Influence of 1-Pentanol on the Micellization of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymers in Aqueous Solutions

Abstract

The critical micellization temperature (CMT) of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) block copolymers in water has been determined by using the temperature-dependent fluorescence intensity of pyrene. The CMT of Pluronic polymers in aqueous solutions is decreased by the addition of 1-pentanol. The thermodynamic parameters of micellization are obtained from the temperature derivatives of the ln(XCMC) (CMC = critical micellization concentration) values. The standard enthalpy and entropy of micellization for Pluronic polymers in water are increased with an increase of the 1-pentanol content. The more PEO component in the Pluronic polymer, the higher the changed values of micellization enthalpy and entropy. The solubilization of 1-pentanol in the micelles was used to explain the relationship between the changed values of thermodynamic parameters of micellization and the composition of Pluronic polymers, in which the water in the micellar cores is replaced by the solubilized 1-pentanol. The I1/I3 ratio of pyrene in aqueous Pluronic polymer solutions at temperatures below the CMT is decreased with an increase of the 1-pentanol concentration. A less polar microenvironment will be formed in the regions of unexpanded PPO coils when more 1-pentanol is added to the aqueous Pluronic polymer solution. The decrease of PPO solubility will result in the shift of CMT toward lower temperature.