摘要:

The viscosity of an epoxy-molding compound (EMC) has been characterized using a parallel-plate viscometer and a specially-developed slit rheometer. In particular, steady-state and dynamic viscosities at low temperatures and shear rates have been determined with a parallel-plate viscometer. Measurements with this instrument indicate that an extended Cox–Merz relation can be used to relate the dynamic viscosity to the steady shear viscosity for the given EMC. For measurements at high shear rates and high temperatures, a special slit rheometer has been built. In this viscometer, the slit is preceded by a disk-shaped reservoir where curing of the specimen takes place. The sample fills the thin reservoir fast and has its temperature raised by heat conduction from the hot wall. Because the reservoir is thin, the specimen quickly reaches the wall temperature with negligible cure and then cures under approximately isothermal conditions. The degree of cure of the sample is measured by quenching the specimen as it is extruded out of the slit and then performing differential-scanning-calorimeter measurements. By combining results from these two viscometers, the viscosity of the EMC has been obtained over the typical processing range of shear rates, temperature and degree of cure encountered during chip encapsulation. The measurement results indicate that the EMC viscosity exhibits a yield-stress behavior at low shear rates and a power-law behavior at high shear rates. The temperature dependence can be described by the WLF (Williams–Landel–Ferry) equation and the degree-of-cure dependence at low cure by an equation proposed by Macosko.

ISSN:0148-6055    

DOI:10.1122/1.550809

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The microstructure and scaling of mechanical properties of dense colloidal gels were investigated as a function of volume fraction and strength of interparticle attraction. Gels were reversibly formed by cooling suspensions of octadecyl silica particles in decalin or tetradecane. Shear history independence of mechanical properties was ensured by preshearing the suspensions in the gelled state. Gelation resulted in suspensions with apparent fractal dimensions of 1.4. Shear densification resulted in an apparent fractal dimension of 2.5 for structures containing many particles. When the gelled suspension was presheared, just after the shear rate was set to zero, elastic moduli were small. Over time, the moduli recovered to a time independent value,G∞′,at a rate α. When measured over a wide range of volume fraction, φ, and strength of interparticle attraction,G∞′,α, and the strain limiting the extent of linear response to oscillatory deformations,γM,fell onto master curves when plotted as a function ofφ/φGfor1.1<φ/φG<5. HereφGis the gel volume fraction that varied from 0.11 to 0.59 for temperatures studied.

ISSN:0148-6055    

DOI:10.1122/1.550812

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The so-calledmatrix modelis a general thermodynamic framework for microrheological modeling. This model has already been proven to be applicable for a wide class of systems, in particular to models formulated at the configuration tensor level of description. For models formulated at the configuration space level of description a matrix formulation is readily obtained, but for the subsequent analysis one still needs an explicit solution of the configuration space distribution functions. In the present paper we describe an approach in which this problem is solved by combining the matrix model with a Lagrangian simulation method in configuration space developed recently by Szeri and Leal. The result is a consistent and unified formulation of stress tensor expressions, including the stress averaging, and the evolution equations. This formulation is also suited for numerical simulations. In this way, the range of applicability of the matrix model is extended substantially. In order to clarify the principles of the method and some aspects of its implementation, a simple example is discussed in some detail.

ISSN:0148-6055    

DOI:10.1122/1.550811

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The development of velocity profile inside a filament was studied for spinning flows of LCPs using a hydrogen-bubble method. Both isotropic (33 wt%) and liquid crystalline (50 wt%) aqueous solutions of hydroxypropylcellulose were used as the test fluids. The velocity profile for the 33 wt% solution varies from a convex profile to a uniform one similar to that for Newtonian fluids. For the 50 wt% solution, however, an anomalous profile of the velocity, that is a concave profile, is found beyond a distance of one radius of the nozzle downstream from the exit. Since this concave profile persists far downstream from the nozzle exit, it may affect the molecular orientation distribution inside a fiber.

ISSN:0148-6055    

DOI:10.1122/1.550861

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical α-brass dies at the same wall shear stresses, indicating near-wall slip. The flow curves were independent of gap spacing. We show through the slip theory of Hill and co-workers [J. Rheol.34, 891–918 (1990)] that a measurable gap dependence of the flow curve is not a necessary consequence of wall slip; the flow curves for both stainless steel and α-brass dies can be fit with the same rheological parameters, with a difference in the work of adhesion accounting for the differences in the flow curves. X-ray photoelectron spectroscopy revealed differences in the chemistry of brass surfaces with different pretreating, corresponding to small differences in flow curves. Fluorocarbon-coated die surfaces showed no more slip than stainless steel, while the flow curve with gold-coated surfaces followed stainless steel at intermediate stress and brass at high stress.

ISSN:0148-6055    

DOI:10.1122/1.550877

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

A suspension (φ=0.2) of Teflon particles in silicone oil exhibited a negative electrorheological response with a dc electric field. This was in keeping with the fact that the conductivity of the host liquidσfwas considerably greater than that of the particlesσp,giving for the complex dielectric mismatch parameter β*=−1/2. Microscopy observations revealed that with application of a dc electric field the Teflon particles migrated to the positive electrode resulting in a structure which consisted of two layers or zones: (a) a relatively pure liquid and (b) a more concentrated suspension. The thickness of the liquid zone increased with field strength, which led to the decrease in the shear stress with field. A numerical analysis was developed for the electrical properties of the segregated suspension, which gave good agreement with measured values.

ISSN:0148-6055    

DOI:10.1122/1.550855

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We have constructed an apparatus which provides enhanced resolution in the measurement of the free surface profile and the axial force exiting the die during spinning of a liquid filament. In this paper we demonstrate how this information can be exploited to give quantitative information about rheological material properties in isothermal elongational flows. The fiber spinning experiments are coupled with mathematical models that serve as inverse problems to deduce material properties when the fiber profile and upstream axial force are experimentally known. We first develop integral and differential forms of the fiber spinning momentum balance which describe how stress varies down the length of the filament for an incompressible material, independent of rheology. These forms are then combined with experiments to deduce the evolution of the elongational viscosity along the spinline and verify the accuracy of free surface measurements. A fiber spinning model specialized to the Giesekus constitutive equation is then combined with spinline measurements to determine material constants within the Giesekus constitutive assumption. In particular, our technique is used to characterize the elongational rheology of a Boger test fluid. We obtain significantly different values for the relaxation time and mobility parameter than we obtain via shear rheometry; the solvent and polymer viscosities deduced from our spinline and shear experiments are essentially the same.

ISSN:0148-6055    

DOI:10.1122/1.550856

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The nonlinear rheology of an aqueous wormlike micellar system made of cetyltrime–thylammonium chloride with sodium salicylate is studied in this work. At high NaCl salt concentration the rheological measurements show a Maxwell behavior for all the solutions. The data of these samples are analyzed with respect to the Spenley, Cates, and McLeish model. The dimensionless quantities σplateau/G0andγ̇cτR, which characterize the nonlinear rheology are compared to the predictions of this model. At a fixed surfactant concentration these quantities are found to be insensitive to changes in salt concentration and in temperature, respectively. The experimental value of the ratio σplateau/G0we find is close at hand to the theoretical one. A discrepancy between the theory and the experiments occurs, however, in comparing the Weissenberg number defined by the productγ̇cτR.

ISSN:0148-6055    

DOI:10.1122/1.550878

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The rheological behavior of fluoroelastomer high-density polyethylene (HDPE) blends was investigated in detail. A capillary rheometer was used to determine the apparent viscosity of the blends as a function of time at various shear rates. The time to attain the steady state and the normalized steady-state apparent viscosity of the blends depend on shear rate and the fluoroelastomer concentration. The reduction in the apparent viscosity is caused by adhesive failure at the interface between the HDPE melt and the fluoroelastomer layer which was formed on the die wall surface during the extrusion of the blends. The presence of the fluoroelastomer layer on the die wall surface was confirmed by x-ray photoelectron spectroscopy and rheological measurements. Based on our previous work and this work, we have discovered that there are at least two different mechanisms—adhesive failure at the interface between the extrudate and the lubricating layer or cohesive failure in the lubricant layer—for viscosity reduction in polymer blends.

ISSN:0148-6055    

DOI:10.1122/1.550805

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

A new model of the electrorheological (ER) effect for immiscible polymer blends is proposed on the basis of microscopic observations. In the absence of an electric field the polymer with high viscosity is dispersed as droplets in the other polymer with low viscosity, while in the presence of an electric field, the droplets stretch and coalesce to form bridges between electrodes, resulting in the increase of the macroscopic viscosity. A few experimental results are given to confirm the validity of this model.

ISSN:0148-6055    

DOI:10.1122/1.550869

出版商:The Society of Rheology    

年代:1997

数据来源: AIP