Conjugated Polymer Chain Conformation in Solution, Bulk, and Thin Films

The chain conformation (i.e., chain rigidity and local backbone planarity) of conjugated polymers dictates the delocalized electron cloud along the conjugated backbone, which greatly influences their optical and electronic properties. Currently, reports on conjugated polymer conformation focus mainly on the solution state. Much less is known about their chain conformations in the bulk state, especially in the thin-film state, which is the state of the materials in devices. Here, we used small-angle neutron scattering to characterize the chain conformation of conjugated polymer in solution, bulk, and ultrathin (less than 100 nm) films. The contrast was achieved by dissolving hydrogenated polymers in deuterated solvents for solution-state measurements and uniformly blending a small amount of hydrogenated polymers with deuterated polymers for the bulk and thin film measurements. The chain conformation in thin films was measured with transmission neutron scattering through a stack of up to 48 identical thin films spin-cast on silicon wafers. In addition, grazing-incidence small-angle neutron scattering (GISANS) was also used to explore the thin films because GISANS afford superior surface sensitivity. We found that the chain conformation is unaffected by confinement in the thin films, or by the packing constraints exerted by the neighboring molecules in the bulk. Chain sizes in thin films are the same as in the bulk and solution-state within experimental uncertainty. Our conclusions are consistent with what was also found for non-conjugated traditional polymers, like polystyrene. Our work provides a novel protocol to probe conjugated polymer conformation in solution, bulk, and thin films and improves our understanding of the conformation of conjugated polymers.