Research On Grafting Modification And Photothermal Conversion Of Cellulose Nanocrystals

With the increasing global energy consumption,non-renewable resources such as fossil and coal are gradually decreasing.People are prompted to replace them with renewable resources.Biomass resources such as cellulose,as one of the most abundant green polymer materials in nature,which are expected to solve these problems by studying their high value utilization.Cellulose nanocrystals(CNCs)are obtained by processing natural cellulose,which has the characteristics of large specific surface area,high crystallinity,and strong hydrophilicity,etc.Most applications of CNCs are simply as enhanced fillers,which cannot meet their applications in intelligence,such as intelligent drug delivery systems and temperature sensors,etc.Therefore,the smart modification of CNCs and the expansion of innovative application prospects of modified CNCs have very important reference value.Due to the large number of hydroxyl groups on the surface of CNCs,it provides the possibility of chemical modification to endow them with functionalized groups.Based on this,in this paper,the modified CNCs are applied to drug release and smart thermal isolation windows by temperature-sensitive modification of CNCs.Firstly,CNCs were prepared by acid hydrolysis.N-isopropylacrylamide(NIPAM)and acrylic acid(AA)were grafted onto the surface of CNCs by single electron transfer living radical polymerization and radical polymerization,respectively,to obtain doubly grafted cellulose nanocrystals,in which the grafting rates of PNIPAM and PAA were64.75%and 9.39%,respectively.The physical and chemical properties,temperature-response and p H-response properties of the modified CNCs were investigated using FT-IR,~1H NMR,TG,TEM,XPS,UV-Vis,XRD and particle size analysis.FTIR,XPS and~1H NMR analysis proved the successful modification of CNCs,TEM showed that the modified CNCs still retained the rod-like crystal morphology but the diameter was widened;XRD resulted in a reduced crystallinity of the modified CNCs;TG showed that the thermal stability of the modified CNCs was significantly improved compared with that of CNCs;the lower critical solution temperature(LCST)of the modified CNCs was tested by UV-Vis at about 37°C,at which the nanoparticles underwent volume contraction and the size of the nanoparticles shrank to 68%of the original size.In addition,the temperature-response is reversible.The UV-Vis test of p H-response shows that the light transmission rate of the suspension decreases significantly at p H<5,indicating that the grafted CNCs are easy to agglomerate under strong acidic conditions,while the hydrophilicity is good under medium alkaline environment,and the p H response is reversibility.Secondly,PNIPAM-grafted CNCs(P-CNCs)were co-dispersed in a sodium alginate(SA)hydrogel matrix with near-infrared absorbing nanoparticles cesium tungsten bronze(CWO),and the SA/CWO/P-CNCs composite hydrogel film dressing was prepared by ionic cross-linking method and applied to drug loading and release.The results showed that when the addition of P-CNCs in the hydrogel film increased,the particle size of P-CNCs shrank when the temperature reached above the LCST,leading to an increase in the pore size of the composite hydrogel film and an increase in the water vapor transmission rate;its hydrophilicity also improved with the addition of P-CNCs,and the mechanical properties improved accordingly.To explore the role of the photothermal conversion properties of the system on drug release,the drug was loaded in the composite gel as diclofenac sodium(DS)and its drug release behavior was observed.The results showed that the addition of CWO could convert the light energy of external NIR light into thermal energy,while the P-CNCs delivered to the gel effectively conferred local temperature sensitization to the composite system.At p H=5.8 and 25°C,there was a significant increase in drug release rate for a period of time with NIR irradiation,and there was a significant photothermal conversion ability that promoted the release rate and cumulative release rate of the composite hydrogels.Finally,the suspension of P-CNCs combined with nanoparticles CWO or CWO film to form a simulated smart thermal insulation window system.Due to the absorption and shielding performance of CWO to NIR in sunlight,the absorption of NIR is converted into thermal energy,which triggers the temperature-sensitive phase transition of P-CNCs in the system to realize the NIR shielding and thermal insulation of the smart window.The results show that the LCST of P-CNCs is less affected by the addition of CWO,and the phase transition temperature of P-CNCs mixed with CWO is about 36.4℃;through the selection of CWO liquid concentration or film and the concentration of P-CNCs,when the addition of P-CNCs is 0.35 wt%and CWO is laminated to the outer layer of window glass in the form of film,the system is ideal The system is an ideal heat shielding material,with a heat insulation temperature difference of 28.6℃between the inside and outside of the glass;while ensuring a certain amount of visible light transmittance,it can also block out most of the effect of heat brought by NIR light.This smart thermal insulation window provides some research value for the application design in the field of energy saving and emission reduction.