Effect Of Temperature Gradient On Diffusion And Crystallization Kinetics In Polymer System

In this dissertation, the crystallizable poly(ethylene oxide) and UCST-type polyolefin blend, poly(ethylene-co-hexene)(PEH) and poly(ethylene-co-butene)(PEB) were selected as model systems. The polarized optical microscopy(POM), thermal stage and the digital image analysis were employed to study the effect of temperature gradient on the polymer interdiffusion in the copolymer blend PEH/PEB and the crystalline morphology and crystallization kinetics of PEO. We have also investigated the effect of the CNTs on the crystallization behavior of PEO sample. The results were shown as blow.It was found that in polymer blends, the thermal diffusion coefficient is dependent on molecular weight and the direction of the shift of interface is influenced by temperature gradient and blend composition in PEB/blend system. If the direction of “thermal force” induced by the temperature gradient is consistent with the pressure gradient caused by the different mobilities of two components, the “thermal force” will enhance the interterdiffuson of polymer system. On the contrary, the “thermal force” will slow down the polymer interdiffusion into each other. The present study also shows the self-diffusion coefficient of faster moving species does indeed vary strongly with blend composition and exhibits a maximum at ?PEB =0.7 in PEB/blend system.We also investigate the effect of the temperature gradient field on the crystalline morphology and crystallization kinetics of PEO. Firstly, we investigate the effect of isothermal crystallization temperature on the spherulite growth rates of the PEO sample. It was found that the spherulite growth rates increased with decreasing crystallization temperatures. When the PEO melt crystallize in a constant temperature, it tends to form spherulites, but under the temperature gradient field the spherulites are oriented along the direction of temperature gradient. A higer orientation of the spherulitic structure has been detected under the moving temperature gradient. As compared to the constant temperature field, the crystallization kinetics under temperature gradient field is significantly enhanced.In the study on the effect of the CNTs on the crystallization behavior of PEO sample, it was found that the neat PEO sample needs a much larger supercooling to nucleate as compared to the PEO/CNTs nanocomposite and the isothermal crystallization kinetics of the PEO with CNTs was substantially enhanced. The enhancement in the overall crystallization kinetics in the PEO/CNTs nanocomposites sample is caused by the super-nucleation effect of CNTs. We believe that the CNTs assist the nucleation of PEO. During the crystallization process, the polymer chains near the surface of the CNTs can easily form nuclei. Our results also show that in the CNTs polymer composite system with low CNTs composition, the overall crystallization rate increases with CNTs composition.