Dehydration behavior of hydrogelsDoctoral thesis
Date of Examination:2024-01-19
Date of issue:2024-01-31
Advisor:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Kai Zhang
Referee:Dr. Yong Wang
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EnglishHydrogels, with their porous structures, steric networks, and high water content, which closely resemble living organisms, are attractive materials for numerous applications, including wound dressing, tissue engineering, and soft actuation. In reality, the dehydration of hydrogels always occurs, thereby modifying the characteristics of materials simultaneously. A thorough grasp of the physicochemical aspects of hydrogel dehydration can advance the use of hydrogels and also provide a new perspective on nature. The object of this research is to investigate the dehydration behavior of hydrogels and to fabricate functional materials via this path. The reorganization of dynamic hydrogels during dehydration were systematically investigated, resulting in the xerogel hollow tubes with unique wall-knot structures via an anisotropic process. Unlike the typical isotropic contraction of hydrogels during dehydration, a fast reorganization of dynamic hydrogels facilitated the migration of polymers within the plane under the induced stress of dehydration. This led to the creation of hollow xerogel tubes with similar density and wall thickness or knots. Specifically, the conformal hydrogel-wall interfaces allowed the isotropic dehydration process to continue steadily. The universal nature of dynamic hydrogel reorganization during dehydration has been demonstrated. To a certain extent, this phenomenon is not influenced by the types of monomer or solvent utilized, dynamic bonds, and can even permeate through the interface between dynamic hydrogels. Combined with the conformality of hydrogel-wall interfaces, it provides a large scale curved surface with nanopatterns. Then, inspired by the formation of heterogeneous structures of natural plants, xerogel fibers with heterogeneous structures were prepared after sequential uniaxial stretching and subsequent air drying. The observation reveals distinct properties of the inner part as core and outer layer as a sheath in one single sample. The outer layer was dense with stronger orientated microstructures, while the inner part was porous and uniform. By adapting Deborah number (De) to the air drying of hydrogels as interaction time, the formation of heterogeneous structures can be correlated with the divergent De, while tuning De values can trigger the transition from homogeneity to heterogeneity in xerogel fibers. Specifically, the turning point was around De = 1. Combining numerical modeling and experimental investigation, the mechanism behind the association between De and heterogeneity is water content-dependent glass transition. By quantitatively analyzing the results in DVS tests, this unique heterogeneous structure was demonstrated to be the key parameter in tuning the water transmission and the capacity of water holding. Herein, based on the physicochemical process in the modified dehydration process of dynamic hybrid hydrogels, we provide a new angle to understand the formation of heterogeneous structures in soft matter. A new method was demonstrated to create a designable optical material system using cellulose nanocrystals (CNCs) that exhibit birefringence through alkaline periodate oxidation (PO-CNCs) and gold nanorods (GNRs) that exhibit surface plasmon resonance (SPR) by uniaxial stretching and hydrogels dehydration. The resulting system allows for adjustable structural colours. The GNRs films show obvious absorbance around 525 nm, while the intensity of absorbance is related to the concentration and the relative angle to the polarizer. The retardation induced by PO-CNCs can be used to manipulate the light intensity transmitted through the polarizer. In addition to direct changing the amounts of nanomaterials in the nanocomposite films, two further strategies can be used to design the structural colors and broaden the color space: stacking the PO-CNCs and GNRs films or changing the rotation angle of GNRs films. In comparison with the directly prepared PO-CNCs + GNRs hybrid films, these two strategies display high flexibility and modularization. For the first time, the PO-CNCs are applied as one important element in constructing optical materials with designable structural colors. This thesis is a cumulative work including 3 publications. Two of them had already been published, and one was being prepared for submission. The background, the objective of the study, results and discussion of the three publications and the conclusions are presented in Section 1-4.