Effects of poloxamer inhomogeneities on micellization in water

Pluronic P85 (poloxamer 235), which is a symmetrical PEO-PPO-PEO triblock copolymer of poly(ethylene oxide) and poly(propylene oxide), has been investigated by differential scanning calorimetry and chromatography. Interaction chromatography at critical conditions for PEO was used for chromatographic characterization and preparative separation of P85 according to PPO block lengths. The results show that the P85 sample is heterogeneous with respect to chemical composition and molar mass. P85 contains 12 wt % lower size impurities, which do not contribute to the main micellization transition. The shapes and widths of the micellization peaks observed in P85 thermograms are influenced by the distribution of PPO block lengths in P85, as shown from measurements on chromatographic fractions. PPO block lengths of the main P85 fraction follow a Poisson distribution closely. The characteristic temperatures of micellization, sphere-to-rod, and phase separation transitions observed in thermograms are influenced by polydispersity.