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Dec 1986

Volume 30, Issue 6, pp. 1085-1186


The Rheological Behavior of HDPE∕LDPE Blends. V. Isothermal Elongation at Constant Stretching Rate

A. Valenza, F. P. La Mantia, and D. Acierno

J. Rheol. 30, 1085 (1986); http://dx.doi.org/10.1122/1.549881 (8 pages)

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Elongational viscosity data taken on high density‐low density polyethylene blends are presented. The low density polyethylene shows the typical strain hardening while the high density polyethylene shows a Troutonian behavior but at high stretching rate. The blends show an intermediate behavior strongly dependent on the composition and stretching rate.
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83.80.Tc Polymer blends
83.50.Jf Extensional flow and combined shear and extension

Are Polymer Melts Visco‐Anelastic?

R. G. Larson and V. A. Valesano

J. Rheol. 30, 1093 (1986); http://dx.doi.org/10.1122/1.549882 (16 pages)

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We raise the question: should polymer melts be described in the usual way as materials that dissipate stress and elastic energy only by gradual relaxation, or are they better described as able to dissipate some stress and elastic energy virtually instantaneously? That is, can they be better described as having relaxing anelasticity? We show that an anelastic description has molecular underpinnings in the tube theory of Doi, has precedent in the empirical constitutive model of Wagner, and has experimental support, both in previous literature on strain recovery, and in data presented here on reversing double‐step shear straining of a linear low density polyethylene.
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83.10.Gr Constitutive relations
83.80.Rs Polymer solutions
83.80.Sg Polymer melts

A Method of Determination of Some Rheological Characteristics of Viscoelastic System

P. Randria and D. Bellet

J. Rheol. 30, 1109 (1986); http://dx.doi.org/10.1122/1.549883 (13 pages)

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The study concerns the unsteady flow of viscoelastic fluids in rigid cylindrical tubes, where one of the ends is either open or submerged in a viscoelastic sink. This shows the possibilities offered by analysis of axial speed or flow rate measurements and by pressure gradient measurements. In both cases, the model serves as a “rheometer” and allows the properties of the fluid to be determined automatically; in the second case, knowledge of the behavior of the fluid and use of axial speed analysis enables rheological modeling of the sink to be effected. This possibility allows “remote” investigations of complex mechanical systems to be done, and leads to numerous applications in various areas.
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47.11.-j Computational methods in fluid dynamics

Heterogeneous Behavior after Yielding of Solid Suspensions

Yoichi Nagase and Kenji Okada

J. Rheol. 30, 1123 (1986); http://dx.doi.org/10.1122/1.549907 (20 pages) | Cited 2 times

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A method of visualizing the temporal changes in the deformation of suspensions simultaneously with rheological measurements was developed. The observed heterogeneity can only be identified by visual means. The diverse results which can normally be obtained only by stress measurements can be explained with the aid of visualization results, thus remarkably contributing to the understandings of the difficult rheological mechanism involved. The following major findings were obtained. It was reconfirmed that suspensions are statistically solid bodies. An initial yielding is difficult to detect unless by visual means. The characteristic properties of the solid body are either a limiting strain within which the suspension behaves as an elastic body (for Na‐bentonite, for example) or a static yield (for kaolin in water, etc.). The former could be recognized to have some binding force between particle and liquid. After yielding, the stress continues to increase, but at slow rotation velocity, deformation begins to converge into local streaming. Thus, the strain and strain rate within the sample are considerably larger than those calculated. By employing the real strain, it was confirmed that the stress increase is attributable to rheopexical hardening, being more important than thixotropic stress reduction. Confirmation was also made that the static yield stress on the elastic limit coincides well with the stress on extrapolation to zero shear rate in the stress‐shear rate relationship.
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83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.80.Iz Emulsions and foams
83.85.Cg Rheological measurements—rheometry
83.10.Gr Constitutive relations

Numerical Comparison of Empirical Rules for Prediction of Nonlinear Rheology from Linear Viscoelasticity

Kurt F. Wissbrun

J. Rheol. 30, 1143 (1986); http://dx.doi.org/10.1122/1.549884 (22 pages) | Cited 1 time

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Three empirical rules for prediction of the shear rate dependence of viscosity and normal stress from linear viscoelastic response are compared with the dependences for a fluid described by the Wagner model. The linear viscoelastic relaxation spectrum used gives a reasonable form for the functional dependence of the viscosity upon shear rate. Further, the integral required for the Bersted approximation can be obtained analytically for this spectrum. The result is a viscosity‐shear rate relation of a form proposed empirically by Bueche and Harding, and which is numerically equivalent to the predictions of the Graessley entanglement model. Over a wide range of power‐law slopes and of shear rates the agreement of the Wagner model and of the three empirical rules is excellent. The reason for the agreement is that for all these rules the rheological functions are computed as integrals over a broad, shallow sloping relaxation spectrum which is truncated by a relatively steep cutoff function. The integrals are therefore not very sensitive to the detailed form of the truncation function.
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83.85.Ns Data analysis (interconversion of data computation of relaxation and retardation spectra; time-temperature superposition, etc.)
83.50.-v Deformation and flow
83.80.Rs Polymer solutions
83.80.Sg Polymer melts

New Representation of the True Stress for Uniaxial Extension of Crosslinked Rubbers

Yong‐Hua Zang, René Muller, and Daniel Froelich

J. Rheol. 30, 1165 (1986); http://dx.doi.org/10.1122/1.549885 (16 pages)

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The elasticity of different natural rubbers and SBR rubbers has been studied in simple extension at constant strain rate. The true stress has been plotted as a function of λ2−λ−1 as suggested by the molecular theory. A series of straight lines which do not pass through the origin have been obtained, and consequently two parameters σ0 and G have been introduced. These results allow one to hypothesize that the tensile stress is the sum of two terms: a Gaussian entropic contribution and a further contribution reaching rapidly a steady state value σ0. The σ0 term is found to depend only on the chemical nature of the rubber and may be ascribed to local interaction of the segments of the network, whereas the value of G depends on the degree of crosslinking and seems to represent the rubbery modulus of the kinetic theory. A number of experimental data for various natural rubbers and PDMS networks from the literature have been replotted in the same way in order to check the validity of our representation, and a satisfactory agreement has been obtained.
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83.80.Va Elastomeric polymers
83.10.Kn Reptation and tube theories
83.10.Mj Molecular dynamics, Brownian dynamics

Note: On the Formulation of Highly Elastic‐Constant Viscosity Liquids

R. K. Gupta, M. E. Ryan, and T. Sridhar

J. Rheol. 30, 1181 (1986); http://dx.doi.org/10.1122/1.549913 (6 pages) | Cited 1 time

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Abstract Unavailable
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83.60.Bc Linear viscoelasticity
83.80.Rs Polymer solutions
83.80.Sg Polymer melts
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