| Biodegradable polymers have attracted tremendous attentions as environmental-friendly polymers. In this dissertation, a series of biodegradable polymer/polyhedral oligomeric silsesquioxane (POSS)nanocomposites were prepared via a solution casting method for the design and modulation of their properties, thereby broadening their application fields.The influence of different kinds of POSS on the crystallization behavior,spherulitic morphology, spherulitic growth rate, mechanical property,hydrolytic degradation behavior and thermal stability of different kinds of biodegradable polymers were fully investigated.1. A series of poly(ethylene succinate) (PES)/octavinyl-POSS (ovi-POSS)nanocomposites with low ovi-POSS loadings were prepared. The crystallized ovi-POSS particles were well dispersed in PES matrix even at the loading of 2 wt%. The nonisothermal and isothermal crystallization behaviors of PES were enhanced by ovi-POSS. However, the nanocomposites and neat PES showed the same crystallization mechanism and crystal structure, despite the presence of ovi-POSS. As neat PES and PES/ovi-POSS nanocomposites had similar spherulitic growth rates at the same crystallization temperature, the enhancement of crystallization rate was attributed to the nucleation agent effect of ovi-POSS. The nanocomposites illustrated greater Young’s modulus and storage modulus values than neat PES, indicating a reinforcing effect of ovi-POSS. Moreover, ovi-POSS accelerated the hydrolytic degradation rates of PES/ovi-POSS nanocomposites.2. Aiming at investigating the miscibility and phase behavior of poly(ethylene glycol)-POSS (PEG-POSS) in PES matrix with different molecular weights, and further studying the influence of PEG-POSS on the crystallization behavior and properties of PES matrix, two series of PES/PEG-POSS nanocomposites with low and high molecular weight PES matrix were first prepared. Low molecular weight PES was miscible with PEG-POSS to some degree. PEG-POSS particles were homogeneously dispersed in the PES matrix. PEG-POSS improved the nonisothermal cold and melt crystallization processes, accelerated the isothermal melt crystallization rate, and increased the spherulitic growth rate of the nanocomposites.Therefore, PEG-POSS enhanced the crystallization behavior of PES by increasing the chain mobility of low molecular weight PES matrix, acting as a plasticizer. However, the isothermal crystallization mechanism or the crystal structure of PES was not modified by PEG-POSS. The thermal stability of low molecular weight PES was slightly decreased by PEG-POSS, but the thermal processability of the nanocomposites was not influenced.Differ from the miscibility of low molecular weight PES/PEG-POSS nanocomposites, PEG-POSS was partially miscible with high molecular weight PES matrix. As the chain mobility of PES was increased by PEG-POSS, the storage modulus of the nanocomposites was decreased.PEG-POSS accelerated the nonisothermal melt crystallization process of PES matrix and increased its isothermal crystallization rate as well. The spherulitic growth rate of PES was drastically increased by PEG-POSS, indicating a distinct plasticizing effect of PEG-POSS. Though the thermal stability of the nanocomposites was slightly decreased by PEG-POSS, the thermal processability of the materials was not influenced.3. A small amount of trisilanolisobutyl-POSS (tsib-POSS) with open cage structure was added into poly(L-lactide) (PLLA) for the preparation of nanocomposite. The increased heating rate would increase the nonisothermal cold crystallization peak temperature of PLLA matrix. Tsib-POSS accelerated the nonisothermal cold crystallization behavior of PLLA matrix, and the Tobin equation better described the nonisothermal crystallization process of the samples than the Ozawa equation did. Moreover, tsib-POSS enhanced the isothermal cold crystallization, nonisothermal melt crystallization and isothermal melt crystallization processes of PLLA matrix without changing its isothermal cold and melt crystallization mechanisms. The nucleation density of PLLA/tsib-POSS nanocomposite was larger than that of neat PLLA,indicating the nucleation agent effect of tsib-POSS. However, the crystal structure of PLLA remained unchanged in the nanocomposite. On the basis of the mechanical properties study, the nanocomposite illustrated a greater tensile modulus value than neat PLLA, indicating that tsib-POSS may act as a kind of reinforcing filler in the PLLA matrix. The thermal stability of the nanocomposite was similar to that of neat PLLA. In addition, tsib-POSS enhanced the hydrolytic degradation process of PLLA/tsib-POSS nanocomposite.4. PEG-POSS was added into PLLA matrix, aiming at simultaneously increase the crystallization rate and ductility of PLLA. PEG-POSS was proved to be miscible with PLLA. PEG-POSS slightly influenced the thermal stability of the nanocomposites, but the nanocomposites were hard to be thermal processed because of the low thermal degradation temperature of PEG-POSS.The nonisothermal cold and melt crystallization behaviors as well as the isothermal crystallization kinetics of PLLA matrix were enhanced by PEG-POSS. PEG-POSS obviously increased the spherulitic growth rate of the nanocomposites, indicating that PEG-POSS increased the chain mobility of PLLA, acting as a plasticizer. However, the isothermal crystallization mechanism and crystal structure of PLLA matrix were not modified by PEG-POSS. In addition, PEG-POSS increased the ductility of PLLA as an efficient plasticizer and still maintained high Young’s modulus.Several kinds of biodegradable polymers were modified by POSS in this dissertation. It was concluded that POSS with different structures or substituents may act as reinforcing filler, plasticizer or nucleating agent in different kinds of polymer matrix, thereby influencing the crystallization behavior and properties of nanocomposites. The results of this dissertation may be helpful for the better understanding of property modulation and practical application of biodegradable polymer/POSS nanocomposites. |
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