NOVEL TRENDS IN RHEOLOGY III: Proceedings of the International Conference
1152(2009); http://dx.doi.org/10.1063/1.3203284View Description Hide Description
The classical Doi‐Edwards (DE) model with Independent Alignment (IA) approximation represents a basic issue for tube theories. In this contribution there is presented alternative approach not taking into account the IA approximation. Under external deformation the polymer sample molecules are deformed affinely, consequently they retract but do not preserve their direction. The end‐to‐end vector of a molecule after retraction becomes distributed with a certain probability. This probability is calculated using geometrical considerations of retracting motion.
1152(2009); http://dx.doi.org/10.1063/1.3203266View Description Hide Description
The pressures exerted by a polymer chain on the walls of an anisotropic confinement are anisotropic. In a tube model with variable tube diameter, this leads to an evolution equation for the chain stretch with an interchain pressure term, which is inverse proportional to the power of the tube diameter a and is characterized by a tube diameter relaxation time [1, 2]. The tube diameter relaxation time in the evolution equation of the Molecular Stress Function (MSF) model  is shown to be equal to 3 times the Rouse time in the limit of small chain stretch. From this result an advanced version of the MSF model is proposed allowing modelling of the transient and steady‐state elongational viscosity data of two monodisperse polystyrene melts without using any nonlinear parameter, i.e. solely based on the linear‐viscoelastic characterization of the melts. The same approach is extended to model experimental data of two polybutadiene solutions  in shear flow. Thus for monodisperse polymer melts and solutions, for the first time a constitutive equation is available to model quantitatively nonlinear extension and shear rheology on the basis of linear‐viscoelastic data alone.
1152(2009); http://dx.doi.org/10.1063/1.3203283View Description Hide Description
Experimental data of two low‐density polyethylene (LDPE) melts at 200° C for both shear flow (transient and steady shear viscosity as well as steady first normal stress coefficient) and elongational flow (transient and steady‐state elongational viscosity) as published by Pivokonsky et al.  were analyzed by use of the Molecular Stress Function (MSF) model for broadly distributed, randomly branched molecular structures. For quantitative modeling of melt rheology in both types of flow and in a very wide range of deformation rates, only three nonlinear viscoelastic material parameters are needed: While the rotational parameter, and the structural parameter, β, are found to be equal for the two melts considered, the melts differ in the parameter describing maximum stretch of the polymer chains.
1152(2009); http://dx.doi.org/10.1063/1.3203285View Description Hide Description
In this contribution, the filtration equations for a cake filtration in viscoelastic fluids are presented. They are based on a capillary hybrid model for the flow of a power law fluid. In order to express the elastic pressure drop excess in the flow of viscoelastic filtrate through the filter cake and filter screen, modified Deborah number correction functions are included into these equations. Their validity was examined experimentally. Filtration experiments with suspensions of hardened polystyrene particles (Krasten) in viscoelastic aqueous solutions of polyacryl amides (0.4% and 0.6%wt. Kerafloc) were carried out at a constant pressure on a cylindrical filtration unit using filter screens of different resistance.
1152(2009); http://dx.doi.org/10.1063/1.3203286View Description Hide Description
In this contribution, the results of experimental determination of the drag coefficient correction factor for spherical particles moving slowly through a Carreau viscosity model fluid are presented. The experimental values of the correction factor, which were evaluated from measurements of terminal falling velocity of seventeen types of spherical particles in viscoelastic polymer solutions, are compared with those calculated numerically using a finite elements method. The results obtained confirm that in the case of shear thinning concentrated polymer solutions the viscoelastic effects are only of minor significance. However, in the case of less concentrated high‐molecular polymer solutions in polyalkylene glycol Emkarox an evident drag enhancement has been observed.
1152(2009); http://dx.doi.org/10.1063/1.3203287View Description Hide Description
In this work, two different HDPEs were investigated from rheological as well as die drool phenomenon point of view. It has been revealed that long‐chain branching and low polymer melt elasticity significantly reduce die drool phenomena at the die exit region.
1152(2009); http://dx.doi.org/10.1063/1.3203288View Description Hide Description
Injection molding represents such a way of polymer processing that requires injection of polymer melt into the mold cavity with very high injection rate. The fluidity of polymers is affected by many parameters (mold design, melt temperature, injection rate and pressure). The main objective of this paper is the study of influence of surface roughness of mold cavity of the polymer melts flow. Evaluation of set of data obtained by experiments where the testing conditions were widely changed shows that quality of cavity surface affects on the length of flow.
1152(2009); http://dx.doi.org/10.1063/1.3203289View Description Hide Description
The aim of this contribution is to present a quasisimilarity solution of helical power‐law fluid flow in concentric annuli with a rotating inner cylinder. Emphasis is paid to the relation axial volumetric flow rate vs. axial pressure gradient. It is shown that for sufficiently broad region of entry parameters (rheological, geometrical, and kinematical) there is a possibility to derive quasisimilarity transformations enabling relatively easy determination of the above relation.
1152(2009); http://dx.doi.org/10.1063/1.3203290View Description Hide Description
Polymer melts experience complex, time variant, stress and deformation fields on their passage through fixed geometries in many conversion operations. Flow complexity is further increased in operations involving the co‐joining of two or more melt streams where one confining boundary is moving and viscoelastic. Such a complex situation arises in coextrusion processes. This work covers experimental studies on polyethylene melt flows in complex coextrusion geometries with a view to understanding the stress fields involved and their effects on flow stability.
A 30° coextrusion geometry is studied using two extrusion arrangements. In one arrangement a single extruder is used to feed a ’bifurcated’ die design wherein the melt stream is split prior to, and rejoined after, a divider plate in the die. In the other design melt streams are delivered to, and converged at 30°, using two independent extruders. In a second die melt streams are brought together at 90°. In each die arrangement melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Velocity ratios of the two melt streams were varied and layer thickness ratios producing instability are determined for each melt for a variety of flow conditions. Stress and velocity fields in the coextrusion arrangements were quantified using stress birefringence and particle image velocimetry techniques.
The study demonstrates conclusively that wave type interfacial instability occurred in the coextrusion geometries when the same low density polyethylene melt is used in each stream. This observation occurred at specific, repeatable, stream layer ratios in each die arrangement. The complex flows were numerical modelled using a modified Leonov model and Flow 2000™ software. There was reasonable agreement between modelled at experimentally determined stress fields. Modelling however provided far more detailed stress gradient information than could be resolved from the optical techniques. A total normal stress difference (TNSD) sign criterion was used to predict the critical layer ratio for the onset of the interfacial instability in one die arrangement and good agreement between theory and experiment has been obtained.
The study conclusively demonstrates wave type interfacial instability in the coextrusion process is not caused by process perturbations potentially introduced by extruder screw rotation but is associated with process‐history dependant differences in melt elasticity.
1152(2009); http://dx.doi.org/10.1063/1.3203259View Description Hide Description
The steady slow motion of solid spheres through a Carreau model fluid contained in a cylindrical tube has been solved numerically using a finite element method by means of the COMSOL software package for steady non‐Newtonian flows From the resulting stress fields, the drag force on the sphere, drag coefficient, drag coefficient corrective factor, and wall correction factor have been evaluated in dependence on the Carreau model parameters and the sphere‐to‐tube diameter ratio. The results of the wall correction factor calculations are presented herein and compared with some experimental data.
Characterization of Microstructure and Molecular Dynamics with High Frequency Oscillatory Techniques1152(2009); http://dx.doi.org/10.1063/1.3203260View Description Hide Description
To characterize the rheological behaviour of complex viscoelastic fluids, polymer melts and other soft materials, motor‐drive controlled rheometers are mainly used, either at constant stress or strain rate, or in the oscillatory mode. The latter has proved advantageous to discover the viscoelastic functions G*, η*, J* as fingerprints of the material under investigation, it’s composition, molecular modelling and applicability. A conclusive analysis of such a viscoelastic spectrum can only be achieved if the amplitudes chosen guarantee linearity and if the frequency range covers more than 6 decades to reach the low kHz‐domain. Investigations of many materials with motor‐drive controlled rheometers are limited at higher frequencies and reach the above mentioned goal by applying the time‐temperature superposition principle, i.e. the mastercurve technique. Since this method is restricted to rheologically simple materials (e.g. some polymer melts), but exclude those of small activation energies and others with temperature‐sensitive chemical/physical structures including phase transitions, oscillating rheometry should be extended into higher real‐frequency ranges, to establish useful linear viscoelastic spectroscopy. Since complex fluids can have structural arrangement over a wide range of lengthscales and their relaxation mechanisms can impact the dynamics over a wide range of timescales, multiple techniques need to be employed in order to accurately and fully establish the links between rheology, microstructure & dynamics. This is also critical information, required for fully validating developed theory and models. In this talk, advantages and limits of classical oscillatory rheometry will be covered, handling and principle of operation of two high frequency options are introduced and typical examples for real frequency spectra on soft matter, such as polymer melts, polymer solutions and weak gels will be shown. A xanthum gum based system has been investigated not only by traditional rheology and high frequency mechanical rheology but also through DWS based optical microrheology. The talk will focus on how unique insights from each of these techniques leads to a better understanding of the overall microstructure‐rheology linkages in this system.
1152(2009); http://dx.doi.org/10.1063/1.3203261View Description Hide Description
In this work, novel patent pending orifice die design for precise extensional viscosity data determination from entrance pressure drop measurements has been developed and tested both, theoretically (through Finite Element Analysis) and experimentally. It has been demonstrated that the proposed novel orifice die allows much more precise extensional viscosity measurements for polymer melts in comparison with conventionally used orifice dies.
1152(2009); http://dx.doi.org/10.1063/1.3203262View Description Hide Description
Two experimental methods, apparent‐wall‐slip (AWS) rotational viscometry with “Morse‐taper” sensors and electrodiffusion (ED) flow diagnostics with auto‐calibrated friction probes, are used to study velocity profiles in aqueous solutions of high‐molecular polysaccharides. By comparing the velocity data from the both methods, estimates are obtained of depleted layer thickness in dependence on wall shear stress.
Fourier Transformation Analysis in Capillary Flow—A New Option to Detect Flow Instabilities (Shark Skin)1152(2009); http://dx.doi.org/10.1063/1.3203263View Description Hide Description
Flow instabilities cause various problems in extrusion blow‐ or flat film processes. Especially shark skin is affected by molecular structure. This phenomenon is analyzed in various scientific works using apparatus that are complex or difficult in operation. A new set‐up, now available is being presented as an option for new and existing capillary rheometers or lab extruders, consisting of a special designed die, a fast responsive transducers system and evaluation software. The software allows the evaluation of diverse pressure frequency regimes causing flow instabilities and generate statistics. Measurement examples are given for different shear rates and polymers. The shark skin effect can be detected even before it is visible on the extrudate. The measured frequency regimes are related to molecular parameters.
1152(2009); http://dx.doi.org/10.1063/1.3203264View Description Hide Description
A modified servo electric injection moulding machine has been used in air‐shot mode with capillary dies fitted at the nozzle to examine the rheology of a number of commercial polymers at wall shear strain rates of up to Shear and extensional flow properties were obtained through the use of long and orifice (close to zero land length) dies of the same diameter. A range of polyethylene, polypropylene and polystyrene melts have been characterized; good agreement was found between the three techniques used in the ranges where strain rates overlapped. Shear viscosity of the polymers studied was found to exhibit a plateau above approximately A relationship between the measured high strain rate rheological behaviour and molecular structure was noted, with polymers containing larger side groups reaching the rate independent plateau at lower strain rates than those with simpler structures.
1152(2009); http://dx.doi.org/10.1063/1.3203265View Description Hide Description
The continuous demand for high performance products is the driving force for the development of state‐of‐the‐art materials with outstanding processability and mechanical strength. In order to fulfill these properties, an appropriate tailoring of the molecular structure is a requirement. Such tailoring can only be achieved provided that suitable and sensitive characterization techniques are available. The dominance of complex flows in most of the industrial processes requires the understanding of the rheological properties under the non‐linear viscoelastic regime. Within the present work Large Amplitude Oscillatory Shear (LAOS) will be applied to a series of industrial polypropylene and polyethylene based materials. This work will provide additional insights on the relationship between given rheological parameters from LAOS flow and molecular topology. The use of the third order Chebyshev elastic and viscous coefficients, and provided an understanding of the physical nature of the non‐linear viscoelastic response.
1152(2009); http://dx.doi.org/10.1063/1.3203267View Description Hide Description
On‐line rheometers are generally inserted between extruder and die and generate data that is typically utilized for quality control purposes. However, on‐line rheometers have also the potential to detect changes in structure, morphology, or composition of a given material system, thus assisting materials research and processing optimization, if they can be used along the axis of the extruder or compounder. The authors have previously developed on‐line capillary and rotational/oscillatory rheometers that can be inserted and used at specific locations along the extruder. Since these devices are operated manually, their manipulation may be cumbersome and data may lack reliability. This work presents new versions of these rheometers, with improved functionalities and motorizessd operation. Details on the validation of one of them is also given.
1152(2009); http://dx.doi.org/10.1063/1.3203269View Description Hide Description
A simple method to measure the local extensional properties of polymer melts using a modified Münstedt extensional rheometer is demonstrated. Real‐time imaging of the specimen during the extension process provides the local rates of strain as a function of the coordinate along the specimen. Simultaneously, the local stresses are assessed by measurements of the sample diameter along the sample and synchronous measurements of the force exerted on the bottom plate of the rheometer. The method provides a quantitative measure of the homogeneity of the deformation states via a kinematic and a dynamic fingerprint of the process. Benchmark homogeneous deformation experiments reveal a good level of agreement between global and integral measurements of the transient extensional viscosity and thus validate the method. Furthermore, this technique is employed in assessing the impact of inhomogeneities in the deformation field on measurements of the transient extensional viscosity. We conclude that in the case of inhomogeneous deformations the extensional properties depend strongly on the position along the specimen and the usual integral measurements of the transient extensional viscosity are highly unreliable and unrepeatable.
1152(2009); http://dx.doi.org/10.1063/1.3203270View Description Hide Description
EPDM and EPM were modified using different processing conditions and amounts of peroxide. Rheological measurements were performed in other to characterize and follow the reactions that took place during processing (branching/crosslinking and/or degradation). The experimental results evidenced that branching/crosslinking occurs for EPDM and degradation is the main reaction for EPM.