Solvation and solvent relaxation in swellable copolymers as studied by time-resolved fluorescence spectroscopy

The fluorescent probe dimethylaminonaphthylsulfonamide is covalently bound to the ends of the pol(ethylene glycol) chains of the swellable block copol ymers poly(ethylene glycol)-polystyrene (PEG-PS) and poly(ethylene glycol)- poly(ethylene imine) (PEG-PEI) to investigate the molecular mobility inside the polymers, swollen by different liquids. Steady-state and time-resolved studies of the Stokes shift between absorption and fluorescence spectra re veal that the probe is solvated by both the polymer matrix and the liquid p hase. The extent of solvation by the liquid and the mobility of the microen vironment of the probe depend on both the swelling volume of the polymer an d the solubility of the probe in this liquid. Steady-state and time-resolve d fluorescence depolarisation measurements show that the polymer matrix for ms a very rigid solvent cage, which almost completely immobilizes the probe . Upon solvation of the probe by the liquid, the mobility of the probe incr eases. In PEG-PEI swollen by polar solvents, the mobilities of the probe it self and of its microenvironment, although not reaching the values observed in homogeneous solution, are significantly higher than in PEG-PS, due to t he hydrophilic nature of the polymeric backbone in PEG-PEI.