Thermotropic Liquid Crystal Polymers Research Articles

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.

Determination of structural changes of dispersed clay platelets in a polymer blend during solid-state rheological property measurement by small-angle X-ray scattering

The poly(ethylene terephthalate)/thermotropic liquid crystal polymer (weight ratio 80:20) blend composites with two different weight percentages of nanoclay were prepared using a twin-screw extruder. The frequency sweep experiments at a constant strain and different temperatures were carried out by a dynamic mechanical analyzer. The temperature sweep experiments of unmodified and nanoclay modified blends were conducted to find out the variation of flexural storage and tan δ values as a function of temperature. Results showed that the modulus of the blend increases after composite formation with nanoclay and the variation of frequencies during temperature sweep experiments do not affect the glass transition temperature values. The small-angle X-ray scattering studies showed that the degree of anisotropy and mean orientation angles of clay platelets in blend matrix were altered significantly after frequency and temperature sweep tests.

Self organisation and photoinduced charge transfer in single-wall carbon nanotubes embedded in a thermotropic liquid crystal polymer

A hybrid material composed by a thermotropic liquid crystal (LCP) polymer (HBA–PET) and single wall carbon nanotubes has been produced in order to study the interaction at the interface matrix/filler for possible applications in electronics and optics. The nanocomposites are characterized by a mosaic-like morphology, with regions of randomly placed LC fibers intercalated with regions formed by aligned polymer fibers, that trigger in turn the alignment of carbon nanotube bundles by means of P stacking interaction. Moreover an effective electronic interaction between the nanocomposite components is demonstrated by combining use of photoluminescence and Raman spectroscopy. The photoinduced charge transfer between SWCNT and polymer could be explain on the basis of the injection of holes (generated in the polymer by light absorption), into the SWCNT valence band and followed by a radiationless decay of the excited polymer’s electron.

Structural Analysis of Liquid Crystal Polymer Based Nanocomposites by X‐Ray Scattering

AbstractThe main theme of this work is to support the unusual melt‐state rheological properties of the clay‐containing nanocomposites of thermotropic liquid crystal polymers by small‐ and wide‐angle X‐ray scattering. The changes of dispersion of silicate particles in the LCP matrix as well as change in crystal growth of LCP were extensively studied by X‐ray scattering in both small‐ and wide‐angle regions. The generalised indirect Fourier transformation technique developed by Glatter was used to study the structural changes in nanocomposites with temperature. This is a new approach recently proposed by us for the quantitative analysis of the nanoparticles dispersion in the polymer matrix.magnified image

Multi-Wall Carbon Nanotube Reinforced Thermotropic Liquid Crystal Copolyester Nanocomposites

Thermotropic liquid crystal polymer (TLCP) nanocomposites reinforced with a multi-wall carbon nanotube (CNT) were prepared by melt compounding in a twin-screw extruder. The incorporation of CNT has a minor effect on the thermal transition temperatures of TLCP/CNT nanocomposites. The enhancement of thermal stability of TLCP/CNT nanocomposites was obtained by the physical barrier effect of the dispersed CNT in the polymer nanocomposites. There was significant dependence of the rheological and mechanical properties of TLCP/CNT nanocomposites on the CNT contents. The TLCP/CNT nanocomposites exhibited more distinct shear thinning behavior relative to pure TLCP over the whole frequency range, and the non-terminal behavior was observed in the TLCP/CNT nanocomposites attributed to the nanotube-polymer interaction. In addition, the improvement in the mechanical properties of TLCP/CNT nanocomposites can be explained by the reinforcement effect of CNT with high aspect ratio as well as the uniform dispersion of CNT in the TLCP matrix.

Internal structure and physical properties of thermotropic liquid crystal polymer/poly(ethylene 2,6-naphthalate) composite fibers

Thermotropic liquid crystal polymer (TLCP)/poly(ethylene 2,6-naphthalate) (PEN) composites and their fibers were prepared by using melt compounding and spin-draw process. Thereafter, they were additionally drawn by various drawing methods. As TLCP contents increased, melt flow index was increased and shear viscosity was decreased because TLCP fibril structure of rigid backbones was formed during melt compounding. Molecular orientation and crystalline structure of the TLCP/PEN composite fibers were improved as a function of draw ratio and the heat–laser drawing method gave synergic effects on internal structure of the TLCP/PEN composite fibers. Thermal stability of the TLCP/PEN composite fiber was improved significantly by incorporating small amount of TLCPs.

EFFECT OF ESTER-AMIDE EXCHANGE REACTION ON THERMAL BEHAVIOUR AND MORPHOLOGY OF NYLON 6/THERMOTROPIC LIQUID CRYSTALLINE POLYMER BLENDS

Binary blends of two grades of nylon 6 and three kinds of commercialized thermotropic liquid crystal polymers (LCPs) (Vectra A950,Vectra B950 and Rodrun LC5000) were prepared by melting blending process,respectively.The ester-amide exchange reaction between nylon 6 and LCPs was analyzed by using diffuse reflectance FTIR spectroscopy (DRIFT) and differential scanning calorimetry (DSC).The blending time,composition and the type of LCPs exerted dramatic influence on the exchange reaction,resulting in the decrease of melting point and crystallinity of nylon 6 in the blends.The increase of blending time promoted the contact and the entanglement of macromolecular chains of the two phases,and therefore,promoted the exchange reaction.The increase of LCP content was another favorable parameter for the exchange reaction.When the content of Rodrun LC5000 was 80 wt%,the melting temperature of nylon 6 in the blend decreased from 219.1℃ to 185.5℃,and the crystallinity was decreased from 37.9% to 20.0%.Compared with Vectra A950 and Vectra B950,Rodrun LC5000 tends to react with nylon 6 more extensively,due to its more flexible molecular chain.The morphology of blends was characterized by using scanning electron microscopy (SEM).It is shown that,unlike other completely incompatible blending systems,the morphology of nylon 6/LCP blends was controlled by blending time as well as the exchange reaction.With the increase of blending time,the morphology of dispersed LCP was developed from droplets to deformed short bars,and finally,fine fibrils with diameter of about 1 μm.

Effect of modified carbon nanotube on physical properties of thermotropic liquid crystal polyester nanocomposites

Polymer nanocomposites based on a very small quantity of carbon nanotube (CNT) and thermotropic liquid crystal polymer (TLCP) were prepared by simple melt blending using a twin-screw extruder. Morphological observations revealed that modified CNT was uniformly dispersed in the TLCP matrix and increased interfacial adhesion between the nanotubes and the polymer matrix. The enhancement of the storage and loss moduli of the TLCP nanocomposites with the introduction of CNT was more pronounced at low frequency region, and non-terminal behavior observed in the TLCP nanocomposites resulted from the nanotube–nanotube and polymer–nanotubes interactions. There is significant dependence of the mechanical, rheological, and thermal properties of the TLCP nanocomposites on the uniform dispersion of CNT and the interfacial adhesion between CNT and TLCP matrix, and their synergistic effect was more effective at low CNT content than at high CNT content. The key to improve the overall properties of the TLCP nanocomposites depends on the optimization of the unique geometry and dispersion state of CNT and the interfacial interactions in the TLCP nanocomposites during melt processing. This study demonstrate that a very small quantity of CNT substantially improved thermal stability and mechanical properties of the TLCP nanocomposites, providing a design guide of CNT-filled TLCP composites with as great potential for industrial use.

Reactive blending of poly(ethylene 2,6-naphthalate) and Vectra A

In situ composites based on poly(ethylene 2,6-naphthalate) (PEN) and a thermotropic liquid crystal polymer (Vectra A950) were prepared by melt blending under different processing conditions. Thermal behaviour, mechanical and physical properties and morphology of the blends were investigated. The DSC analysis indicates that, as expected, Vectra enhances the crystallization process of PEN. Moreover, mechanical and thermal tests evidence the significant role of 20 wt% Vectra on increasing the material performances; tensile properties, coupled with SEM, show that strength and modulus of PEN are significantly improved when Vectra domains are long and continuous fibrils. The overall results can be attributed to the compatibilisation of the system induced by the mixing conditions that affect the extent of transreactions occurring in the melt. To deepen this aspect, the soluble and insoluble fractions in a PEN solvent were isolated and thoroughly characterized: both the fractions contain PEN/Vectra copolymers. The results underline the potentiality of the PEN/Vectra system for different high-performance applications requiring superior strength and modulus, heat stability or barrier properties.

A multi-stream flow technique to obtain isotropic texture in extruded thermotropic liquid crystalline polymer melts

In this article, we present a simple method toward achieving global orientation isotropy in a capillary-extruded thermotropic liquid crystal polymer (TLCP), that is, a random copolyester of 73 mol% hydroxybenzoic acid (HBA) and 27 mol% hydroxynaphthoic acid (HNA), by passing it through porous, flow altering metal foam inserts placed in the pathway of extrusion. TLCP melt flow through foam metal inserts causes a rapid creation of large number of small streams of varying cross-section that subjects all regions of the flowing body to intense shearing leading to high orientation in individual streams. These individual streams of varying velocity and direction interact to form a structure similar to a woven rope. As a direct result of this randomization effect of these small streams, the extruded liquid crystal polymer tends toward isotropic macro properties. The degree of orientation anisotropy is evidenced by scanning electron microscopy (SEM) and micro beam wide angle x-ray diffraction (WAXD) techniques and contrasted in samples extruded with and without the presence metal foam inserts. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers

Structure and properties of thermotropic liquid crystal polymer and poly(ethylene 2,6‐naphthalate) blend fibers

AbstractBACKGROUND: The melt blending of thermotropic liquid crystal polymers (TLCPs) using conventional thermoplastics has attracted much attention due to the improved strength and tensile modulus of the resulting polymer composites. Moreover, because of their low melt viscosity, the addition of small amounts of TLCPs can reduce the melt viscosity of polymer blends, thereby enhancing the processability.RESULTS: In this study, TLCP/poly(ethylene 2,6‐naphthalate) (PEN) blend fibers were prepared by melt blending and melt spinning to improve fiber performance and processability. The relation between the structure and the mechanical properties of TLCP/PEN blend fibers and the effect of annealing on these properties were also investigated. The mechanical properties of the blend fibers were improved by increasing the spinning speed and by adding TLCP. These properties of the blend fibers were also improved by annealing. The tensile strength of TLCP5/PEN spun at a spinning speed of 2.0 km h−1 and annealed at 235 °C for 2 h was about three times higher than that of TLCP5/PEN spun at a spinning speed of 0.5 km h−1. The double melting behavior observed in the annealed fibers depended on the annealing temperature and time.CONCLUSION: The improvement of the mechanical properties of the blend fibers with spinning speed, by adding TLCP and by annealing was attributed to an increase in crystallite size, an increase in the degree of crystallinity and an improvement in crystal perfection. The double melting behavior was influenced by the distribution in lamella thickness that occurred because of a melt‐reorganization process during differential scanning calorimetry scans. Copyright © 2007 Society of Chemical Industry

Novel CO2 laser drawing of thermotropic liquid crystal polymer and poly(ethylene 2,6‐naphthalate) blend fibers

AbstractThermotropic liquid crystal polymer (TLCP)/poly(ethylene 2,6‐naphthalate) (PEN) were prepared by a melt blending, and were melt spun by a spin‐draw process. In this study, we suggest novel drawing technology using the CO2 laser that can directly and uniformly heat up fiber inside to prevent the formation of ununiform structures in conventional heat drawing process. The properties of the heat/laser drawn TLCP/PEN blend fibers were superior to those of any other handled fibers, and were rather more excellent than those of TLCP/PEN blend fibers annealed at 135°C for 10 min. It was confirmed that the CO2 laser drawing made it possible to achieve the optimal drawing effect by draw ratio. The combined heating and CO2 laser‐drawing method has a great potential for industrial applications as a novel fiber‐drawing process, and it can also be applied continuously to conventional spin‐draw system. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 205–211, 2007

Development of The Novel Screen for Ultra-thin Film Printing Using Monofilaments of Thermotropic Liquid Crystal Polymer

Development of The Novel Screen for Ultra-thin Film Printing Using Monofilaments of Thermotropic Liquid Crystal Polymer

Thermotropic liquid crystal polymer reinforced poly(butylene terephthalate) composites to improve heat distortion temperature and mechanical properties

Thermotropic liquid crystal polymer (TLCP)-reinforced poly(butylene terephthalate) (PBT) composites were prepared by melt processing. The improvement in the mechanical properties and the processability of the PBT/TLCP composites was attributed to the reinforcing effect by TLCP phase and its well distribution in the PBT matrix. X-ray diffraction results demonstrated that a slow cooling process leads to the thicker lamellar structures and the formation of more regular crystallites in the composites. The incorporation of TLCP improves not only the tensile strength and flexural modulus but also the heat distortion temperature (HDT) of the PBT/TLCP composites. The HDT values of the composites were dependent on TLCP content. The improvement in the HDT values of the PBT/TLCP composites may be explained in terms with the increased flexural modulus, the development of more regular crystalline structures, and the enhancement of the ability of the composites to sustain the storage modulus by TLCP phase. In addition, the simple additivity rule makes it possible to predict the HDT values of the PBT/TLCP composites.

Preparation and mechanical properties of TLCP/ UP / GF in-situ hybrid composites

One kind of novel reactive thermotropic liquid crystalline polymer-methacryloyl copolymer (LCMC) containing polyester mesogenic units was synthesized. Its structure, morphology and properties were investigated systemically by Ubbelohde viscometer, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffractometry (WAXD) and polarizing optical microscopy (POM). The results indicate that it is one kind of nematic thermotropic liquid crystal polymer (TLCP). The impact strength, bending strength and the morphologies of impact fracture surface of LCMC, unsaturated polyester (UP) and glass fiber (GF) in-situ hybrid composites were studied by Izod impact tester, universal testing machine and scanning electron microscopy (SEM), respectively. The results show that the impact and bending strength of composites containing LCMC are improved, especially the composites containing 5% LCMC increases most obviously. These results with SEM results reveal that LCMC plays an important role in the improvement of interfacial adhesive between matrix and fiber.

Strengthening of highly heat-resistant liquid-crystal polymer fibers upon heat treatment

Fibers of fully aromatic thermotropic liquid-crystal polymers containing mesogenic groups in the backbone are prepared on laboratory machines that make it possible to vary the conditions of jet spinning and subsequent heat treatment. The thermal and mechanical properties of the polymer fibers are investigated using differential thermal and thermomechanical analyses and strength measurements. It is found that, after spinning under optimum conditions, the strength of the fibers at room temperature is approximately equal to 1 GPa. After heat treatment under specially chosen conditions, the strength increases by a factor of two or three due to an increase in the activation energy of fracture. The role of crystallization and cross-linking in strengthening of liquid-crystal polymer fibers is elucidated.

Structure and property relationship of thermotropic liquid crystal polymer and polyester composite fibers

AbstractPoly(ethylene 2,6‐naphthalate) (PEN) and poly(ethylene terephthalate) (PET) composite fibers reinforced with a thermotropic liquid crystal polymer (TLCP) were prepared by the melt blending and spinning process to achieve high performance fibers with improved processability. Polymer composite fibers consisting of cheap polyester and small quantity of expensive TLCP are of interest from an economic point of view and from an industrial perspective. The increase in the birefringence and density of the TLCP/PEN/PET composite fibers with the spinning speed was attributable to the enhancement of the molecular orientation and effective packing between chains in the TLCP/PEN/PET composite fibers. Annealing process resulted in the formation of more ordered and perfect crystalline structure and higher crystallinity, improving the mechanical properties of the TLCP/PEN/PET composite fibers. The increase in the crystallite size and the degree of chain extension with increasing spinning speed resulted in the gradual increment of the long period for the TLCP/PEN/PET composite fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006.

In Situ fibril formation of thermotropic liquid crystal polymer in polyesters blends

AbstractPolymer blends based on poly(ethylene 2,6‐naphthalate) (PEN) and poly‐(ethylene terephthalate) (PET) reinforced with a thermotropic liquid crystal polymer (TLCP) were prepared using a melt blending process. Polymer blends consisting of conventional cheap polyester with a small quantity of expensive TLCP are of interest from an economic point of view. The shear viscosity of the TLCP and polyester blends decreased with increasing shear rate and depended on TLCP content. The lower values of the structural viscosity index for the TLCP and polyester blends were attributed to the formation of fibrillar TLCP structures having elongated fibrils in the polyester matrix. The TLCP/PEN blends exhibited long TLCP fibrils that had smaller average diameters and narrower distributions of the diameter compared with those of the TLCP/PET blends. The higher shear force and lower viscosity ratio observed may favor the in situ TLCP fibril formation in the polyester matrix. The viscosity ratio was the most crucial factor in controlling the morphology of the TLCP phase in the TLCP and polyester blends. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3600–3610, 2005

The linear viscoelastic behavior of a series of molecular weights of the thermotropic main-chain liquid crystal polymers HBA/HNA 73/27

In this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid (HBA)/2,6 hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30 000– > 51 000. The thermotropic main-chain liquid crystal polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes, as detected in strain sweep experiments performed at a constant frequency of 1, 5, 10, 30, 50, and 100 rad/s. The onset for nonlinearity (γc) significantly decreases (from ∼35% to ∼5%) as the applied frequency (ω) increases, independently of the molecular weight. When the results are plotted in terms of G*/G0* (the complex modulus normalized to the corresponding linear viscoelastic value) as a function of the stress amplitude, the different curves do not seem to collapse upon each other in the large frequency range investigated in this paper. Indeed, the stress onset for nonlinearity increases as the applied frequency (ω) increases (σc∼700 Pa at ω=1 rad/s to σc∼2000 Pa at ω=100 rad/s for Mw=51 000), opposite to the trend shown by the strain onset. The behavior of these TLCPs seems, therefore, intermediate between having a characteristic strain and stress level for the occurrence of nonlinearity. The nonlinear onset dependence on the frequency has been, then, analyzed by horizontally shifting the normalized G*(γ) profiles curves at different ω values and a master curve has been obtained. The frequency behavior of these TLCPs has also been studied showing that the lower molecular weight samples are characterized by a prevalent viscous behavior at high frequencies (G″ > G′), while decreasing ω the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples (Mw=51 000 and Mw > 51 000) show a linear viscoelastic behavior that resembles that of conventional flexible polymers with a terminal regime at the lower frequencies and a transition to the rubber-like region at higher ω. Our results support, then, the hypothesis suggested in the literature [Romo-Uribe (2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic copolyester, whose contribution increase with increasing the molecular weight.

Fiber property and structure development of polyester blend fibers reinforced with a thermotropic liquid‐crystal polymer

AbstractTernary blend fibers (TBFs), based on melt blends of poly(ethylene 2,6‐naphthalate), poly(ethylene terephthalate), and a thermotropic liquid‐crystal polymer (TLCP), were prepared by a process of melt blending and spinning to achieve high‐performance fibers. The reinforcement effect of the polymer matrix by the TLCP component, the fibrillar structure with TLCP fibrils of high aspect ratios, and the development of more ordered and perfect crystalline structures by an annealing process resulted in the improvement of the tensile strength and modulus for the TBFs. An increase in the apparent crystallite size with the spinning speed was attributed to the development of larger crystallites and more ordered crystalline structures in the annealed TBFs. The birefringence and density of the TBFs increased with increasing spinning speed, the TBFs becoming more oriented and the crystal packing becoming more enhanced. The molecular orientation was an important factor in determining the tensile strength and modulus of the TBFs. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 395–403, 2004