Preparation Of Chitin Nanofibers And Study On Amphiphobic Modification Mechanism Of Their Transparent Coatings

Chitin is a kind of natural polysaccharide polymer and mainly exist in the exoskeleton of crustaceans.Since chitin is non-toxic and renewable,and it is an important biomass resource.Chitin nanofibers(ChNFs)have a high aspect ratio,and the nanoscale width of ChNFs meets the basic requirements for constructing transparent coatings.In order to make the ChNFs transparent coatings meet the application requirements and maintain its transparency,hydro-oleophobic modification of ChNFs coatings is necessary.The coating surfaces constructed by the rod-like ChNFs possess the micro/nano structure which is beneficial for formation of hydro-oleophobicity,moreover,the active groups of ChNFs can be using to graft low surface energy chemicals.All of these provide an advantage to prepare transparent coatings with hydro-oleophobicity and anti-fingerprint performance.In this work,maleic anhydride(MA)and oxalic acid(OA)were used to acylate chitin,and maleic anhydride modified-chitin nanofibers(MA-ChNFs)and oxalic acid modified-chitin nanofibers(OA-ChNFs)were prepared by ultrasonic treatment.Then,transparent MA-ChNFs coatings and OA-ChNFs coatings were fabricated by casting the ChNFs suspensions on glass slides,and trichlorosilanes were used to modify the ChNFs transparent coatings in order to make the surfaces have hydro-oleophobicity and anti-fingerprint performance.Based on the analysis of the differences in hydrooleophobicity and anti-fingerprint performance between the trichlorosilanes modified MA-ChNFs coatings and OA-ChNFs coatings,the wettability and anti-fingerprint performance of the PFDTS modified MA-ChNFs coatings with different DS(carboxyl content)were investigated and analyzed.The grafting reaction mechanism of PFDTS on the ChNFs coatings was studied.(1)MA-ChNFs and OA-ChNFs were prepared,respectively,by using the MA acylated chitin and OA acylated chitin combined with ultrasonic treatment.When the amount of MA was lower than the mass of chitin,the size of the MA-ChNFs decreased with increase of the amount of MA.When the amount of MA was equal to the mass of chitin,the mean length and width of the MA-ChNFs were 209 ± 75 nm and 19.5 ± 7.8nm,respectively.The length of the MA-ChNFs was not significantly affected by increasing the amount of MA(P > 0.05),but the width continued to decrease.The DS of MA-ChNFs increased with the increase of MA amount and reached 0.46 when the amount of MA was five folds of the chitin mass.When the concentration of oxalic acid was 25 wt%,even the amount of oxalic acid was 50 times of the chitin mass and the reaction time was 10 h,the nano-scale OA-ChNFs could not be prepared by 500 W ultrasonic treatment.However,when the concentration of OA was 50 wt%,both of OA amount and acylation time affected the size of the OA-ChNFs.When the amount of OA was 50 and 25 times of the chitin mass and the acylation time was 7.5 h and 10.0 h,the main peak position in the hydration kinetics diameter distribution curves of the obtained OA-ChNFs was less than 100 nm.When the amount of OA was reduced to 12.5 times of the chitin mass,OA-ChNFs with different sizes could be prepared by adjusting the acylation time,the mean length and width of the OA-ChNFs prepared after 10.0 h of the acylation were 227 ± 58 nm and 30.7 ± 11.2 nm,respectively.The DS of OA-ChNFs was only 0.06,which was much lower than that of MA-ChNFs.It was difficult to prepare the OA-ChNFs with higher DS(with more carboxyl groups)by OA acylated chitin.The prepared MA-ChNFs suspensions(pH = 7)and OA-ChNFs suspensions(pH= 10)had good stability.(2)Both the MA-ChNFs and the OA-ChNFs were used to prepare transparent coatings on surface of glass slides,and the transmittance of the coatings increased with decrease of the ChNFs size.The transmittance of glass slide coated with the MAChNFs reached 93% and was higher than that of glass slides(91%),indicating that the MA-ChNFs coating had anti-reflection performance.The transmittance of slide coated with the OA-ChNFs was only 91%.The adhesion between the MA-ChNFs coating and the glass slide reached level 2,while the adhesion between the OA-ChNFs coating and the glass slide was only level 3.This was because the MA-ChNFs had a smaller size,higher specific surface area,and more exposed polar groups(hydroxyl and carboxyl),which enhanced the adhesion between the MA-ChNFs coating and the glass slide.(3)Vapor deposition of trichlorosilane was more suitable for modification of the ChNFs coating surface than casting and spinning of trichlorosilane ethanol solution.Based on the properties of the MA-ChNFs coatings modified with methyl trichlorosilane(MTS),perfluorooctyltrichlorosilane(PTOTS),and perfluoroodecyltrichlorosilane(PFDTS)via vapor deposition,it was concluded that the shorter the carbon chain at the hydrophobic end of the trichlorosilane,the smaller the impact on the transmittance of the MA-ChNFs coatings.The longer the carbon chain at the hydrophobic end,the more the hydro-oleophobicity of the MA-ChNFs coatings was enhanced.The transmittance of the glass slide coated with the MA-ChNFs after PFDTS modification was 89%(2% lower than that of the slide),and the water contact angle and oleic acid contact angle of the coating surface were 149° and 117°,respectively.The modified MA-ChNFs coating had anti-fingerprint performance.After being wiped by anhydrous ethanol cotton,the modified MA-ChNFs coating also had hydrooleophobicity,indicating that PFDTS were not adsorbed on the surface of MA-ChNFs coating.The results of ATR-FTIR and XPS confirmed that PFDTS was grafted onto the MA-ChNFs transparent coating.The water contact angle and oleic acid contact angle of the OA-ChNFs coating modified by PFDTS were only 89° and 56°.(4)The change in surface roughness of the ChNFs coating introduced by the vapor deposition of PFDTS had no significant effect on the hydro-oleophobicity of the coating surface,the hydro-oleophobicity was mainly determined by surface energy,i.e.,the amount of PFDTS grafted onto the coating surface.The ATR-FTIR and XPS results showed that the more the carboxyl groups in the MA-ChNFs coatings,the more PFDTS was grafted onto the MA-ChNFs coating surface,which indicated that the amount of carboxyl groups(DS)was the main factor determining the hydro-oleophobicity of the ChNFs coatings modified by vapor deposition with PFDTS.However,when both of the MA-ChNFs and the OA-ChNFs had DS of 0.06,the water contact angle and oleic acid contact angle of the MA-ChNFs coatings modified by vapor deposition with PFDTS were significantly higher than that of the OA-ChNFs coatings,indicating that the grafting efficiency of PFDTS onto the ChNFs coatings was not only influenced by the amount of carboxyl groups but also the structure of carboxyl groups.The more the carboxyl groups and the longer the molecular chains of carboxyl groups in the ChNFs,the higher the grafting efficiency of PFDTS onto the ChNFs coatings.