Modeling of the Sorption and Swelling Behavior of Semicrystalline Polymers in Supercritical CO2

The effect of semicrystallinity on the sorption and swelling behavior of polymers in supercritical CO2 is investigated. In particular, the experimental behavior measured for four different polymers in a previous work, i.e., poly(methyl methacrylate), poly(vinylidene fluoride), poly(tetrafluoroethylene), and the copolymer tetrafluoroethylene-perfluoromethylvinyl ether, is modeled using four different thermodynamic approaches: the Sanchez-Lacombe method in its nonequilibrium and equilibrium version, a modification of the latter for cross-linked polymers, and the statistical association fluid theory (SAFT) approach. The four considered polymers have an increasing crystallinity from 0% (poly(methyl methacrylate)) to about 50% (poly(vinylidene fluoride)). The results in terms of sorption and swelling are compared and the parameter evaluation procedure is discussed. It is observed that the nonequilibrium Sanchez-Lacombe method performs better than the others in describing the semicrystalline polymers at least from a qualitative point of view. This supports the interpretation that semicrystallinity induces a nonequilibrium effect, since the polymer structure, made more rigid from the presence of the crystallites, does not recover the original compactness after sorption of CO2.