Browsing by Subject "Exfoliation"

Now showing 1 - 3 of 3
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    Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane
    (2013-10) Agrawal, Kumar Varoon
    In the wake of the energy crisis, an efficient separation technology such as membrane is required to replace the energy intensive processes like distillation. High performance zeolite membrane can be fabricated by coating of a thin film of high-aspect-ratio zeolite nanosheets on a porous support. However, the synthesis of highly crystalline and morphologically intact zeolite nanosheets by the direct hydrothermal synthesis has been challenging. Successful reports on the synthesis of zeolite nanosheets by the exfoliation of their layered structure exist, but the synthesis routes provided in these reports often lead to significant damages to the structure and the morphology of nanosheets. This dissertation focuses on the development of a scalable method for the synthesis of zeolite nanosheets, while preserving their structure and the morphology. MWW and MFI nanosheets were prepared by polymer melt compounding of their layered precursors with polystyrene. Zeolite nanosheets were extracted out of the polymer matrix by solution processing of the zeolite-polymer nanocomposite. Exfoliated nanosheets and their coatings were then characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). A compact, oriented, 300-nm thick zeolite film was fabricated on a symmetric alumina support by a one-step filter coating method. This nanosheet film demonstrated molecular sieving capabilities after a mild hydrothermal treatment. Density gradient centrifugation was used to purify the zeolite nanosheets from the polymer matrix, and the large unexfoliated particles, resulting in a two-fold increase in the yield of nanosheets in the final coating suspension. Sub-100 nm thick films of these nanosheets were made on a symmetric alumina supports. Nanosheet films with thickness ranging from 10 nm to 100 nm were prepared on an asymmetric silica supports. In-plane secondary growth of these films by the impregnation growth method led to b-oriented, 100-150 nm thick zeolite film that separated xylene isomers with separation factors of 100-800, while providing a high permeance of p-xylene (4 x 10-7 moles/m2-s-Pa).</
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    Exfoliated zeolite sheets and block copolymers as building blocks for composite membranes.
    (2009-08) Maheshwari, Sudeep
    Mixed matrix materials, comprising of zeolites incorporated in suitable matrix (polymeric or inorganic), are promising as future membrane materials with high permselectivity. However, they suffer from the drawback of low productivity due to increase in the membrane thickness by incorporation of micron-sized zeolites crystals as well as the low-permeability matrices employed currently. Nanocomposite membranes, consisting of thin zeolite sheets (~2 nm) embedded in an appropriate matrix, can provide a solution to this problem. This thesis addresses some of the material challenges to make such nanocomposite membranes. A high permeability polymer was synthesized by combining the glassy polystyrene (PS) with the rubbery polydimethylsiloxane (PDMS) in a block copolymer architecture. The mechanical toughness of the material was optimized to facilitate the fabrication of thin free standing films and its gas transport properties were evaluated. The PS-PDMS-PS triblock copolymers were successfully hydrogenated for the first time to obtain the PCHE-PDMS-PCHE triblock copolymers (PCHE stands for polycyclohexylethylene). The hydrogenation reaction proceeded without any polymer chain breaking and the resultant polymer showed some interesting, rather unexpected thermodynamic properties. These polymeric materials are potentially useful as the matrix of nanocomposite membranes. Highly crystalline zeolite sheets were obtained by exfoliation of zeolite lamellae. Preservation of crystal morphology and pore structure, which presents a major challenge during the exfoliation process, was successfully addressed in this work by judicious choice of operating conditions. Lamellae were exfoliated by surfactant intercalation and subsequently melt processing with polymers, resulting in polymer nanocomposites containing thin zeolite sheets (~2.5 nm) with well preserved pore structure. A method to obtain polymer-free exfoliated sheets was also developed to facilitate the fabrication of inorganic composite membranes. These zeolite sheets can be used as the selectivity-enhancement additive in composite membranes.
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    Two-dimensional clay and graphene nanosheets for polymer nanocomposites and energy storage applications
    (2013-08) Qian, Yuqiang
    Clay and graphene nanosheets are attractive to materials scientists due to their unique structural and physical properties and potentially low cost. This thesis focuses on the surface modification and structure design of clay and graphene nanosheets, targeting special requirements in polymer nanocomposites and energy storage applications. The high aspect ratio and stiffness of clay and graphene nanosheets make them promising candidates to reinforce polymers. However, it is challenging to achieve a good dispersion of the nanosheets in a polymer matrix. It is demonstrated in this study that organic modifications of clay and graphene nanosheets lead to better filler dispersion in polymer matrices. A prepolymer route was developed to achieve clay exfoliation in a polyurethane-vermiculite system. However, the phase-separated structure of the polyurethane matrix was disrupted. Intragallery catalysis was adopted to promote the clay exfoliation during polymerization. With both catalytic and reactive groups on the clay modifier, the polyurethane-vermiculite nanocomposites showed a significant increase in modulus and improved barrier performance, compared to neat polyurethane. The toughening effect of graphene on thermosetting epoxies and unsaturated polyesters (UPs) was also investigated. Various types of graphene with different structures and surface functionalities were incorporated into the thermosetting resin by in situ polymerization. The toughening effect was observed for epoxy nanocomposites at loading levels of less than 0.1 wt%, and a peak of fracture toughness was observed at 0.02 or 0.04 wt% of graphene loadings for all epoxy-graphene systems. A microcrack-crazing mechanism was proposed to explain the fracture behavior of epoxy-graphene systems based on fractography observations. Similar peak behavior of fracture toughness was not observed in UP system. UP nanocomposites with modified graphene oxide showed better mechanical performance than those with unmodified graphene oxide, which was attributed to better graphene dispersion and a stronger UP-graphene interface. Graphene has also been extensively studied in energy storage applications, due to its high conductivity and surface area. In order to utilize the benefits of graphene, macroscopic graphene/V2O5 films and graphene aerogels were fabricated from the self-assembly of graphene materials. The unique 2D structure of graphene helped to maintain the integrated film morphology in graphene/V2O5 composites and the monolithic macroporous structure in graphene aerogels. Good conductivity was obtained by incorporation of graphene sheets in the structure, which results in good electrochemical performance as electrode materials for batteries or supercapacitors. The facile preparation methods allow good control of the composition and thus the properties of the macroscopic graphene nanostructures.