(261b) Bicontinuous Microemulsions in Homopolymer-Block Copolymer Blends

A bicontinuous microemulsion (BmE) is a familiar state of structured soft material enabled by the interfacial action of properly configured surfactants in combination with immiscible oil-water mixtures. Similar interpenetrating domains with dimensions ranging from 10 nanometers to 1 micron can be created by blending appropriate amounts of two thermodynamically incompatible homopolymers with the corresponding diblock copolymer. The resulting phase prism, containing three composition variables and temperature, is characterized by various nanoscale ordered phases and macroscopic phase separation at high and low copolymer concentrations, respectively. Increasing temperature leads to disorder (DIS) and a state of phase mixing. We have explored the phase behavior of a model ternary system comprised of poly(cyclohexylethylene) (PCHE), poly(ethylene) (PE) and the associated PCHE-PE diblock copolymer. These materials were synthesized by anionic polymerization of styrene and butadiene followed by heterogeneous catalytic hydrogenation. The BmE state exists within a relatively narrow portion of the phase prism bounded by ordered lamellar (LAM) and the 2-phase region at concentrations of PCHE and PE that result in zero mean interfacial curvature. Small-angle neutron and x-ray scattering (SANS and SAXS) and transmission electron microscopy (TEM) have been employed to probe the structure of the BmE and the line of congruent LAM-DIS transitions, which define the peak temperature separating the LAM and DIS states, terminating at the BmE. Dynamic mechanical spectroscopy (DMS) experiments have established the viscoelastic behavior of the mixtures as a function of temperature and composition. These results will be discussed in the context of self-consistent mean-field theory (SCFT), highlighting the role of fluctuation effects.

* Research conducted in collaboration with Robert Hickey, Timothy Gillard, Matthew Irwin, David Morse, and Timothy Lodge.