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|Title:||Study on stimulus-responsive cellulose-based polymeric materials||Authors:||Luo, Hongsheng||Degree:||Ph.D.||Issue Date:||2012||Abstract:||Cellulose is the most abundant natural polymeric material and has been widely used by human beings for several thousands of years. Exploration of novel and new materials based on the "old" material is of great academic and industrial significance. This thesis presents the first attempt to explore stimulus-responsive polymeric materials based on the natural cellulose by physical and chemical approaches, namely polymeric nano-composites, blending/semi-interpenetrating network and chemical modification. The thermal, structural, mechanical and morphologic properties of the samples were comprehensively investigated by Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), Wide angle X-ray Diffraction (WXRD), Dynamic Mechanical Analysis (DMA), tensile tests, Scanning Electron Microscope (SEM) and so on. Two categories, five types of stimulus-responsive polymeric materials were exploited based on the natural cellulose. Water-sensitive shape memory effect (SME), programming-structure-property relationship and underling mechanisms were emphasized in this study. Some new concepts, such as heterogeneous-twin-switch, path-dependent multi-shape, rapidly switchable water-sensitive SME were established. For the first category, cellulose nano-whiskers (CNWs) were incorporated into crystalline shape memory polyurethane (SMPU) and thermal plastic polyurethane (TPU). The CNW-SMPU nano-composites had heterogeneous twin switches, i.e. the thermo-sensitive switch due to the PCL crystals and the water-sensitive switch due to the percolating cellulose network (PCN). The water-sensitive switch was further hypothesized to divide into the shielded and the naked parts. Triple-and multi-shape effects were achieved for the CNW-SMPU nano-composites by applying into appropriate thermal-aqueous-mechanical programming. When the composites were in the dry state, the thermally triggered shape recovery was tuneable according to the PCN content. Furthermore, theoretical prediction along with numerical analysis was conducted, providing evidence on the proposed microstructure of the CNW-SMPU nano-composites. The CNW-TPU nano-composites were found to possess rapidly switchable water-sensitive SME due to the PCN whose hydrogen bonding was able to be reversibly regulated by water. A novel mechanism model was established based upon systematic investigation of the composites in terms of the morphological, thermal, structural and mechanical properties.
The explored stimulus-responsive cellulose-based polymers in the second category consisted of cellulose-polyurethane (PU) blends, cellulose-poly(acrylic acid) (PAA) composites and modified cellulose with supramolecular switches, featuring the requirement of homogeneous cellulose solution in the synthesis process. The reversible behaviours of the cellulose-PU blends in wet-dry cycles as well as the underlying shape memory mechanism were characterized and disclosed. The micro-patterns of the blends were found to be self-similar and have fractal dimensions. As for the cellulose-PAA semi-interpenetrating networks, the emphasis was put on the mechanical adaptability in wet-dry cycles. A thermally reversible quadruple hydrogen bonding unit, ureidopyrimidinone (UPy), was used to react with the cellulose as pendent side-groups, which may impart the modified cellulose with thermal sensitivity. The findings in this project are beneficial for extending the potential application of cellulose in smart materials field. Moreover, it contributes to the development of shape memory polymers. The combination of multiple disciplines in this study also benefits the applications of nanotechnology and supramolecular chemistry in materials field.
Hong Kong Polytechnic University -- Dissertations
|Pages:||xxvi, 224 p. : ill. ; 30 cm.|
|Appears in Collections:||Thesis|
View full-text via https://theses.lib.polyu.edu.hk/handle/200/6792
Citations as of May 22, 2022
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