Effects of added CO ₂ 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-PDMSPy molecules appear to aggregate below ∼0.2 CO ₂ mole fraction. Above ∼0.2 CO ₂ mole fraction, the PyPDMS-Py dynamics appear to obey a Birks-like model. As we increase the CO ₂ mole fraction in the PyPDMS-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.