(797c) Phase Behavior of Tapered Diblock Copolymers From Self-Consistent Field Theory

It is well known that polymers composed of two connected segments/blocks of chemically different monomers A and B can microphase separate into various ordered structures. The fraction of A monomers and the quantity χN together determine whether phase separation is preferred and the details of the resulting morphology, where N is the polymer length and χ is the Flory parameter related to how unfavorable the AB interactions are. Thus, it’s inherently difficult to independently tune material properties such as the bulk modulus (with increases with N) and the microphase separated state (related to χN). This is unfortunate specifically in the search for high modulus materials with a bicontinuous network structure such as the double gyroid phase, which is generally less favorable at high χN. Such materials are of interest in applications such as solid battery electrolytes where penetrant transport occurs through one continuous phase while the other phase provides mechanical strength.

An attractive way to get around this problem is to use tapered block copolymers. These are composed of a pure A block, a linear gradient “block” whose composition varies from fully A to fully B (tapered) or B to A (inverse-tapered), and a pure B block. The resulting copolymer then contains a new parameter (the fraction of gradient/tapered block) that can be used to tune the microphase separated state independent of N. To determine how this strategy can best be employed, we compute phase diagrams of tapered and inverse-tapered diblock copolymers via self-consistent field theory. Their composition is approximated using a multi-block model in which the tapered region consists of alternating A and B blocks of appropriate lengths to approximate the gradient. Phase diagrams were produced for varying sizes of the tapered region, and show a shift of the ordered phases to higher χN for larger tapered regions (and an even greater shift for inverse-tapered systems). Interestingly, the typically narrow gyroid region widens for direct tapered systems, by about a factor of two at high χN.