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Effect of salt on the phase behaviour of F68 triblock PEO/PPO/PEO copolymer

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Original languageEnglish
Pages (from-to)4461-4470
Number of pages10
JournalJournal of Physics: Condensed Matter
Issue number19
Publication statusPublished - 17 May 2006


We studied the effect of sodium chloride on the phase behaviour of the symmetric triblock copolymer F68 in water using direct visual observation, viscometry, small-angle x-ray scattering and microscopy. We found that salt has a destabilizing effect. The transitions from unimer to micellar solution and from micellar solution to BCC micellar crystal occurred some 20 degrees C lower in 1 M NaCl than in the salt-free system. Salt also induced a region of liquid liquid coexistence in the phase diagram at F68 concentrations less than or similar to 30 wt%, with the lower critical solution temperature (or critical point) occurring at similar to 80 degrees C in 1 M NaCl. At this salt concentration and similar to 85 degrees C, liquid-liquid phase separation becomes metastable. A homogeneous micellar solution at this temperature first separated into coexisting dilute and dense micellar liquids, and then the dense liquid would transform into a transparent (less than or similar to 85 degrees C) and turbid (greater than or similar to 85 degrees C) mesophase. We tentatively identified these as the BCC cubic and lamellar (L-alpha) phases respectively. A more complex sequence of observations at a Pluronic concentration of 34 wt% confirms this picture. A phase diagram for F68 in 1 M NaCl consistent with all of our observations is proposed, as well as a 'rule of thumb' for interpreting the effect of salt on Pluronic phase behaviour.

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