| Superabsorbent polymers(SAPs) have gained enormous attention in recent years due to their combined water absorption and retention capacity, and have strong potentials to be widely used in areas such as medical treatment, food and agriculture science. In particular, the development of biodegradable SAPs has become a research frontier since it possesses some important additional advantages such as biodegradability and biocompatibility, and it could be potentially used as functional materials with high additional value, such as slow release carrier materials, tissue engineering repair materials, biointerface materials. Utilizing advanced technique to incorporate hydrophilic monomers into natural polymer matrix is one of the key factors to develop biodegradable SAPs. Batch mixer, as an extensively used method to mix and synthesize polymer, could handle high viscosity polymer via its high stress shear and enhance the reactions between the components. By virtue of this, the range of viscosity reaction system used to produce SAPs could be enlarged, and also the reaction efficiency could be improved. To realize the goal of preparing starch-based SAPs with regulatable properties effectively and efficiently by batch mixer, this thesis was mainly focused on how some factors(i.e. system concentration, initiator content, crosslinking agent content, starch microstructure, and various amylose/amylopectin ratios) affecting the structure and properties of starch-based SAPs prepared via high concentration and high shear stress system, and further exploring the key factors to endow alterations to its structure and properties. Based on which, the synthesis mechanism of starch-based SAPs would be disclosed. The details and main conclusions are shown in the following.In this thesis, starch-based SAPs were successful synthesized in high starch concentration system with shear stress via Haake mixer and how the water concentration in system affecting the molecular structure and properties of starch-based SAPs was explored. The results demonstrated that the increase of water in mixer chamber had no obvious effect on its polyacrylamide(PAM) content, but increased the graft ratio and decreased the average length of PAM. Yet, uneven gird in three-dimensional network induced by less water content improved the water imbibing rate and weakened its water absorbent capacity(WAC) and hydrogel strength.The effects of applied initiator and crosslink agent amounts on molecular structure and properties of starch-based SAPs were also investigated. As the amount of initiator was increased, PAM contents in starch-based SAPs were increased firstly and then decreased, and the GR was decreased gradually. Based on the above two molecular parameters, the average length of PAM chain could be calculated and the results revealed that an increased trend was induced by the increased amount of initiator. Regarding its properties, WAC increased firstly and then decreased, and the water absorbent rate showed an enhanced trend, while hydrogel strength increased firstly and then no further alterations caused, with the increasing of initiator. While for the increasing of crosslink agent, there was no significant change on PAM content. Yet, the average length of PAM was longer, accompanying by the increased GR. Furthermore, the reduction of WAC was induced and rise of water uptake rate and hydrogel strength was brought about as a function of applied crosslink agent amount.The influences of starch botanical origins on the molecular structure, micromorphology, colloidal fractal structure and its properties were also explored. The sequence of GR and PAM contents were as follows, maize-SAP > potato-SAP > cassava-SAP, maize-SAP ? cassava-SAP > potato-SAP. As the result, potato-SAP possessed the shortest average length of PAM chain, while the longest one was occurred on cassava-SAP. Regarding the micromorphology, SEM results demonstrated that the size of pores on the maize-SAP and potato-SAP was almost the same, while the cassava-SAP showed the larger one. In the nanometer scale, the hydrogel of potato-SAP displayed largest radius of gyration and fractal structure was relatively loosest, while cassava-SAP showed the opposite trend, that is, the smallest radius of gyration and densest fractal structure. As for water uptake properties, potato-SAP could absorb largest amounts of water but possessed slower water imbibing rate, while cassava-SAP showed lowest water absorbent capacity but highest water uptake rate. Since the yield of potato-SAP was much more less than maize-SAP and cassava-SAP, and the water absorbent capacity of maize-SAP was a little less than potato-SAP, maize was chose as materials in the following part.The effects of starches with various amylose/amylopectin ratios(G80、G50、maize、waxy) on the molecular structure, micromorphology, colloidal fractal structure and its properties were also evaluated. With the increasing of amylopectin contents, the reduction of PAM content and rise of GR led to the decrease of the average length of PAM chain. For the micromorphology, as the chains formed the pore walls of dried hydrogel were organized more compactly, less and less pores could be observed with the increasing of amylopectin content. As for the nanoscale structure, waxy-SAP showed largest radius of gyration and G80-SAP displayed the smallest one. Regarding the water uptake properties, G50-SAP had the highest WAC and waxy-SAP possessed the lowest one.In summary, predominantly two factors, i.e. “cage effect” and crosslinking degree, governed the properties of SAP(e.g., WAC, water imbibing rate and hydrogel strength of starch-based SAPs) by affecting the average length of PAM chains, the fractals on nano scale and the micro-pore structure. This research undoubtedly enriches theoretical foundation for the relationships between the molecular structure and properties, and provides theoretical and technical supports for the application of high concentration system with shear stress and biodegradable superabsorbent polymer. |
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