Static and dynamic scattering from ternary polymer blends: Bicontinuous microemulsions, Lifshitz lines, and amphiphilicity

Ternary polymer blends, comprising two homopolymers and the corresponding d iblock copolymer, have been examined by small-angle neutron scattering (SAN S) and dynamic light scattering (DLS). Two chemical systems have been emplo yed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(et hylethylene-b-dimethylsiloxane), and another containing polyethylenepropyle ne, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The mo lecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (B muE) phase; the homopolymer molecular weights and volume fractions were approximately equa l. The SANS intensity was compared quantitatively with the Teubner-Strey st ructure factor, and interpreted via the amphiphilicity factor f(a). The tra nsition from a fully disordered mixture at higher temperatures to a well-de veloped B muE upon cooling did not correlate well with either the disorder line (f(a)=+1) or the total monomer Lifshitz line (f(a)=0). However, DLS pr ovided a clear signature of this transition, via a distinct maximum in the temperature dependence of the dynamic correlation length. We hypothesize th at this maximum is closely correlated to the homopolymer/homopolymer Lifshi tz line. The structure of the interfaces in the B muE was further examined in terms of the curvature and the copolymer coverage, as functions of copol ymer concentration and temperature. (C) 2001 American Institute of Physics.