PLURONIC-P105 PEO-PPO-PEO BLOCK-COPOLYMER IN AQUEOUS UREA SOLUTIONS -MICELLE FORMATION, STRUCTURE, AND MICROENVIRONMENT

The effects of urea on the micellization properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PP O-PEO) copolymer (commercially available as Pluronic P105) and on the structure and microenvironment of the micelles are reported. Critical micellization concentration (cmc) and temperature (cmt) values for the amphiphilic block copolymer dissolved in urea/water mixtures (urea co ncentration 0, 1, 2, and 4 M) were obtained using a dye solubilization method and corroborated with data from surface tension, density, and fluorescence spectroscopy experiments. Urea increased the cmc and cmt of the PEO-PPO-PEO copolymer; the effect of urea on the cmt was more p ronounced at low copolymer concentrations and diminished at concentrat ions of similar to 2.5%. The thermodynamic parameters of micelle forma tion in the presence of urea were estimated using a closed association model; the enthalpy of micellization was positive (endothermic) and d ecreased upon increasing the urea concentration. The surface activity and the partial specific volume of the block copolymer decreased with an increase in the urea concentration, whereas the hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained unaffected by the presence of 4 M urea in the solution. The m icropolarity in copolymer solutions in urea/water was probed as a func tion of temperature using the I-1/I-3 intensity ratio of the pyrene vi brational fine structure recorded in fluorescence emission spectra; a small decrease in the micropolarity of the micelle core was observed i n the presence of urea. The microviscosity in the micelle interior, es timated from the intramolecular excimer fluorescence of the hydrophobi c probe bis(1-pyrenylmethyl) ether (dipyme), also exhibited a small de crease with an increase in the urea concentration. The findings presen ted here are discussed in the context of the molecular mechanism under lying the effects of urea.