Shear and nonisothermal elongational characterization of a liquid crystalline polymer

A commercial liquid crystalline polymer (LCP) sample was characterized both in shear and in nonisothermal elongational flow over a wide range of temperatures (240–350°C). The LCP sample employed was a copolyesteramide of 2,6 hydroxynaphthoic acid, terephthalic acid, and aminophenol manufactured by Celanese and commercially known as Vectra B950. The viscosity decreases dramatically at temperatures above the crystal-nematic transition of the polymer. At lower temperatures, the extrusion of the sample is mainly due to plastic flow. Die-swell values are very small and contraction of the extrudate is also revealed, but the sample shows high elasticity. The polymer is not spinnable at very low temperatures, and the melt strength rapidly decreases with increasing temperature.     

Flow behavior of mesophase pitch

The objective of this work was to provide a comprehensive rheological and structural study of AR mesophase pitch. The shear viscosity of AR mesophase pitch was found to be qualitatively similar to results from prior investigations of mesophase pitch and followed the typical trend for liquid crystalline fluids. However, a distinct hesitation, or kink, was observed in the shear viscosity curve near the shear thinning to constant viscosity transition. This behavior has been seen previously for some low molecular weight and polymeric liquid crystals and was thought to be due to a transition between tumbling and steady alignment of the uniaxial director. Optical studies of mesophase pitch revealed an orientation change of the poly-domain structure at shear rates where the kink was observed. This orientation change results in a high viscosity to low viscosity transition. This viscosity transition, and not tumbling, is responsible for the kink phenomenon. The optical studies also reaffirmed that shear flow reduces the size of the poly domain structure and elongates the domains along the flow direction. Also under quiescent conditions, the poly-domain size increased with increasing relaxation time.     

Reactive compatibilization and in‐line morphological analysis of blends of PET and a thermotropic liquid crystalline polymer

The reactive extrusion of a compatibilized polyethylene terephthlate and thermotropic liquid crystalline copolyester blend system is investigated via in‐line optical microscopy and light scattering. The reactivity, morphology, rheological behavior and mechanical properties were studied. It was found that a droplet‐fiber transition in the liquid crystalline polymer can occur, and this transition is dependent upon the reactivity, the viscosity ratio of the blend components and the shear fields. The fibers are strongly oriented along the flow direction when specific processing conditions are met. An enhancement in the mechanical properties correlates with the morphology. The formation, deformation and stability of the fiber microstructure under different flow conditions are discussed.     

Steady viscosity of a polymer/liquid crystal biphasic system

Steady shear viscosities of blends of poly(butyl acrylate) and cholesteryl oleyl carbonate were studied under a cone and plate fixture. Unique shear behavior was observed for the polymer/low molar mass liquid crystal mixture. The viscosity of the liquid crystal‐rich phase increases with polymer content until a maximum is reached. The height of the viscosity maximum decreases with the magnitude of the shear stress and disappears when the stress reaches 200 Pa. Addition of liquid crystal to the polymer‐rich phase causes a viscosity reduction, and at higher stress levels, the viscosity reduction becomes more effective with the same amount of liquid crystal addition. The viscosity reduction may be related to the fibril morphology of the liquid crystal and the viscosity maximum can be interpreted by the emulsion effect being counteracted by a viscosity reduction effect. A shear‐thickening behavior was observed in the intermediate shear rates for the blends with a volume fraction of poly(butyl acrylate) between 9.4 and 49.8%. This is a novel liquid–liquid system that exhibits a shear thickening behavior. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 25–30, 2004     

Melt viscosity reduction of polycarbonate with low molar mass liquid crystals

The phase behavior and rheological behavior of low molar mass liquid crystal (LLC) and polycarbonate blends is firstly reported. The results of small angle light scattering (SALS) indicate that the LLC is miscible in the mixture for weight fractions of LLC less than 6%. Mixtures of two different liquid crystals with two different molecular weight of polycarbonate were prepared inside the miscible regime of the blends. Both the complex and steady shear viscosities of the blends were found to be significantly decreased upon addition of small amounts of liquid crystal (1% by weight). At low shear rate, the steady state shear viscosity was similar to the pure polycarbonate, whilst, at higher shear rates, three further regimes of behavior, as has been described for liquid crystals and liquid crystal polymers, were found despite the low concentration of LLC; hence, the rheological properties of the blends can be significantly modified by small concentrations of LLC (as low as 1%). The decrease in melt viscosity of polycarbonate that we observe upon addition of LLC is not due to lubrication effects at the interfaces, as shown by reproducible oscillatory shear flow sweeps. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Viscosity profile prediction of a heavy crude oil during lifting in two deep artesian wells

It has been known that the productivity of artesian wells is strongly dependent on the rheological properties of crude oils.This work targets two deep artesian wells (＞5000 m) that are producing heavy crude oil.The impacts of well conditions including temperature,pressure and shear rate,on the crude oil rheology were comprehensively investigated and correlated using several empirical rheological models.The experimental data indicate that this heavy oil is very sensitive to temperature as result of microstructure change caused by hydrogen bonding.The rheological behavior of the heavy oil is also significantly impacted by the imposed pressure,i.e.,the viscosity flow activation energy (Eμ) gently increases with the increasing pressure.The viscosity-shear rate data are well fitted to the power law model at low temperature.However,due to the transition of fluid feature at high temperature (Newtonian fluid),the measured viscosity was found to slightly deviate from the fitting data.Combining the evaluated correlations,the viscosity profile of the heavy crude oil in these two deep artesian wells as a function of well depth was predicted using the oilfield producing data.     

中国神木煤灰渣在高温下的流变特性

利用高温流变仪测定了中国神木煤灰渣在不同温度下的流变特性,并利用光学显微镜观察了煤灰渣在高温下冷激后的微观形貌,同时利用热力学软件FactSage预测了煤灰渣在不同温度下结晶粒子的相对含量与其均相熔体组成随温度的变化。实验结果表明:当实验温度为煤灰渣的全液相温度时,其流型表现为牛顿性流体;当温度低于其全液相温度时,随着温度的降低,其流型依次变为Bingham流型与Casson流型。同时随着温度的降低,由于煤灰渣中的固含量在不断增加,使得煤灰渣表现出明显剪切变稀的流变现象。最后利用Urbain模型与改进后的Einstein-Roscoe模型分别对煤灰渣中均相熔体黏度与实际黏度进行了模拟,其模拟值与实验值有良好的相关性。     

STEADY SHEAR FLOW PROPERTIES OF HIGH SOLIDS SOFTWOOD KRAFT BLACK LIQUORS: EFFECTS OF TEMPERATURE, SOLIDS CONCENTRATIONS, LIGNIN MOLECULAR WEIGHT AND SHEAR RATE

The steady shear flow properties of several softwood kraft black liquors (slash pine) from a two level, four variable factorially designed pulping experiment were determined for solids concentrations from 50% to 85%, temperatures from 40°C to 140°C and shear rates up to 10,000 s-1 by using Instron capillary and Haake coaxial cylinder rheometers. It was shown that the slip velocity at the wall of the capillary is insignificant and that a two capillary method can be used to determine the viscosity of the samples. At high solids, black liquor can exhibit non-Newtonian behavior dependent upon temperature, solids concentrations, solids composition and shear rate. In general, the liquors behave as pseudoplastic fluids. The exact level of viscosity at any given condition is dependent upon the solids composition which will vary from liquor-to-liquor. The flow behavior of the liquors was described using power-law, Cross and Carreau-Yasuda models. Superposition principles developed for polymer melts and concentrated polymer solutions were applied to obtain reduced correlations for viscosity behavior of the liquors. By using a suitable reference temperature, related to the glass transition temperature of black liquors, a generalized WLF type shift factor was obtained for the liquors used in this study and can be used to obtain a reduced plot of viscosity behavior of other black liquors.     

60PHB／PET热致液晶共聚酯的流动特征相转变及热处理的影响

在２９０℃下经熔融共聚制得６０ＰＨＢ／ＰＥＴ热致液晶共聚酯。用毛细管流变仪测定流动性能，表明此共聚酯为切力变稀流体；其非牛顿指数、屈服应力在２６０℃均有突变。结合ＤＳＣ，ＤＬＩ，ＴＯＴ等研究，共聚酯在２５５～２５８℃为ＰＨＢ微晶向向列型液晶转变点。流变特征与相态有关。经高温处理（＞２６０℃）的共聚酯熔体在较低温度下呈保持高温流动特征。而经不同温度热处理后冷却样品，其相转变热焓、相转变温度均随热处理温度的增加而增加，尤其是在２６０℃以上热处理后冷却样品，其相转变热焓增加较大，而２８０℃以上热处理后冷却样品的相转变热焓达到最大值。     

Molecular dynamics study on the viscosity of glass-forming systems near and below the glass transition temperature

Temperature-dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (T_g) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear-induced variation in shear modulus or potential energy of the glass-forming system. The shear viscosity as a function of steady-state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero-strain-rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near T_g of four glass-forming systems (Kob–Andersen system, silica, Cu_45.5Zr_45.5Al₉, and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below T_g; the upper-bound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~10³² and 10²⁵ Pa·s, respectively.     

Effects of shear rate,viscosity ratio and liquid crystalline polymer content on morphological and mechanical properties of polycarbonate and LCP blends

Studies were conducted on the effects of shear rate, viscosity ratio and liquid crystalline polymer (LCP) content on the morphological and mechanical properties of polycarbonate (PC) and LCP blends. The LCP (LC5000) used was a thermotropic liquid crystalline polymer consisting of 80/20 of parahydroxybenzoic acid and poly(ethylene terephthalate) (PHB/PET). The viscosity ratio (viscosity of LCP: viscosity of matrix) was varied by using two processing temperatures. Due to the different sensitivity of materials to temperature, variation in the processing temperature will lead to varying viscosity of the components in the blends. Based on this principle, the processing temperature could be manipulated to provide a favourable viscosity ratio of below unity for fibre formation. To study the effect of shear rate, the flow rate of the blend and the mould thickness were varied. The shear rate has a significant effect on the fibrillation of the LCP phase. The effect was more prominent when the viscosity ratio was low and the matrix viscosity was high. At 5 wt% LCP, fibrillation did not occur even at low viscosity ratios and high shear rates. It was also observed that the LCP content must be sufficiently high to allow coalescence of the dispersed phase for subsequent fibrillation to occur. © 2002 Society of Chemical Industry     

The Smectic Rheology of a Polysiloxane Side Chain Liquid Crystalline Polymer

The rheology of a side chain liquid crystalline polymer (SLCP) with a polysiloxane backbone was investigated. The dynamic shear moduli of the SLCP in a smectic phase did not show the normal terminal behavior as the homogenous polymeric melts did, and instead, they tended to level off in the low frequency terminal zone. Time–temperature superposition failed for both dynamic moduli in the low frequency terminal zone and the departure from the superposition became more evident in the vicinity of smectic/isotropic transition. The plateau-like moduli in the terminal zone indicated the layer structure of the smectic phase. The steady shear viscosities of the smectic phase exhibited a shear thinning behavior over the shear rates investigated. The shear thinning was lost at low shear rates when the temperature passed the smectic/biphasic border. The shear viscosity and the dynamic moduli showed a divergence in the neighborhood of the smectic/isotropic temperature. The activation energies of the shear viscosity and the moduli were smaller than that of the SLCP with polymethacrylate backbone. The rheological behavior of the SLCP at low frequencies and low shear rates was dominated by the smectogen.     

Investigation of the effect of double‐walled carbon nanotubes on the curing reaction kinetics and shear flow of an epoxy resin

In this article, the effect of combined temperature‐concentration and shear rate conditions on the rheology of double‐walled carbon nanotubes (DWCNTs)/RTM6‐Epoxy suspension was investigated to determine the optimum processing conditions. The rheological behavior and cure kinetics of this nanocomposite are presented. Cure kinetics analysis of the epoxy resin and the epoxy resin filled with DWCNTs was performed using Differential Scanning Calorimeter (DSC) and parameters of the kinetics model were compared. The DWCNTs have an acceleration effect on the reaction rate of the epoxy resin but no significant effect is noted on the glass transition temperature of the epoxy resin. This study reveals that the effect of shear‐thinning is more pronounced at high temperatures when DWCNTs content is increased. In addition, the steady shear flow exhibits a thermally activated property above 60°C whereas the polymer fluid viscosity is influenced by the free volume and cooperative effects when the temperature is below 60°C. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms

A controlled shear stress–shear rate rheometer was used to determine the viscoelastic behavior of cement paste incorporating various superplasticizers and subjected to prolonged mixing at high temperature. At a low applied shear stress range, the oscillatory shear strain/stress curve of cement paste was characteristic of a linear elastic solid; while the higher stress range was characteristic of a viscous liquid exhibiting a linear strain increase with increasing applied shear stress. The transition from solid-like to liquid-like behavior occurred over a very narrow stress increment. This transition stress corresponded to the yield stress parameter estimated from conventional flow curves using the Bingham model. The yield stress from oscillatory shear stress tests was estimated using the intersection between the viscous part of the oscillatory shear strain/stress curve and the oscillatory shear stress axis. In this study, equations describing the variation of shear strain versus shear stress beyond the solid–fluid transition for cement pastes incorporating various superplasticizers at different ambient temperatures and mixing times were developed using genetic algorithms (GA). The yield stress of cement pastes was subsequently predicted using the developed equations by calculating the stress corresponding to zero strain. A sensitivity analysis was performed to evaluate the effects of the mixing time, ambient temperature, and superplasticizer dosage on the calculated yield stress. It is shown that the computed yield stress values compare well with corresponding experimental data measured using oscillatory rheology.     

Flow and heat transfer characteristics of structural viscosity fluids

Laminar friction loss and heat transfer characteristics for a non-Newtonian fluid described by a general viscosity function of the structural type have been computed for tube and slit geometries. The model includes most other models for fluids without a yield stress as special cases. The Graetz problem for constant wall temperature and temperature independent properties has been solved, including viscous dissipation, for the transverse and axial temperature distribution. Both momentum and energy transport characteristics are shown to be strongly dependent upon a dimensionless fluid time constant, which characterizes the transition from Newtonian to non-Newtonian shear thinning behavior. A direct and exact analogy between the momentum and energy transport mechanisms is demonstrated for hydrodynamically and thermally fully developed flow, provided the heat transfer rate is represented by a suitably defined Nusselt number.     

Investigation of the three region flow curve of polymeric liquid crystals by rheo-optical methods

Steady flow behavior of bulk liquid crystals and polymeric liquid crystals are summarized into a three region flow curve. Changes in the internal structure during deformation and flow were investigated by rheo-optical methods. Two different optical geometries were used. In one method, a light beam passed through perpendicular to the shear plate, and in the other method a light beam passed perpendicular to the edge of the lamina flow. The results indicate that the details of the internal texture do not necessarily appear in the rheological properties, such as viscosity, and that the molecular orientation mechanism under shear force is fundamentally different in a liquid crystal of rod-like molecules and a suspended solution of rod-like molecules. In the isotropic solution, the rod-like molecules float or are suspended in a solvent and are oriented by the flow field of the solvent fluid. However, in a liquid crystal, the torque propagates directly on the individual rod-like molecules, affecting their orientation. They flow as if it were a plastic solid.     

Rheology of diallyl phthalate polymers below the gel point

The analysis of molding operations for thermosetting polymers requires information on the reaction rates and rheology of the materials. The purpose of this research was to measure kinetic and rheological data on diallyl phthalate resins and to develop models describing the flow behavior. The rheological data were measured with a mechanical spectrometer equipped with eccentric rotating discs. For the region below the gel point of the polymer, the elastic modulus and viscosity were correlated with molecular weight, temperature and shear rate through fairly simple models. By combining kinetic and rheological correlations, the viscosity of a reacting thermoset can be predicted as a function of time, temperature and shear rate.     

Rheological properties of aqueous polymer dispersions

Aqueous dispersion of nearly monodisperse charged polymer spheres of 250 nm diameter have been investigated in steady and oszillatory shear flow. Polydisperse latices were included for comparison. By using different types of rotational rheometers and a capillary viscometer viscosity measurements could be performed over a shear stress range from 10^-3 Pa to 10⁵ Pa. At shear stresses below 10^-2 Pa the viscosity is independent of the shear rate but strongly increases with qrowing volume fraction ?. In the stress range of 0.1 to 10 Pa a transition to a lower viscosity level is observed. For ??0.2 this transition is negligible and the dispersion behaves Newtonian. The viscosity decrease (shear-thinning behaviour) becomes significant at higher volume fractions and reaches a magnitude of more than 3 decades for ? = 0.50. Polydispersity increases the viscosity level at small stresses. The latter is also increased by growing particle charge (higher pH-value) but reduced by the addition of NaCl (screening of the charge). For volume fractions ? ?0.4 shear-thickening is found up to a viscosity maximum. The maqnitude of shear-thickening can be reduced by addition of NaOH (increase of particle charge) and broadening of the particle size distribution. The concentrated dispersions behave viscoelastic. They show an instantanious elastic deformation at the onset of shear creep tests and exhibit recoverable strains after unloading. In oscillatory shear the storage modulus approaches a constant value with decreasing angular frequency indicating the formation of a three-dimensional lattice structure.     

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     

A Survey of Rheological Properties of One-Component Epoxy Adhesives

Flow characteristics of seven commercially available one-component epoxy adhesive pastes were measured using a controlled shear stress rheometer and a controlled shear rate rheometer over a temperature range from 5°C to 60°C. Combining data obtained from both controlled rate and controlled stress experiments over a wide range of shear rates, we observed Newtonian flow (shear stress proportional to shear rate) at very low shear rates, a plateau “shear thinning” region at intermediate shear rates, and a second region of linear dependence of shear stress on shear rate at high shear rates. The adhesive pastes exhibited a very broad range of rheological behavior. Two flow parameters important to adhesive application technology, the plastic viscosity and the apparent yield stress, were measured for each adhesive. The plastic viscosity ranged from 11.6 to 329.5 Pa. s; the apparent yield stress ranged from 56.2 to 413 Pa. The temperature dependence of the rheological parameters of the epoxy adhesive pastes was also determined. The results are reported as the activation energies, E_η and E_σ, of plastic viscosity and apparent yield stress, respectively. The apparent yield stress of each adhesive paste was much less sensitive to changes in temperature than was the plastic viscosity. This suggests that the processing characteristics are likely to show qualitative as well as quantitative changes with temperature.