(14f) Layer-By-Layer Assembly for Water Desalination and Gas Separation

1^st Year Postdoctoral Fellow

Research Interests:

Layer-by-layer (LbL) assembly is a bottom-up manufacturing technique that uses complementary interactions between components to deposit materials one layer at a time. Since a single layer is usually 1-100 nm thick, the structure of LbL assembly can be controlled with nanometer precision. This level of precision is unobtainable by traditional processing techniques, such as melt blending and solution mixing. Additional customization of the LbL assembly can be realized by changing its components and altering complementary interactions between the components. Besides being precise and versatile, LbL assembly is also very simple, allowing industrial-scale production through conventional dipping, spraying, or spin-coating procedures. These collective advantages make LbL assembly suitable for many applications, including anti-fogging, anti-microbial, drug delivery, and energy generation and storage. I am particularly interested in making advanced LbL assemblies for high-performance water filtration and gas separation applications.

Teaching Interests:

I will kindle studentsâ?? interest in polymer science by relating classroom content with popular subjects among young people. Customized teaching methods will also be utilized to effectively reach every student as well as encourage the participation of underrepresented groups in science. Throughout my career, I plan to continually improve my teaching by seeking student feedback. Upon completing my course with a final group project, I expect my students to have a strong understanding of the fundamentals of polymers (or even fall in love with polymers), and have improved their critical thinking ability and communication skills. Based on this highly customizable teaching plan, my training in STEM teaching, and my background in polymer science and materials science, I am well prepared for teaching several polymer science and engineering courses, including polymer physics, membrane and separation technology, surface and interfaces, materials science and engineering, and multifunctional materials.

Completed Proposal:

I was a co-principal investigator for a proposal to the U.S. Department of the Interior, Bureau of Safety and Environmental Enforcement (E16PS00024); this proposal currently is under second-round review.

Postdoctoral Project:

â??Layer-by-Layer Assemblies with Advanced Gas Separation Propertiesâ?

Under supervision of Dr. David Hopkinson, Department of Energy, National Energy Technology Laboratory

PhD Dissertation:

â??Improvements in Processing and Stretchability of Super Gas Barrier Multilayer Thin Filmsâ?

Under supervision of Prof. Jaime Grunlan, Department of Mechanical Engineering, Texas A&M University

M.S. Thesis:

â??Modification of Immiscible HDPE/PA6 Blends using Carbon Nanotubesâ?

Under supervision of Prof. Yong Wang, School of Materials Science and Engineering, Southwest Jiaotong University

Research Experience:

My research centers on structure-property relationships of polymeric materials. My first research project involved toughening immiscible polymer blends using carbon nanotubes (CNTs). In that project, I improved the toughness by selectively dispersing CNTs onto the interfaces between polymer domains. However, I was unable to control the dispersion of all of the CNTs, due to their strong Brownian motion in molten polymer blends. The desire to fully control the dispersion of nanofillers in polymer matrices propelled me to search for a new solution. I found the answer to this problem in LbL assembly, which can be used to disperse two-dimensional nanosheets in a polymer matrix with very high degrees of exfoliation and orientation. This unique layered-structure imparts exceptional gas barrier by creating a tortuous path for diffusion gas molecules, and improves the mechanical property of the thin film beyond predictions of the Halpin-Tsai model. By replacing rigid nanoparticles with soft polymers that combine with each other through H-bonding, I produced stretchy gas barrier coatings that remained functional under high-strain conditions. My current research on gas-separation membranes helps me better understand the transportation behaviors of different gases within polymeric thin films, and inspires me to develop new LbL assembly membranes for advancing separation technologies for gases and liquids.

Teaching Experience:

I actively sought opportunities to improve my teaching skills. I was a teaching assistant for an undergraduate-level Materials Science and Engineering course for one year, and enjoyed helping students solve problems. I also learned the â??dos and donâ??tsâ?? of teaching, based on student feedback. I received training from a Science, Technology, Engineering and Math (STEM) teaching program, and guest-lectured an undergraduate-level class on ceramic materials. I have mentored several undergraduate students in my research groups, and almost all of my mentees continued their studies in graduate schools.

Future Direction:

I anticipate building a research group that uses state-of-the-art nanotechnology to counter pressing global challenges, such as water shortage, clean energy generation, and climate change.

Using LbL assembly for preparing water desalination membranes has attracted significant attention because LbL assembly features a multi-bipolar structure. This structure can be used to separate monovalent and divalent ions based on â??Donnan exclusionâ?. However, current LbL membranes are either too thick to permit high water flux, or too thin to provide sufficient salt rejection. Using rational post-treatment procedures, we can design a LbL assembly that features a nanoporous skin layer and a macroporous support layer, which impart high salt rejection and high water flux properties, respectively. This technology would improve our understanding on water desalination and should help reduce the global shortage of fresh water.

Besides separation in liquid media, I am also interested in gas separation. Specifically, I plan to produce multilayer membranes for hydrogen purification and carbon dioxide separation (the latter is also known as carbon capture). The hydrogen purification membrane exploits the difference between the kinetic diameters of different gas molecules. Specifically, graphene oxide sheets with tunable nanopores will be used as embedded molecular sieving nanofillers to generate high-purity hydrogen for fuel cells. For carbon capture membranes, I plan to use LbL assembly in conjunction with supported ionic liquid membranes to combine their advantages in mechanical and separation properties. Properly designed and developed films can be used to capture carbon dioxide to help counter global climate change.

The techniques and strategies summarized above can also be used to prepare other kinds of LbL assemblies with designed nanostructures and properties that are suitable for chemical sensing, self-healing, and self-cleaning purposes.

Select Publications:

F. M. Xiang, D. Parviz, T. M. Givens, P. Tzeng, E. M. Davis, C. M. Stafford, M. J. Green, J. C. Grunlan. Stiff and Transparent Multilayer Thin Films Prepared Through Hydrogen-Bonding Layer-by-Layer Assembly of Graphene and Polymer. Advanced Functional Materials, 2016, 26, 2143.

C. Y. Cho, F. M. Xiang (co-first author), K. L. Wallace, J. C. Grunlan. Combined ionic and hydrogen bonding in polymer multilayer thin film for high gas barrier and stretchiness. Macromolecules, 2015, 48, 5723.

F. M. Xiang, T. M. Givens, S. M. Ward, J. C. Grunlan. Elastomeric polymer multilayer thin film with sustainable gas barrier at high strain. ACS Applied Materials & Interfaces, 2015, 7, 16148.

F. M. Xiang, T. M. Givens, J. C. Grunlan. Fast spray deposition of super gas barrier polyelectrolyte multilayer thin films. I&EC Research, 2015, 54, 5254.

F. M. Xiang, S. M. Ward, T. M. Givens, J. C. Grunlan. Structural tailoring of hydrogen-bonded poly(acrylic acid)/poly(ethylene oxide) multilayer thin films for reduced gas permeability. Soft Matter, 2015, 11, 1001. Featured on Soft Matter Blog as a hot article for January 2015.

F. M. Xiang, S. M. Ward, T. M. Givens, J. C. Grunlan. Super stretchy polymer multilayer thin film with high gas barrier. ACS Macro Letters, 2014, 3, 1055.

F. M. Xiang, P. Tzeng, J. S. Sawyer, O. Regev, J. C. Grunlan. Improving the gas barrier property of clay-polymer multilayer thin films using shorter deposition times. ACS Applied Materials & Interfaces, 2014, 6, 6040.

For more information, please visit my personal website http://fangmingxiang.weebly.com