Liquid Crystal Polyesters Research Articles

New liquid crystal polyethers and polyesters based on diphenylbutadiene mesogens

AbstractNew liquid‐crystalline (LC) polymers and small molecules have been synthesized based on the 1,4‐diphenylbutadiene mesogen, a chromophore with χ(3) nonlinear optical properties. Polyethers were formed by phase‐transfer reaction, and polyesters were prepared by Schotten‐Bauman polymerization from these 4,4'‐bisphenolic mesogenic structures. Liquid crystallinity was examined using differential scanning calorimetry (DSC) and optical microscopy. Most of the model compounds were found to be nematic. All mesomorphic polymers were nematic and displayed LC ranges of up to 50°C as well as a wide diversity of melting points. The melting points varied with the flexible (methylene) spacer length and were strongly dependent upon the presence of substituents in the mesogenic core. Crosslinking and cis‐trans isomerization were observed under the influence of heat and ultraviolet radiation. © 1992 John Wiley & Sons, Inc.

Miscibility and crystallization kinetics in blends of poly(ethylene terephtalate) and thermotropic liquid crystal polyesters

AbstractBinary blends of poly(ethylene terephtalate) (PET) and thermotropic liquid crystal polyester (TLCP) have been prepared by both solution and melt blending methods. The TLCPs utilized were Vectra (Hoechst Celanese), TR‐4, a TLCP synthesized in our laboratory, and a block copolymer consisting of three TR‐4 units followed by three PET units. The phase behavior of the blends was studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and optical microscopy. The results show that none of the blends is miscible, but significant interactions exist between the PET phase and the TLCP phase in the case of TR‐4 and TR‐4 block copolymer blends. These interactions lead to a different nucleation mechanism in these blends compared to that in PET/Vectra.

Main-chain side-chain liquid crystal polymer blends for improved physical properties

Previous work has demonstrated that compatible blends of main-chain liquid crystal polymers (LCP) with side-chain LCP's can be prepared. The present work was carried out to extend the scope and application of this finding to liquid crystal polyesters, similar to commercially available LCP's and to demonstrate the effect of the blend composition upon the physical and mechanical properties. A series of melt-processable main-chain polyesters was prepared as well as a series of acrylic polymers and copolymers which possess a side-chain mesogenic unit, similar to that of the main-chain LCP's. While the results of the blending studies of these high molecular weight materials were somewhat ambiguous as to compatibility, there are strong indications that the presence of the side-chain mesogen containing polymers does result in an improvement in the overall mechanical properties of the main-chain LCP.

Thermal transitions of liquid crystal polyesters: Poly(decamethylene-4,4?-diphenoxy terephthalate)

The thermal transitions of liquid crystalline poly (decamethylene-4,4′-diphenoxy terephthalate) have been studied by differential scanning calorimetry, X ray diffraction and thermoptical microscopy. At room temperature a three dimensional order and a smectic mesophase coexist. The crystal-smectic mesophase transition occurs at 240°C and at higher temperature, 250–260°C, a transition from smectic C to nematic has been observed, before the isotropic transition at 290°C. The no previously known smectic C-nematic transition has been detected by microscopy, observing bars and striated textures, and by X ray diffraction patterns obtained in the whole range of temperatures.

Structural changes through phase transitions in liquid-crystal polymers: 1. A spectroscopic study of polyesters

Previous work on lattice dynamics and vibrational spectra of disordered chain molecules has allowed the identification of spectroscopic signals characteristic of the molecular microstructure. These data are used for the identification from the infra-red spectrum of the structural changes with temperature of a ‘symmetric’ liquid-crystal polyester and of a few model compounds. The KS phase transition is prepared by the introduction of GTG′ defects in the flexible spacers in a temperature range well inside the K phase. The S phase is characterized by the generation of a larger concentration of GTG′ conformational kinks while no GG defects are observed. Thus the flexible chain does not coil in a ‘liquid-like structure’. The same structural situation is observed for the I phase. These results have interesting implications for the description of the structural changes of this polymer molecule through phase transitions. A mechanism for the transition is proposed.

Rheological and structural features of liquid crystal polyalkylene fumaroyl- bis-4-oxybenzoates

The rheological, thermodynamic, and structural properties of liquid crystal polyesters, i.e. polyhexa- and polydecamethylene fumaroyl- bis-4-oxybenzoates and of the statistical copolymer based on them are studied. All these polymers form a smectic phase in the liquid crystal region. The transition from homo- to copolymer results in widening of the region over which the liquid crystal phase exists. A feature of the copolyester is the high sensitivity of its structure to the thermal prehistory in the melting region. In this case protracted annealing results in a fundamental rearrangement of the copolymer structure, beginning from the melt, and indicated by a sharp increase of viscosity with time. When the shear stresses are large a high degree of molecular orientation along the flow direction is realized, the smectic layers being located perpendicular to this direction. In the melting region the existence of a metastable network with crystalline lattice sites is superimposed on the special features of the flow of the homo- and copolyesters. The structure of the elementary cell of the crystallites depends on the composition and thermal prehistory.

Processing and Properties of Liquid Crystal Polyesters

Processing and Properties of Liquid Crystal Polyesters

Fourier transform Raman spectroscopic study of main-chain thermotropic liquid crystalline polyesters

The Fourier transform infrared and Raman spectra of the semi-flexible main-chain thermotropic liquid crystal polyester, poly(heptamethylene terephthaloyl-bis-4-oxybenzoate) are presented, and tentative band assignments given. The polymer is investigated as a function of its thermal history, and Fourier transform Raman spectra are recorded with temperature using a high-temperature cell. The results are discussed along with calorimetric and X-ray scattering data. Spectral evidence is presented which is related to both the sample crystallinity, and the crystal-to-liquid crystal phase transition.

Study and control of phase morphology in liquid crystal polyester–poly(alkylene terephthalate) blends

AbstractScanning electron microscopy (SEM) reveals that the morphologies of poly(hexamethylene terephthalate) (PHMT) and poly(p‐oxybenzoate co‐ethylene terephthalate) (LCP6/4) blends are biphasic. When LCP6/4 is the minor component, liquid crystalline domains are embedded in the matrix. The reaction in the melt between the polymers apparently increases the adhesion of the matrix to the embedded domains. As the reaction proceeds, the chemical composition of the matrix progressively becomes modified. High reaction temperatures, small particle size, mixing coupled with particle dispersion and high LCP6/4 content in the mixture promote the melt reaction and decrease the PHMT content in the polymer formed. At 260°C, the coaddition of di‐n‐octadecyl phosphite (DNOP) at a 2% weight fraction with PHMT and LCP6/4 inhibits the reaction between the polymers for 2 h.

Liquid crystal polymer‐carbon fiber composites. Molecular orientation

AbstractThe objective of this work has been to study composite systems in which carbon fibers are dispersed in a liquid crystal polymer matrix. The fundamental point of interest here has been the interfacial response that fiber surfaces can potentially induce in self‐ordering polymers. The matrix material used was a thermotropic liquid crystal polyester synthesized in our laboratory from the monomers p‐acetoxybenzoic acid, diacetoxyhydroquinone, and pimelic acid. The aromatic‐aliphatic polymer was characterized by NMR as a chemically disordered polymer of the three structural units which exhibits a nematic phase at temperatures above 150°C. Breadline proton NMR above the solid to liquid crystal transition was used to measure the rate of magnetic alignment of molecules in the matrix and polarized optical microscopy was used to analyze interfacial zones in composite samples. Fiber surfaces were found to influence the orientation and orientational dynamics of a liquid crystal polymer matrix. This was revealed by enhanced rates of magnetic orientation in the polymer melt when carbon fibers are dispersed in the medium. Fiber surfaces were also found to stabilize nematic ordering of the polymer as the melt was heated towards complete isotropization. The phenomena discovered here may originate in the development of zones around fibers with a common molecular orientation anchored by the carbon surface.

Thermal properties of some fully aromatic thermotropic liquid crystal polyesters

AbstractWholly aromatic liquid crystalline main chain polyesters derived from terephthalic acid, phenyl‐ or (1‐phenylethyl)hydroquinone modified with either 3,4′‐ or 4,4′‐dicarboxydiphenylether and p‐hydroxybenzoic acid, have been prepared by acidolysis and thermally investigated. All prepared polyesters exhibit excellent thermal stability up to about 400°C, however, the (1‐phenylethyl)hydroquinone polyesters generally showed lower stability. Melting points could be decreased to around 200°C without any decrease in the thermal stability or the nematic range.

サーモトロピック液晶ポリマー同士のポリマーブレンド

サーモトロピック液晶ポリマー同士のポリマーブレンド

The Effect of Chain-kinks on the Phases and Structure of a Series of Thermotropic Liquid Crystal Polyesters Prepared by a Novel Non-aqueous Dispersion Route

A series of aromatic polyesters have been synthesized by a non-aqueous dispersion route, comprising monomer residues of x parts p-hydraxybenzoic acid and (1- x)/2 parts of each of hydroqzcinone and isophthalic acid. The nascent polymers are in a novel particulate form with a molecular organization that differs front melt-processed polymer. By decreasing x, increasing amounts of kinked meta phenyl units are intro duced into an otherwise para-phenyl chain. The phase diagram deduced from DSC and hot stage microscopy shows the series spans frorn isotropic to liquid crystalline behaviour with an interesting borderline region. Three crystal forms are observed. One is associated with cooling from the mesophase and contains the kinked isophthalic acid units; this exhibits a relatively high Enthalpy of Fusion. Mouldings from the series show a gradual change in modulus with x.

Infrared study of phase transitions in thermotropic liquid crystal polyesters

AbstractThree samples of semiflexible thermotropic liquid‐crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal‐to‐liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal‐to‐isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy techniques.

Synthesis of Segmented Anisotropic-Isotropic Poly(Aryl Ester)Poly(Arylene Ether) Copolymers

AbstractCarboxyl functional poly(arylene ether) oligomers of controlled number average molecular weight were synthesized via nucleophilic aromatic substitution step polymerization using p-hydroxy benzoic acid as the molecular weight and end group controlling reagents. The subsequent functionalized polyarylene ethers were copolymerized with acetoxy benzoic acid or acetoxyphenoxy benzoic acid via melt acidolysis procedures. Successful copolymers were generated using chlorobenzene as a solvent for the first s age of the reaction followed by melt reaction at temperatures as high as 325°C. Resulting copolymer composition and segment size were investigated. It was possible to prepare improved solvent resistant copolymers through incorporation of the liquid crystal polyester (LCP) segment. Extraction tests using boiling chloroform showed that a high percentage of segmented copolymer were generated. Swelling characteristics were also noted and were observed to decrease as a function of the LCP concentration utilized. Multiple transitions were observed in differential scanning calorimetry consistent with Tg's, Tm's and possibly with liquid crystal transitions. Optical microscopy showed an ordered microstructure developed which is consistent with the presence of liquid crystalline phases. The materials were successfully compression molded to afford tough coherent films at all compositions, implying the likelihood of improved compressive strength relative to the LCP homo- or copolyesters.

Compatibility of blends of poly(butylene terephthalate) and liquid crystal polymers: a DSC study

A calorimetric study of blends of poly(butylene terephthalate) (PBT) with two liquid crystal polyesters, one aliphatic-aromatic poly(biphenyl-4,4'-ylene sebacate) (PB8) and the other wholly aromatic (Vectra B 950), has been made for the prime purpose of obtaining information on the interphase. The dependence of the transition temperatures and enthalpies on blend composition, together with examination of the chemical interactions occurring at high temperature at the interface, showed that the compatibility between PBT and Vectra is much lower than that between PBT and PB8. Electron scanning microscopy confirmed this conclusion.

Interfacial effects on the viscosity and the morphology of a thermotropic liquid crystalline polymer

The apparent viscosity of nematic liquid crystal polyester measured by using capillary-tube viscometers showes a dependence on tube diameter and a yield stress in the low shear rate region called Region I. When the wall thickness measured by SEM is subtracted from the tube diameter, the viscosity becomes independent of tube diameter but still showes the Region I. This behavior may be caused by the molecular orientation at the wall of the capillary-tube.

The relationship of chain linearity of aromatic liquid-crystal polyesters to molecular orientation and stiffness of mouldings

Three aromatic liquid-crystal polyesters have been synthesized from monomer residues of x parts p-hydroxybenzoic acid and 1 2 (1 − x) parts each of isophthalic acid and hydroquinone, with x = 0.15, 0.27 and 0.36. Chain linearity therefore increased with x. They were injection-moulded into tensile bars which had axial moduli that were found to increase with x. The bars were sectioned to reveal the oriented layer structure formed by the injection process. The molecular orientation within the layers was characterized by X-ray diffraction and then input into the Ward aggregate model to calculate predicted moduli for comparison with experimentally measured moduli. The analysis shows that, for higher x, the increase in modulus is mainly a consequence of the increase in molecular orientation within the layers.

Pressure-induced phases in a thermotropic polyester

Abstract : Studies of a copolyester of 20% hydroxybenzoic acid, 40% isophthalic acid and 40% hydroquinone, a mesomorphic polymer, were carried out under pressure. The phase behavior was characterized by the x-ray diffraction, thermal analysis and high pressure DTA techniques. It was found that pressure induced a new crystal habit in the solid and a new mesophase in this polymer. All thermal transitions have been summarized in a proposed phase diagram. The finding of pressure induced mesomorphism confirms the possibility of extending the range of polymers which might exhibit liquid crystallinity. Keywords: thermotropic Liquid Crystal Polyester, Thermal Analysis, Pressure induced, Mesomorphism.

Jacketing of Optical Fibers with Ordered Thermotropic Liquid Crystal Polyesters

Abstract Thermotropic liquid crystal polyesters (LCPs), based on improved polyethylene terephthalate (PET) and para-hydroxybenzoic acid (PHB), are investigated as a secondary jacketing material for enclosing optical fibers. Because of shear- and deformation-induced molecular orientations produced during extrusion, the newly developed LCPs exhibit low thermal expansion coefficients and high Young's moduli. As a result, the LCP-jacketed optical fibers show small loss increases in the −60°C to 80°C temperature range and high lateral load resistance, compared with conventional nylon-jacketed optical fibers. Furthermore, the low melt viscosity and significantly low thermal shrinkage allow us to coat the LCPs on the optical fibers at high speeds above 300 m/min.