The Preparation And Properties Of Cellulose Based Lightweight Materials With Tunable Fluorescence

Natural macromolecular cellulose has the advantages of biodegradability,biocompatibility,etc.,and it can improve the material strength and modulus while maintaining the shape stability of the material.Therefore,the study of the application of cellulose-based composite materials in lightweight materials is not only the expansion of the high-value utilization of natural polymer biological resources,but also the introduction of sustainable raw materials in the field of lightweight materials,which is in line with the concept of green chemistry.In this thesis,mononuclear nanocomposites were prepared by reacting carboxymethyl cellulose?CMC?and hydroxypropyl carboxymethyl cellulose?HPCMC?with rare earth metal ions Tb³⁺and Eu³⁺,respectively.The fluorescent paper with the characteristics fluorescent color of Tb³⁺and Eu³⁺were prepared by coating.Further,a carboxyl group-containing cellulose derivative/Tb-Eu dinuclear co-doped nanocomposites were pre-frozen at-20°C and liquid nitrogen-198°C,respectively,to prepare a flexible and high modulus aerogel.The main conclusions of this paper are as follows:?1?Through the rheological test of the composite suspension,the n values of the CMC/Eu???,CMC/Tb???,HPCMC/Eu???,and HPCMC/Tb???complex suspensions are less than 1,exhibiting a characteristic of shear-thinning of a typical pseudo-nematic fluid similar to ligands;as the temperature increases,the viscosity of the suspension decreases.It is indicated that the four composite suspensions can be used as a sizing agent for paper coating,and the temperature can be adjusted according to the production process.Combined with the SEM and physical property test of the coated fluorescent paper,it was found that the coating of the composite suspension had a remarkable film forming effect on the surface of the paper,the mechanical properties of the fluorescent paper were improved,and the tensile strength and toughness could be simultaneously improved.In the fluorescence emission spectrum of fluorescent paper,the characteristic red ⁵D₀?⁷F₂ transition intensity of Eu³⁺at 615nm was observed to be the highest,and the characteristic green ⁵D₄?⁷F₅ transition intensity of Tb³⁺at 545nm was the highest.Combining the photos of the coated paper under UV light,it can be concluded that the fluorescent paper coated with CMC/Eu???or HPMCC/Eu???can emit characteristic red fluorescence of Eu³⁺,and the fluorescent paper coated with CMC/Tb???or HPCMC/Tb???can emit characteristic green of Tb³⁺.The emission peaks of the fluorescent paper prepared under the experimental conditions are narrow and sharp,and the fluorescence signal is accurate,suggest that the fluorescent paper can be applied in the signal detection.?2?The composites aerogels prepared by pre-freezing at-20°C has a shape of white sponge and a low density,and it has good macro integrity,compressive capacity and flexibility.SEM image shows the three-dimensional fiber network skeleton structure and a large number of pores in the aerogel system.Part of the aerogel chain skeleton can be entangled to form two-dimensional sheet structure.Combined with the compressive stress-strain curve,it is found that the composites aerogels have high compressive capacity and high modulus,and the modulus can reach MPa level.Composite aerogel with concentration of 2.0%can withstand a pressure of up to 6 MPa while having an elastic modulus of more than 180 MPa.The FT-IR shows that the composites aerogrels have the same chemical structure with the composites,while the aggregation state of the macromolecules is changed.Conbining the PL and UV-vis,the ultraviolet absorption region of the ligand overlaps with the excitation spectrum of the central ions,illustrating that the ligand CMC can transfer energy to the central rare earth through the metal-O bond:Tb???-O^2-,Eu???-O^2-,that is"antenna effect",and the fluorescent intensity of the rare earth ions is enhanced.Through the PL and fluorescence lifetime of CMC/Tb-Eu composite aerogels with different rare earth ion doping ratios,it is found that the intensity and the lifetime of the characteristic emission peak of Tb³⁺at545 nm are decreased with the increase of the molar concentration of Eu³⁺,indicating that the energy transfer from Tb³⁺to Eu³⁺exists in the CMC/Tb-Eu composite aerogel system.While changing the doping ratio of Tb³⁺to Eu³⁺the fcuorescent colors of the composites aerogels can be adjusted from green to yellow,orange,and red.?3?The aerogel prepared by liquid nitrogen pre-freezing has a smooth appearance with a smaller volume shrinkage.Higher the concentration is,the more likely the regular crack is generated.And the pressure resistance is improved with the flexibility is remarkably lowered.The SEM image showed that the orientation of the fiber network skeleton structure of the aerogel prepared by liquid nitrogen pre-freezing was more consistent,so the difference in elastic modulus measured under the pressure of two mutually perpendicular directions was greater.The compressive stress-strain curves show that the aerogels prepared under different conditions exhibit a typical open-cell foam structure,but the aerogel prepared by liquid nitrogen pre-freezing begins to plastically harden at a lower deformation?less than 50%?.By comparing the FT-IR spectra of the composite aerogel,it was found that the rapid pre-freezing of liquid nitrogen had an effect on the strength of the coordination bond,but did not change the coordination form of the central rare earth ion and the CMC ligand molecular chain.Combined with PL analysis,it was found that rapid freezing easily caused the fluorescence transition of rare earth ions to split,which affected the peak position of the fluorescence emission spectrum of the composite aerogel,but did not change the characteristic transition peaks of Tb³⁺and Eu³⁺at 545 nm and 612 nm,respectively.It is indicated that the composite aerogel can emit characteristic fluorescent colors of Tb³⁺and Eu³⁺.