Contents

On the determination of yield surfaces in Herschel-Bulkley fluids

Gilmer R. Burgos, Andreas N. Alexandrou, and Vladimir Entov

Flow development of Herschel-Bulkley fluids in a sudden 3-D square expansion

Gilmer R. Burgos and Andreas N. Alexandrou

The yield stress of concentrated flocculated suspensions of size distributed particles

Zhongwu Zhou, Michael J. Solomon, Peter J. Scales, and David V. Boger

Steady shear and dynamic rheological properties of xanthan gum solutions in viscous solvents

M. A. Zirnsak, D. V. Boger, and V. Tirtaatmadja

A new technique allowing the quantification of diffusion at polymer/polymer interfaces using rheological analysis: Theoretical and experimental results

Hua Qiu and M. Bousmina

Tumbling dynamics in a nematic surfactant solution in transient shear flows

Franklin E. Caputo and Wesley R. Burghardt, and Jean-François Berret

Calculation of the persistence length of solubilized starches from intrinsic viscosity measurements

C. J. Carriere and E. B. Bagley

Bulk crosslinking of diallyl phthalate monomers

C. Heydel, P. Cassagnau, and A. Michel

Modified hybrid closure approximation for prediction of flow-induced fiber orientation

Kyeong-Hee Han and Yong-Taek Im

Piston-driven flow of highly concentrated suspensions

Ralf B. Lukner and Roger T. Bonnecaze

A novel processing aid for polymer extrusion: Rheology and processing of polyethylene and hyperbranched polymer blends

Ye Hong, J. J. Cooper-White, M. E. Mackay, C. J. Hawker, E. Malmström, and N. Rehnberg

The rigid-rod model for nematic polymers: An analysis of the shear flow problem

V. Faraoni, M. Grosso, S. Crescitelli, and P. L. Maffettone

Transient flow experiments in a model immiscible polymer blend

Mario Minale, Paula Moldenaers, and Jan Mewis

Segment connectivity, chain-length breathing, segmental stretch, and constraint release in reptation models: Shear flows

Chi C. Hua, Jay D. Schieber, and David C. Venerus

The rheology and microstructure of concentrated, aggregated colloids

L. E. Silbert, J. R. Melrose, and R. C. Ball

Linear response of regular asphalts to external harmonic fields

J. Stastna and L. Zanzotto

Capillary thinning of polymeric filaments

Mette Irene Kolte and Peter Szabo

Stress and birefringence measurements during the uniaxial elongation of polystyrene melts

D. C. Venerus, S.-H. Zhu and H. C. Öttinger

The role of LCP rheology on the evolving morphology of immiscible blends containing LCPs

William A. Kernick and Norman J. Wagner

Rotational magnetic particle microrheometry: The Newtonian case

George J. Besseris, Irving F. Miller, and Donovan B. Yeates

On the determination of yield surfaces in Herschel-Bulkley fluids

Gilmer R. Burgos and Andreas N. Alexandrou
Semisolid Metal Processing Laboratory
Metal Processing Institute, WPI, Worcester MA 01609 USA

Vladimir Entov
Institute for Problems in Mechanics of Russian Academy of Science
pr. Vernadskogo, 101, 117526, Moscow, Russia

Abstract

Herschel-Bulkley fluids are materials that behave as rigid solids when the local stress t is lower than a finite yield stress t₀, and flow as nonlinearly-viscous fluids for t > t₀. The flow domain then is characterized by two distinct areas, t < t₀ and t > t₀. The surface t = t₀ is known as the yield surface. In this paper, by using analytic solutions for antiplane shear flow in a wedge between two rigid walls, we discuss the ability of regularized Herschel-Bulkley models such as the Papanastasiou, the bi-viscosity and the Bercovier and Engelman models in determining the topography of the yield surface. Results are shown for different flow parameters and compared to the exact solutions. It is concluded that regularized models with a proper choice of the regularizing parameters can be used to both predict the bulk flow and describe the unyielded zones. The Papanastasiou model predicts well the yield surface, while both the Papanastasiou and the bi-viscosity models predict well the stress field away from t = t₀. The Bercovier and Engelman model is equivalent to the Papanastasiou model provided a proper choice of the regularization parameter d. It is also demonstrated that in some cases the yield surface can be effectively recovered using an extrapolation procedure based upon an analytical representation of the solution.

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Flow development of Herschel-Bulkley fluids in a sudden 3-D square expansion

Gilmer R. Burgos and Andreas N. Alexandrou*
Semisolid Metal Processing Laboratory
Metal Processing Institute, WPI, Worcester MA 01609 USA

*Corresponding author

Abstract

The flow development of Herschel-Bulkley fluids in a sudden three-dimensional square expansion is studied numerically. The flow is modeled using the Mixed-Galerkin finite element formulation to solve the conservation of mass and momentum equations. The Herschel-Bulkley material behavior is described using a regularized model based on the Papanastasiou model. Solutions are obtained for a downstream-to-upstream expansion ratio of 2:1, and for wide range of pressure gradient values and rheological parameters. The results show that during the evolution of the flow, two core regions and dead zones at the corners are formed. The extent of the core regions decreases with the pressure gradient and the Reynolds number, and increases with the power-law index. It is also found that the volume flow rate at steady flow increases with the pressure gradient, power-law index, and Reynolds number.

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The yield stress of concentrated flocculated suspensions of size distributed particles

Zhongwu Zhou, Michael J. Solomon, Peter J. Scales, and David V. Boger*
Advanced Mineral Products Research Centre
Department of Chemical Engineering
University of Melbourne, Parkville, Vic. 3052, Australia

*Corresponding Author
E-mail: d.boger@chemeng.unimelb.edu.au

Abstract

An investigation of shear yield stress is made on well-characterised alumina suspensions of different distributed particle sizes at the vicinity of the particle isoelectric point (IEP) across a wide range of volume fractions. Experimental results are compared with recently developed models (Kapur et al., 1997; Scales et al., 1998) and structural effects on the yield stress are examined. The models predict the magnitude order of the yield stress below a volume fraction of approximate 0.42, suggesting that inter-particle forces play a dominant role in determining the network strength in this concentration region. Deviations between experimental results and theoretical predictions are explained in terms of structural effects being controlled by a competition between weak particle-particle linkages and geometric resistance on the network strength. At higher volume fraction, the effect of geometric resistance to the deformation of suspensions becomes more pronounced. A number of models for the yield stress of size distributed suspensions are then proposed. Results suggest that the effect of polydispersity of particles on the yield stress of suspensions can be well characterised by a surface area average diameter and the broad size distributed suspension exhibits a higher yield stress than the narrow size distributed suspension of same volume average diameter.

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Steady shear and dynamic rheological properties of xanthan gum solutions in viscous solvents

M. A. Zirnsak,* D. V. Boger,** and V. Tirtaatmadja
Department of Chemical Engineering
University of Melbourne, Parkville, Vic. 3052, Australia

*Current address: Comalco Research and Technical Support
P.O. Box 316, Thomastown 3174, Australia

**Corresponding Author
E-mail: d.boger@chemeng.unimelb.edu.au

Abstract

An extensive examination of the first normal stress difference and linear viscoelastic properties of xanthan gum solutions has been conducted in relation to molecular theories in the literature. The first normal stress difference, storage modulus and loss modulus are reported for 0.01 to 0.04% w/w solutions of xanthan gum in high viscosity (wheat syrup and water) solvents. The average length of the xanthan molecules used was determined by light scattering to be (1.25±0.05)×10³ nm. The storage and loss moduli obtained show a frequency dependence consistent with theories that included a relaxation time spectrum, while the first normal stress difference exhibits dependence on shear rate consistent with theories of suspensions of rigid particles. Both the first normal stress difference and the solute contribution to the storage modulus were found to vary linearly with concentration and with the solvent viscosity to the power of 2/3. Extensional viscosity measurements of a xanthan gum solution are in good agreement with the predictions for semi-dilute suspensions of rigid rods.

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A new technique allowing the quantification of diffusion at polymer/polymer interfaces using rheological analysis:
Theoretical and experimental results

Hua Qiu and M. Bousmina*
Department of Chemical Engineering, CERSIM
Laval University, Sainte-Foy, Québec (QC), G1K 7P4 Canada

* Corresponding Author
E-mail: bousmina@gch.ulaval.ca

Abstract

A new technique allowing the quantification of diffusion at polymer/polymer interfaces and also measurement of self-diffusion coefficient of polymer melts using rheological tools has been developed. The technique consists of measuring the dynamic moduli as a function of time for a multilayer sandwich-like assembly at temperature slightly above T_g. The technique was tested on polystyrene/polystyrene (PS/PS) system sheared in oscillatory mode under small amplitudes of deformation for different times of welding. The experimental results showed that the dynamic complex shear modulus increased with the elapsed time of welding. This increase was attributed to the diffusion of polymer chains at the superimposed interface layers. Based on the reptation theory, an analytical expression for the self-diffusion coefficient as a function of polymer rheological material functions was derived. Graessley's expression for bulk self-diffusion coefficient is recovered as a special case. The obtained self-diffusion coefficient for polystyrene was found to be in good agreement with the literature data reporting the self-diffusion coefficient of PS measured by classical techniques.

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Tumbling dynamics in a nematic surfactant solution
in transient shear flows

Franklin E. Caputo and Wesley R. Burghardt*
Department of Chemical Engineering
Northwestern University, Evanston, lL 60208 USA

Jean-François Berret
CC026 / Groupe de Dynamique des Phases Condesées
Université de Montpellier II, F-34095 Montpellier cedex 05, France

*Corresponding Author:
E-mail: w-burghardt@nwu.edu
Phone: 847-467-1401; Fax: 847-491-3728

Abstract

We report rheological and structural studies of a nematic surfactant solution (CPCl/Hex) in transient shear flows. Upon step changes in steady shear flow conditions, the shear stress exhibits damped oscillations that scale with shear strain, attributed to director tumbling under shear. In situ SAXS is used to measure changes in average micellar orientation during similar transient shear protocols; the measured orientation parameter also exhibits damped oscillations reflecting the underlying tumbling dynamics. Stress and structure data are compared with predictions of the Larson-Doi tumbling polydomain model. The model predictions are qualitatively similar to the experimental observations. In particular, orientation is observed to initially decrease upon shear flow reversal in CPCl/Hex, in agreement with the Larson-Doi predictions but in contrast to previous results for model lyotropic solutions of poly(benzyl glutamate). Shear stress and average orientation are closely related during the oscillatory response to transient flows. The data suggest that higher orientation in the flow direction leads to low instantaneous stresses, and vice versa. The Larson-Doi model does not predict this relationship.

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Calculation of the persistence length of solubilized starches from intrinsic viscosity measurements*

Abstract

The persistence lengths for two different commercial starches were estimated from intrinsic viscosity measurements. The intrinsic viscosities of the starches were measured at 25°C using 90/10 DMSO/water (weight/weight) as the solvent. The calculations were conducted using the equations developed by Yamakawa and Yoshizaki and corrected for polydispersity using the Schultz-Flory distribution. Values for the persistence lengths were 8.1 ± 0.6, and 6.6 ± 0.05 nm for a high-amylose potato, and a high-amylose maize starch, respectively. The values for the persistence length obtained for the two starch samples reflect the stiffness of the amylose chains in DMSO, which is due to the helical conformation of amylose in the solvent.

*Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable.

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Bulk crosslinking of diallyl phthalate monomers

C. Heydel, P. Cassagnau, A. Michel
Laboratoire des Matériaux Plastiques et Biomatériaux, UMR5627
Bâtiment 303, Université Claude Bernard, Lyon 1
43 Bvd du 11 Novembre 1918, 69622 Villeurbanne Cédex, France

Abstract

The radical cross-linking of ortho diallyl phthalate was studied with dynamic mechanical analysis and infra-red spectroscopy, the combination of which provides an in situ method for the elucidation of the effects of the reaction temperature. Studies of the side reactions (degradative transfer) showed that the reaction temperature must be lower than 170°C to obtain the optimal mechanical properties when dicumyl peroxide is used as a initiator. Above 170°C, at the end of polymerization, the extent of reaction increases with the reaction temperature whereas the cross-linking density decreases. These phenomena can be explained by an intramolecular cyclization reaction which consumes the allyl bond without formation of a cross-linking point. Furthermore, the gel point behavior was well characterized from the divergence of zero shear rate viscosity and the power law in dynamic moduli. The value of the relaxation exponent (D=0.75) was found to be close to the theoretical percolation t value. However, the exponent increases slightly with the reaction temperature, suggesting a more open network structure. This phenomenon was also explained by an intramolecular reaction that evolves as a function of temperature.

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Modified hybrid closure approximation for prediction
of flow-induced fiber orientation

Kyeong-Hee Han and Yong-Taek Im*
Computer Aided Materials Processing Laboratory
Department of Mechanical Engineering ME3227
Korea Advanced Institute of Science and Technology
373-1 Kusong-dong, Yusong-gu, Taejon 305-701, South Korea

*Corresponding Author

Abstract

Distribution of fiber orientation in flow molding processes with short fiber reinforcements is of great importance because it affects mechanical properties of molded parts. Due to trade-off between the computational efficiency and accuracy a second-order orientation tensor has been widely used to describe fiber orientation distribution. For calculation of this fiber orientation tensor, a closure approximation has been introduced to reduce a higher fourth-order orientation tensor to a lower second-order. In the present investigation, a hybrid closure approximation has been modified. Two parametric forms of distribution function which accurately describe random-in-space, random-in-plane, and uniaxial distributions of fiber orientation were linearly interpolated. The interpolating factor was obtained as a function of the fiber interaction coefficient by fitting distribution function calculations. Test simulation in homogeneous flow and non-homogeneous flow fields, respectively, showed that proposed closure approximation gives good performance for a wide range of C₁ values without showing non-physical behavior.

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Piston-driven flow of highly concentrated suspensions

Ralf B. Lukner and Roger T. Bonnecaze*
Department of Chemical Engineering
The University of Texas at Austin
Austin, Texas 78712-1062 USA

*Corresponding Author

Abstract

The piston-driven flow of highly concentrated suspensions (55% or 59% by volume solids) of dense spheres was investigated as a function of piston speed, liquid viscosity, particle material and particle size. The drag of the suspension was found to be independent of piston speed for 0.5 and 1.4 mm glass or PMMA spheres in liquids with viscosities from 1 to 641 cP. For suspensions prepared from 55,000 cP liquids, the drag was piston-speed dependent. In the speed-independent range, the drag decreased with increasing liquid viscosity and was significantly larger than that predicted by existing models for suspensions. In the speed-dependent range, the drag increased with viscosity. A macroscopic model based on lubricated frictional contacts between the particles and the wall was successfully applied to correlate the shear-rate independent data for suspensions prepared from liquids with viscosities from 1 to 641 cP. It was possible to estimate the drag of the suspensions prepared from 1.4 mm spheres by using tabulated friction coefficients and the macroscopic model. Results from flow visualization confirmed the presence of significant wall slip and the presence of fountain flow.

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A novel processing aid for polymer extrusion: Rheology and processing of polyethylene and hyperbranched polymer blends

Ye Hong, J. J. Cooper-White, M. E. Mackay*
Materials Characterisation and Processing Centre
Department of Chemical Engineering
The University of Queensland, Brisbane, QLD 4072, Australia

C. J. Hawker
IBM Almaden Research Laboratories
San Jose, CA, USA

E. Malmström
Department of Polymer Technology
Royal Institute of Technology
S-100 44 Stockholm, Sweden

N. Rehnberg
Perstorp AB, Perstorp, Sweden

*Corresponding Author.
Present Address:Department of Chemical, Biochemical and Materials Engineering
Stevens Institute of Technology, Hoboken, NJ 07030 USA
E-mail: mmackay@stevens-tech.edu

Abstract

The use of hyperbranched polymers (HBPs) as a processing aid for linear low density polyethylene (LLDPE) was investigated. Various generation (or pseudogeneration) HBPs were used which had either 16 carbon atom alkanes or a mixture of 20/22 carbon atom alkanes on the end groups. In addition, the degree of end group substitution was studied. Blends of up to 10% HBP content were mixed via extrusion at 170°C to produce 1 mm diameter fibres. Processability, surface appearance and the rheological properties of the blends were evaluated. It was found the power requirement for extrusion was significantly decreased as a result of reduced blend viscosity, and also, the mass flow rate for a given extruder speed was greater than virgin LLDPE for all HBP blends. Melt fracture and sharkskin of the blends was successfully eliminated, and minimal preprocessing time was required for the effect to take place. Surface analysis using XPS and TEM techniques were performed with both showing that the HBP had a preference to accumulate at the fibre surface. Rheological experiments were similarly affected, therefore, the blend viscosity is really a composite of a HBP rich phase and a neat LLDPE phase. It is hypothesized that the HBP rich phase acted as a lubricating layer at the polymer/die wall interface. The HBP with a greater degree of end group substitution acted better as a processing/rheological property aid. Blends of LLDPE and paraffin wax were also studied. The surface appearance of HBPs/LLDPE blends was superior to those blends mixed with paraffin wax, as was the extruder performance. The results suggest that HBPs, at trace levels (» 500 ppm), may offer a number of advantages when used as a processing aid for LLDPE.

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The rigid-rod model for nematic polymers:
An analysis of the shear flow problem

V. Faraoni, M. Grosso, S. Crescitelli
Dipartimento di Ingegneria Chimica Università Federico II
Piazzale Tecchio I -80125 Napoli, Italy

P. L. Maffettone*
Dipartimento di Scienza dei Materiali ed Ingegneria Chimica
Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129, Torino, Italy

*Corresponding Author
E-mail: maffetto@athena.polito.it

Abstract

The rigid-rod model is capable of predicting several rheological features of rodlike polymers in the nematic phase. The model is formulated in terms of a nonlinear partial differential equation that describes the evolution of an orientational distribution function. The morphological properties and the rheological response of the sample can be determined once the distribution function is known. In this paper the rigid-rod model is thoroughly analyzed with tools typical of bifurcation analysis for the case of shear flows. New flow regimes, both stationary and periodic, are found and illustrated. The detailed description of the model bifurcation structure allows some considerations about up to date closure approximations.

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Transient flow experiments in a model immiscible polymer blend

Mario Minale*, Paula Moldenaers, and Jan Mewis
Katholieke Universiteit Leuven, Department of Chemical Engineering
de Croylaan 46, B 3001 Heverlee, Leuven, Belgium

*Present address: Seconda Università di Napoli
Department of Aerospace Engineering
E-mail: mario.minale@unina2.it

Abstract

The rheological and morphological changes that are induced in emulsions or simple polymer blends by simple flow histories are relatively well understood. Here these results are extended to more complex flow histories, which not only approach more closely real processing conditions but also provide more critical tests in model assessment. For this purpose a semi-concentrated, incompatible, model blend is subjected to a stepwise increase in shear rate followed by a flow reversal. The blend morphology consists of droplets immersed in a matrix. Two different types of behavior are identified, depending on the instant at which the flow is reversed. When this occurs after the droplets are broken up, the stress transients resemble those of the corresponding step-up experiment. When flow is reversed prior to the droplet break-up, the stress transients are much longer than those in simple step-up experiments, the normal stress curves also become more complex. A model is presented for flow reversal prior to droplet break-up. A satisfactory agreement, albeit only qualitative, is obtained assuming that the drops deform affinely during the entire transient. As expected, tumbling of the droplets does not seem to provide an adequate description of the observed transients. The experimental results show that a residual interfacial stress is always present, thus indicating a residual mean deformation of the droplets in the flow direction during the whole transient. Polydispersity of the droplet population is proposed as a possible explanation for this phenomenon. Light scattering and conservative dichroism measurements, performed on a similar blend undergoing the same type of flow history, confirm indeed that a residual mean stretching of the drops persists throughout the flow reversal.

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Segment connectivity, chain-length breathing, segmental stretch, and constraint release in reptation models: Shear flows

Chi C. Hua
Chemical Engineering Department
National Chung Cheng University, Chia-Yi 621, Taiwan

Jay D. Schieber and David C. Venerus
Center of Excellence for Polymer Science and Engineering
and Chemical and Environmental Engineering Department
Illinois Institute of Technology, Chicago, IL 60616-3793 USA

Abstract

A previously proposed self-consistent reptation model that includes chain stretching, chain-length fluctuations, segment connectivity, and constraint release (CR) is used to predict transient and steady shearing flows. Quantitative comparisons are made with the concentrated solution data considered in the previous papers of the series. The model is able to capture quantitatively all features of experimental data considered, including overshoot in both shear and first normal stresses, the strain-rate dependence of the strain magnitude at maximum stress, the steady-state viscosity and first-normal-stress coefficient as functions of shear rate, the viscosity curves for different molecular weight, the transient and steady-state behavior of the extinction angle, and the stress relaxation in cessation of steady shear flow. The model can describe all aspects of the data very well except the magnitude of the overshoot in stress at high shear rates, where the model is somewhat over-predictive. A new method of analysis for shear stress decay following cessation of steady shear is proposed, based on the physics of the model.

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The rheology and microstructure of concentrated, aggregated colloids

L. E. Silbert* and J. R. Melrose
Polymers and Colloids Group, Cavendish Laboratory
University of Cambridge, Madingley Road, Cambridge, CB3 OHE, UK

R. C. Ball
Department of Physics, University of Warwick
Coventry, CV, UK

*E-mail: leo.silbert@phy.cam.ac.uk

Abstract

The rheology of concentrated, aggregated colloidal suspensions is determined through particulate simulations. Aggregating systems experience a large viscous enhancement over non-aggregating systems, this being due to the increase in the component of the viscosity arising from the repulsive colloid (thermodynamic) forces when attractive forces are present. The shear behaviour of aggregating systems, for colloid volume fraction 0.47 £ f_c £ 0.57, is characterised in the steady-state regime over a wide range in shear rate, and is found to be power law, shear thinning h ~ f(f_c)(g-dot)^-a, where the shear thinning index a = 0.84 ± 0.01. The effect of volume fraction enters as f(f_c) = (1 - f_c/f_max)^-1, with f_max = 0.64, the value of random close packing; similarly, the viscosity also scales with the potential well depth as a power law, of index a. Consequently, we are able to deduce the full constitutive relation for this power law behaviour. The associated structural features which emerge as a result of the imposed shear are identified with the rheology. The shear thinning regime crosses over into a state of ordered phase flow at high shear rates likewise simulations of hard sphere fluids. We also show that the high-shear ordered configurations appear to be a function of colloid concentration, with a transition from string phase order through to layered phases as f_c increases.

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Linear response of regular asphalts to external harmonic fields

J. Stastna and L. Zanzotto
Bituminous Materials Chair, Faculty of Engineering, University of Calgary
2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

Abstract

Dielectric permitivity and mechanical complex compliance of regular asphalts, at T = 323 K, is studied. Both the loss compliance and the loss of permitivity functions can be decomposed into two parts. The first part represents the viscous deformation and the ohmic conductivity, respectively. The second part then describes a "pure" response of the material to external harmonic fields. By this decomposition one can observe a relatively strong transition of regular asphalts into their relaxed state. A simple fractional model of the response function can be applied to both mechanical and dielectric transitions.

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Capillary thinning of polymeric filaments

Mette Irene Kolte and Peter Szabo*
Danish Polymer Centre, Department of Chemical Engineering
Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark

*Corresponding Author

Abstract

The capillary thinning of filaments of a Newtonian polybutene fluid and a viscoelastic polyisobutylene solution are analysed experimentally and by means of numerical simulation. The experimental procedure is as follows. Initially, a liquid sample is placed between two cylindrical plates. Then, the bottom plate is lowered under gravity to produce a specified strain. The sample is thereby stretched into a filament. Provided the filament is sufficiently long, surface tension will induce a thinning of the filament until break-up in finite time. The numerical simulations are performed with a Lagrangian finite element method and show good agreement with the experiments.

A comparison of the results with existing theory in the literature reveals differences between the theoretical predictions and the real behaviour, both for Newtonian and viscoelastic fluids. The origin to the divergence is analysed and quantified.

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Stress and birefringence measurements during the uniaxial elongation of polystyrene melts

D. C. Venerus*, S.-H. Zhu and H. C. Öttinger
ETH Zürich, Department of Materials, Institute of Polymers
CH-8092 Zürich, Switzerland

*Corresponding Author. Current Address:
Department of Chemical and Environmental Engineering and
Center of Excellence in Polymer Science and Engineering,
Illinois Institute of Technology, Chicago, IL 60616 USA
E-mail: chevenerus@iit.edu

Abstract

A rheometer for generating uniaxial elongations in molten polymers (RME) has been modified to allow for the simultaneous measurement of stress and flow-induced birefringence. Tensile stress s and birefringence Dn' data in flows at constant strain rates up to one s^-1 were collected on a polydisperse polystyrene melt at temperatures of 160°C and 170°C. From these data, the stress-optic rule was followed for stresses below roughly one MPa. For stresses less than one MPa, the stress-optic coefficient |C| = |Dn'|/s was found to have a value of 4.8 x 10^-9 Pa^-1, which was independent of strain, strain rate and temperature. At stress levels higher than one MPa, |C| decreased indicating a failure of the stress-optic rule. A criteria for failure of the stress-optic rule was formulated using simple arguments from network models and characteristic times suggested by the tube model. This criteria, which is based on the hypothesis that failure of the stress-optic rule is the result of significant chain stretching, was found to be consistent with the data reported in this study and with data from previous studies on polystyrene melts.

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The role of LCP rheology on the evolving morphology of immiscible blends containing LCPs

William A. Kernick and Norman J. Wagner*
Center for Molecular and Engineering Thermodynamics
Department of Chemical Engineering
University of Delaware, Newark, DE 19716 USA

*Corresponding Author
E-mail: wagner@che.udel.edu

Abstract

A new method is demonstrated that applies the Porod limit of small angle neutron scattering to measure the interface morphology in concentrated, immiscible blends in situ under simple shear. The effect of viscoelastic contrast on rheology and blend microstructure is probed for a model dispersion of liquid crystalline polymer dispersed in a linear polymer matrix. Comparison to theories demonstrates the complex morphology evolution is a consequence of the unique rheology of liquid crystalline polymers. The results at low shear rates are used to predict the onset of stable micro-fibrillation at higher shear rates through consideration of the contrast in first normal stress differences.

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Rotational magnetic particle microrheometry: The Newtonian case

George J. Besseris¹, Irving F. Miller², and Donovan B. Yeates*
Department of Medicine and Department of Chemical Engineering
The University of Illinois at Chicago, Chicago, IL 60680 USA
and
Veterans Affairs Chicago Health Care System
Chicago, IL 60612 USA