Novel nanostructured materials from cellulose esters
by Jiaxiu Wang
Date of Examination:2020-10-06
Date of issue:2021-03-04
Advisor:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Carsten Mai
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EnglishCellulose, the most abundant natural polymer on earth, constitutes virtually an inexhaustible source for the fabrication of nanostructured materials. Combining its biocompatibility and biodegradability, cellulose-based nanostructured materials possess significant benefits when compared to conventional nanostructured materials. Nanostructured materials, such as cellulose nanocrystals and cellulose nanofibers, have been isolated from cellulose, via top-down approaches. The present thesis focuses on the bottom-up approach for constructing nanostructured materials with cellulose derivatives, which are chemically functionalized with improved solubility and processability in comparison with cellulose. Specifically, cellulose esters were utilized in this study, to construct nanostructured materials including nanoparticles (NPs) and nanostructured membranes. The solvent-responsive swelling behaviors of surface-modified NPs made from cellulose 10-undecenoyl ester (CUE) were investigated. Swelling solvents, which have diverse interactions with the NPs, including the surface-functionalized part (SFP) and the interior non-functionalized part (INP), were added to the colloidal systems. Distinct swelling modes of NPs were obtained by tracking the sizes of NPs via dynamic light scattering (DLS). (1) The sizes of the NPs increased exponentially when using the swelling solvent, which has good interactions with both the SFP and the INP. (2) A logarithmic increase of the sizes of NPs was observed when using the swelling solvent, which has good interaction with the SFP, but inferior interactions with the INP. Excess use of this type of swelling solvent can cause slight shrinking of the INP and thus the decrease of the sizes of NPs after substantial swelling. (3) A dispersant with a smaller viscosity resulted in NPs with tight structure, which impeded the swelling at the starting stage. Thus, the size of NPs increased in a sigmoidal mode, even when the swelling solvent, which has good interactions with both the SFP and INP, was utilized. The investigation of the solvent-responsive swelling behaviors of NPs provided a route for adjusting the sizes of NPs according to practical requirements. Self-compounded multifunctional nanocomposite membranes were obtained from cellulose cinnamate (CCi). Two distinct morphologies: CCi-NPs and CCi matrix were obtained in self-compounded CCi nanocomposite membranes, with CCi-NPs either firmly embedded in CCi matrix or fused with adjacent NPs. This unique self-compounded compact structure imparted good mechanical and barrier properties to the membranes with tensile strength and Young’s modulus of 93.9 ± 4.6 MPa and 3.1 ± 0.2 GPa, respectively, while the oxygen permeability, water vapor permeability and oil permeability were as low as (8.48 ± 2.39) ×10-13 cm3·cm/cm2·s·cmHg, (0.94 ± 0.03) × 10-11 g·m-1·s-1·Pa-1 and 0.008 ± 0.003 g·mm·m-2·day-1, respectively. Moreover, CCi membranes also demonstrated excellent heat and humidity resistance, with decrease of storage modulus < 10 % when humidity increased from 21% RH to 94% RH and storage modulus value > 2.0 GPa even at temperature of 98 °C. CCi membranes also demonstrated good UV-shielding properties by total shielding of UVB and UVC as well as partial shielding of UVA. Furthermore, the CCi membranes also demonstrated exceptional photothermal conversion functions. The temperature of CCi membranes can increase from room temperature to 96 °C in 10 s with UV irradiation. Especially, the humidity sensitivity of CCi with low degree of substitution (DS) inspired the development of a new type of eco-friendly plastics: hydroplastics. Self-compounded nanocomposite membranes from CCi with DS of 0.27 were fabricated. The membranes were mechanically strong and stiff with a tensile strength of 92.37 ± 2.18 MPa and Young´s modulus of 2.56 ± 0.03 GPa, which were superior than most of the widely-used plastics, including thermoplastics (e.g. PE, PP, PET and PVC) and thermosetting plastics (e.g. epoxies and phenolics). Moreover, the membranes were programmed into versatile two dimensional and three dimensional shapes via a facile, eco-friendly hydrosetting process, i.e. using water to manipulate the plasticity of membranes for shaping. This hydrosetting process endowed the reversible and random transformation between various shapes of CCi membranes. The programmed shapes maintained stable for at least 6 months. Moreover, one same membrane can be programmed for multiple times (at least 15 times) without fatigue, thus the plastics can be reused with extended life-cycles. This thesis is a cumulative work with 3 publications. One of them is published, one under review and another one submitted. The background and objectives of this study, results and discussions, general conclusions and perspectives are shown in sections 1-4.
Keywords: cellulose esters; nanostructured materials