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Journal of RheologyVolume 44, Issue 6 (November-December 2000) |
Contents
Rheological properties and reactive compatibilization of immiscible polymer blendsM. Moan, J. Huitric, and P.
Médéric J. Jarrin AbstractThe effects of addition of a reactive compatibilizer, a random terpolymer, on morphological and rheological properties of blends of polyamide dispersed in a polyethylene matrix were studied experimentally. This addition leads to smaller size and narrower size distribution of the dispersed phase. This has been related to the presence, at the interface, of copolymers, formed in situ by reaction between the polyamide and the terpolymer, which form an interphase between the nodules and the matrix. The linear viscoelasticity of these compatibilized blends reveals that, in addition to the form relaxation process (relaxation time ld) even observed in absence of compatibilizer, another relaxation process, which has been related to the dynamics of nodule-matrix interphase, is observed at a characteristic time lint higher than ld. The concentration and molecular weight dependence of this characteristic time lint, and the intensity Hint of the relaxation mechanism associated to lint has been investigated. The steady shear measurements exhibit an unusual behavior never observed before in blends of immiscible polymers. Indeed, the viscosity curve shows two plateau regions, respectively, at low and moderate shear rates, connected by a rather steep portion of curve, which is associated to a characteristic time of the same order of magnitude as lint This behavior has been interpreted by considering that the response of the system at the lowest shear rates is dominated by the molecular dynamics of the nodule-matrix interphase, whereas at higher shear rates the system behaves as an emulsion with hydrodynamic particle-particle interactions. The viscosity excess, due to the slowest relaxation process and expressed as the difference between the two plateau viscosities, shows a marked dependence on both concentration and molecular weight of the compatibilizer. Creep compliance-time behavior and stability of bitumen in water emulsionsNilia Romero, Antonio Cárdenas, and Hercilio Rivas AbstractThe effect of bitumen concentration, presence of electrolytes, storage temperature, mean droplet diameter, and distribution and the length of the ethylene oxide chain in nonionic surfactants, on the viscoelastic parameters of bitumen in water emulsions, was investigated. Viscoelastic parameters were derived from the creep compliance-time response of the emulsions at a constant stress. All the emulsions exhibited an increase in the instantaneous elastic modulus and in the Newtonian viscosity, when the mean droplet diameter was decreased at constant bitumen fraction, or when the bitumen fraction was increased at constant mean droplet diameter. In all cases studied, the storage temperature and the presence of a magnesium nitrate in the aqueous phase, promoted a decrease in the values of the viscoelastic parameters. Results are explained based on the interaction between the hydrophilic chains of the surfactant molecules adsorbed on adjacent bitumen droplets. Emulsion stability, measured, as a function of the increase in mean droplet diameter with the storage time, is closely related to the variation in the values of the viscoelastic parameters. Strain-hardening behavior of polycarbonate in the glassy stateT. A. Tervoort L. E. Govaert AbstractThis paper presents an experimental characterization of the three-dimensional strain-hardening response of polycarbonate in the glassy state. Using a special mechanical conditioning technique, large homogeneous deformations were obtained in tension, compression, and shear. The experimental results are compared to a number of existing network models. It was found that the state-of deformation dependence of the strain-hardening response was adequately described by neo-Hookean behavior with a shear modulus G = 26 MPa. Up to the deformations applied in this study, no sign of a finite extensibility of the entanglement network was observed. Viscosity of bimodal charge-stabilized polymer dispersionsF. M. Horn and W. Richtering AbstractMonodisperse electrostatically stabilized polymer latices with particles diameters of 310 and 120 nm were synthesized and used to prepare binary mixtures with bimodal size distribution. The zero shear viscosity of these bimodal, charge stabilized polymer latices was studied at different salt concentrations. In contrast to hard sphere colloidal suspensions, no minimum in viscosity was found as a function of the mixing ratio of small and large particles. Instead the viscosity increased when the fraction of small particles increased, which is due to the direct Coulomb interaction. In order to compare the results with data from hard sphere systems, we used an effective volume fraction feff. The experimentally determined volume fraction at the divergence of the zero shear viscosity is compared with the hard sphere value in order to define feff. The effective volume of the particles was then used to calculate the effective volume fraction of the binary mixtures containing small and large particles. When using feff, a minimum of the viscosity was found at a composition of ~30% of small particles similar to the behavior of bimodal hard-sphere suspensions. The volume fraction at maximum packing could be calculated by theoretical models only at high salt concentration. The models underestimate the maximum volume fraction at low salt concentration. A thermodynamically admissible reptation model for fast flows of entangled polymers. II. Model predictions for shear and extensional flowsJiannong Fang, Martin Kröger, and Hans Christian
Öttinger AbstractNumerical predictions of a previously proposed thermodynamically consistent reptation model for linear entangled polymers are presented for shear and extensional flows. Comparisons with experimental data and two alternative molecular-based models are given in detail. The model studied in this paper incorporates the essence of double reptation, convective constraint release, and chain stretching, and it avoids the independent alignment approximation. Here, no use is made of the ingredient of anisotropic tube cross sections of the previously proposed model. Simulation results reveal that the model at a highly simplified level with few structural variables, i.e., four degrees of freedom, is able to capture qualitatively all features of the available experimental observations and is highly competitive with recently proposed models in describing nonlinear rheological properties of linear entangled polymers. The correlation of experimental surface extrusion instabilities with numerically predicted exit surface stress concentrations and melt strength for linear low density polyethyleneRulande Rutgers Malcolm Mackley AbstractExperimental data on the onset and magnitude of surface instabilities are reported for two grades of linear low density polyethylene. Numerical simulation of the flow is presented and the magnitudes of surface stress concentrations established. The onset of experimentally observed surface instabilities is then correlated with the magnitude of the surface stress concentrations at the exit and also with the melt strength of the polymer. Solving the inverse problem of Couette viscometry by Tikhonov regularizationY.
Leong Yeow, Woan C. Ko, and Pannie P. P. Tang AbstractMost of the existing procedures for converting Couette viscometry data into a shear stress t versus shear rate g-dot material function rely on the small annular gap assumption or require the algebraic form of the t-g-dot curve to be prespecified. Furthermore most of the existing procedures are not particularly suitable for fluids with yield stress. In this investigation the problem of converting Couette viscometry data into a t-g-dot material function is formulated as a Volterra integral equation of the first kind. A method based on Tikhonov regularization is then developed to solve this equation for the t-g-dot curve. The method does not depend on the small gap assumption or require prespecification of the algebraic form of the t-g-dot relationship. It is equally applicable to fluids with and without yield stress. For fluids with yield stress, provided the data include one or more points where the fluid in the annular gap is partially sheared, the method will also extract the yield stress from the data. The performance of this general method is demonstrated by applying it to synthetic Couette viscometry data with added random noise and to experimental data taken from the literature. Molecular weight distribution dependent kernels in generalized mixing rulesWolfgang Thimm, Christian Friedrich, and Tobias Roths Josef Honerkamp AbstractIn this article a model is proposed for the kernel in the generalized mixing rule recently formulated by Anderssen and Mead [Anderssen, R. S. and D. W. Mead, J. Non-Newtonian Fluid Mech. 76, 299-306 (1998)]. In order to derive such a model, it is necessary to take into account the rheological significance of the kernel in terms of the relaxation behavior of the individual polymers involved. This naturally leads to examining how additional physical effects, which depend on the molecular weight distribution, can be included in the mixing rule. The advantage of this approach is that, without changing the generality of the Anderssen and Mead (1998) rule, the choice of the kernel can be used to enhance the physical and rheological significance of their mixing rule. Nonlinear rheology of immiscible polymer blends: Step strain experimentsM.
Iza and M. Bousmina AbstractRelaxation experiments after simple shear flow were performed on (50/50) PB/PDMS poly(1-butene)/polydimethylsiloxane immiscible model blends and the results were compared to the predictions of the Doi-Ohta and Lee-Park models. Three situations of flow were examined: (i) first the variation of stress relaxation was followed in time at various step strain amplitudes, (ii) variation of stress relaxation as a function of the amplitude of preshear rate at a fixed strain, and (iii) at a fixed strain and preshear rate, the relaxation of the stress was studied as a function of the time elapsed between the end of the preshear and the step strain. After application of step strains of various magnitudes, the stress relaxation modulus G(t,g) at short times was found to obey the Wagner time-strain separability [Wagner (1976)]. It was possible to separate linear effects from the nonlinear ones via a damping function h(g) of sigmoidal form. After cessation of steady shear flow of different magnitudes, the linear stress relaxation modulus at long time scale was found to be very sensitive to the shear flow conditions and to the elapsed time between the end of the preshear and the step strain. The morphology evolution characterized by the droplet radius extracted from emulsion models as a function of the steady shear rate was found to be fairly described by the empirical partially mobile interface coalescence model and did not obey the inverse proportionality to shear rate as predicted by the Doi-Ohta theory. After cessation of steady shear flow, the blend morphology continues to evolve until a steady state was reached. Two kinetics seem to govern the establishment of a stable morphology: a rapid retraction process of elongated droplets leading to an increase of terminal relaxation time followed by breakup via Rayleigh instabilities and end-pinching mechanisms as was confirmed by in situ morphological observations carried out between two sliding plates at equivalent deformation. Drop shape dynamics under shear-flow reversalStefano
Guido Mario
Minale Pier
Luca Maffettone AbstractThe shape evolution of a liquid drop immersed in an immiscible liquid is studied under transient flow conditions. The drop is Newtonian and buoyancy free; the external liquid is Newtonian and subjected to shear flow reversal. Three model systems, polydimethylsiloxane/polyisobutylene, polybutene/silicone oil, and silicone oil/polybutene, all Newtonian under the experimental conditions investigated, have been selected to have a range of viscosity ratios. The three drop axes and the drop orientation within the shear plane are independently measured. The results are compared with the predictions of a phenomenological model. The agreement between experimental results and theory is good. Peculiar behavior of the orientation angle has been observed and correctly predicted. The results are also used to explain some rheological features typical of immiscible polymer blends. Breakup of filaments in blends during simple shear flowPeter Van Puyvelde, Hong Yang, Jan Mewis, and Paula
Moldenaers AbstractIn this paper the breakup of droplets under shear in polymer blends is studied by means of linear conservative dichroism and small angle light scattering. More specifically breakup of long fibrils by interfacial instabilities is considered. Measurements are performed on dilute model systems containing nearly Newtonian components in transient flows that involve a sudden increase or decrease in shear rate. The experimental results are used to evaluate the Khakhar and Ottino theory [Khakhar and Ottino (1987)]. In this theory breakup times are calculated starting from the onset of the instability. It is demonstrated that the scaling derived from the Khakhar and Ottino theory also holds for the more readily accessible total breakup time, calculated from the onset of shear flow in a startup flow. The development of interfacial disturbances is studied in a flow history, which consists of generating fibrils by suddenly applying a shear rate followed by a sudden drop in shear rate during which the breakup process is observed. The evolution of the dominant wavelength during this process turns out to be time controlled rather than strain controlled as suggested by the theory. Effects of particles on the steady state and transient rheology of lyotropic hydroxypropylcellulose solutionsValerie Hartmann, Jan Vermant, Emmanuel Heinrich, Jan
Mewis, and Paula Moldenaers AbstractThe rheological effects of adding particles to main-chain liquid crystalline polymers (LCPs) are studied. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose in water are used. Even very small amounts of particles seem to eliminate negative normal stress differences. The complex and unsteady flow field in between particles might interfere with the director orientation and in this manner affect the occurrence of negative normal stresses. Flow reversal experiments on filled systems produce damped oscillatory responses for both the shear stress and the first normal stress difference. In these transients the scaling of time with shear rate, a characteristic of LCPs, is preserved. This indicates that the LCP texture still determines the time scale of the rheological response of the filled system. The textural changes are more rapid when particles are present. This has been observed in recoil and relaxation experiments as well as in flow reversals. When adding particles the decrease in time scales closely follows the increase in viscosity. The observed phenomena can be rationalized by considering how the presence of particles affects the macroscopic textural length scale, the amount of LCP material present and the director orientation during flow. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |