RATIONAL DESIGN AND ADVANCED CHARACTERIZATION OF NANOCOMPOSITE GLASSY POLYMER MEMBRANES WITH ENHANCED STABILITY AND SELECTIVITY IN AGGRESSIVE GAS AND LIQUID SEPARATIONS

Abstract

Polymer membranes are transforming modern separation technologies by providing substantial energy and cost advantages over traditional thermal-based processes. Among them, glassy polymers stand out for their outstanding molecular size-sieving capabilities. However, three fundamental limitations continue to hinder their widespread use: the permeability–selectivity tradeoff, physical aging, and plasticization. Overcoming these interconnected challenges is critical for enabling robust, high-performance membrane systems for chemically aggressive industrial separations. To address these issues, this work reports a class of nanocomposite membranes in which glassy polymers are blended with a rigid porous organic filler—the triptycene–isatin porous polymer network (PPN). This PPN is an optimized, highly crosslinked network material developed by Iglesias et al. in 2018 that demonstrates extreme rigidity due to the incorporation of triptycene, a highly rigid and contorted 3D monomer; high BET surface areas (~790 m2 g-1), and an excellent distribution of micropores that enable size-sieving separation performance. Importantly, PPNs are fully organic, allowing compatibility with organic polymers and facile fabrication of defect-free nanocomposite membranes. Triptycene-isatin PPNs were synthesized and processed into nanoparticles exhibiting sizes of 150-700 nm, then incorporated into two different polymers. The first polymer used was carboxylated polymer of intrinsic microporosity 1 (CPIM), a rigid ladder polymer with pendant carboxylic acid groups. While CPIM has excellent separation performance, it suffers from plasticization by CO2 at relatively low pressures (5-10 atm). To solve this problem, the CPIM-based nanocomposite membranes containing up to 30 wt.% PPN were fabricated and subjected to a 200 °C treatment for 24h under vacuum, resulting in the CPIM crosslinking to itself as well as to the PPN particles by radical decarboxylation of the acid groups in CPIM, resulting in what is termed "inter-crosslinked membranes" due to the CPIM/PPN interfacial crosslinking. It was found that particle incorporation alone nor crosslinking of neat CPIM significantly enhanced plasticization resistance. Only inter-crosslinked membranes, which exhibit the polymer/particle interfacial crosslinks, had significantly enhanced plasticization resistances, with these membranes exhibiting no signs of plasticization up to CO2 partial pressures of 50 atm. In addition, these PPN-containing membranes had enhanced gas separation performance relative to neat CPIM. The individual and synergistic roles of PPN incorporation and crosslinking were elucidated using a variety of material characterization and transport measurement techniques. The second polymer that was blended with the triptycene-isatin PPN was poly(1-trimethylsilyl-1-propyne) (PTMSP), a highly permeable, microporous glassy polymer that exhibits rapid physical aging and that, as such, has been used, over the years, as a model polymer for aging studies. Thus, PPN incorporation into PTMSP (5-20 wt. %) was studied and found to slow physical aging: over the course of three weeks, neat PTMSP lost 41 % of its N2 permeability, while PTMSP blended with 5 wt. % PPN lost only 15% of its N2 permeability. Positron Annihilation Lifetime Spectroscopy (PALS), free volume measurements, cross polarization/magic angle spinning (CP/MAS) 13C NMR T1 measurements, transport measurements and molecular dynamics (MD) simulations revealed that an adsorptive interaction at the PTMSP-PPN interface suppressed backbone carbon relaxations, slowing the collapse of larger micropores, and that decreases in gas diffusivity due to micropore-blocking induced by PPN incorporation can be offset by increasing PPN loading, resulting in an increased sorption capacity. The exceptional rigidity and stability of the inter-crosslinked CPIM/PPN membrane platform prompted further exploration for challenging and aggressive chemical separations, particularly for organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO) in hydrocarbon media. Hydrocarbons severely soften and plasticize glassy polymers, limiting their long-term stability and separation efficiency, yet CPIM/PPN membranes were able to fractionate hydrocarbons with greater efficiencies than observed in conventional polymeric membranes in OSN tests and were exceptionally stable, exhibiting enhanced OSN performance after 6 months of exposure to liquid toluene. Remarkably, after almost two years of exposure to liquid toluene, inter-crosslinked CPIM/PPN membranes still exhibited excellent hydrocarbon fractionation performance in OSRO tests. As a matter of fact, the material platform discovered in this project exhibits unprecedented long-term stability in the harshest chemical environments, creating promising opportunities for fundamental understanding and translational purposes. In addition to the inter-crosslinking, a novel interfacial interaction between primary amine functionalities on the porous support and the acid groups in the CPIM were found to form a thin interfacial layer that significantly enhances the overall membrane selectivity. Transport modeling, liquid dilation tests, and correlation of literature data were used to elucidate the transport mechanism of these membranes, offering insights into future optimization and utilization of the chain immobilization and functionalization approaches developed in this work. Overall, this dissertation combines synthesis, advanced characterization, transport modeling, and long-term stability studies to develop a mechanistic understanding of how porous polymer networks (PPNs) can stabilize and enhance the separation performance of glassy polymer membranes in aggressive chemical environments. The findings establish design principles for creating nanocomposite membranes that simultaneously achieve high selectivity, permeability, and resistance to aging and plasticization. These insights aid the broader goal of expanding the implementation of polymer membranes in chemically aggressive gas and liquid separation environments, providing a foundation for more energy-efficient industrial separations.