Structure-Processing-Property Interrelationships of Vapor Grown Carbon Nanofiber, Single-Walled Carbon Nanotube and Functionalized Single-Walled Carbon Nanotube – Polypropylene Nanocomposites
Type of Degreedissertation
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This dissertation describes the first use of a design of experiments approach to investigate the interrelationships between structure, processing, and properties of melt extruded polypropylene (PP) carbon nanomaterial composites. The effect of nanomaterial structure was evaluated by exploring the incorporation of vapor grown carbon nanofibers (VGCFs), or pristine or functionalized single-walled carbon nanotubes (SWNTs or C12SWNTs) in polypropylene, while the effect of processing was investigated by studying the influence of melt extrusion temperature, speed, and time. The nanomaterials and PP were combined by an initial mixing method prior to melt extrusion. The nanocomposite properties were characterized by a combination of morphological, rheological, and thermal methods. Preliminary investigations into the effects of the initial mixing method revealed that the distribution of nanomaterials obtained after the mixing had a considerable influence on the properties of the final melt extruded nanocomposite. Dry mixing (DM) resulted in minimal adhesion between nanomaterials and PP during initial mixing; the majority of nanomaterials descended to the bottom. Hot coagulation (HC) mixing resulted in extremely high degrees of interaction between the nanomaterials and PP chains. Rotary evaporation (RE) mixing resulted in nanomaterial distribution uniformity between that obtained from DM and HC. Employing design of experiments to investigate the effects of structure and processing conditions on melt extruded PP nanocomposite properties revealed several interesting effects. The effect of processing conditions varied depending on the degree of nanomaterial distribution in PP attained prior to melt processing. Increasing melt extrusion temperature increased the decomposition temperature (Td) of PP/C12SWNT obtained from HC mixing but decreased Td of PP/C12SWNT obtained from RE mixing. Higher melt extrusion screw speed, on the other hand, significantly improved the nanocomposite crystallization behavior in RE nanocomposites, while not being a major processing factor in HC nanocomposites. The variations in nanocomposite properties with processing conditions were the result of complex interactions between the degree of dispersion, polymer degradation, and stability of the nanocomposite microstructure effected by the nanomaterial structure and processing conditions. Most importantly, this investigation revealed that the optimum melt processing conditions to be employed varied depending on the materials being used and the property of interest.