Preparation And Properties Of Functional Nanocomposites Based On Bacterial Cellulose

Nanofibers with diameters less than 100 nm have attracted much more attention because of their potential applications in environmental conservation,filtration, biomedical products,protective clothing.Biosynthesis is also a method to fabricate nanofibers except electrospinning.Bacterial cellulose(BC) produced by Acetobacter xylinum is a typical example.BC is a novel natural nano-biomaterial.Compared with other plant cellulose in nature,BC displays unique physical-chemical properties,such as high Young's modulus,the tensile strength,high purity(free of lignin and hemicelluloses),high crystallinity,high water-holding capacity,ultra fine three dimensional(3D) network with a distinct tunnel and pore structure,well-separated nano- and microfibrils creating an extensive surface area and good biocompatibility,biodegradability. Benefiting from its unique properties,BC has been applied in many fields such as food,medical materials,paper and so on.In this thesis,BC was synthesized by Acetobacter xylinum in static and agitated culture and its structure and properties were characterized.Then BC obtained from static culture was chosen as matrixes for synthesizing nanocomposites.The CdS/BC nanocomposites and AgCl/BC nanocomposites were prepared and characterized.Moreover,the formation mechanism of nanoparticles was studied.The main work was presented as follows:(1) BC was prepared through cultivating in static andagitated culture.The structure and properties of BC were studied.The results showed that BC cultivated in static culture at 30℃were hydrogel membranes with a fine 3D network comprised of 20～80 nm width cellulose micro fibrils,water-holding capacity of 98.5%,tensile strength of 56.2 MPa and tensile modulus of 831.1 MPa.BC synthesized in agitated culture at 30℃were loose snow-like consisting of 1～10μm microfibrils. While cultivated in agitated culture at 14℃,BC synthesized appeared as spheres and the microfibrils twisted intensively which was proved to be amorphous cellulose.(2) CdS/BC nanocomposites were prepared by in situ process and chacterized by SEM,XRD,TGA,UV-Vis absorption and photoluminescence spectra.The results showedthe CdS particles were 300 nm and well distributed in the BC when the concentration of Cd(NO₃)₂ and Na₂S was 10 mM,.When the concentration of Cd(NO₃)₂ and Na₂S was reduced to 1 mM,the size of CdS particles were reduced to about 30 nm.The results of UV-Vis absorption showed a absorption peak at 426 nm was observed with a 90 nm blue-shift when compared with the characteristic absorption of the corresponding band-gap of bulk CdS(515 nm),reflecting the quantum confinement effect of the CdS nanocrystal.The photoluminescence spectrum of the CdS/BC nanocomposite showed two emission peaks at 417 nm and 437 nm which blueshifts about 60 nm from that of CdS bulk materials.(3) AgCl/BC nanocomposites were prepared through alternating dipping of BC membranes in the solution of AgNO₃ or NaCl followed by a rinse step.The structure and properties were characterized by SEM,XRD,TGA,etc.The size of AgCl particles in BC can be adjusted by the concentration of solution.When the concentration of AgNO₃ and NaCl solutions was 10 mM,large amounts of Ag⁺ ions were adsorbed on cellulose fibers,leading to large and widely distributed particles.When the concentration of AgNO₃ and NaCl was changed to 1 mM,the size of AgCl particles in BC were about 20～100 nm.TGA showed that the content of AgCl in this composite was 12.6%.The antibacterial activity of never-dried AgCl nanoparticles impregnated BC membrane was measured by shake flask test method.The results showed there was a 100%reduction in viable E.coli and S.aureus on the sample membrane.