Effects of PEO−PPO Diblock Impurities on the Cubic Structure of Aqueous PEO−PPO−PEO Pluronics Micelles:  fcc and bcc Ordered Structures in F127

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We report on structural properties of PEO−PPO−PEO type of triblock block copolymers (Pluronics F127) with special emphasis on the effect of diblock PEO−PPO impurities on the ordered gel phase. Commercial F127 polymers contain as received roughly 20% PEO−PPO diblock and 80% PEO−PPO−PEO triblock copolymers. Aqueous solutions of F127 copolymers used as received form fcc ordered micellar structure. Copolymers depleted with respect to the diblock impurity, resulting in a pure PEO−PPO−PEO triblock copolymer system, form bcc ordered micelles within the major parts of the gel phase. However, close to the disorder-to-order transition, the purified F127 copolymer system also forms fcc structure, thus giving rise to a novel sequence of micellar phases: disorder−fcc−bcc. This is the phase sequence predicted theoretically for pure block copolymer melts.