Fluorescence studies of pyrene-labelled, pH-responsive diblock copolymer micelles in aqueous solution

Fluorescence spectroscopic techniques, and time-resolved anisotropy measurements (TRAMS) in particular, have provided valuable information regarding micelle formation in luminescently labelled pH-responsive diblock copolymers of 2-(diethylamino)ethyl methacrylate (DEA) and 2-(dimethylamino)ethyl methacrylate (DMA). A pyrenyl derivative, located at the DEA block, allowed motion of this site to be monitored via TRAMS in aqueous solution: a significant reduction in the mobility of this label was apparent at concentrations in excess of the critical micelle concentration, CMC, of the diblock copolymer. This is consistent with the labelled DEA block being located in the core of the micelles. At concentrations below the CMC, unimers were detected in solution. The micelle size estimated from TRAMS is approximately half of that determined from dynamic light scattering measurements. This suggests that the chain ends of the block copolymer are not “frozen” into position but that limited motion may occur due to fluidity within the micelle core. This is reasonable given the low Tg of the DEA block. Alternatively, a model is proposed which suggests that the interior of the micelle is a hard sphere, surrounded by flexible, fast-moving corona, which imparts little viscous drag on the core.