MICELLIZATION OF POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK COPOLYMERS IN AQUEOUS-SOLUTIONS - THERMODYNAMICS OF COPOLYMER ASSOCIATION

The critical micellization temperature (cmt) and critical micellizatio n concentration (cmc) values of 12 Pluronic poly(ethylene oxide)-poly( propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, covering a wide range of molecular weights (2900-14 600) and PPO/PEO r atios (0.19-1.79), were determined employing a dye solubilization meth od. A closed association model was found to describe adequately the co polymer micellization process for the majority of the Pluronics and us ed to obtain the standard free energies (DELTAG-degrees), enthalpies ( DELTAH-degrees), and entropies (DELTAS-degrees) of micellization. It w as determined that the micellization process is entropy-driven and has an endothermic micellization enthalpy. The hydrophobic part of the Pl uronics, PPO, was responsible for the micellization, apparently due to diminishing hydrogen bonding between water and PPO with increasing te mperature. The cmc dependence on temperature and size of headgroup (PE O) of Pluronics follows a similar trend with lower molecular weight C( i)E(j) nonionic surfactants, the effect of temperature being more pron ounced with the Pluronics. The PEO-PPO-PEO block copolymers were compa red to PPO-PEO-PPO block and PEO-PPO random copolymers, in an attempt to probe the effect of molecular architecture in the formation of mice lles. No micelles were observed in aqueous PPO-PEO-PPO block copolymer solutions with increasing temperature, up to the cloud point.