| Cellulose is a kind of renewable resource,which is widely distributed in nature.It is widely distributed in plants,animals and bacteria.CNCs is made from natural cellulose.It has double effect and excellent performance.At present,CNCs has broad prospects in many fields such as materials,biology and chemistry.It plays a very important role in improving the added value of biological resources and solving the problem of continuously wasting natural resources.However,because of the strong hydrogen bonding between cellulose molecules and molecules,the CNCs prepared by sulfuric acid hydrolysis will have agglomeration and instability when preparing composite materials,reinforcing agents,filling agents and stabilizers.In order to improve the nature of CNCs,this article began to explore from the most basic source.That is to say,when sulfuric acid was used as a hydrolytic agent,it would react with CNCs' s alcohol hydroxyl group and produce negatively charged ionized sulfate half ester.On this basis,we studied the important effects of sulfate coverage on the formation of CNCs.The main methods were:(1)using MCC as raw materials,sulfuric acid hydrolysis method as basic process and single factor experiment method to study the reaction temperature,time and concentration of sulfuric acid on CNCs sulfate content;then determining the sulfate groups of CNCs by conductometric titration,elemental analyzer and X photoelectron spectroscopy,and analyzing the relationship between the yield of CNCs and sulfate group content.(2)on the basis of single factor experiments,the response surface method was used to modulate the CNCs with a large difference in the coverage rate of the surface sulphuric acid group.The microstructure of CNCs was observed by transmission electron microscopy(TEM),Malvin laser particle size analyzer and atomic force microscope(AFM).The chemical structure of CNCs was determined by Fourier infrared spectroscopy(FT-IR)and X ray diffraction(XRD).The thermal properties of CNCs were studied by thermogravimetric analysis(TGA)and differential scanning calorimetry(DSCT).The storage stability of CNCs was determined by ultraviolet visible spectrophotometry(UV-VS).(3)microemulsions were prepared by three different kinds of CNCs,and microemulsions were characterized by optical microscopy,laser particle size analyzer and centrifuge.The effect of CNCs with different content of sulphuric acid group on the properties of microemulsion was studied.The fundamental mechanism of the stability of the microemulsion was further analyzed by the difference of the surface coverage rate of the sulphuric acid group and the difference of the CNCs properties.The results showed that:(1)the sulphuric acid group content and yield of CNCs increased and then decreased with the increase of three factors(temperature,time and sulfuric acid concentration),that is,the similarity between them showed that the two had a certain relationship.Temperature,time and sulfuric acid concentration had a significant effect on the content of CNCs sulfate group.The interaction between the factors was obvious,and the order of influence was sulfuric acid concentration > time > temperature.The preparation conditions of CNCs functionalized with different sulfate groups were CNCs-L(55 C,11 h,3 M),CNCs-M(55 C,5 h,6 M)and CNCs-H(55 C,3 h,9 M).The sulfur content of CNCs obtained from the two experiments was 1.42%,5.88% and 8.14%,respectively,and the yield was 35.21,69.44 and 76.01%,respectively.Therefore,the CNCs obtained under the three reaction conditions can be used in the next experiment.(2)the average particle sizes of CNCs-L,CNCs-M and CNCs-H were 64.74,134.78 and 216.08 nm,respectively;PDI were 0.286,0.266 and 0.209,respectively;ZP were 20.49,27.35 and 40.96 m V,respectively;The crystallinity were 62.44,67.32 and 71.26%,respectively.This shows that CNCs-H(8.14%)had the best micromorphology,particle size distribution and crystallinity.The transmittance of the upper layer of CNCs-L suspension at 400-800 nm is significantly lower than that of its lower layer,and the transmittance curve of CNCs-M and CNCs-H almost coincides with that of the upper layer.This shows that the order of the three storage stability is: CNCs-L > CNCs-M > CNCs-H.Due to the increase in CNCs binding of sulfate groups,although the residual ash increased,its thermal degradation temperature and heat value of Han was reduced,showing that the thermal stability decreased.(3)with the increase of the amount of CNCs,the size of the Pickering emulsion droplets firstly decreased and then gradually tended to the equilibrium value.From CNCs-L and CNCs-M to CNCs-H,the emulsion volume of Pickering emulsion increased continuously;the residual nanoparticles in the residual CNCs suspension and the droplet size decreased;and the distribution became more uniform.Three kinds of CNCs type Pickering emulsions showed shear dilution behavior,which belonged to non Newtonian fluid and formed a network structure of weak droplet action.From CNCs-L,-M to-H,the viscosity of the emulsion increased continuously,which could inhibit the unstable state of the emulsion.Three kinds of CNCs type Pickering emulsion showed a gel like behavior,which could be placed for a long time under the normal temperature,with no precipitation and aggregation.And from CNCs-L,CNCs-M to CNCs-H,the energy storage modulus(G ')and loss modulus(G')of the emulsion decreased in turn.It was indicated that the emulsion prepared by small size nanocrystals was more stable.Therefore,cellulose nanocrystals with different content of sulfuric acid groups have different effects on the preparation of microemulsions due to their different particle size,Zeta potential and crystallinity.CNCs-H has the best emulsifying stability.Therefore,this paper studies the mechanism of the content of sulfuric acid groups on the physical and chemical properties of CNCs and its stabilizing effect on microemulsion,which can provide more theoretical support for the surface modification of CNCs and the stability of microemulsion,and greatly expand its application in food,medicine and other fields. |
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