotation of the benzene ring residues about the 1‐4 substitution direction. At more elevated temperatures the observed NMR spectra are consistent with rotation of both HBA and HNA groups.