Preparation Of Decrystallized Nanocellulose By Solvent Pretreatment

Because of its unique structure and properties,nanocellulose has been widely used in many fields and has become the most promising direction in the field of cellulose.To improve the functionality of nanocellulose,researchers have done a lot of work in the preparation and modification of nanocellulose such as adjust the aspect ratio,specific surface area,crystallinity of nanocellulose,and improve its surface activity.Although many research results have been made in the research of nanocellulose,it is still a research hotspot in the field of cellulose.On a one hand,is caused by the complexity of the problem.For example,to obtain nanocellulose with different aspect ratios,different preparation methods are often required.Therefore,the specific surface area,surface activity,and other aspects of nanocellulose will also be different.The function of the obtained nanocellulose needs to be comprehensively evaluated in conjunction with its application.On the other hand,it's to expand the application fields of nanocellulose.For example,when nanocellulose is used in the fields of reinforcing materials,food colloids,Pickering emulsifiers,etc.,it is often required to have high surface activity.Traditional chemical modification methods often make nanocellulose with specific groups,which limits the application field of the modified nanocellulose.In recent years,by reducing the crystallinity of cellulose and even realizing its amorphization,the method of improving the reaction accessibility and surface activity of the hydroxyl groups on the cellulose macromolecular chain has caught the attention of researchers.But so far,there is no report about the purposeful preparation of nanosized low-crystallinity or even amorphous nanocellulose,and the study of its structure,properties,and application effects.Based on the above analysis,this paper uses the nanocellulose crystals(CNC),microcrystalline cellulose(MCC),the solvent system is(TBAH/DMSO/water)or(TBAH/DMAc/water),and the crystal is eliminated through the dissolution-regeneration method and attempts to prepare nano-sized cellulose with low-crystallinity and even amorphous.While discussing and analyzing its structural morphology and performance changes,the application prospects of it in water suspension,Pickering emulsion,and PMMA nanocomposite are discussed respectively.Specific research contents are as following:(1)Using(TBAH/DMSO/water)as the solvent system,the CNC and MCC prepared by the sulfuric acid hydrolysis method were pretreated.By adjusting the concentration of the solvent,a series of cellulose particles with different crystallinity and sizes were obtained,and an atomic force microscope(AFM),Fourier infrared(FTIR),X-ray diffractometer(XRD),and thermogravimetric analysis(TGA)were used to characterize and analyze the morphological structure and thermal properties of the obtained samples.The results indicate that as the concentration of the pretreatment solvent increases,the decrystallizing effect of CNC increases,and its morphological structure changes.The thermal stability of the particles increases first and then decreases.while the crystallinity of MCC reduces when the solvent concentration reaches a certain level.When the value is higher,nanoization is also realized,and its thermal stability gradually decreases.The stability study of aqueous suspension shows that CNC series samples can form a stable dispersion system,while in MCC series samples,only nanometerized samples can achieve stable dispersion.Rotational rheometer(DHR)analysis shows that as the crystallinity decreases,the suspension transitions from"liquid-like"to"gel-like"behavior,showing obvious rheological characteristics of shear-thinning.After analyzing the rheological properties of the low crystallinity P_4:1 CNC and P4 MCC suspensions,it is found that the P_4:1CNC suspension has a higher degree of gelation,and its viscosity and viscoelasticity are higher than those of the P4 MCC suspension.Besides,P_4:1 CNC suspension and P4 MCC suspension show a"gel-like"fluid behavior and shear thinning in a wide p H range(p H 4?10)and high salt concentration.Rheological properties.interfacial tension analysis shows that,As the crystallinity of the particles decreases,the interfacial tension of the decrystallization nanocellulose suspension at the oil-water interface also decreases.(2)Pickering emulsion was prepared by using decrystallized nano cellulose with different crystallinity,and the stability of the emulsion was explored,and then the Pickering emulsion was prepared by selecting low crystallinity P_4:1 CNC with the best emulsification effect.The oil-water ratio and particle size were systematically studied.The influence of concentration,salt concentration,p H value,and temperature on the stability and rheological properties of P_4:1CNC stabilized emulsion.When the oil-water ratio is 6:4,as the particle concentration increases,the size of the emulsion droplets decreases,and the stability and gelation degree of the emulsion increase.when the particle concentration is the same,the oil-water ratio is 6:4 or 1:9 has the best stability.when the oil-water ratio is 6:4 and the particle concentration is 1 wt%,the prepared Pickering emulsion exhibits good stability in a wide p H range(4?10).Besides,under high salt concentration(1000 m M)and high temperature,the emulsion exhibits extremely high stability.(3)Using(TBAH/DMAc/water)as the solvent system,citrate esterified microcrystalline cellulose(C-MCC)was subjected to solvent pretreatment.By adjusting the concentration of the solvent,a series of citrate esters with different crystallinity and sizes were obtained.The morphological structure and thermal properties of the obtained samples were characterized and analyzed by AFM,FTIR,XRD,and TGA.The results showed that the MCC after citrate esterification was pretreated with a certain concentration of solvent to achieve nanometerization.At the same time,as the concentration of the pretreatment solvent increased,the crystallinity and thermal stability of C-CNC were significantly reduced.FTIR,titration test,and ¹³C solid nuclear magnetic(NMR)analysis showed that the citric acid groups were successfully grafted on MCC after citric acid chemical modification,and the C-CNC prepared after solvent pretreatment retained these citric acid groups.Then PMMA/C-CNC composite membranes were prepared by solution casting method.TGA,stretching,and scanning electron microscopy(SEM)analysis of pure PMMA film and PMMA/C-CNC composite films show that C-CNC_0.6,C-CNC_1.0,and C-CNC_1.4can all improve the performance of PMMA composite film Mechanical properties and thermal stability.C-CNC_1.0improves the mechanical properties and thermal stability of PMMA composite membranes most dramatically when the amount of particles added is the same.An appropriate amorphous zone can drastically improve the phase between the particles and the PMMA interface.Compatibility,when 3%added amount of C-CNC_1.0 is compounded with PMMA,the tensile strength and elongation at break of the composite film are increased by 140.1%and 392.6%,respectively,compared with the pure PMMA film.The optical transparency test shows that the addition of particles will reduce the optical performance of the PMMA composite film,and C-CNC_1.0 has the least impact on the optical performance of the composite film.When the additive amount of C-CNC_1.0 is 3%,the light transmittance of the composite film at 550 nm is 86.9%,which is 6.4%lower than that of the pure PMMA film.