2022 Theses Doctoral

Preparation Effects on the Morphology of Polymer-grafted Nanoparticle Membranes for Gas Separation Applications

Chan, Sophia

About a quarter of all industrial energy consumption in the US is from distillation to separate chemicals such as carbon dioxide from a natural gas stream. Unfortunately, distillation requires huge amounts of thermal energy, space, maintenance, and costs. Gas separation membranes use 90% less energy than distillation, save significant space, and are relatively simple to maintain. Polymers are the main platform for these membranes, but they are often hindered by an intrinsic trade-off between how fast a gas flows through the membrane (permeability) and how effectively the membrane can separate two or more gases (selectivity).

One method of overcoming the permeability-selectivity trade-off is to use polymer-grafted nanoparticles (grafted NPs or GNPs) which chemically graft polymer chains from the surface of nanoparticles. These GNP-based membranes have demonstrated significant gas permeability enhancements relative to its neat analogue with a well-defined transport maximum as a function of graft chain length (MWg). They also have shown gas selectivity improvements up to two orders of magnitude greater than the neat with the addition of small amounts of neat polymer. Recently, we discovered that the preparation methods of these GNP-based membranes strongly affect their gas transport properties. Understanding the effects of preparation methods on nanostructure and, in turn, gas transport properties is critical for the commercialization of these gas separation membranes.

This thesis is divided into six chapters that investigate how preparation methods may affect the GNP structure with and without the addition of homopolymer, and how these structural changes may affect gas transport. The main questions we answer in this thesis are:

• How does the nanostructure of matrix-free GNPs (i.e., GNPs with no free chains) change with increasing graft chain length? How do these changes affect gas transport?
• How do evaporation rate, casting method, film thickness, annealing time, and annealing temperature affect the GNP structure? How are these changes related to gas transport?
• How does the structure of matrix-free GNPs change upon addition of small amounts of homopolymer? How might these changes relate to gas transport?

Chapter 2 presents the experimentally-based model of a multi-GNP system that changes in structure between different regimes of MWg. We discover these changes are energetically driven and suggest different layers of the polymer brush have varying favorability for transport that yield the observed macroscopic properties. Chapter 3 and 4 explores the effects of evaporation rates, casting methods, and annealing temperatures on localized GNP packing with a micro-focused SAXS beam and on global GNP packing with pair-wise distribution functions, respectively. We find that evaporation rates show no effect, but melt-pressing a solution-cast GNP film causes greater disorder with a broader distribution of interparticle spacings whereas annealing a GNP film to higher temperatures reduces disorder. Chapter 5 explores the effects of annealing temperatures, annealing times, film thickness, and MWg’s on the interparticle spacings of GNP thin films. Chapter 6 presents the localized GNP packing on several series of GNP “blends” (i.e., adding small amounts of homopolymer to GNPs), showing that GNP blends increasingly swell with added homopolymer fractions compared to their parent GNPs in all studied cases.
Most notably, the addition of short chains to a GNP with MWg below the transport maximum swell similarly to that of the loading of a matrix-free GNP with solvent.

This suggests these short chains also act akin to a loaded solvent, isotropically filling the GNP free volume pockets. The Conclusions and Future Work chapter details what questions were answered in this thesis and which questions were only partially answered. We then discuss suggestions for future experiments to ascertain the relationships among preparation method, nanostructure, and macroscopic gas transport.