Hierarchical microphase separation in bicontinuous ternary polymer blends

Abstract

We describe the phase behavior of a ternary blend of poly(ethylene-alt-propylene) (P), poly(cyclohexylethylene-b-ethylene-b-cyclohexylethylene) (CEC), and a CECEC–P amphiphilic hexablock where both CEC and CECEC–P are ordered in the bulk. In this system, designed to mimic A/B/A–B ternary polymer blends, CEC and P were mixed with CECEC–P along the 50 : 50 CEC/P volumetric isopleth, and the phase behavior of select blend compositions was probed with a combination of rheology, SANS, TEM and SEM. At low volume fractions of CECEC–P and above the TODT of CEC, the phase behavior is very similar to that of A/B/A–B systems, with a progression from swollen lamellae to microemulsion phases. Cooling through the TODT of CEC yields a striking change in morphology, precipitating a transition to microemulsion phases at a broader range of blend compositions. Within the CEC-rich domain of such a bicontinuous microemulsion, we identify alternating C–E lamellae which are oriented perpendicularly to the CEC/P interface. SEM images of porous templates of this phase created by selective removal of P confirm the bicontinuous structure. The mechanical properties of these templates are poor due to the alignment of C–E lamellae across the CEC-rich domain. These results illustrate the generalizability of A/B/A–B ternary blend phase behavior to significantly more complex linear architectures containing multiple ordering transitions, and offer a route for templating functional multiblock copolymers with the bicontinuous microemulsion morphology.