Surface Modification And Functionalization Of Bacterial Cellulose

Bacterial cellulose (BC) is internationally recognized as a type of natural bio-nanomaterial with a fine three-dimensional network. Due to its "nano effect", it has high water absorbing and holding capacity, large specific surface area, high gas-liquid permeability, and excellent mechanical properties. So far, BC has been successfully applied in the food and medical fields with a large-scale production. However, the current research on BC materials mainly focuses on the optimization of biosynthesis process and the low-cost preparation. The modified BC with differentiated structure and properties and the related functional nanomaterials are rarely studied. So far, the reported BC-based functional nanomaterials concentrate on the reinforcing and medical fields, the application field of BC needs to be further expanded. For the derivatization and functionalization of BC, the surface characteristic plays a crucial role which determines the selection of modification methods and the construction strategies of functional nanocomposite system. However, the surface properties and the relationship between the structure and properties of BC have not been intensively investigated.Therefore, in this thesis, we conducted a systematic study on the surface properties of BC, including the amount of accessible hydroxyl groups, the availability of different hydroxyl groups, hydrogen bonding system on the accessible surfaces andζ-potentials, in order to understand the soft template and nanoreactor effects of the BC nanofibers and realize the controllable preparation of modified BC materials and functionalized BC-based nanocomposites.Based on the application requirements, we have explored the structural design and preparation techniques to obtain novel modified BC nanomaterials, in order to achieve the controlled design of surface functional groups of BC nanofibers with adjustable characteristics. In addition, based on the BC and modified BC nanomaterials and the controllable reactive sites mechanism in the in-situ synthesis process, we used different in-situ preparation methods to establish the novel BC-based functionalized nanocomposite system. These studies have further provided a more efficient synthetic method and a new way for the controllable preparation of large-scale and low-cost BC-based nanocomposites applied in sensor, photocatalytic, and photoelectric fields.The detailed studies on surface characteristic, modification and functionalization of BC are as follows.1. The accessibility of surface hydroxyl groups and the relationship between structural features and properties of BC have been studied, in order to provide a theoretical basis for the surface modification of BC and the preparation of functional BC-based nanocomposites. The amount of accessible surface hydroxyl group has been found to be1.28(maximum of3) using dynamic vapor sorption method. The chemical microstructure analysis has shown the O(2)H has the maximum availability in the three kinds of hydroxyl groups, which has implied it has no or minimum contribution to the hydrogen bonding system on accessible surface. The O(3)H has lowest availability which is due to the highly ordered intra-molecular hydrogen bonding with O(5’).BC has a low isoelectric point at pH3.7and a ζplateau of-7.5mV. The template effect of BC can be exerted sufficiently in neutral and alkaline conditions due to the dissociation of its surface hydroxyl groups, which can serve as effective reactive sites to control the distribution and growth of the nanoparticles and nanowires in the BC matrix. The BC film has a high specific surface area of55.37m2/g, and the specific surface area and morphology can be adjusted by different drying and post-processing methods.2. With a combination of unique surface characteristics and practical application of BC material, a novel highly stable and sensitive humidity sensor based on beating-treated BC coated quartz crystal microbalance (QCM) has been successfully fabricated. The results showed that the sensors possessed good sensing characteristics by increasing more than two orders of magnitude with increasing relative humidity (RH) from5to97%, and the Log (Δf) showed good linearity. The sensitivity of sensors coated with BC membranes was four times higher than that of the corresponding cellulose membranes at97%RH. In addition, the sensors exhibited a good reversible behavior and good long term stability.3. The acetylated BC and amidoximated BC (Am-BC) films with hydroxyl groups of BC partially substituted by acetyl groups and amidoxime groups have been prepared by using chemical modification methods. These modifications can expand the application prospects of BC in hydrophobic enhancing materials and metal ion adsorption fields. BC preserving the microfibrillar morphology was partially acetylated by the solvent-free acetylation method using iodine as a catalyst. The obtained acetylated BC membrane shows more hydrophobic surface and good mechanical properties (Young’s modulus is13.4GPa and tensile strength is225.8MPa), which is in favor of enhancing the hydrophobic non-polar polymeric matrix. The Am-BC hydrogel can be obtained under moderate conditions by the successive polymer analogous reactions using acrylonitrile in an alkaline solution medium. The results revealed that the amidoximated modification could enhance the interactions between guest metal ions and host BC nanofiber while preserving the microfibrillar morphology, which can enrich the template effect of BC nanofibers.4. By introducing different functional components to interact with the large number of hydroxyl groups of BC, we have used simple surface modification methods to effectively expand the application fields of BC. The photochromic BC nanofibrous membranes were successfully prepared by surface modification of BC nanofibers with spiropyran photochromes. UV/vis spectrometry of the resulting BC-NO2SP revealed that the membranes show reversible photochromic property by changing their color from colorless to pink forming a merocyanine structure upon UV irradiation, and returning back again to colorless spiropyran structure by visible light. A novel formaldehyde sensor based on nanofibrous polyethyleneimine (PEI)-modified BC membranes coated QCM has been successfully fabricated. The sensor showed high sensitivity with good linearity and exhibited a good reversibility and selectivity towards formaldehyde in the concentration range of1-100ppm at room temperature, which make possibility for the application of BC in gas sensor fields.5. Using the soft template and nanoreactor effect of BC and Am-BC, we have successfully in-situ prepared photocatalytic ZnO/Am-BC and ZnO/Am-BC nanocomposites and flexible luminescent CdSe/BC nanocomposite membranes. The results have shown the structure and properties of the nanocomposites were affected by the preparation method and the concentration of reaction solution. Under optimized conditions, the spherical ZnO and CdSe nanoparticles with a size of20-50nm were homogeneously dispersed on the BC nanofibers. The introduced amidoxime group of Am-BC has provided more effective active sites for the nucleation and growth of ZnO nanoparticels, so the ZnO/Am-BC composite film with higher loading of ZnO nanoparticles exhibited improved photocatalytic efficiency (91%at120min) for the degradation of methyl orange solution compared to the ZnO/BC nanocomposite film. The resultant composite film can be applied to the treatment of organic wastewater, and can be easily reused. The CdSe/BC nanocomposite membranes emitted green fluorescence upon UV excitation, which are promising for applications in the fields of security papers, sensors and flexible luminescent membranes.The novel conductive flexible polyaniline (PANI)/BC nanocomposite membranes have been synthesized in situ by oxidative polymerization of aniline with ammonium persulfate as an oxidant and BC as a template. It was found that the PANI nanoparticles deposited on the surface of BC connected to form a continuous nanosheath by taking along the BC template, which greatly increases the thermal stability of BC. In addition, the acids remarkably improve the accessibility and reactivity of the hydroxyl groups of BC. The results indicate that the composites exhibit excellent electrical conductivity (the highest value was5.0×10-2S/cm) and good mechanical properties (Young’s modulus was5.6GPa and tensile strength was95.7MPa). Moreover, the electrical conductivity of the membrane is sensitive to the strain. This work provides a straightforward method to prepare flexible films with high conductivity and good mechanical properties, which could be applied in sensors, flexible electrodes, and flexible displays. It also opens a new field of potential applications of BC materials.