Molecular simulation of carbon dioxide sorption in nanoporous crystalline phase of sydiotactic polystyrene

Nanoporous crystalline δe-form of syndiotactic polystyrene (sPS) is characterized by the rather peculiar behavior of being able to absorb considerable amounts of low molecular weight penetrants, in contrast to the general behavior reported for the crystalline phase of polymers that is impervious to penetrants. In particular, the δe nanoporous crystalline form of sPS displays a sorption capacity of penetrants that is several times higher than the one of the amorphous phase of sPS. In this paper, sorption thermodynamics of carbon dioxide in the δe nanoporous crystalline form of semicrystalline sPS is analyzed by means of Grand Canonical Monte Carlo (GCMC) molecular simulation methods, evaluating sorption isotherms as well as isosteric heats of sorption based on a semi-empirical molecular force-field. In fact, in the last years, this technique has been successfully used to investigate sorption properties of periodic crystals of a wide range of materials, including zeolites and polymers, supplying reliable estimates and representing a valid support to the experimental activity. While computational techniques allow direct determination of sorption properties of purely crystalline systems, experimental characterization of sorption in a semicrystalline polymer, as is the case of sPS under investigation, does not give a straight estimation of sorption capacity of the crystalline phase since sorption measurement incorporates other contributions related to the amorphous phase and interphases, as well as to possible defects of the crystalline phase. However, in the case of sPS at the investigated gas pressures, contribution of the amorphous and non-crystalline phases to carbon dioxide sorption can be neglected, and it has been possible to directly compare simulation predictions with experimental results, showing that GCMC computations supply excellent estimates for sorption isotherms and isosteric heats of sorption.