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The performance of organic-inorganic composite membranes (or mixed matrix membranes) is greatly influenced by the nature and structure of the interfaces between the two dissimilar phases. Thus it is important to understand the interaction between the organic and inorganic phases at their interface, ultimately enabling design of the interface to fully realize the potential for dramatically improved membrane performance by using highly selective phases in a polymer matrix. It has, however, proven very difficult to examine the interfacial region in these membranes in part because of their small volume and also because of the limitations of conventional characterization techniques. This two-year new investigator project focuses on developing model systems and applying non-destructive techniques, fluorescence spectroscopy and Raman spectroscopy, to gain a quantitative understanding of the structure and properties of these interfaces. The working hypothesesof the proposed work are: 1) the interfaces between micro-/meso-porous metal oxide and polymer phases (in mixed matrix membranes) are very different from those between non-porous oxides and polymer phases and 2) the interfacial structures can be controlled by the surface chemistry as well as the surface nanostructures of metal oxides and the polymer chemistry.

The first objectiveis to develop two dimensional model systems of micro-/meso-porous oxides with tunable surface chemistry and surface nanostructures, and to determine the structures of polymers in the interfacial regions using fluorescence and Raman spectroscopies. Understanding the interfacial structures of polymers with micro-/meso-porous metal oxides provides a fundamental basis to intelligently tune and rationally design the interfaces to successfully realize the full promise of mixed matrix membranes.

The second objectiveis to study the effects of the surface chemistry of the metal oxides as well as their surface nanostructures on the interfacial structures. For maximum performance enhancement, the glass transition temperature (Tg) shift and the thickness of polymer in the interfacial regions is to be minimized while maintaining good adhesion between polymer and oxide surface.

The work that has been conducted as part of this grant makes two contributions to achieve these objectives: (1) develop two-dimensional model systems, and (2) construct a custom-made sample stage with temperature control and incorporate into a fluorescence spectrometer. In the second year, we have focused on the first part of the work. This is primarily due to the fact that it took a while to get a new student since the student working on this project has left. Note that the PI has requested for the no-cost extension of the project.

The first objective of this work has dealt with developing new methods. In the first year, we have developed two methods, “micro-tiles-and-mortar-joint method” and “thermal seeding method”. Due to the simplicity as well as the controllability of the preferred orientation of zeolite crystals, we decided to focus on the micro-tiles-and-mortar-joint method. However, improving the method developed in the first year, we have realized that the metal coating used to prevent the out-of-plane growth introduces grain boundary defects upon the dissolution of the metal. In the second year, a self-assembled organosilane monolayer is used as passivation layer. The molecular-scale chemical modification of the large surface area of seed crystal surfaces is obtained by micro-contact printing method. The flat facets of seed crystals was passivated by transferring hydrophobic organosilane molecules using polydimethylsiloxane (PDMS) stamp. The key of this method is to deposit passivating organosilane molecules on the flat facet of seed crystals where the PDMS stamp has close contacts, while the side faces of seed crystals are free of passivation molecules where the PDMS stamp does not have contact.  A part of this portion of the work has been conducted by Inho Lee and Miral Shah. Inho Lee obtained his PhD and now works at Dow Chemical. Miral Shah is a PhD student. One journal paper has been published and another accepted for publication.

In summary, the grant PRF# 48884-DNI5 partially supported one PhD student in the second year and resulted in two journal papers that acknowledge ACS-PRF support.

Bibliography  Journal Publications

[1] H.-K. Jeong, “Zeolite and Zeolite/Polymer Composite Membranes: Promises and Challenges”, Appl. Chem. Eng., 2010, 21(5), 481

[2] M. Shah, M. C. McCarthy, S. Sachdeva, A. K. Lee, and H.-K Jeong, “Current Status of Metal-Organic Framework Membranes for Gas Separations: Promises and Challenges”, Ind. Eng. Chem. Res. 2011, accepted.

[3] M. Shah, I. Lee, and H.-K Jeong, “Highly b-Oriented MFI Zeolite Films by Controlled Secondary Growth Using Micro-contact Printing Passivation”, in preparation (2011)