MODIFICATION OF THE MICROSTRUCTURE IN POLOXAMER BLOCK COPOLYMER-WATER-OIL SYSTEMS BY VARYING THE OIL TYPE

Amphiphilic block copolymers can self-assemble in the presence of sele ctive solvents (''water'' and ''oil'') into a variety of different mic rostructures. The role that ''oil'' plays in the phase behavior and st ructure of copolymer-oil-water systems is the focus of the present inv estigation. We examined a poloxamer-type poly(ethylene oxide)-poly(pro pylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127: E100P70E100) in ternary isothermal systems with water and each of thre e ''oils'' (of varying polarity), p-xylene, n-butyl acetate and butan- 1-ol. A total of six phases with different microstructures have been i dentified in the ternary systems studied here: normal (oil-in-water) m icellar cubic (I-1), normal hexagonal (H-1), lamellar (L-alpha), and r everse (water-in-oil) hexagonal(H-2) lyotropic liquid crystals, and no rmal micellar (L-1) and reverse micellar (L-2) solutions. The number a nd types of phases, as well as the composition range of their stabilit y, are different for each ternary system, reflecting the effects of oi l. Systematic small-angle X-ray scattering (SAXS) measurements have be en performed in order to establish the structure of the phases and det ermine their characteristic length-scales. The location of the ''oils' ' in the different self-assembled microstructures has been deduced fro m the structural dimensions obtained from SAXS and the constraints imp osed by the copolymer total volume fraction and interfacial area: all xylene is present in the interior (apolar core) of the oil-in-water mi crodomains and butanol is located at the interfacial region between po ly(propylene oxide) and poly(ethylene oxide), while butyl acetate cont ributes to both the core and the interfacial domains.