Clay Surface Modification And Polyethylene Nano-composite Materials

Polyethylene/clay nanocomposites have recently attracted significant attention both in scientific research and industrial exploitation because of their widespread applications. However, due to the nonpolar backbone of polyethylene, which is immiscible with conventional ammoniums intercalated clays, the aim to obtain nanocomposites by melt compounding process remains a major challenge. In this dissertation, the studies on clay surface modification and Polyethylene/clay nanocomposites have been performed. The relationship between clay modification and structure and properties of polyethylene/clay nanocomposites has been investigated on the basis of the clay modification involving optimized interlayer structure and edge grafting. The main investigations and contributions are listed as follows.A surface modification approach involving grafting reaction of silanes with hydroxyl groups on the clay edge was successfully performed. The influences of reaction time, dispersion medium and structure properties of silanes (e.g. polarity, chain length and functional end groups) on the grafting reactions have been studied. In addition, the structures and surface properties of the modified clays and silane-clay interaction modes have been investigated. The ammonium intercalated organoclays have also been modified by silanes, which presented superiorities of thermal stability and hydrophobicity over the traditional intercalated organoclay. The dispersibility of modified clay in xylene solvent and HDPE-g-MA has been studied, which suggested that the combination of ion-exchange and grafting was crucial for dispersion.Polyethylene/clay nanocomposites have been successfully prepared by melt compounding. A unique orientation structure has been formed in PE nanocomposites due to the improved miscibility. The orientation degree was dependent on the grafted silane amounts, alkyl chain length of silane and melt processing condition. The tunable structure of PE/clay nanocomposites has been proposed by controlling the clay modification procedures, such as modification sequence and mediums, and melt processing conditions. The mechanism of clay dispersion in PE nanocomposites has been proposed based on the research results, which indicated that the clay dispersion is not only the result of peeling due to shear stress, but also the result of diffusion of polymer chains into optimized clay interlayer.The relationship between clay modification or clay dispersion and properties of PE nanocomposites has been investigated. The different nanocomposites structures lead to different extent of mechanical properties. The orientation structure resulted in improved mechanical properties, while the uniform dispersed intercalation/exfoliation structure has no superiority for mechanical enhancement. Effect of the clay surface grafting, interlayer ammonium and nanocomposites structure on the photo-oxidation, thermal degradation and thermal-oxidative degradation of PE nanocomposites has been investigated. The results indicated that the grafted silane on clay surface covering the active sites has positive influence on the environmental stability, and the Hofmann elimination of ammoniums has negative catalysis effect especially during photo-oxidation and thermal degradation, while the orientation and uniform dispersion of clay lead to significant enhancement of the environmental stability of PE/clay nanocomposites.