Preparation And Characterization Of Optically Transparent Nanocomposites Based On Bacterial Cellulose

Bacterial cellulose (BC) is a new kind of biological nanomaterials, generated as an ultrafine nanosized3D fibrous network comprised of bundles of nanofibers which are aggregates of semicrystalline extended cellulose. BC therefore could serve as a reinforcement of polymer matrix to fabricate optically transparent organic/inorganic nanocomposites due to its unique potentials. Firstly, incorporation of BC into optically transparent resin could maintain high transparency with the nanosized BC fibers. The rigid nanofiber networks of BC could improve the flexibility and ductility of the polymer matrix. Otherwise, BC has a nanoporous structure and a great deal of hydroxyl groups along nanofibers, hence, BC could be used as "biotemplate" to synthesize inorganic nanoparticles through in-situ method.In this paper, we provide a novel preparation method to fabricate optically transparent nanocomposites based on BC. Firstly, spherical ZnS nanoparticles were successfully synthesized using BC as templates through in situ homogeneous precipitation, then impregnated with epoxy resin (E56) to fabricate the final transparent BC/ZnS/E56nanocomposites. The dependency of BC/E56nanocomposites properties on BC fiber content and the effect of the zinc precursor concentrations on the structures and properties of BC/ZnS, BC/ZnS/E56nanocomposites are investigated. The main work is presented as follows:1. Preparation and characterization of BC/ZnS nanocompositesThe zinc precursor concentrations have the significant impact on the determination about the size and distribution of ZnS nanoparticles. The single ZnS crystals assemble to form uniform spherical polycrystalline ZnS nanoparticles which can be observed homogeneously dispersing on BC nanofibers. When the zinc precursor concentrations range from0.1to5wt%, the average diameters of the nanoparticles on BC surface are approximately30～40nm,50～60nm,80～100nm,150～200nm,200～300nm, respectively. ZnS nanoparticles anchored on the BC have strong interactions with the hydroxyl groups of BC nanofibers. The resultant nanocomposites possessed excellent mechanical properties.2. Preparation and characterization of optically transparent nanocomposites based on BC.Optiaclly transparent BC/E56nanocomposites are successfully fabricated and the dependency of BC/E56nanocomposites properties on BC fiber content are investigated. BC/E56nanocomposites (BC fibers content2.6～32.7wt%) display high visible light transparence (70-85%at400～700nm), with refractive index (RI) between1.52～1.56(632.8nm) according to BC fiber content. The coefficient of planar thermal expansion is reduced from81.6ppm/K (E56) to42.6ppm/K even at ultralow fiber content (2.6wt%) due to the dense and finely branched BC nanofiber networks in the planar direction. However, such a restriction does not chang the coefficient of volume thermal expansion of E56matrix, which results in a high increase in coefficients of thermal expansion of BC/E56nanocomposites in thickness direction (155.9-210.8ppm/K). Therefore, thermal optical coefficients (TOC) of BC/E56nanocomposites increase from-144.6ppm/K to-265.6～-360.8ppm/K in thickness direction. The mechnical properties of BC/E56nanocomposites are much improved compared with the pure E56.The final optically transparent BC/ZnS/E56nanocomposites are fabricated and the effect of the zinc precursor concentrations on the structures and properties of the nanocomposites is investigated. The spherical ZnS nanoparticles uniformly disperse inside the nanocomposites observed on the section from TEM. BC avoids the interparticle aggregation effectively and maintains the uniform dispersion of ZnS nanoparticle inside the nanocomposites during the E56curing process due to the strong interaction between the hydroxyl groups along BC nanofibers and ZnS nanoparticles even at high zinc precursor concentration. When the zinc precursor concentrations range from0.1to5wt%, the average diameters of the nanoparticles are approximately15～20nm,20～30nm,30～40nm,40～50nm,70～80nm, respectively. When the zinc precursor concentrations are0.1wt%to1wt%, nanocomposites display high visible light transparence (75～80%at600nm). But when the zinc precursor concentrations increase to2.5,5wt%, the interparticle agglomerates of ZnS nanoparticles on BC surface occur forming the irregular aggregates, which results in opaque. RI and TOC of the nanocomposites are1.55-1.58and-310.1ppm/K～-360.8ppm/K, increase with the increament of the ZnS content. The mechanical properties of the nanocomposites decline and display brittleness with zinc precursor concentrations.