Effect of sodium chloride on association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solutions.

The effect of sodium chloride (NaCl) upon the thermally induced association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer, Pluronic P103, has been investigated using pyrene fluorescence spectroscopy. The critical micellization temperature (CMT) of Pluronic P103 in aqueous solution is decreased by the addition of NaCl. The standard enthalpy and entropy of micellization for Pluronic P103 in water are increased in the presence of small amounts of NaCl, but further addition of NaCl decreases the standard enthalpy and entropy of micellization. The I1/I3 ratio of pyrene in aqueous Pluronic P103 solutions at temperature below the CMT decreases with increases of NaCl concentration, which is related to the decrease of PPO solubility. The decrease in polarity of the PPO shifts the CMT toward lower temperature.