摘要:

Capillary instability and breakup of jets of magneto‐rheological fluids are studied theoretically in the framework of one‐dimensional theory. Linear instability is considered and the non‐linear jet profile evolution is followed using numerical modelling. The magnetic field is assumed to be trapped within the liquid and both produces magnetic pressure and causes an increase in viscosity due to the magneto‐rheological effect. Both effects of the magnetic field result in slowing down the necking and lead to development of a beads‐on‐string structure at the late stage of jet evolution. It is shown that the magneto‐rheological effect by itself may prevent early breakup even in suspensions with strongly shear‐thinning rheological behavior.

ISSN:0148-6055    

DOI:10.1122/1.550761

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A new model is derived for the mechanical behavior of linear viscoelastic materials subjected to physical aging. According to the model, a viscoelastic medium is treated as a system of elastic springs (links between polymeric molecules) which replace each other. Two types of links are distinguished: links arisen at the instant of quenching (type I), and links emerging in the quenched material at a constant temperature (type II). The mechanical behavior of an aging medium is determined by three material functions which characterize (i) the collapse of links of type I, (ii) the breakage of links of type II, and (iii) the rate of emergence of new links of type II. We derive integral equations for these functions and find their solutions using data of the standard relaxation tests. To verify the model, we calculate the material response in the creep tests and compare results of numerical simulation with experimental data for an epoxy adhesive. The results obtained demonstrate fair agreement between experimental observations and their prediction. By using the model developed, we analyze numerically the behavior of a viscoelastic medium under time‐varying loads. We study elongation of a specimen with a constant rate of strain, its recovery after creep tests, and steady shear oscillations of a layer. In the latter case, numerical results are compared with experimental data for polypropylene samples.

ISSN:0148-6055    

DOI:10.1122/1.550762

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

An important characteristic of a polymer is its molecular weight distribution (MWD). The MWD affects its processing characteristics and its solid state performance. The traditional analytical method for determining MWD is gel permeation chromatography (GPC). However, GPC results depend strongly on the skill and experience of the operator and are not sensitive to small amounts of high molecular weight material that can have an important effect on processing and product properties. It is known that rheological properties depend on MWD, and it has been proposed that viscosity or complex viscosity data can be used to infer the MWD for commercial polymers. Although there has been some controversy as to whether this is feasible, recent results of Shaw, Tuminello, and others indicate that for linear polymers this is, in theory, possible. However, limitations in the accessible frequency range, as well as inevitable experimental errors, pose serious barriers to the use of such a procedure. We have investigated the severity of these barriers and have shown that within certain limits, complex viscosity data can be used to infer a realistic MWD for linear polyethylenes.

ISSN:0148-6055    

DOI:10.1122/1.550763

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

In this study a phenomenological constitutive model is proposed to describe the finite, nonlinear, viscoelastic behavior of glassy polymers up to the yield point. It is assumed that the deformation behavior of a glassy polymer up to the yield point is completely determined by the linear relaxation time spectrum and that the nonlinear effect of stress is to alter the intrinsic time scale of the material. A quantitative three‐dimensional constitutive equation for polycarbonate as a model polymer was obtained by approximating the linear relaxation time spectrum by eighteen Leonov modes, all exhibiting the same stress dependence. A single Leonov mode is a Maxwell model employing a relaxation time that is dependent on an equivalent stress proportional to the Von Mises stress. Furthermore, a Leonov mode separates the (elastic) hydrostatic and (viscoelastic) deviatoric stress response and accounts for the geometrical complexities associated with simultaneous elastic and plastic deformation. Using a single set of parameters, the multi‐mode Leonov model is capable of describing realistic constant strain rate experiments, including the strain rate dependent yield behavior. It is also capable of giving a quantitative description of nonlinear stress‐relaxation experiments.

ISSN:0148-6055    

DOI:10.1122/1.550755

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The shear history dependence of the viscosity of a depletion flocculated dispersion of colloidal spheres was studied using two different rheometrical geometries. The observed rheological behavior is found to depend on the geometry, due to effects of thixotropy and sedimentation. By comparing the results of a cone‐plate and a Couette geometry, we were able to obtain reliable data. The shear history dependence is explored by measuring a flow curve before and after subjecting the aggregated dispersion to a constant shear rate for one hour. The viscosity values of the flow curve after this hour turned out to be considerably lower than the initial flow curve. The results were interpreted with a microrheological model for fractal aggregation in shear flow. The drop in viscosity is attributed to a shear induced compaction of the aggregates. Combination of this model and the concept of compaction results in a satisfactory description of the experimental results.

ISSN:0148-6055    

DOI:10.1122/1.550783

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Viscoelastic properties of paving grade asphalt cements (AC) were measured on a Rheometrics RDA II. Thermal properties were studied with a Mettler TA 2000 B differential scanning calorimeter. A review of the literature showed that most authors proposed that the time‐temperature superposition principle applies to asphalt over the whole temperature range. However, the results presented herein demonstrate that it indeed fails at high temperature, especially for high asphaltene content and high crystalline content materials. This paper focuses on the role of asphaltenes, which are, by definition, insoluble inn‐heptane. Asphaltenes are suspended in a colloidal manner within the AC matrix and are peptizided by the species called resins. Thus, a temperature dependent solid fraction exists within a liquid (or glassy) matrix. This paper shows that the viscoelastic properties of asphalt are bimodal: they are governed by both solid and continuous phases.

ISSN:0148-6055    

DOI:10.1122/1.550764

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The behavior under transient uniaxial elongation of a homologous series of variable extensibility polystyrene‐based Boger fluids has been investigated in an apparatus similar to that developed by Sridhar and co‐workers. The test fluids are dilute solutions of either 2.0×107or 2.0×106g/mol monodisperse polystyrene dissolved in poor, dioctyl phthalate‐based or good, tricresyl phosphate‐based solvents. This rational manipulation of Boger fluid solvent quality (assessed based on previously reported light scattering and intrinsic viscometry measurements) and chain length yields a unique opportunity to correlate viscoelastic extensional response to finite extensibility and molecular interactions: here we search for such effects in transient filament extension between parallel plates, an approximation of purely extensional flow. The test device, which can impart a maximum Hencky strain, ε, of 4.5 at rates, ε̇, between 0.3 and 3.0 s−1, is similar to that reported by Sridharetal. (1991). In agreement with results communicated there and in Tirtaatmadja and Sridhar (1993), large strain hardening is observed for these polymer solutions, as well as certain deviations of the experimental flow from ideal uniaxial extension. As opposed to Tirtaatmadja and Sridhar (1993), no steady‐state extensional viscosities are obtained for the comparatively small strains of our experiment. The transient experimental results are roughly consistent with simple one‐dimensional FENE–P calculations. However, uncertainties due to the nonideal nature of the flow at short times and the sensitivity of the measured extensional stress growth coefficient to the details of the imposed elongation make it difficult to unambiguously assignLbased solely on short time filament stretching results. These results are considered in light of the substantial effects solvent quality and molecular weight have on the measured drag in flow past a sphere [Chmielewskietal. (1990); Solomon and Muller (1996b)].

ISSN:0148-6055    

DOI:10.1122/1.550770

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A Hookean dumbbell model for polymers in dilute solutions undergoing homogeneous flow is generalized to include arbitrary imposed temperature profiles. In order to obtain the ‘‘nonisothermal diffusion equation’’ for the probability density in polymer configuration space we generalize the approach of Schieber and Öttinger [J. Chem. Phys.89, 6972–6981 (1988)] to Brownian motion out of equilibrium. In addition, we derive the polymer contributions to the mass‐flux vector, stress tensor and heat‐flux vector by means of the kinetic theory approach of Curtiss and Bird [Adv. Polym. Sci.125, 1–101 (1996)] for the case of a slowly varying temperature gradient, and we find coupled constitutive equations for the mass, momentum and energy fluxes. For a simple steady shear flow it is then possible to calculate the heat‐flux vector explicitly, at least for small temperature gradients and shear rates. We compare our approach and results with previous works on this subject, and we finally discuss some extensions.

ISSN:0148-6055    

DOI:10.1122/1.550765

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

This paper explores the molecular mechanism for sharkskin formation on extrudate of linear low density polyethylenes (LLDPE) and investigates the rheological origin of a characteristic curvature (i.e., a slope change) in the flow curve of LLDPE. Rheological measurements, performed at various temperatures from 160 to 240 °C with a controlled‐pressure capillary rheometer and a variety of dies, suggest that the slope change in the flow curve, interpreted by many as demonstrating wall slip in the die land, arises from a combination of interfacial slip and cohesive failure due to chain disentanglement, first initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains within a certain stress range below the critical stress for the global stick–slip transition, a partial slip flow cannot sustain itself and occurs only periodically. This time‐dependent molecular entanglement–disentanglement fluctuation produces the sharkskin like extrudate in the regime where the slope change takes place. Sharkskin dynamics are found to precisely correlate with chain relaxation processes. Specifically, the characteristic time scale τ (i.e., the sharkskin periodicity) is found to be of the same magnitude and have the same WLF (Williams–Landel–Ferry) temperature dependence as that of the characteristic molecular relaxation time τ* as determined by oscillatory shear measurements in a parallel‐plate flow cell. The LLDPE resins are also observed to undergo interfacial stick–slip transitions as well as a rarely seen cohesive slip–slip transitions at various temperatures.

ISSN:0148-6055    

DOI:10.1122/1.550766

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Reversible shear thickening is measured in model colloidal suspensions of silica that exhibit near hard‐sphere behavior. Flow dichroism measurements and the colloidal stress‐optical relationship prove that hydrodynamic interactions drive the shear thickening transition. Turbidity and flow‐small angle neutron scattering (flow‐SANS) demonstrate that particles cluster reversibly in the shear thickened state. Further, SANS measurements show that shear thickening occurs without any shear‐induced order‐disorder transition, in contrast to observations for dispersions of charged colloids. A simple force balance predicts the scaling of the critical stress for the onset of shear thickening with particle size and volume fraction. Measurements on bimodal mixtures verify the scaling laws derived from the force balance and provide a strategy for controlling the shear thickening transition.

ISSN:0148-6055    

DOI:10.1122/1.550767

出版商:The Society of Rheology    

年代:1996

数据来源: AIP