Confinement-induced miscibility in polymer blends

The use of polymer thin films in technology is increasingly widespread—for example, as protective or lithographic surface coatings, or as active (electronic or optical) elements in device architectures. But it is difficult to generate films of polymer mixtures with homogeneous surface properties, because of the tendency of the polymers to phase-separate1,2. Copolymer compatibilizers can induce miscibility in polymer blends, but only with chemical components that are either close to a critical point in the phase diagram3 or which have an attractive interaction between them4,5. Instead of manipulating the chemical composition of the blend, we show here that complete mixing can be obtained in polymer blends by the physical effect of confinement in thin films. The compatibilization results from entropic inhibition of phase separation into micelles, owing to confinement. The result is an intimately mixed microemulsion with a perfectly flat surface and a two-dimensional maze-like structure with columnar domains that extend through the film.