Structure and properties of nano-confined main chain liquid crystalline polymers
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Main chain liquid crystalline polymers (MCLCPs) possess many interesting properties, including extraordinary gas/water barrier properties, chemical resistance and mechanical strength, all which imparted by the special chain ordering in liquid crystal phases. Hence, MCLCPs are very attractive candidates in a variety of applications (e.g. high performance gas barrier and separation membranes, solar cells and electronic devices, etc.). Previous studies have shown that the structure and thermophysical properties of polymers can be dramatically affected by nano-confinement. However, most of the studies performed in this area are focused on the effect of nano-confinement of conventional amorphous or semi-crystalline polymers. A comprehensive understanding of structure and ordering of nano-confined MCLCPs is still lacking. Nano-confined MCLCPs (e.g. nanoscale MCLCP thin films) have been widely used in organic electronic devices and fuel cells, therefore, a better fundamental understanding of their structure and properties is key for controlling and improving performance in these applications. In this dissertation, various systems (i.e. multilayer coextrusion, spin coating and nano-imprint lithography) were employed to study the effect of nano-confinement on the structure and properties of MCLCPs. High quality MCLCP multilayer films were fabricated through coextrusion, which could be potentially used as high performance gas/water barrier packaging films. Further studies on spin coated single layer films have shown that the nano-confined MCLCP chains adopted a highly ordered in-plane structure with their chain axis aligned parallel to the film plane. Such unique chain ordering imparted more than an order of magnitude lower gas permeability when compared to that of the bulk due to the increased tortuosity of the gas molecule diffusion pathway. Furthermore, nanoimprint lithography was performed to simultaneously control the chain structure and alignment by nano-confinement and the squeeze flow present during processing.