摘要:

The rheological response of a class of single fibers of natural origin is considered from a phenomenological point of view on the basis of the well‐known super‐position principle in viscoelasticity. A rheological model is presented which aims at the determination of both the creep and relaxation functions over the considered range of applied stress, or strain, and the extent of time covered. The proposed model is illustrated numerically for the case of the first creep of dry summerwood fibers of a longleaf pine holocellulose pulp and for the stress relaxation of cotton fibers. The model is presented in a general form and could be applied to a large class of viscoelastic materials.

ISSN:0148-6055    

DOI:10.1122/1.549935

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

We report a low‐cost electronic circuit which improves the precision of conventional analog methods for the measurement of linear dynamic moduli in the Weissenberg rheogoniometer. The performance of the circuit was tested by comparing the results obtained on a polymer melt with both the Weissenberg rheogoniometer and the Rheometrics mechanical spectrometer.

ISSN:0148-6055    

DOI:10.1122/1.549951

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

An experimental study was conducted to investigate the effect of salt on viscoelastic properties of partially hydrolyzed polyacrylamide Pusher 700 (Dow Chemical) in aqueous solutions. Viscosity η and primary normal stress differencesN1,were measured on a Weissenberg rheogoniometer and a Contraves low‐shear viscometer. The experimental results show the well known stabilizing effect of salt on the solutions' viscosity (i.e., the solutions become less viscous and less shear thinning) up to a critical value of shear rate,γ̇c,which depends on salt and polymer concentrations. For values of shear rate greater thanγ̇c,both η andN1exhibit appreciable increases with increasing shear rate. The amplitude of the so‐called shear‐thickening phenomenon increases with increasing salt concentration. Forγ̇>γ̇cthe presence of salt has no apparent effect on either η orN1of the concentrated polymer solutions. For lower polymer concentrations, the addition of salt increases markedly the values of η andN1.The investigated solutions also exhibit, forγ̇>γ̇cin stress growth experiments, a long transition from a pseudo‐steady state to an equilibrium steady state. The shear thickening reported here in purely shear flow is less pronounced than the “elongational thickening” observed in “entry flows.” Intrinsic viscosity data show that salt decreases the apparent size of macromolecules in solution. Physical arguments based on the conformation of macromolecules in solution are proposed to interpret the experimental results.

ISSN:0148-6055    

DOI:10.1122/1.549959

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

The recent development of new types of sliding‐plate rheometers for concentrated polymeric liquids makes it possible to generate a shearing deformation in which the strain is exponential in time. Such a deformation is a strong flow in the sense that the deformation kinematics have a strong tendency to stretch out the molecules of a polymeric liquid. Previous applications of the concept of strong flows have been limited to motions with constant stretch history, that is, deformations where the components of the strain rate tensor are constant with respect to time. For example, steady simple extension is a strong flow that has attracted much attention because of its ability to generate a high level of molecular stretching. However, it is a deformation that is difficult to generate in the laboratory. We make use of both a kinematic criterion for strong flows and one based on the elastic dumbbell model to show that exponential shear is likely to generate a high degree of molecular stretching. In addition, we propose the definition of a material function for representing data from exponential shear experiments. We also show that in the nonlinear viscoelastic range, exponential shear is likely to exhibit an unbounded growth in an appropriately defined function similar to the strain‐hardening behavior observed in the tensile stress growth coefficient of polymeric liquids with a branched molecular structure.

ISSN:0148-6055    

DOI:10.1122/1.549936

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

Polymer melts show “thinning” in shear deformation and “hardening” in simple elongation; the latter phenomenon is especially pronounced for melts of branched polyethylene (LDPE). Elongation hardening and shear thinning are closely connected with the deformation histories in simple elongational and simple shear flows. These flows are used for material characterization, and they are adequate for purely viscous and purely linear viscoelastic liquids. For elastic liquids with slowly fading memory, however, the first principal stretch rateλ̇1(t)turns out to be a better reference than the components of the strain rate tensor, andλ̇1(t)is very different for the two conventional test modes, viz. simple elongation and simple shear. For a more relevant comparison of the rheological behavior in elongation and shear, we therefore deformed a LDPE melt at 150°C such thatλ̇1(t)was the same in both test modes. The LDPE melt investigated showed hardening in elongation as well as in shear whenλ̇1increased exponentially with time, and the melt response was thinning in both elongation and shear whenλ̇1was kept constant, except for small total strains and strain rates at which the melt exhibited linear viscoelasticity.

ISSN:0148-6055    

DOI:10.1122/1.549937

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

We present an experimental study of the rheology of polystyrene latices phase separated by dissolved dextran polymer. Our phase‐transition experiments are guided by a statistical mechanical theory which predicts this phase transition and the densities of each phase. Measurements of the steady shear viscosity and the linear viscoelastic response at fixed polymer concentration well into the two‐phase region define the dependence on latex volume fraction. Viscoelastic properties of the separated dense phase near the phase boundary (at equilibrium) are also presented. A low frequency plateau inG′and an apparent yield stress appear for samples in the two‐phase region and increase exponentially with volume fraction of particles.

ISSN:0148-6055    

DOI:10.1122/1.549938

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

An explanation is presented for the isoclinic band‐spreading patterns associated with the measurement of flow birefringence in a two‐dimensional slit‐flow geometry. Band spreading is attributed to the positional dependence of the shear stress in the vicinity of the side wall viewing windows leading to a range of positions along the channel depth at which a given stress level may exist. Analyses are presented for the stress fields for Newtonian and power law flow which enable comparison to experimental data for the band spread. Results show that for a given rotation of the polars, the inside line of the resulting band represents the stress characteristic of the one‐dimensional centerline flow to be used for the evaluation of the isoclinic angle. Data are also presented for the stress‐optic coefficient of poly(vinylidene fluoride).

ISSN:0148-6055    

DOI:10.1122/1.549939

出版商:The Society of Rheology    

年代:1987

数据来源: AIP