Probing the size of Pluronic® P84 block copolymer micelles in aqueous solution by means of positron annihilation lifetime spectroscopy

The size of a Pluronic block copolymer, P84, forming micelles in aqueous solution (12.5 wt.%) is investigated as a function of temperature using triplet positronium (o-Ps) as a probe in positron lifetime spectroscopy experiments (PALS). This triblock copolymer has the formula (EO)19(PO)43(EO)19, where EO and PO correspond to ethylene oxide and propylene oxide groups. This system is potentially more complex than, e.g., sodium dodecyl sulfate solutions, because the poly(ethylene oxide) (PEO) chains can expand out into the water around the poly(propylene oxide) (PPO) micellar core, thus possibly affecting some properties of water. To assess the influence of the PEO chains on the o-Ps lifetime in water, τ3, a solution of poly(ethylene glycol) (PEG) was first studied, and shows an increase in τ3 with increasing PEG concentration, and a decrease to a plateau on increasing temperature. For the P84 case, using values of τ3 extrapolated from these data and values of the aggregation number, Nag, from the literature gives information on the variation of the PPO micellar core radius with temperature. These results are in excellent quantitative agreement with what can be calculated for spherical PPO micelles.