Liquid Crystalline Polyesters Research Articles

Enhanced intrinsic thermal conductivity of liquid crystalline polyester dispersed films through hydrogen bond interaction

Polymer-dispersed liquid crystal (MHxFy) films comprising liquid crystal polymer (LCPH and LCPF) were successfully obtained by the method of solution casting & thermal compressing. At a LCPH mass fraction of 67 %, thermal conductivity (λ) value in through-plane (λ⊥) and in-plane (λ∥) direction of MH2F1 films reach up to 0.305 W/(m·K) and 2.879 W/(m·K), respectively. In comparison to pure LCPF, the thermal conductivity of MH2F1 exhibited an increase of 4.42 times, which is 14.4 times than that of conventional thermoplastics. Meanwhile, the crystallinity of MHxFy films around 50 % which is indicate the crystal phase become more integrated and regular. The FTIR spectra reveals a significant blue shift (3432 cm−1 to 3421 cm−1) in the absorption peak of the –OH group in MH2F1, suggesting a robust interaction between LCPF and LCPH. The primary cause of elevated thermal conduction in these films was determined to be the consistent presence of “thermal bridges” and a high degree of crystallinity. Furthermore, the T5%, Tmax and residual carbon at 800 °C of MH2F1 were 347.8 °C, 409.4 °C and 8 %, respectively. This is attributed to the ordered distribution of LCPH in LCPF matrix through hydrogen bonding interaction, forming an interpenetrating network of LCPF-LCPH. The result show that MHxFy films demonstrating superior intrinsic thermal conductivities, thermal stabilities, and mechanical properties.

Degradation of a Wholly Aromatic Main-Chain Thermotropic Liquid-Crystalline Polymer Mediated by Superbases.

Plastic circular economy needs to be established to solve environmental issues related to plastic waste. Superengineering plastics such as liquid-crystalline (LC) polymers exhibit excellent thermal and mechanical properties, resulting in poor degradability in natural environment. Herein, we report the degradation of a wholly aromatic thermotropic LC polyester, poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid) (Vectra) mediated by superbases. Methanolysis and hydrolysis of Vectra yield its monomeric compounds, 4-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, and their methyl esters. Among several transesterification catalysts explored, 1,5,7-triazabicylco[4.4.0]dec-5-ene (TBD) is the most suitable for the methanolysis of Vectra. The complete degradation of Vectra is achieved under reflux. The degradation proceeds heterogeneously via a surface erosion mechanism, preferentially starting from less chain-packed regions. Model reactions using aryl arylates reveal that monomeric compound-superbase complexes could mediate the cleavage of the ester bonds in both homogeneous and heterogeneous systems. The ester bonds of Vectra have inherent poor reactivity and are protected by oriented robust structures of the polymer. Nevertheless, the superbases enable the degradation of Vectra via the cleavage of the ester bonds by methanol. These outcomes open the way for recycling high-performance plastics as well as demonstrate the feasibility of recovering precious aromatic compounds from plastic waste as aromatic feedstock.

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2.

Dicarboxylic acids and derivatives are important building blocks in organic synthesis, biochemistry, and the polymer industry. Although catalytic dicarboxylation with CO2 represents a straightforward and sustainable route to dicarboxylic acids, it is still highly challenging and limited to generation of achiral or racemic dicarboxylic acids. To date, catalytic asymmetric dicarboxylation with CO2 to give chiral dicarboxylic acids has not been reported. Herein, we report the first asymmetric dicarboxylation of 1,3-dienes with CO2 via Cu catalysis. This strategy provides an efficient and environmentally benign route to chiral dicarboxylic acids with high regio-, chemo-, and enantioselectivities. The copper self-relay catalysis, that is, Cu-catalyzed boracarboxylation of 1,3-dienes to give carboxylated allyl boronic ester intermediates and subsequent carboxylation of C-B bonds to give dicarboxylates, is key to the success of this dicarboxylation. Moreover, this protocol exhibits broad substrate scope, good functional group tolerance, easy product derivatizations, and facile synthesis of chiral liquid crystalline polyester and drug-like scaffolds.

Photocatalytic dicarboxylation of strained C[sbnd]C bonds with CO2via consecutive visible-light-induced electron transfer

Dicarboxylic acids have a wide range of applications in the polymer industry to construct valuable materials. Photocatalysis has recently emerged as an efficient and sustainable strategy to generate dicarboxylic acids. However, photocatalytic dicarboxylation with CO2 is mainly limited to unsaturated bonds, and the dicarboxylation of CC single bonds still remains a challenge. Herein, we report a photocatalytic dicarboxylation of CC single bonds in strained rings with CO2 units via consecutive photo-induced electron transfer (ConPET). It is also the first photocatalytic reductive ring-opening reaction of cyclobutanes. Notably, this transition-metal-free protocol exhibits good functional group tolerance, broad substrate scope, facile scalability, and easy product derivatizations. Moreover, diacids can easily be derivatized to main-chain liquid crystalline polyesters.

Synthesis of a polyester with a liquid crystalline silsesquioxane-contained backbone Chain

A one-pot multistep growth melt procedure utilizing multistep feeding has been developed to enable the synthesis of thermoplastic polymers based on 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA)with greater numbers of alternating units in their backbone chains. The decreased melting point (Tm) and glass transition temperature (Tg) indicated that the multistep feeding procedure helped to enhance the flexibility of the backbone chain due to the increasing number of alternating blocks, due to a reduction in the stacking between different main-chains, thus enabling the formation of liquid crystalline polyester (LCP) fibers with oriented structures. The covalent incorporation of rigid cage-like silsesquioxane groups led to a significant reduction of the melting point, resulting from the inevitable transformation of the original symmetrical backbone structure to a less ordered structure, due to the steric effect and hybrid feature of silsesquioxane. The presentation of silsesquioxane groups also enhanced the aggregation of ultrafine fibers with well-aligned orientation in the molten state and produce stronger fiber bundles.

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.

Characterization of Molecular Structures and Polymerization Reactions of Fully Aromatic Liquid Crystalline Polyesters Containing Aminophenol Units by Thermally-Assisted Hydrolysis and Methylation-GC and MALDI-MS

Characterization of Molecular Structures and Polymerization Reactions of Fully Aromatic Liquid Crystalline Polyesters Containing Aminophenol Units by Thermally-Assisted Hydrolysis and Methylation-GC and MALDI-MS

Soluble Liquid Crystalline Poly(ester imide)s with High Glass Transition Temperatures and Improved Dielectric Properties

At present, high-performance liquid crystalline polyesters with a good processability, low dielectric constant, and low dielectric loss at a high frequency (GHz) are in high demand ascribed to the rapid development of high-speed communication technology. Here, an imide dicarboxylic acid (IA) with a bulky phenyl side group is designed and synthesized. It is then copolymerized with 4-hydroxybenzoic acid (HBA), isophthalic acid (IPA), 4,4′-dihydroxybipheny (BP), and 4,4′-dihydroxydiphenyl ether (ODP) via solution polycondensation to obtain liquid crystalline poly(ester imide)s (LCPEIs). By controlling the feeding ratio of IA, the solubility, thermal stability, mechanical properties, and dielectric properties of the LCPEIs are tunable. When the molar fraction of IA reaches or exceeds 5%, the obtained LCPEIs are soluble in ordinary organic solvents, such as chloroform, N,N-dimethylformamide, N-methyl-2-pyrrolidinone, and so forth, which allows a simple solution casting to prepare isotropic LC films. The nematic LCPEIs show high glass transition temperatures (Tg > 200 °C) but relatively low melting temperatures and isotropization temperatures (Tm and Ti, <340 °C). The LCPEI prepared with 10 mol % IA shows excellent combined properties, whose ηinh, Mn (GPC), Tg, Tm, Ti, Td5%, tensile strength, tensile modulus, water uptake, dielectric constant, and dielectric loss (at 10 GHz) are 0.92 dL g–1, 20,900 g mol–1, 218 °C, 268 °C, 330 °C, 458 °C, 80.5 MPa, 4.0 GPa, 0.39%, 3.07, and 0.006, respectively. These LCPEIs can be promising candidates for flexible copper-clad laminate substrates.

Performance of liquid crystalline polyester composite burn-in board connectors under cyclic high-temperature condition

The performances of connectors for burn-in tests made of various glass fibre/liquid crystalline polyester composites were analysed using mechanical testing and finite element analyses. Three different composites (E6006L, E6007LHF, and E6808UHF) were evaluated, containing one of two fibre types (chopped or milled) at different contents by weight. Material characterisations of the composites were accomplished by static tensile and fatigue testing under three temperature conditions (23 °C, 160 °C, and 220 °C). The tensile strength, failure strain, and S–N curves of the materials were identified and applied in finite element analyses of the connector structure. The structural integrity of each connector subjected to cyclic high temperature condition was then evaluated using stress and fatigue analyses, and the fatigue life was calculated according to material. Finally, the 40 wt% milled glass fibre/liquid crystalline polyester composite was identified as the most appropriate material for the burn-in test board connector structure.

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.

Highly Toughened and Heat-Resistant Poly(lactic acid) with Balanced Strength Using an Unsaturated Liquid Crystalline Polyester via Dynamic Vulcanization

Poly(lactic acid) (PLA) is a well-known sustainable plastic but with an inherent brittleness, slow crystallization rate, and poor heat resistance, which significantly limits its application. In this work, toughened PLA with a balanced tensile strength, fast crystallization rate, and great heat resistance was obtained by dynamic vulcanization with an unsaturated liquid crystalline polyester (PBDPSM) in the presence of a free radical initiator of dicumyl peroxide (DCP). PBDPSM is efficient in enhancing the tensile toughness of PLA in that the elongation at break of PLA significantly increased from 15 to 179% while adding only 1 wt % PBDPSM. When adding 8 wt % PBDPSM, the PLA/PBDPSM8 blend has an elongation at break of 265%, tensile toughness of 107.2 MJ/m3, and notched Izod impact strength of 22.5 kJ/m2. Moreover, it keeps a high tensile strength of 62.6 MPa. In addition, the crystallization rate of PLA has dramatically accelerated due to the apparent nucleation effect induced by DCP and PBDPSM. PLA/PBDPSM3 shows the highest crystallinity with 44.4%. Furthermore, it shows a high Vicat softening temperature of 152.2 °C, which is 92.2 °C higher than that of neat PLA. Finally, highly toughened PLA materials with balanced tensile strength and good heat-resistant properties were realized.

Liquid crystalline hyperbranched polyesters with phosphorus functional groups

Liquid crystalline hyperbranched poly(aryl ester)s (A2B3) were prepared by polycondensation reaction of 2-(6-oxido-6H-dibenz&lt;c,e&gt;&lt;1,2&gt;oxaphosphorin-6-yl)1,4-naphthalene diol with 1,3,5-benzenetricarbonyl trichloride, taken in two different molar ratios. The chemical structure of the newly synthesized hyperbranched polymers was confirmed by FTIR, 1H NMR, 13C NMR spectroscopy. The polymers exhibited high thermal stability with initial decomposition temperature above 410–435°C and char yield at 700°C higher than 40%. Combined differential scanning calorimetry, polarized optical microscopy and wide-angle X-ray diffraction measurements were carried out to closely examine their thermal behavior and phase transitions.

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.

High-Performance Liquid Crystalline Polyesters: Influence of the Synthetic Pathway on the Thermal Properties

Liquid crystalline (LC) copolyesters derived from 4-hydroxybenzoic acid, terephthalic acid, and bisphenol A were synthesized under a variety of reaction conditions. Carboxylic groups in the raw materials were involved in either direct polycondensation or transesterification reactions. Protecting the carboxylic groups via methyl ester or phenyl ester formation was found to increase the yield and the molecular weight of the samples. It turned out that the maximal value of the degree of polymerization and, consequently, the highest heat resistance was observed for the samples prepared using phenyl esters of carboxylic acids.

Synthesis and Characterization of Liquid Crystalline Polyesters Containing α,β-unsaturated Ketone Moiety in the Main Chain Derived from 2,6-bis(4-hydroxybenzylidene)cyclohexanone

Synthesis and Characterization of Liquid Crystalline Polyesters Containing α,β-unsaturated Ketone Moiety in the Main Chain Derived from 2,6-bis(4-hydroxybenzylidene)cyclohexanone

Significant enhancement of thermal conductivity in graphite/polyester composite via interfacial π–π interaction

AbstractInterfacial thermal resistance between matrix and filler is one of the most serious factors hindering heat transfer in composites. Here, a type of liquid crystalline polyester (LCP) containing phenyl pendant groups was intended to blend with pristine graphite by interfacial interaction. The intensity at 26.6° of the wide angle X‐ray diffraction pattern which exceeded that of pristine graphite indicated the existence of a strong interfacial π–π interaction. Both DSC and XRD tests showed that the ordered structure of the LCP matrix is directly affected by the mass fraction of graphite, indicating the interfacial interaction between LCP and graphite. By increasing the content of graphite, the thermal diffusivity showed a sharp increment by 1004%. The maximum thermal conductivity of the composite reached 28.613 W m−1 K−1, which was seven times that of traditional thermoplastic blended with graphite. Compared with the data calculated using effective medium theory, interfacial interaction plays a significant role in enhancing the thermal conductivity of the composites. Furthermore, the maximum tensile strength of this series of composites reached 13.3 MPa and the maximum Young's modulus reached 1067 MPa, exhibiting a potential guideline for further applications in flexible electronics. © 2019 Society of Chemical Industry