Thermotropic Liquid Crystal Polymers Research Articles

3D Printing of Liquid Crystal Polymers for Space Applications

AbstractFused Filament Fabrication is a promising manufacturing technology for the circularity of space missions. Potential scenarios include in‐orbit applications to maximize mission life and to support long‐term exploration missions with in situ manufacturing and recycling. However, its adoption is restricted by the availability of engineering polymers displaying mechanical performance combined with resistance to space conditions. Here, a thermotropic Liquid Crystal Polymer (LCP) is reported as a candidate material with extrusion 3D printing. To expand its scope of applicability to structural parts for space applications, four different exposure conditions are studied: thermal cycling under vacuum, atomic oxygen, UV, and electron irradiations. While 1 MeV‐electron irradiation leads to a green coloration due to annealable color centers, the mechanical performance is only slightly decreased in dynamic mode. It is also found that increased printing temperature improves transverse strength and resistance to thermal cycling with the trade‐off of tensile stiffness and strength. Samples exposed to thermal cycling and the highest irradiation dose at lower printing temperatures still display a Young's modulus of 30 GPa and 503 MPa of tensile strength which is exceptionally high for a 3D‐printed polymer. For the types of exposure studied, overall, the results indicate that LCP 3D‐printed parts are well suited for space applications.

3D Printing of Flow-Inspired Anisotropic Patterns with Liquid Crystalline Polymers.

Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained by utilizing directionality of the nematic flow the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can becombined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed,offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicatingintricate patterns observed in nature, such as wood or turbulent flow using 3D printing.

Branched thermotropic liquid crystal polymer with favourable processability and dielectric properties

Thermotropic Liquid Crystal Polymer (TLCP) has emerged as a promising material for 5G mm-wave antennas due to its low dielectric constant and loss at high frequencies. However, its low melt viscosity, inadequate melt elasticity, and rigid rod-like macromolecular chain with easy orientation pose a significant challenge in the production of isotropic films. In this study, in order to improve the melt viscosity and melt elasticity of TLCP, a multifunctional epoxide-based branching agent (ADR 4468) was applied to produce branched TLCP. The impact of the branched structure on linear viscoelastic, thermal, mechanical, and dielectric performance was investigated. Results from linear rheological characterizations indicated that complex viscosity and storage modulus of modified TLCP samples were significantly enhanced, indicating the presence of long-chain branches. The mechanical performance of modified samples was scarcely affected by the concentration of ADR 4468, with a tensile strength of approximately 245 MPa, and Charpy notched impact strength of approximately 30 KJ/m2. However, with an increase in ADR 4468 content, the dielectric constant increased (from 3.25 to 3.30 at 10 GHz) as well as the dielectric loss (from 1.36 × 10−3 to 2.14 × 10−3 at 10 GHz). Based on the overall performance, the optimal content of ADR 4468 was found to be about 2–3 wt%. These findings offer insights into the potential applications of modified TLCPs as substrates for 5G mm-wave antennas.

Implication of Freeze-Thaw Erosion and Mechanism Analysis of High-Performance Aromatic Liquid Crystal Fibers.

According to the demand for high-performance fibers for high-latitude ocean exploration and development, this paper selects representative products of high-performance liquid crystal fibers: thermotropic liquid crystal polymer fibers (TLCP) and poly p-phenylene terephthalamide (PPTA) fibers. Through a series of freeze-thaw (F-T) experiments for simulating a real, cold marine environment, we then measure the retention of mechanical properties of these two kinds of fibers. Before that, due to the difference in their chemical structures, we tested their Yang-Laplace contact angle (YLCA) and water absorption; the results suggested that PPTA fibers would absorb more moisture. Surprisingly, then, compared with thermotropic liquid crystal polymer (TLCP) fibers, the retention of the mechanical properties of poly p-phenylene terephthalamide (PPTA) fibers decreased by around 25% after the F-T experiments. The Fourier-transformed infrared (FT-IR) analysis, the attenuated total reflection (ATR) accessory analysis and the degree of crystal orientation measured by two-dimensional wide-angle X-ray diffraction (2D-WAXD) confirm that no changes in the chemical and the orientation structure of the crystal region of the fibers occurred after they underwent the F-T cycles. However, as observed by scanning electron microscopy (SEM), there are microcracks of various extents on the surface of the PPTA fibers, but they do not appear on the surface of TLCP fibers. It is obvious that these microcracks will lead to the loss of mechanical properties; we infer that the moisture absorbed by the PPTA fibers freezes below the freezing point, and the volume expansion of the ice causes the collapse of the microfibrillar structure. The two sorts of fibers subjected to the F-T experiments are immersed in a sodium chloride solution, and the amount of water infiltrated into the PPTA microfibrillar structure is evaluated according to the content of sodium ions in the fiber surface and subsurface layers through X-ray spectroscopy (EDS) elemental analysis. From the above analysis, we found that TLCP fibers can more effectively meet the operating standards of the severe and cold marine environment.

Rotational blow molding of thermotropic liquid crystal polymers and resulting orientational structure and mechanical properties

Liquid crystal polymers(LCP)is one of the most promising choices of 5G antenna material in the future, but its rigid rod-like macromolecular chain and the lack of melt elasticity make it difficult to produce film. In this paper, LCP film is successfully prepared based on a blow molding device developed in-house which the mandrel rotates in reverse with the outer die and the effect of annular shear on the orientation structure and mechanical properties of LCP film is investigated. SEM graphics show that obvious self-reinforcing fibers and the fiber cross structure are formed in the LCP film under the synergistic effect of annular shear flow field and axial extrusion. Polarization infrared pole diagram and 2D-WAXD results prove that the orientation degree of LCP film tends to be uniform with the increase of rotational speed. The tensile results show that the die rotation greatly reduces the gap between the longitudinal and transverse mechanical properties of the film, and even the transverse properties exceed the longitudinal properties at the rotational speed of 55 rpm. The LCP film with uniform mechanical properties is obtained eventually. • LCP film was successfully prepared based on a self-made blow molding device which the mandrel rotates in reverse with the outer die. • The rotation of the die made the LCP film formed a fibers cross structure. • The rotation die can effectively promote the orientation of the LCP molecular chain in all directions. • The transverse tensile strength of the RLCP55 exceeds the longitudinal strength.

The effect of a compatible liquid crystal polymer on homogeneous reinforcement of mesogenic and nonmesogenic rigid-rod monomers

Discrete amount of main-chain Liquid Crystal Polymers (LCP) could mechanically reinforce the mesophase of rigid-rod monomers to form a supramolecular homogeneous composite for Liquid Crystal Display (LCD) materials. Moreover, a homogeneously dispersed LCP may induce an enantiotropic transition in monotropic or non-mesogenic rigid-rod compounds. In this work, we studied the blends of a flexible main-chain nematic LCP with an enantiotropic nematic, a monotropic nematic, and two non-mesogenic rigid-rod compounds as model systems. The results indicate that homogenous reinforcement of thermotropic LCP in these monomers is a valid concept and could lead to improvement of mesogenic stability, orienational and mechanical properties of rigid-rod materials for various applications.

Analytical methods for the stress measurement of the resin

Raman spectroscopy has been used to investigate the stress of resin sheet made from thermotropic liquid crystal polymer. The nominal strain and stress dependencies of the 1730cm-1 band due to stretching of the ester binding, have been studied in detail and found that a number of Raman bands show shifts with stress following tensile deformation.

Synthesis and properties of high performance aromatic thermotropic liquid crystal copolyesters based on naphthalene ring structure

In order to obtain thermotropic liquid crystal polymers (TLCP) with excellent comprehensive performance, naphthalene monomers (amount exceeding 50 mol%) of 6-hydroxy-2-naphthoic acid (HNA) and 2,6-naphthalene dicarboxylic acid (NDA) were used to prepare wholly aromatic TLCP with strong molecular interaction via “one-pot” melt polymerization method in this work. The structure and properties of the copolyesters were fully investigated by experimental measurements and molecular simulation. The simulation results indicated that the naphthalene TLCPs had less chain mobility and stronger molecular interactions, and resulted in a linear rigid chain conformation, endowing the copolyesters with superior properties. As compared with the control copolyester M without naphthalene units, the resulted copolyesters had higher glass transition and melting temperature, higher melt tension, better thermal stability and fiber mechanical property. Overall, the rigidity, molecular interaction and properties of the obtained copolyesters could be efficiently improved after incorporation of the naphthalene units, which is beneficial for its processing and application.

Effects of dye doping on electro-optical, thermo-electro-optical and dielectric properties of polymer dispersed liquid crystal films

In this study, polymer dispersed liquid crystal (PDLC) films comprising a phase separated low molecular weight thermotropic liquid crystal (LC) and high molecular weight polymer were, prepared using the polymer–induced phase separation method. In order to enhance the optical efficiency of the PDLC films, LC was doped with dichroic azo dye. Various characterization techniques were employed to determine the properties of the components and, the operating principles of the dye–doped PDLC (DPDLC) films. Morphological studies using polarizing optical microscopy and scanning electron microscopy indicated that the LC droplet configuration changed with various dye concentrations and voltages. Electro-optical analysis showed that the performance of the DPDLC film with a low dye concentration (0.007%) was excellent, with a high contrast ratio (241) and transmittance difference (ΔT = 98.57%), low threshold (VTH = 0.85 V/μm) and saturation voltages (VSAT = 1.45 V/μm), and a minimum hysteresis effect. The effects of temperature on the electro-optical properties of the DPDLC films were studied. Absorbance analyses were conducted using ultraviolet–visible spectrophotometry to complement the transmittance analyses. Dielectric relaxation spectroscopy was employed to measure relaxation frequency/time of DPDLC films. Debye and Cole – Cole modeling were conducted to understand nature of the relaxation process. The zero values of the distribution parameter (α) for all of the DPDLC composite films confirmed their Debye type relaxation process.

Synthesis and properties of wholly aromatic phosphorus-containing thermotropic liquid crystal copolyesters with excellent fibre formation ability

ABSTRACT In this study, wholly aromatic phosphorus-containing thermotropic liquid crystal polymers (TLCP) derived from 6-hydroxy-2-naphthenic acid (HNA), terephthalic acid (TA) and 10-(2,5-Dihydroxypheny)-10 h-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO-HQ) are successfully synthesised via one pot melt polymerisation method. Although, the resulted copolyesters are amorphous, high glass transition temperature (182–196 °C) is observed due to the existence of kinked DOPO-HQ unit and crankshaft HNA monomer. The nematic liquid crystal behaviour is found for all of the copolyesters under POM at relatively low temperature (190–207 °C). The copolyesters show good thermal stability and high char yield under both nitrogen and air atmosphere, which of the initial degradation temperature (T5%) and char yield are above 450 °C and 50%, 439 °C and 29%, respectively. Dynamic rheology results show that viscous modulus of the obtained copolyesters are much more prominent under both frequency and temperature sweeping mode, indicating have excellent processing ability. The fibres can be easily drawn from the melt, and exhibit well oriented fibrillated structure. The convenient polymerisation method, excellent thermal stability and fibre formation ability of the phosphorus-containing TLCP endow them much potential use in the safety protection field.

Design and Synthesis of Polysiloxane Based Side Chain Liquid Crystal Polymer for Improving the Processability and Toughness of Magnesium Hydrate/Linear Low-Density Polyethylene Composites.

In this study, a polysiloxane grafted by thermotropic liquid crystal polymer (PSCTLCP) is designed and synthesized to effectively improve the processability and toughness of magnesium hydroxide (MH)/linear low-density polyethylene (LLDPE) composites. The obtained PSCTLCP is a nematic liquid crystal polymer; the liquid crystal phase exists in a temperature range of 170 to 275 °C, and its initial thermal decomposition temperature is as high as 279.6 °C, which matches the processing temperature of MH/LLDPE composites. With the increase of PSCTLCP loading, the balance melt torque of MH/LLDPE/PSCTLCP composites is gradually decreased by 42% at 5 wt % PSCTLCP loading. Moreover, the power law index of MH/LLDPE/PSCTLCP composite melt is smaller than 1, but gradually increased with PSCTLCP, the flowing activation energy of PSCTLCP-1.0 is lower than that of MH/LLDPE at the same shear rate, indicating that the sensitivity of apparent melt viscosity of the composites to shear rate and to temperature is decreased with the increase of PSCTLCP, and the processing window is broadened by the addition of PSCTLCP. Besides, the elongation at break of MH/LLDPE/PSCTLCP composites increases from 6.85% of the baseline MH/LLDPE to 17.66% at 3 wt % PSCTLCP loading. All the results indicate that PSCTLCP can significantly improve the processability and toughness of MH/LLDPE composites.

Characterization of injection-molded high-strength/high-stiffness thermoplastic hybrid materials containing thermotropic liquid crystal polymer (LCP), polyphenylene sulfide (PPS) with carbon fibers

In this study, thermotropic liquid crystal polymer (LCP)-based hybrid materials with various carbon fiber (CF) and polyphenylene sulfide (PPS) contents were developed to replace the heavy metal brackets used for liquid crystal displays (LCDs) in thin, light mobile devices. To determine the physical properties of the composite, several characterization methods, including mechanical, thermal, and morphological tests, were used. As the CF content increases, the tensile and flexural moduli also increase because of the high modulus values of CF. However, some strength-related properties, such as the tensile, flexural, and impact strength, decrease because of the lack of interfacial compatibility between the CF and matrix polymers. Additionally, as the thickness of the LCP-based hybrid material decreases, the flexural strength and flexural modulus increase because of the highly oriented characteristics of LCPs at the surface. To improve the interfacial strength between the CF and LCP, epoxy is a good compatibilizer. In addition, PPS can be used to reduce the weld line formation of LCP and improve the processability of LCP-based hybrid materials with high CF contents.

The influence of CF and TLCP co-reinforced on the mechanical properties of PA6-based composites

The purpose of this study was to investigate the effects of carbon fiber (CF) and thermotropic liquid crystal polymer (TLCP) as co-reinforcements on the mechanical properties of composites. The TLCP fibers were produced by melt exclusion. CF and TLCP co-reinforced PA6 composites (CF/TLCP/PA6) were prepared by the lamination molding method. The influence of molding temperature, TLCP content, compatibilizer, and pre-impregnation process on microstructure and mechanical properties of the CF/TLCP/PA6 composites were investigated with Electronic Microscopy and Mechanical tests (flexure and interlaminar shear) by Scanning Electron Microscope (SEM) and Universal Test Machine. It was found that the optimum molding temperature was 240 °C. The mechanical properties of the composites first increased and then decreased with an increase of TLCP content. The introduction of a compatibilizer obviously improved the mechanical properties of the composites. When the TLCP content was 15 wt%, the molding temperature was 240 °C, and 5 wt% compatibilizer was added, the comprehensive mechanical properties of the composites in terms of the flexural and interlaminar shear strengths were the best, which were 363.5 ± 4.4 MPa and 44.9 ± 2.9 MPa, respectively. The addition of TLCP fibers could prevent the propagation of microcracks in the composites, which further improved their mechanical properties due to the synergistic effect with CFs.

Shear-induced assembly of graphene oxide particles into stripes near surface

ABSTRACTWe report the shear-induced assembly of graphene oxide (GO) particles into periodic stripe-like patterns near the surface. These stripe-like patterns, which have an average periodic length of 100–250 μm, are aligned in a wavy manner along the normal to the flow direction. The self-assembled GO structures are investigated at different depths using three different analysis methods, namely, reflective microscopy observations of the photonic-crystalline GO dispersion, polarized optical microscopy, and fluorescence confocal laser scanning microscopy. The surface microstructures observed in reflection mode are different from the shear-induced band structures formed in bulk thermotropic liquid crystals and liquid crystal polymers, in terms of the shape and scale of the stripes. Further, there is also a difference in terms of the dependence of the stripe width on the shear rate. The observations suggest that the stripes are formed because of a competition between the stable surface-field-induced planar alignment of the GO particles near the surface and their relatively unstable shear-induced vertical alignment in the bulk. The findings of this study should advance our understanding of GO assembly under shear stress. Further, the proposed method is a novel one for inducing the assembly of GO particles into microstructures shaped as thread-like stripes.

Structure Evolution of Thermotropic Polymers by Thermal Annealing. A Light and X-Ray Scattering Study

ABSTRACTSmall-angle light scattering (SALS) and wide-angle X-ray scattering (WAXS) were used to study the influence of heat treatment on the texture and microstructure of extruded films of high-performance thermotropic liquid crystal polymers (LCPs). The microstructure was correlated with tensile mechanical properties. LCPs based on random units of hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) were supplied by the former Hoechst Celanese Corp. as 50 μm thick extruded films. The LCPs, denoted B-N, COTBP and RD1000, have B and N as common co-monomers and vary the other co-monomers. Thus, this study also enabled the investigation of the influence of monomer composition on microstructure and mechanical properties. Heat treatments were carried out at temperatures close to the solid-to-nematic transition (Ts→n) for periods up to 5 h, under dry air conditions. The thermal treatment produced either two endotherms or a small increase of Ts→n (B-N and RD1000), or Ts→n increased significantly (COTBP). Moreover, when heat treatment was carried out approximately 40°C below the respective Ts→n, the mechanical Young’s modulus, E, along the extrusion axis increased for all LCPs. Strikingly, for COTBP, E increased over 100% relative to the as-extruded film. The results also showed that the optimum treatment time for improving the Young modulus was between 3 and 4 h. Wide-angle X-ray scattering showed a significant sharpening of crystalline reflections and concentration of the 002 meridional reflection as a result of thermal treatment, suggesting the elimination of defects and a better alignment of the molecular chains along the extrusion axis. This would explain the increase in tensile modulus.

Simulation Study of Thermotropic LCPs and Prediction of Normal Stress Difference at High Shear Rate

Abstract The shear viscosity and normal stress difference of two filled and two unfilled thermotropic liquid crystal polymer (TLCPs) were studied. The rigid and rod like molecules of TLCPs orientate differently at different shear rates. Under low shear rate, the molecules tend to align in the direction of the flow but also tumble and wagging on their own axis. The abnormal orientation of the molecules also depends upon temperature, fillers contents, aspect ratio and elastic nature of LCP molecules. These behaviors lead to unusual rheological properties of LCPs, such as negative first normal stress difference for filled LCPs at low shear rates. But with the increase of shear rate, the molecules are oriented in the direction of flow, which lead to isotropic flow at high shear rates. The complicated rheological properties and characteristic anisotropic properties of LCPs are modelled by recently developed Leonov's viscoselastic constitutive equations. Simulation has been carried out using Mathematica software and the characteristic anisotropic properties of LCPs have been identified. The experimentally measured viscosities at high shear rate have been compared with model predictions. Moreover, the normal stress differences using at high shear rates have been estimated using Leonov's model, which is experimentally not accessible.

SALS, WAXS and mechanical properties of heat‐treated thermotropic polymers

The influence of heat treatment on the texture, microstructure and tensile mechanical properties of extruded thin films of a series of high‐performance thermotropic liquid crystal polymers (LCPs) was investigated. LCPs based on random units of hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) were kindly supplied by the former Hoechst Celanese Corp as 50 µm thick extruded tapes. The LCPs, denoted B‐N, COTBP and RD1000, have B and N as common comonomers and vary the other comonomers. Thus, this study also enables the investigation of the influence of monomer composition on microstructure and mechanical properties. Heat treatments were carried out at temperatures close to the solid‐to‐nematic transition (Ts→n) for periods up to 5 h, under dry air conditions. The thermal treatment produced either two endotherms or a small increase of Ts→n (B‐N and RD1000), or increased significantly Ts→n (COTBP). Moreover, when heat treatment was carried out approximately 40°C below the respective Ts→n, the mechanical Young's modulus, E, along the extrusion axis, increased for all LCPs. Strikingly, for COTBP, E increased over 100% relative to the as‐extruded film. The results also showed that the optimum treatment time for improving the Young modulus, under dry air atmosphere, was between 3 and 4 h. Wide‐angle X‐ray scattering showed a significant sharpening of crystalline reflections and concentration of the 002 meridional reflection as a result of thermal treatment, suggesting the elimination of defects and a better alignment of the molecular chains along the extrusion axis. This would explain the increase in tensile modulus. Copyright © 2013 John Wiley & Sons, Ltd.

Synthesis, Characterization, and Properties of Thermotropic Novel Liquid Crystal Photosensitive Polymers Containing Mesogen and Photocrosslinkable Group

A series of side chain liquid crystalline photosensitive polymers containing methyl methacrylate followed by the spacer and chalcone moiety were synthesized and characterized. Monomers, namely 4-[4{biphenyl oxy carbonyl phenyl oxy carbonyl} phenyl carbonyl ethylene phenyl] oxy butyl methacrylate (BOCPOCPCEPOBMA) and 4-[4{1-methoxy phenyl oxy carbonyl phenyl oxy carbonyl} phenyl carbonyl ethylene phenyl] oxy octyl methacrylate (MPOCPOCPCEPOOMA) was synthesized and subsequently polymerized using benzoyl peroxide as an initiator. The chemical structures and purity were characterized by FT-IR, 1H- NMR, and 13C-NMR analyses. The photocrosslinking studies of the polymers were investigated by UV spectroscopy. The rate of crosslinking increased with increase in the length of the methylene chain. The results of differential scanning calorimeter (DSC) analysis confirmed the formation of mesophase in the polymers. Hot stage optical polarized microscopy (HOPM) showed presence of nematic phase for poly (BOCPOCPCEPOBMA) and poly (MPOCPOCPCEPOOMA).

The Influence of Thermal Annealing on Microstructure and Mechanical Properties in High Performance Liquid Crystal Copolyesters

ABSTRACTIn this research we have focused on the influence of thermal treatment for periods of time on the thermal and mechanical properties of extruded films of a series of high-performance thermotropic liquid crystal polymers (LCPs). The dependence of microstructure, thermal and mechanical properties on the extent of thermal treatment is investigated. Especially synthesized wholly aromatic LCPs based on hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) are kindly supplied by Hoechst Celanese Research Corp in the form of 50 μm thick extruded films. Thus, the influence of monomer composition is also studied in order to contrast the influence of molecular conformation. Thermal treatments are carried out at temperatures close to the solid-to-nematic transition (Ts→n) for up to several hours under dry air conditions. The results show a profound influence of thermal annealing on morphology and mechanical modulus when annealing is carried out c.a. 40ºC below Ts→n, where solid-to-nematic transition and Young’s modulus are significantly increased.

Liquid crystal polymers and ionomers for membrane applications

Transport phenomena through polymeric composite membranes depend on several complex factors, involving not only their chemical composition but also their microstructure. Due to the combination of mesomorphic and macromolecular behaviour, thermotropic liquid crystal polymers (LCP) may enhance the transport properties in membranes by improving parameters related to the order, orientation, connectivity and distribution of the different domains arising from phase separation. This paper provides an overview of some existing and potential applications of liquid crystals (LC) in polymeric membranes as effective structure-directing templates or as components providing a dynamic response to external stimuli. The first part is a description of the composition and phase behaviour of LCP containing ionic and ionogenic groups, the so-called liquid crystal ionomers (LCI). The second part is focused on the description of LCP and LCI used in transport applications, such as proton-conducting films in fuel cells, ionic polymer transducers and hydrogels. The paper highlights the relationships between the composition and the LC properties of the materials and their potential as membranes for ionic and solvent transport, with particular attention paid to covalently linked materials prepared by copolymerisation, due to their higher versatility and stability.