Rheological characterization of polypropylene filled with magnesium hydroxide

Consideration is given to the dynamic viscoelastic and shear flow properties of magnesium hydroxide-filled polypropylene, at a filler concentration of 60 wt%. Five variants of magnesium hydroxide were used, one surface-treated with magnesium stearate. The results reported illustrate the effects of filler particle size, morphology and surface coating on the rheology of the composites. The presence of magnesium hydroxide caused a significant increase in the shear viscosity of polypropylene relative to unfilled polymer, although this was much less pronounced using surface-treated filler, particularly at low shear rates. Complex viscosity and storage modulus data, obtained at very low shear rates (0.002 s^–1), demonstrated the presence of a critical shear yield stress for flow to occur, which was greatest for compositions containing uncoated fillers with small particle size. These observations are discussed in terms of structure formation between the particles. Results obtained from capillary and dynamic measurements of melt flow were found to follow the Cox-Merz rule.     

Anisotropic rheology of continuous fibre thermoplastic composites

The anisotropic rheology of continuous fibre thermoplastic composites has been resolved into along fibre and transverse components using a balanced pair of off-axis specimens in a commercial rotational rheometer with a parallel disc geometry. Existing theory has been used to demonstrate a small but consistent difference between the components of shear modulus and viscosity obtained from dynamic viscoelastic measurements. Transverse viscosity and yield stress are higher. The use of maximum shear rate (product of maximum shear strain and angular frequency), used here with apparent Maxwell viscosity, eliminates the strain dependence of the rates and correlates with steady shear viscosity and shear rate data. Differences in yield stress and viscosity are associated with the fibre-polymer system and the fibre volume fractions.     

Time-Dependent and Flow Properties of Foams

Foams have been prepared from water added with a surfactant and a polymer. A controlled stress rheometer was used to study the changes of their rheological properties during ageing by the mean of different rheological tests: shear viscosity measurements, creep compliance tests at a constant low shear stress and dynamic experiments have been performed. It has been observed that apparent viscosity decreases with ageing. A thixotropic behaviour was also found, loading and unloading curves present an hysteresis. Then, the choice of the stress ascent time is of primary importance to study the time-dependent properties of the foam. The viscosity was found to decrease with the stress ascent time while the thixotropic area decreases. Creep flow and harmonic tests have shown important viscoelastic properties of the foams.The second part of this work is concerned with the study of the flow properties of the foams. Pressure drops were measured during flow in capillary tubes in laminar regime. Pronounced wall slip effects were found. While the Mooney method for slip correction is not applicable, the Oldroyd–Jastrzebski method leads to satisfactory results. The Metzner and Reed correlation method was found to be applicable in the case of the corrected data.     

聚丙撑碳酸酯的熔融流变特性

研究了不同分子量的聚丙撑碳酸酯(PPC)的熔融流变行为。流变分析表明,在流变测试过程(140℃~170℃)中PPC结构稳定;通过时温等效原理(TTS)得到了150℃的动态模量主曲线;随着分子量的增加,PPC的线性粘弹区范围增大,剪切变稀行为明显,高分子链松弛时间延长。PPC的流变行为符合"Cox-Merz"规则,由此可以通过动态测试获得高剪切速率下的PPC稳态粘弹特性。由Carreau-Yasuda(Osc)公式得到PPC的零切黏度(η0),确定了η0与分子量之间的关系为η0∝-Mw2.8。     

Rheological Behaviour for Polymer Melts and Concentrated Solutions Part Ⅱ: Material Function with Nagai Chain Constraints and Determination of Their Parameters from Flow Curves

An integral constitutive equation and a set of material functions for describing the strain history of polymer melts were formulated in terms of the Cauchy-Green and Finger tensors. A simple memory function and the dependence of ηo and τt on M3.4 were derived from the theory of non-linear viscoelasticity with constraints of entanglements for polymer melts and substituted into the Oldroye-Walters-Fredickson constitutive equation. An integral constitutive equation for polymer melts was consequently obtained. Some material functions of the constitutive equation related to certain "test flow" are examined as follows : (1) simple steady shear flow; (2) steady elongation flow; (3) small-amplitude oscillatory shear flow; (4) stress growth upon the inception of steady shear elongation flow; (5) stress relaxation (modulus and compllance). These theoretical relations for simple steady shear flow were compared with experimental data from our laboratory and references for various polymer melts and concentrated solutions. A good agreement between the theory and experiment was achieved.     

一种基于粘性耗散的动态黏度计算方法

基于剪切功率的耗散机理,提出了一种计算振动剪切流场中聚合物熔体动态黏度的粘性耗散法,并建立了理论模型。运用粘性耗散法,计算了简单振动剪切流场中Maxwell流体的动态黏度,得到了与传统方法相一致的结果,从而验证了理论模型的正确性。通过讨论振动叠加流场中Maxwell流体的动态黏度,分析了粘性耗散法的应用局限性。最后,通过动态流变实验,发现粘性耗散法对于小振幅范围的振动剪切流具有较好的预测能力。     

Thermal stability and melt rheology of poly(<Emphasis Type="Italic">p</Emphasis>-dioxanone)

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO’s thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples’ viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (M _w = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox–Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.     

Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Abstract

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10^?3 s^?1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.

MST/978     

Rheological Behaviors of Polymer Melts and Concentrated Solutions

The memory function and elastic modulus were introduced into the O-W-F type of constitutive equations with the Cauchy-Green and Finger tensors for simple fluid, and then the concrete constitutive equation and material function for polymeric suspensions in the different flow fields were formulated: 1) Viscosity and first normal-stress difference coefficient in the shear flow field; 2) Tensile viscosity at the uni-axial elongation flow field; and 3) Dynamic viscosity and modulus in the dynamic field. Then the dependence of the material function on the deformation rate and weight fraction of fillers was discussed, and the shear flow curves with four characteristic regions ( I-low shear rate plateau region, II-apparent yielding region, III-intermediate rate plateau region, and IV-shear thinning region.) were explained theoretically. Finally, it was verified by a number of experimental values for the rheological suspensions and the viscoelastic and mechanical behaviors can be predicted by the molecular theory of non-linear viscoelasticity for polymeric suspensions.     

Melt-state shear flow and elasticity of a thermoplastic fluorosulphonated—PTFE copolymer

Some rheological properties of a perfluorosulphonated PTFE copolymer have been measured in order to characterise the melt-state viscous and elastic behaviour of a thermoplastic precursor of Nafion®, an ion-selective membrane used in the electrochemical industry. Steady-state shear viscosity measurements show conventional pseudoplastic flow behaviour over a wide temperature range, under high shear conditions. These data have been modelled to a high level of accuracy using polynomial simulations to obtain Carreau model coefficients and flow activation energies. Using an orifice die in a capillary rheometer, calculated extensional viscosity data are shown to decrease with stress and are more temperature-sensitive than shear viscosity. Although die swell increases with shear rate in a conventional manner, unusual and complex die swell data (at a fixed shear stress) have been obtained in response to an increase in process temperature. This behaviour is attributed to the breakdown of a small-scale network of domains and ordered crystalline material in the amorphous matrix of the precursor, as exemplified by a very broad melting endotherm. Different modes of deformation have been proposed to explain the rheological data observed across the process temperature range. The observed changes to the flow mechanism and elastic character of the melt carry practical implications for the extrusion processes and developed microstructure of film products manufactured from this precursor copolymer.     

注射成型中聚合物熔体应力分布的研究

对聚苯乙烯在注射成型过程中全展区的熔体进行了应力分布的研究计算结果与理论预测和实验值具有较好的一致性。     

Particulate flow simulation using Lattice Boltzmann Method: A rheological study

This work deals with calculating rheological properties of a suspension of particles in a fluid. A suspension of mono- and poly-disperse circular particles in shear flow is studied using two different methods for application of shear force: (a) by placing parallel walls at the top and bottom of the domain which are moving in opposite directions with the same velocity, and (b) using the Lees–Edwards boundary condition. The system which starts moving from rest, is allowed to reach a statistically steady state. Rheological properties namely, bulks shear stress, effective viscosity and normal stress difference of the suspension at different particle-based Reynolds numbers and different mean particle area fractions are calculated. Furthermore, the effect of size distribution on the relative effective viscosity of the suspension is investigated. Comparison of the present results with empirical formulations found in the literature shows reasonable agreement.     

The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows

This paper is concerned with the rheological modelling of both chemically treated and untreated carbon nanotube (CNT) suspended in a Newtonian epoxy resin. CNT suspensions generally exhibited shear-thinning characteristic—the apparent viscosity decreases as shear rate increases—when subject to steady shear flows. Chemically treated CNT suspensions with little optical microstructure were found to exhibit a less significant shear-thinning effect compared with untreated CNT suspensions where clear optical aggregates were observed. In the case of treated CNT suspensions, the shear-thinning characteristic could be described using a Fokker–Planck based orientation model. The model assumed that the treated CNTs behaved as high aspect ratio rods and that shear flow was able to align the CNTs in the flow direction, thereby resulting in a decrease in the shear viscosity. Despite the success in describing the rheological response of treated CNT in steady shear flows, the orientation model failed to explain the more pronounced shear-thinning effect observed in untreated CNT suspensions having a hierarchy of aggregate structures. A new model called the aggregation/orientation (AO) model was formulated by modifying the Fokker–Planck equation. The AO model considered elements of aggregation as well as CNT orientation and it was capable of capturing the steady shear response of untreated CNT suspensions.     

Dynamic asymptotic mode III crack tip field in rate dependent materials

The asymptotic field at a dynamically growing crack tip in strain-rate sensitive elastic-plastic materials is investigated under anti-plane shear loading conditions. In the conventional viscoplasticity theory, the rate sensitivity is included only in the flow stress. However, it is often found that the yield strength is also affected by previous strain rates. The strain rate history effects in metallic solids are observed in strain rate change tests in which the flow stress decreases gradually after a rapid drop in strain rate. This material behavior may be explained by introducing the rate sensitivity in the hardening rule in addition to the flow rule. The strain-rate history effect is pronounced near the propagating crack where the change of strain rates take place. Effects of the rate dependency in the flow rule and the hardening rule on the crack propagation are analyzed. The order of the stress singularity in the asymptotic field is determined in terms of material parameters which characterize the rate sensitivity of the material. The results show that an elastic sector is present in the wake zone when the rate-dependency is considered only in the hardening rule. Terminal crack propagation speed is determined by applying the critical stress fracture criterion and the critical strain criterion to the asymptotic fields under the small scale yielding condition.     

Slow steady-shear of plastic bead rafts

Experimental measurements of the response of a two dimensional system of plastic beads subjected to steady shear are reported. The beads float at the surface of a fluid substrate and are subjected to a slow, steady-shear in a Couette geometry. The flow consists of irregular intervals of solid-like, jammed behavior, followed by stress relaxations. We report on statistics that characterize the stress fluctuations as a function of several parameters including shear-rate and packing density. Over a range of densities between the onset of flow to the onset of buckling (overpacking) of the system, the probability distribution for stress fluctuations is essentially independent of the packing density, particle dispersity, and interaction potential (varied by changing the substrate). Finally, we compare the observed stress fluctuations with those observed in other complex fluids.     

PMMA/PBA/BaTiO3复合电流变液的制备及其性能研究

通过溶胶-凝胶法合成了钛酸钡溶胶，在此基础上合成了PMMA/PBA/BaTiO3复合材料，并对其进行了结构和性能表征，把PMMA/PBA/BaTiO3复合材料经过直接减压蒸馏得到电流变液，通过稳态剪切测试了复合材料的电流变效应．实验结果表明：稳态剪切下，剪切应力随着电场的变大而变大，而表观粘度随着电场的增强也增大，整个体系属于剪切稀化体系，且动态屈服应力随电场的增强而变大；同时在相同电场下，剪切应力和表观粘度均随着粒子含量的变大而变大。     

Organoclay/thermotropic liquid crystalline polymer nanocomposites. Part IV: organoclay of comparable size to fully extended TLCP molecules

Organoclay with a size of 100–200 nm was successfully prepared by a combination of wet ball milling and ultrasonication methods without changing its physico-chemical properties. A nanocomposite (TC3 FS) of 3.0 wt% treated organoclay in thermotropic liquid crystalline polymer (TLCP) was prepared. The treated organoclay was of comparable size to the fully extended TLCP molecules and it formed weak interactions with them. The liquid crystallinity of the TLCP was not greatly affected by the treated organoclay at the nematic temperature of the TLCP. Rheological characterization demonstrated that the viscosity of the TC3 FS was less than one order of magnitude higher than that of the TLCP in the linear viscoelastic region, and the steady shear viscosity of the two materials was comparable in steady shear experiments. Thus, TC3 FS is a promising viscosity reduction agent for high molecular mass polyethylene (HMMPE), functioning similarly to TLCP. The 1.0 wt% TC3 FS in HMMPE has more efficient viscosity reduction ability than the 1.0 wt% TLCP in HMMPE, with lower yielding stress and yield-starting shear rate, as well as a narrower yield shear rate region. The viscosity reduction ability of the TLCP was enhanced by the treated organoclay.     

Flow of electro-rheological materials

Summary An electro-rheological fluid is a material in which a particulate solid is suspended in an electrically non-conducting fluid such as oil. On the application of an electric field, the viscosity and other material properties undergo dramatic and significant changes. In this paper, the particulate imbedded fluid is considered as a homogeneous continuum. It is assumed that the Cauchy stress depends on the velocity gradient and the electric field vector. A representation for the constitutive equation is developed using standard methods of continuum mechanics. The stress components are calculated for a shear flow in which the electric field vector, is normal to the velocity vector. The model predicts (i) a viscosity which depends on the shear rate and electric field and (ii) normal stresses due to the interaction between the shear flow and the electric field. These expressions are used to study several fundamental shear flows: the flow between parallel plates, Couette flow, and flow in an eccentric rotating disc device. Detailed solutions are presented when the shear response is that of a Bingham fluid whose yield stress and viscosity depends on the electric field.     

Experimental and computational validation of Hele-Shaw stagnation flow with varying shear stress

An in vitro flow model system with continuous variation of fluid shear stress can be used to test cell responses to a range of shear stresses. In this investigation, we validated such a flow system computationally for steady and unsteady flow conditions and experimentally for steady flow conditions. The unsteady flow validation is important for studying cells such as endothelial cells that experience unsteady flow conditions in their native environment. The system is capable of exposing cells in different regions of the chamber to steady or unsteady shear stress conditions with average values ranging linearly from 0 to 30 dyn/cm $^{2}$ . These tests and analyses demonstrate that the variable-width parallel plate flow system can be used to test the influence of a range of steady and unsteady fluid shear stress levels on cell activities.     

In-process rheometry as a PAT tool for hot melt extrusion

Real time measurement of melt rheology has been investigated as a Process Analytical Technology (PAT) to monitor hot melt extrusion of an Active Pharmaceutical Ingredient (API) in a polymer matrix. A developmental API was melt mixed with a commercial copolymer using a heated twin screw extruder at different API loadings and set temperatures. The extruder was equipped with an instrumented rheological slit die which incorporated three pressure transducers flush mounted to the die surface. Pressure drop measurements within the die at a range of extrusion throughputs were used to calculate rheological parameters, such as shear viscosity and exit pressure, related to shear and elastic melt flow properties, respectively. Results showed that the melt exhibited shear thinning behavior whereby viscosity decreased with increasing flow rate. Increase in drug loading and set extrusion temperature resulted in a reduction in melt viscosity. Shear viscosity and exit pressure measurements were found to be sensitive to API loading. These findings suggest that this technique could be used as a simple tool to measure material attributes in-line, to build better overall process understanding for hot melt extrusion.