摘要:

Rheological and rheo‐optical studies on nematic solutions of the rodlike poly (1,4‐phenylene‐2,6‐benzobisthiazole), PBT, are discussed in terms of theoretical predictions based on the use of the Ericksen–Leslie constitutive equation for nematic fluids. The behavior of both aligned and unaligned solutions is discussed. Experiments on the former show that the nematic solutions of PBT do not align in a slow shear flow, and that reorientation of the director in a magnetic field is coupled with local flow, producing a phase grating. The grating, which is visualized by its diffraction of light, may also be observed in flow. The textural features of initially unaligned solutions are discussed, and the flow behavior is considered in terms of certain theories based on a polydomain structure in the nematic fluid.

ISSN:0148-6055    

DOI:10.1122/1.550176

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

An explanation is given for the surface defect, commonly referred to as sharkskin, considered in the present study to occur at the die exit. It will be shown numerically that large negative pressures can exist at the die exit by simulating the flow of a linear polydimethylsiloxane melt, which showed significant sharkskin, through a capillary using a finite element program. Furthermore, a series of photographs indicates that the cracks on the surface of the extrudate originate at the die exit and not further down the extrudate. It will then be postulated that negative pressure (hydrostatic tension) cavitates the polymer melt very close to the die lip (within a micron) at the die exit on the surface of the die or in the bulk of the polymer depending on the respective strengths of adhesion to the die surface at the exit or of the cohesion of the bulk of the polymer. The growth and coalescence of these voids would then lead to cracks (sharkskin) at the die exit.

ISSN:0148-6055    

DOI:10.1122/1.550177

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The flow properties of concentrated polymer dispersions which exhibit extreme shear thickening have been investigated in steady and transient shearing flows using various types of rheometers. A phenomenological characterization based on few fluid parameters is proposed and pertinent test modes for their determination are discussed. The dispersions consist of monodisperse, electrostatically stabilized, solid spherical particles (diameter<0.5 μm) dispersed in glycols. They show a reversible steplike viscosity transition of nearly three powers of ten when an apparent critical shear rate γ̇cis exceeded. This jump from a low to a high viscosity state shows typical features of a shear‐rate‐induced phase transition, including hysteresis and the existence of a metastable low viscosity state above γ̇c. The viscosity step can be traced continuously by using stress controlled rheometers. Pronounced fluctuations of the rate of deformation with time are observed at the critical shear rate. The apparent critical shear rate depends on the rheometer geometry. Its change with the gap width in Couette flow and with the radius of circular dies can be interpreted by taking into account a true critical shear rate γ̇0and a wall slip velocityvs. Periodic switching to the high viscosity state in oscillatory shear only occurs if both a critical rate amplitude and a critical shear amplitude are reached during the cycle. Applications of these dispersions for simple speed controlling or selectively damping mechanical elements are briefly introduced.

ISSN:0148-6055    

DOI:10.1122/1.550257

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

In order to use either a linear or nonlinear model of viscoelasticity to calculate the stress response of a material to various deformations, it is usually necessary to have available an explicit equation for the linear relaxation modulusG(t). The most popular procedure is to use the data from a small‐amplitude oscillatory shear experiment to determine the parameters of a generalized Maxwell model. However, this is an ill‐posed problem and is not at all a straightforward curve‐fitting operation. We compare three procedures for determining a set of relaxation times and discrete moduli that can then be used as empirical fitting parameters in fluid mechanics computations. These are linear regression, with and without regularization, and nonlinear regression. Nonlinear regression is found to give a good fit of the data with a minimum number of parameters.

ISSN:0148-6055    

DOI:10.1122/1.550164

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Certain polyelectrolytes, when rigorously dried and dispersed into a nonconducting liquid, produce dispersions which may be electrorheologically active at 23 and 100 °C, at 100 °C but not at 23 °C, or not active at either temperature. Further, fluids apparently ER inactive at room temperature but active at elevated temperatures have not previously been reported. In this paper, three primary chemical characteristics of the polyelectrolyte inclusions have been examined to help in understanding this behavior and on a broader scale to aid in better understanding of fundamental mechanisms responsible for ER effectiveness. These include dissociation constants of the polyelectrolytes, counter ion type, and matrix resin type of the macroions. The results show a strong relationship between ER behavior andpKa, whereas no clear dependencies seem to exist with the other parameters. This additionally supports the hypothesis that ER activity is associated with mobile charges bound to the particulate phase. Additionally, the fact that some of these materials work much better at 100 °C than at room temperature further supports the contention that the behavior is not water mediated and another mechanism is involved, i.e., a mechanism intrinsic to the chemistry of the particulate phase.

ISSN:0148-6055    

DOI:10.1122/1.550165

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

In this study, the effect of hydrogenated styrene butadiene diblock copolymers on the rheological properties of high‐density polyethylene (HDPE)/high impact polystyrene (HIPS) blends is investigated. The blends were prepared in the melt state at 180 °C for 5 min in a batch mixer. The results show that the rheological properties of the blends were very sensitive to the copolymer concentration present in the blend mainly in the low frequency region. At low copolymer content (e.g., 1 wt. %), an important decrease of the zero‐shear viscosity (η0) was observed in the case of HIPS rich blends. For the HDPE rich blends, this decrease was not observed when modified with the pure diblock copolymer. As the copolymer content increases, and depending on the copolymer structure, an increase or a further decrease of η0was obtained. This behavior was interpreted as due to the change in the copolymer state in the blend, i.e., saturation of the interface and micelles formation. Predictions of an emulsion model of two viscoelastic liquids including an interfacial tension (σ) contribution to the elastic properties were close to the experiments for the 90/10 and 20/80 HDPE/HIPS unmodified blends, but unsatisfactory in the other cases. According to the general features of polymer suspensions and to the predictions of this model, it is concluded that phase interactions as produced by the addition of the copolymer are very important. This aspect must be included in any rheological model for copolymer modified immiscible blends.

ISSN:0148-6055    

DOI:10.1122/1.550166

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The viscosity of concentrated shear thickening dispersions was measured as a function of shear rate, Couette cylinder size, and time. The level of the low shear rate viscosity, which was found to be independent of system size and time, could be predicted by the equation of Frankel and Acrivos. At shear rates above the critical shear rate for shear thickening in highly concentrated (φ≳0.57) dispersions of monodisperse particles strong viscosity instabilities were detected, together with a dependence on cylinder size. The instabilities are attributed to reversible order–disorder transitions, e.g., from strings to clusters. This dependence on cylinder size is due to wall slip, slipping planes in the dispersion, and even plug flow in the gap. With less concentrated or polydisperse dispersions the effects are much less severe but there is thixotropy, probably due to a reordering of the dispersion.

ISSN:0148-6055    

DOI:10.1122/1.550167

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Statically cooled samples of waxy crude oil possess a complex yielding behavior that cannot be described by existing yield stress fluid models nor by the description and modeling of thixotropic materials. Three distinct characteristics of the yielding process, namely, a solid (Hookean) behavior, a slow deformation (creep), and a sudden failure of the sample that closely resembles the brittle or ductile fracture of solids, have been identified by four different techniques−the vane technique, the cone and plate viscometer (constant rotation), constant stress rheometry, and oscillatory (dynamic) testing. A capillary viscometer or pilot scale pipeline presents technical difficulties which make it unsuitable for investigating the yielding behavior of waxy crude oils. The shear stress at fracture is the value of most interest to pipeline designers and the one most often taken as the yield point. Means of obtaining reproducible yield stress and fracture stress data are discussed.

ISSN:0148-6055    

DOI:10.1122/1.550168

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Steady planar shear flow is considered as a solution of a boundary value problem for the equation of a Giesekus fluid for various values of the mobility parameter. It is shown that if the mobility parameter is greater than 1/2 then such a solution either does not exist or is notrealizablefor large values of the shear rate (if there is no solvent viscosity contribution) or for an entire finite range of shear rates (if there is a small contribution from the solvent viscosity). If the mobility parameter does not exceed 1/2 then the solution exists and isrealizablefor all values of the shear rate. The conclusions follow from a one‐dimensional linear stability analysis of the appropriate boundary value problem and from an admissibility criterion that stems from restrictions imposed on the configuration tensor which arises from the molecular model description of the polymer liquid. We consider a solutionrealizableif it is stable and admissible. Thus a clearly distinct behavior of the Giesekus model is observed for values of the mobility parameter between 0 and 1/2 as opposed to values between 1/2 and 1. This may have implications on the suitability of the model with values of the mobility parameter exceeding 1/2.

ISSN:0148-6055    

DOI:10.1122/1.550169

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Transient and steady rheological measurements of shear and normal stresses are reported for a liquid crystal copolymer of 73 hydroxybenzoic acid/27 hydroxynaphthoic acid. Dynamic time sweeps indicate a sensitiveness of the rheological parameters to thermal history, probably related to polymerization phenomena. In the shear rate range 0.1–1 s−1atT=310 °C the normalized stress overshoots vs strain curves nearly superpose. The steady values of the viscosity seem to follow the three region scheme of Onogi and Asada. The steady values of the first normal stress difference are positive in all the shear rate range analyzed. Constant stress measurements are also reported to prove that the low shear rate data are not significantly affected by the polymerization phenomena. Moreover, the constant stress data show that a true yield stress cannot be detected in the LCP studied.

ISSN:0148-6055    

DOI:10.1122/1.550170

出版商:The Society of Rheology    

年代:1991

数据来源: AIP