Layer-by-layer Self-assembly Of Chitosan Thin Films And Their Properties

Chitosan is the only cationic polysaccharide in nature and bears with functional groups such as hydroxyl groups and amino groups.Through chemical modifications,chitosan can be processed into various derivatives and used for a variety of applications ranging from food,smart materials,agriculture & environment to biomedicine.Layerby-layer self-assembly(LBL)has helped to expand the synthesis of materials from macroscopic views to microcosmic prospects,thus achieving the precise control over the structures and properties of the prepared architectures.Due to its advantages in material design,LBL method has been widely used to introduce functional or multifunctional materials with self-healing,anti-fogging,drug carrier,filtrations & separation properties.In the present study,chitosan was dissolved in Li OH/ KOH/ urea alkaline solution in low temperature and further modified with acrylamide to produce acrylamide modified chitosan(AMCS),and the AMCS was employed as building block while LBL technique was used to construct transparent,self-healable and fog-resistant polysaccharide films.The prospects of the films for food packaging application were investigated and the details of the project are listed below:(1)Chitosan powder was dissolved in Li OH/ KOH/ urea mixture by freezing-thawing method and then transparent chitosan solution was received.The AMCS3 was obtained by taking advantage of Michael reaction between acrylamide and chitosan(molar ratio at 3).The structures and properties of the resultant were evaluated via Fourier transform infrared spectroscopy(FT-IR),X-ray photoelectron spectra(XPS),1H nuclear magnetic resonance(1H-NMR),Emission scanning electron microscope(SEM).The results demonstrated that acrylamino groups and carboxyl groups were grafted on chitosan backbone and was accompanied with a degree of substitution of 0.54.The received AMCS has porous morphology and can form solution in water and diluted HCl solution.(2)The alginate dialdehyde(ADA50)was prepared by oxidizing sodium alginate with sodium periodate(molar ratio is 2:1).The structures and morphologies of the product were investigated by FT-IR,XPS,Ultraviolet-visible spectroscopy(UVvis)and SEM.The results showed that the hydroxyl groups on C-2,C-3 of the pristine alginate were oxidized into aldehyde groups,and the oxidation degree is 44.75%.Due to the oxidation reaction,alginate was endowed with loose structure.In addition,layered polysaccharide films composed of acrylamide modified chitosan(AMCS3)and alginate aldehyde(ADA50)were fabricated via LBL manner and evaluated by UV-vis,SEM,FT-IR,XPS,and water contact angle test(WCA),respectively.The results demonstrated that the peaks at 249 nm of UV-vis spectra and at 401.52 e V(N1s)of XPS of the films were resulted from Schiff base linkage,revealing the mentioned bond is the driving force for the growth of the films.The film thickness increased linearly with the increase of deposition cycles,but this effect has relatively little influence on WCA(~25°)and transmittance(>95%)as seen from the wettability and high transmittance of the films.(3)The chitosan based films were activated by water drops after damaged by blade or abraser,and the self-healing phenomenon of films were recorded by Microscope,SME,WCA test,and UV-vis spectra.The results depicted that the movement of the molecule chain and the reconstruction of Schiff base linkage within the networks gave the films a repetitive self-healing property,and the repeated damage-healing of the films was thickness-dependent.Except for the excellent self-healing property,various temperatures & humidity were set to investigate the fog-resistant feature of chitosan based films.It was found that the films had fog-resistant ability,and the anti-fogging ability of the films was also thickness-dependent.Besides,due to their self-healing capability to repair the damage caused by mechanical abrasion,the films could regain their transmittance and fog-resistant property.Furthermore,chitosan films with 6 bilayers were coated on PE films and used to packed strawberry,and their fog resistant property was recorded by digital camera.It was noticeable that the coated PE films were fog-free and transparent enough to present the packed strawberry.In summary,these self-healable and fog-resistant polymeric films which fabricated by LBL technique may have potential applications in food packaging.