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Nov 1990

Volume 34, Issue 8, pp. 1199-1414


Processing preshear and orientation effects on the rheology of an LCP melt

Paul D. Frayer and Paul J. Huspeni

J. Rheol. 34, 1199 (1990); http://dx.doi.org/10.1122/1.550082 (17 pages)

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Independent LCP melt rheological measurements were performed using both capillary and slit rheometer die equipment. The latter was attached to the end of a laboratory extruder so that the attending rheological measurements were obtained under actual processing conditions. The viscosity–shear rate behavior determined by these two methods for the subject LCP melts was significantly different. For a specified melt temperature and shear rate, the viscosity measured by the slit rheometer was as much as 10× lower than that measured by capillary rheometry. The above measured viscosity differences appear real since corresponding viscosity measurements on conventional polymers such as LLDPE and HDPE by these two methods produced equivalent results. Thus, the rheology of these LCP melts is strongly dependent upon preshear history, die geometry, and residence time. This dependence is attributed to the highly anisotropic nature of the LCP melt, the tendency of the polymer ‘‘domains’’ or chains to orient when processed and a long relaxation time for the oriented structure. Knowledge of LCP rheological behavior under actual processing conditions is of paramount importance to properly design extruder or injection molding screws, as well as corresponding dies and molds.
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62.60.+v Acoustical properties of liquids
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
46.80.+j Measurement methods and techniques in continuum mechanics of solids
46.35.+z Viscoelasticity, plasticity, viscoplasticity

Nematic phase of rodlike polymers. I. Prediction of transient behavior at high shear rates

G. Marrucci and P. L. Maffettone

J. Rheol. 34, 1217 (1990); http://dx.doi.org/10.1122/1.550083 (14 pages) | Cited 7 times

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The two‐dimensional model introduced in a previous paper to describe the nematic phase of rodlike polymers is used to obtain predictions of transient rheological response at high shear rates. First, the transient behavior of a single domain is considered. It is found that, between the tumbling and nontumbling regimes, an intermediate range of shear rates exists where the director continuously oscillates between two orientations close to the shear direction, a result independently obtained in a recent work by Larson, who has considered the full three‐dimensional case. Polydomain structure, typical of polymeric liquid crystals, is then considered. In this paper, results for this case are given for the nontumbling regime only, whereas the tumbling and oscillating regimes are considered in the next paper.
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62.60.+v Acoustical properties of liquids
61.30.Cz Molecular and microscopic models and theories of liquid crystal structure
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Nematic phase of rodlike polymers. II. Polydomain predictions in the tumbling regime

G. Marrucci and P. L. Maffettone

J. Rheol. 34, 1231 (1990); http://dx.doi.org/10.1122/1.550084 (14 pages) | Cited 8 times

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The two‐dimensional model previously considered is here used to obtain predictions in the complex situation of the tumbling regime which prevails at low shear rates. Although each domain is permanently in a time‐dependent, periodic regime, the macroscopic response can be stationary in time because of the well‐known polydomain structure. First, the average steady rheological response for this situation is calculated by taking a polydomain structure which neglects interdomain interactions. Although the steady state predictions thus obtained favorably compare with the experimental results throughout the range of shear rates, the transient start‐up responses do not, because of the crucial role played by the interactions in such a case. These interactions, due to Frank elasticity, are then introduced in the model in the simplest possible way, i.e., by use of a mean field potential. Under this assumption, also the predictions of transient behavior in the tumbling regime show the correct qualitative features.
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62.60.+v Acoustical properties of liquids
61.30.Cz Molecular and microscopic models and theories of liquid crystal structure

The effect of extrusion operating conditions on the on‐line apparent viscosity of 98% Amylopectin (Amioca) and 70% Amylose (Hylon 7) corn starches during extrusion

L. S. Lai and J. L. Kokini

J. Rheol. 34, 1245 (1990); http://dx.doi.org/10.1122/1.550085 (22 pages)

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The effects of extrusion processing variables (such as melt tempeature, screw speed, and moisture content), Amylopectin/Amylose ratio as well as extent of starch conversion on the apparent viscosity of starch dough have been studied through the application of a capillary die and a slit die viscometers in conjunction with a single screw extruder, covering a shear rate range of at least two orders of magnitude. The results suggested that increasing melt temperature or moisture content reduced the viscosity. Both shear force and thermal energy contribute to starch conversion. Higher temperature and lower moisture content gave rise to a higher extent of starch conversion. 70% Amylose starch (Hylon 7) gave a higher overall viscosity and was gelatinized less than 98% Amylopectin starch (Amioca), which was attributed to the structural difference between these two corn starches. A simple linear modification of the temperature‐dependent constant and the moisture‐dependent constant in Harper’s viscosity model was shown to fit the data better. Expansion of this model including a power function dependence of viscosity on the extent of starch conversion further improved the prediction of viscosity, significantly in the case of high Amylose corn starch.
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46.35.+z Viscoelasticity, plasticity, viscoplasticity

An investigation of the shear thickening and antithixotropic behavior of concentrated coal–water dispersions

D. S. Keller and D. V. Keller

J. Rheol. 34, 1267 (1990); http://dx.doi.org/10.1122/1.550086 (25 pages) | Cited 2 times

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A study of the flow characteristics of a deflocculated, shear thickening, coal–water suspension indicated a significant antithixotropic behavior. The effect of applied shear rate, particle concentration, and shear zone dimensions were examined using a Couette flow viscometer. The dispersion was composed of refined coal particles with mass median diameter of 2.9 μm suspended in water with sufficient anionic dispersant. The rheological properties of eight dilutions of this suspension, from 47.7 to 54.5 vol %, were studied in detail. Shear stress was observed as a function of shear rate in ramp tests in which γ̇ was increased from 10 to 1000 s−1, and as a function of time at several constant shear rates. All tests were conducted using three rotors to test the effect of annular gap width. The characteristics of the reversible, shear thickening with time phenomenon (antithixotropy) that occurs when sufficient shear rate is applied, is described in terms of three regions. Quantitative relationships that allow the determination of stress as a function of shear rate, particle concentration, and shearing time are developed for one of the rotors. An examination of the anisotropic nature of this phenomenon is also presented.
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62.10.+s Mechanical properties of liquids
62.60.+v Acoustical properties of liquids

Transient and steady‐state rheology of a liquid crystalline hydroxypropylcellulose solution

Nino Grizzuti, Silvana Cavella, and Paolo Cicarelli

J. Rheol. 34, 1293 (1990); http://dx.doi.org/10.1122/1.550139 (18 pages) | Cited 12 times

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Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.
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62.60.+v Acoustical properties of liquids

The measurement of interfacial tension in polymer/polymer systems: The breaking thread method

P. H. M. Elemans, J. M. H. Janssen, and H. E. H. Meijer

J. Rheol. 34, 1311 (1990); http://dx.doi.org/10.1122/1.550087 (15 pages) | Cited 9 times

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When blending incompatible polymers in the melt, the resulting morphology is strongly dependent on the interfacial tension. One stage of the mixing process is now used to determine this interfacial tension: in the absence of an overall flow field, extended liquid threads in a liquid matrix exhibit sinusoidal disturbances which cause the threads to disintegrate into lines of droplets. From the growth rate of these disturbances, the interfacial tension between the thread phase and the matrix phase is calculated. For molten polymers, this so‐called ‘‘breaking thread method’’ is relatively fast and simple since it does not require density data for the two phases. Upon addition of a diblock copolymer to the thread phase, a considerable decrease in interfacial tension is measured.
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68.03.Cd Surface tension and related phenomena
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials

Technical note: Correlation between the Hamaker constant and the flow behavior index of a concentrated suspension

S. C. Tsai and K. Ghazimorad

J. Rheol. 34, 1327 (1990); http://dx.doi.org/10.1122/1.550088 (6 pages)

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Abstract Unavailable
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62.60.+v Acoustical properties of liquids

Addendum: Rheological behavior of polydimethylsiloxane/polyoxyethylene blends in the melt. Emulsion model of two viscoelastic liquids [J. Rheol. 34, 193 (1990)]

D. Graebling and R. Muller

J. Rheol. 34, 1333 (1990); http://dx.doi.org/10.1122/1.550089 (1 page)

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Abstract Unavailable
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62.60.+v Acoustical properties of liquids

Lubricant elasticity and the performance of dynamically loaded short journal bearings

A. Rastogi and R. K. Gupta

J. Rheol. 34, 1337 (1990); http://dx.doi.org/10.1122/1.550090 (20 pages) | Cited 3 times

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A theoretical analysis is developed for flow of lubricants in a dynamically loaded short journal bearing. The upper convected Maxwell model is chosen to study the influence of lubricant elasticity on the motion of the shaft center within the bearing. An order of magnitude analysis reveals that the fluid relaxation time does not influence bearing performance for steady loads. For time varying loads, however, numerical calculations show that the pressure distribution, the minimum oil film thickness, the attitude angle, and the shaft orbit are all significantly affected by the presence of fluid elasticity.
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81.40.Pq Friction, lubrication, and wear

Elastohydrodynamic lubrication of point contacts with a Ree–Eyring fluid: Film thickness and traction

F. Vergne, J. P. Chaomleffel, and G. Dalmaz

J. Rheol. 34, 1357 (1990); http://dx.doi.org/10.1122/1.550140 (15 pages) | Cited 1 time

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The study of elastohydrodynamic (EHD) contacts is primarily concerned with the film thickness and the traction force developed between two, heavily loaded, solid surfaces in relative motion with each other. The aim of this paper is to calculate simultaneously film thickness, pressure, and shear stress distributions in isothermal EHD point contacts lubricated with a Ree–Eyring fluid under steady state conditions. A numerical method is developed to solve the EHD point contact problem for moderate and high loads. Theoretical results show that film thicknesses do not vary from those computed for a Newtonian fluid, except in the cases of very small values of the Eyring reference stress which lead to a significant reduction in film thickness. Calculated traction forces in rolling–sliding contacts are similar to those measured experimentally. The theory is compared with experiments made on a rolling–sliding point contact apparatus in which film thicknesses and traction forces are measured simultaneously. Good agreement is found.
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81.40.Pq Friction, lubrication, and wear

Flow of multigrade motor oils in an extensional flow field

R. K. Gupta, R. C. Chan, and A. K. Deysarkar

J. Rheol. 34, 1373 (1990); http://dx.doi.org/10.1122/1.550091 (14 pages) | Cited 2 times

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A spinline viscometer was used to measure the apparent extensional viscosity of eight motor oils consisting of two SAE grades at two different temperatures. Oils corresponding to a 5W30 grade appeared to be slightly more elastic than 10W40 oils in extension. However, none of the oils was very elastic; measured Trouton ratios were less than 20 in each case. In fact, the 10W40 oils exhibited Newtonian behavior at low stretch rates. All the oils became slightly less elastic as the temperature of measurement was raised. In addition, an oil formulated using a synthetic base stock was found to behave differently from oils containing solvent extracted or hydrotreated base stocks.
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66.20.-d Viscosity of liquids; diffusive momentum transport
62.10.+s Mechanical properties of liquids
81.40.Pq Friction, lubrication, and wear

High pressure rheology for high pressure lubrication: A review

D. Berthe and Ph. Vergne

J. Rheol. 34, 1387 (1990); http://dx.doi.org/10.1122/1.550092 (28 pages) | Cited 2 times

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High pressure lubrication, also called elastohydrodynamic lubrication (EHL) is a branch of tribology which has been considerably developed since 1960. Mechanisms running under this regime are characterized by high contact pressures (≳0.7 GPa) and very high values (up to 4 GPa) are now found. The advances have been realized in the fields of metallurgy, tooling, surface treatments and roughness control, lubricant elaboration, and fluid rheology. In most EHL problems, the main parameters governing the global contact behavior are the fluid film thickness which separates the solids and the dissipated shear energy which is usually expressed by the traction coefficient. Aside from solid properties, kinematic and geometric data, the lubricant high pressure rheology controls these important parameters. In this paper, we show how high pressure rheology can answer EHL problems. To illustrate this, we present experimental results on high pressure viscometry and high pressure rheometry. Simultaneously, we mention analytical and numerical work in which the EHD flow is modeled, showing the historical evolution of shear models, from Newtonian behavior to the Montrose et al. model. Other works related to EHL needs are presented, for instance, ultrasonic propagation in lubricants. The existence of phase transition effects in EHL is considered.
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46.55.+d Tribology and mechanical contacts
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