Aromatic Copolyesters Research Articles

Aromatic copolyesters with stilbene mesogenic groups. 1. Liquid-crystalline properties of compounds containing a stilbene, terephthaloyl, or hydroquinone central group

Model compounds (S n ) of stilbene-unit containing copolymers are studied and compared with compounds T n and H n derived from terephthalic acid and hydroquinone, respectively. They have the following structures: Ar[COOC 6 H 4 COO(CH 2 ) n CH 3 ] 2 , where Ar=-C 6 H 4 CH=HC 6 H 4 (S n ) and Ar=-C 6 H 4 -(T n ); Ar[OOCC 6 H 4 COO(CH 2 ) n CH 3 ] 2 , where Ar=-C 6 H 4 - (H n ). Their liquid crystalline behaviors are established by DSC, optical microscopy, and X-ray diffraction. The lower homologues in the T n and H n series show only a nematic phase. When the alkyl group is lengthened, both nematic and S A phases are formed

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.

Banded texture decoration of disclinations in a main chain thermotropic aromatic copolyester observed by optical microscopy

Abstract A new method for the study of texture and director patterns in a main chain thermotropic aromatic copolyester with a flexible spacer is described, whereby the molecular chain or director orientation of the nematic mesophase becomes decorated by the formation of a banded texture during quenching, without being subjected to a shear. The pattern of the decorated banded texture may be observed directly by polarizing optical microscopy, revealing the complete texture of molecular chain orientation across the whole specimen. The molecular director orientation lies perpendicular to the long axis of the bands. Various types of disclination, including an inversion wall, in the nematic mesophase of a thermotropic aromatic copolyester have been observed. This decorating technique is particularly suited for non-crystallizable main chain liquid crystalline polymers, where the lamellar decoration technique fails.

Detailed Investigations of Structural Layering Phenomena in Injection Molded Thermotropic LCP

Abstract Effects of processing variables on the structural gradients developed in the injection molding of thermotropic wholly aromatic copolyester parts were investigated using optical macro and microscopy combined with image analysis techniques at various sections of the molded LCP parts. The studies indicate the existence of a multi-layer structure from skin to core of the parts. These include: a skin layer, a transition zone composed of a sub-skin layer, a shear layer and a core region. The effects of processing conditions including injection speed, mold temperature, and mold size on the structural features were evaluated. An interesting buckling phenomenon resembling features observed in jetting at core regions near the end of the cavity after the divergence section was observed for the samples molded at the lowest injection speed and mold temperature. Digital image analysis of the images of the sectioned parts revealed that there are more layers than one observes macroscopically. In certain instances as many as 15 layers were detected. Orientation behavior and the positional light reflection intensity appear to correlate with each other particularly away from the skin regions. This may be the manifestation of high visible light absorption anisotropy exhibited by these aromatic polymers.

Observation of disclinations in the nematic state of a thermotropic aromatic copolyester by lamellar decoration

Abstract The frozen nematic state of a thermotropic aromatic copolyester exhibits a schlieren texture with two and four dark brushes emanating from a point observed under a polarizing optical microscope; these correspond to disclination strengths s = ± ½ and s = ± 1, respectively. The combined techniques of lamellar decoration and ruthenium tetraoxide staining have been used to reveal the director orientation around disclinations in the frozen nematic state of polymers by transmission electron microscopy. All six disclinations in the nematic mesophase of a single liquid-crystalline aromatic copolyester have been observed. Every disclination interacts and is connected with its neighbouring disclination so that disclination exist in pairs usually of opposite sign in their disclination strengths. Some pairs with the same sign disclination have also been observed.

The processing of ternary LCP/LCP/thermoplastic blends

The feasibility of introducing a second thermotropic liquid crystalline polymer (TLCP) as a compatibilizer, or coupling agent, in order to improve the adhesion and dispersion between components of incompatible TLCP/thermoplastic blends was investigated. Two TLCP blends of a wholly aromatic copolyester (K161) with poly(ethylene terephthalate) (PET) and polycarbonate (PC) were prepared by a two-step melt-blending process, using an aliphatic containing TLCP, PET/60% poly(hydroxybenzoate) (PHB60), as the coupling phase. Preliminary morphological evidence indicated that the K161 reinforcing phase in the ternary systems exhibited improved adhesion and dispersion in a much finer scale than in binary blends of K161/PET and K161/PC prepared under the same processing conditions. It was found that the adhesion between the reinforcing K161 and the matrix PET phases depended on both the concentration of the coupling PET/PHB60 phase and the relative composition of the two TLCPs. The improved adhesion between K161 and PC matrix with the inclusion of PET/PHB60 phase was further delineated by comparing the phase morphology and mechanical properties of the ternary blends after a cold-drawing process with those of the binary systems. While the binary systems remained practically unaffected by the cold-drawing process, the ternary blends exhibited a well aligned and elongated K161 fibrillar phase with a diameter of around 1 μm or less. An increase of both initial modulus and breaking strength suggests that the enhanced adhesion between K161 and PC facilitates a better stress transfer for the deformation of the dispersed K161 particles into oriented, ellipsoidal domains. Other characterizations, including both differential scanning calorimetry and dynamic mechanical analysis, were conducted to elucidate the nature of the interactions between the components.

Processing of thermotropic liquid crystalline polymers and their blends—analysis of melt‐spinning TLCP fibers

AbstractIsothermal melt‐spinning of two thermotropic liquid crystalline polymers (TLCPs), a wholly aromatic copolyester KU‐9211 (also named K161 from Bayer AG) and an aliphatic containing TLCP, PET/PHB60 (Tennessee Eastman), was studied to analyze the effect of processing conditions on fiber properties. Fibers were melt‐spun from a capillary rheometer equipped with an isothermal chamber in which cross‐flowed air was used as the cooling medium. The processing variables studied included the extrusion temperature, the extrusion rate, the cooling conditions, and the draw ratio. As‐spun fibers were characterized by measuring storage moduli and molecular orientation parameters as a function of draw ratio under various processing conditions. Among the processing variables studied, the draw ratio was the primary factor in determining both the fiber modulus and the molecular orientation. The extrusion rate did not appear to affect the fiber properties within the range studied. The properties of K161 fibers were also dependent on the extrusion and cooling temperatures, while PET/PHB60 fibers were rather insensitive to the processing temperatures within data scatter and temperatures studied. A composite model based on a rigid‐rod rotation mechanism and the deformation of nematic domains in an elongational flow field was used to model the experimental results and was compared with other theories available. Conformance of data to the composite model was obtained by use of a single temperature dependent parameter n, suggesting that the rigid‐rod rotation mechanism could be used to predict the orientation development of TLCPs. The Halpin‐Tsai equations and the orthotropic equation for angular dependence were used to describe the elastic properties of the TLCP fibers.

Conformational analysis of some aromatic copolyesters in solution by means of 1H 1H nuclear Overhauser effect experiments

Conformational analysis was performed on some aromatic copolyesters in solution by means of 1H 1H nuclear Overhauser effect (NOE) measurements, using the cyclic dimer of isophthalic acid and ethylene glycol (DIMIS) as the model compound. From the observed NOE, a folding of the ethylene glycol towards the aromatic ring can be assumed for all the polyesters containing an isophthalic ring. In addition, the spatial interaction between adjacent aromatic rings, put forward in order to explain the chemical shift distribution observed in the 1H n.m.r. spectra of these copolymers, has been demonstrated with steady-state NOE experiments. The average distance between protons on adjacent aromatic rings has been calculated by means of dynamic NOE measurements. The most probable conformation, giving rise to these NOE effects, has been postulated on the basis of a formalism introduced in a previous work.

Solid State Ordering of Wholly Aromatic Copolyesters of Random Monomer Sequence

ABSTRACTX-ray diffraction and computer molecular modeling Methods are being used to investigate the ordering of random sequence copolyesters when cooled for the nematic Melt. Data will be presented for the copolymers prepared from p-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid. The non-periodic diffraction patterns arising from the random monomer sequences are also indicative of the presence of limited three dimensional order. For both polymer systems, we can generate the qualitative feature of the x-ray data using models in which short non-identical segments of 10–12 Monomers on adjacent chains are approximately in register at their center. Molecular Mechanics calculations show that mese non-identical sequences can be packed on the observed hexagonal lattice with only small energy differences when compared to analogous homopolymer structures. Data will also be presented showing how x-ray diffraction can be used to follow transesterification of copoly (HBA/HNA) in the nematic MelL

1H NMR investigation of some aromatic copolyester

AbstractThe microstructure of aromatic copolyesters prepared by bulk polymerization was investigated by means of 1H NMR spectroscopy. The aromatic region of the spectra appears to be sensitive to the sequence distribution of aromatic rings. Line shape simulation and resolution enhancement of the time domain spectra with a Gaussian window function allow a quantitative evaluation of the relative intensities. Sequence distribution in terms of triads and diads formed by aromatic rings was then derived and compared with the one calculated on the basis of a random distribution. The results show that the distribution, as expected for this class of copolymers, is random.

Structure hierarchy in liquid crystalline polymers

Structure models have been developed for the liquid crystalline polymers (LCPs), showing the existence of fibrillar hierarchies for both the lyotropic aramids and the thermotropic aromatic copolyesters. Hierarchies of structure have also been observed for biological materials. The nature of the smallest nanostructure that aggregates, typically microfibrils, and their interaction, are important in understanding the behavior of the material. This paper discusses the first application of scanning tunneling microscopy (STM) and field emission scanning electron microscopy (FESEM) to image the microfibrils in LCPs, in the 1-10 nm size range, resulting in a new LCP structural model.The structure model proposed earlier, was based on the study of Vectra® thermotropic LCP moldings and extrudates, and Vectran® and Kevlar® fibers. The model resulted from characterization by light microscopy, and transmission and scanning electron microscopy. Recent studies of similar fibers by STM and low voltage FESEM has provided additional insights. Details of single microfibrils and their aggregation into fibrils and macrofibrils was shown.

Processing of thermotropic liquid crystalline polymers and their blends—analysis of an in‐situ LCP composite system

AbstractThermotropic LCP/LCP fiber blends were prepared by a combination of meltblending and hot‐drawing, using a wholly aromatic copolyester KU‐9211 (also called K161 from Bayer A.G.) and an aliphatic containing LCP (liquid crystalline polymer) PET/PHB60 (from Kodak Tennessee Eastman). Morphological evidence, including scanning electron (SEM) and transmission electron microscopy (TEM), showed that the dispersed phase consisted primarily of highly oriented, 0.5 to 2 μm diameter rigid‐rods of aromatic fibers imbedded in a matrix of predominantly aliphatic LCP fibrils with diameters in the range of 20 to 50 nm. An interphase of approximately 50 nm strongly bonded the two phases together. The fiber blends were characterized using dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA), gas chromotography/mass spectroscopy (GC/MS), and rheological measurements. It appears that the processing conditions employed for melt blending had caused PET/PHB60 to undergo chain scission, thereby creating chemical interactions between the two LCP components during the melt blending process. Differential scanning calorimetry (DSC) thermograms as well as nuclear magnetic resonance (NMR) spectra of the extracted fraction from the mixture of 30 wt% K161/70 wt% PET(PHB60) confirmed the chemical interaction between the two thermotropic liquid crystalline polymers.

Structure/property relationships in LCP/PEI blends revealed with a new selective etching process

An etching technique for making visible the morphology of blends composed of polyetherimide and a liquid crystalline fully aromatic copolyester (LCP) has been developed. The four organic solvents-tetrachloroethane, N-methylpyrrolidone (NMP), dimethylformamide (DMF) and dimethylacetamide (DMAc)-selectively etch the polyetherimide without attacking the LCP component. By adjusting the etching conditions to the respective conditions prevailing within the samples, all the four solvents yielded equally good results across the entire concentration range investigated (from 20/80 to 80/20 percent by weight of LCP/PEI). The viscosith of the LCPs decreases markedly with increasing shear rate. The large shear rate gradient from the mould edges to the centre results in different deformations of different volume elements within the melt. The resulting frozenin morphologies can be visualized with the new etching technique. This allows one to explain certain correlations between morphology and mechnical properties of the respective samples.

Thermotropic Liquid Crystalline Aromatic Copolyesters with Flexible Spacers

AbstractTwo series of thermotropic liquid crystalline polymers were prepared from hydroquinone, methylhydroquinone, 2,5‐dichloroterephthaloyl chloride and four terephthalates with varying length substituents. Some of the properties of the terephthalates and polymers were characterized by IR, elementary analysis, differential scanning calorimetry (DSC), X‐ray diffraction and polarized hot‐stage microscopy.

Influence of chain flexibility on polymer mesogenicity

Molecular modelling procedures have been used to determine statistical parameters of isolated chains, with the objective of achieving a simple description which defines the tendency of a molecule to form a liquid-crystalline phase, its mesogenicity. Initial attention is focused on the persistence length parameter as the most promising in this context, while a related parameter, the turn-round length, is introduced to provide a description of mesogenicity which is equally applicable to molecules which consist of rigid segments separated by flexible sequences. A range of known mesogenic molecules has been modelled using Monte Carlo routines based on carefully determined bond-rotation potentials. For each molecule type a large number of chains are built for each temperature, and the chain parameters determined as averages over the models. Evidence is presented to suggest that for liquid-crystalline polymer molecules other than those containing flexible sequences, the nematic to isotropic transition temperatures occurs when the ratio of the persistence length to diameter (the persistence ratio) reaches a value of 5. The predictive possibilities of this criterion are explored in the estimation of the nematic–isotropic transition temperatures of one or two common mesogenic polymers which are too high to be accessed experimentally. It is also applied to polyethylene, giving a value of 150 K for the onset of liquid crystallinity, a transition which is of course not normally seen owing to crystallization. Modelling of a series of aromatic copolyesters which contain different lengths of flexible (alkane) sequences, shows that the critical persistence ratio, calculated at the experimentally observed transition temperature, drops from 5 to 2.5 when the flexible sequences are long enough to decouple the orientation of neighbouring rod segments, and from comparatively low-energy hairpin folds in the mesophase. The introduction of the turn-round length/diameter parameter is promising and appears to have a true predictive capability to cover series of molecules ranging from worm-like to jointed rigid rod (Kuhn)-like.

Thermal sequence randomization of a wholly aromatic copolyester having an ordered sequence

A wholly aromatic copolyester having an ordered comonomer sequence was prepared from 2,7-naphthalenediyl bis(4-hydroxybenzoate) and terephthalic acid following a method reported previously. This polymer was subjected to thermal treatment at 305°C under nitrogen atmosphere and the sequence randomization process was quantitatively analysed by 13C n.m.r. spectroscopy. The melting temperature of the original polymer was 290°C. According to the results of analysis for various triad formation, the ester bonds in the triad of terephthaloyl- p-oxybenzoyl-oxynaphthyleneoxy (TPN) sequence were found to undergo very fast exchange reactions at the treatment temperature. The formation of PPP triad was evident after 30 min at 305°C, whereas PP dyad was formed after only 10 min. Although the sequence changes occurred rather rapidly, complete randomization was not attained even after 1 h.

Liquid crystalline properties of aromatic copolyesters modified with bisphenol–A

Liquid crystalline properties of aromatic copolyesters modified with bisphenol–A

Blends of thermotropic polyester with poly(phenylene oxide)

AbstractA thermotropic liquid crystalline polymer (LCP) based on wholly aromatic copolyesters based on hydroxynaphthoic and hydroxybenzoic acid was melt‐blended with a thermoplastic poly(phenylene oxide) by corotating twin screw extruder. Rheological properties, temperature transitions, dynamic and mechanical properties, and electron microscopy study have been performed. Rheological study indicated significant viscosity reductions with increasing LCP content leading to ease of processing. From the differential scanning calorimeter (DSC) and dynamic mechanical thermal analyzer results, these blends showed incompatibility for the whole range of concentrations. Mechanical properties were found to be slightly improved at low LCP and dramatically improved at above 50% LCP contents. In addition, impact strength was significantly increased up to two times after adding 10% LCP into the matrix. The morphology of blends was affected by composition. Droplets and stubby fibrils structures caused lower tensile strength, whereas fibrillar structure improved this property.

Structure and properties of molded polyblends containing liquid crystalline polymers

AbstractThe relationship between the microstructure developed during injection molding of liquid crystalline polymers (LCPs) containing blends and their mechanical properties, was studied. A wholly aromatic copolyester LCP was melt blended in various levels with polycarbonate (PC), poly(butylene terephthalate) (PBT), Nylon 6 (N‐6), and amorphous nylon (AN). In all cases the LCP was the minor component. The resulting injection molded structure had a distinct skin core morphology, where elongated fibrous LCP particles comprised the skin layer and spherical and ellipsoidal ones composed the core section. The highest elongation and the finest diameter LCP fibrils were obtained with AN/LCP system, followed by PC/LCP. PBT/LCP blends showed a coarser morphology, while N‐6/LCP system did not correlate with the tensile moduli of the injection molded specimens. AN/LCP blends demonstrated the highest moduli values, consistent with the highest orientations observed using electron microscopy, followed by PC/LCP, PBT/LCP, and N‐6/LCP. Finally, tensile strength levels were correlated with both orientation levels and interfacial adhesion between the polyblend components. AN/LCP that exhibited the highest orientation and good adhesion appearance gave the highest tensile strength values followed by PC/LCP, PBT/LCP, and N‐6/LCP polyblends.

Compressional behavior of liquid-crystalline wholly aromatic copolyester fibers.

High performance fibers with high modulus and high strength have been reported to exhibit poor compressional properties. The present work is concerned with the axial compressional behavior of liquid crystalline wholly aromatic copolyester (ACPE) fiber (Ekonol type). ACPE fibers were embedded in 2-hydroxypropyl methacrylate and were compressed manually with a vise. Compressed ACPE fibers taken out from the resin block were examined by polarizing optical microscopy (POM) and scanning electron microscopy. In the initial stage of compression, kink bands are formed on the surface of ACPE fibers at an angle of ca 60° to the fiber axis. It was confirmed by POM that all molecular chains within the band were sharply bent from their original direction by an angle of 30-40°. It can be predicted that two slip systems, (110) 〈001〉 and (100) 〈001〉, are responsible for the formation of the kink band. With progressive compression, new kink bands were formed near the kink bands that were formed in the initial stage of the compression, indicating that compressional stress is concentrated at regions near the kink bands. When compressed further, massive lateral displacement occurred from place to place along fiber axis and finally ACPE fiber was almost squeezed out of shape. Intermicrofibrillar slip played an important role at this stage of compression.