Melt Expansion and Thermal Transitions of Semiconducting Polymers in Thin Films

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.