Carbon dioxide-induced swelling of poly( dimethylsiloxane)

A new experimental device is used to monitor in situ the swelling behavior of poly (dimethylsiloxane) melts in contact with supercritical carbon dioxide. The effects of pressure, temperature, and sample molecular weight on the kinetics and extent of swelling are examined using this experimental technique. The swelling kinetics of all polymer samples exhibit two distinct regimes: an initial region of large swelling in which the diffusion of CO2 into the polymer follow Fickian behavior and a subsequent region of small volume increase asymptotic to an equilibrium swelling value. Diffusion coefficients of CO2, obtained from the initial swelling kinetics data, are found to be relatively insensitive to pressure, increase with temperature, and decrease with polymer molecular weight with the latter exhibiting a power-law dependence with an exponent of similar to-2. The extent of swelling increases with both pressure and molecular weight but exhibits different trends with temperature depending on system pressure. For pressures below 15 MPa the extent of swelling decreases monotonically with temperature. However, for pressure above this threshold, a maximum in swelling is observed with temperature increments. The maximum with temperature is thought to be a result of large variations in the physical properties of CO2 near its critical point. The results of equilibrium swelling have been modeled using the Sanchez-Lacombe equation of state and found to be in good agreement with the thermodynamic theory.