A phenomenological model for the viscosity reductions in blends of HMMPE containing a small quantity of thermotropic liquid crystalline copolyester HBA/HQ/SA

In this paper, we present a phenomenological model for the viscosity changes in bulk high molecular mass polyethylene (HMMPE) due to the addition of a very small quantity of a main chain longitudinal thermotropic liquid crystalline polymer (TLCP) containing flexible spacers. The chain alignment in the elongated TLCP domains causes chain alignment and disengagement in the neighboring HMMPE melt. In converging capillary flows, this occurs at a certain critical centerline velocity. After the onset of such transition, melt of elongated chain conformations forms from the center core and expands towards the capillary wall with increasing flow rates. The model successfully predicts both the drastic viscosity reduction effects and the critical yield stress in the HMMPE+TLCP blends without any adjustable parameters. Our model is also applicable to other systems that undergo flow-induced phase transitions, e.g. in biphasic liquid crystalline polymer melts.     

Fibrillation of thermotropic liquid crystalline polymer enhanced by nano-clay in nylon-6 matrix

The influence of well-dispersed nano-clay filler on the morphology of thermotropic liquid crystalline polymer (TLCP) in nylon-6 matrix was investigated by melt extrusion process. The good dispersion of clay in the hybrid blends was confirmed by X-ray diffraction, transmission electron microscopy and rheological measurement. Morphological observation showed that the clay platelets had dramatic influences on the dispersion and deformation of TLCP phase. The TLCP droplets got smaller at the clay content ≤3 wt%, and deformed into fibrils at the clay content up to 5 and 7 wt%. The morphology evolution of TLCP in the hybrid blends, especially at 7 wt% of clay loading, was consistent well with the prediction based on the micro-rheology parameters such as the viscosity ratio of the dispersed phase to the matrix (η_d/η_m) and the ratio of capillary number to the critical capillary number (Ca/Ca_crit). This enhanced fibrillation of TLCP droplets was attributed to the role of nano-clay particles as a compatibilizer to improve the interfacial adhesion and to suppress the interfacial slip between TLCP and nylon phases in the melt, so that the shear stress was effectively transferred to the dispersed TLCP phase.     

Experimental measurement and numerical simulation of viscosity reduction effects in HMMPE containing a small amount of exfoliated organoclay-modified TLCP composite

A small amount (1 wt%) of organoclay-modified thermotropic liquid crystalline polymer (TLCP) acting as a viscosity reduction agent in high molecular mass polyethylene (HMMPE) was characterized and compared with purified TLCP (1 wt%) in HMMPE at 190 °C and 230 °C, respectively, where the TLCP displayed nematic and nematic-isotropic biphase structures. In the TLCP/PE blend at 190 °C and 230 °C, dramatic reductions in viscosity were observed with significant improvement in extrudate surface smoothness and an enlarged processing window. For the organoclay-modified TLCP in PE, the viscosity reduction ability of TLCP was further enhanced with viscosity dropped by up to >98.5% and >97.4% at 190 °C and 230 °C and processing window enlarged to >700 s⁻¹ and >900 s⁻¹ respectively in comparison to that of PE. Moreover, yielding stress, initial transition shear rate and transition region decreased to lower magnitudes than those of the TLCP/PE blend. A phenomenological model was applied to elucidate the mechanism of organoclay, TLCP and PE conformation before and after yielding in the confined capillary environment. A binary flow pattern model was applied to successfully predict the rheological behavior of the blends at 190 °C.     

Inhibited transesterification and enhanced fibrillation of TLCP by nano-SiO2 in polycarbonate matrix

Hybrid composites composed of a thermotropic liquid crystalline polymer (TLCP), nano-SiO₂ and polycarbonate (PC) were prepared by melt blending in a twin-screw extruder. Infrared spectroscopy analysis indicated that the transesterification between PC and TLCP molecules during melt blending was significantly reduced in TLCP/PC blends filled with nano-SiO₂, compared to the unfilled TLCP/PC one. Scanning electron microscopy (SEM) observation showed that better compatibility and finer TLCP dispersion were reached in the unfilled blend, which made the fibrillation of TLCP difficult in capillary flow even at high shear rate. In contrast to this, well-developed TLCP fibrils were formed by capillary flow in nano-SiO₂ filled TLCP/PC blends. By increasing the nano-SiO₂ concentration and shear rate, the fibrillation of TLCP was significantly enhanced. Thermodynamically the interfacial tension between these components and dynamically the viscosity ratio of TLCP to PC were used to investigate the mechanism of nano-SiO₂ in inhibiting the transesterification and enhancing the fibrillation of TLCP droplets in these hybrid composites.     

Hierarchical structure of thermotropic liquid crystalline polymer formed in blends jointly by dynamic and thermodynamic driving forces

The hierarchical structure of thermotropic liquid crystalline polymer (TLCP), especially microfibrils with an average diameter of 30 nm has been obtained in polyamide 6 (PA6)/TLCP/glass bead (GB) ternary blends by capillary flows. Thermodynamically the different interfacial tensions between PA6 and GB, and between TLCP and GB, make the glass beads migrate to the vicinity of the TLCP melt droplets. Then the strong extensional flow field formed by the micro-rollers of these glass beads exerts strong extensional action on TLCP coils so that results in the formation of TLCP microfibrils, which are usually generated with neat TLCP melt only. The hierarchical structure of thermotropic liquid crystalline polymer (TLCP) in PA6/TLCP/GB ternary blends can enhance mechanical performance of such blends.     

A comparative study of thermotropic LCP and organoclay as fillers in high molecular mass polyethylene with different blending sequences

The influences of thermotropic liquid crystalline copolyester (TLCP) on viscosity reduction in high molecular mass polyethylene (HMMPE) filled with organoclay were investigated by controlling the blending sequence. The interactions between organoclay and TLCP in HMMPE create different morphologies and influence rheological properties of the clay/TLCP/HMMPE blends. When the organoclay was blended with TLCP first, large amounts of organoclay formed partially intercalated structures in TLCP, with phase separation occurring at the temperature when TLCP was in the nematic phase, corresponding an antagonistic effect which weakens viscosity reduction ability of TLCP for HMMPE. However, with first blending of TLCP with HMMPE and then adding organoclay into the blend, most of the organoclay enriched on TLCP surfaces in the blend. Such interaction prevents TLCP droplets from coalescing at high shear stresses, enlarging the processing window. A phenomenological model, originally for HMMPE/TLCP systems, was successfully adopted to predict the flow behaviors of clay/HMMPE/TLCP blends. POLYM. ENG. SCI., 50:1679–1688, 2010. © 2010 Society of Plastics Engineers     

Effects of convergent flow on in situ fibrillation of TLCP in PEN

A polymer melt entering a capillary die from a cylinder undergoes a convergent flow in which there is a complex combination of extensional and shear flows. The convergent flow plays an important role in controlling the in situ fibrillation of thermotropic liquid crystalline polymer (TLCP) in a thermoplastic matrix melt. This study examines effects of the convergent flow on development of TLCP fibrils in a TLCP/poly(ethylene naphthalate) (PEN) blend. A capillary rheometer was used and the extent of the convergent flow was varied by changing capillary dimension and shear rate. With a given capillary die, the TLCP fibrillation was found to increase with increasing shear rate because of the increased deformation of TLCP droplets. The establishment of a fully developed shear velocity profile by using a relatively long die is considered to be necessary to retain the TLCP fibrils initiated in the convergent flow region. At a given high shear rate, TLCP fibrillation improves with increasing capillary diameter (≤2 mm) because of the increased difference in velocity between the capillary and the cylinder. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1505–1513, 2004     

The interfacial tension between a thermotropic liquid crystalline copolyester and polyethersulfone: A comparison of methods

Three dynamic methods to determine the interfacial tension between the thermotropic liquid crystalline polymer (TLCP) Vectra A900 and polyethersulfone were evaluated: (1) thread breakup, (2) fiber retraction and (3) dynamic shear rheometry. The thread breakup and retraction methods, were suitable for measuring the interfacial tension, provided that the shear thinning flow behavior of the TLCP was taken into account. The viscosity of the TLCP during breakup or retraction was estimated from steady-shear measurements at the observed overall rate of deformation during growth of capillary instabilities or during retraction. The calculation of the interfacial tension from breakup rates of TLCP threads was improved by accounting for transient flow behavior during distortion growth using a single-element Maxwell model. Determination of the interfacial tension by oscillatory shear measurements on TLCP/PES dispersions using the emulsion model of Palierne, was not applicable for this system. Only for dispersions containing low TLCP volume fractions (e.g. 9 vol%) was there reasonable agreement between the emulsion model and measurements. At higher volume fractions agreement was poor, possibly because of different dynamic flow behavior of the TLCP in the pure form and in blends. The interfacial tension values obtained from thread breakup and fiber retraction ranged from 4 to 6 mN/m, which demonstrate that in-situ determination of the interfacial tension is possible for blends containing TLCPs, despite their complex flow behavior.     

Study on morphology development for in situ fiber–reinforced composites by blending polyolefin and polycaprolactone

The morphology developments and interfacial properties of extruded polyethylene/polycaprolactone and polypropylene/polycaprolactone blends were investigated. The interfacial thicknesses of both polymer blends were thin and this was investigated by interfacial tension measurement in the melt state. The aspect of boundary area was observed by AFM, and a clear line could be observed at the interface area as a result of thin interfacial thickness. The in situ fiber formation of the dispersed phase was remarkably generated under elongational flow (between die exit and solidification) rather than under shear flow (in the cylinder and die). Drawing ratio was varied at three levels to study its effect on elongation of the dispersed phases. The dispersions dramatically changed from spherical to spheroidal and filament shapes depending on the drawing ratio. Reduced capillary number (Ca*) was used to characterize droplet deformation. The deformation mode under shear flow was classified as nondeformation mode due to the fact that the Ca* was almost 0. On the other hand, the deformation mode under elongational flow was classified into filament shape mode (Ca* > 4). This classification was in agreement with the SEM images. The tensile properties were increased at the border line where the Ca* was 4.0. The melt interfacial tensions of polyolefin/polycaprolactone were relatively large, and a clear line could be observed at the interface area as a result of little affinity of polymer interface. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 500–508, 2005     

The effect of TLCP melt structure on the bulk viscosity of high molecular mass polyethylene

A thermotropic liquid crystalline polyester (TLCP), denoted as TCLP3 and based on hydroxybenzoic acid, hydroquinone and sebacic acid, was used as a processing aid in the extrusion of high molecular mass polyethylene (HMMPE). This TLCP is in the nematic phase at 179.6–182°C. Capillary rheometry experiments were carried out at two processing temperatures: 190°C and 230°C. At 190°C TLCP3 is predominantly nematic and at 230°C it is predominantly isotropic. It is an effective processing aid for HMMPE, particularly at 190°C, with viscosity reductions in excess of 90% with a 1 wt% TLCP3/HMMPE blend. The rheological characteristics of the blends have been linked to the optical textures of the TLCP3 using hot-stage optical microscopy. From the experimental observations speculations are made about the mechanisms of viscosity reduction. Initial viscosity reductions are caused by TLCP3 structure effects (fibrillation and phase change), giving rise to fibrillation-induced molecular orientation in the neighboring HMMPE phase. This is only effective when nematic structures are present. Above a critical wall shear stress value the lubrication effect due to TLCP3 migration is dominant.     

Influence of viscosity ratio on processing and morphology of thermotropic liquid crystal polymer‐reinforced poly(ethylene 2,6‐naphthalate) blends

Thermotropic liquid crystal polymer (TLCP) microfibril‐reinforced poly(ethylene 2,6‐naphthalate) (PEN) composites with various intrinsic viscosities were prepared by a melt compounding method. Polymer composites consisting of bulk cheap polyester with a small amount of expensive TLCP are of interest from a commercial perspective. The TLCP acts as a nucleating agent in the TLCP/PEN composites, enhancing the crystallization of the PEN matrix through heterogeneous nucleation. The structural viscosity index of the TLCP/PEN composites was lower than that of PEN and TLCP, which was attributed to the formation of TLCP fibrillar structures with elongated fibrils in the PEN matrix. The TLCP/PEN composites with higher intrinsic viscosity than the polymer matrix contained these elongated fibrils, and had a TLCP component with a smaller average diameter, and a narrower diameter distribution than TLCP/PEN composites with lower intrinsic viscosity. The higher intrinsic viscosity of the polymer matrix, the higher shear rate and the lower viscosity ratio of TLCP to PEN can all favour TLCP fibrillation in the polymer composites. Copyright © 2006 Society of Chemical Industry     

Epitaxy growth and directed crystallization of high-density polyethylene in the oriented blends with isotactic polypropylene

Various lamellar orientations of high-density polyethylene (HDPE), due to competition between bulk nucleation and interfacial nucleation, have been realized in its melt drawn blends with isotactic polypropylene (iPP) upon cooling after subjected to 160 °C for 30 min. Directed crystallization, with heterogeneous nucleation in the bulk (within domains), is defined as lamellar growth along boundary of anisotropic domains and is favored in larger domains at higher temperature (slow cooling), since overgrowth of lamellae can feel the interface rather than impingement with neighbor ones as a result of scare nuclei at higher temperature. Moreover, lamellar growth caused by directed crystallization is dependent of dimension of confinement. Due to 2D confinement of cylindrical domains, lamellae can only grow along the axis of cylinder and thus b-axis orientation is formed. While in the layered domains with 1D confinement, however, lamellae grow with the normal of (110) plane along the melt drawn direction. On the other hand, epitaxial growth of HDPE chains onto iPP lamellae is related to the surface-induced crystallization and dominated by the interfacial nucleation. Only interfacial nucleation is preferred can epitaxial growth occur. Therefore, retarded crystallization, realized by either strong confinement in finer domains or rapid cooling or both, is favorable for it.     

The cutting of multi-walled carbon nanotubes and their strong interfacial interaction with polyamide 6 in the solid state

The cutting of multi-walled carbon nanotubes (MWCNTs) using solid state shear milling (S³M) method and their strong interfacial interaction with polyamide 6 (PA6) in the solid state were studied. Transmission electron microscopy showed that after milling, the CNTs were greatly reduced in length, and disentangled, being straighter with open ends. Fourier transform infrared spectra and differential scanning calorimeter analysis indicated the existence of strong interfacial interactions between MWCNTs and PA6 of the pan-milled PA6/CNTs powder. It was further quantified by thermogravimetric analysis that about 30 wt.% of PA6 formed a strong combining force with CNTs after pan-milling. The mechanism of cutting CNTs and the reason for their strong interfacial interactions with PA6 in the solid state were discussed. A fine and homogeneous dispersion of CNTs throughout PA6 matrix was observed by scanning electron microscopy. The tensile properties of the composites prepared by the S³M method were significantly improved compared to those of pure PA6 and composites prepared by conventional melt mixing. Upon incorporation of only 1.5 wt.% MWCNTs, the tensile modulus of PA6 was enhanced from 2448 MPa to 4439 MPa, by about 80%, and the tensile strength was increased by about 23%.     

Effects of polyamide 6 incorporation to the short glass fiber reinforced ABS composites: an interfacial approach

The properties of 30 wt% short glass fiber (SGF) reinforced acrylonitrile-butadiene-styrene (ABS) terpolymer and polyamide 6 (PA6) blends prepared with extrusion were studied using the interfacial adhesion approach. Work of adhesion and interlaminar shear strength values were calculated respectively from experimentally determined interfacial tensions and short beam flexural tests. The adhesion capacities of glass fibers with different surface treatments of organosilanes were evaluated. Among the different silanes tested, γ-aminopropyltrimethoxysilane (APS) was found to be the best coupling agent for the glass fibers, possibly, because of its chemical compatibility with PA6. Tensile test results indicated that increasing amount of PA6 in the polymer matrix improved the strength and stiffness of the composites due to a strong acid–base interaction at the interface. Incorporation of PA6 to the SGF reinforced ABS reduced the melt viscosity, broadened the fiber length distributions and increased the toughness of the composites. Fractographic analysis showed that the incorporation of PA6 enhanced the interactions between glass fibers and the polymeric matrix.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Liquid crystalline copolyester/polyethylene in situ composite film: Rheology,morphology, molecular orientation,and tensile properties

A thermotropic liquid crystalline copolyester (TLCP) was blended with low density polyethylene using a corotating twin screw extruder and then fabricated by extrusion through a miniextruder as cast film. Rheological behavior, morphology, and tensile properties of the blends were investigated. Melt viscosities of neat components and blends measured by using plate‐and‐plate and capillary rheometers at 240°C, in the shear rate range 1–10⁴ s^?1, showed similar shear thinning effect. The viscosity values measured by the two techniques in the overlapping range of shear rate are found to be identical, which is in accord with the Cox–Merz rule. Addition of TLCP slightly reduces the matrix melt viscosity. TLCP dispersed phase in the extruded strand appeared in the form of spherical droplets. These droplets were elongated into fibrils with high aspect ratio (length to width) at the film extrusion step. As a result, the Young's modulus in machine direction (MD) of the composite film was greatly enhanced. At 20 wt % of TLCP, the MD Young's modulus was found to be about 16‐fold increase compared to that of the neat polyethylene film. However, the elongation at break sharply dropped with the increase of TLCP content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 561–567, 2002; DOI 10.1002/app.10307     

Interfacial properties of PP/PP composites

All‐polypropylene (PP/PP) composites, consisting of homoisotactic PP fibers and a propylene–ethylene random copolymer matrix, were manufactured. The interfacial morphologies and mechanical deformation of the composites were investigated. It was found that a transcrystalline layer was formed in the matrix depending on the molding conditions. This layer is composed of two regions with different morphologies: In one region close (<300 nm) to the fibers, lamellae exist very densely and the c‐axes of the crystal lattices are oriented along the longitudinal direction of the fiber. In the other region, a little away (>300 nm) from the fiber, a cross‐hatched structure was observed. The well‐developed transcrystalline layer could be highly elongated, and no debonding was observed at the fiber/matrix interface by the tensile stress in the direction perpendicular to the fiber. It is conceivable that this is the reason for the observed improvement of the transverse tensile properties in the PP/PP composite with a transcrystalline layer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2875–2883, 2003     

Thermotropic liquid crystalline polymer (Rodrun LC5000)/polypropylene in situ composite films: rheology, morphology, molecular orientation and tensile properties

In situ composite films were prepared by a two-step method. First, polypropylene and thermotropic liquid crystalline polymer (TLCP), Rodrun LC5000 (80 mol% p-hydroxy benzoic acid (HBA)/20 mol% polyethylene terephthalate (PET)), were melt blended in a twin-screw extruder and then fabricated by extrusion through a mini-extruder as cast film. Rheological behavior of the blends, morphology of the extruded strands and films, and tensile properties of the in situ composite films were investigated. Rheological behavior of the blends at 295 °C studied using a plate-and-plate rheometer revealed a substantial reduction of the complex viscosity with increasing TLCP content, and all specimens exhibited shear thinning behavior. Over the angular frequency range of 0.6-200 rad/s, the viscosity ratio (dispersed phase to matrix phase) was found to be very low, in the range of 0.03-0.07. Morphologies of the fracture surfaces of the blend extrudates and the film surfaces etched in permanganic solution were investigated by scanning electron microscope (SEM). The TLCP droplets in the extruded strands were seen with a progressive deformation into fibrillar structure when TLCP content was increased up to 30 wt%. In the extruded films, TLCP fibrils with increasing aspect ratio (length to width) were observed with increasing TLCP concentration. Orientation functions of each component were determined by X-ray diffraction using a novel separation technique. It was observed that the Young's modulus in machine direction of the extruded film was greatly improved with increasing TLCP loading, due to the increase in fiber aspect ratio and also molecular orientation.     

The Effect of Surface Treatment on the Strength and Adhesion Characteristics of Phoenix dactylifera-L(Date Palm) Fibers

ABSTRACT

There is a growing interest in the use of natural/bio-fibers as reinforcing components for thermoplastics and thermosets. However, they do suffer from a few limitations, such as lower compatibility with relatively hydrophobic polymer matrixes. Thus, improvement of the interface and interphase interactions in natural fiber–polyester composites is essential. In this research date palm (Phoenix dactylifera-L) fibers were modified by surface treatment using chemical method in order to improve their adhesion to polyester matrixes. Alkaline treatment, as an example of dissolution and treatment with silane coupling agents were performed. Furthermore, a combination treatment of alkaline and silane coupling agents was also carried out. Fiber modifications were monitored by Scanning Electron Microscopy (SEM). In addition to that, the improvement of adhesion and strength between date palm–modified fibers and polyester matrix was investigated by single filament pull-out test as well as tensile tests. It was found, from interfacial shear strength values, that substantial improvements in fiber-matrix compatibility have been achieved. According to single filament pull-out test results, interfacial shear strength increased for all treated fibers as compared to non-treated fibers. Particularly, combination of alkaline and silane coupling agents resulted in substantial adhesion improvement to the polyester matrix in comparison to the untreated fibers and fibers treated by alkaline and silane methods only.     

Interfacial compatibilization for PSF/TLCP blends by a modified polysulfone

Maleic anhydride-grafted polysulfone (PSF-g-MAH) was prepared by copolymerization in a solution. Compared to polysulfone, the surface tension and its polar fraction of PSF-g-MAH were increased by modification. The PSF-g-MAH was used to compatibilize polysulfone (PSF)/thermotropic liquid crystalline polymer (TLCP) (Vectra B950) blends. DRS-FTIR investigations showed that the special interaction between PSF and TLCP phases was enhanced due to the incorporation of PSF-g-MAH. The X-ray photoelectron spectroscopy (XPS) and PLM studies revealed that some chemical reactions had occurred between PSF-g-MAH and TLCP components, which produced PSF-VB copolymers. It is suggested that the improvement of compatibility should be attributed to some interactions including both chemical and physical ones. The improvement of compatibility was also confirmed by DMA and TGA analysis. Morphological observation showed that the addition of compatibilizer significantly reduced the size of the dispersed TLCP phase, and in case of injection molding, PSF-g-MAH promoted the TLCP fibrillation and improved their dispersion within the matrix. The processabilities of PSF/TLCP blends before and after compatibilization were investigated. It was found that the melt viscosities were increased by the addition of PSF-g-MAH, which also gave an evidence of the occurrence of interaction between phases.