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  • 1
    Electronic Resource
    Electronic Resource
    Role of director tumbling in the rheology of polymer liquid crystal solutions (1991)
    s.l. : American Chemical Society
    Macromolecules 24 (1991), S. 2546-2555 
    Details
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ma00009a063
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  • 2
    Electronic Resource
    Electronic Resource
    Comparison of optical and mechanical measurements of second normal stress difference relaxation following step strain (1995)
    Springer
    Rheologica acta 34 (1995), S. 221-234 
    Details
    ISSN: 1435-1528
    Keywords: Second normal stress difference ; birefringence ; tube model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Relaxation of the second normal stress difference (N 2) following step strain of a concentrated monodisperse polystyrene solution has been studied using mechanical and optical rheometry. Measurements of normal thrust in a parallel plate geometry are corrected for strain inhomogeneity and combined with independent measurements of the first normal stress difference (N 1) to determine N 2. Optical experiments were performed using a novel configuration where flow birefringence data collected using multiple light paths within the shear plane are combined with the stress-optical law to determine all three independent stress components for shearing deformations. This technique eliminates end effects, and provides an opportunity to oversample the stress tensor and develop consistency checks of experimental data. N 2 is found to be nonzero at all accessible times, and relaxes in roughly constant proportion to N 1. This reflects nonaffine distribution of chain segments, even well within the regime of chain retraction at short times. Data collected with the two techniques are reasonably consistent with each other, and with results of previous studies, generally lying between the predictions of the Doi-Edwards model with and without the independent alignment approximation. The normal stress ratio −N 2/N 1 shows pronounced strain thinning in the nonlinear regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00396013
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular orientation in channel flows of main-chain thermotropic liquid crystalline polymers (2000)
    Springer
    Rheologica acta 39 (2000), S. 247-258 
    Details
    ISSN: 1435-1528
    Keywords: Key words Liquid-crystalline polymer ; Channel flows ; Molecular orientation ; WAXS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract  In situ wide angle X-ray scattering is employed for quantitative measurements of flow-induced molecular orientation in channel flows of thermotropic hydroxypropyl-cellulose (HPC). An extrusion die, constructed to allow X-ray access, generates slit flow, slit flow with superimposed contractions and expansions in cross-section, and slit flow past an obstruction. In slit flow, weak molecular orientation develops slowly with downstream position. Superimposed extension associated with contraction flows leads to a strong enhancement in orientation, which persists with distance further downstream of the contraction. Conversely, transverse extension present in expansion flows generates a bimodal orientation state and substantial reductions in average molecular orientation. These results are compared to earlier measurements on a commercial fully aromatic thermotrope. HPC is found to respond more strongly to superimposed extension, and more weakly to the prevailing inhomogeneous shear flow than the commercial material.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003970000077
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  • 4
    Electronic Resource
    Electronic Resource
    Polydomain model predictions of liquid crystalline polymer orientation in mixed shear/extensional channel flows (2000)
    Springer
    Rheologica acta 39 (2000), S. 259-270 
    Details
    ISSN: 1435-1528
    Keywords: Key words Liquid-crystalline polymer ; Channel flows ; Orientation ; Larson-Doi model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract  The Larson-Doi (LD) polydomain model is used to simulate orientation development along the centerline of slit-expansion and slit-contraction flows of liquid crystalline polymers (LCPs). Orientation is computed using the LD structural evolution equations, subject to an imposed velocity field that accounts for the spatial variation of both shear and extension rates characteristic of this class of flows. Computed axial distributions of orientation averaged through the sample thickness are qualitatively similar to birefringence and X-ray scattering measurements of molecular orientation in similar flows of lyotropic and thermotropic LCPs. In slit-expansion flows, the simulations predict a 90∘ flip in orientation direction near the midplane due to transverse stretching in the expansion region. Far away from the midplane where shear gradients dominate, orientation remains primarily along the flow direction. Within the LD model, tumbling and flow aligning materials respond in a qualitatively similar manner to mixed shear and extension, although tumbling materials are systematically more susceptible to the effects of extension.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003970000078
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  • 5
    Electronic Resource
    Electronic Resource
    Unstable slit flow of a liquid crystalline polymer solution (1997)
    Springer
    Rheologica acta 36 (1997), S. 384-396 
    Details
    ISSN: 1435-1528
    Keywords: Hydroxypropyl-cellulose ; slit flow instability ; birefringence ; x-ray scattering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract A lyotopic solution of 27 wt% hydroxypropylcellulose [HPC] in m-cresol has been studied in pressure-driven slit flow. At high flow rates an instability leads to large wavelength disturbances in fluid structure. A combination of image analysis and time signal processing is used to determine the velocity at which the structural disturbances are convected downstream, which is shown to be equal to the independently measured and predicted centerline velocity. This implies that the disturbance structure is confined near the midplane of the slit flow. Upstream of the onset point of the wavy fluid structures, the fluid exhibits unusual optical properties when viewed between crossed polarizers that are rotated relative to the flow direction. Specifically, the optical properties indicate that there must be some variation in the macroscopic optical axis of the sample as light passes through the slit flow. A discrete optical model consisting of birefringent elements twisted away from and back to the flow direction as a function of depth in the sample is able to predict the essential optical characteristics; however, independent x-ray scattering measurements show that the macroscopic molecular alignment is along the flow direction. The wavy textures apparently emerge as a result of an inhomogeneous transition of orientation back to the flow direction, trapping thin bands of fluid in the twisted configuration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00396325
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  • 6
    Electronic Resource
    Electronic Resource
    Field-induced anisotropy in concentrated systems of rigid particles and macromolecules (1991)
    Springer
    Journal of statistical physics 62 (1991), S. 1025-1039 
    Details
    ISSN: 1572-9613
    Keywords: Concentrated suspensions ; suspensions ; liquid crystals ; polymer liquid crystals ; electric field orientation ; hydrodynamic orientation ; dichroism ; polarimetry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This paper presents experimental results on the use of spectroscopic optical polarimetry to study structure in dense systems of rigid particles and rigid polymer liquid crystals. These measurements probe microstructural anisotropy induced by the application of electric fields in the case of dense suspensions of rigid spheres, or flow fields in the case of polymer liquid crystals. It is demonstrated that conservative linear dichroism can measure moments of the particle pair distribution function in dense suspensions. In liquid crystals, the dichroism is a result of field-induced anisotropy in the defect structure of the material.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01128175
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  • 7
    Electronic Resource
    Electronic Resource
    Second normal stress difference relaxation in a linear polymer melt following step-strain (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 2671-2675 
    Details
    ISSN: 0887-6266
    Keywords: second normal stress difference ; melt rheology ; Doi-Edwards model ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Flow birefringence is used to study stress relaxation following step strain deformations of a well entangled polyisoprene melt. The optical method employs multiple light paths to fully sample the three-dimensional stress tensor, and hence provides measurements of all three independent shear material functions (shear stress and both first and second normal stress differences). Experiments are complicated by multiple orders in retardation. However, data show that the ratio of the second to the first normal stress difference is a strain thinning function, with magnitude intermediate between the predictions of the Doi-Edwards model with and without independent alignment. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2671-2675, 1998
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers (1998)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 199 (1998), S. 471-488 
    Details
    ISSN: 1022-1352
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: This article summarizes recent experiments relating measurements of molecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques are flow birefringence and x-ray scattering. Since LCPs usually exhibit a “polydomain” texture, measurements of flow-induced orientation reflect both the local distribution of molecular orientation around the director and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Deborah number to flow alignment at high Deborah number. Rheo-optical measurements of the full refractive index tensor in PBG allow the orientation predictions of microstructural theories for LCP rheology to be quantitatively tested. At low shear rates the two model materials differ: PBG solutions exhibit significant orientation, while HPC solutions show little orientation. This is correlated with the presence of so-called “Region I” shear thinning in HPC solutions. Conversely, in PBG solutions of high concentration, x-ray scattering measurements demonstrate that Region I arises from the presence of a hexagonal phase. The model systems are further differentiated in relaxation. Molecular orientation increases in PBG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of dynamic properties. Finally, structural investigations of a PBG solution and a nematic surfactant solution during step changes in shear conditions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 9
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    Microphase Separation and Shear Alignment of Gradient Copolymers: Melt Rheology and Small-Angle X-Ray Scattering Analysis (2008)
    American Chemical Society
    Details
    Publication Date: 2008-08-01
    Print ISSN: 0024-9297
    Electronic ISSN: 1520-5835
    Topics: Chemistry and Pharmacology , Physics
    Published by American Chemical Society
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  • 10
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    Thermoreversible, Epitaxialfcc↔bccTransitions in Block Copolymer Solutions (2002)
    American Physical Society
    Details
    Publication Date: 2002-10-31
    Print ISSN: 0031-9007
    Electronic ISSN: 1079-7114
    Topics: Physics
    Published by American Physical Society
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