摘要:

In the bimodal model of the viscoelastic behavior of a bimodal suspension with a colloidal size fine fraction and a noncolloidal size coarse fraction, the two fractions behave independently of each other when the particle size ratio is large. The colloidal fraction is solely responsible for the non-Newtonian behavior while only hydrodynamic interaction takes place between the coarse particles. Measurements performed on a bimodal suspension and on a suspension of noncolloidal particles in a polymeric liquid indicate that the non-Newtonian behavior originates from the colloidal fraction in the former and from the polymeric liquid in the latter. The results support the validity of the concepts behind the bimodal model.

ISSN:0148-6055    

DOI:10.1122/1.550868

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

A slender one dimensional (1D) model for filaments of liquid crystalline polymers (LCPs) is applied to simulate isothermal fiber spinning of materials with internal orientation. The focus is on the hydrodynamic-orientation interactions in spinning flows, isolated from other significant spinline effects of temperature, crystallization, and phase changes. Spun fiber orientation (in particular, birefringence) is deduced from first principles along with fiber diameter and velocity. One result of our modeling and simulations is that, in isothermal spinning, the microstructure (orientation tensor) is weakly radially dependent and can be calculated from 1D models. Families of numerical steady state fiber spinning solutions, together with their linearized stability, are presented. These calculations reveal upper bounds on throughput in terms of thecritical draw ratio, above which the process is unstable. The effects on stability due to LCP parameter changes are thoroughly investigated. We find enhancement of the effects of LCP kinetic energy or relaxation can either stabilize or destabilize steady spinning solutions, whereas enhanced anisotropic drag is always destabilizing. Evidence is given for a preferred degree of upstream LCP alignment at which the critical draw ratio achieves a maximum, indicating an important role played by near-spinneret conditions in increasing throughput.

ISSN:0148-6055    

DOI:10.1122/1.550821

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Two methods have been extended (those due to Tuminello and to Shaw), among the several currently available, for calculating the molecular weight distribution functionW(M)of linear polymers from their rheology data. An extension of Tuminello’s method is proposed by using the terminal Rouse relaxation time as a scaling factor. The 79th percentile of the zero-shear viscosity normalized flow curve is used to evaluate the terminal Rouse relaxation time. Using this technique, it is possible to convert frequency to molecular weight, without the use of an external standard. An extension to Shaw’s numerical technique is also proposed leading to an analytical expression for the polydispersity ratio. For this purpose the Elbirli–Yasuda–Carreau viscosity model is used to describe the flow curve. The development is carried out for both the logarithmic and the power law mixing rules considered by Shaw. Due to the approximate nature of both the original methods that Tuminello and Shaw started out with and the subsequent approximations, the resulting distribution should only serve as a first order approximation to the actualW(M)and the calculated moments ofW(M)may be off by as much as about 50% from their true values. Since the more rigorous inverse problem techniques that are also currently available can offer greater accuracy, justification for the use of the approximate methods discussed in this work rests solely on their relative simplicity and ease of implementation.

ISSN:0148-6055    

DOI:10.1122/1.550822

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The sharkskin defect appearing during the capillary extrusion of three low-density polyethylene resins with different molecular structures has been characterized. Using complementary techniques, profilometry, optical microscopy, and observation of cross sections, the amplitude and the period of the defects have been measured accurately. The influence of flow rate, temperature, and die geometry has been quantified. The specific behavior of the orifice die has been put in evidence. It shows that, if the role played by the stress field is evident, the wall shear stress is not the unique determinant of the sharkskin process. The influences of molecular structure and elongational behavior on sharkskin are discussed. It appears that resins exhibiting long chain branching and strain hardening are less sensitive to sharkskin.

ISSN:0148-6055    

DOI:10.1122/1.550837

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Nonequilibrium Brownian dynamics simulations are applied to model a highly concentrated dispersion of charged particles at high ionic strength. Through the use of hydrodynamic preaveraging, as applied to the overdamped Langevin equation, and correlations appropriate for hard spheres, good predictions are obtained for the suspension shear viscosity at low to moderate shear rates. At higher shear rates, the influence of near-field hydrodynamic interactions on the suspension microstructure invalidates the preaveraging approach.

ISSN:0148-6055    

DOI:10.1122/1.550840

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The nonlinear viscosity of the wormlike surfactant system cetyl pyridinium chloride/sodium salicylate (60 mM/100 mM in water) has been investigated in both pipe and cylindrical Couette geometries, using nuclear magnetic resonance to image both velocity and diffusion. In pipe flow we observe transitions from Newtonian to non-Newtonian viscosity, to spurt, to unstable flow, and then to a regime where fluctuations are rapid on the timescale of a few milliseconds. In the Couette cell we observe apparent slip at the inner wall as well as a high shear rate band located away from the wall in the body of the fluid. The banding phenomenon, which has its counterpart in the pipe flow, is consistent with double valuedness in the stress versus rate of strain relationship for this fluid.

ISSN:0148-6055    

DOI:10.1122/1.550864

出版商:The Society of Rheology    

年代:1997

数据来源: AIP