Functional Cellulose Materials Based On Aqueous Alkali Hydroxide/urea Solution

Cellulose,as a ubiquitous biomass resource in nature,has attracted widespread attention due to renewability,biocompatibility and biodegradability.Cellulose-based materials with special hierarchical structure and excellent properties can be applied in many fields including flexible electronic devices,biological platforms and energy storage materials.Regenerated cellulose may be prepared by physical dissolution and regeneration in“green”solvents（alkali/urea aqueous solution）,an environmentally friendly process avoiding the consuming of chemicals since most of the reagents may be recycled and reused.The novel dissolution method for the preparation of the functional regenerated cellulose materials have been developed successfully,showing great potentials in the fields of natural polymer and materials.The regenerated cellulose materials can produce different materials according to different structural designs such as cellulose fibers,films,microspheres,hydrogels,aerogels,bioplastics and polymer composites,which is of potential values in applications of textiles,packaging,biomedicine,water treatment,flexible devices,agriculture and food,etc.Then,the chemical structure and physical properties of nanocellulose can be modified by various modification methods because of a large number of hydroxyl groups on the surface of nanocellulose,which also provides new applications for nanocellulose materials.At present,many researches are about microcrystalline cellulose,cellulose nanowhisker,nanofibrillated cellulose and bacterial cellulose,which are cellulose I crystal structure.However,regenerated nanoporous cellulose hydrogel（NCG）prepared from alkali/urea aqueous solution is the type of cellulose II crystal form.It is composed of intertwined cellulose nanofibers with excellent mechanical properties,high porosity,high transparency and easy processing and molding.Therefore,this thesis aims to build more new functional cellulose-based materials in the alkali/urea aqueous solution,so as to broaden the application fields of cellulose-based materials.The main innovations of this work are as follows.（1）Grafting of polyglycidyl methacrylate（PGMA）by surface-initiated atom transfer radical polymerization（SI-ATRP）on the surface of NCG with three-dimensional network nanoporous structure.Grafting hydrophobic C₇F₁₅ groups on hydrolyzed NCG-g-PGMA nanocomposites to prepare hydrophobic NCG-g-PGMA nanocomposites,and evaluation the structure and properties of hydrophobic NCG-g-PGMA nanocomposites.（2）Hydrophobic nanoporous cellulose aerogels were prepared by silanization on NCG,which has a variety of properties（density,high strength,large specific surface area,low thermal conductivity and high oil-water separation capacity）.（3）Construction of elastic cellulose aerogels by chemically crosslinking method using epichlorohydrin.Investigation the effect of epichlorohydrin to cellulose dehydrating glucose unit molar ratios on the structure and mechanical properties of elastic cellulose aerogels.（4）Preparation of nanofibrillated cellulose by"bottom-up"mode and hydrogen bond self-assembly method,and evaluated its basic physical andchemical properties;（5）Nanofabrillated cellulose（NFC）/waterborne polyurethane（WPU）nanocomposites were successfully constructed by using self-assembled cellulose nanofibers to enhance waterborne polyurethane,and evaluated the relationship between the structure and properties of NFC/WPU nanocomposites.The main contents and conclusions of this thesis are divided into the following parts.NCG-g-PGMA nanocomposites were fabricated via SI-ATRP of GMA monomer on the cellulose nanofibrils of the NCG.Additionally,based on the PGMA functional polymer,the NCG-g-PGMA nanocomposite grafted with C₇F₁₅ groups was hydrophobic,in contrast to the hydrophilic NCG.The grafting temperature and time played an important role in grafting PGMA on NCG and hence dramatically affected the microscopic morphology and structure of the hydrophobic NCG-g-PGMA nanocomposites.The interconnected nanofibrillar network structure of the NCG was deposited with PGMA after SI-ATRP.Further grafting with C₇F₁₅COCl did not change the microscopic morphology and structure of the NCG-g-PGMA nanocomposites.Moreover,the hydrophobic NCG-g-PGMA nanocomposites demonstrated a remarkable improvement in the tensile storage modulus above the glass transition temperature and moderate thermal stability.The presented results illustrated the usefulness of this synthetic strategy for converting NCGs into novel multifunctional materials,which would lead to the development of new materials for potential applications in packaging,textiles,adsorbents,biomedical materials and many others.Hydrophobic nanoporous cellulose aerogels were successfully prepared by silanization and freeze drying technique.First,the shape and microstructure of hydrophobic nanoporous cellulose aerogel has hardly change after silanization and it has higher hydrophobic contact angle（160°）,high tensile and compressive strength（6.4MPa and 6.4 MPa）,low density(0.012⁰.35 g cm^-3)and high surface area(122³26m² g^-1).Secondly,the hydrophobic nanoporous cellulose aerogels also have good moisture resistance,reusability and low thermal conductivity(0.0096 Wm^-1K^-1),as well as high absorption capacity to organic solvents.Therefore,hydrophobic nanoporous cellulose aerogel can be applied in the fields of thermal insulation and oil-water separation.Elastic cellulose aerogels were prepared by dissolving in“green”solvent and then being chemically crosslink using epichlorohydrin（ECH）.LiOH/urea solution was used to dissolve high molecular weight cellulose,and then controlled the ECH to anhydroglucose unit（AGU）of cellulose molar ratio ranged from 2 to 6.Chemically cross-linked cellulose hydrogels have high moisture content（>95%）and better recoverability.The pore structure and mechanical properties of the chemically cross-linked cellulose aerogel can be controlled by adjusting the molar ratios of ECH and AGU in the chemical cross-linked process and the ethanol concentration of the regeneration in the physical cross-linked process.Compared to physically crosslinked cellulose aerogels,chemically crosslinked cellulose aerogels exhibit lower density(0.015 g cm^-3),larger pores,and good recoverability.The elastic cellulose aerogels has potential applications in sensors,oil-water separation and catalyst carriers.Nanofibrillated cellulose was fabricated by etherification using ClCH₂COONa in cellulose solution and then self-assembly using"bottom-up"method.We can regulate the diameter and length of cellulose nanofibers by regulating the degree of substitution.The NFC with degree of substitution of 0.095 was prepared using ClCH₂COONa to AGU molar ratio of 5 at 55 ^oC for 5 hours,and its width and length were 9±1 nm and385±5 nm,respectively.The microscopic morphology of the nanofibrillated cellulose can be observed from SEM and TEM images.According to the results of FT-IR,XRD and solid state ¹³C NMR,nanofibrillated cellulose existed in the crystalline form of cellulose II.We can also use the nanofibrillated cellulose to prepare cellulose aerogels,cellulose membranes and other materials.It was shown that this cellulose aerogel has a higher specific surface area(172 m² g^-1)and better thermal stability.Therefore,this type of nanofibrillated cellulose material is expected to be applied in the fields of catalyst carriers,adsorbents and cosmetics,ect.NFC/WPU nanocomposites were fabricated by mixing and heating,and can adjust its transparency and mechanical properties by controlling the content of NFC（1wt%²0 wt%）.The results of FT-IR show strong hydrogen bonding interactions between NFC and WPU in NFC/WPU nanocomposites.The DSC and DMA show that the glass transition temperature（T_g）of the soft segment of WPU decrease and remarkable increase in tensile storage modulus of the nanocomposites above glass transition temperature（T_g）of the soft segment of WPU.These results are due to NFC interferes with the interaction between the soft and hard segments of WPU and the phase separation between NFC and WPU.When the temperature is higher than the T_g of the soft segment of WPU,the storage modulus of the nanocomposite containing 20wt%of NFC is 610 times higher than that of the pure WPU.These results indicate that the NFC has a very significant improvement to mechanical strength for the WPU.In addition,NFC/WPU nanocomposites have good solvent resistance with increasing NFC content.Therefore,the NFC/WPU nanocomposites material have potential applications in packaging materials,electronic devices,and biomedical applications,etc.This thesis developed several kinds of functional cellulose materials based on alkali/urea aqueous solution.The physical and chemical properties of different functionalization of regenerated cellulose materials were explored and it could provide important theoretical and experimental basis for the preparation and application of functional cellulose materials.The use of renewable biomass resources to build new functional materials has innovative and scientific value.Therefore,this thesis exhibits important academic value and application prospects.