Micellar Packing of Pluronic Block Copolymer Solutions: Polymeric Impurity Effects

Abstract

Small angle X-ray scattering (SAXS), dynamic light scattering (DLS), and high performance liquid chromatography (HPLC) experiments are performed to support that the inter-micellar distance of Pluronic cubic structures in aqueous solutions is governed by the poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblock copolymer concentration (not the overall polymer concentration) in the solutions. The "as-received (AR)" and " purified (Pure)" F108 solutions show a separate concentration dependence of body-centered cubic (BCC) lattice spacing, when the overall polymer concentration is used as a micellar packing parameter in aqueous solution. When the 22 wt% of non-micellizable polymeric impurities in the AR Pluronic F108 is taken into account, however, a universal concentration dependence of the BCC lattice spacing is observed, unifying results from both AR and Pure F108 solutions. When the PEO-PPO-PEO triblock copolymer concentration from the HPLC analysis is employed as an effective polymer concentration parameter, the universal relationship is observed to provide strong evidence that the polymeric impurities in AR F108 locate themselves in the less dense parts of the interstitial regions on the BCC lattice points, where were occupied by the triblock copolymer micelles. Although the polymeric impurities in AR F108 do not affect the actual triblock concentration dependence of the lattice spacing, they do shift the onset concentration of BCC micellar ordering. In the Pure F108, the onset of BCC packing occurs at the point where the nearest-neighbor radius (R-nn) in the BCC lattice is approximately equal to the hydrodynamic radius (R-h), indicating that lattice formation begins upon " hydrodynamic contact" between micelles. In the AR F108, the onset of packing occurs when R-nn/R-h is approximately 0.9, indicating that, in the presence of the polymeric impurities, micelles must be forced together beyond the point of hydrodynamic contact for the BCC packing.