Study On The Preparation And Properties Of Bacterial Cellulose-Based Composites

As a kind of bio-polymer material, bacterial cellulose(BC) has many unique properties,such as high crystallinity and mechanical strength, good water-holding capacity, biocompatibility,biodegradability and controlled synthesis. It is considered as a potential material that may has a multitude of applications in food, paper, biomedical material and other fields. BC attacts more and more attentions in recent years because of its excellent properties. The current research on BC materials mainly focus on the optimization of biosynthesis, modification of structure,preparation of composites and so on. In this thesis, BC was synthesized in a static culture medium by Acetobacter xylinum. BC was chosen as a matrix material for preparing bacterial cellulose-graphene(BC-G), bacterial cellulose-silver(BC-Ag), aminoalkyl-grafted bacterial cellulose(A-g-BC) and quaternary ammonium-grafted bacterial cellulose(G-g-BC) composites.The thermal stabilities, mechanical properties and antibacterical abilities were investigated. And the main results in this work are listed as follows.Firstly, the wet BC membranes were disintegrated by high speed homogenizer to achieve BC fiber slurry, which was blended with graphene suspension. The mixed suspension was filtered to obtain BC-G composites. Scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and Raman spectroscopy results showed that graphene nanosheets were evenly distributed in the denser network structure of BC with a good compatibility. Water contact angles of BC-G composites increased with increasing graphene loadings. BC-G composites exhibited enhanced thermal stability compared with BC.Thermogravimetric(TG) analysis results showed that the decomposition temperatures increased from 338.9 ℃ to 360.7 ℃. The mechanical properties of BC-G composites got improvements with increasing graphene loadings. The tensile strength of pristine BC was 96 MPa, while its composite with a loading of 8 wt% graphene exhibited the strength of 162 MPa with an improvement of 68.8 %.Secondly, BC-Ag nanocomposites were prepared in situ reduction with BC acting as templates. BC was immersed in the Ag NO3 solution that silver ions could penetrate into BC through the porous structure and bound to its microfibrils probably via electrostatic interactions.After reduction in aqueous Na BH4, silver ions were reduced to form Ag NPs on the microfibrils of BC. The BC and as-prepared BC-Ag nanocomposites were characterized by SEM, XRD,infrared spectroscopy(FTIR) and TG. The results showed that spherical Ag NPs with particle size of 100 nm well dispersed on the surface and inside of BC matrix. The releases of Ag~+ at different pH values were studied, which showed BC-Ag nanocomposites had the highest releases of Ag ion over 9 days and the silver ion release was strongly pH dependent, ion release rates decreased with increasing pH. The antibacterial performances of BC-Ag nanocomposites were evaluated with Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Candida albicans.The biggest antibacterial ratios were 98.4 %, 100 %, 100 % and 89.6 %, and the maximum zones of inhibition were 14.1 mm, 18.7 mm, 24.3 mm and 16 mm, respectively. It was found that antibacterial ability improved with increasing Ag NPs loadings in the nanocomposites. The experimental results showed that BC-Ag nanocomposites have excellent antibacterial activities,thus confirming its utility as potential wound dressings.Thirdly, antibacterial bacterial cellulose membranes(A-g-BC) were prepared by chemical grafting of aminoalkyl groups onto the surface of its 3D network. A-g-BC membranes were characterized by FTIR, SEM, XPS and TG. The tested results showed that A-g-BC membranes became thicker and denser because of the coverage with aminoalkylsilane groups. With the increase of grafting yield of the BC membranes, water contact angle of A-g-BC membranes increased from 48.25 o to 86.37 o due to the covalent linkage with hydrophobic aminoalkyl groups.A-g-BC membranes display effective antibacterial and antifungal activities against E. coli,S. aureus, S. subtilis and C. albicans. The biggest antibacterial ratios were 100 %, 99.4 %, 99.9 %and 98.7 %, and the maximum zones of inhibition were 25.0 mm, 26.1 mm, 20.4 mm and 23.0mm, respectively. At the same time, A-g-BC membranes showed different sensitivity to different strains, antibacterial capabilities were shown as follows: gram-positive bacterium > gramnegative bacteria > fungus. The resultant functionalized BC membranes exhibited excellent antibacterial property, thus confirming its utility as potential wound dressings and other biomedical applications.Lastly, quaternized bacterial cellulose membranes(G-g-BC) were prepared by nucleophilic addition method of grafting 2,3-epoxy propyl trimethyl ammonium chloride(GTMAC) onto the surfaces of BC fiber. G-g-BC membranes were characterized by SEM, XPS, FTIR and so on. The tested results showed that the surface charge turned into a positive charge from negatively charge with the increase of grafting yield of the BC. The SEM pictures of G-g-BC membranes exhibited a fluffy network structure. The BC ribbon-shaped microfibrils became thicker with the introduction of GTMAC and arranged regularly with the same orientation. The swelling speed and water loss speed decreased with increasing grafting ratio of GTMAC in the membranes. The swelling rates of G-g-BC membranes increased from 1001 % to 5511 %, which proved G-g-BC membranes have better water-absorbing and water-holding capacities than pristine BC membrane.G-g-BC membranes displayed effective antibacterial and antifungal activities against E. coli,S. aureus and C. albicans. All of the biggest antibacterial ratios were 100 %, and the maximum zones of inhibition were 26.5 mm, 28.2 mm and 18.3 mm, respectively. Therefore, G-g-BC membranes showed more active against gram-positive bacteria than gram-negative bacteria and fungus. This research proved the potential of G-g-BC antibacterial membranes for contact active antimicrobial surface applications in active food packaging, medical dressings or in health and cosmetic field.