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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Colloid & polymer science 273 (1995), S. 1156-1162 
    ISSN: 1435-1536
    Keywords: Electrorheological fluids ; colloids ; rheodielectric spectroscopy ; light transmission
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Structural relaxations of an electrorheological fluid (ERF) due to changes in the applied electrical field strength or shear rate are observed on time scales 1 s〈t〈40 000 s. Commercial ERFs consisting of mesoscopic polyurethane particles in a silicone oil matrix were studied by three different experimental techniques in order to obtain and compare the characteristic relaxation times. It is demonstrated that dielectric spectroscopy, viscosimetry and light transmission experiments represent the same results concerning the structural relaxation phenomena of ERFs when electrical fields are applied. The tendency of strong induced dipoles to align the particles in the direction of the field increases the effective dipole moment and therefore Δε, the shear viscosity ν and the amount of light transmitted along the field direction in an ITO/glass sandwich cell. The optical experiment is capable of resolving fast processes within the first 1 ms if large electrical fields are applied. The effects of electrophoresis and shearing, which both counteract the field induced structures, are also addressed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1435-1536
    Keywords: Structure ; phase transitions ; x-ray scattering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The structure of a fully aromatic thermotropic liquid crystalline (LC) copolyester poly-[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenylhydroquinone, andp-hydroxybenzoic acid at a molar ratio of 45/45/10, respectively, was studied at ambient and elevated temperatures by means of x-ray diffraction and differential scanning calorimetry as a function of molecular weight. On heating of PES fibers with fixed ends an irreversible phase separation process takes place above the glass transition point and two different crystalline phases are formed. A model is proposed where the phases are assumed to contain the constituents of the statistical copolymer in different amounts. The relative volume fraction of the two crystalline modifications depends on the molecular weight of the investigated fibers. At higher temperatures the melting of the two crystalline phases and their transition to a LC nematic mesophase is observed.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Colloid & polymer science 273 (1995), S. 848-856 
    ISSN: 1435-1536
    Keywords: Electrorheological fluids ; dielectric spectroscopy ; colloidal suspensions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The technique of rheodielectric spectroscopy is used to investigate the dielectric behaviour of electrorheological fluids (ERF) as a function of external electrical DC-field and/or shear rate. Commercial ERF's consisting of mesoscopic polyurethane particles in a silicone oil matrix were studied. The particles contain a salt which leads to strong dipole moments via the Maxwell-Wagner-Polarization (MWP) if subjected to an external electrical field. In an electrical field the dipoles and, concomittantly, the particles motion become correlated leading to the formation of solid-like structures and significant changes in the viscosity. We demonstrate that dielectric spectroscopy is capable of monitoring the field and shear rate effects in terms of relaxation strength and relaxation time of the MWP. In electrical or shear fields dipole-dipole correlations increase the MWP's relaxation strength, so that we are able to observe structure formation with dielectric spectroscopy, especially the time resolved response of the ERF to changes in the electrical field or the shear rate.
    Type of Medium: Electronic Resource
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