Fluorescence studies of polymer adsorption . 1. Rearrangement and displacement of pyrene-terminated polyethylene glycol on colloidal silica particles

Abstract

Adsorption of pyrene end-labeled poly(ethy1ene glycols) (Py-PEG-Py) on colloidal silica particles and their subsequent rearrangement and/or displacement by the addition of unlabeled PEG are studied in water by using photostationary fluorescence and time-resolved fluorescence decay. The tagged polymers we use in this study have molecular weights 4250 and 8650 based on the PEG backbone. The molecular weight of the displacing polymer (PEG) ranges from 1470 to 22000. Experimental conditions for the adsorption of Py-PEG-Py are chosen to ensure that a negligible amount of Py-PEG-Py exists in free solution. For the adsorption of Py-PEG-Py (8650) of 1 X lo4 M on silica particles at 0.1 wt % concentration there is an overshoot in the excimer to monomer intensity ratio, Ie/Im, relative to that of bulk solution, (Ie/Im)o, as the displacer (PEG (22000)) concentration is increased. This overshoot is not detected for the adsorption of Py-PEG-Py (4250). Analysis of the supernatant solution shows that the overshoot may be attributed to the rearrangement of Py-PEG-Py (8650) on the silica surface due to the adsorption of incoming displacer prior to displacement of the tagged PEG into solution. We speculate that there is a competition between rearrangement of Py-PEG-Py on the surface and displacement into bulk solution that depends upon the absolute number of Py-PEG-Py segments actually in contact with the surface. The effects of the molecular weight of the displacing polymer and the initial concentration of Py-PEG-Py on (Ie/Irn)/(Ie/Irna)r,,e also studied. The measurement of excitation spectra and time-resolved fluorescence decays for both monomer and excimer emission provides insight into the state of the Py-PEG-Py when adsorbed on a silica surface.