Intrinsically Microporous Oligomers and Polymers for Facilitated Molecular Transport and Separation

Abstract

Polymers of intrinsic microporosity (PIM-1) have rather rigid intramolecular micropores with an average diameter in the range of 0.6~0.9 nm, resulting in extremely large amount of free volume. Such nanopores are formed because of spirocenters in molecular structures of PIM-1. The intrinsic pores can provide effective pathways for transport of gases, ions and vapors, and consequently they can act a selective gate or sieve for mass transport depending on their relative size, i.e., called sieve mechanism. Therefore, PIM-1 has been utilized as a strong separation medium such as membranes for gas or ion separation because of its intrinsic and rather rigid pores for high gas permeability and ion conductivity, in addition to its high thermal stability (> 350℃), for Li metal batteries, vanadium redox batteries, gas separation membranes, pervaporation and ion exchange membranes. When we attempt to apply PIM-1 for improvement of the mass transport property through a conventional polymeric medium, its miscibility with the polymer will be important in determining the amount of its micropores. In this regard, we regulate the molecular weight of PIM-1 to have around 2,500 ~ 3,000 (oligomers of intrinsic microporosity, OIM) because the lower the molecular weight of PIM-1 and the higher is their miscibility. In this dissertation, synthesis of PIM-1 was first intensively studied to control its molecular weight, in particular in the range of around 2,500 ~ 3,000 of OIM-1. The functional groups of the OIM-1 chain ends were also controlled to investigate their effects on the miscibility and the microporosity. PIM-1 was then applied as an artificial solid interphase for Li metal anode, allowing the transport of Li+ while prohibiting the transport of other components including solvents and the other ions. OIM-1 was utilized as an organic porogen for mixed-matrix membranes to improve their separation performance. Furthermore, olefin/paraffin separation membranes with silver nanoparticles (Ag NPs) were also included. Only olefin molecules like ethylene, propylene have reversible interactions with surface activated silver nanoparticles (S-Ag NPs). These specific and reversible interactions help facilitate the transport of olefin molecule only and consequently to improve the separation performance of olefin/paraffin mixtures. In Chapter 1, polymerization, especially condensation polymerization, was intensively reviewed in terms of how to control the molecular weight and the functional group of the chain ends. Molecular transports though porous materials were subsequently introduced for improvement of molecular separation. In particular, effects of the free volume in both PIM-1 and OIM-1 having high fractional free volume on the increase in the molecular permeability were emphasized along with their several applications. Finally, historical advance of facilitated transport for separation of olefin/paraffin mixtures was reviewed. In Chapter 2, synthesis and characterization of PIM-1 and OIM-1 with various methods were attempted along with the monomer purification and the separation of high molecular weights of PIM-1. Molecular weights and polydispersity of both PIM-1 and OIM-1 are various with synthesis conditions. In particular, molecular weights and end groups were regulated by adjusting the monomer ratio following the Carothers equation. In Chapter 3, PIM-1 film was used as an artificial solid interphase for Li metal anodes. The unique pore size and hydrophobicity of PIM-1 could reduce the solvation number of Li ions in electrolytes, which suppress the transport of solvent and unexpected chemical reaction between Li metal and electrolytes. Consequently, the cycle life of batteries with PIM-1 coated Li metal anodes was extended. Desolvated Li ions were detected by nuclear magnetic resonance (NMR) and suppressed Li dendrite formation and dead Li also observed by microscopic images. In Chapter 4, OIMs were utilized as an organic porogen which was incorporated into a polysulfone (PSU) matrix to make mixed-matrix membranes (MMMs) for gas separation. OIM with controlled molecular weights and end groups were synthesized for improvement for both their compatibility with PSU and consequently gas separation properties. OIM/PSU MMMs shows better compatibility than PIM-1/PSU MMMs. Also, The MMMs with OIM/PSU show enhanced permeability with little sacrifice of ideal separation factor. In Chapter 5, facilitated olefin transport membranes with surface-activated silver nanoparticles were introduced. Silver nanoparticles (Ag NPs) were synthesized by polyol reduction among polyvinylpyrrolidone (PVP), ethanol (EtOH) and silver tetrafluoroborate (AgBF4). Silver nanoparticles were characterized by UV-Vis spectra and transmission electron microscopy (TEM) images. 7,7,8,8-tetracyanoquinodimethane (TCNQ) were used for electron acceptors to activate surface of silver nanoparticles. The PVP/Ag NPs/TCNQ shows high olefin/paraffin separation performances by interaction between silver nanoparticles and olefin molecules. In conclusions, intrinsically microporous polymers with high molecular weights and regulated molecular weights were successfully synthesized. The synthesized microporous materials were applied for Li metal batteries and gas separation membranes. Unique micropores in PIM-1 separated Li ion from its solvation shells, improving the cycle life of Li metal batteries. Also, the microporosity of OIM-1 effectively created microvoids in many polymer matrices and consequently the fractional free volume of the OIM/PSU matrix were increased. The extra microvoids enhanced the gas permeability with little expenses of the ideal separation factor. PVP/Ag NPs/TCNQ composites successfully separated olefin/paraffin mixtures with a high selectivity. It was confirmed that 7,7,8,8-tetracyanoquinodimethane (TCNQ) as surface polarizers could induce partially positive charge on the surface of silver nanoparticles.