The Study On Preparation And Properties Of Nanocellulose Hydrogels

Nanocellulose(CNC)is a kind of nanoscale rod-like particles obtained from acidolysis of cellulose.It has many advantages,such as high Young’s modulus,high strength and rigidity,surface modification,etc.Since the end of the 20 th century,nanocellulose has attracted more and more attention with its functional exploration.Nanocellulose can be used as dopants to improve the mechanical properties of hydrogels,improve the strength of hydrogels and increase the load of drugs.In addition,the hydrogels can also have a certain stimulus response through specific interaction forces to produce controllable drug release.Nanocellulose can also form photonic thin films with chiral nexal phase structure through evaporation-induced self-assembly(EISA),which can selectively reflect left-rotation light,and can be applied to functional materials such as chiral catalysis,anti-counterfeiting,sensors and so on.In this context,the functional utilization of nanocellulose crystals is studied in this thesis.On the one hand,based on polyacrylic acid(PAAc)-polyacrylamide(PAAm)traditional hydrogel,CNC-doped functional gel materials with multi-response of drug loading and drug release were prepared.On the other hand,flexible chiral photonic composite films with circular polarization fluorescence luminescence enhancement effect were prepared through the method of "dilatation-in-situ insertion".It opens up a new way for the further utilization of nanocellulose in the field of functional materials.In this work,we report a multiresponsive upper critical solution temperature(UCST)composite hydrogel based on poly(acrylic acid-co-acrylamide),PAAc-co-PAAm,sequentially cross-linked by acid-hydrolysis cellulose nanocrystals(CNCs).Scanning electron microscopy(SEM)observations demonstrated that the hydrogels are formed by densely cross-linked porous structures.The PAAc/PAAm/CNC hybrid hydrogels exhibit swelling and shrinking properties that can be induced by multiple stimuli,including temperature,p H,and salt concentration.The driving force of the volume transition is the formation and dissociation of hydrogen bonds in the hydrogels.A certain content of CNCs can greatly enhance the shrinkage capability and mechanical strength of the hybrid hydrogels,but an excess addition may impair the contractility of the hydrogel.Furthermore,the hydrogels can be used as a matrix to adsorb dyes,such as methylene blue(MB).MB may be partly discharged from hydrogels by saline solutions,especially by those with high ionic strength.Notably,through temperature-controlled hydrogel swelling and shrinking,doxorubicin hydrochloride(DOX-HCl)can be controllably adsorbed and released from the prepared hydrogels.In addition,the flexible chiral photonic composite films with circular polarization fluorescence enhancement were prepared by matrix swelling and in-situ insertion"method.Evaporation-induced self-assembly(EISA)can be used for the preparation of chiral nematic co-assemblies composed of cellulose nanocrystals(CNCs)and compatible polymers,but fails using incompatible polymers,especially those cationic polyelectrolyte,such as poly(ethylene imine)(PEI).Owing to strong electrostatic attraction,phase separation or gelation occurred when mixing CNC dispersions with PEI solutions,thus unable to further prepare their ordered co-assemblies through EISA.Here,a fabrication strategy on basis of intercalating PEI into the as-prepared swollen chiral nematic polyethylene glycol(PEG)doped CNC(PCNC)matrix was proposed,resulting in PCNCPEI nanocomposites with well-defined chiral nematic structure.The crucial step is the creation of swollen PCNC matrix using an excellent solvent of cellulose,i.e.,Nmethylmorpholine-N-oxide(NMMO),to swell as-prepared chiral nematic PCNC films.Then,PEI was intercalated into the interstitial free space of the swollen PCNC matrix,without disturbing the intrinsic long-range chiral nematic structure.As a result,the filling of PEI allowed the increase of helical pitch and red shift of structural colors in the composite films.Because of the intercalating of PEI,the PCNC-PEI composite films exhibit a great increase in ultimate stress,ultimate strain,toughness,as well as extremely high folding endurance.Moreover,the intercalating of PEI in PCNC matrix also leads to the fluorescence enhancement effect induced by physical immobilization,which possesses potential utilizations in circularly polarized luminescence,optical response,and encrypted materials.