Carboxylated multiwall carbon nanotube-reinforced thermotropic liquid crystalline polymer nanocomposites

Thermotropic liquid crystalline polymer (TLCP) nanocomposites reinforced with carboxylated multiwall carbon nanotube (c-MWCNT) were prepared through melt compounding in a twin screw extruder. The thermal stability of TLCP/c-MWCNT nanocomposites increased with even a small amount of c-MWCNT added. The rheological properties of the TLCP/c-MWCNT nanocomposites were depended on the c-MWCNT contents. The contents of c-MWCNT have a slight effect on the complex viscosity of TLCP/c-MWCNT nanocomposites due to the high-shear thinning of TLCP. The storage modulus of TLCP/c-MWCNT nanocomposites was increased with increasing c-MWCNT content. This result can be deduced that the nanotube–nanotube interactions were more dominant, and some interconnected or network-like structures were formed in the TLCP/c-MWCNT nanocomposites. Incorporation of very small amount of c-MWCNT improved the mechanical properties of TLCP/c-MWCNT nanocomposites, and this was attributed to the reinforcement effect of c-MWCNT with high aspect ratio and their uniform dispersion through acid treatment in the TLCP matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microstructure of carbon nanofiber/thermotropic liquid crystalline polymer composites

The properties and microstructure of a thermotropic liquid crystalline polymer (TLCP, Vectran V400P) were investigated in the presence of carbon nanofibers (CNF). The electrical conductivity of TLCP increased with an addition of CNFs. The thermal analysis of pure TLCP and its composites revealed that a glass transition at ～ 110°C did not change significantly. However, a decrease of tensile modulus and strength was observed with the addition of CNFs. WAXD studies showed a decrease of Herman's orientation parameter, indicating reduction of anisotropy of TLCP. Further, the disruption of molecular orientation of TLCPs was inferred by SEM and TEM analysis. SEM micrographs revealed a fibrillar structure for pure TLCPs at a macro‐scale (2–5 μm). However, this structure was not observed in composites at the same scale even though micro‐size fibrils (0.05 μm) were found with the addition of CNFs. TEM micrographs displayed banded structures of pure TLCPs, but these structures were not significant in the vicinity of CNFs. These observations confirmed that a decrease of molecular alignment and disruption of fibrillar structure of TLCP, in the presence of nanofibers, are attributed to a significant decrease in tensile modulus and strength. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Separation of a thermotropic liquid crystalline polymer from polypropylene composites

This work is concerned with determining how to effectively recycle wholly thermoplastic composites comprised of a polypropylene (PP) matrix reinforced with a thermotropic liquid crystalline polymer (TLCP). A novel reclamation process was developed in which the TLCP could be recovered from the PP matrix. Reactive extrusion was used to reduce the molecular weight of the PP (Montell 6523) and to facilitate phase separation. The melt was then extruded into a heated mineral oil bath, which separated the TLCP (DuPont HX8000) from the matrix by dissolving the PP. It was found that greater than 70 wt% of the TLCP could be reclaimed from the PP matrix at a purity of greater than 96 wt%. In order to determine the ability to reuse the reclaimed HX8000, injection molded in situ composites were generated and their mechanical properties were determined. When the neat HX8000 component was partially replaced with reclaimed HX8000, the injection molded TLCP/PP composites showed no discernible difference in mechanical properties.     

Elastic moduli of a liquid crystalline polymer and its in-situ composites

The elastic moduli of a liquid crystalline polyesteramide (LCP) and polycarbonate/LCP in-situ composites with 10 to 80 wt% of LCP have been measured as functions of draw ratio λ from 1 to 15 by an ultrasonic method. For the LCP, the sharp rise of the axial Young's modulus E₃ and the slight decreases of the transverse Young's modulus E₁ and the axial (C₄₄) and transverse (C₆₆) shear modulus with increasing λ result from the alignment of chains along the draw direction. E₁, C₄₄, and C₆₆ follow the lower bounds calculated using the series coupling scheme of the aggregate model. Although E₃ lies close to the lower bound at low λ, it follows the upper bound calculated according to the parallel coupling scheme at λ > 3. The elastic moduli of the composites have similar draw ratio dependences as those of the LCP. The strong increase in E₃ with increasing λ arises from the higher aspect ratio of the LCP domains in the composites and the improved molecular orientation within the domains. The reinforcement effect on the other moduli is much weaker, with E₁ and C₄₄ of the composites only 5 to 30% higher than those of polycarbonate at λ = 15. Since C₆₆ of the LCP decreases to a value below that of polycarbonate at λ > 2, there is a positive reinforcement effect at low λ but a negative effect at high λ.     

Orientation development in molding of short-fiber liquid crystalline polymer composites

The orientation development mechanisms that take place during molding of short-fiber reinforced liquid crystalline polymer composites, have been analyzed and consequently the orientation development and properties distribution were studied for the case of thin wall large area components. Two factors associated with orientation development were investigated. The first is related to the melt front shape for in-plane orientation distribution, and the second to the thickness of the skin layer where the alignment of the orientable liquid crystalline matrix and the rigid reinforcing fibers are of importance. The two basic factors were first verified by flow calculations using a computer assisted design (CAD) system and then experimentally validated. Experimental results for thin wall large area plaques have shown that the control over the melt front shape could be achieved with a narrow restrictor-type gate and that the skin layer thickness is determined by the heat transfer processes during filling of the mold. Furthermore, a clear correlation has been established between the measured tensile properties and the ratio of skin layer to total thickness of the plaque. Such correlations should be generally developed for short-fiber polymer composites for cases where rigidity and load bearing capabilities are of importance.     

Liquid crystalline epoxy resins in polymer dispersed liquid crystal composites

Liquid crystalline epoxy resins can be used as a matrix to encapsulate low molecular weight liquid crystalline droplets to produce PDLC devices. The presence of the low mass liquid crystal decreases the rate of reaction of the resin, that also hardens in a mesomorphic structure for high concentrations of liquid crystal. The optical characterization of the PDLC indicates an improvement of the angular transmission of visible light. ©1997 SCI     

Elastic modulus of in-situ composites of a liquid crystalline polymer and polycarbonate

In this paper, we model the elastic modulus of in-situ composite fibers from polymer blends where a fibrous liquid crystalline polymer (LCP) phase is induced by drawing. We propose a composite model to account for the change of the elastic moduli of the reinforcing LCP phase with the draw ratio of the composite fibers. We envisage the LCP phase as a composite of a perfectly oriented chain aggregate and a randomly oriented chain aggregate which are connected in series. We then derive equations for the longitudinal and the transverse elastic moduli of the composite fibers based on the well-known Halpin-Tsai equation and the composite model of the reinforcing LCP phase. Using this approach, we are able to make a number of predictions including the transverse elastic modulus and mechanical anisotropy. Our results show that theoretical predictions of the longitudinal elastic modulus agree fairly well with experimental results for polycarbonate/Vectra composites. The proposed modulus equations will be useful in providing guidelines for fabrication and applications of this new class of polymeric materials.     

Alignment and properties of carbon nanotube buckypaper/liquid crystalline polymer composites

Carbon nanotubes (CNTs) have been recognized as a potential superior reinforcement for high‐performance, multifunctional composites. However, non‐uniform CNT dispersion within the polymer matrix, the lack of adequate adhesion between the constituents of the composites, and lack of nanotube alignment have hindered significant improvements in composite performance. In this study, we present the development of a layer‐by‐layer assembly method to produce high mechanical performance and electrical conductivity CNT‐reinforced liquid crystalline polymer (LCP) composites using CNT sheets or buckypaper (BP) and self‐reinforcing polyphenylene resin, Parmax. The Parmax/BP composite morphology, X‐ray diffraction, mechanical, thermal, and electrical properties have been investigated. SEM observations and X‐ray diffraction demonstrate alignment of the CNTs due to flow‐induced orientational ordering of LCP chains. The tensile strength and Young's modulus of the Parmax/BP nanocomposites with 6.23 wt % multi‐walled carbon nanotube content were 390 MPa and 33 GPa, respectively, which were substantially improved when compared to the neat LCP. Noticeable improvements in the thermal stability and glass transition temperature with increasing CNT content due to the restriction in chain mobility imposed by the CNTs was demonstrated. Moreover, the electrical conductivity of the composites increased sharply to 100.23 S/cm (from approximately 10^?13 S/cm) with the addition of CNT BP. These results suggest that the developed approach would be an effective method to fabricate high‐performance, multifunctional CNT/LCP nanocomposites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of normal stress difference on polymer drop deformation

Polypropylene drops of varying viscosity and elasticity were sheared in a polystyrene matrix. Two transparent, counter-rotating parallel disks provided simple shear flow. By adjusting the speed of one disk the drop center was fixed in the laboratory frame and deformation followed via high magnification video camera. It was found that with high matrix elasticity drops of the minor phase stretched perpendicular (x₃) to the flow direction (x₁). This is the first report of widening of drops in shear flow. An analytical relation was established between the second normal stress differences of the phases and degree of widening. The formation of sheets and the phenomena of widening results in a larger than affine area generation.     

Phase behavior of thermotropic liquid crystalline/conducting polymer blends

Summary Liquid crystalline/conducting polymer blends have been prepared. The conductingpolymer [poly(2,5-dimethoxyphenylene vinylene)] retards the liquid crystallinity of the liquid crystalline polymer (hydroxypropyl cellulose), while the liquid crystalline polymer reduces the conductivity of the conducting polymer. However, blends with 17% conducting polymer were both liquid crystalline and conductive. Dedicated to Prof. Dragutin Fleš on the occasion of his 70th birthday     

聚丙烯/热致性液晶聚合物原位复合材料的研究进展

聚丙烯是一个通用塑料,为了提高聚丙烯的性能,拓宽其应用领域,人们对聚丙烯进行了改性研究.笔者简述了热致液晶聚合物原位复合改性聚丙烯的研究进展,并讨论热致液晶聚合物对聚丙烯结晶与熔融行为、微观结构、流变性及力学性能的影响,介绍了改进TLCP与PP之间相容性的常用方法.最后对TLCP/PP原位复合材料的发展提出一些看法.     

Morphology and mechanical properties of thermoplastic composites containing a thermotropic liquid crystalline polymer

Blends of two thermotropic liquid crystalline polymers (TLCPs), with brittle and ductile matrix materials were both injection molded and spun into fibers, in order to investigate the mechanism of in-situ mechanical reinforcement. In the injection molded samples, the TLCP was only moderately elongated into fibrils, and the mechanical properties were below predictions of the rule of mixtures. Fibers spun out of the blends contained numerous fine fibrils with nearly infinite aspect ratio, and as expected, the modulus increased linearly with the TLCP volume fraction, obeying the Tsai-Halpin equation for transversely isotropic composites. Wide angle X-ray diffraction measurements, as well as determination of the fiber-moduli, revealed that during spinning not only a macroscopic elongation of the fibrils was achieved, but also a considerable molecular orientation within the TLCP domains.     

Thermal behavior and laser marking of a novel nematic liquid crystalline polymer

The thermal behavior and laser marking of a novel nematic liquid crystalline polymer is explored in this report. The glass transition and mesophase transition temperatures of this material are at 45 and 68°C, respectively, and allow the ready annealing of the nematic phase as well as conversion of the nematic to the isotropic phase by pulsed laser irradiation of metallized or dyed films of the polymer.     

Modifications of carbon for polymer composites and nanocomposites

The various forms of carbon used in composite preparation include mainly carbon-black, carbon nanotubes and nanofibers, graphite and fullerenes. This review presents a detailed literature survey on the various modifications of the carbon nanostructures for nanocomposite preparation focusing upon the works published in the last decade. The modifications of each form of carbon are considered, with a compilation of structure-property relationships of carbon-based polymer nanocomposites. Modifications in both bulk and surface modifications have been reviewed, with comparison of their mechanical, thermal, electrical and barrier properties. A synopsis of the applications of these advanced materials is presented, pointing out gaps to motivate potential research in this field.     

Annealing of drawn monofilaments of liquid crystalline polymer vectra/vapor grown carbon fiber nanocomposites

The annealing treatment of drawn LCP nanocomposite monofilaments is discussed here. Upon annealing, an unstable structure is generated which presents a strong dependence on the annealing time sequence and temperature as observed by DSC. Moreover, it seems that increasing the draw down of the fiber restricts the formation of this new (unstable) structure. Based on dynamic mechanical analysis and on solid state ¹³C NMR, the nature of the newly-formed structure is elucidated, and based on sequence ordering within the non-oriented amorphous phase in the copolymer, a 10% change in the monomer proportion in the ordered phase is observed after annealing.     

Crystallization behavior of polyphenylene sulfide in blends with a liquid crystalline polymer

Blends of polyphenylene sulfide (PPS) with a commercial, wholly aromatic, liquid crystal copolyesteramide (Vectra-B950) have been prepared by meltblending. The crystallization behavior of neat and blended PPS has been studied by differential scanning calorimetry (DSC), under both non-isothermal and isothermal conditions. It has been found that blending PPS with Vectra-B leads to an increase of the temperature of non-isothermal crystallization and to a pronounced acceleration of the isothermal crystallization, without any reduction of the degree of crystallinity. All these effects have been found to occur independent of the Vectra-B concentration, within the investigated range (2 to 20%, w/w). The results have been interpreted in terms of an incrased nucleation density of the blends, probably due to heterogeneous substances, initially present in the Vectra-B bulk, which dissolve to saturation in the PPS phase, during melt-blending.     

Thermoforming of liquid crystalline polymer sheets

Liquid crystalline polymer (LCP) extruded sheets were further processed by the conventional thermoforming method. The available processing temperature range was defined through the structural, thermal, and elevated temperature mechanical characterization of the extruded sheet. This temperature range was found for LCP to be quite narrow, in the proximity of the crystal-mesophase transition. The structural changes imposed on the LCP sheet during forming and its thermal stability were investigated using wide angle X-ray diffraction, mainly for the determination of the chain orientation distribution, DSC, and dynamic mechanical analysis. Thermoforming onto a symmetrical male mold was found to enhance the orientation in the extrusion machine direction and even change the preferred orientation in the extrusion transverse direction to orientation along the thermoforming direction. Annealing at the thermoforming temperature range results in a more ordered and thermally stable structure accompanied by just a slight orientation loss.     

Effect of fumed silica nanoparticles on glass fiber filled thermotropic liquid crystalline polymer composites

Glass fiber filled thermotropic liquid crystalline polymer (gLCP)/silica composites were prepared by melt compounding. The total torque of the gLCP/silica composites decreased and the melt flow index increased with increasing silica content, which indicates that the fumed silica nanoparticles act as good processing aids and enhance the processing behavior of gLCP/silica composites. The rheological properties of the gLCP/silica composites were significantly dependent on the silica content. The complex viscosity and storage modulus (G′) of the gLCP/silica composites decreased with increasing silica content. This was attributed to the ability of the silica nanoparticles to break the glass fiber–glass fiber interactions in the gLCPs. The storage modulus and loss modulus (G″) of the gLCP/silica composites increased with increasing frequency, and the increment was more significant at low frequency. Incorporation of a small quantity of silica nanoparticles improved the thermal stability and mechanical properties of gLCP/silica composites. However, at high silica nanoparticle content the mechanical properties of gLCP/silica composites decreased because of the aggregation of silica nanoparticles. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Properties of ternary in situ polycarbonate/polybutylene terephthalate/liquid crystalline polymer composites

Ternary in situ polycarbonate (PC)/polybutylene terephthalate (PBT)/liquid crystalline polymer (LCP) composites were prepared by injection molding. The liquid crystalline polymer used was a versatile Vectra A950. The matrix of composite was composed of PC/PBT 60/40 by weight. A solid epoxy resin (bisphenol type‐A) was used as a compatibilizer for the composites. Dynamic mechanical analysis (DMA) showed that epoxy resin was effective to improve the compatibility between PC and PBT, and between PC/PBT and LCP, respectively. Tensile tests revealed that the stiffness of composites shows little change with the LCP content up to 10 wt %. Above this concentration, the stiffness tended to increase with increasing LCP content. Furthermore, the tensile strengths appeared to increase with increasing LCP content, and their values were close to those predicted from the rule of mixtures. Scanning electron microscopic examination showed that LCP ribbons and short fibrils were developed in the composites containing LCP content ≤10 wt %. However, fine and elongated fibrils were formed in the skin and core sections of the composites when the LCP content reached 25 wt % and above. Thermogravimetric analysis indicated that the thermooxidative stability of the PC/PBT 60/40 blend tended to improve with increasing LCP content. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1827–1835, 1999