摘要:

In an effort to quantitatively examine the effect of coupling between multiple relaxation modes, a new model involving two coupled Maxwell modes is developed as a generalization of the upper‐convected Maxwell and the Giesekus models. The model contains, in addition to the parameters inherent to a Maxwell model with two uncoupled modes (i.e., λ1,λ2and η1≡G1λ1, η2≡G2λ2), a dimensionless coupling coefficient θ that multiplies a quadratic coupling term. In the two characteristic limits θ=0 or (η1,λ1)=(η2,λ2), the Maxwell model with two uncoupled relaxation modes or the Giesekus constitutive model is obtained, respectively. The rheological behavior of the model is investigated in the linear and nonlinear deformation‐rate regimes. Calculation of the linear viscoelastic behavior shows that the linear stress relaxation modulus is the sum of two decaying exponentials with characteristic times and preexponential factors that are quite different from λ1, λ2andG1,G2, respectively. In slow, slowly varying flows, the zero shear‐rate ratio Ψ02/Ψ01assumes small negative values when θ takes on small positive values. The nonlinear rheological behavior of the model is examined under the imposition of shear and extensional flow fields, from both a steady‐state and transient perspective. The qualitative behavior observed is remarkably rich in describing the experimental trends seen in polymer melts and Boger fluids for a constant value of θ≊0.1.

ISSN:0148-6055    

DOI:10.1122/1.550768

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

In contrast to the usual electrorheological behavior manifesting in an increase in viscosity of suspensions of various materials in electrical field, a negative electrorheological effect, i.e., a decrease in viscosity of suspensions of magnesium hydroxide particles in silicone oil was observed.

ISSN:0148-6055    

DOI:10.1122/1.550769

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The shear stress and the first normal stress difference in steady and transient shear flows of the commercial thermotropic liquid crystalline polymer (LCP) Vectra B950 are measured at 300 °C. The viscosity curve shows the typical three region flow curve of LCPs. The first normal stress difference is everywhere positive and its steady‐state value increases linearly with the shear rate throughout regions I and II. The growth curves of the shear and the normal stresses for shear rates in region II show slow oscillations that scale with strain. The stress transients during flow reversal and stepwise increase or decrease of the shear rate also scale with strain. The damping of the transients is slower than what is found in other thermotropic LCPs. The oscillations ofN1after reversal and after stepwise increase of the shear rate do not have a clear phase difference. The relaxation of the stresses after cessation of flow shows a fast initial part and a long strain scaling tail. The elastic texture contribution to the total stress is lower than in other semiflexible systems. Either the viscous/elastic molecular contribution or an additional viscous contribution of the defects seems to be responsible for this low value.

ISSN:0148-6055    

DOI:10.1122/1.550788

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

In this work, the rheology of PBG/m‐cresol systems is reevaluated with the specific goal of understanding the influence of concentration on low shear‐rate rheology. Comparisons of steady‐state and transient rheology and the behavior after cessation of shear allow for the separation of the contributions to the stress from the nematic fluid and defect texture. Stress relaxations demonstrate that the textural contribution to the stress increases nonlinearly from 30% to 55% as the concentration increases from 12 to 37 wt %. Recoil shows a similar nonlinear trend in the magnitude of the ultimate recovered strain γ∞. Stress relaxations and recoil, once normalized, have universal dynamics dependent only on γ̇ot and independent of concentration. Two different regions of concentration are identified and correlated to a distinct change in the nature and behavior of the textural contribution to the stress.

ISSN:0148-6055    

DOI:10.1122/1.550797

出版商:The Society of Rheology    

年代:1996

数据来源: AIP