Melt Expansion and Thermal Transitions of Semiconducting Polymers in Thin Films

Abstract The molecular self‐organization and reorganization and thus volumetric and density changes during structural relaxation and melting of modern organic semi‐conducting materials remains largely unknown, particularly in the device relevant thin film geometry where the initial state may be structurally quenched away from equilibrium. Here, the apparent mass‐thickness of a range of semi‐conducting polymeric or molecular model materials systems is measured through in situ ellipsometry. Surprisingly, the volume changes upon melting correlate inversely, with a few exceptions, to the quality of crystallinity found via x‐ray methods (i.e., directly correlate with the paracrystalline g‐parameter) rather than the melting enthalpy that is possibly a proxy for the degree of crystallinity. This study also observes changes in orientation and/or density due to segmental relaxation during the first heat, thus complementing other characterization methods that measure relaxation or reorganization transitions. Semiconducting materials exhibit very large melt expansion and a richer phase‐behavior compared to commodity polymers, presumably due to their complex chemical structure. The results delineate an important and novel structure‐function relation that, together with simulations constrained by these results, will lead to better rational design of semi‐conducting materials.