Thermotropic Liquid Crystalline Polymer Research Articles

Effect of functionalized CNTs and liquid crystalline polymer on thermo-oxidative stability of polyethylene-based hybrid composites

Hybrid composites based on polyethylene (PE) filled with thermotropic liquid crystalline polymer (LCP) and pristine or hydroxyl- or carboxylic-functionalized CNTs were prepared using melt extrusion process. Thermo-oxidative decomposition under dynamic and isothermal conditions was investigated using thermogravimetric analysis (TG). From TG profiles and activation energy analysis, the highest thermo-oxidative stability revealed from both dynamic and isothermal investigations was clearly observed for the PE-based hybrid composite filled with LCP and small amount of carboxylic-functionalized CNT. The melt rheological investigation showed that the effect of LCP and CNTs loading on viscoelastic characteristics of the composites was clearly pronounced at low frequency and the viscous part was found as a dominant factor. Complex viscosity, storage and loss moduli of the hybrid composite containing carboxylic-functionalized CNT showed the highest values compared among all samples examined. The morphological results observed using scanning electron microscopy and transmission electron microscopy indicated the improved dispersion of both LCP and carboxylic-functionalized CNT fillers. The obtained results indicated the importance of LCP-assisted dispersion of carboxylic-functionalized CNT through interface interaction, resulting in the remarkable improvement in thermo-oxidative stability of the composite.

Isothermal crystallization of blends containing two thermotropic liquid crystalline polymers

The intention of this work is to determine how the isothermal crystallization behavior of a thermotropic liquid crystalline polymer (TLCP) can be adjusted by blending it with a pure TLCP of lower melting point. One TLCP (Tm ∼350°C) used is a copolyester synthesized from terephthalic acid, 4-hydroxybenzoic acid, hydroquinone and hydroquinone derivatives. The other TLCP (Tm ∼280°C) is a copolyesteramide of 60mol% hydroxynaphthoic acid, 20mol% terephthalic acid and 20mol% 4-aminophenol. As the content of the low melting TLCP increases in the blends, the temperature at which isothermal crystallization occurs decreases. Comparing with pure TLCPs, the blend of 75% low melting TLCP crystallizes at a lower temperature than the pure matrix, and the blend remains as a stable super-cooled fluid in the temperature range from 220 to 280°C for 1200s. Because of the low energy released during the phase transition, small amplitude oscillatory shear (SAOS) is more sensitive to detecting the onset of isothermal crystallization than differential scanning calorimetry (DSC).

The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Abstract To determine the processing temperature of a representative thermotropic liquid crystalline polymer (TLCP) in the super-cooled state, small amplitude oscillatory shear (SAOS), the startup of shear flow and differential scanning calorimetry (DSC) measurements were performed. Good agreement on the onset crystallization temperature was obtained from the SAOS and the startup of shear flow. Compared with DSC, the rheological measurements were more sensitive for detecting crystallization in the super-cooled state. The rheological study suggested that crystallization did not occur unless the TLCP was cooled 30 °C below its melting point, and its viscosity was significantly increased as the material was super cooled. Additionally, super cooling the TLCP did not significantly affect the relaxation of shear stress after preshearing. However, the recovery of transient shear stress in the interrupted shear measurements was suppressed to a great extent in the super-cooled state.

In situ composites based on blends of PEEK and thermotropic liquid crystalline polymer

ABSTRACTA series of blends were prepared by mixing pellets of the thermotropic liquid crystalline polymers (TLCP) with poly (ether ether ketone)(PEEK) in the dry state under injection mouldingmolding conditions. The mechanical properties of blends were measured, and it is proved that the mechanical properties of blends are better than those of their matrix. Compared with the thermoplastic matrix, the tensile modulus and strength of the blends increased obviously, with the addition of only 2 wt% LCP. At the same time, the addition of TLCP decreased the melt viscosity of the blends.

Morphology evolution of a thermotropic liquid‐crystalline polymer in a polyamide 6,6 matrix regulated by graphene

ABSTRACTA two‐step process, thermotropic liquid‐crystalline polymer (TLCP) premixing with reduced graphene oxide (RGO) followed by blending with polyamide 6,6 (PA66), was used to prepare ternary TLCP/RGO/PA66 blends. The rheological behaviors, morphology, and mechanical properties of the blends were investigated. The results show that RGO migrated from the TLCP phase to the interface between the TLCP and PA66 phase during melt blending; this was due to a similar affinity of the RGO nanosheets to both component polymers. The dimensions of the dispersive TLCP domains were markedly reduced with the mounting RGO content; this revealed a good compatibilization effect of RGO on the immiscible polymers. The hierarchical structures of the TLCP fibrils were found in both the unfilled TLCP/PA66 blends and TLCP/RGO/PA66 blends. This supposedly resulted from the extensional and torsional action of unstable capillary flow. With the addition of RGO, the viscosities of the blends decreased further, and the fibrillation of TLCP and the mechanical performance of TLCP composites were both enhanced. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43735.

Study on the Isothermal Crystallization Behaviors of PEN/TLCP Blends

Abstract: The isothermal crystallization behaviors of blends of poly(ethylene naphthalate) (PEN) and a thermotropic liquidcrystalline polymer (TLCP) were investigated by differential scanning calorimetry (DSC) as functions of crystallization tem-perature and blend composition. Avrami analyses were applied to obtain information on the crystal growth geometry andthe factors controlling the rate of crystallization. The crystallization kinetics of the PEN/TLCP blends followed the Avramiequation up to a high degree of crystallization, regardless of crystallization temperature. The calculated Avrami exponentsfor PEN/TLCP revealed three-dimensional growth of the crystalline region in each blend. The crystallization rate of eachblend increased as the crystallization temperature decreased, and decreased as the TLCP content increased. The crystalli-zation of PEN in the blend was affected by the addition of TLCP, which acts as a nucleating agent.Keywords: Avrami equation, isothermal crystallization kinetics, nucleating agent, PEN/TLCP blend

In-depth analysis on thermal phase transition mechanism of main-chain phosphorus-containing thermotropic liquid crystal copolyester

ABSTRACTIn order to surmount drawbacks of the infrared spectroscopy (IR) itself during investigating the mesophase-transition behaviours and mechanism of the thermotropic liquid crystalline polymers (TLCPs), the elemental phosphorus as an internal marker was introduced into the main-chain TLCPs. The detail mechanism of the glass transition and mesophase phase transition of the phosphorus-containing aromatic liquid crystalline copolyester (poly(-hydroxybenzate-co-DOPO-benzenediol dihydrodiphenyl ether terephthalate) [PHDDT]) was revealed through tracing the internal marker with the perturbation correlation moving window 2-dimensional correlation and 2-dimensional correlation analysis (2DCOS) correlation IR spectra. The results showed that the phosphorus-containing unit did not participate in the glass transition of the PHDDT. The results of the 2DCOS showed that the PHDDT mesophase phase transition took place through adjustment of the phosphorus-containing units. Simultaneously, the adjustment of the phosphorus-containing unit also can induce the motion of the other groups, and the sequential orders of the spectral changes were Ar–O–Ar → ester C–O → C=O. However, the sequential orders of the spectral changes were converse during the PHDDT glass transition.

High-performance liquid-crystalline polymer films for monolithic “composites”

Highly oriented, free-standing films of a thermotropic liquid-crystalline polymer (LCP) were prepared using a new processing technique, referred to as “foil-spintrusion”. These films underwent solid-state fusion at elevated temperatures to yield homogeneous, self-reinforced monolithic “composites” without the use of additional adhesives. In this study, unidirectional LCP monoliths were evaluated for their mechanical performance. The monoliths exhibited highly anisotropic behavior, with stiffness and tensile strength along the direction of orientation of around 65GPa and 1.7GPa, respectively, combined with excellent damping properties in the viscoelastic regime. In addition, experiments showed that the material has a high tensile strength (>2.2GPa) at high strain rates, indicating promising behavior under high impact conditions. As expected for such highly oriented polymer systems, the monoliths were found to exhibit poor compressive and shear strengths due to low internal coherence of the material itself. However, it was shown that the LCP monoliths could be useful for applications requiring high specific stiffness, for example as facings for composite sandwich panels.

Synthesis and characterization of thermotropic liquid crystalline polyimides

Non-symmetrical and linear dyad-based mesogens were synthesised containing imine or ester bridging group. In the present work, due to the absence of branching in diamine-based mesogen, the structure has—rigidity inversely imine/ester bridging groups between two benzene rings imparts—flexible property to the mesogen and consequently rigid–flexible property has been balanced. The synthesised mesogens were characterized by different techniques including nuclear magnetic resonance and Fourier transform infrared spectroscopy. Liquid crystalline polymers (LCPs) were synthesised using pyromellitic dianhydride and 4-[(4-aminobenzylidene)amino]aniline or 4-aminophenyl-4-aminobenzoate. Subsequently, thermotropic liquid crystalline polymers (TLCPs) have also been evaluated to obtain optical microscopy textures at different temperatures which demonstrated interesting and notable changes. It is worth noting that marble-like textures were observed upto 200°C.

Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a Liquid Crystalline Triblock Copolymer in Ionic Liquid

Ion gels with tunable storage moduli are prepared through the self-assembly of an ABA triblock copolymer AOA-12 comprised of a thermotropic liquid crystalline (LC) polymer as the end-block A and poly(ethylene oxide) (PEO) as the mid-block B in a room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMIM][TFSI]). The PEO mid-block is well-solvated by this ionic liquid, whereas the LC polymer end-blocks aggregate and serve as the physical cross-linkers. For comparison, a triblock copolymer AOA-0 containing a non-LC side-chain polymer with the same mesogen is also synthesized, and its corresponding ion gel is prepared. The ion gels with relatively high concentrations of the LC triblock copolymer have hierarchical structures with different microphase-separated nanostructures and the LC arrangement of the LC blocks. By incorporating the azobenzene mesogen in the side chains, transparent AOA-n/[EMIM][TFSI] (where n is the number of carbon atoms in the spacer between ...

액정 고분자/에틸렌-아크릴산 이오노머 블렌드의 반응상용화에 관한 연구

This paper describes the reactive compatibilization of blends of a wholly aromatic thermotropic copolyester liquid crystalline polymer (TLCP) with random copolymers of ethylene and acrylic acid (EAA) and their salts. Blends were prepared by melt mixing in an intensive batch mixer, and the formation of a graft copolymer due to acidolysis between the TLCP and the acrylic acid group of the ionomer was evaluated. Chemical reaction was assessed by torque measurement during melt mixing and by thermal analysis and morphological observation. The Na-salt of the EAA ionomers was especially effective at promoting a grafting reaction. The extent of reaction depended not only on the cation, but also composition of the ionomer and reaction time. The product of the grafting reaction between the TLCP and a sodium-neutralized ionomer proved to be an effective compatibilizer for TLCP and EAA ionomers.

Liquid crystalline polymer nanocomposites reinforced with in-situ reduced graphene oxide

In this work liquid-crystalline polymer (LCP) nanocomposites reinforced with in-situ reduced graphene oxide are investigated. Graphene oxide (GO) was first synthesized by the Hummers method, and the kinetics of its thermal reduc- tion was assessed. GO layers were then homogeneously dispersed in a thermotropic liquid crystalline polymer matrix (Vec- tran ® ), and an in-situ thermal reduction of GO into reduced graphene oxide (rGO) was performed. Even at low rGO amount, the resulting nanocomposites exhibited an enhancement of both the mechanical properties and the thermal stabil- ity. Improvements of the creep stability and of the thermo-mechanical behavior were also observed upon nanofiller incor- poration. Furthermore, in-situ thermal reduction of the insulating GO into the more electrically conductive rGO led to an important surface resistivity decrease in the nanofilled samples.

Synergistic Reinforcement of Phenol-Formaldehyde Resin Composites by Poly(Hexanedithiol)/Graphene Oxide

In this paper, the preparation of graphene oxide was achieved by Hummers method and the surface modification was achieved by poly(hexaneditiol), which was a synthetic thermotropic liquid crystalline polymer. The c-PHDT/GO/PF composites were prepared by blending, rolling and compression molding techniques. Then, the as-prepared samples were characterized by FTIR, Raman, XRD, TGA and POM to obtain information on their structures and properties. After that, the effects of c-PHDT/GO content on the mechanical properties, friction performance and dynamic mechanical performance of c-PHDT/GO/PF composites were studied by Mechanical and Dynamic Mechanical Analysis (DMA) methods. Also, Scanning Electron Microscope (SEM) was used for the characterization of wear and fracture surface morphology. The results revealed that the reinforcing effect of c-PHDT/GO was significant as a considerable enhancement on the mechanical performance of c-PHDT/GO/PF composite as compared to pure phenol-formaldehyde composites was observed: the impact strength, bending modulus and bending strength increased from 1.63 kJ/m2, 8.61 GPa and 41.55 MPa to 2.31 kJ/m2, 10.16 GPa and 54.40 MPa respectively at the c-PHDT/GO content = 0.75%. Moreover, the initial storage modulus increased by 28.4%, while the wear mass loss decreased by 17.8%. More importantly, the reinforcement by c-PHDT/GO was further enhanced as compared to GO/PF and p-PHDT/GO/PF composites, the impact strength of c-PHDT/GO/PF composite increased by 27.6% and 11.1%, the bending strength increased by 11.8% and 7.6%, the initial storage modulus increased by 16.2% and 4.2% and the mass loss due to wear decreased by 12.7% and 8.8%, respectively. Based on these results, we can conclude that the surface modification of GO by poly(hexanedithiol), which includes synergistic effect by c-PHDT and GO, improves the interfacial adhesion between GO and the resin matrix, thus reinforcing the composites.

Incompatible blends of liquid crystalline polymers and engineering thermoplastic

The increasing demand of the high technology market for new applications in the aerospace industry has led to research on high performance thermoplastics as composite matrices. The inclusion of solid fibres in this class of materials during manufacturing may result in processing conditions that are too demanding. High temperature and pressure in stamping or injecting tools may result in thermal degradation of the organic matrix. Moreover, the load transfer through the fibre–matrix interface produces fibres breaking into fragments shorter than the critical length, affecting their aspect ratio, and consequently reducing the final performance of the product. The possibility of producing the reinforcing filler during the cooling cycle of the composite processing has suggested the definition of 'in situ' composite. In this paper the addition of thermotropic liquid crystalline polymer to engineering thermoplastics has been investigated. X–ray diffraction techniques have been used to evaluate the molecular orientation of the dispersed phase. The rheology and the mechanical properties of the in situ composites is related to the morphology of the material and to the processing operations.

Polarimetry-controlled fluorescent color in oriented LC biopolyesters

Oriented films of the thermotropic liquid-crystalline (LC) polymer, poly{3-benzylidene amino-4-hydroxybenzoic acid (3,4-BAHBA)-co-trans-4-hydroxycinnamic acid (4HCA)} (Poly(3,4-BAHBA-co-4HCA)), were prepared by shearing the LC melt. X-Ray and polarized microscopy analyses indicated that the main chains were oriented perpendicularly to the shear direction, and the benzylidene amino side chains were parallely oriented. Furthermore, the oriented film showed polarized fluorescence in which the analyzer rotation changed the emission color when the polarizer was set perpendicular to the sheared direction. Open image in new window

Thermal Degradation and Kinetic Studies of New Flame-Retardant Phosphorus-Containing Polymers with Liquid Crystalline Properties

In this study, two new thermotropic liquid crystalline phosphorus-containing polymers were prepared by polycondensation reaction of 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)carbinol) with two different phosphonic dichlorides, namely ethyl dichlorophosphate and phenyl dichlorophosphate. The polymers were characterized by means of FT-IR, 1H NMR, differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction, and thermogravimetric analysis. Thermogravimetric analysis was performed in nitrogen atmosphere by heating the samples over a range of temperatures from 25° to 700°C using five different heating rates. The kinetic processing of data was achieved using the Freeman-Carroll, ASTM E1641, and Kissinger methods.

Thermotropic Polyesters from 2,5-Furandicarboxylic Acid and Vanillic Acid: Synthesis, Thermal Properties, Melt Behavior, and Mechanical Performance

In this work, we address the synthesis of novel aromatic–aliphatic biobased polyesters showing thermotropic behavior in the melt. Successful incorporation of different biobased monomers such as 2,5-furandicarboxylic acid (2,5-FDCA), suberic acid (SuA), and vanillic acid (VA) in thermotropic liquid crystalline polymers (TLCPs) is made possible by performing synthesis at low temperatures. The chemical structures, molecular weights, phase transitions, thermal behavior, and mechanical performance of the synthesized polymers are studied using polarization optical microscopy, WAXD, DSC, TGA, DMTA, solid-state NMR spectroscopy, rheology, and tensile tests. It is shown that the incorporation of the rigid, aromatic 2,5-FDCA moiety enhances the formation of blocky copolymers, whereas the VA moiety tends to decrease the block formation. However, when combined, nonblocky TLCPs containing 2,5-FDCA and VA with high aromatic content can be obtained. These materials show a low temperature transition from the crystalline ...

An Easy Route to Lower-Destructive and Highly Functionalized Multi-Walled Carbon Nanotubes with Thermotropic Liquid Crystalline Polymer on a Large Scale

An easy and less-destructive method has been developed to highly functionalize multi-walled carbon nanotubes (MWCNTs) with thermotropic liquid crystalline polymer (TLCP) via an elimination reaction between TLCP terminated phenolic hydroxyl groups and p-benzyl chloride groups of active poly(4-chloromethyl styrene) (PCMS) on the surface of tubes, where MWCNTs were functionalized with PCMS through the esterification reaction of carboxylate of the former and p-benzyl chloride groups of the latter in the presence of a phase-transfer catalyst. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) confirmed liquid crystal behavior of TLCP. The high density of covalent bonding between TLCP and nanotubes was confirmed by FTIR and Raman, resulting in a high grafting efficiency, as high as 18 wt% of TGA. Microscopic observations clearly displayed thick TLCP layers formed on the MWCNT surface.

Phosphorus-containing thermotropic liquid crystalline polymers: a class of efficient polymeric flame retardants

The flame retardation of polymeric materials can be achieved by addition of small-molecule flame retardants. However, traditional small molecule flame retardants exhibit practical drawbacks during application, such as migration problems, the deterioration of polymer performance, as well as potential persistence, bio-accumulation and toxicity (PBT), etc. High molecular weight molecules have been found to be less accessible by living organisms and so have an automatically lower PBT profiles than small molecules. Phosphorus-containing thermotropic liquid crystalline polymers have proven to be a class of efficient polymeric flame retardants, which can overcome the aforementioned drawbacks of small molecules and have potential industrial applications to replace some of the existing small molecule flame retardants. The recent relevant developments are reviewed in this article.

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.