Methanol Sorption in Poly(ether imide): Molecular Insights from a Multiscale Study Combining Experiments, Theory, and Simulations

The objectives of this work are 2-fold. First, we aim to provide a comprehensive description of the poly(ether imide) (PEI)–methanol system, linking atomistic insights to macroscopic thermodynamic behavior, with explicit consideration of the hydrogen-bonding interactions governing the system. Second, by presenting this paradigmatic case, we intend to illustrate how a microscopic description validated through vibrational spectroscopy can be effectively exploited to establish a sound physical basis for developing a predictive macroscopic thermodynamic model. Sorption thermodynamics of methanol vapor in poly(ether imide) (Ultem 1000) has been thoroughly investigated through a synergistic multiscale theoretical approach combined with in situ FTIR spectroscopy. Density Functional Theory (DFT) calculations provided estimates of methanol–methanol and PEI–methanol interaction energies as well as simulated FTIR spectra. These results showed good agreement with the experimental outcomes of in situ FTIR spectroscopy, both in terms of the collected spectra and of the energies of hydrogen bond formation within the methanol-PEI system exposed to methanol vapor at different pressures and at a temperature of 30 °C. The analysis of the experimental FTIR spectra of the polymer phase enabled a quantitative assessment of the concentration of methanol molecules hydrogen-bonded to imide groups of the polymer and of methanol molecules hydrogen-bonded to the former methanol molecule (the so-called “first-shell” and ‘second-shell’ methanol molecules, respectively). The presence of these two “populations” was quantitatively confirmed by Molecular Dynamics (MD) simulations. The detailed picture emerging from these complementary approaches was then exploited to refine the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state model, which was used to interpret the sorption thermodynamics of methanol in PEI. In particular, the PC-SAFT model was extended, adopting the Dry Glass Reference Perturbation Theory (DGRPT), to address the nonequilibrium nature of the glassy PEI-methanol system and to account for the swelling associated with methanol sorption. Predictions made using the DGRPT-PC-SAFT theoretical framework showed a very good agreement with both the FTIR experimental results and the MD simulations, successfully predicting the concentrations of first- and second-shell methanol molecules in PEI as a function of methanol activity in the vapor phase.