Liquid Crystalline Polyesters Research Articles

Synthesis and characterization of thermotropic liquid crystalline polyester/multi-walled carbon nanotube nanocomposites

Thermotropic liquid crystalline polyester (TLCP) was synthesized via low-temperature solution polycondensation from 1,4-Bis(4-Hydroxybenzoyloxy)butane and terephthaloyl dichloride. Polymer nanocomposites based on a small quantity of multi-walled carbon nanotubes (MWNTs) were prepared by in situ polymerization method. The wide-angle X-ray diffraction (WAXD) results suggested that the addition of MWNTs to TLCP matrix did not significantly change the crystal structure of TLCP. The interactions between the molecules of the TLCP host phase and the carbon nanotubes were investigated through Raman spectroscopy investigations. We detected a distinct wave number shift of the radial breathing modes, confirming the carbon nanotubes interacted with the surrounding liquid crystal molecules, most likely through aromatic interactions (π-stacking). The interactions between liquid crystal host and nanotube guests were also evident from a polarizing microscopy (POM) study of the liquid crystal–isotropic phase transition in the proximity of nanotubes. The thermal properties and the morphological properties of the TLCP/MWNTs nanocomposites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). TGA data demonstrated the addition of a small amount of MWNTs into TLCP matrix could improve the thermal stability of TLCP matrix. DSC results revealed that melt transition temperatures and isotropic transition temperatures of the hybrids were enhanced.

Free-radical chain generation of ketene during the synthesis of liquid crystalline aromatic polyesters

At the high temperatures used for the preparation of liquid crystalline aromatic polyesters (LCPs), ketene cleavage occurred from acetoxyarenes. Ketene is known to oligomerize to colored oligomers which may be responsible for an undesirable yellow color in LCPs. Thermolysis of the model compound p-acetoxybenzoate gave ketene oligomers and methyl p-hydroxybenzoate, which oligomerized. A free-radical chain reaction mechanism for ketene formation was demonstrated by the reaction of tributyltin hydride with haloacetoxyarenes.

Synthesis and Characterization of Thermotropic Liquid Crystalline Polyesters/Glass Fiber by <i>In Situ</i> Process

Liquid crystalline polyesters containing central biphenylene moiety in the mesogenic unit with ester linkages to the different lengths of flexible spacers were synthesized using melt polycondensation reaction. Glass fiber, as a reinforce, was meltpolymerized with 4-acetoxybenzoic(ABA),4,4＇-diacetoxybiphenyl and terephthalic acid(TA) to make TLCPs with a good mechanical properties. All the obtained compounds were characterized by conventional spectroscopic methods. The thermal behavior of the polymers has been characterized using polarized optical microscopy and differential scanning calorimetry. The effect of monomer structure and mechanical properties has been investigated on the prepared polyesters and is also compared with that of the analogous polyester by other processing mode. Optical microscopy showed a highly threaded liquid crystalline texture to a high birefringent Schlierene texture characteristic of the nematic phase for all polyesters. DSC experiments were also found in accord with mesophase formation. The melting points of polymers having a central 4,4’-biphenyldicarboxylate residue have two broad endotherms. The glass transition temperature values of polyesters are not effect by the glass fiber. According to the SEM result showed that the composite had strong fiber-matrix adhesion. From the mechanical test result, it showed that TLCP are reinforced by rigid fiber further improvement of properties depending on fiber content, fiber fabric construction, fiber type. The mechanical properties of GF/TLCP had been improved greatly as the content of glass fiber was increased,but GF/TLCP mechanical properties do not improve remarkably as glass fiber content more than 30wt%.The effect of copolymerization with added glass fiber in synthesis process is compared with that of the anlogous homopolyesters by other processing mode. Our finding shows that the copolymers exhibit reduced technical process and maintained mechanical properties.

Transesterification-controlled compatibility and microfibrillation in PC–ABS composites reinforced by phosphorus-containing thermotropic liquid crystalline polyester

For polymer/liquid crystal polymer (LCP) blend systems, the in-situ fibrillation of LCP in polymer matrix can result in the self-reinforcement of polymer/LCP composites. How to control the microfibrillation of LCP in matrix is a key to enhance the mechanical properties of composites. In this paper, we investigated the transesterification-controlled compatibility and microfibrillation of phosphorus-containing thermotropic liquid crystalline polyester, poly( p-hydroxybenzoate- co-DOPO-hydroquinone ethylene terephthalate) (PHBDET) in the PC/acrylonitrile–butadiene–styrene copolymer blend (PC–ABS) during the melt processing. A standard mode and temperature-modulated differential scanning calorimetry (DSC and TMDSC) and 13C nuclear magnetic resonance ( 13C NMR) were used to investigate the transesterification and compatibility of PHBDET with PC–ABS. Microstructures, rheological and mechanical properties of the composites were also studied via scanning electron microscopy (SEM), dynamic rheological measurement and universal material testing machine. The results showed that the extent of transesterification could influence the compatibility of PHBDET with PC–ABS, and could be controlled by processing temperature and time. The improved compatibility was not always favorable for the microfibrillation of PHBDET in PC–ABS, but a certain extent of transesterification showed a positive influence on the tensile properties of the composites. Therefore, there existed an optimal extent of transesterification, in which the composite could show a good balance of compatibility and tensile properties.

Synthesis and characterization of a thermotropic liquid crystalline hyperbranched polyester

BACKGROUND: Hyperbranched polymers have received increasing attention in the fields of medicine, homogeneous catalysis and materials science. Hydroxyl-functional aliphatic polyesters are one of the most widely investigated families of hyperbranched polymers. The research reported here is based on the preparation of a novel hyperbranched polyester and the modification of its terminal hydroxyl groups by biphenyl mesogenic units. RESULTS: 2,2,6,6-Tetramethylolcyclohexanol as a core and 8-[4′-propoxy(1,1-biphenyl)yloxy]octanoic acid as a mesogenic unit were synthesized. A hyperbranched polyester (HPE) was synthesized in one step and subsequently substituted by reaction of its terminal hydroxyl groups with the biphenyl mesogenic units to yield a novel liquid crystalline hyperbranched polyester (HPE-LC). The chemical structures of all compounds were confirmed using Fourier transform infrared, 1H NMR and 13C NMR spectroscopy. The thermal behavior and the mesogenic properties of the biphenyl mesogenic unit and HPE-LC were investigated using differential scanning calorimetry, polarized optical microscopy and wide-angle X-ray diffraction. The results demonstrated that the degree of branching of the HPE is ca 0.63. Both HPE-LC and the biphenyl mesogenic unit exhibit mesomorphic properties, but HPE-LC has a lower isotropic transition temperature and a wider transition temperature range than the biphenyl mesogenic unit. CONCLUSION: A novel liquid crystalline hyperbranched polyester was successfully synthesized, which exhibits mesomorphic properties. This polymer has good solubility in highly polar solvents and good thermal stability. Copyright © 2009 Society of Chemical Industry

Liquid crystalline matrix polymers for aramid ballistic composites

AbstractFive semi‐flexible thermotropic liquid crystalline (LC) polyesters and poly(ester‐amide)s were synthesized and used as matrix resins for Twaron™ aramid‐based ballistic fabrics. The ballistic performance was investigated as a function of the neat resin content. For the most successful liquid crystalline polyester system, the effect of blending with styrene‐ethylene‐butylene‐styrene (SEBS) and polyvinyl butyrate (PVB) rubber was also explored. The best neat resin V50 values were obtained for 20 wt% LC polyester (LCPE)‐Twaron™ composites, that is, 418 m.s−1, whereas SEBS/LCPE and PVB/LCPE modifications resulted in maximum V50 values of 460 and 466 m.s−1, respectively. It was found that the ballistic impact resistance is strongly influenced by the elastic modulus of the resin component and to a lesser extent to the level of adhesion between the resin and fabric. The effect of resin content, resin strength, elongation‐at‐break, and resin toughness on the ballistic impact resistance was found to be small. The best ballistic protection could be obtained when the Young's modulus of the LC resin was in the range of 0.01–1 GPa. This result seems to be in agreement with existing inter‐yarn friction models. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Spontaneous deformation of main-chain liquid-crystalline elastomers composed of smectic polyesters

We observe the spontaneous shape change of a uniaxially deformed liquid-crystalline elastomer composed of smectic main-chain liquid-crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracts by about 115% on heating up to the isotropic phase, the sample length recovers by 55% on cooling to room temperature in the first heating–cooling process, and the elastomer exhibits an almost complete reversible deformation in the second heating–cooling process. By a comparison of the results of sample observation with those of X-ray analysis, we recognise that the strain λ was linearly coupled with the orientational order parameter S. In addition, the results of the X-ray analysis imply that a cybotactic nematic state, in which smectic clusters lie scattered in a nematic-like matrix, emerges after exposure to the isotropic phase.

Molecular engineering of photoactive liquid crystalline polyester epoxies containing benzylidene moiety

AbstractA series of photoactive liquid crystalline polyester epoxies incorporating bisbenzylidene segments as photoactive mesogenic cores were synthesized by polyaddition of diepoxy monomers and terephthalic acid/trimesic acid. To investigate the influence of structural parameters such as, molecular architecture, structural rigidity of mesogenic unit and substituents on thermal, mesogenic, and photoactive properties, the bisbenzylidene segment was incorporated into one acyclic and two cycloalkanone units with two and four substituents, respectively in both linear and hyperbranched architectures. Degree of branching of hyperbranched polymers was found to be in the range of 0.49–0.62. All polymers exhibited nematic mesophase (nematic droplets). Photo induced (2π + 2π) cycloaddition reaction, upon exposure to light at 365 nm, was examined. Inter molecular photocycloaddition was confirmed by photoviscosity measurement of UV irradiated polymer solutions. Faster photo induced reactivity of polymers in hyperbranched architecture was observed when compared to linear structure. Acyclic units facilitated photocycloaddition, and five‐membered ring showed higher photoactivity compared to six‐membered ring. The steric hindrance caused by substituents decreased the photoactivity of polymers. Refractive index change was found to be in the range of 0.015–0.024. Substantial variation of refractive index indicates that these polymers could be used for optical recording. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7637–7655, 2008

Callimatic and T-shaped liquid crystalline compounds with fluorinated terminal groups

A series of liquid crystalline polyesters with a rigid T-shaped or a stick-like mesogenic fragment and fluorinated terminal groups of various structures was synthesized and studied by means of polarization optical microscopy, differential scanning calorimetry, and IR and 1H NMR spectroscopy. It was shown that the effect of the fluorinated group on the thermostability of the mesophase depends on the shape of the mesogenic fragment. The thermostability of the mesophase in callimatic liquid crystalline compounds with perfluorinated terminal groups is higher and in those with difluoromethyl terminal groups containing one hydrogen atom is lower compared with their alkyl analogs. This is connected with the possibility of weak hydrogen bonding that destroys the liquid crystalline order. The thermostability of the mesophase in compounds with a T-shaped mesogenic fragment and any fluorinated terminal groups is always higher compared with their alkyl analogs.

Novel thermotropic esters‐based polymers with broad nematic processing windows

AbstractIn this study, a new series of semiflexible liquid crystalline (LC) polyesters and poly(ester‐amide)s were synthesized and characterized. Polymers based on 4‐hydroxybenzoic acid (4‐HBA), 6‐hydroxy‐2‐naphthoic acid (HNA), suberic acid (SUA), and sebacic acid (SEA) were modified with hydroquinone (HQ) and different concentrations of 4‐acetamidophenol (AP) to obtain a polyester and two poly(ester‐amide)s, respectively. All polymers were successfully prepared using conventional melt‐condensation techniques. The polymers were characterized by inherent viscosity measurements, SEC, hot‐stage polarizing microscopy, DSC, and TGA. The mechanical behavior was investigated using DMTA and tensile testing. All linear polymers have Tgs in the range of 50–80 °C and melt between 120 and 150 °C. Our polymers display good thermooxidative stabilities (5% wt loss at ∼ 400 °C) and exhibit homogeneous nematic melt behavior over a wide temperature range (ΔN ∼ 250 °C). The liquid crystal phase was lost when high concentrations of nonlinear monomers such as 3‐HBA (>27 mol %) and resorcinol (RC) (>23 mol %) were incorporated. The LC polyester based on 4‐HBA/HNA/HQ/SUA/SEA could easily be processed into good quality films and fibers. The films display good mechanical properties (E′ ∼ 4 GPa) and high toughness, that is, ∼ 15% elongation at break, at room temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6565–6574, 2008

Fullerene C60 as a stabilizer in the synthesis of liquid-crystalline polyesters

It has been shown that owing to the use of fullerene C60 (∼0.01 wt % based on the monomer weight) as a stabilizer in the synthesis of a liquid-crystalline (LC) polyesters via high-temperature acceptorless poly-condensation, the temperature interval of reaction may be widened to 220°C and the crosslinking side reactions may be eliminated.

Dipole architecture of molecules and mesomorphic behavior of liquid crystals with rigid T-shaped mesogenic fragment

A number of liquid crystalline polyesters having a rigid T-shaped mesogenic fragment and differing in dipole architecture were synthesized and examined by polarizing optical microscopy, differential scanning calorimetry, and IR and 1H NMR spectroscopy. The thermal stability of the mesophase in the given series of compounds was shown to increase with extension of arms in the T-shaped mesogenic fragment, as well as on replacement of the terminal ester groups (COOAlk) therein by ether moieties (OAlk). Such replacement also enhances smectogenic properties.

Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene) cycloalkanone units

AbstractPhotoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene)cycloalkanone moieties were synthesized and characterized by structural, thermal, mesomorphic, and optical measurements. The bis(benzylidene) cycloalkanone chromophores in the main chain can constitute both as a mesogen and photoreactive center, whereas 1,3,4‐oxadiazole is a well‐known fluorophore. The thermal properties of polymers were found to be inversely proportional not only to the spacer length but also to ring‐size of cycloalkanones. Hot stage polarized optical microscopic investigations displayed enantiotropic nematic liquid crystalline phases and development of grainy to schlieren textures depends on the length of flexible spacer in the polymer backbone which was in accordance with DSC analysis. Both photoisomerization and photodimerization are observed from the absorption spectra and discussed. The fluorescence spectra in solution state at various concentrations showed that the polymers show blue‐emission maxima and the Stokes shifts being 48–49 nm. The energy transfer occurred when increasing the concentration of the solution. The band gap energies calculated from the absorption spectra are in the range of 3.17–3.41 eV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5760–5775, 2008

Thermotropic liquid crystalline polyester nanocomposites via in situ intercalation polycondensation

AbstractA thermotropic liquid crystalline polyester (TLCP)/organoclay nanocomposite was synthesized via in situ intercalation polycondensation of diethyl‐2,5‐dihexyloxyterephthalic acid and 4,4′‐biphenol in the presence of organically modified montmorillonite (MMT). The organoclay, C18‐MMT, was prepared by the ion exchange of Na+‐MMT with octadecylamine chloride (C18‐Cl−). TLCP/C18‐MMT nanocomposites were prepared to examine the variations of the thermal properties, morphology, and liquid crystalline phases of the nanocomposites with clay content in the range of 0–7 wt%. It was found that the addition of only a small amount of organoclay was sufficient to improve the thermal behavior of the TLCP hybrids, with maximum enhancement being observed at 1 wt% C18‐MMT. Copyright © 2008 John Wiley & Sons, Ltd.

The photochemistry of some main chain liquid crystalline 4,4′-stilbene dicarboxylate polyesters

Several aspects of the photochemistry and photophysics of four main chain liquid crystalline polyesters with a rigid trans-stilbene 4,4'-dicarboxylate mesogen as chromophore and flexible spacer groups are reported. The three polymers with the longest 'spacer' groups are liquid crystalline at room temperature, two have smectic phases. Chromophore aggregation has a dramatic effect on the photophysics and photochemistry of these polymers. Each of the polymers in poor solvents or as films has greatly perturbed UV-Vis absorption and fluorescence spectra due to aggregation of the stilbene chromophore. These effects are more pronounced upon annealing above the glass transition temperature, T(g), and in the mesophase. Film fluorescence is excitation wavelength dependent and is suppressed at elevated temperatures. The stilbene 'environment' in both films and solution is clearly heterogeneous and energy transfer processes relatively slow. The dominant photochemical reaction upon direct excitation above 300 nm is 2 + 2 photocycloaddition rendering polymer films insoluble. No significant trans-to-cis photoisomerization can be detected upon initial irradiation of the polymer films. There is evidence for the formation of aldehyde and carboxylate functionality upon irradiation in the presence of air. Loss of the aggregate UV-Vis absorption and fluorescence occurs during irradiation. Difference UV-Vis spectra of irradiated films suggest preferential initial consumption of dimeric aggregates. Loss of stilbene UV-Vis absorption upon irradiation above 300 nm can be partly photoreversed upon subsequent 254 nm irradiation. The rate of stilbene chromophore loss from films increased significantly above Tg and in the smectic phase above room temperature.

Unusual Formation of Smectic A Structure in Cross-Linked Monodomain Elastomer of Main-Chain LC Polyester with 3-Methylpentane Spacer

The main-chain BB/PDA-5(3-Me) polyester containing a photo-cross-linkable p-phenylenediacrylate (PDA) moiety was synthesized and cross-linked in the transparent monodomain smectic CA (SCA) state by UV irradiation. The monodomain SCA elastomer contracted to about 60% of the original length on heating up to the isotropic phase and recovered only 5% of the original length at the liquid crystallization on cooling. Irrespective of such a small recovery in the length, the liquid crystal was well formed with a high orientation, but the formed smectic structure was of a smectic A (SA) type with the layer spacing 2 times larger than that of the original SCA phase. By elongation of this contracted elastomer, the SA structure transformed to the original SCA one, showing a quasi-plateau in the stress–strain curve. The connection between the structural transformation and the stress–strain behavior lets us propose an unsusal SA structure in which the polymer chain folds every two repeat units. Such a characteristic fol...

機能性高分子材料の最新技術動向 液晶ポリマー

Liquid crystalline polyester (hereafter designated as LCP) represents peculiar physical properties and provides excellent injection moldability for accurate electro and electronic parts, such as connector, relay, bobbin, etc., due to its intrinsic liquid crystallinity.Recently the market of commercial LCP has been rapidly growing and has shown almost 15% growth of the shipment volume by making good use of the excellent performance. In the future the market is expected to expand more and more rapidly because miniaturization and thin wall moldability of electronic parts still more proceed in the front electro and electric parts fields.In this review we will proceed from generalities into particulars on liquid crystalline polyesters. The latest situation of research and development of commercial LCP will be reported through not only molecular design and physical properties of our typical LCP product but also introduction of our newly developed isotropic fabrication method such as solvent-casting, micro powder compression, and nano-scale fiber non-woven cloth.

Improvement of Adhesion Between Liquid Crystalline Polyester Films by Plasma Treatment

Surface modification of thermotropic liquid crystalline aromatic polyester (LCP) films was carried out by low-pressure plasma treatment to improve the initial adhesion as well as the long-term adhesion reliability, a measure of durability between the LCP films used as substrates for printed circuit boards. Plasma irradiation was carried out in various plasma gases with different plasma modes such as reactive-ion-etching, and direct-plasma (DP) with pressures ranging from 6.7 Pa to 26.6 Pa. The introduction of polar groups on the film surface such as phenolic hydroxyl groups and carboxyl groups enhanced the initial adhesion by increased chemical interaction. However, if the concentration of polar groups became too high, the longterm adhesion reliability estimated by the pressure cooker test was degraded due to the acceleration of the penetration of water molecules into the interface. A large surface roughness was also effective in preventing the decrease in the long-term adhesion reliability. However, too much increase in surface roughness decreases the long-term adhesion reliability. The DP-treatment in the O2 atmosphere at a gas pressure of 6.7 Pa was found to be the best plasma condition for both the initial adhesion as well as the long-term adhesion reliability between the LCP films.

Improvement of adhesion and long‐term adhesive reliability of liquid crystalline polyester film by plasma treatment

AbstractSurface modification of thermotropic liquid crystalline aromatic polyester (LCP) films was examined by low‐pressure plasma treatment to improve initial adhesion and long‐term adhesive reliability between the LCP film and an epoxy bonding sheet for printed circuit boards. Plasma irradiation was carried out in various plasma gases with different plasma modes as reactive‐ion‐etching (RIE), direct‐plasma (DP) with pressures ranging from 4.0 to 26.6 Pa. Although the initial adhesion of the DP‐treated film increased, the long‐term adhesive reliability estimated by pressure cooker test (PCT) decreased with decreasing the plasma gas pressure in every gas. The higher concentration of the generated polar groups such as phenolic hydroxyl group and carboxyl group enhanced the initial adhesion by the increase in the chemical interaction; however, it damaged the long‐term adhesive reliability due to the acceleration of the penetration of water molecules into the interface. The large surface roughness was effective to enhance the initial adhesion and the long‐term adhesive reliability. The RIE‐treatment generated the polar groups and the larger surface roughness than the DP‐treatment. The RIE‐treatment in the O2 atmosphere at the gas pressure of 13.3 Pa was the best plasma condition for both the initial adhesion and the long‐term adhesive reliability. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Effect of molecular architecture and size of mesogen on phase behavior and photoactive properties of photoactive liquid crystalline hyperbranched polyester epoxies containing benzylidene moiety

AbstractA series of photoactive liquid crystalline linear and hyperbranched polyester epoxies were synthesized by polyaddition of photoactive bis benzylidene alkanone diol monomers and terephthalic acid and trimesic acid respectively with good yield. The effect of molecular architecture (linear and hyperbranched), size of mesogenic unit (cyclic and acyclic units) on the physicochemical, thermal, mesogenic, and photoactive properties of hyperbranched polymers were studied and compared. Degree of branching of hyperbranched polymers was found to be in the range of 0.46–0.49. Monomers containing cyclic moieties only exhibited nematic mesophase, while all polymers exhibited typical nematic mesophase. Intermolecular photo cycloaddition reaction was studied by ultraviolet–visible spectra (UV–vis) and NMR spectroscopy and photo viscosity measurement of UV irradiated polymer solutions. Faster photo induced behavior of hyperbranched polymers containing acyclic alkanone moiety, as compared to polymers containing cycloalkanone moieties, was observed. The change in the refractive index was found to be in the range of 0.02–0.024. Substantial variation of refractive index indicates that this polymer could be used for optical recording. All the polymers were also found to be fluorescent in nature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 552–563, 2008