In-Situ IR spectroscopy and ab initio calculations to study polymer swelling by supercritical CO2

The CO2 sorption and polymer swelling of hydroxytelechelic polybutadiene (HTPB) and poly(ethylene glycol) (PEG) have been investigated as a function of temperature and CO2 pressure by combining in situ near-infrared spectroscopy with molecular modeling. The results reported here for the PEG−CO2 system are in a very good agreement with literature data hence validating our experimental procedure. It has been found that CO2 sorption and swelling effect is more important for PEG than for HTPB. For both polymers, an increase of temperature leads to a strong decrease of both the CO2 sorption and swelling. In order to identify at a molecular level the nature and strength of intermolecular interaction occurring between CO2 and the polymers, ab initio calculations have been performed on model structures, representative of the main functional group of the polymer, and their complex with CO2. Trans-3-hexene (3-Hex), propyl methyl ether (PME) and methoxytrimethylsilane (MTMS) have been selected to mimic the functional groups of HTPB, PEG and polydimethyl siloxane (PDMS), respectively. The last system has been chosen since previous works on the swelling of PDMS by high pressure CO2 have revealed the high ability of CO2 to swell both uncrosslinked and crosslinked PDMS. The calculated stabilization energies of the MTMS−CO2, PME−CO2, and 3-Hex−CO2 dimers indicate that CO2 interacts specifically with the three moieties through a Lewis acid−Lewis base type of interaction with the energies displaying the following order: E(MTMS−CO2) = −3.59 > E(PME−CO2) = −3.43 > E(3-Hex−CO2) = −2.5 kcal/mol. Since the solubility of CO2 in the corresponding homopolymers follows the same order, it is evidenced that the stronger the interaction between CO2 and the polymer, the higher the CO2 sorption. Therefore, even if one cannot exclude the influence of free volume and chain flexibility of the polymer, it appears that the solubility of CO2 in the polymer is predominantly governed by the interaction between CO2 and the polymer. Although the same trend is observed for the swelling of the polymer as a function of the CO2 pressure, we have found that for a given value of CO2 sorption, the swelling of the polymer depends on its nature, meaning that the swelling is not only governed by the CO2−polymer interaction but also by other intrinsic properties of the polymer