Preparation And Application Of Nanocellulose/Boron Nitride Nanosheet Composites

As an ideal thermal conductive filler,the preparation and dispersion of boron nitride nanosheet(BNNS)are very difficult.It is a common strategy to exfoliate BNNS from bulk boron nitride(BN).However,most exfoliation techniques suffer from several issues,such as high cost,long time,not environmental friend,small lateral size,and many defects,which greatly limit the large-scale production and application of BNNS.In recent years,cellulose nanofiber(CNF),as a green dispersing agent,has been introduced into the exfoliation and dispersion of BNNS.In this thesis,CNF/BNNS composites were prepared with cellulose fiber(CF)and boron nitride(BN),and their application were explored.The main research works are as follows:Firstly,CNF was obtained via a synergistic ball milling and high-pressure homogenization after the 2,2,6,6-tetramethylpiperidinyl-1-oxide(TEMPO)oxidized pretreatment of CF.By introducing a ball milling between the TEMPO oxidation and the high-pressure homogenization,the energy consumption and the clogging problem of the homogenization process are effectively reduced.The stress,strain and toughness of CNF-500-H are 148.0 MPa,7.12%and 7.27 MJ·m-3,respectively,which are 53%,127%and 186%higher than those of CNF-500.Based on this,CNF/BNNS composites with high dispersity in water were prepared via the combination of ball-milling and high-pressure homogenization.The obtained CNF/BNNS dispersions can be stable for more than 30 days without the observation of sediments.The CNF as a medium for ball milling can protect the exfoliated BNNS from being re-aggregated and severely destroyed in violent collisions,and the BNNS and CNF can promote their mutual exfoliation.The hydrophilicity and electrostatic repulsion of CNF,as well as the strong interfacial interactions and stereo-hindrance effects between one-dimensional(1D)CNF and two-dimensional(2D)BNNS are the main reasons for the high dispersity of CNF/BNNS composites in water.The composite dispersions were then transformed into paper-like films by the vacuum assisted filtering and drying.The excellent dispersion stability of composite dispersions in turn leads to the favorable mechanical properties of the formed films.Although the introduction of BNNS lowers the stress,strain and toughness of the neat CNF films,their modulus increases due to the stiffness of BNNS.However,the thermal conductivities of the homogenized samples are inferior to those of the unhomogenized samples,which are ascribed that the decreased lateral size and improved dispersibility of BNNS do not favor the formation of connective path of thermal conduction.Then,in this thesis,the effect of pure water,sodium dodecyl sulfate(SDS),and cetyltrimethylammonium bromide(CTAB)on the exfoliation of bulk BN was compared.It was found that the BNNS obtained with CTAB assisted exfoliation has a larger lateral size and thinner thickness,as well as the highest exfoliation yield and dispersion stability.In order to improve the stability of BNNS dispersion in water,dopamine-induced one-step polymerization method was employed to fabricate the polydopamine/poly(3-sulfopropyl methacrylate potassium salt)(PDA-PSPMA)conjugate coating on the surface of BNNSCTAB to engineer the functional nanosheets(BNNS@PDA-PSPMA).After grafting PDA-PSPMA,the zeta potential of the nanosheets changes from+27.3 mV to-13.9 mV,the average hydration particle size is increased from 309 nm to 445 nm,and the dispersion stability of BNNS in water is significantly improved.Based on the exfoliation of BNNS in the previous research,CTAB-assisted exfoliating BNNS was further applied into surface enhanced Raman scattering(SERS)with silver nanowires(AgNWs)as SERS signal enhancement material,CNF as the substrate,and BNNS as the spacer and protective layer for AgNWs.The enhanced signals of SERS sensors prepared by filtration and spin coating were compared and explored.It can be found that the detection signal of SERS sensor via spin coating method for Rhodamine 6G(R6G)molecules was much higher than that of filtration method.In addition,AgNWs and BNNS were spin-coated layer by layer on the CNF film,and SERS sensors with different layer numbers of spin coatings were fabricated.As the layer numbers of spin coatings are increased,the distribution uniformity of AgNWs and BNNS is gradually increased,and the BNNS amounts interspersed between AgNWs are also increased.These results lead to the significant increase in the hot spot numbers between AgNWs,further verifying the remarkable improvement for the adsorption amount of detected molecules between AgNWs and the effective enhancement on the SERS signal.