Micellization to gelation of a triblock copolymer in water: Thermoreversibility and scaling

Aqueous solutions of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, Pluronic F108 (PEO133PPO50PEO133), ranging from 1 to 35 wt %, were studied with differential scanning microcalorimetry and rheology. The thermoreversible micellization and gelation were examined through a heating process and a subsequent cooling process at a fixed rate of 1 degreesC/min. The critical micellization temperature (CMT), determined by the onset temperature of the endothermic peak in the heating process, was a decreasing function of the F108 concentration. A small secondary endothermic peak appeared only when the polymer concentration was 22.5 wt % or higher, indicating that there was a sol-gel transition but that the gelation was a nearly athermic process. Upon heating, an abrupt increase was observed in both the dynamic storage modulus (G') and dynamic loss modulus (G) within a narrow temperature range. T-G', the temperature for the transition in G', was a linear decreasing function of the polymer concentration and different from CMT. T-G' tended to approach CMT with an increasing F108 concentration. Beyond this transition, G' reached a plateau, and the plateau increased in height and broadened with the polymer concentration. The value of G' at 70 degreesC (G'(70)) could be approximately scaled with concentration c by G'(70) proportional to c(7.3). In addition, the definition for a gel to obey G' > G was valid only when c was greater than 22.5 wt %, and this was in agreement with the secondary endothermic peak found with differential scanning calorimetry. (C) 2004 Wiley Periodicals, Inc.