FLUORESCENCE PROBE STUDIES OF PLURONIC COPOLYMER SOLUTIONS AS A FUNCTION OF TEMPERATURE

The temperature variation of the I-1/I-3 ratio of pyrene vibrational f ine structure has been studied in aqueous poly(ethylene oxide)-block-p oly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolyme r solutions, in bulk PEG, PPO, and various organic solvents. Linear de creases were observed in organic solvents and in mixtures of bulk PEO and PPO. The data in aqueous PEO-PPO-PEO copolymer solutions reveal th ree distinct solution conditions: at low temperatures, the linear decr ease with temperature parallels that for water; over a small intermedi ate temperature range there is a sharp decrease in the ratio, attribut ed to the formation of micelles; and at higher temperatures, a linear decrease is again observed. Values in this region are strongly depende nt on the PPO block and are consistent with the representation of mice lles with a core dominated by PPO. These decreasing values for the int ensity ratio in the latter region indicate a less polar environment fo r pyrene and were considered to be a linear combination of the tempera ture effects on the polarity observed in bulk poly(ethylene oxide) and poly(propylene oxide) homopolymers. The composition of the micelle co re was estimated using these data and indicated an increase of poly(et hylene oxide) as the polymer becomes more hydrophilic. In addition, th e size of the micelles and their aggregation numbers were determined u sing light scattering and pyrene luminescence decay studies, respectiv ely. These data showed an increase of the aggregation number with temp erature while the hydrodynamic radius remained constant and were inter preted in terms of dehydration of the PEO block.