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.     

Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro‐magnetized die of an extrusion rheometer

This article investigates the radial extrudate swell and velocity profiles of polystyrene melt in a capillary die of a constant shear‐rate extrusion rheometer, using a parallel coextrusion technique. An electro‐magnetized capillary die was used to monitor the changes in the radial extrudate swell profiles of the melt, which is relatively novel in polymer processing. The magnetic flux density applied to the capillary die was varied in a parallel direction to the melt flow, and all tests were performed under the critical condition at which sharkskin and melt fracture did not occur in the normal die. The experimental results suggest that the overall extrudate swell for all shear rates increased with increasing magnetic flux density to a maximum value and then decreased at higher densities. The maximum swelling peak of the melt appeared to shift to higher magnetic flux density, and the value of the maximum swell decreased with increasing wall shear rate and die temperature. The effect of magnetic torque on the extrudate swell ratio of PS melt was more pronounced when extruding the melt at low shear rates and low die temperatures. For radial extrudate swell and velocity profiles, the radial swell ratio for a given shear rate decreased with increasing r/R position. There were two regions where the changes in the extrudate swell ratio across the die diameter were obvious with changing magnetic torque and shear rate, one around the duct center and the other around r/R of 0.65–0.85. The changes in the extrudate swell profiles across the die diameter were associated with, and can be explained using, the melt velocity profiles generated during the flow. In summary, the changes in the overall extrudate swell ratio of PS melt in a capillary die were influenced more by the swelling of the melt around the center of the die. Polym. Eng. Sci. 44:2298–2307, 2004. © 2004 Society of Plastics Engineers.     

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     

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

A 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.     

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.     

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.     

A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (～1.9) and low (～0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.     

Rheological properties of poly(trimethylene terephthalate) in capillary flow

The shear viscosity, extensional viscosity, and die swell of the PTT melt were investigated using a capillary rheometer. The results showed that the PTT melt was a typical pseudoplastic fluid exhibiting shear thinning and extensional thinning phenomena in capillary flow. There existed no melt fracture phenomenon in the PTT melt through a capillary die even though the shear rate was 20,000 s^?1. Increasing the shear rate would decrease the flow activation energy and decline the sensitivity of the shear viscosity to the melt temperature. The molecular weight had a significant influence on the flow curve. The flow behavior of the PTT melt approached that of Newtonian fluid even though the weight‐molecular weight was below 43,000 s^?1 at 260°C. The extensional viscosity decreased with the increase of the extensional stress, which became more obvious with increasing the molecular weight. The sensitiveness of the extensional viscosity to the melt temperature decreased promptly along with increasing the extensional strain rate. The die swell ratio and end effect would increase along with increasing the shear rate and with decreasing the temperature, which represented that the increase of the shear rate and the decrease of temperature would increase the extruding elasticity of the PTT melt in the capillary die. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 705–709, 2005     

高分子材料动态成型过程中熔体非线性行为的研究

借助流变测量和连续介质理论,不依赖已有的本构关系,对平行叠加正弦振动条件下高分子熔体经毛细管的动态挤出过程进行了理论分析。以低密度聚乙烯(LDPE)为原材料,实验测量LDPE熔体在一定振动频率和振幅下毛细管入口压力、体积流量和挤出胀大的瞬态值,即可得到动态成型过程中高分子熔体剪切应力、剪切速率和表观粘度的变化规律:随振幅和频率的变化,LDPE熔体的表观粘度呈非线性变化趋势;在不同的振幅和频率下动态挤出LDPE熔体,跟稳态挤出时一样,壁面剪切应力与壁面剪切速率也成非线性比例关系。     

Kinematics of the stick-slip capillary flow of high-density polyethylene

A study of the kinematics of the stick-slip capillary flow of high-density polyethylene has been carried out in this work by using particle image velocimetry (PIV). The experiments covered a wide range of shear rates and the velocity maps and profiles across the die were obtained for the different regimes of the discontinuous flow curve. In the low shear rate region, the melt exhibited shear thinning without slip. In the unstable stick-slip regime, an alternating behavior between full adhesion and slip was observed, whereas both, the maximum velocity and the slip velocity of the melt, changed continuously during pressure oscillations. In addition, non-homogenous slip, characterized by regions with and without slip at the die wall, was occasionally observed during the oscillations. In contrast to the general assumption, the flow in the high shear rate region was found to be unstable, and characterized by high frequency pressure oscillations. A steep rise of the slip velocity took place from the onset of the stick-slip regime and reached values higher than 70% of the maximum velocity for the profiles in the high shear rate branch. However, a true plug flow was never observed due to shear thinning of the melt. Finally, a direct proof of the Mooney hypothesis to account for slip in polymer melts is given on the basis of the comparison of velocity profiles measured in the low and high shear branch.     

管壁厚度对微挤出成型的影响分析

基于Bird-Carreau黏度模型,运用有限元方法对三维等温微管挤出成型流动模型进行了数值分析,主要研究了管壁厚度对微管挤出成型过程中挤出胀大、速度分布、剪切速率和口模压降等重要指标的影响。结果表明,当熔体入口体积流率相等时,随着管壁厚度的增大,挤出物挤出胀大率和横截面尺寸变化量增大;口模出口端面上熔体的二次流动增强,但挤出速度和剪切速率减小;熔体在口模内的压力降明显下降;适当增加管壁厚度,有利于提高微管挤出质量。     

Relationship between shear stress and shear strain of polymer melt under vibrating force field

Abstract

When a sinusoidal vibration was superimposed in parallel on the flow direction of a polymer melt being extruded through a capillary, the shear stress and shear rate of the polymer melt were analysed with a constant velocity type dynamic rheometer of capillary (CVDRC) devised by the authors. By measuring the instantaneous data of capillary entry pressure, capillary volume flux (or absolute velocity of piston rod) and their phase difference in a vibrating force field, it was found that the relationship between the pulsating amplitude value coefficient of entry pressure and that of volumetric flowrate was an approximate power series; the wall shear stress and wall shear rate of low density polyethylene (LDPE) melt extruded dynamically under various amplitudes and frequencies also exhibited a non-linear proportional relationship.     

A study of extrudate distortion in controlled-rheology polypropylenes

The melt fracture characteristics of controlled-rheology polypropylenes (CRPP) were studied by means of capillary rheometry experiments. CRPPs were produced through reactive extrusion of a commodity polypropylene resin using various peroxide concentrations. These materials exhibited lower molecular weights and narrower molecular weight distributions than those of the starting commodity resin. The CRPP materials studied were found to exhibit only gross melt fracture. At extremely high shear rates and relatively low temperatures, a sigmoidal flexure was observed in the flow curve of certain CRPPs. Generally, it was found that the severity of melt fracture decreased with increasing shear rate for a given material and temperature and in some cases, the extrudates exhibited completely smooth surfaces. Also, the severity of surface distortions was reduced when high L/D dies were employed at a given shear rate. The critical shear stress for the melt fracture onset was found to increase with decreasing molecular weight and polydispersity, and correlations have been developed between the critical stress values and the polymer polydispersity and shear compliance.     

微孔塑料挤出快速降压口模的数值模拟

应用FLUENT软件对微孔塑料连续挤出成型过程中的快速降压口模内的熔体流动进行数值模拟,经过简化及边界处理,分别研究了微孔塑料在不同CO2浓度、不同流量和不同温度条件下微孔塑料连续挤出过程中快速降压口模中的压力和速度分布情况。结果表明:压力降随熔体温度和CO2浓度的升高而降低,随熔体流量的升高而增大。导管中的速度也几乎均匀分布,在毛细管人口处中心线速度突然增大。熔体流量和CO2浓度的变化对口模压降和口模速度的影响比较大,而温度的变化对其影响要小得多。     

An in‐process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts

The orientation and relaxation behaviors of a low‐density polyethylene melt and polypropylene melts with different melt indices undergoing a shear flow in a restricted channel were investigated by using ultrasound. A capillary rheometer was used to force the polymer melt through a slit die equipped with pressure, temperature, and ultrasound sensors, and the variation of ultrasound velocity traversing the melt was measured. Experimental results revealed that due to different mechanisms involved, the relaxations of orientation and disorientation processes show different dependences of ultrasound velocity on shear rate, temperature, and melt index. POLYM. ENG. SCI., 2008. Published 2008 Society of Plastics Engineers     

Extrusion blow molding of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion blow molding of bottles from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated in elongational flow fields that occur in polymer processing operations and to determine if TLCP reinforced bottles could be produced by extrusion blow molding of microcomposites. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of TLCP concentration and in situ composite strand properties on the mechanical properties and anisotropy of bottles made from microcomposites. Specifically, strands having three different draw ratios are used to produce bottles at 10 and 20 wt% TLCP. Increasing the in situ composite strand modulus is shown to cause an increase in both the machine and transverse direction moduli of the composite bottles. The mechanical properties of the bottles increase with increasing TLCP composition. Finally, the machine and transverse direction properties are not balanced in the composite bottles produced in this study (degrees of anisotropy ranging from 1.5 to 1.8). The mechanical anisotropy is probably the result of a low blow up ratio (2) in the bottles and the TLCP fibers being oriented primarily in the machine direction due to the shear flow in the die.     

The effct of asymmetries of die exit geometry on extrudate swell and melt fracture

Experimental investigations were performed to see how the die exit geometry and the extrusion velocity influence on extrudate swell and melt fracture for several polymer melts [low-density polyethylene, styrene-butadiene rubber (SBR) and SBR/HAF (carbon black) compound]. Four different types of die exit geometry were considered; 0° (symmetric. usual capillary die), and 30°, 45° and 60° (asymmetric dies) were chosen for the die exit angle. Extrudate diameters were measured without draw-down under isothermal condition. Polymer melts were extruded into an oil that has the same density and temperature as those of the extrudate. Extrudate swells from dies with different diameters were correlated with volumetric flow rates. It was observed that the extrudate swell increases with increasing volumetric flow rate and exhibits through a minimum value at about 45° die exit angle. As to the fracture phenomena, it was observed that the critical shear for the onset of melt fracture increases with the increasing die exit angle up to 45°. However, for 60° die exit angle, the onset of melt fracture is again similar to that of 0° exit angle.     

Effect of convergent–divergent flow on thermal and crystallization properties of PP/MWCNTs nanocomposites

A melt‐mixing process based on convergent–divergent flow has been used to prepare PP/MWCNT composites with a self‐built convergent–divergent die (C‐D die) composed of different numbers of convergent plates. Dynamic extensional deformation was generated in the C‐D die, which improved the mixing effect and mixing efficiency of the composites during extrusion. The C‐D die acted as a mixer for composites when mounted onto a capillary rheometer. The residence time of PP/MWCNTs melt in the extensional flow field is adjusted by changing the numbers of convergent plates and the velocity of the ram. The intensity of extensional flow field is controlled by the structure of the convergent plate and the ram velocity. Influences of convergent–divergent flow on PP/MWCNTs composites were characterized in terms of transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). TEM results showed that MWCNTs disperse more homogeneously with the increase of convergent plates. DSC showed that the crystallinity of PP/MWCNTs composites increased and the crystallization temperature shifted to higher temperature with the increase of the numbers of the convergent plates. TGA showed that the thermal stability of composites improved remarkably. The decomposition temperature increases from 381 to 408.2°C when the numbers of convergent plates increased from 2 to 8. In addition, the increase of ram velocity also has the same influences on the dispersion of MWCNTs in the resin and the properties of PP/MWCNTs nanocomposites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42330.     

Thermotropic liquid‐crystalline copolyester/thermoplastic elastomer in situ composites. I. Rheology,morphology, and mechanical properties of extruded strands

A thermotropic liquid‐crystalline polymer (TLCP), a copolyester with a 60/40 molar ratio of p‐hydroxy benzoic acid and poly(ethylene terephthalate), was blended with a styrene/ethylene butylene/styrene thermoplastic elastomer with a twin‐screw extruder. The rheological behavior, morphology, and mechanical properties of the extruded strands of the blends were investigated. The rheological measurements were performed on a capillary rheometer in the shear rate range of 5–2000 s^?1 and on a plate‐and‐plate rheometer in the frequency range of 0.6–200 rad s^?1. All the neat components and blends exhibited shear thinning behavior. Both the shear and complex viscosities of all the blends decreased with increasing TLCP contents, but the decrease in the shear viscosity was more pronounced. The best fibrillar morphology was observed in the extruded strands of a blend containing 30 wt % TLCP, and a lamellar structure started to form at 40 wt % TLCP. With an increasing concentration of TLCP, the tensile modulus of the blends was greatly enhanced, whereas the tensile strength was almost unchanged. The elongation at break of the blends first slightly decreased with the addition of TLCP and then sharply dropped at 40 wt % TLCP. The tension set measured at 200% deformation slightly increased with increasing TLCP contents up to 30 wt %, over which the set value was unacceptable for a thermoplastic elastomer. A remarkable improvement in the dynamic mechanical properties of the extruded strands was observed in the blends with increasing amounts of TLCP. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2676–2685, 2003     

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.