Coil−Globule Transition of Pyrene-Labeled Polystyrene in Cyclohexane:  Determination of Polymer Chain Radii by Fluorescence

The coil−globule transition of a polystyrene chain (M w = 113 000, M w/M n = 1.03) randomly labeled with a pyrene derivative was studied in cyclohexane under dilute conditions by fluorescence. The coil−globule transition temperature was found to be around 25 °C from the presence of a break point in the pyrene excimer-to-monomer fluorescence intensity ratio plot with temperature. Analysis of the pyrene monomer fluorescence intensity decays with the fluorescence blob model modified to consider the presence of ground-state pyrene pairs allowed for the determination of the chain radii at several temperatures between 12 and 50 °C and confirmed the occurrence of a coil−globule transition. Fluorescence was found to be a reliable technique for studying the coil−globule transitions in dilute solutions of low-molecular-weight polymers where stable globules exist without the interference of polymer aggregates in a large temperature interval.