Determining order-to-disorder transitions in block copolymer thin films using a self-referencing fluorescent probe

The thin film geometry of block copolymers (BCPs) required by many applications often leads to a shift of their order-to-disorder transition temperature (T_ODT) due to the presence of air-polymer and substrate-polymer interfaces. A challenge associated with measuring BCP thin film T_ODTs is the lack of a simple, generalizable, non-destructive characterization method, which is of critical importance for exploiting BCP self-assembly in nanotechnology. In this work, we present a non-invasive fluorescence characterization method for determining the T_ODT of BCP thin films. Fluorescent pyrene molecules were covalently attached at nearly trace levels to one block and used as a probe of the local polymer matrix. The BCP T_ODT can be determined by a discontinuous change in the intensity ratio I₁/I₃ of pyrenyl vibronic emission bands as a function of temperature, which is induced by a change of the nanoscopic chemical environment around the pyrene labels at the ODT. When the BCP film thickness is sufficiently small, an increase of T_ODT compared to bulk is observed. This increase of T_ODT in thin films is mainly attributed to attractive polymer-substrate interactions, the effects of which can be varied by adjusting the film thickness and substrate surface hydrophobicity. These results not only demonstrate that fluorescence characterization can be used as a generalizable approach for determining BCP thin film ODTs, but also provides additional insights on manipulating functional BCP nanopatterns by controlling the substrate surface functionality.