| Due to the global exhaustion of non-renewable resources such as oil and the deterioration of ecological environment,the development and utilization of renewable resources have been attached great importance.Cellulose,the most abundant renewable natural polymer biomass resource in the world,has become a research hotspot due to its reproducibility,biocompatibility and biodegradability.However,due to the large number of intermolecular and intramolecular hydrogen bonds and its highly structured supramolecular structure,cellulose cannot be fused and is difficult to dissolve in common solvents,which greatly limits the development and utilization of cellulose.At present,the common solvent systems of cellulose mainly include alkali urine system,ionic liquid solvent system,N-methylmorpholine-N-oxide（NMMO）system,Li Cl/DMAc,quaternary ammonium solvent system,etc.However,these solvent systems may have limited solubility,high cost,harsh conditions,toxic,difficult recovery and other problems.Therefore,it is necessary to develop an efficient,quick,green and simple cellulose solvent system for high-value conversion and utilization of cellulose.This research has proposed a green and efficient cellulose dissolution method.Cellulose with different degree of polymerization（DP 180～4080）can be dissolved rapidly（2-120 min）in Al Cl3/Zn Cl2 aqueous solution at room temperature.This paper systematically studied the specific conditions of cellulose dissolution in Al Cl3/Zn Cl2 aqueous solvent system,the specific influencing factors,and studied the dissolution mechanism by means of NMR and infrared.Regenerated cellulose hydrogels were prepared and the influencing factors of electrical conductivity and mechanical properties were studied.Finally,the system was used as a new method to measure the degree of polymerization of cellulose,and its Mark-Houwink equation parameters were derived.The specific research contents and conclusions of this paper include the following parts:（1）A new compound inorganic salt solvent system was found,which can rapidly dissolve cellulose with a wide range of polymerization degree（DP）at room temperature（DP up to 4080）.This method has little degradation of cellulose,which is beneficial to subsequent solvent recovery and material construction.The specific conditions of Al Cl3/Zn Cl2 dissolution of cellulose were investigated by optical microscopy and rheology,including water content and ion ratio,and the influence of polymerization degree and solid content of cellulose on dissolution.The results showed that the best solvent solution was Al3+:Zn2+:H2O=0.1:0.9:4.The solubility of cellulose is proportional to the degree of polymerization of cellulose.With the increase of the degree of polymerization of cellulose,the time required for dissolution is longer and the viscosity of the solution is greater.The solid content of cellulose has no effect on the dissolution effect,and the required time does not change,and the solution viscosity increases with the solid content.The solvent recovery was tested,the solvent system can achieve college recovery,the recovery rate is up to 95%,the solvent can still dissolve cellulose after recycling ten times.The dissolution mechanism of cellulose was studied by using 13C-NMR,DIFFERENTIAL scanning calorimeter（DSC）,field emission scanning electron microscopy（SEM）,infrared and X-ray diffraction（XRD）.The results show that the Al Cl3/Zn Cl2 aqueous solvent system is a non-derivatization solvent system of cellulose,and cellulose is directly dissolved without derivatization reaction.The dissolution of cellulose in inorganic solvent system is related to the size of inorganic metal ion radius and hydration ion radius.The smaller the ionic radius,the greater the charge density,the greater the water and energy,and the stronger the ability to dissolve cellulose.（2）Al Cl3/Zn Cl2 aqueous solvent system is expected to be a new system for the determination of cellulose polymerization degree because of its strong ability to dissolve cellulose,environmental protection,economic and fast.The Al Cl3/Zn Cl2 aqueous solvent system was used to dissolve cellulose from different sources,and the time change of the solution flowing out of the Ubbelohde viscometer was obtained by the change of concentration.The specific viscosity and logarithmic viscosity of specific concentration were calculated by the formula,so as to deduce the intrinsic viscosity.According to the known Mark-Houwink equation for the determination of cellulose in copper ethylenediamine solution,compared the characteristic viscosity of the two systems and the degree of polymerization of cellulose,thus the parameters of the Mark-Houwink equation of the system were deduced,The Mark-Houwink equation of the system is derived as[η]=4.812×103?M??η?0.61.A new method for the measurement of the degree of polymerization of cellulose was obtained.（3）Using Al Cl3/Zn Cl2 aqueous solvent system to dissolve cellulose,water,ethanol,isopropyl alcohol as coagulating bath to prepare regenerated cellulose hydrogel.SEM,rheometer,tension machine and electrochemical workstation were used for analysis and characterization.The results showed that the network structure of regenerated cellulose hydrogels was related to the degree of polymerization,solid content and coagulation bath of cellulose.The network structure of regenerated cellulose hydrogels became more compact and uniform with the degree of polymerization and solid content of cellulose.The structure of gel networks in different coagulation baths was isopropanol>water>ethanol,and the mechanical properties and viscoelastic energy also followed the above rules.Because the solvent system contains a large number of inorganic ions,it has the properties of water retention,frost resistance and electric conductivity.Electrochemical workstation and DSC were used to analyze and characterization the results showed that with the increase of ion concentration in the gel,its frost resistance and electrical conductivity were improved.In addition,the conductivity and water retention of the gel are also related to the degree of polymerization and solid content of cellulose,and the conductivity is inversely proportional to the degree of polymerization and solid content of cellulose.Water retention is proportional to polymerization degree and solid content.In order to further improve the performance of the gel,epichlorohydrin（ECH）,polyethylene glycol diglycidyl ether（PEGDE）and cellulose were chemically crosslinked to prepare a double crosslinked hydrogel.The results showed that the gelation of double-crosslinked cellulose hydrogels was related to time,ECH content and cellulose solid content.With the increase of ECH content or cellulose solid content,the crosslinking winding between cellulose and ECH increased and the gel time decreased.The mechanical properties of cellulose/ECH/PEFGDE double chemical crosslinked hydrogels are enhanced with ECH content.By regulating ECH content,the gelation time and gel properties can be well regulated,and a gel that meets the requirements can be designed.This paper has made a certain contribution to the development of green solvent for cellulose,which is in line with the national goal of sustainable green development and the development requirements of"double carbon economy",so it has important academic value and application prospect. |
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