Preparation And Properties Of Quaternized Cellulose Nanocrystals/Polyvinyl Alcohol Antibacterial Composite Films

Bacterial infections have been a concern for a long time.Therefore,it is particularly important to develop new antibacterial materials which are safe,efficient and environmentally friendly.As the smallest physical structural unit of lignocellulose,cellulose nanocrystals（CNC）have a large number of hydroxyl groups on their surface,which allow for the preparation of materials with antibacterial properties through chemical modification.In addition,CNC has excellent physical properties such as high mechanical strength,large specific surface area and biodegradability.Therefore,CNC shows good prospects in the construction of functional nanocomposites.In this study,purified cellulose extracted from wood powder was used as raw material to prepare CNC by hydrochloric acid hydrolysis,and surface modified CNC containing bromine atoms was prepared by esterification.Finally,quaternized with alkyl imidazole and open chain tertiary amines of different alkyl chain lengths to obtain quaternized cellulose nanocrystals with antibacterial groups.The quaternized cellulose nanocrystals were used as antibacterial nanofillers to improve the properties of polyvinyl alcohol（PVA）matrix,and quaternized cellulose nanocrystals/polyvinyl alcohol composite films were prepared.The effects of quaternized cellulose nanocrystals with different contents on the optical transmittance,mechanical properties and antibacterial properties of PVA nanocomposite films were investigated.The main contents and results were as follows:（1）Purified cellulose extracted from poplar wood powder was used as raw material,cellulose nanocrystals（CNC）were obtained by acid hydrolysis of cellulose with hydrochloric acid.Surface 6-bromohexyl cellulose nanocrystals（ECNC）with bromine atoms were prepared by surface esterification of CNC with 6-bromohexanoyl chloride,and then reacted with alkyl imidazole and open chain tertiary amines with different alkyl chain lengths,respectively.Imidazole type quaternized cellulose nanocrystals（ECNC-Im_n,n=4,8 and 12,n representing the number of carbon atoms of the alkyl chain）and open chain quaternized cellulose nanocrystals（ECNC-Q_n,n=8,12,14 and 16,n representing the number of carbon atoms of the alkyl chain）were obtained.The chemical structures and morphologies were characterized by FTIR,¹³C NMR,XRD and TEM.The results showed that the quaternized cellulose nanocrystals were successfully prepared and that the products existed in the form of rod-like nanoparticles and retained the crystalline structure.The antibacterial activities of the quaternized cellulose nanocrystals were evaluated using the colony-counting method.The antibacterial rates of ECNC-Im₈ and ECNC-Q₈against Escherichia coli（E.coli）and Staphylococcus aureus（S.aureus）were more than 99%,which have the best antibacterial effect.（2）In order to solve the problems of the poor mechanical properties and lack of antimicrobial activity of PVA,ECNC-Im_n nanofillers were blended with PVA matrix and the nanocomposite films（ECNC-Im_n/PVA）were prepared by the solvent casting method,and the optical transmittance,mechanical properties and antibacterial properties of the nanocomposite films were tested.The results showed that the addition of ECNC-Im_n had no significant effect on the optical transmittance of the nanocomposite films.The mechanical properties of the nanocomposite films were most significantly improved when the addition of ECNC-Im_n was 5%,and the maximum tensile strength and elongation at break of the composite films reached 46.0MPa and 296.5%,respectively,which were increased by 41.5%and 90.8%compared with pure PVA film.The antibacterial results showed that the antibacterial rates of ECNC-Im₄/PVA,ECNC-Im₈/PVA and ECNC-Im₁₂/PVA composite films against E.coli were 51.2%,98.6%and66.0%,respectively.In conclusion,the addition of ECNC-Im_n improved the mechanical properties of the PVA matrix and imparted antibacterial properties to the PVA.（3）Due to the poor antibacterial effect of ECNC-Im₄/PVA and ECNC-Im₁₂/PVA nanocomposite films against E.coli,nanocomposite films（ECNC-Q_n/PVA）were prepared by compounding ECNC-Q_n with PVA.The effects of ECNC-Q_n on the optical transmittance,mechanical properties and antibacterial properties of the PVA matrix were investigated.The results showed that the optical transmittance of all nanocomposite films was above 90%.The maximum tensile strength and elongation at break of ECNC-Q_n/PVA nanocomposite films were45.4 MPa and 318.4%,respectively,which were 37.2%and 94.3%higher than those of pure PVA film.In addition,the nanocomposite films had excellent antibacterial properties,and the nanocomposite films with 5%ECNC-Q_n addition had antibacterial rates of over 90%against E.coli.Therefore,the addition of ECNC-Q_n improved the tensile strength and toughness of the PVA matrix and imparted good antibacterial properties to the PVA matrix.In this study,environmentally friendly nanocomposite films with good mechanical properties and excellent antibacterial properties were prepared by modifying cellulose nanocrystals with quaternary ammonium and compounding them with PVA.The prepared nanocomposite films have potential applications in the packaging field.This study opens up an effective way for the high value utilization of poplar wood powder and the preparation of biodegradable antibacterial materials.