Flow in micellar cubic crystals |
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Authors: | Erika Eiser François Molino Grégoire Porte Xavier Pithon |
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Institution: | (1) European Synchrotron Radiation Facility BP 220, F-38043 Grenoble, France e-mail: eiser@esrf.fr, FR;(2) Groupe de Dynamique des Phases Condensées, Université de Montpellier II F-34095 Montpellier Cedex 05, France, FR |
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Abstract: | We investigate and compare the mechanical properties of two micellar cubic crystals. Both are obtained from aqueous triblock-copolymer
solutions of similar block structure, (PEO)76(PPO)29(PEO)76 and (PEO)127(PPO)48(PEO)127, designated Pluronic F68 and F108 respectively. Extensive small angle X-ray scattering experiments under shear and common
rheometry provide the following results: (i) the solid phases of the two polymeric solutions are constituted of micelles that
are ordered in a bcc (F68) and fcc (F108) structure. The overall appearance of both unsheared samples is polycrystalline;
(ii) SAXS showed that both systems undergo a structural reorientation under shear, with the dense planes arranged parallel
to the velocity - vorticity plane, thus facilitating flow; (iii) F68 showed a clear transition in the flow curve, associated
with a textural change, but the F108 system exhibited a continuous evolution; (iv) the bcc crystal appears to have no measurable
yield stress and flows even for low applied stresses. This was in contrast to the fcc sample which showed a clear yield stress,
separating creep and flow regimes.
Received: 30 November 1999/Accepted: 30 November 1999 |
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Keywords: | Micellar cubic phase Shear-banded flow Textural transition |
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