Thermal and thermomechanical properties of clay containing polymer nanocompsites
Makhatha, Mamookho Elizabeth
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The main focus of this research project was to understand the effect of clay particles incorporation on the thermal and thermomechanical behaviour of biodegradable polymers. Clay minerals, due to their unique layered structure, rich intercalation chemistry and availability at low cost, are promising nano-particles reinforcement for polymers to manufacture low cost, light weight, and high performance nanocomposites. Up to this date very few attention has been given to using nano-dimentional clay particles as a means of increasing the crystallinity and improving the thermal and mechanical properties. Understanding the structure-property relationship in polymer-clay layered silicate nanocomposites is of fundamental importance in designing materials with desired properties. To understand the relations, in the case of poly(ethylene succinate) (PES), poly(butylene succinate) (PBS) and the organically modified layered silicates: montmorillonite (MMT) and synthetic fluorine mica (SFM), wide-angle x-ray diffraction (WAXRD), small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) analyses were conducted for the structural and morphological analysis. The PES/OMMT nanocomposite was prepared by a solution-intercalation-film-casting technique. SAXS and TEM observations show the homogeneous dispersion of silicate layers in the PES matrix. The crystallization and melting behaviour of the PES matrix in the presence of the dispersed silicate layers were studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and WAXRD. The results show that the incorporation of OMMT stops the super-cooling effect and significantly accelerates the mechanism of nucleation and crystal growth of the PES matrix. The incorporation of OMMT also significantly improves the thermal stability of the PES. The effect of the change of variables like temperature, time, and heating rate on the crystallization behaviour was studied. It was observed that the double melting behaviour of the PBS matrix is a function of these variables. Various models namely the Avrami method, the Ozawa method, and the combined Avrami-Ozawa method, were applied to describe the kinetics of the pure PBS and its nanocomposite samples during non-isothermal crystallization. The Ozawa equation did not provide an adequate description of the non-isothermal crystallization kinetics of PBS and its nanocomposite. In contrast, the Avrami analysis modified by Jeziorny and the method developed by Liu et al. were successful in describing the non-isothermal crystallization kinetics of pure PBS and its nanocomposite. The results show that the crystal growth of the PBS matrix retards in the presence of dispersed intercalated organoclays and supports the reduced crystallization of the PBS matrix in the presence of Cloisite® 30B nanoclay. The structure and morphology of the PBS nanocomposites, with three different weight ratios of organically modified synthetic fluorine mica (OMSFM) were also characterized. We observed the homogeneous dispersion of the intercalated silicate layers into the PBS matrix. The thermal properties of pure PBS and the nanocomposite samples were studied by both conventional and temperature modulated differential scanning calorimetry (DSC) analysis, which shows multiple melting behaviour of the PBS matrix. The investigation of the thermomechanical properties was performed by dynamic mechanical analysis (DMA). The results reveal a significant improvement in the storage modulus of the PBS in the presence of OMSFM. The tensile modulus of the PBS is also significantly increased in the presence of OMSFM. However, the yield strength and the elongation at break of the PBS systematically decrease with the loading of OMSFM. The thermal stability of the nanocomposites compared to that of the pure polymer sample was examined under both pyrolytic and thermo-oxidative environments. The thermal stability of PBS increased moderately in the presence of 3 wt% of OMSFM, but there is no significant effect on further silicate loading in the oxidative environment. In the nitrogen environment, however, the thermal stability systematically decreased with increasing clay loading.