摘要:

The prediction of transient network theory is in reasonable agreement with the biaxial shear behavior of a polyisobutylene and a low density polyethylene melt in two different biaxial shear tests. Transient network theory provides a proper description of the development of melt anisotropy for highly nonlinear behavior in biaxial shear. The use of the nonaffine deformation hypothesis gives an improvement for the case of low density polyethylene, at high shear rate.

ISSN:0148-6055    

DOI:10.1122/1.550690

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We report results of Brownian dynamics simulations of the steady state and transient rheological behavior of bead and spring chain models in elongational flow. The simulations include examination of the effects of excluded volume, hydrodynamic interactions, chain length, and maximum bond extension. Hydrodynamic interactions are modeled by the Rotne–Prager–Yamakawa tensor. The steady state results for the rheological properties of finitely extensible nonlinear elastic (FENE) chains in the absence of hydrodynamic interactions and excluded volume interactions are compared with the analytic FENE‐PM theory of Wedgewood and co‐workers. For bead and spring models with short bond lengths, excluded volume and hydrodynamic interactions are shown to affect the steady state rheological properties significantly.

ISSN:0148-6055    

DOI:10.1122/1.550722

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We present a comprehensive nonlinear numerical analysis of the temperature dependent nematorheological shear behavior, using experimentally measured material parameters of typical nematic, low molar mass, rodlike liquid crystals. We consider a typical aligning nematic (5CBP:4‐n‐pentyl‐4’‐cyanobiphenyl) and a typical nematic (8CBP:4‐n‐octyl‐4’‐cyanobiphenyl) exhibiting both aligning and nonaligning behavior during shearing. The accuracy of the numerical results is established by a thorough validation procedure based on recently available experimental rheological data corresponding to the same model materials. Very good semiquantitative agreement between experiments and simulations is found for both the aligning and nonaligning materials. Despite the simplified model geometry, the simulations are able to capture both details and salient transient and steady rheological features that characterize the multiple regime transitions of nonaligning nematics, as they are cooled towards the smectic A phase.

ISSN:0148-6055    

DOI:10.1122/1.550691

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

This paper presents results of a series of experiments on the settling velocity of spheres in two‐component solid–liquid suspensions. Both sedimenting and fluidized suspensions are considered. Particular emphasis has been given to the effective values of the density and viscosity of the mixture which allow us to describe slow settling of a test sphere using the Francis [Physics4, 403–406 (1933)] modification of the Stokes formula for the settling of a single sphere in a Newtonian fluid with wall effects. Our experimental results show that values of the effective density and viscosity are close to the average density of the mixture and to the viscosity of the mixture predicted by a correlation suggested by Thomas [J. Colloid. Sci.20, 267–277 (1965)] when the test particles are of the same size as the suspended particles, or larger, but not too much smaller. The signature of the failure of the concept of effective fluid properties is traced to an increase of the variance of the experimental results and is not associated merely with a different than average value of the density; in particular, none of our results are well described by the use of the fluid density in the expression for the buoyant force. We also studied the settling of spheres in fluidized suspensions at high Reynolds numbers. The Barnea and Mizrahi [Chem. Eng. Sci.5, 171–189 (1973)] model for the expansion of uniformly fluidized beds was modified in two different ways to describe the settling of particles in the fluidized suspensions. It is not possible to describe the modified drag laws with an effective Newtonian theory if for no other reason than the effective densities which enter into buoyancy are not in principle those which control inertia. The modified drag law is in agreement with experiments in sedimentation columns. Coarse agreement between the predicted and measured drag is found in the fluidized suspension even when the test and the fluidized particles are the same size.

ISSN:0148-6055    

DOI:10.1122/1.550692

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

This paper describes a method for extracting useful information from small‐scale pipe viscometer measurements of foam rheology. The rheology of a foamed polymer solution at a given temperature, pressure, and quality was determined in pipes of five diameters. The flow curves showed a marked dependence on the diameter of the pipe. The concept of apparent slip could be used to explain the phenomenon. The classical slip correction of Mooney was not applicable, but the method developed by Jastrzebski (based on the previous work of Oldroyd) provided a consistent means of apparent slip correction. The geometric interpretation of the two slip correction methods revealed the possible reason for the difference of their performance. The slip corrected measurements were interpreted in the framework of the volume equalization principle.

ISSN:0148-6055    

DOI:10.1122/1.550701

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The low shear rate slip behavior of monodisperse polystyrene melts was characterized with the parallel plate geometry in a torsional rheometer. It was found that all melts studied exhibited some slip behavior. Tool material of construction showed no appreciable effect. Results were analyzed according to deGennes’ theory of slip, and values of the characteristic slip dimension,b, were calculated for each molecular weight. The slip dimension was found essentially proportional to the molecular weight to the first power. A slider block and spring model is proposed in an attempt to explain slip phenomena in the low shear rate limit.

ISSN:0148-6055    

DOI:10.1122/1.550702

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550703

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We present here a short summary of two rather different, but complementary, simulations of plastic deformation in a flexible chain glassy polymer. In the first, atomistic simulation, a molecular structure model is used in which well‐established force fields between atoms and atom groups on a typical chain polymer are introduced to account for the most relevant molecular degrees of freedom that govern both structural and shear relaxations. The principal result of this simulation which has been described in great detail elsewhere [Mottetal., Philos. Mag.67, 931–978 (1993a)] is that shear relaxations are in the form of abrupt shear transformations occurring in volume elements of ∼10 nm size and resulting in transformation shear strains of ∼2%. This simulation has established that the chemically specific conformational constraints are internal to the volume elements and that the interaction of the elements with their surroundings is elastic, suggesting that the phenomenon can be meaningfully simulated by a two‐dimensional mesoscopic model. In the second part of the present communication we summarize the most important findings of the extensive two‐dimensional mesoscopic simulation which we [Bulatov and Argon, Model. Sim. Mater. Sci. Eng.2, 167–184 (1994a); 185–202 (1994b); 203–222 (1994c)] have performed. These findings include the important effects of disorder related misfit stresses in governing both quasihomogeneous flow at elevated temperatures and localized shear flow at lower temperatures. In addition, we demonstrate that these misfit stresses are the key ingredient that governs the initial deformation transients, as well as the well‐known distributed kinetics of the deformation process in amorphous media in general.

ISSN:0148-6055    

DOI:10.1122/1.550728

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Several authors have shown that many aspects of the yieldlike response of polymers can be described by the constitutive framework of nonlinear viscoelasticity. This arises through the use of a reduced time which causes stress relaxation to be accelerated by increasing deformation. Moreover, the history dependence inherent in the constitutive equation leads to different yield response under strain and stress control conditions, as is observed experimentally. Previous work has been concerned with yield during large extensional deformations under elongation control conditions, or small shear deformations under strain or stress control. The present work is concerned with a numerical study of yieldlike response under large shear deformations. Of particular interest is the influence of volume changes associated with the normal stress effects induced by shear deformations. A nonlinear single integral constitutive equation for a viscoelastic compressible solid is developed for the purpose of the study. Its initial elastic response and the long time equilibrium response are given by a Beatty–Blatz–Ko model for a nonlinear elastic compressible solid. A reduced time is introduced in which the shift function depends on the volume change and/or the shear deformation. Results are presented of numerical simulations of experiments involving simple shear deformations and shear deformations in the absence of normal tractions. The latter allows the influence of volume changes arising from the absence of normal tractions to be studied. Both the shear deformation and shear stress control conditions are examined.

ISSN:0148-6055    

DOI:10.1122/1.550704

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The work described in this article is a more quantitative development of previous work on three grades of polyethylene which identified two yield points under deformation over a range of conditions. This behavior was modeled as two processes acting in parallel such that the total stress is the sum of the two individual stresses, termed the recovery stress and the effective stress. The individual stresses on each arm were found by use of a strain‐dip experiment, which enabled the yield stress for each arm to be found as a function of the temperature and strain rate. The effective stress measurements on one of these polymers (medium density polyethylene) have been extended over a wider range of temperatures and strain rates. The results obtained were then modeled in two different ways: firstly assuming that the stresses on the two arms were each modeled by a single activated rate process; and secondly assuming that the recovery stress is strain‐rate independent and the effective stress was modeled by a cooperative jump process. The results show that the second approach is more successful, but further analysis has to be carried out to confirm this proposition. Activation volumes found using the two methods are in agreement with previous results. However the activation energies found are smaller than previously measured due to either differences in data analysis or in definitions of yield points. A tentative assignment of the effective stress to an interlamellar shear process has been made.

ISSN:0148-6055    

DOI:10.1122/1.550705

出版商:The Society of Rheology    

年代:1995

数据来源: AIP