Credit: © 2010 ACS

Conjugated polymers such as polyacetylene and polythiophenes have useful electronic and optical properties and the potential for widespread use in many devices. The ease with which they can be processed gives them advantages over traditional solid-state inorganic materials with similar properties. Controlling their morphology on the nanoscale is crucial to controlling their properties. Phase separation is a common way to do this for many polymers, but this is much harder for conjugated ones, as attempts to alter the molecular structure to control phase separation can reduce the conjugation.

Now, Dwight Seferos and colleagues from the University of Toronto have made conjugated copolymers from blocks of different heterocycles that phase separate. The two heterocycles are similarly-functionalized thiophene and selenophene, both of which form conjugated polymers on their own. The team made a controlled block copolymer of the two and, for comparison, a statistical — or random — copolymer as well as the two homopolymers.

The presence of large blocks of polyselenophene in the block copolymer was confirmed by its absorption edge being almost identical to that of the polyselenophene homopolymer. The thiophene block dominated the fluorescence spectrum, because of its greater intensity. The statistical copolymer emitted weakly at an intermediate frequency. Atomic force microscopy and scanning tunnelling electron microscopy images showed that the block copolymer phase-separated into sulfur- or selenium-rich domains, even though the two components are so structurally similar.