Weak Interactions between Poly(ether imide) and Carbon Dioxide: A Multiscale Investigation Combining Experiments, Theory, and Simulations

Abstract

This contribution aims at an understanding, at a molecular scale, of the state of CO2 molecules absorbed in glassy poly(ether imide) (PEI). This issue has been challenged by combining different approaches that tackle the problem from both the experimental and the theoretical viewpoints and providing a comprehensive physicochemical picture. In situ FTIR spectroscopy and gravimetry were exploited to gather relevant experimental information, while molecular dynamics (MD), density functional theory (DFT), and statistical thermodynamics approaches were used to model the behavior of the binary system at different scales. Based on the findings of FTIR spectroscopy and on DFT and MD calculations, it was determined that, among the possible interaction configurations, some are prevailing. In particular, the carbon atom of carbon dioxide molecules establishes relatively weak interactions prevalently with the carbonyl groups of PEI. A quantitative estimate of such interaction has been provided by MD calculations. The system was also analyzed using a lattice fluid model, specifically developed to deal with sorption of low molecular weight compounds in glassy polymers, that is rooted on statistical thermodynamics, determining the values of the isosteric heat of sorption and carbon-dioxide–polymer interaction energy. Finally, experimental data of CO2–PEI mutual diffusivity have been interpreted using a semiempirical theoretical model accounting for the effects of the penetrant concentration, of energy barriers associated with the occurrence of an effective diffusive jump, and of a thermodynamic factor.

Sorption of CO2, CH4 and their mixtures in Amorphous Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO)

“…In the present case, the ν2 bandshape is symmetrical and can be reliably simulated by a single gaussian function (Figure 5B) which confirms the absence of vibrationally detectable interactions. The present band profile is coincident with that observed in an ethylene-propylene copolymer, a fully inert matrix [26].…”

“…The Non-Equilibrium Thermodynamics of Glassy Polymers -Non Random Hydrogen Bonding (NETGP-NRHB) model has been developed and successfully implemented by our group [16,[19][20][21][22][23][24][25][26] to the aim of modelling the two-phase equilibrium, at fixed pressure, P, and temperature, T, established between a polymer-penetrant amorphous mixture "frozen" in a non-equilibrium (glassy) state and a multicomponent penetrant fluid phase mixture of assigned composition, under the assumption that the polymer is not soluble in the penetrant phase. This Pseudo-Equilibrium nature of phase condition refers to the solubility of penetrants within a non-equilibrium glassy polymer at a temperature far below the glass-to-rubber transition temperature, Tg, of the polymer-penetrants mixture so that it can be assumed that the mass density of the polymer within the glassy mixture is kinetically locked at an out-of-equilibrium value that, in the case of light gases at low pressure as is the case of the present investigation, can be identified with the value of the unpenetrated polymer mass density 𝜌𝜌 𝑝𝑝,0 , right before the start of the sorption process.…”

“…In particular, the NETGP extension procedure is consistently built in such way that the proper out-of-equilibrium expressions of the chemical potentials in the glassy phase, being related to a first order derivative of the Gibbs energy, G, collapse into the corresponding equilibrium expressions when the glassy state approaches the corresponding equilibrium state at the same P, T and composition, i.e. when the phase volume is dictated by the EoS of the equilibrium thermodynamic model adopted in the NETGP procedure [26]. Since the mass of polymer is constant in the polymer-based phase, the collapse into the equilibrium expressions can be equivalently expressed imposing that the out-of-equilibrium value of 𝜌𝜌 𝑝𝑝,0 attains the corresponding equilibrium value dictated by the EoS of the thermodynamics model adopted in the procedure of extension.…”

“…Since the systems investigated in the present contribution do not exhibit strong specific interactions, as also confirmed by FTIR analysis, in the following we adopt here a simplified version of the general NETGP-NRHB framework, that is the one where any HB term is canceled out in the calculation of the Pseudo-Equilibrium expressions of chemical potentials [19,26]. Here and in the following we refer to this version of the model as NETGP-NR (where NR stands for the corresponding pure "mean-field" version of NRHB model).…”

“…This effect activates distinct in-plane and out-of-plane modes and the signal splits in two fully or partially resolved components, depending on the degree of distortion. The occurrence of a two component bandshape provides a clear signature of an existing interaction between the probe and active sites on the polymer backbone, and has been detected in numerous matrices including PMMA, poly(butyl methacrylate), poly(vinyl acetate), poly(vinyl fluoride) and, more recently, in an amorphous polyetherimide [26,41]. In the present case, the ν2 bandshape is symmetrical and can be reliably simulated by a single gaussian function (Figure 5B) which confirms the absence of vibrationally detectable interactions.…”