Phase behavior of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) triblock copolymer melt and aqueous solutions

Structural studies are presented on the Pluronic-R copolymer 25R8, which has a central poly(ethylene oxide) block (80 wt % of the copolymer) symmetrically surrounded by poly (propylene oxide) blocks. The studies include small-angle neutron scattering, static and dynamic light scattering, and rheology. Complex aggregation behavior is observed as a function of temperature and copolymer concentration. At low temperatures and low copolymer concentrations the (PO)_n(EO)_m(PO)_n copolymers are dissolved as independent macromolecules. In a wide temperature range of dilute concentrations, the scattering patterns indicate formation of large domains of networks of copolymer strands interconnected randomly through the hydrophobic poly (propylene oxide) end blocks. At higher copolymer concentrations an interconnected network of micelles is formed in which micellar cores of hydrophobic polypropylene oxide) are interconnected by polyethylene oxide) strands. For concentrations above 50 wt % this network constitutes the whole sample, resulting in a transparent homogeneous phase. Close to 60% copolymer concentration, the micellar network forms a low-temperature ordered solid-like mesophase. At higher polymer concentrations and low temperatures an elastic two-phase system is formed of a coexisting swollen lamellar and micellar network. In the pure melt, the 25R8 triblock copolymer forms a lamellar structure below T_m = 55 °C.