Interactions between Water and Cellulose Esters
von Wenbo Chen
Datum der mündl. Prüfung:2024-01-17
Erschienen:2024-02-05
Betreuer:Prof. Dr. Kai Zhang
Gutachter:Prof. Dr. Kai Zhang
Gutachter:Prof. Dr. Carsten Mai
Dateien
Name:Thesis- Wenbo Chen.pdf
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Diese Datei ist bis 17.07.2024 gesperrt.
Zusammenfassung
Englisch
Unique interactions between cellulosic assemblies and water is something which needs to be harnessed in the drive toward a green and sustainable world for a range of applications, where the replacement of fossil-based materials is of paramount importance and need. Notably, it is crucial to recognize that the physicochemical and mechanical properties of cellulose assemblies are substantially influenced by the interactions with water. These interactions play a pivotal role in performing the mechanical functions, and ultimately determines the application prospects of the product. Along with this pressing concern, the current manufacturing of new intelligent bio-based products designed for specific tasks, an ability to accurately predict their properties and how these properties change in different environments, is also highly desirable. Building on the fundamental exploration and practical need, this study shed focused light on the beneficial role of water in regulating the mechanical properties of lignocellulosic materials. Specifically, cellulose esters with low degree of substitution (DS) of 0.2-0.3 were adopted to fabricate nanostructured materials. The interactions between cellulose esters and water were mainly investigated by static and dynamic mechanical analysis. This, in turn, will contribute to the establishment of the relationship between the microscopic structure and macroscopic properties under the water responsive framework. While developing green approaches, we reveal sustainable polymers, herein cellulose phenoxyacetate as a typical example, with unusual water-responsive dual-mechanic functionalities via a chronological water training strategy. While a bulk mechano-responsiveness was evident after 3 minutes water exposure, an unprecedented differential mechano-responsiveness due to the formation of spatially distributed mesostructures after 3 hours water training. This endows the materials with multiple recoverable shape-changes during water and air training, and consequently even underlines the switchability between the pre-loaded stable water shapes (> 13 months) and the sequentially fixed air shapes. Our hydrosetting sustainable polymers, which exploits the water training initiated competitive mechanics, promoting a paradigm shift from conventional passive plastics processing into emergent plastics with actively spatially regulated mesostructures afforded by their internal differential mechanical attributes. Insights into the molecular changes represents a considerable fundamental innovation, which can be generally applicable to a wide range of biomass. Although breakthrough have been made towards unveiling that the dual mechano-responsiveness of CPaE under water-responsive framework was dominated by the enhanced elasticity, the molecular mechanisms underlying the elastic properties of hydrated cellulose esters remain obscure. Here, hydrous CUE0.3 and RC were selected as typical examples to understand how the water governs both the microscopic hydrogen bonding interactions and microstructures, which is in turn manifested in the macroscopic mechanical responses. Through the combination of mechanical experimental results and molecular dynamic simulation (MD), we demonstrated that the water-related viscoelastic responsive behaviors are determined by their physical nature, that is the dynamic hydrogen bonds system and microstructure. Our fundamental discovery provides insights into the relationship between water-related microscopic physical nature and mechanical response, which could be applicable to the broad hygroscopic material systems. Despite offering a degree of flexibility in the geometry manipulations, however, the practical application of hygroscopic materials still faces challenges. This is due to their mechanical properties are severely limited by the intrinsic tie between their reversible hydroplasticity and the geometric shapes. Therefore, we report moist-defined biopolymers, herein thiolated cellulose undecenoyl esters cross-linked with imine-coordinated boroxine (TCUE-IBs), with tunable multisteady mechano-responsiveness. The plasticity due to the exchange of dual dynamic bonding of boroxine and Schiff base trigger shape-changing upon binding sufficient moisture, while the viscoelastic nature enabled by their reassociation plays a key role in maintaining the geometry of materials with moist-triggered stress adjustment under various relative humidity conditions. Such an unusual equilibrium of the plasticity and viscoelasticity within one single bio-based polymer system allows an unprecedented tunable multisteady mechano-responsiveness and shape-changing. Such mechanical features offer great prospects where water is present for geometric manipulation, such as multifunctional devices in biomedicine and environment. These achieved outcomes provide considerable constructive inspirations for the future design, processing and applications of nanostructured bio-based materials with water-responsive functionalities. The thesis is a cumulative work including 3 publications. All papers were submitted in peer-reviewed journals, with one under review, one submitted and one accepted. The background, objective of the study, results and discussion, and general conclusion and perspectives are presented in Section 1-4.
Keywords: cellulose esters; mechanical property