Thermotropic Liquid Crystalline Polyesters Research Articles

Short aramid fiber reinforced thermotropic liquid crystalline polyester composites: Fabrication, thermal, and mechanical properties

We conducted a study to analyze the impact of short aramid fibers (AFs) on the melt-rheological behavior, thermal transition, thermal stability, and mechanical durability of thermotropic liquid crystal polyesters (TLCPs). By using different AF loading contents ranging from 3–15 wt%, we produced TLCP matrix composites through masterbatch-based melt-compounding and injection-molding. The SEM images and FT-IR spectra demonstrate that the AFs are dispersed in the TLCP matrix with a microfibrillar structure through good interfacial adhesion caused by specific intermolecular interactions between the TLCP and AFs. As a result, the complex viscosity, shear storage/loss moduli, and thermal transition (melt-crystallization, glass transition, and melting) temperatures of the composites increase with increasing AF filler content. However, the melt-crystallization and melting enthalpies increase only at low AF loading contents of 3–5 wt%. At high AF contents of 7–15wt%, the enthalpies decrease owing to the partial aggregation of AF fillers. The thermogravimetric analysis proves that the thermal stability of TLCP/AF composites improves when the AF filler is introduced. The dynamic mechanical analysis using the stepped isothermal method shows that the addition of 5 wt% AF to the TLCP leads to an approximately 150% improvement in elastic moduli and long-term mechanical durability at elevated temperatures.

Effects of Mesogenic Cycloaliphatic Units on the Microstructure and Properties of Thermotropic Liquid Crystalline Polyesters

AbstractWe report the synthesis and structure‐property relationship of new thermotropic liquid crystalline polyesters (TLCPs) with different mesogenic cycloaliphatic unit content in the mainchain. For this purpose, a series of TLCPs composed of three different repeating units based on 4‐hydroxybenzoic acid (HBA, 75.0 mol%), 2‐hydroxy‐6‐naphthoic acid (HNA, 25.0–15.0 mol%), and terephthalic acid/1,4‐cyclohexanedimethanol (TPA/CHDM, 0.0–10.0 mol%) were synthesized via two‐step bulk polymerization. The structure, thermal and mechanical properties of the TLCPs were investigated by considering the effects of size and content of TPA/CHDM‐derived repeating units, in comparison to HNA‐based repeating units. Infrared spectroscopic data showed that TPA/CHDM‐derived repeating units were successfully incorporated into HBA/HNA‐based TLCPs. The polarized optical and electron microscopic images revealed the existence of thermotropic liquid crystalline features and associated fibrillar structures for all TLCPs. The melting temperatures of the TLCPs decreased with increasing the TPA/CHDM‐based repeating units, whereas the glass transition temperature increased, which is due to the trade‐off effect of the segmental mobility restriction in the amorphous phase and the destruction of the thermotropic liquid crystalline phase by the introduction of TPA/CHDM‐derived repeating units in the TLCP mainchain. In addition, the maximum elastic storage modulus was attained for the TLCP with 2.5 mol% TPA/CHDM‐derived repeating units.

Synthesis and structure-property relationship of thermotropic liquid crystalline polyesters containing thiophene-aromatic and cycloaliphatic moieties

ABSTRACT We report the structure and property characterization of a series of thermotropic liquid crystalline polyesters (TLCPs) composed of three different repeating units based on 1,4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), and 2,5-thiophenedicarboxylic acid/1,4-cyclohexanedimethanol (TDCA/CHDM), which are synthesized through two-step melt polymerization. The effect of the TDCA/CHDM-based repeating unit (0.0–10.0 mol%) on the structure, thermal and mechanical properties of the TLCPs with 75.0 mol% HBA and 25.0–15.0 mol% HNA were investigated. The polarized optical and electron microscopic images confirmed the presence of thermotropic liquid crystalline and fibrillar structures for all synthesized TLCPs. The glass transition and melting temperatures as well as the crystallinity index of the TLCPs were found to increase with increment of the TDCA/CHDM unit up to 5.0 mol% due to the improved interchain packing, but they decreased at higher amounts of 7.5–10.0 mol% TDCA/CHDM units owing to the enhanced flexibility by the cyclohexylene ring and the mismatching in the length between HNA and TCDA/CHDM units. In addition, there was a crystal lattice transition of TLCPs from pseudo-hexagonal to orthorhombic phases. Although the thermal decomposition temperature and elastic storage moduli of the TLCPs decreased with the amount of the TDCA/CHDM unit, all TLCPs remained stable up to 300°C and had excellent elastic moduli above 4.1 GPa at 50°C.

In Situ Preparation of Thermotropic Liquid Crystalline Polyester and Nylon 10T

Poly-decamethylene terephthalamide (PA10T) is one of high temperature special engineering plastics, which is widely used in aviation automobile, and electronic industries. However, its bad processability, poor crystallinity, and toughness may affect its further applications. In this work, the melting polycondensation synthesis of thermotropic liquid crystalline polyester (TLCP) prepolymer was explored. This synthesized TLCP was then added into PA10T via in-situ compounding, to obtain the polydecamethylene terephthalamide / thermotropic liquid crystalline polyester composites (TLCP/PA10T). The TLCP was found easy to be oriented, which can reduce the entanglement of PA10T and improve the processing properties of PA10T. Moreover, the anisotropy under tensile conditions after the TLCP orientation can form microfibers and improve the mechanical properties of the composites. The relationship between crystallization properties, tensile strength and TLCP content of TLCP/PA10T composites was also studied. Such TLCP/PA10T composites can serve as a high temperature special engineering plastic in industrial manufacturing.

Influences of reactive compatibilization on the structure and physical properties of blends based on thermotropic liquid crystalline polyester and poly(1,4‐cyclohexylenedimethylene terephthalate)

AbstractWe report the effects of reactive compatibilization on the morphology, microstructure, and physical properties of thermotropic liquid crystalline polyester/poly(1,4‐cyclohexylenedimethylene terephthalate) (TLCP/PCT, 75/25 by wt%) blends, which are fabricated via melt‐compounding in the presence of different catalyst types and contents. Among three different catalysts, titanium butoxide (TBT) is found to be most effective in the reactive compatibilization of TLCP/PCT blends, which is confirmed by SEM images. FT‐IR spectroscopic analysis reveals the formation of copolyesters by catalyst‐induced transesterification between TLCP and PCT components during melt‐compounding with 0.5 and 1.0 phr TBT loadings. Nonetheless, X‐ray diffraction analysis confirms that the crystal structures of TLCP/PCT blends are not affected by reactive compatibilization. The melting and crystallization transition temperatures of the PCT component in the compatibilized blends decrease owing to the shortening of crystallizable PCT segments by the transesterification. TGA data show that the residue at 800°C increases for the blends melt‐compounded with higher TBT catalyst loadings. The shear moduli and complex viscosity of compatibilized TLCP/LCP blends at a melt state are found to be even higher than neat TLCP and PCT. Although the elastic storage moduli of compatibilized TLCP/PCT blends are slightly lower than neat TLCP, they are far higher than neat PCT.

Effects of Poly(ethylene-co-glycidyl methacrylate) on the Microstructure, Thermal, Rheological, and Mechanical Properties of Thermotropic Liquid Crystalline Polyester Blends.

In this study, a series of thermotropic liquid crystalline polyester (TLCP)-based blends containing 1–30 wt% poly(ethylene-co-glycidyl methacrylate) (PEGMA) were fabricated by masterbatch-assisted melt-compounding. The scanning electron microscopy (SEM) images showed a uniformly dispersed microfibrillar structure for the TLCP component in cryogenically-fractured blends, without any phase-separated domains. The FT-IR spectra showed that the carbonyl stretching bands of TLCP/PEGMA blends shifted to higher wavenumbers, suggesting the presence of specific interactions and/or grafting reactions between carboxyl/hydroxyl groups of TLCP and glycidyl methacrylate groups of PEGMA. Accordingly, the melting and crystallization temperatures of the PEGMA component in the blends were greatly lowered compared to the TLCP component. The thermal decomposition peak temperatures of the PEGMA and TLCP components in the blends were characterized as higher than those of neat PEGMA and neat TLCP, respectively. From the rheological data collected at 300 °C, the shear moduli and complex viscosities for the blend with 30 wt% PEGMA were found to be much higher than those of neat PEGMA, which supports the existence of PEGMA-g-TLCP formed during the melt-compounding. The dynamic mechanical thermal analysis (DMA) analyses demonstrated that the storage moduli of the blends decreased slightly with the PEGMA content up to 3 wt%, increased at the PEGMA content of 5 wt%, and decreased again at PEGMA contents above 7 wt%. The maximum storage moduli for the blend with 5 wt% PEGMA are interpreted to be due to the reinforcing effect of PEGMA-g-TLCP copolymers.

Thermotropic Liquid Crystalline Polyesters Using Aromatic Rigid Diols, Unsaturated Fumaric Acid and Flexible Sebacic Acid

In the present study, seven different series of thermotropic liquid crystalline polyesters (TLCPs) containing unsaturated fumaroyl chloride and flexible sebacoyl chloride were synthesized using the interfacial polymerization methodology. Homopolyesters prepared with saturated flexible spacer such as sebacoyl chloride were mesomorphic whereas homopolyesters synthesized using unsaturated aliphatic spacer such as fumaroyl chloride that was non-mesomorphic. Aromatic diad and triad based mesogenic diols were selected as rigid moiety for liquid crystalline phase formation. Within each series, copolyesters were synthesized by varying relative mole ratio of the above two aliphatic diacid chlorides. Most of the liquid crystalline polyesters showed solubility in phenol:tetrachloroethane (60:40) at 40 ᵒC.

A Correlation between Thermal Diffusivity and Long Period in Thermotropic Liquid Crystalline Polyesters

High thermal conductive polymers are critical for solving the heat dissipation problem confronted by high-performance, compact electronic devices. However, relations between the thermal conductivit...

Rheological Behaviors and Properties of Poly (Ether Ether Ketone) In-Situ Blends: Effect of Chemical Structure of Added Thermotropic Liquid Crystalline Polyarylates

Two kinds of thermotropic liquid crystalline polyesters (TLCP) with different chemical structures were used as flow modifiers during the melt spinning of poly (ether ether ketone). One of TLCPs, named PAT-S, was a kind of aromatic polyester synthesized from 4-acetoxybenzoicacid and 6-hydroxy-naphthoic acid, and the other one named, PEEKAR, was a block copolymer based on poly (ether ether ketene) (PEEK) oligomers and PAT-S oligomers. The effects of the chemical structures of the prepared TLCPs on the rheological behavior of the TLCP/PEEK blends were measured by rheological measurements, and then the structure and properties of the TLCP/PEEK in-situ blend fibers were studied by a series of methods, including differential scanning calorimetry (DSC), wide angle X-ray diffraction (WRXD), optical microscopy (OM), scanning electron microscopy (SEM), orientation degree tests, mechanical properties tests, etc. The results showed that the melt viscosity of PEEK could be effectively reduced by nearly 20% by adding a small amount of PEEKAR or PAT-S with the viscosity reducing effect of PEEKAR on PEEK being greater than that of PAT-S; this was mainly caused by the better compatibility between PEEK and PEEKAR because of the similar segment structures. The crystallinity of the PEEK fibers increased by nearly 50% with addition of both TLCPs, indicating that both the PEEKAR and PAT-S could serve as nucleating agent as well. The results from the morphology analysis clearly proved that PEEKAR had a better compatibility with the PEEK resin compared with PAT-S. The tensile strength of the PEEK fibers could be improved to some extent by adding PEEKAR, while it was obviously reduced after adding PAT-S.

Thermotropic Liquid Crystalline Polyesters with Mesogenic Fragments Based on the p-Oxybenzoate Unit

Main trends in the development of thermotropic liquid crystalline (LC) polymers based on p‑hydroxybenzoic acid are described in this review. The relationship between the structure of the main chain of copolymers and their thermal and mechanical properties is shown. The possibility of creating biodegradable LC polymers is demonstrated.

Physio- and chemo-dual crosslinking toward thermoand photo-response of azobenzene-containing liquid crystalline polyester

Combining the stability of chemical crosslinking and the processability of physical crosslinking is a well-established strategy to design new materials with desirable stimuli–responsive properties. Herein, a series of azobenzene-based thermotropic liquid crystalline polyesters were synthesized by introducing mesogenic dial named 4,4ʹ-bis(6-hydroxyhexyloxy)azobenzene (BHHAB), 2-phenylsuccinic acid (PSA), and different contents of 1,2,3-propanetricarboxylic acid (PTA) as the chemical crosslinker. All these polyesters showed good thermal stability and smectic liquid crystalline phase. Wide-angel X-ray diffraction (WAXD) and the fluorescence emission spectra confirmed the existence of π–π stacking interactions as the physical crosslinking in the polymer chains, particularly at the lower content of PTA. However, when the PTA content increased, the chemical crosslinking changed the chain conformation, and thus the intensity of physical crosslinking slackened gradually. Combining the physical and chemical crosslinking, these polyesters showed the thermoplastic processability, thermal shape memory, heat-assisted healing and photoresponsive behaviors. Taking advantages of these features, these multiple stimuli–responsive polymers can bring more chances for smart materials such as soft actuator.

Thermotropic liquid crystalline polyesters derived from 2-chloro hydroquinone

Synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology is presented. Synthesised polyesters consist of bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene as a mesogen and aliphatic diacid chloride as flexible spacer. The length of oligomethylene units in the polymer was varied from the trimethylene to the dodecamethylene groups. Synthesized polyesters were characterized by differential scanning calorimetry and optical microscopy. The transition temperatures and thermodynamic properties were studied for all these polymers. These polyesters exhibited thermotropic liquid crystalline behavior and showed nematic texture except decamethylene spacer. Decamethylene spacer based polyester showed marble texture of smectic C. Mesophase stability of these polyesters was higher than $$123{^{\circ }}\hbox {C}$$ (except first heating cycle of PE-1). SYNOPSIS The present study deals with the synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology.

Dielectric relaxation of thermotropic liquid crystalline polyesters based on α,ω-alkylene-di-4-hydroxybenzoates and 4,4′-alkylenedioxy-dibenzoic acid

Two structurally isomeric homologous series of semiflexible thermotropic liquid crystalline polyesters were analysed by dielectric spectroscopy in temperature intervals varying from 253 to 403 K. Each series is characterised by a well-distinct behaviour of the dielectric loss versus temperature with a minor diffuse and a more confined relaxation occurring at lower and higher temperatures, respectively. Extrapolations from semilog plots of frequency (f) versus reciprocal absolute temperature (1/T) indicated that the principal relaxation is related to the glass transition temperature as defined by DSC. The analysis of data according to WLF equation indicated that the size and the kind of the polymethylene groups influence the apparent activation energy. A parity effect, connected to the length of polymethylene segments within each series, was finally detected on the relative position of the maximum relaxation, the odd numbers showing in any case higher temperature values.

Azobenzene-containing liquid crystalline polyester with π–π interactions: diverse thermo- and photo-responsive behaviours

An azobenzene-containing thermotropic liquid crystalline polyester showing unique thermo- and photo-responsive behaviours was synthesized.

Improving Stiffness, Strength, and Toughness of Poly(ω-pentadecalactone) Fibers through in Situ Reinforcement with a Vanillic Acid-Based Thermotropic Liquid Crystalline Polyester

We report on the morphology and performance of melt-drawn poly(ω-pentadecalactone) (PPDL) fibers reinforced with a vanillic acid-based thermotropic liquid crystalline polyester (LCP). The in situ reinforced PPDL/LCP fibers developed in this work are considered to be renewable in nature, given the fact that the feedstock for both polymers can be obtained from natural resources. To prepare these fibers, the polymers were mixed in a small scale twin-screw extruder, followed by melt-drawing of the extrudate. It is demonstrated that the tensile modulus and tensile strength of the fibers increase with increasing LCP orientation and concentration. Despite the brittle nature of the pure LCP component, melt-spun PPDL/LCP fibers maintain their ductile deformation for fibers containing up to 30 wt % LCP. The improved stiffness and strength of these PPDL/LCP fibers in combination with their ductile nature ensure improved energy absorption during deformation and effectively increases their toughness compared to the pu...

Annealing studies on a thermotropic liquid crystalline polyester meltblown fabric

Meltblown fabrics composed of a thermotropic liquid crystalline polyester were subjected to heat conditioning at various temperatures. Physical effect of the treatment was investigated by tensile testing of the fabrics and the individual fibers. The fabrics exhibited increased tensile strength by more than 100% after the heat conditioning due to inter-fiber bonding in the fabric structure and morphological reorganization of the thermotropic polymer. The calorimetric behavior of the polymer was further investigated to obtain information about the internal structure. Structural change during the annealing was also visually observed under a polarized light microscope.

Thermotropic liquid crystalline polyesters derived from bis-(4-hydroxybenzoyloxy)-2-methyl-1,4-benzene and aliphatic dicarboxylic acid chlorides

A series of thermotropic liquid crystalline polyesters derived from bis-(4-hydroxybenzoyloxy)-2- methyl-1,4-benzene (BHBOMB) and aliphatic dicarboxylic acid chlorides were investigated. All these polyesters were synthesized by interfacial polycondensation method and characterized by differential scanning calorimetry and wide-angle X-ray diffractometer. These polyesters consist of BHBOMB as a mesogenic diol and aliphatic diacid chlorides were used as flexible spacers. The length of oligomethylene units in polymer was varied from the trimethylene to the dodecamethylene groups. The transition temperatures and thermodynamic properties were studied for all these polymers. All these polyesters were soluble in chlorinated solvents such as chloroform, dichloromethane, dichloroethane, etc. More importantly, all these polyesters exhibited very large mesophase stability.

Study on in situ reinforced composites of thermoplastic resins

Typical crystalline thermoplastic resin polypropylene (PP) and amorphous thermoplastic copolymer acrylonitrile–butadiene–styrene (ABS) were respectively blended with self-made novel low-melting point thermotropic liquid crystalline polyester (TLCP) that contained phosphorus and nitrogen elements (PN-TLCP). Then, the PP/PN-TLCP and ABS/PN-TLCP in situ-reinforced composites were prepared. The effects of PN-TLCP on mechanical property, microstructure, processability, and thermal stability of these two composites were investigated. The results showed that the strength and rigidity of matrix were improved, indicating that PN-TLCP played a role of enhancement. Meanwhile, PN-TLCP could form microfibrillar structure in PP and ABS matrix, which was the main reason of the formation of in situ composites. In the forming process, PN-TLCP could induce PP resin to form β-crystal, which was why the toughness of PP was improved. In addition, PP and ABS exhibited better processing flowability and their melt flow rates were respectively increased 18% and 56% after blending with PN-TLCP. Besides, PN-TLCP was highly beneficial to improving the thermal stabilities of matrix. Various tests showed that this kind of TLCP was suitable for crystalline thermoplastic resin PP and its composites would have more outstanding comprehensive properties.

Experimental understanding of the viscosity reduction ability of TLCPs with different PEs

In this study, two thermotropic liquid crystalline polyesters (TLCPs) synthesized by polycondensation of phydroxybenzoic acid /hydroquinone/ poly dicarboxylic acid were used as viscosity reduction agents for polyethylene (PE). The TLCPs had different thermal, rheological and other physical properties that were quantitatively characterized. The two TLCPs were blended with high density PE (HDPE) and high molecular mass PE (HMMPE) by simple twin screw extrusion under the same weight ratio of 1.0 wt% and were each rheologically characterized at 190 o C. The TLCPs acted as processing modifiers for the PEs and the bulk viscosity of the blends decreased dramatically. However, the viscosity reduction ability was not identical: one TLCP had obviously higher viscosity reduction ability on the HDPE, with a maximum viscosity reduction ratio of 68.1%, whereas the other TLCP had higher viscosity reduction ability on the HMMPE, with a maximum viscosity reduction ratio of 98.7%. Proposed explanations for these differences are evaluated.

Chain folding in main-chain liquid crystalline polyesters: from π–π stacking toward shape memory

A series of main-chain thermotropic liquid crystalline polyesters were synthesized by polycondensation from mesogenic dial such as 4,4′-bis(6-hydroxyhexyloxy)biphenyl (BHHBP) and various diacids with different substituents such as succinic acid (no side group), 2-methylsuccinic acid (aliphatic side group) and 2-phenylsuccinic acid (aromatic side group), named poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl succinate) (PBDS), poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl methylsuccinate) (PBDMS), and poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl phenylsuccinate) (PBDPS), respectively. Liquid crystalline behaviours were investigated through differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and small angle X-ray scattering (SAXS) and the thermal stability of the polyesters was determined via thermogravimetric analysis (TGA). PBDS, PBDMS and PBDPS showed a SmF, SmB and SmA phase, respectively. On enlarging the side group, the d-spacing of the smectic layer increased, indicating folding packing of the polymer chains. Therefore, the adjacent phenyl rings in side groups stacked well together and formed strong π–π interactions even when the temperature was higher than Ti. The special structure of PBDPS could provide good mechanical properties. Thus, PBDPS exhibited the maximum tensile stress (28.6 MPa) and the highest elongation at break (1060%). Furthermore, the strong π–π interaction can act as netpoints; therefore, PBDPS exhibited excellent shape fixing (>99%) and shape recovery ratio (>99%) with large strain (>220%).