Baldanza, Antonio (2022) Sorption thermodynamics and mass transport in glassy and rubbery polymers. [Tesi di dottorato]

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The study of mass transport of low molecular weight molecules in polymers assumes a great interest both for theoretical, technology and engineering field. For example, the technologic importance of glassy organic polymers is well known, in fact application fields are extremely diverse, and they range from structural (hyperbaric windows) to environmental (membranes for industrial gas separation), or moreover to electronic field (ionic conductors, surface coatings for printed circuit boards). To better describe this phenomenon a synergic experimental and theoretical approach is needed, in fact the aim of this research is to validate a model which allows to describe satisfactorily mass transport of gases (or vapors) in glassy and rubbery polymers. For this purpose, Non-Randomness Hydrogen-Bonding (NRHB) model is selected, since it has a great capability to predict thermodynamic sorption in rubbery polymer and it is also able to take into account for specific interactions (as hydrogen bonding), unlike other similar Lattice Fluid models present in literature as Sanchez-Lacombe model. In addition, this latter has been demonstrated to be thermodynamically inconsistent, as ideal gas state approaches, and the correction, proposed in literature onto the original Sanchez-Lacombe model, only partially circumvent the inconsistency. Conversely, during this study the thermodynamic consistency of the NRHB model has been demonstrated as ideal state approaches. Furthermore, to describe the behavior of thermodynamic sorption involving glassy polymers, NRHB is extended to Non-Equilibrium Thermodynamic Glassy Polymer (NETGP), and, for the first time, an explicit expression of the NETGP-NRHB multicomponent chemical potential is found. The achieved results have allowed to predict relevant thermal quantities, i.e., the isosteric heat of sorption and the polymer-penetrant interaction energy of PEI/CO2 system. The predicted value of isosteric heat of sorption is compared to the one evaluated experimentally while the value of the intermolecular interaction energy is compared to the results of Density Functional Theory (DFT) calculations and applied to some ternary mixtures to predict satisfactorily thermodynamic solubility coefficients. Moreover, the equilibrium NRHB and the NETGP-NRHB are used in combination to study the glass transition temperature as function of the amount of penetrant gas in PMMA/CO2 and Nylon 6,6/H2O systems, this latter displaying specific interactions like hydrogen bonding. To complete the description of the mass transport, i.e., the diffusion or the permeation phenomena, an approach based on Free Volume theories is applied to NETGP-NRHB. The predictions of solubility and permeability of binary CO2/CH4 and CO2/C2H4 mixtures in glassy polymeric membranes are compared with experimental data available in literature. The theoretical research cannot be completed without an appropriately experimental approach. At this aim, a new hyphenated technique based on concurrent pressure decay and in situ FT-NIR vibrational spectroscopy measurements is implemented to study sorption of a low molecular weight compound in a polymeric membrane. The pressure-decay method is used to provide a quantitative information on the concentration of the penetrant within the polymer. This latter, once combined with the information gathered from vibrational spectroscopy about the IR absorbance of specific peaks associated to the penetrant absorbed within the polymer, allows a quite accurate estimate of the molecular absorptivity of the analytic peaks. As a test case, thermodynamic and kinetic sorption at 35°C of CO2 in Polydimethylsiloxane (PDMS) at pressure values up to 9 bar and up to up to 5.976 bar, respectively, are investigated. The results are compared with available literature data to validate the technique. Finally, the technique has been extended to the case of absorption of CO2/CH4 mixtures within PDMS at different temperature and the results of measurements are compared with the few data available in literature.

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