Construction, Properties And Application Of Chitin Nanofiber-Based Hydrogels

With the increasing attention on sustainable development, reasonable utilization of biomass resources became more and more important. Chitin is the second most abundant biomass materials after cellulose, mainly existed in the shells of crab and shrimp. However, most chitin is discarded as industrial waste (waste crab and shrimp shells) without effective utilization. It is not only taking heavy burden to the natural environment of humans, but also spending a lot of precious natural resources. The biomass fiber materials are supported with nano fiber element. Because nanofiber has prospective properties such as high aspect ratio, high surface area, nanonetwork structure, high strength and very low coefficient of thermal expansion, while the chitin was effectively nanofibrillated, they are expected to be utilized as a kind of green nanomaterials providing environmental protection and biological compatibility. Hydrogels, as unique "soft" materials, which prepared from nanofiber, exhibited special performance such as good mechanical properties and degradability. Hence, these hydrogels are useful in expanding the application area of natural materials.The innovation of this work was as follows:1. chitin nanofiber was prepared using "one-pass grinding" from chitin powder. The resultant uniform chitin nanofiber with high aspect ratio was formed into film materials.2. The dissolution of a-chitin was always a research challenge. In this work, by avoiding the dissolution of chitin, tough a-chitin nanofiber-based hydrogels were obtained. These hydrogels shows both high crystallinity and high mechanical properties; 3. Alternatively, mild conditions was found to prepare the tough hydrogels either using calcium chloride dehydrate-saturated methanol (Ca solvent) at room temperature; 4. Effective utilization of the above high strength nanofiber network structure was conducted to be incorporated with carbon nanotubes. By applying with gelation treatment, a three-dimensionally chitin nanofiber/carbon nanotube hydrogel was fabricated with enhanced electrical conductivity.The primary content and conclusion of this work was as follows:1. Preparation of chitin nanofiber and its film-like materials using simple methodIn the crab shell, chitin exists as "fiber bundles", forming skeleton structure, to support the living body. This finding was carried out by the observation of its micro structure before and after purification. Chitin nanofiber with average diameter of 50 nm was prepared using "one-pass grinding" method; centrifugation treatment can further decrease the diameter to 10～20 nm. Characterization and analysis proved that mechanical grinding only change the morphology of chitin, its chemical composition and functional groups were still stable. Due to "effective nanofibrillation" of the chitin nanofibers after centrifugation, they exhibited high light transmittance of 86%, at 600 nm wavelength. Chitin nanofiber films also showed excellent mechanical properties, which can reach 160 MPa at maximum. The above preparation of chitin nanofiber are green and sustainable without use of any special chemicals, hence the natural characteristic of chitin is remained.2. Preparation of high strength chitin nanofiber-based hydrogelsFor the first time, both a-chitin powders and nanofibers could be dissolved only by contacting 20 wt% NaOH at low temperatures. After coagulation, the regenerated chitin hydrogel exhibited a homogeneous micro-porous structure with low crystallinity that was caused by dissolution. To improve the samples’properties, cold ethanol was used during the initial stage of neutralization, which effectively prevented dissolution and decrease in crystallinity and preserved the original nano-network structure. Unlike conventional regenerated hydrogels, this gelation behavior was due to interdigitation between neighboring nanofibers in the cold alkali solution. Although recently there have been a few other reports on the preparation of chitin hydrogels, the high crystalline gel formed via interdigitated a-chitin nanofibers without dissolution is a unique and interesting finding. We expect this new type of hydrogel could be promoted to wide applications and researches as novel green nanomaterials.3. Finding mild conditions to prepare high strength chitin nano fiber-based hydrogelsAs the next step, we are struggling to find a mild method to prepare high performance chitin hydrogels. In this study, both a-chitin powder and nanofibers could be made into hydrogels using calcium chloride dehydrate-saturated methanol. This system is mild in comparison with the conventional methods involving concentrated alcoholates or copper ammonium solutions. During the reaction of chitin powder in Ca solvent at reflux conditions, dissolution seems occur after the nanofibrillation. However, similar with our previous study, this method is energy-intensive. For improvement, WChNF and ChNF sheets were treated in Ca solvent and then prepared hydrogels at room temperature. XRD and FE-SEM results indicated that the water inside the nanofiber sheet was the main factor for preserving the original nanofiber network structure and reducing the loss of crystallinity. Unlike conventional regenerated hydrogels, both chitin hydrogels prepared from 20 wt% NaOH solutions (1.8 MPa) and Ca solvent (1.3 MPa) showed high tensile properties in the wet state. Although recently there have been a few reports on the modification of the mechanical performance of chitin hydrogels, the method here was relatively easier without use of any specific solvents or cross-linking agents and can be conducted at room temperature.4. Effective utilization of the tough chitin nanofiber-based hydrogelsConductive ChNF/MWCNT nanocomposites with three-dimensional network structure were prepared by vaccum-filtration method which was a kind of ideal biomass base materials. The application of gelation process further provided enhanced conductivity to ChNF/MWCNT which can be used as electrodes or current collector without the use of binders or conventional metal coating. FE-SEM images revealed that the MWCNTs incorporated in ChNF network provided integrated electrical channels that permitted electron transfer. Conductivity measurements indicated the modified ChNF/MWCNT gel-film shows almost twice higher conductivity due to the assembling of MWCNTs during the shrinking. Specially, Specific capacitance of ChNF/MWCNT gel-film is about 50.2 F/g under 5 mv/s scanning rate, which is almost twice higher than the film without gelation treatment. After 1000 cycles, it still can maintain 112% of the initial capacity showing high stability. Combined, this work demonstrated the preparation of foldable ChNF/MWCNT nanocomposite with higher electrical conductivity using hybrid hydrogel system. This unique gelation treatment is expected to further expand the application area of natural polymer composite hydrogels by capturing the features of nano-networks structure.