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

Abstract

We studied the effect of sodium chloride on the phase behaviour of the symmetric triblockcopolymer F68 in water using direct visual observation, viscometry, small-angle x-rayscattering and microscopy. We found that salt has a destabilizing effect. The transitions fromunimer to micellar solution and from micellar solution to BCC micellar crystal occurred some20 °C lower in 1 M NaCl than in the salt-free system. Salt also induced a region of liquid–liquidcoexistence in the phase diagram at F68 concentrations wt%, with the lower critical solution temperature (or critical point) occurring at∼80 °C in 1 M NaCl. At this salt concentration and , liquid–liquid phase separation becomes metastable. A homogeneous micellar solution atthis temperature first separated into coexisting dilute and dense micellar liquids, and thenthe dense liquid would transform into a transparent () and turbid () mesophase. We tentatively identified these as the BCC cubic and lamellar(Lα) phases respectively. A more complex sequence of observations at a Pluronic concentrationof 34 wt% confirms this picture. A phase diagram for F68 in 1 M NaCl consistent with all ofour observations is proposed, as well as a ‘rule of thumb’ for interpreting the effect of salton Pluronic phase behaviour.