Date of Award

8-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Husson, Scott M

Committee Member

Wagener , Earl H

Committee Member

Hirt , Douglas E

Committee Member

Kitchens , Christopher L

Abstract

Membrane technologies have been used widely in industrial practices such as waste water treatment, sea water desalination, gas separations, pharmaceutical separations, and food and beverage processing to name a few. Among the features that make them attractive are that they are cost effective, highly efficient, scalable, and environmentally benign. The focus of this dissertation is to contribute to the understanding and use of polymeric membranes for carbon dioxide separations.
Currently, the largest carbon capture demand lies in the natural gas industry. However, since global warming has become a severe concern for human existence, flue gas cleaning is potentially another very important market for carbon capture. Therefore, I chose to address the membrane-based separation of carbon dioxide from nitrogen, a mixture that results from post-combustion processes using air as the oxygen source.
The work was done in close collaboration with Tetramer Technologies, LLC (Pendleton, SC). In addition to intellectual capital, Tetramer provided their perfluorocyclobutyl (PFCB) polymers, which are a new class of fluorinated polymers with promising application in O2/N2, CO2/methane, and flue gas separations. I used a representative polymer, biphenylvinyl-PFCB, as the selective layer in the development of thin-film composite membranes for CO2/N2 separations.
Different from ultrafiltration and microfiltration membranes, polymeric thin-film composite (TFC) membranes usually are nonporous. Their separation ability is not just size based but also based on the interactions between gases and the polymer(s). A solution-diffusion mechanism is the most popular theory explaining the transport phenomena inside these membranes. Oftentimes, glassy polymers such as PFCB are used as the selective layer in TFC membranes, as they have higher selectivity than rubbery polymers. However, glassy polymers, including PFCB, are associated with the problems of physical aging and plasticization. Many researchers have found that these two phenomena are more significant in thin polymer films. Unfortunately, flux of gas through polymeric membranes is inversely related to polymer thickness; thus, thin polymer films (<1000 >nm) are more practical than bulk films for industrial applications. A goal of my work was, therefore, to elucidate structure-property relationships for Tetramer PFCB polymers that provide a foundation for TFC membranes with resistance to CO2 plasticization and physical aging. Specifically, my doctorate research consists of three parts: 1) study of PFCB thin film formation, 2) development and characterization of PFCB thin-film composite membranes, and 3) elucidation of the roles that plasticization and physical aging play on PFCB thin-film performance.
In part 1, I conducted comprehensive research to understand how PFCB thin films form by the method of dip coating. Through the control of solvents, polymer solution concentrations, and withdrawal speeds, a series of PFCB thin films were formed on silicon wafers. Film thickness and refractive index were characterized by ellipsometry. Results suggested that when the withdrawal speeds are higher than 50 mm/min, film thickness increases with increasing withdrawal speeds, as it is predicted in the proposed extension of the Landau-Levich model. When the withdrawal speeds are lower than 50 mm/min, film thickness increases with decreasing withdrawal speeds, which could be explained by the phenomenon of PFCB surface excess. Subsequent surface tension studies proved the existence of this surface excess. Surface images of these films were measured by atomic force microscope. Films prepared from tetrahydrofuran and chloroform yielded uniform nanolayers. However, films prepared using acetone as solvent yielded a partial dewetting pattern, which could be explained by a surface depletion layer of pure solvent between the bulk PFCB/acetone solution and the substrate.
Based on the knowledge generated in part 1, I developed, from scratch, procedures to prepare PFCB TFC membranes that were free of major defects. I used mathematical models based on resistance in series to predict composite membrane performance. In many cases, surface defects are the major reason for poor separation ability of TFC membranes. Mathematical analysis showed that the surface defects are less critical in thinner films but are still an important factor causing selectivity loss. Surface defects occur mainly from polymer dewetting on the support substrate. A method of plasma treatment was developed to modify the surface of the proprietary gutter layer, which is highly permeable but not very selective. Forced wetting was proposed to guide the degree of surface modification that was needed. Finally, PFCB TFC membranes were successfully produced with excellent reproducibility. The selective PFCB layers were between 10 and 59 nm. Measured CO2 permeance through PFCB layers were as high as 1700 GPU (gas permeation units) with CO2/N2 selectivity close to 20. Membrane performance was highly reproducible.
In the last part of this dissertation, plasticization and physical aging were evaluated on different PFCB thin-film selective layers. Measurement of plasticization curves was the focus of this part of the study. PFCB thin-film selective layers were prepared from solutions with different PFCB concentrations using four different dip-coating withdrawal speeds. I discovered that thickness played a more important role than coating solution concentration on PFCB thin-film plasticization when film thickness is below 35 nm; whereas, concentration played a more important role when film thickness is above 35 nm. Impacts of different thermal treatments and physical aging on film plasticization also were analyzed. Thermal annealing effectively enhanced the plasticization pressure at the expanse of permeance loss. PFCB films aged for longer times had lower plasticization pressure. Long-term flux studies revealed a complicated relationship between plasticization and physical aging and their roles on membrane permeance. During the early stage of membrane exposure to CO2, plasticization has a competing effect with physical aging, but, at longer times, plasticization appears to accelerates physical aging.
In all, my doctorate research provides a systematic method to fabrication TFC membranes with high performance and protocols to evaluate PFCB polymer thin film plasticization and physical aging.

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