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Layer multiplying “forced assembly” micro and nanolayer coextrusion is used to produce films with thousands of layers containing two or more alternating polymers. This dissertation focuses on the structure-property relationships in layered polymeric systems for applications including confined crystallization for high gas barrier applications and multilayer dielectric films for high energy density capacitors.CHAPTER 1: A review of recent progress of confined polymer crystallization using forced assembly nanolayer coextrusion is described. Confinement of crystalline polymer materials in layer thicknesses ranging from hundreds to tens of nanometers thick resulted in multilayer films possessing enhanced gas barrier properties. The enhanced gas barrier has been attributed to nanolayer confinement of the crystalline polymer resulting in a highly ordered intralayer lamellae orientation extending over micron or larger scale areas. Research into the confined crystallization mechanism of the multilayered polymer films has resulted in several material case studies as well as an understanding of the chemical and thermodynamic parameters that control the degree and rate of the confinement in multilayer polymer systems. This review highlights our recentstudies on the confinement of poly(ethylene oxide), poly(caprolactone), polypropylene, and poly(vinylidene fluoride) polymers in multilayered films. CHAPTER 2: The structure and morphology of polyethylene terephthalate (PET) and poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDFTFE)] were investigated under nanolayer confinement. In addition, both biaxial stretching and isothermal melt recrystallization were used as an additional approach to manipulate the morphologies of these confined polymers. The synergistic combination of nanolayering, biaxial stretching, and isothermal recrystallization facilitated the formation of unique morphologies in the P(VDFTFE) layers. These structures, specifically the high aspect ratio in-plane P(VDFTFE) crystals, resulted in a 50-100x reduction in the oxygen and water permeability properties by increasing the diffusion pathway, or tortuosity, around these confined structures.CHAPTER 3: Commercial polymer films for capacitors include biaxially oriented polypropylene and biaxially oriented PET. These materials possess rather low energy densities (5-6 J/cc) due to their low dielectric constants (2-4). Our approach to improve the energy density of polymer films is to combine a high resistance, high breakdown strength polymer (PET) with a high dielectric constant polymer (P(VDF-TFE), dielectric constant ~ 12) in an ABAB nanolayered film. These multilayer films exhibit enhanced breakdown properties as compared to the single layer materials. Mimicking the commercial materials, biaxial orientation was also used on the mulitlayer films, which resulted in an even further enhancement in the breakdown and energy density properties, with measured values as high as 16 J/cc for the biaxially oriented multilayer samples. This enhancement was attributed to the ability of the layered structure to discharge along the layer interface resulting in a “treeing” fracture mechanism unique to the layered materials. The confined structures of PET and P(VDF-TFE) were also correlated to the dielectric and breakdown properties. It was found that the crystal orientation of the P(VDF-TFE) layer played a critical role in the determination of the bulk dielectric constant of the overall film. The on-edge P(VDF-TFE) crystal orienatation greatly enhanced the dielectric constant of the P(VDF-TFE) layer. In addition, this also increased the dielectric contrast between the PET and P(VDFTFE) layers resulting in larger tree areas, larger tree branches, and enhanced breakdown properties. |
