http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Gas Barrier and Separation Behavior of Layer-by-Layer Assemblies
Thin films with the ability to control gas permeability are crucial to packaging and purification applications. The addition of impermeable nanoparticles into neat polymers improves barrier/separation properties by creating a tortuous pathway, but particle aggregation occurring at high filler loading can reduce transparency of these composites as well as barrier/separation improvement. Layer-by-layer (LbL) assembly allows full control of morphology at the nanoscale, so barrier/separation properties can be precisely controlled and the films remain flexible and transparent. A three component recipe, consisting of polyvinylamine, poly(acrylic acid) and montmorillonite clay was deposited as repeating PVAm/PAA/PVAm/MMT quadlayers (QL) via LbL assembly. By adjusting solution pH and varying the placement of polycation layers, polymer interdiffusion and clay concentration were controlled, as well as oxygen barrier. Another QL assembly, with a PEI/PAA/PEI/MMT repeating sequence, was deposited using LbL to create light gas barrier films. Transmission electron microscope images revealed a highly-oriented nanobrick wall structure. A 5 QL coating on 51 [micro]m polystyrene (PS) is shown to lower both hydrogen and helium permeability three orders of magnitude relative to bare PS, demonstrating better performance than ethylene vinyl-alcohol (EVOH) copolymer film and even metallized plastic. Graphene oxide sheets, along with chitosan and PAA were used in a CH/PAA/CH/GO QL assembly. pH deviation between CH and PAA ionizes the counter ion, creating a more interdiffused polymer matrix and resulting in higher GO loading. Thermal reduction of GO, provides improvement in barrier performance under humid conditions and better H₂/CO₂ separation behavior. Finally, a PEI/PAA multilayer membrane exceeding that of the current state-of-the-art gas separation membranes was performed. This ionically crosslinked assembly exhibits H₂/N₂ and selectivities beyond Robeson's upper-bound limit, which are superior to the properties of most organic, inorganic or mixed-matrix membranes reported in the open literature, making it a significant advance in polymeric membranes for gas separation. In conclusion, barrier and separation behavior were improved via either a polymer-polymer or polymer-nanoplatelet assembly. Several treatments, including interdiffusion or thermal reduction proved to enhance film performance. Future works focus on applying LbL techniques in Bragg reflectors and CO₂/N₂ separation. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/155066
Doctor of Philosophy Texas A & M University 2015 http://hdl.handle.net/1969.1/155066