Self-Diffusion of Polystyrene in a CO2-Swollen Polystyrene Matrix: A Real Time Study Using Neutron Reflectivity

The self-diffusivities of high molecular weight polystyrene chains in CO 2 -swollen polystyrene matrices were measured in real time using neutron reflectivity. Bilayer films of hydrogenated and deuterated polystyrene (PS) were prepared on silicon substrates and exposed to compressed CO 2 . The broadening of the interface between the films as a function of time was determined from the reflectivity profiles, yielding the chain diffusivity. Diffusivity was studied as a function of polymer molecular weight, concentration of CO 2 in the polymer film, and temperature. Nearly an order of magnitude enhancement in the diffusivity of polystyrene chains ( M = 2 × 10 5 ), from 1.62 × 10 -16 to 9.35 × 10 -16 cm 2 /s, was found with a modest increase in the concentration of CO 2 in the polystyrene (from 8.9 to 11.3 wt %) at 62 °C. This concentration dependence was modeled using the Vrentas−Duda free volume theory. 50-52 At a constant temperature and CO 2 pressure the polystyrene diffusivity scaled as M -2.38 . The scaling of the self-diffusivity of PS in CO 2 -swollen PS with T − T g, where T g is the glass transition temperature depressed by the presence of the solvent, is discussed.