Abstract
The miscibility in binary blends of ferroelectric copolymers of vinylidene fluoride (VF~2~) and trifluoroethylene ( F~3~E) has been investigated over the full range of blend compositions for three different random copolymers containing 52, 65, or 73 mol % VF~2~. We found these copolymers miscible in the melt with each other but not with the parent homopolymer, PVF~2~. In thin solid films we demonstrated, for all blend compositions, the very rare phenomenon of isomorphic cocrystallization in both the ferroelectric and paraelectric phases of the 52 + 65 and the 65 + 73 mol % VF~2~ copolymers. However, the blend having the largest compositional difference (52 + 73 mol % VF~2~) was found to consist of two separate ferroelectric and paraelectric phases. These results demonstrate the very close chemical and structural similarities required for coexistence of two different macromolecules within a single crystalline lattice. Moreover, they allow us to probe the compositional limits for such similarity in both the ferroelectric and the paraelectric lattices. Phase diagrams of these blends have been constructed and are discussed. The Curie transition is shown to be an exceptionally sensitive probe of compatibility in the ferroelectric phase.