Primary normal-stress difference for two liquid crystalline copolyesters

Gotsis, A. D.; Baird, Donald G.

doi:10.1007/bf01357954

Cited by 41 publications

(12 citation statements)

References 21 publications

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“…This is an important experimental finding in that most flows of interest are, in fact, inhomogeneous and in the case of liquid crystals of low molecular weight, such an identity does not occur [25]. A similar observation has recently been reported by Gotsis and Baird [26] for thermotropic polymeric liquid crystals; their data are for a parallel plate geometry with the gap spacing varying from 0.0125 to 0.1 cm. The ability of the 5-constant continuum theory of Ericksen-Leslie-Parodi [1][2][3]25] to predict effects of flow field inhomogeneity, unlike the Doi formulation (which is valid only for homogeneous flow fields), thus does not seem to be a major advantage where polymeric liquid crystals are concerned.…”

Section: Discussionsupporting

confidence: 62%

The rheology of polymeric liquid crystals

Doraiswamy

Metzner

1986

Rheol Acta

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The molecular theory of Doi has been used as a framework to characterize the rheological behavior of polymeric liquid crystals at the low deformation rates for which it was derived, and an appropriate extension for high deformation rates is presented. The essential physics behind the Doi formulation has, however, been retained in its entirety. The resulting four-parameter equation enables prediction of the shearing behavior at low and high deformation rates, of the stress in extensional flows, of the isotropic-anisotropic phase transition and of the molecular orientation. Extensional data over nearly three decades of elongation rate (10 . 2 -101 ) and shearing data over six decades of shear rate (10 -2-104 ) have been correlated using this analysis. Experimental data are presented for both homogeneous and inhomogeneous shearing stress fields. For the latter, a 20-fold range of capillary tube diameters has been employed and no effects of system geometry or the inhomogeneity of the flow-field are observed. Such an independence of the rheological properties from these effects does not occur for low molecular weight liquid crystals and this is, perhaps, the first time this has been reported for polymeric lyotropic liquid crystals; the physical basis for this major difference is discussed briefly.

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Section: Discussionsupporting

confidence: 62%

The rheology of polymeric liquid crystals

Doraiswamy

Metzner

1986

Rheol Acta

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Section: Introductionsupporting

confidence: 76%

“…Similar behavior was reported for 80 mol % PHB/PET but the transition temperature was 328 .C. 8 The point of the above work is that the stress growth behavior of thermotropic LCP's is highly dependent on the temperature at which the measurements are made.The purpose of this work is to investigate further the transient behavior of thermotropic LCP's. In particular, we would like to know whether there is anything unique about the rheology of fluids composed of rigid chain molecules.…”

supporting

confidence: 76%

“…There was no observed dependence of the results on the cone angle. However, in the case of the parallel plate device Gotsis and Baird 8 have reported experiments in which the steady state normal stresses were gap dependent for a certain temperature range. As long as the samples are properly dried, they were thermally stable for periods up to 45 min.…”

Section: Rheological Measurementsmentioning

confidence: 99%

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Transient flow of thermotropic liquid crystalline polymers in step strain experiments

Done¹,

Baird²

1990

Journal of Rheology

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SynopsisThe transient response of two thermotropic liquid crystalline polymers (a copolyester of 60 mol % p-hydroxybenzoic acid, PHB, and 40 mol % poly(ethylene terephthalate), PET, and a copolyester of PHB and 2-hydroxy-6-naphthoic acid, following step strains up to 20 strain units was measured. The relaxation curves typically show an initially rapid decay followed by a long relaxation tail. The lower the temperature, the more remarkable is the length of the long relaxation tail. This behavior makes the LCP uniquely different from most flexible chain polymers such as PET and polystyrene, which show a relaxation modulus which decreases continuously having no discontinuity in the slope. This behavior is probably due to unmelted solid phase which exists in the melt up to quite high temperatures. When the LCP's are heated to temperatures well above their melting points as determined by DSC, then the long relaxation tails are eliminated. Furthermore, on cooling the sample rapidly down from a temperature well above the melting point the relaxation modulus resembles that at the higher temperature as a result of the supercooling effect. The relaxation modulus was also determined for samples subjected to lubricated squeezing flow. Whereas for polystyrene the relaxation modulus determined in lubricated squeezing flow was equal to that determined in step shear strain experiments, this was not the case for the LCP's. It is not known whether the behavior reported here is common to nematic LCP's or to the multiphase structure (crystallites, nematic phase, isotropic phase) which might be present. INTRODUCTIONThermotropic liquid crystalline polymers (LCP's) have been of considerable interest in the last few years because they can be processed to form high modulus materials. For example, the flexural modulus of Copyright by the American Institute of Physics (AIP). Done, D; Baird, DG, "transient flow of thermotropic liquidcrystalline polymers in step strain experiments," J. Rheol. 34, 749 (1990); http://dx.doi.org/10.1122/1.550149 750 DONE AND BAIRD injection molded plaques can exceed that found for fiber reinforced materials. 1 Furthermore, the modulus of fibers processed from thermotropic LCP's is similar to that reported for steel.' The exceptional properties are related to the high degree of molecular orientation generated during melt processing. Hence, it is apparent that polymer systems with liquid crystalline order (LCO) can be oriented much more readily than isotropic polymer systems.It is suggested by theories such as those of Leslie and Ericksen 3 ,4 and Doi 5 that the change in transient rheological properties is due to changes in the orientation of the rod-like molecules. Although under some conditions it is predicted by Ericksen's theory that the rod-like molecules will tumble, in general it is predicted that the molecules will orient in both shear and extensional flow. In studies by Viola and Baird 6 it was found for a thermotropic copolyester that in isothermal shear flow there was no evidence of molecular orienta...

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“…If the viscosity decreases as the shear rate is increased, the fluid is called shear‐thinning. Liquid‐crystal polymers usually exhibit shear‐thinning behavior 32–34. They can also exhibit Newtonian fluid behavior over a narrow range of low shear rates 32–34.…”

Section: Resultsmentioning

confidence: 99%

Effects of carbon fillers on the rheology of highly filled liquid‐crystal polymer based resins

King

Tambling

Morrison

et al. 2008

J of Applied Polymer Sci

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Adding conductive carbon fillers to insulating resins increases the composite electrical and thermal conductivity. Often, enough of a single type of carbon filler is added to achieve the desired conductivity while still allowing the material to be molded into a bipolar plate for a fuel cell. In this study, various amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid-crystal polymer. The rheological properties of the resulting single-filler composites were measured. In addition, the rheological properties of composites containing combinations of different carbon fillers were studied via a factorial design. In all cases, the viscosity increased with increasing filler volume fraction and followed a shear-thinning power-law model. The factorial design results indicated that each of the single fillers and all the filler combinations caused a statistically significant increase in the composite viscosity when compared at a shear rate of 500 s 21 or at a stress of 10 5 Pa. For composites containing synthetic graphite particles and/or carbon fiber, the viscosity variation with the volume fraction of carbon followed a modified Maron-Pierce equation. When compared at a constant volume fraction of carbon, composites containing carbon black showed viscosity enhancement above and beyond that shown by the other composites.

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Primary normal-stress difference for two liquid crystalline copolyesters

Cited by 41 publications

References 21 publications

The rheology of polymeric liquid crystals

The rheology of polymeric liquid crystals

Transient flow of thermotropic liquid crystalline polymers in step strain experiments

Effects of carbon fillers on the rheology of highly filled liquid‐crystal polymer based resins

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