摘要:

Nuclear magnetic resonance imaging was used to measure fluid velocity and fluid fraction in suspensions flowing into an abrupt four-to-one contraction in pipe diameter, through a section of smaller diameter pipe, and out of an abrupt expansion back to the original pipe size. Suspensions of 50% by volume of particles in a Newtonian liquid were forced to flow by a plunger moving at a constant, slow velocity. Two sizes (100 and 675 μm diameter) of suspended spheres were studied. Conditions were such that buoyant, inertial, Brownian, and surface forces could be assumed to be negligibly small. Little change in particle concentration was seen in the region of the contraction until the plunger was within about one pipe diameter of the contraction. The particles in the small diameter section of pipe migrated toward the pipe axis, the region of lowest shear rate. Particle concentration varied downstream of the pipe expansion, especially in a suspension of the larger particles. Over time, particles were partially swept out of the region immediately downstream of the expansion joint. Although Reynolds numbers based on average suspension properties were identical in the two suspensions, the velocity fields in the expansion region differed, showing that demixing may markedly influence the downstream flow field in systems with complex geometry.

ISSN:0148-6055    

DOI:10.1122/1.550865

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The influence of molecular weight on the ordering processes and linear viscoelastic properties of a series of thermotropic main-chain liquid crystalline polymers has been investigated. The polymers are wholly aromatic copolyesters based on random units of 75 mol % 1,4-hydroxybenzoic acid (B) and 25 mol% 2,6-hydroxynaphthoic acid (N). The thermal characterization, performed between−50 °Cand 360 °C, showed that annealing below 290 °C gives rise to a secondary endotherm. However, only one endotherm is observed when annealing is carried out above 290 °C, showing that all transitions are completed by 310 °C. Hot-stage wide-angle x-ray scattering demonstrates that the second endotherm is associated with a solid-solid transformation from pseudohexagonal to orthorhombic. The orthorhombic phase, found on slow heating or annealing below the melting point, has a higher melting point than the pseudohexagonal phase. The increase in melting point which can result from such thermal treatments is avoided in all the experiments reported here by comparatively rapid heating of the specimens into the melt.In situoptical microscopy and x-ray scattering measurements show that the “as-molded” samples for rheological experiments exhibit preferred orientation, which is associated with their mechanical history. However, holding the sample in the molten state over periods of time leads to a relaxation of the degree of orientation, until a macroscopically unoriented (textured) state is obtained. This reduction of degree of orientation is correlated with an increase of the complex viscosity, where a plateau value is reached in the final unoriented state. The rheological characterization, on textured samples, show that B–N copolyesters exhibit a linear viscoelastic (LVE) regime similar to that observed in common flexible chain polymers. However, it is also found that this linearity extends only up to strains of about 10%, and is independent of the molecular weight. Dynamic oscillatory experiments in the LVE regime reveal a rubberlike region (minimum in delta loss angle), suggesting that the B–N thermotropic copolyesters behave like lightly cross-linked materials, adding thus support to the elastic network hypothesis.

ISSN:0148-6055    

DOI:10.1122/1.550820

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The effects of a large strain (600%) reciprocating steady shear on the lamellar orientation and the order–disorder transition of a nearly symmetric poly(ethylene propylene)–poly(ethylethylene)–poly(ethylene propylene) (PEP–PEE–PEP) triblock copolymer have been studied using small-angle neutron scattering and rheological measurements. Shearing in the ordered state produced parallel lamellae under all steady-state conditions investigated, and the lamellar-to-disorder transition temperature decreased dramatically with increasing shear rate. Both phenomena are qualitatively different from the documented behavior of PEP–PEE diblock copolymers. However, cooling the disordered material while shearing at a relatively low deformation rate initially induces perpendicular lamellae, analogous to the diblock response. These observations are considered in the context of theory and other experimental studies.

ISSN:0148-6055    

DOI:10.1122/1.550819

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We have used fluorescence microscopy to observe the effect of shear on rigid two-dimensional structures residing in a Langmuir monolayer. Monolayers of 12-NBD stearic acid undergo a two-dimensional liquid-to-solid transition on the water surface. The domains of the solid phase that form in the coexistence region are elongated (needle-shaped) crystallites. We observe the effect of shear within the monolayer in two geometries: two-dimensional Poiseuille flow and simple shear flow created by moving bands. The technique allows us to determine the average orientational order parameter as well as the details of the rotational kinematics, which are, as expected, well-described by a Jeffery orbit. We propose that this technique of direct observation in Langmuir monolayers will be a useful method for the study of systems of rigid particles under flow.

ISSN:0148-6055    

DOI:10.1122/1.550814

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

We report experimental results on the deformation of a single drop suspended in a medium under uniaxial elongational flow along the central axis of a converging conical channel made of Plexiglas. Both the drop and the continuous phases consist of constant viscosity elastic fluids, so-called Boger fluids. This study reveals several interesting features about the role played by both the drop and matrix elasticities on the drop deformability. In a given matrix fluid, the drop deformation decreases as its elasticity increases. For a given drop fluid, the matrix elasticity has the opposite effect: the drop deformation increases with increasing matrix elasticity. An empirical relation between the drop and matrix deformations is established as a function of the drop and matrix characteristic elastic times.

ISSN:0148-6055    

DOI:10.1122/1.550853

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The linear viscoelastic behavior of various polydisperse linear polymers in the melt is used to predict their average molecular weights and polydispersity index. The method is based on simplified molecular dynamics and has been previously shown to enable a correct description of the dynamic moduli of polypropylenes from the knowledge of their molecular weight distribution (MWD). This so-called forward calculation only requires a few parameters, namely the scaling law for the zero-shear viscosity of narrow fractionsη0=f(M),the plateau modulusGN0,and the value of the molecular weight between entanglementsMe.The main goal of the present work is to find a solution to the “inverse” problem. To avoid the problem of becoming ill-posed, the shape of the MWD has to be prescribed. Using the assumption of a typical logarithmic bell-shaped Wesslau MWD, the method has been proven to be successful for the recovery of the weight average molecular weight and of the polydispersity index of many linear polymers in a large range of molecular weights and polydispersity indices. Rough estimates ofMzhave been obtained for well characterized polypropylenes by the use of a generalized exponential distribution (GEX). Attention has also been focused on some requirements for the frequency window which are necessary for reasonable accuracy of the values of the parametersMwandIp.It was found that, because of tube renewal and constraint release, three or four decades is generally sufficiently for a wide range which can be easily achieved with routine rheological dynamic measurements.

ISSN:0148-6055    

DOI:10.1122/1.550815

出版商:The Society of Rheology    

年代:1997

数据来源: AIP