Effects of Added CO2 on the Dynamics of Poly(dimethylsiloxane) Oligomers Dissolved in a ϑ Solvent and a Poor Solvent

We report on the tail−tail cyclization and unfolding kinetics of a poly(dimethylsiloxane) oligomer that is end-labeled with pyrene (Py−PDMS−Py) when it is dissolved at low concentration in liquid ethyl acetate (a ϑ solvent for PDMS) or methanol (a poor PDMS solvent) at 308 K as a function of added CO 2 (0−1 mole fraction). In all liquids, the tail-to-tail cyclization rate increases as the CO 2 mole fraction increases. The tail-to-tail unfolding rate is CO 2 independent, and generally slower in comparison to the cyclization rates. As CO 2 is added, toluene (a good solvent for PDMS) and ethyl acetate move toward becoming poorer PDMS solvents. At a given CO 2 mole fraction in the liquid phase, the ϑ solvent (ethyl acetate) quality for PDMS decreases ahead of and in greater magnitude than the good solvent (toluene). In methanol, the Py−PDMS−Py molecules appear to aggregate below ∼0.2 CO 2 mole fraction. Above ∼0.2 CO 2 mole fraction, the Py−PDMS−Py dynamics appear to obey a Birks-like model. As we increase the CO 2 mole fraction in the Py−PDMS−Py/methanol system above ∼0.2, there is not a significant change in the Py−PDMS−Py tail-to-tail distance; the internal polymer dynamics increase. The overall effects of solvent and CO 2 mole fraction on the Py−PDMS−Py dynamics in CO 2 -expanded liquids are captured by a simple model.