Theses Doctoral

Gas Transport Mechanisms in Polymer-Grafted Nanoparticle Membranes

Tannenbaum, Robert J.

Carbon capture and related gas separation processes are critical tools in our efforts to combat climate change. While polymer membranes are seen as a central construct to achieve these goals, their performance needs further improvement to meet current sustainability objectives. It is in this context that membranes composed of polymer-grafted nanoparticles (GNPs) become highly germane. Chemically tethering the available polymer to the nanoparticle (NP) surface in GNP systems helps mitigate difficulties controlling nanoparticle dispersion common when incorporating inorganic filler NPs into polymer (i.e., mixed matrix membranes (MMMs)). Previous work has shown that gas transport in pure GNP membranes can be strongly enhanced relative to that in the corresponding neat polymer. Additionally, we demonstrated that larger gases display greater degrees of permeability enhancement than smaller ones. This work explores the underlying mechanisms governing the unique gas transport behavior observed in GNPs, with the goal of designing materials possessing superior transport properties that can be known and manipulated a priori.

We begin with the identification of transport mechanisms for penetrants of different sizes through an exploration of the heterogenous nature of GNPs. In the limit of moderate-to-high grafting density (the number of chains tethered per unit surface area), the chains are overcrowded near the surface and assume extended conformations termed “polymer brushes”. These brushes comprise two regimes: (1) a dry zone of higher polymer stretching closer to the NP surface and (2) the interstitial spaces in the multibody packing of lower polymer density. We find that larger penetrants such as CH₄, with low solubilities, preferentially sorb into the interstitial spaces in the NP packing prior to diffusing through stretched chains in the dry brush region. The nature of small gas permeability enhancement, on the other hand, is due primarily to enhancements in penetrant diffusion through the stretched chain region close to the NP surface – this is because these gases have high enough solubilities to be present everywhere in the polymer layer.

Such solubility differences enable the direct control over penetrant transport through the disparate regions of the polymer brush in mixed-gas environments relevant to operation. Elevated CO₂ content, through increasing feed concentrations at higher pressures, yields increased CH₄ permeability and an associated reduction in mixed-gas selectivity relative to ideal gas analogs. Additionally, high-pressure conditioning with CO₂ evidently dilates the material (due to gas adsorption) in a manner that is apparently not recoverable after a pressure decrease.

An alternative handle to control penetrant transport is to manipulate the physical brush structure. Such morphological control is accomplished through variations in preparation methodology; in particular, the rate of solvent evaporation in solution-cast samples plays a significant role in dictating the final structure of the jammed colloidal glass. Utilizing high-pressure conditioning in CO₂ as a concentration quench, we combine morphological control over the brush structure with selective penetrant manipulation to dilate the overcrowded brush regime and enhance gas transport performance. Leveraging the colloidal glass nature of GNPs in this way enables the formation of quasi-equilibrium structures with even greater amounts of “free volume”.

The remaining chapters focus on employing our knowledge of the gas transport mechanisms in these materials to aid in future experimental design and to form mechanically resilient materials. Implementing a simulated design-of-experiments loop, we find that a surprisingly minimal amount of experimental data is necessary to effectively model the transport properties of new materials to within practical experimental error. Selectively altering the chemistry of specific chain regions achieved slight enhancement in membrane selectivity while significantly improving material toughness and ultimate utility. Our enhanced understanding of gas transport mechanisms in polymer-grafted nanoparticle membranes will aid in the design and implementation of membranes with tunable separation performance through direct control of how penetrants transport and via morphological changes to the brush structure.

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More About This Work

Academic Units
Chemical Engineering
Thesis Advisors
Kumar, Sanat K.
Degree
Ph.D., Columbia University
Published Here
September 6, 2023