Effects of Added CO2 on the Conformation of Pyrene End-Labeled Poly(dimethylsiloxane) Dissolved in Liquid Toluene

Abstract

We report on the tail−tail cyclization and unfolding kinetics of poly(dimethylsiloxane) that is end-labeled with pyrene (Py−PDMS−Py) when it is dissolved at low concentration in liquid toluene (a good solvent) as a function of added CO2 (0−204 bar). The pyrene excimer emission provides information on the PDMS tail−tail cyclization and unfolding kinetics and the conformation of the polymer chains. Under the aforementioned conditions, the Py−PDMS−Py excimer emission is entirely intramolecular in nature; there is no evidence for any inter- or intramolecular ground-state preassociation of the pyrene residues. However, the pyrene excimer-to-monomer intensity ratio (E/M) increases by ∼5-fold as we increase the CO2 pressure from 0 to 70 bar. E/M begins to decrease gradually as the CO2 pressure is increased above 70 bar. Time-resolved fluorescence spectroscopy reveals three important points. First, the rate of Py−PDMS−Py tail−tail unfolding (kunfolding) is essentially independent of added CO2. Second, the rate th...