Sensing the glass transition in thin and ultrathin polymer films via fluorescence probes and labels

Fluorescence was used to characterize the glass transition in thin and ultrathin supported polymer films with common chromophores. The temperature dependence of the fluorescence intensity exhibits a transition or break upon cooling from the rubbery state to the glassy state, and this is identified as the glass transition. A variety of chromophores are investigated including pyrene, anthracene, and phenan-threne either as dopants, covalently attached to the polymer as a label, or both. The particular choice of the chromophore as well as the nature of the attachment, in the case of labels, have significant impact on the success of this method. Problematic cases include those in which the excited-state chromophore undergoes significant photochemistry in addition to fluorescence or those in which the particular attachment of the chromophore as a label may allow for conformational interactions that affect the fluorescence quantum yield in a nontrivial way. Polymers that have an intrinsic fluorescence unit, for example, polystyrene, may allow for the fluorescence sensing of the glass transition without added dopants or labels. Finally, it is demonstrated that this technique holds promise for the study of the glass transition in polymer blends and within specific locations in multilayer films.