Interfacial structure of polymers near a surface: a molecular dynamics study

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The efficient separation of gases is a subject of considerable interest due to economic and environmental threats associated with air pollution, and is an imperative to meet energy demands of the world. Membrane based gas separation is considered as an efficient, productive, readily scalable, and environmentally friendly process that can operate in a continuous fashion. The recent advances have shifted towards the development of mixed matrix membranes (MMM), due to the challenges with the current spectrum of polymeric and inorganic membranes. MMMs have been commonly prepared by incorporating inorganic fillers such as zeolites or metal organic frameworks in a continuous polymer matrix. However, the success of MMMs depends greatly on the screening and selection of suitable polymer matrix, inorganic filler and interaction between them. Though the past decade has witnessed substantial progress in both the fundamental and application aspects of MMMs in gas separation, interface problems such as the formation of non-selective voids, rigidified polymer and pore blockage due to poor interaction between the polymer and inorganic filler are still challenging. Hence, understanding and minimising interfacial barriers between the polymer and the inorganic filler are critical to the design and optimisation of MMMs; however, trial and error experimentation is required to address these non-ideal interface issues. On the other hand, atomistic simulations have become an important tool in the screening and selection of suitable materials in MMMs. The present thesis aims to develop a fundamental knowledge of the polymer structure near a surface, and thus facilitate the design of MMMs, especially for gas-separation.Firstly, the morphology of the polyimide (PI) polymer membrane is characterized by exploring the volume-temperature relations, distribution of free volume elements in the polymer and available free volume analysis. Then, the separation performance of a PI membrane in pure gas conditions is investigated. Gas sorption isotherms were extracted via a two-step methodology considering the dynamics and structural transitions in the polymer matrix upon gas sorption using a combination of equilibrium molecular dynamics (EMD) in the constant pressure ensemble and grand canonical Monte Carlo simulations. The gas transport behavior in the polymer membrane is evaluated by extracting Maxwell-Stefan diffusivities, and found to be consistent with experimental evidence.Further, the separation performance of a polymer membrane in mixed gas conditions is investigated by considering an equimolar mixture of CO2 and CH4 in a fluorinated PI polymer membrane. Significant swelling of the polymer in the presence of CO2 is found, as a result of which the predictions of traditional models such as ideal adsorption solution theory and dual mode sorption for mixed gases in mixed gas conditions are inaccurate, particularly for CH4. The Onsager coefficients indicate that in mixed gas conditions finite correlations exist between the diffusing species in the polymer membrane. Further, the swollen membrane is diffusive selective for CH4 at high pressures in mixtures due to availability of large pores, in contrast to pure gas conditions where the membrane is diffusive selective for CO2 over CH4 at all pressures. Analysis of membrane behavior under practical conditions using EMD-based transport coefficients shows that while the CO2/CH4 perm-selectivity increases with increase in pressure based on pure component data, the trend is opposite for mixture data. Thus, the commonly used approach of screening membrane materials based on pure component data can be misleading, as it overlooks the correlation effects arising from the presence of other species in the mixture.Subsequently, the structure of the PI in the vicinity MFI-zeolite, and its CO2/CH4 transport properties is investigated. It was found that incorporation of MFI zeolite into PI results in the formation of a densified polymer layer near the surface, having thickness around 1.2 nm, contradicting empirical suggestions of an approximately 1-micron thick interface between the polymer and filler. This interfacial region offers extra resistance to gas diffusion, which increases with kinetic diameter. Consequently, significant increase in CO2/CH4 selectivity as well as gas permeability is observed in the PI-MFI composite membrane compared to the pure PI polymer membrane, which is correlated with the high selectivity of the rigidified interfacial layer in the polymer. Thus, while enhancing transport resistance, the rigidified layer can be beneficial to membrane selectivity.Finally, the structure of a PI in the vicinity of the ZIF-8 surface is investigated. It is seen that incorporation of ZIF-8 into PI results in formation of sub-nanometer voids as defects near the polymer-filler interface. We then identified an ionic liquid (BMIM-BF4) which has favorable interactions with both ZIF-8 as well as polymer to achieve a defect-free interface, thus exhibiting superior gas separation performance compare to the pure polymer membrane.In summary, this thesis has developed a nanoscale understanding of polymer structure near a surface for the information necessary to design MMMs. This investigation also includes strategies to minimise the interfacial defects such as nano-scale voids to achieve separation performances surpassing the Robeson upper bound limit in MMM membranes.