| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 89 === In this thesis, the micronecking mechanism of the craze is correlated with the molecular chain network and the entanglement density of the chain network will affect the deformation of polymer thin film. In addition, a new model of the strain localization of polymer thin film is explored. By suppressing the trend of strain localization the brittle polymer thin film can be toughened and demonstrate a super-plastic behavior. The void fraction in the fully necked craze region was determined and a close-packed fibril structure was then concluded. The local stress and strain within the craze were obtained from AFM topographic data by the Bridgman’s plasticity analysis. The stress / strain curve of craze fibrillation was subsequently determined where an apparent strain softening was found in the initiation of fibrillation. Strain rate was found to peak at the craze boundaries, consistent with the surface drawing mechanism from TEM results. We further exploited the necking characteristic of crazing by sandwiching the craze-forming brittle polymer film between two ductile polymer films to examine the deformation behavior of the brittle polymer which necking is suppressed. The super-plastic behavior is remarkably dependent on the thickness of the outer ductile polymer layers. When the outer-layer thickness is less than a critical thickness, the brittle polymer film in combination with the sandwich structure demonstrated a different degree of strain localization with the critical strain increased with the thickness of the outer-layer. A sharp ductile-brittle transition in the sandwich thin film structure was observed as the PPO thickness decreased. A simple mechanical model built upon the competition between the necking force, associated with crazing, and the constraining force, due to the ductile films, was utilized to analyze the stability of this super-plasticity. The result of the mechanical analysis is in good agreement with the experimental data.
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