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

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 deg. 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 ≤30 wt%, with the lower critical solution temperature (or critical point) occurring at ∼80 deg. C in 1 M NaCl. At this salt concentration and ≥85 deg. 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 (≤85 deg. C) and turbid (≥85 deg. C) mesophase. We tentatively identified these as the BCC cubic and lamellar (Lα) 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