Interaction of Binary Pluronic Block Molecules

Water-soluble poly(ethylene oxide)-block-poly(propylnen oxide)-block-poly-(ethylene oxide) (PEO-PPO-PEO) copolymers are nonionic macromolecular surface-active agents, available commercially as Pluronics. Recently, the micellization of these copolymers has been focused because of a strong interest in Pluronic micelles as drug-microcarriers in therapeutic creams and ointments. However, micellization of Pluronic copolymers is very complex compared with those conventional surfactants. Due to this reason, interaction of binary Pluronics has been hardly studied up to now. This paper reported the micellization of binary Pluronic molecules P94/L92 and F108/L92 which have similar length in PPO blocks and different length in PEO blocks, as well as P94/L64 which have different length in PPO blocks. I₂ was used as a probe to determine the critical micelle concentration (cmc) of the mixtures in aqueous solutions. This method has been confirmed to be very sensitive to the determination of cmc. The results showed that P94/L92 or F108/L92 binary components interact each other to form mixed micelles over the range of all concentration ratio. With addition of L92 molecules with a shorter PEO block, hydration of P94/L92 or F108/ L92 mixed micelles was weakened duo to a decrease in the number of EO groups within the shell. When the mass fraction of L64, w_L64 < 0. 4, the micellization in P94/L64 mixtures was hindered and the cmc of P94 was enhanced to a higher value due to the formation of P94/L64 mixed pre-micelles in aqueous solution. Theses mixed pre-micelles have weaker hydrophobicity compared with those pre-micelles formed with individual P94 molecules. Over the range of w_L64 < 0. 4, therefore, there was only the micelles formed with individual P94 molecules when the micellization took place. With further increasing w_L64, the mixed micelles of P94/L64 started to be formed and the critical point of mixed micelle formation was determined at w_L64=0. 4.