Study On Fabrication, Structure And Properties Of Bacterial Cellulose/Carbon Nanotube Composite Materials

Owning to their unique and superior physical properties,including mechanical, electrical,chemical properties and high aspect ratio,carbon nanotubes(CNTs) have been uesd to fabricate performanced composite materials by embedded into the polymer matrix as reinforcing and electric materials.In this study,the bacterial cellulose(BC) nanofibers with unique properties were synthesized by Acetobacter xylinum(A.xylinum) which consumed the glucose in static and agitated culture.The BC/CNTs nanocomposites were manufactured using BC microfibrils and carbon nanotubes via in situ synthesis in CNTs-containing culture medium,immersing BC membrane in CNTs suspension and homogenizing BC membranes into slurry to mix the CNTs suspension.The fabricating methods, structure and properties ofnanocomposites were investigated as follows:The Acetobacter xylinum 1.1812 strains were cultivated in Hestrin & Schramm's static medium.The effects of inoculum amount,cultivation temperature and time on the BC yield were systematically studied.The optimization cultivation conditions for the greatest BC productivity and transfer efficiency were ascertained. The high quality and properties BC was obtained.The elemental analysis data showed that the residual culture medium and bacterial cell debris in the nascent BC membrane were removed efficiently by boiling in a 4w/v%aqueous solution of NaOH for 1 h.The alkali treatment did not destroy the crystal microstructure of the BC ribbons from the X-ray Diffraction(XRD) analysis.The scanning electron microscopy(SEM) analysis showed that the BC membrane was fabricated into layer-by-layer network pellicles by microfibrils with about 40～100 nm width and several microns length.The porous BC membrane had high water holding capacity(WHC) with 98.5%.The data from by Fourier transform infrared spectroscopy(FT-IR),CP/MAS ¹³C NMR and XRD analysis indicated the alkali treatment BC had a high crystalline index with 84.3%(CrI^XRD) and high cellulose I_αcontent with 87.4%(f_α^NMR),higher than that synthesized by BRP2001.The dried BC membranes had great tensile strength with 56.2 MPa and tensile modulus with 831.1 MPa.The morphology and microstructure of BC strictly depended on the culture conditions.The snow-like assemblies consisting of loose microfibrils were synthesized in agitated medium at 30℃and could absorb the 150 times water than its dried weight,which WHC was changed from 98.5%into 99.34%.The SEM images showed that the microfibrils assemblies changed from cellulose ribbons into flat cellulose bands.Comparing with the cellulose synthesized in static culture,the snow-like assemblies decreased the crystalline index,crystallite size and cellulose l_αcontent determined by FT-IR,NMR and XRD analysis,which may be ascribed that the agitated stress influenced the secretion,assembly and crystallization of BC microfibrils.While cultivation in agitated culture at 14℃,the BC microfibrils twisted intensively and assembled a hollow spheres,which was amorphous cellulose determined by FT-IR,NMR and XRD analysis.The CNTs were fluxed by concentrated H₂SO4 and HNO₃ and the chemical structure was analyzed by FT-IR.As a result,amount of functional aqueous groups such as carbonyl,carboxyl and hydroxyl groups were introduced onto the surface of CNTs,which increased their surface activities and adsorbability and improved their dispersibility and stabilizability in water.The acid-treated CNTs were dispersed uniformly in the culture medium and the A.xylinum strains could grow and synthesize cellulose continuously,which indicated that the acid-treated CNTs had good biocompatibility.On the basis of systematic research on the culture methods and conditions,black composite membranes could be in situ synthesized in static medium containing CNTs at 30℃.The SEM images showed that the CNTs were incorporated into the BC microfibrils network and formed nano-network.The BC microfibrils interwound with the MWNTs,and constructed the three-dimensional reticular tissue. By SEM,AFM,FT-IR,XRD and NMR analysis,the results showed that the CNTs in the medium influenced the assembly and crystallization of microfibrils,changed the morphology,resulting in flat BC bands with 400～900 nm width and the crystalline index changed from 84.3%into 80.65%,and the cellulose I_αcontent changed from 87.4%into 79.6%.In agitated medium containing CNTs at 30℃,the BC/CNTs composites could also be in situ synthesized.However,the BC microfibrils packed the CNTs and formed fibrous assemblies.Comparing with the static culture,the fibrous assemblies synthesized in agitated CNTs medium had lower crystalline index and cellulose I_αcontent.Interestingly,the BC microfibrils and CNTs could assemble a hollow sphere in agitated medium at 14℃.The BC microfibrils and CNTs arranged uniformly along the radial direction.The spherical BC microfibrils were amorphous cellulose determined by FT-IR,XRD and NMR analysis.The purified BC membranes were immersed into the CNTs suspension and a layer of MWNTs were adsorbed onto the surface of BC membrane,resulting in the BC/CNTs composites.The higher the CNTs content was and the more time BC membrane was immersed,the more CNTs were adsorded.The agitating or ultrasonic treatment could help the CNTs adhere to the BC membrane.The CNTs on the surface of BC could construct electrical conductive networks,and formed the low electrical resistivity composites with 0.865Ω·cm.The electric conductive mechanism of composite membranes was also investigated.The BC microfibrils were disintegrated by a double-cylinder type homogenizer and formed the suspension,which was rapidly blended with the CNTs suspension. The mixed suspension was filtered using a Buckner funnel and a composite film was obtained.The BC microfibrils and CNTs impenetrated each other and built a new 3-dimensional reticulate structure by SEM and AFM analysis.The CNTs could enhance the mechanical properties,thermal stability and the conductivity of composites.When the CNTs content in the composites was above 5wt%,the CNTs could construct a continuous network and enhance sharply the tensile strength and modulus,and reduce the electrical resistance.