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, whi ch 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 scatt ering, and rheology. Complex aggregation behavior is observed as a fun ction of temperature and copolymer concentration. At low temperatures and low copolymer concentrations the (PO)n(EO)m(PO)n copolymers are di ssolved as independent macromolecules. In a wide temperature range of dilute concentrations, the scattering patterns indicate formation of l arge domains of networks of copolymer strands interconnected randomly through the hydrophobic poly(propylene oxide) end blocks. At higher co polymer concentrations an interconnected network of micelles is formed in which micellar cores of hydrophobic poly(propylene oxide) are inte rconnected by poly(ethylene oxide) strands. For concentrations above 5 0 wt% this network constitutes the whole sample, resulting in a transp arent homogeneous phase. Close to 60% copolymer concentration, the mic ellar network forms a low-temperature ordered solid-like mesophase. At higher polymer concentrations and low temperatures an elastic two-pha se system is formed of a coexisting swollen lamellar and micellar netw ork. In the pure melt, the 25R8 triblock copolymer forms a lamellar st ructure below T(m) = 55-degrees-C.