摘要:

The stress jump after steady shear for a semidilute, semirigid macromolecule, xanthan gum, in a viscous solvent, was measured and are the first such data reported. The amount of stress instantaneously dissipated, σv, depends on the shear rate and xanthan gum concentration. It is less dependent on the concentration than the amount of stress retained on cessation, σe, yet, the concentration scaling does not follow that for existing theories possibly due to the polydispersity and finite flexibility of the macromolecule. It was found that the stress jump ratio (i.e., the amount of stress retained divided by the total stress due to the polymer prior to cessation) does not initially deviate from the dilute solution prediction which suggests that existing theories may need modification. The stress jump was calculated for a variety of rheological equations of state, including recent theories for liquid crystal polymers, demonstrating that this phenomenon is not without precedent.

ISSN:0148-6055    

DOI:10.1122/1.550439

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Manneville and Dubois‐Violette’s linear stability analysis of the roll‐cell instability in nematics is extended to account for the effect of finite rates of roll‐cell growth. The extended theory is applied to shearing flows of tumbling liquid crystalline polymers, in which the Ericksen number is typically a decade or more above the critical value for formation of the roll cells. It is found that, starting from a monodomain with director oriented perpendicular to both the flow and the flow gradient, at asymptotically high Ericksen numbers a wide band of wave numbers is unstable and the widthlmof the fastest growing roll cells is proportional to γ̇−1/4D1/2, where γ̇ is the shear rate andDis the gap between the shearing surfaces. This prediction of thin roll cells whose width depends only weakly on shear rate is consistent, in part, with roll‐cell observations of Srinivasarao and Berry for poly(1,4‐phenylene‐2,6‐benzobisthiazole), and with characteristic texture spacings measured in sheared poly(γ‐benzyl‐glutamate) solutions. These results suggest that the roll‐cell instability may provide a mechanism for texture refinement and director turbulence in shearing flow of tumbling nematics.

ISSN:0148-6055    

DOI:10.1122/1.550440

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The dynamic oscillatory response of electrorheological suspensions is investigated using a molecular dynamics‐like simulation method, where suspensions are modeled as hard, monodisperse, neutrally buoyant, dielectric spheres contained in a dielectric, Newtonian fluid between parallel‐plate electrodes. The response is described by frequency‐dependent moduli, which display a significant relaxation due to competition between hydrodynamic and electrostatic forces on spheres within thick clusters. For small amplitude deformation of monodisperse suspensions, the direct electrostatic contribution to the response obeys time‐electric field strength superposition analogous to time–temperature superposition in polymer rheology. The response for monodisperse suspensions is dominated by a single relaxation time, in contrast to the broad dispersions commonly observed. Possible explanations for this discrepancy are discussed.

ISSN:0148-6055    

DOI:10.1122/1.550441

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Many previous investigations have established and/or quantified yield stresses in suspensions of interacting (colloidal or surface active) particles. In this work it is shown for the first time that highly filled suspensions of noncolloidal particles can also exhibit a static yield stress σs. The determination of static yield stresses in these systems was made possible by conducting very long time (up to 6000 h) tensile creep tests. These tests are particularly suitable for high viscosity systems such as the suspensions examined here and also have the advantage that there is minimal influence on the results due to the measurement apparatus. Through these experiments, it is possible to observe an unambigious change of material response above and below the static yield stress σs, with viscoelastic solid‐like behavior exhibited below σs, and viscoelastic liquid‐like behavior above σs. The dependence of this static yield stress on the volume concentration filler is examined in this work.

ISSN:0148-6055    

DOI:10.1122/1.550442

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

To illustrate a procedure for optimizing an industrial fiber spinning process through modeling, we make two choices: (1) we adopt the viscoelastic fiber‐spinning model of Denn, Petrie, and Avenas; and (2) we choose draw ratio as the process variable to be optimized. We begin with an analysis of the time‐dependent generalization of the Denn, Petrie, and Avenas model to determine all well‐posed boundary and initial conditions for this coupled system of nonlinear equations. Next we give thegeneralanalyticalsolutionto the steady dimensionless form of the problem, which we exploit to analyze the response of the solution toallpossiblesteady‐statevariationsin the dimensionless process parameters available within the Denn, Petrie, and Avenas model. We then predict both the conditions which optimize draw ratio in this model for the fiber spinning process, and how to achieve the maximum. This determination is first obtained in the dimensionless formulation and then transferred to the more practical dimensional formulation.

ISSN:0148-6055    

DOI:10.1122/1.550443

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

A continuum tensorial theory was formulated to describe the isothermal, incompressible flow of uniaxial rodlike nematic liquid crystalline polymers. The tensor theory was reduced to a vector theory that describes the microstructure of the uniaxial phase by specifying the director orientation and the scalar order parameter. The reduction establishes useful relationships between this theory and the Leslie and Ericksen theory. The model was solved for a given steady simple shear flow, assuming spatial homogeneity. Two types of orienting modes are predicted: (a) the simple aligning mode, in which the microstructure reaches a stable shear dependent steady state for all shear rates, and (b) the complex mode, which at sufficiently high nematic potentials exhibits three flow regimes (tumbling, oscillating, and stationary) according to the strength of the imposed shear flow. For sufficiently low nematic potentials, the complex mode predicts the existence of a single stationary mode. Bifurcation theory was used to fully characterize the existence and transitions between the three regimes of the complex mode. Guidelines for future simplifications of the constitutive equations are given. The frequently reported changes in the sign of normal stress differences with shear rate are captured by the model.

ISSN:0148-6055    

DOI:10.1122/1.550444

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Superposed plane Poiseuille flow of a compatible polymer system consisting of linear low‐density polyethylene (LLDPE) and high‐density polyethylene (HDPE) has been investigated. Using the apparatus and procedures developed in our previous study [G. M. Wilson and B. Khomami, J. Rheol.37, 315 (1993)], we have investigated the role of viscosity ratio, layer depth ratio, and disturbance wave number on the stability of the interface. We have presented experimentally determined stability diagrams and growth rate curves and compared these with theoretical predictions based on the linear stability analysis using a modified Oldroyd‐B constitutive equation. Additionally, we have compared our experimentally determined growth rate curves to those presented earlier for the incompatible system of polypropylene (PP) and HDPE [G. M. Wilson and B. Khomami, J. Non‐Newt. Fluid Mech.45, 355 (1992)]. These comparisons reveal an unexpectedly low disturbance growth rate for the LLDPE/HDPE system which is attributed to diffusive and convective mixing in the vicinity of the interface. We have shown that mixing can occur on both small and large scales. Small scale mixing occurs because of oscillatory currents that form near the interface when an interfacial wave of any amplitude is present. On the other hand, when the interfacial waves reach a critical amplitude and wave number, they bifurcate supercritically which causes a portion of the wave to be pulled into the adjacent layer (i.e., large‐scale mixing).

ISSN:0148-6055    

DOI:10.1122/1.550446

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

A new approach has been adopted to study the sharkskin phenomenon and to characterize the extrudate surface roughness observed in polymer melt extrusion of a linear low‐density polyethylene through capillary dies. A previously proposed method, for the calculation of slip velocities as a function of both wall shear stress and pressure, was extended for polymer melts with a pressure‐dependent viscosity. In this work, the surface roughness of solidified extrudate samples was analyzed by means of digital image processing techniques. The fractal dimension of the extrudate surface was used as a quantitative measure of roughness and the effect of the wall shear stress on the severity of the sharkskin phenomenon was studied. The onset of surface roughness could be directly determined and it was found to lag behind the onset of slip in the die. The main advantage of the present method is the scale independence of the roughness measure. In addition, since only digital images of extrudates are required for the detection of surface roughness and due to short processing times involved, this method could potentially be employed in on‐line monitoring systems for the production of films with controlled roughness.

ISSN:0148-6055    

DOI:10.1122/1.550447

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The theme of this presentation is the steady‐state flow behavior of filled plastic melts. These are suspensions in which the continuous phase consists of a viscoelastic liquid. The disperse phase comprises solid particles. A concept for the description of such materials is introduced, which allows the influence of the two phases on the shear flow behavior to be approximately separated. The fluid is characterized by its shear stress and first normal stress function, whereas the influence of the solid phase is described by two shift factorsBτ(cv) andBN(cv). Both are functions of the volume concentrationcv. The prerequisite of the proposed concept is the dominance of fluid dynamic forces over direct particle–particle interactions. The concept has been experimentally tested by suspending the same limestone fraction in four different high molecular weight polymeric fluids. The zero shear viscosities of the suspending fluids varied from η0=0.12 to 80 kPa s; the volume concentrations up to 45%. The steady‐state shear stress and the first and second normal stress differences of the suspensions and the pure suspension fluid were measured. The observed deviations between the concept and the experiments are discussed. Finally the Bagley end effect behavior is documented.

ISSN:0148-6055    

DOI:10.1122/1.550449

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550450

出版商:The Society of Rheology    

年代:1993

数据来源: AIP