Design,Constrction And Application Of All Cellulose Nanocomposites

With the arising of non–renewable resources shortage and environmental protection,more and more attentions have been paid to develop and utilize environmentally friendly materials recently.As a derivative of the most abundant,green resource of cellulose,cellulose acetate（CA）exhibit renewable property,good biodegradability and biocompatibility.Unfortunately,it usually shows poor thermal and mechanical properties,weak hydrophilicity and so on compared with oil–base materials.To overcome these limits,the enhancement of CA with organic nanofiller is an effective route.Recently cellulose nanocrystals（CNCs）as unique organic nanoreinforcers extracting from cellulose raw materials have attracted more and more attentions because of their nanoscale size,low density,high specific area and aspect ratio,high reactivity and excellent mechanical.However,there are also some shortcomings such as low yield and poor thermal properties in the preparation of the CNCs.Moreover,the abundance of surface hydroxyl groups would result in the aggregation of the CNCs,which would restrict the application of CNC in composites.Therefore,we would focus on the efficient extraction and surface functionalization of the CNCs based on hydrochloric acid hydrolysis under hydrothermal conditions,and the enhancing mechanism resulting from the interaction between different kinds of the CNCs and CA.Furthermore,the feasibility of as–prepared nanocomposites as sustained drug release systems would be evaluated.The main researches were summarized as follows:（1）Enhancing effect of CNCs prepared through hydrochloric acid hydrolysis catalyzed by metal salts under hydrothermal conditions on electrospun CA nanofibers.To reduce the energy consumption and efficiently control the degradation products,six inorganic salts including FeCl₃,AlCl₃,CuCl₂,MnCl₂,Fe₂（SO₄）₃ and Al₂（SO₄）₃ were introduced into HCl hydrolysis of microcrystalline celluloses（MCC）at relatively low acid concentration.The as–prepared CNCs were investigated by dynamic light scattering（DLS）,field emission scanning electron microscope（FE-SEM）,transmission electron microscopy（TEM）,X–ray diffraction（XRD）,Fourier transform infrared（FT-IR）,and thermogravimetric analysis（TGA）.The role of inorganic chlorides including FeCl₃,AlCl₃,CuCl₂,MnCl₂,Fe₂（SO₄）₃ and Al₂（SO₄）₃ in the extraction and properties of high quality CNCs was determined.It is observed that the introduction of inorganic salts obviously enhanced the hydrolysis process through faster degradation of the disordered region of cellulose.Compared with that obtained in pure HCl reaction medium,the CNCs prepared by salt–catalyzed hydrolysis not only showed rod–like shape but also smaller diameter and larger length to diameter ratio,which could contribute to greater enhancement on the mechanical properties of electrospun CA nanofibrous menbranes.It is found that these inorganic salts as Lewis acid exhibited excellent catalytic activity and obviously promoted the hydrolysis process.A possible mechanism was proposed for understanding the role of the inorganic salts played on the rapid salt–catalyzed hydrolysis of cellulose.The CNC prepared in the presence of FeCl₃ exhibited the largest aspect ratio of 16.8,the average diameter of CNC was 10±3 nm and the length was 168±25 nm.Meanwhile,the yield of CNC could be maintained at a high level of 73%.Moreover,it is found that the highest reinforcing effects among all the nanocomposites were achieved in the presence of FeCl₃,benefiting from the largest length to diameter ratio.Compared with the pure CA nanofibrous membrane,the tensile strength of nanocomposite increased from 10.7 MPa to 13.6 MPa,while the Young’s modulus of nanocomposite increased from 0.79 GPa to 1.16 GPa.These results showed that the use of salt–catalyzed hydrolysis especially FeCl₃ had a significant improvement in achieving the energy–efficient and cost–effective conversion of cellulose starting materials into high quality CNCs.（2）One–step extraction and oxidative carboxylation of CNC through hydrothermal reaction by using mixed inorganic acids.A facile and rapid one–step approach was presented to prepare CCNCs with high suspension stability and enriched carboxyl groups,which would make these nanoparticles more reactive and extend their potential applications.By using the mixed acid of HCl and HNO₃ the conversion of the cellulose primary hydroxyl groups to carboxyl groups can be achieved within short oxidation time without special pre–or post–treatment.The influences of HNO₃ fraction on the morphology,microstructure and properties of CCNCs were discussed.The results showed that the combination of the mixed acid and hydrothermal reaction could speed up the process of CCNC preparation,and then high quality of the product could be obtained at relatively low acid concentration.It is found that the addition of HNO₃ could not only promote the conversion of surface groups on the CNCs,but also have significant influences on the yield,particle size and microstructure of CNCs.For the volume ratio of HCl/HNO₃ of 7:3,the as–prepared CCNCs exhibited the largest length to diameter ratio and narrowest dimension distributions as well as maximum degree of oxidation of 0.12.The average length and width of the CCNCs were 176±6 and 11±2 nm,respectively,the Zeta potential of the CCNCs was-45.3 mV.High dispersion stability for the CCNCs could be observed due to the existence of negative carboxyl groups.Although the thermal stability of CCNC decreased after carboxylation,as–prepared CCNC was stable when the temperature was less than 260 ℃.This approach based on one–step oxidative carboxylation greatly simplified the preparation of CCNCs with high yield and high crystallinity under mild hydrothermal condition.（3）Preparation and thermal properties of solid–solid phase change nanoparticles by grafting poly（ethylene glycol）onto the surface of the CNCs.Poly（ethylene glycol）（PEG）was grafted onto CNC prepared by using different inorganic salts as catalyst,and toluene diisocyanate（TDI）,diphenylmethane diisocyanate（MDI）and isophorone diisocyanate（IPDI）were chosen as coupling agents.Nine kinds of nanoparticles with solid–solid phase change properties were prepared.As shown in FT–IR and ¹H NMR spectra,it could be found that PEG had been grafted onto the surface of the CNC successfully.Compared with pure PEG,as–prepared phase change nanoparticles showed similar cross spherulites but lower crystallinity,smaller crystal size and enthalpy of phase transition.Because the movement of PEG chains was limited by rigid CNC,the crystallization behavior of PEG was restricted.Largest crystallinity and crystal size,higher grafting degree and phase transition enthalpy were obtained by using MDI as the coupling agent because of the highest activity of isocyanate group in MDI.Meanwhile,the CNCs prepared in the present of FeCl₃ exhibited smaller size,larger specific surface area and higher activity of surface hydroxyl,which were benefit for the grafting reaction.At the heating and cooling temperature rate of 15 ^o C/min,the T_m,ΔH_m,T_c andΔH_c of CNC_Fe–MDI–PEG were 55.3 ℃,135.2J/g,31.6 ℃ and 133.1 J/g,respectively.Moreover,the thermal conductivity of phase change nanoparticles was improved due to the introduction of highly crystalline CNC.In addition,the solid–solid phase change nanoparticles showed good thermal stability,exhibiting great potential as phase change materials.（4）Enhancement effects of various modified CNCs on the electrospun CA nanofibrous membranesAll–cellulose nanocomposites were prepared by using CA as matrix material and CNC as nanoreinforcers through electrospinning method.The optimized electrospinning conditions were as follows:the spinning solvent was acetone/DMAc mixed solvent with mass ratio of 2:1,the flow rate was 1 mL/h,the high potential voltage was 20 kV,the distance between needle and collector was 15cm,and the mass fraction of CA was 15 wt%.As shown in FE–SEM images,neat CA nanofibrous membranes exhibited smooth surface and large diameter of 331±102 nm.With the addition of CNC,CCNC and CNC–g–PEG nanoparticles,the diameter of the composite nanofibers decreased firstly and then increased.When the content of CNC,CCNC,and CNC–g–PEG were 3%,7%and 16%respectively,the as–prepared composite nanofibers showed a narrow size distribution and the diameters were 260±64,121±63 and 97±30 nm.Meanwhile,the tensile strength and Young’s modulus of the composite nanofibrous membranes increased from 10.7±0.15 MPa and 0.79±0.03GPa to 13.6±0.10 MPa and 1.16±0.02 GPa,17.7±0.15 MPa and 1.55±0.02 GPa,21.6±0.15MPa and 1.99±0.03 GPa,respectively.These results showed that the addition of CNC could improve the mechanical properties of CA nanofibrous membranes significantly.At the same time,the modified CNC showed excellent dispersibility and better interfacial compatibility with the polymer matrix which were benefit for the enhancement effects.Moreover,the hydrophily of CA nanofibrous membranes was improved by introducing nanoparticals especially CNC–g–PEG.The water contact angel was decreased from 136±4^o to 54±4^o by introducing 20%CNC–g–PEG.（5）The feasibility of CNC-reinforced CA nanofibrous membranes as potential controlled release drug delivery systems.The drug–loaded CA nanofibrous membranes and all–cellulose nanocomposites reinforced with unmodified or modified CNC with 5%,15%,25%,35%and 50%drug loading were prepared by electrospinning method.The effects of the addition of nanoparticles on the structure and properties of drug loading CA were investigated.The CA nanofibrous membranes exhibited smooth morphology and uniform size distribution under low drug loading as seen in SEM images.With the increase of drug loading,beads and spindle structure would appear.However,the composite nanofiber could maintain a smooth morphology and uniform size distribution even under high drug loading.It is worth noting that CA/CCNC and CA/CNC–g–PEG CA nanofibers showed a narrow size distribution even at high drug loading.Meanwhile,the drug loading efficiency and hydrophilicity could be improved by introducing these nanoparticals.The drug release property of all–cellulose nanocomposites under different environment temperature was also investigated.At the environment temperature of 37℃,CA nanofibrous membranes exhibited a fast drug release behavior at the initial stage,resulting in the phenomenon of"burst release".When nanoparticles were introduced,the maximum cumulative drug release of all–cellulose nanocomposites was improved significantly.Due to the strong hydrogen bonding interaction between the nanoparticles and the drugs,the drugs could be dispersed well inside the CA nanofibers which would result in a sustained drug release property.When the temperature increased to 40℃,the ability of the movement for drug molecules was enhanced,and the maximum cumulative drug release and the drug release rate were both increased.Moreover,the maximum cumulative drug release and drug release rate of CA/CNC–g–PEG nanofibrous membrane increased obviously.More importantly,CA/CNC–g–PEG drug loaded nanofibrous membrane presented a temperature–sensitive drug release characteristic,which was mainly attributed to the phase transition property of CNC–g–PEG.