A fluorescence-based method for determining order-disorder transition temperatures in block copolymers

We present a technique for observing the order-disorder transition (ODT) in block copolymers through changes in the temperature-dependent behavior of an optically fluorescent probe. This technique provides potential advantages over existing strategies such as scattering and rheology, as it is non-invasive and can be directly applied to thin films. Pyrene molecules were covalently attached to the styrene block of a PS-b-PMMA copolymer through addition of trace levels of functionalized monomer during synthesis by controlled radical polymerization. We observed a discontinuous change in the intensity ratio of the pyrene vibronic emission bands as the polymer was heated through its ODT. As this intensity ratio is sensitive to the chemical environment surrounding these molecules, we hypothesized that this change occurs as the pyrenes are in contact with only PS when the polymer is ordered, but experiences an averaged effect of both PS and PMMA in the disordered state. We then applied this ODT measurement technique to thin films, where an increase in T_ODT is observed when the substrate preferentially interacts with the PMMA and the film thickness is sufficiently small. Further experiments tuned this effect by modulating the surface energy of the substrate.