Generalization of Surface Freezing in P3AT Thin Films

We directly observe surface freezing in 90–95% regioregular poly(3-alkylthiophene)s (P3ATs) of different molecular weights and side-chain lengths, i.e., a free surface-induced crystallization event occurs at temperatures 25–50 °C above bulk crystallization. For all studied P3ATs, surface freezing involves the development of highly edge-on oriented crystallites within 20 nm of the air–polymer interface; however, nucleation and growth of 2D spherulites (“circulites”) during surface freezing can be optically resolved only in P3ATs with relatively low circulite nucleation densities. For poly(3-hexylthiophene) (P3HT), molecular weight and regioregularity impact circulite nucleation density as well as surface and bulk crystallization kinetics. Both free surface and bulk crystallization temperatures are nearly insensitive to molecular weight for 95% regioregular P3HT but strongly decrease with increasing chain length for 90% regioregular P3HT. Overall, the results indicate that surface freezing and high edge-on orientation at the free surface are general phenomena in P3ATs, but circulites are not always observable.