摘要:

Torque and normal force measurements in torsional experiments provide a powerful technique for the efficient measurement of material properties. Over the past several decades NIST has been active in pioneering the use of such measurements for the characterization of polymer material behaviors, particularly in transient experiments. In a recent series of tests using a force rebalance transducer, anomalies in normal force measurements were observed. It was determined that the active nature of the transducer, which leads to excellent zero stability, causes thermal heating and consequent expansion of the system. The transducer length changes are shown to be the cause of the normal force anomalies and, in collaboration with the equipment manufacturer, suggestions for working around the problem and potential transducer redesign are discussed.

ISSN:0148-6055    

DOI:10.1122/1.550745

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

In this work we thoroughly explore the meanings of dissipation (sometimes referred to as viscous dissipation) and stress power. To do this we utilize the Cauchy momentum equations and the first and second laws of thermodynamics. First, the generalized engineering Bernoulli equation (GEBE) is derived from the Cauchy momentum equations and it is clearly shown to have nothing to do with a balance of energy. Next, the first law of thermodynamics or energy balance is discussed and a combined equation by subtracting the two is derived which we refer to as the mechanoenergy balance (sometimes referred to as the ‘‘equation of thermal energy’’). The fact that a difference exists further reinforces that the GEBE is not related to a balance of energy. Finally, the second law of thermodynamics is presented and the concept of dissipation introduced. An example is presented to demonstrate the utility of these equations which will hopefully eliminate some confusion in the literature.

ISSN:0148-6055    

DOI:10.1122/1.550746

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Starches with various amylose contents (0%–70%) were processed on a twin‐screw extruder equipped with a special slit die rheometer. Relationships between thermomechanical treatment and starch macromolecular degradation are defined, and flow curves are discussed in order to determine the role of moisture content, product temperature, and mechanical energy on melt viscosity. The viscous behavior is described by a power‐law expression. Viscosity is more sensitive to moisture content and macromolecular degradation at lower amylose contents. Using multiple regression analysis, expressions for the different starches are proposed to describe the influence on the viscosity of amylopectin, which is the macromolecular component with short chain branching. The main differences observed when decreasing the amylose content are a lower viscosity and less pronounced shear thinning. These effects are interpreted in terms of entanglements.

ISSN:0148-6055    

DOI:10.1122/1.550747

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

In this work we analyze the possibility of improving the reliability of the conversion from the viscosity function to the molecular weight distribution (MWD) for linear flexible polymers. A constrained inverse procedure has been applied to guide the inverse problem to the correct solution, i.e., an analytical MWD function utilizing the generalized exponential function (GEX) has been assigned. The self‐consistency of this flow curve‐MWD constrained conversion has been tested on copolyacetal samples of different molecular weights by determining the χ2surface in the space of the GEX parameters around the minimum. The result is that the constrained inverse method is capable of yielding meaningful results in terms of MWDs and molecular weight averages provided that the viscosity data fall into the commonly accepted experimental error range and cover an adequately wide shear rate range. Discrepancies between the MWDs calculated from rheological measurements and the corresponding GPC data have been detected in the case of the polymers of lower molecular weight. This event probably occurs because of the lack of viscosity data at very high shear rates due to experimental difficulties. In addition, failure can occur in the rheological model utilized for the lower molecular weight samples.

ISSN:0148-6055    

DOI:10.1122/1.550748

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The influence of flow rate and die geometry on the observable flow rate/pressure relationship of a linear high density polyethylene is investigated using a capillary rheometer. The experimental results are applied to an adapted version of the relaxation–oscillation model proposed by Molenaar and Koopmans for describing the oscillating flow regime. The current model allows for a quantitative description of the hysteresis cycle in the oscillating flow regime in terms of the main experimental variables, such as imposed flow rate, reservoir (barrel) volume, and material compressibility.

ISSN:0148-6055    

DOI:10.1122/1.550749

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

We have carried out a set of experiments using anhydrous carbonaceous particles to determine the effect on the electrorheological properties of blending particles of different conductivities. We find that there is a significant dip in the shear stress under an electric field as the concentration of higher conductivity particles is increased, showing that uniformity of the electrical properties among the particles is a most important factor in achieving optimum electrorheological fluid (ERF) performance. We explain this behavior by a simple model based on the strength of particle–particle interactions. Measurements of the dielectric permittivity show that the α parameter of the Cole–Cole expression provides an excellent way to express the degree of uniformity in the electrical properties. This gives a convenient method to evaluate the potential performance of an ERF.

ISSN:0148-6055    

DOI:10.1122/1.550750

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

This article advances ideas and presents experiments on the flow characteristics of concentrated emulsions of Venezuelan bitumen in water plus surfactant. These emulsions are studied under a variety of flow conditions, namely, between rotating cylinders, in a colloid mill, and in pipes. The ideas advanced here concern the modeling of the highly viscous bitumen drops as solid spheres and their fracture under contact forces between neighboring drops, as in comminution, rather than break‐up by hydrodynamic forces. Further, we observe and discuss the local inversion of an emulsion due to local increases of the bitumen fraction induced by flow and the conditions that lead to slip flow, in which the drag is reduced by the formation of a lubricating layer of water at the wall. We believe that the results presented here unveil mechanisms that take place in the pumping and pipelining of oil‐in‐water emulsions and therefore contribute to the understanding of the dynamic stability of these systems.

ISSN:0148-6055    

DOI:10.1122/1.550751

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A flexible and robust approach to nuclear magnetic resonance (NMR) based capillary rheometry has been developed. The precision of the technique has been explored, with particular regard to the capability for measurement of apparent wall slip velocities. The practical implications of alternative NMR rheometry protocols are discussed with regard to robustness and speed. NMR rheometry results are presented for a Newtonian fluid (aqueous 50.4% sucrose), a shear thinning solution (0.2% aqueous xanthan gum), and a particulate system composed of 5–50 μm irregular, soft agar gel particles. In all cases, fully developed Poiseuille flow was studied in a 4‐mm‐i.d., glass capillary. Radial velocity profiles were measured by NMR velocimetry; radial differentiation provides shear rate values, which have been scaled by the associated radial positions and measured pressure drops to determine viscosity as a function of shear rate. Agreement with cone‐and‐plate or parallel plate rheometry has been established for each system. The xanthan solution shows power‐law behavior, and no evidence of significant apparent wall slip. The particulate gel system exhibits power‐law behavior, but with a pronounced apparent wall slip. Apparent slip velocities determined by extrapolation of the NMR velocimetry data to the wall surface and by a Mooney analysis of flow through four different diameter capillaries agree.

ISSN:0148-6055    

DOI:10.1122/1.550752

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The rheological behavior of a 1% w/w commercial hydrophobically modified (hydroxypropyl)guar (HMHPG) in water solution is examined in the presence of a low molecular weight nonionic surfactant. The linear and nonlinear response to steady and oscillatory shear flow, at different surfactant concentrations around the cmc, is mainly compared with the response of a 1% w/w aqueous solution of HMHPG alone, following a rheological approach previously used by the authors. Below the cmc of the free surfactant in water, the rheological results show a reinforcement of the reversible associating network mainly due to an increase of the number of intermolecular hydrophobic associations, with hardly no modification of the average ‘‘strength’’ of the hydrophobic junctions. Above the cmc, the rheological data show that there is a progressive weakening leading to the disruption of the intermolecular hydrophobic junction network. Those results are interpreted in terms of interactions of surfactant molecules with the regions of intermolecular hydrophobic association.

ISSN:0148-6055    

DOI:10.1122/1.550753

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Thermodynamics provides a convenient framework to analyze the electrorheology of suspensions of non‐Brownian, noncolloidal particles in Newtonian carriers. Aided by simple constitutive relations linking suspension architecture to its electrical response, we examine the evolution of the microstructure upon cessation of flow and the linear‐elastic behavior in the preyield region. In the case where the particles and the fluid are both nonconducting, a rigorous proof can be worked out to show that, upon cessation of flow, the suspension microstructure will evolve towards a ‘‘pillared’’ configuration, whereby chains of particles spanning the flow gap are formed. Also, a maximum in the linear‐elastic shear modulus as a function of solid concentration is predicted in this case, under certain conditions. For finite solid and carrier conductivities, where Maxwell–Wagner–Sillar polarization can occur, the results are less rigorous; although some necessary conditions for stability of the pillared structure can still be inferred.

ISSN:0148-6055    

DOI:10.1122/1.550735

出版商:The Society of Rheology    

年代:1996

数据来源: AIP