Crystalline Monolayer Ordering at Substrate-Polymer Interfaces in Poly-3-hexylthiophene Ultrathin Films

The nanocrystalline structure of thermally annealed P3HT ultrathin films was studied as a function of the substrate properties using AFM, GIXRD, and static contact angle measurements which showed that an edge-on structure is formed on both types of substrate. Substrate-induced ordering is observed at the interface with air only when the film matches the average lamellae thickness, approximate to 30nm in the present case, while, when the film thickness is higher than a single lamella size, i. e., approximate to 60nm the substrate nature does not affect the lamellar ordering at the film/air interface. In other words, the substrate properties seem able to induce the ordering of the only first crystalline layer. These results are discussed in terms of simple relationships between interface free energy and chain mobility.