Preparation Of Chitin Nanofibers And Construction Of Their Coating Along With Analysis Of Coating’s Light Transmittance

Chitin is an important bioactive polysaccharide,as one of the most abundant natural polymers in the ocean,it is mainly sourced from marine animals(crabs,shrimps and shellfish)and has excellent biocompatibility,non-cytotoxicity,biodegradability and regenerability.Chitin nanofiber is a new type of nanomaterial stripped from natural chitin,which inherits the excellent characteristics of chitin such as biocompatibility and non-cytotoxicity;as a nanofiber,it has a large length-to-diameter ratio and also has excellent mechanical properties.Therefore,chitin nanofibers have great potential for application in many fields such as drug delivery,biomedical materials,emulsion stabilizers,structural reinforcers,water purification,food packaging,etc.Currently,the mainstream solution for the preparation of chitin nanofibers is chemical modification of chitin combined with mechanical crushing.Commonly used chemical modification methods include TEMPO oxidation,maleic anhydride esterification and partial deacetylation;commonly used mechanical crushing methods include ultrasonic crushing,grinding,high pressure homogenization,etc.Transparency-enhancing(antireflective)coatings are in demand in the photovoltaic industry,optical components,electronic screens,etc.They can be constructed in different ways on substrates based on the thickness requirements of the "quarter wavelength" theory and the refractive index requirements,on the one hand,the optical thickness of the coating should be one-quarter of the incident wavelength,and on the other hand,when the substrate is float glass(refractive index of 1.52),the refractive index of the coating should be 1.23.Chitin nanofibers have nanometer diameter(much smaller than the visible wavelength),therefore,they do not cause light scattering,and the coating constructed with them as the main material can ensure a high degree of transparency.Moreover,because of the high aspect ratio and excellent mechanical properties of chitin nanofibers,the irregular accumulation and winding of nanofibers will not only introduce pores inside the coating and thus reduce the refractive index,but also ensure the high mechanical wear resistance of the coating to a large extent.Based on the previous methods of esterification and deacetylation of chitin,this paper creatively proposed a scheme of esterification + deacetylation combined with ultrasonic crushing to prepare chitin nanofibers.A series of chitin nanofibers were prepared by esterification,deacetylation,esterification and then deacetylation combined with ultrasonic crushing,and the effects of modification methods,modification conditions and ultrasonic crushing power on the properties of chitin nanofibers were studied.By changing the p H of chitin nanofiber dispersion,the effect of chitin nanofiber dispersion on the stability of chitin nanofiber dispersion was analyzed.Finally,the chitin nanofiber coating was constructed by casting method and thermal adsorption method of heating substrate,and the light transmission of the coating was adjusted by changing the process parameters of the coating to make it have antireflective effect.The main conclusions are as follows:(1)The equivalent particle size of chitin nanofibers prepared by partial deacetylation combined with ultrasonic crushing is larger and the particle size distribution is uneven.However,the equivalent particle size of chitin nanofibers can be significantly reduced and the particle size uniformity can be improved by adding one step of esterification modification(without changing other preparation conditions)before deacetylation.Increasing the deacetylation time can reduce the equivalent particle size of double modified chitin nanofibers and promote the particle size homogenization.Increasing the concentration of Na OH in the process of deacetylation can have a similar effect.(2)The chitin nanofibers prepared by the double modification scheme were positively charged in acidic and weakly alkaline conditions,negatively charged in alkaline conditions,and the isoelectric point of positive and negative charge balance appeared in weakly alkaline conditions.During the change of p H from 2.5 to 12,the absolute potential of chitin nanofibers increased at first and then decreased,and then gradually increased and then decreased.The peak potential of chitin nanofibers in acidic condition was between p H = 3 and 4,while that in alkaline condition was between p H = 10 and 11.The absolute potential of double modified chitin nanofibers under acidic condition is generally higher than that under alkaline condition.The longer the time of deacetylation modification or the higher the concentration of Na OH used during modification,the more unstable the dispersion of double modified chitin nanofibers in alkaline conditions,and the final uniform dispersion can only be achieved in acidic conditions;on the contrary,short deacetylation time or low concentration of Na OH will make double modified chitin nanofibers have two discontinuous p H regions(one in acidic environment and the other in alkaline environment).(3)When the chitin nanofiber coating is prepared by casting method,the antireflective effect can be achieved when the p H of the dispersion is 3.5,and when the p H is 7 and 10.5,the light transmittance of the original glass slide will be greatly reduced.Controlling the drying temperature at 30 °C and the dispersion concentration of 0.1 wt% or 0.05 wt% are more beneficial to the antireflection of the coating.In this study,the antireflective effect of double modified chitin nanofiber coating can be up to 2%-3% in the 390-800 nm band(only one side of the glass film is coated).