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

Abstract

Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(ethylethylene-b-dimethylsiloxane), and another containing polyethylenepropylene, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (BμE) phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner–Strey structure factor, and interpreted via the amphiphilicity factor fa. The transition from a fully disordered mixture at higher temperatures to a well-developed BμE upon cooling did not correlate well with either the disorder line (fa=+1) or the total monomer Lifshitz line (fa=0). However, DLS provided a clear signature of this transition, via a distinct maximum in the temperature dependence of the dynamic correlation length. We hypothesize that this maximum is closely correlated to the homopolymer/homopolymer Lifshitz line. The structure of the interfaces in the BμE was further examined in terms of the curvature and the copolymer coverage, as functions of copolymer concentration and temperature.