Effect of sodium chloride upon micellization and phase separation transitions in aqueous solutions of triblock copolymers: A high-sensitivity differential scanning calorimetry study

The effect of sodium chloride upon the thermally induced transitions in aqueous solutions, of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymers, has been investigated using high-sensitivity differential scanning calorimetry (HSDSC). The addition of salt to these systems lowers the onset temperature of micellization (critical micellization temperature-CMT). Moreover the accompanying decrease in phase separation temperature (cloud point) has provided an opportunity to investigate the calorimetric details of phase separation. The temperature region over which micellization occurs, in agreement with previously reported findings, is broad and indicates that micelles and unimers coexist over an extended temperature range. The cloud paint on the other hand is characterized, calorimetrically, by a narrow and at times complex peak, indicating that the cooperativity of the transition is very high. For some systems, most notably P235 (EO27-PO39-EO27), a third transition-of low enthalpy-occurs at temperatures intermediate between the micellization and cloud point transitions. Extensive literature evidence suggests that this transition represents a change in the shape of the micelles from spheres to prolate ellipsoids (rods).