Polybenzimidazoles, a class of aromatic heterocyclic polymers, are well known due to their remarkable thermal stability, mechanical properties and chemical resistance which are often required in extreme operation conditions. Because of these properties, polybenzimidazoles are excellent candidates in various application areas including proton exchange membrane fuel cells, gas separation membranes, reverse osmosis and nanofiltration, and high performance coatings. The following studies are focused on the synthesis, characterization and related properties of polybenzimidazoles and polybenzimidazole based materials.
A novel sulfonyl-containing tetraamino-substituted monomer (3,3',4,4'-tetraaminodiphenylsulfone) was synthesized and polymerized with three different diacid monomers to make polybenzimidazoles. The new monomer synthesis route with reduced steps relative to the existing literature method increased the overall yield by a factor of three. The sulfonyl-containing polybenzimidazoles have enhanced solubilities in common organic solvents including dimthylsulfoxide, dimethylacetamide and N-methyl-2-pyrrolidone in comparison with the commercial polybenzimidazole, Celazole®, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole). The improvements in solubility are attributed to the introduction of polar sulfonyl linking moiety in the monomer. Remarkable thermal stabilities (high T<sub>g</sub>, > 428 °C) were demonstrated through Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 μm thick) for gas transport measurements. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation.
There is a trade-off relationship between gas permeability and selectivity through dense, non-porous polymer membranes that was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve both permeability and selectivity simultaneously. Gas permeability is related to the free volume between polymer chains. In order to improve gas permeability, we hypothesized a concept that increasing free volume could be achieved by thermally degrading sacrificial components and volatilizing their byproducts from a glassy matrix. Volatile components were introduced into the films to preoccupy the spaces between polymer chains. Once they were degraded and removed through the thermal treatment, it was hypothesized that the preoccupied spaces would remain empty due to the glassy nature of the matrix at the heat treatment temperature, thus resulting in more free volume. Two post- modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. The H₂ permeability of the post-modified TADPS-OBA polybenzimidazole increased from 3.1-6.2 Barrers to 5.2-7.5 Barrers (up to 66% increase). The selectivity for H₂/CO₂ increased from 7.5-10.5 to 10.1-13.0 (up to 33% increase). The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix. / Ph. D. / Polybenzimidazoles represent a class of polymeric high performance materials due to their remarkable thermal stability, mechanical properties and chemical resistance. They are competitive material candidates for applications involving extreme conditions including high pressure and high temperature. The following studies are focused on the synthesis, characterization and properties of polybenzimidazoles and polybenzimidazole based copolymers and blends. Of particular importance to this dissertation are the gas transport properties. The new materials are excellent candidates for making non-porous membranes that can separate very small molecules such as nitrogen, oxygen, carbon dioxide, and hydrogen. The non-porous membranes achieve separations of such small molecules by having the gases solubilize in the upstream side of a membrane, diffuse through it, then evaporate from the downstream side. This mechanism is known as the solution-diffusion mechanism.
The monomer, 3,3’,4,4’-tetraaminodiphenylsulfone, was synthesized via our designed synthesis method that was simpler than previous methods described in the literature and with a 3 times higher yield. A series of polybenzimidazoles with systematically varied chemical structures were prepared and it was demonstrated that they all had enhanced solubilities in common organic solvents over the only known commercial polybenzimidazole, Celazole®. This is particularly important for membrane materials because they must be fabricated into thin films from solution. Remarkable thermal stabilities for polymeric materials with glass transition temperatures above 400 °C were found for these polybenzimidazoles. A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 µm thick) and their gas transport properties were measured. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation.
The gas transport properties involve two important parameters, permeability and selectivity. A trade-off relationship between the two parameters was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve permeability and selectivity at the same time. In order to improve gas permeability, we hypothesized a concept that increasing permeability could be achieved by creating more spaces between the polymer chains in non-porous films. Sacrificial components were introduced into the films, then thermally degraded and the byproducts were volatilized to remove them from the film. It was further hypothesized that conducting the heat treatment process at a temperature where the matrix polymer was in the glassy state would allow the matrix polymer to preserve the free volume introduced by the volatization. Two post-modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. This is an important separation that could economically be carried out at elevated temperatures (~250°C) if the polymer membrane would withstand such a temperature. It could be utilized to separate H₂ from CO₂ in pre-combustion syngas. This is the major method for H₂ production worldwide. The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/96200 |
| Date | 29 June 2018 |
| Creators | Liu, Ran |
| Contributors | Chemistry, Riffle, Judy S., Lesko, John J., Liu, Guoliang, Turner, S. Richard, Davis, Richey M. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0027 seconds