摘要:

Constitutive equations for an incompressible elastic fluid are specialized for small elastic deformations. These equations are consistent with the dissipation inequality only if the elastic strain tensor is determined by a corotational evolution equation. An appealing special case of this theory is similar to the corotational Jeffreys model. This modified Jeffreys model predicts normal stresses in shear and significant extensional viscosities in uniaxial elongation. These predictions describe several dilute polymer solutions over a wide range of flow conditions. Limits are suggested on applying the proposed constitutive equations to more elastic solutions.

ISSN:0148-6055    

DOI:10.1122/1.550081

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

Rheological and rheo‐optical properties are studied for homogeneous blends of the rodlike poly(1,4‐phenylene‐2,6‐benzobisthiazole), PBT, and poly(hexamethylene adipamide), nylon‐66. The linear recoverable creep compliance shows that the distribution of retardation times is similar for all the blends studied, though shifted on the time scale by large variation in the viscosity among the blends. A single‐integral (BKZ‐type) constitutive equation is found adequate to represent the dependence on shear rate of the nonlinear steady‐state viscosity, the recoverable compliance, and the flow birefringence (save for certain anomalous behavior at low shear rate in the latter case). Effects on the viscosity are discussed in terms of a simplified model for behavior when a characteristic rheological time constant is longer for the rodlike (PBT) than for the flexible chain (nylon‐66) component.

ISSN:0148-6055    

DOI:10.1122/1.550108

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

This paper develops a rheological model for flow of polymer compounds highly filled by small interacting (attractive) particles. The properties of the particle phase in the compound are described by introducing a viscoelastic ‘‘particle mode’’ with an additional kinetic equation which describes the processes of rupture and restoration of flocs of particles. The elastic deformations in the particle mode are assumed to be small enough to consider the geometrical nonlinearities as second‐order effects. The rheological behavior of the polymer matrix is modeled by means of nonlinear viscoelasticity, as in the case of pure polymer melts. It was shown that all the basic features of the yield‐like and thixotropic behavior of the filled polymer compounds, including the appearance of yield values in steady state flows, stress overshoots in start up flows and effects of ‘‘frozen memory’’ during relaxation could be described by this approach without using the mathematical yield criteria. Some preliminary comparisons of the theory with steady state data are also demonstrated.

ISSN:0148-6055    

DOI:10.1122/1.550109

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The Bird–Carreau model was used to predict the steady viscosity η, the primary normal stress coefficient ψ1, and the small amplitude oscillatory properties η’and η‘/ω for a hard wheat flour dough containing 40% total moisture in the region of frequencies of 0.01 to 100 rad/s for the dynamic viscoelastic properties and a region of 10−5through 103s−1for steady shear properties. The model accurately represented the experimental data in the shear rate/frequency region tested.

ISSN:0148-6055    

DOI:10.1122/1.550110

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

What is the simplest adequate model of shear thinning and extension thickening of polymer solutions at the intense local deformation rates in coating and related flows? We examine a combination of the Carreau equation of shear sensitivity with an analogous equation of extension sensitivity. The two contributions are mediated by the rate at which the principal straining axes turn with respect to the liquid itself, a relative rotation rate that is frame independent, vanishes in purely extensional flow, and harks back to Lumley’s (1969) idea of persistent molecular strain rate, which was adumbrated by Takserman‐Krozer (1963) and named by Frank and Mackley (1976). We indicate how the new equation can be used to extract viscosity‐in‐extension from the extension‐rich mixed flow between opposed nozzles. We illustrate how the new equation can be applied to the complex mixed flow in slide coating.

ISSN:0148-6055    

DOI:10.1122/1.550075

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The development of new experimental methods in recent years has made possible the study of the response of melts and concentrated polymer solutions to transient deformations in which markedly nonlinear behavior is exhibited. To provide a basis for the interpretation of nonlinear data it is a common practice to compare nonlinear data with the response predicted by the linear theory of viscoelasticity for the same deformation. Such a comparison has led to the use of the term ‘‘strain hardening’’ to describe a positive deviation from linear behavior in the response of some polymeric liquids to the start‐up of tensile extension and to exponential shear. Likewise, ‘‘strain softening’’ is defined as a negative departure from the linear prediction. In both cases, it is demonstrated that the usual methods used to calculate the linear behavior are inappropriate. When valid reference cases are used for comparison, all available data indicate that polymeric liquids are strain softening rather than strain hardening.

ISSN:0148-6055    

DOI:10.1122/1.550077

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The viscoelastic and thixotropic properties of 30% polystyrene latex with Na–caseinate suspensions were investigated. Oscillatory measurements showed that dynamic rigidities of the suspensions were slightly affected by the addition of Na–caseinate within the range of 1.4% to 5.37%CN(ratio of the weight of added Na–caseinate to the weight of polystyrene latex). The addition of Na–caseinate changed the rheology of the latex suspension from nonthixotropic to thixotropic. The greatest degree of thixotropy was found at theCNof 2.65% which was between the maximum degree of flocculation and the saturation plateau of Na–caseinate adsorption of the latex (CNabout 5%). Empirical equations for constant structure flow curves of the different suspensions were also obtained.

ISSN:0148-6055    

DOI:10.1122/1.550078

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

Since most polymer processing operations involve inducing flow at the same time that the material is being heated or cooled (so‐called ‘‘nonisothermal rheology’’), there is a strong need for systematic studies to develop nonisothermal constitutive equations. In prior work we pulled small (fiber‐like) samples in the rubbery state at the same time that we cooled them (up to 4.5 °C/s). In the present work constrained (clamped) samples were cooled rapidly from about 20 °C aboveTgto 20 °C below it, and the stress build‐up measured. Accompanying independent studies (to establish the relaxation spectrum) were done in a cone and plate viscometer in the rubbery state and in a Rheovibron in the transition and glassy states. As in the earlier (pulling) work simple time–temperature superposition of thetimeconstantsdoes not work: the measured stresses are higher than predicted at high cooling rates. Several efforts based on multiple or experimentally adjusted shift factors were tried but seemed implausible. We conclude finally that themodulithemselves must depend on thermal history. By using a plausible, but finally empirical, adjustment of the moduli as a function of cooling rate, both sets of experiments (the pulling data at high temperatures and the constrained stress data at lower temperatures) were brought together in a single framework.

ISSN:0148-6055    

DOI:10.1122/1.550079

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

A commercial engineering polymer of polyetherimide (ULTEM), a liquid crystal copolyester (TLCP), and blends with different percentages of TLCP in ULTEM have been rheologically analyzed in this work. Rheological measurements have been performed both at low and high shear rates. In particular, by means of a cone and plate rheometer, the transient shear flow behavior has been studied. The time necessary to reach the steady stress value was much longer in the case of the TLCP than for the amorphous ULTEM. Steady state rate sweeps as well as dynamic sweeps have been also carried out and discussed. Moreover, data obtained at high shear rates revealed that ULTEM viscosity is possibly pressure dependent. A procedure is shown for correlating the results obtained with the different techniques. The possibility of improving the processability of ULTEM has been also studied. In this regard the effect of the addition of low percentages of TLCP on the rheological properties of the thermoplastic matrix has been analyzed.

ISSN:0148-6055    

DOI:10.1122/1.550080

出版商:The Society of Rheology    

年代:1990

数据来源: AIP