Preparation And Characterization Of Nanographite/Polymer Conducting Composite Film

Conductive polymer composite materials are usually obtained by mixing conductive fillers with polymer matrix in a certain way.They can be obtained via solution compounding, mixing or in-situ polymerization.Commonly used conductive fillers are conductive metals such as gold,silver,copper and aluminum,and non-metallic materials such as carbon black, graphite,etc.In this paper,we have chosen the expandable graphite as a material,which was properly handled to prepare graphite nanosheets(NanoG),oxidized NanoG(ONG) and reduced NanoG(RNG).Polyvinyl alcohol,cellulose and polystyrene were appropriately modified in order to increase the interface compatibility with a variety of nano-graphite conductive fillers. And then nine kinds of conductive composite materials were fabricated.The performance of conductive composite materials was all determined,and also their structures were characterized.The work is divided into six parts:The expandable graphite wastreated at a high temperature(1050℃) to obtain expanded graphite,which was sonicated to prepare NanoG.The NanoG was further reduced or oxided to obtain RNG and ONG,respectively.Some pieces of NanoG can be observed with a thickness of about 40-70nm and width of more than 2μm through scanning electron microscopy(SEM), and the pores among irregularly stacked NanoG are irregular and in unequal size.Although NanoG sheet is very thin,it is not a single layer of graphite.According to the SEM micrographs and X-ray diffraction(XRD) patterns,it is concluded that a piece of NanoG sheet with a thickness of 40-70nm consists of 80-140 graphite layer,based on the assumption of a single carbon layer with a thickness of 0.5nm.The SEM micrographs also indicate no marked difference in morphologies of three kinds of graphites.But there are differences in oxygen contents and electrical conductivites.The electrical conductivity of NanoG(0.1311 S·cm^-1) is greater than that of ONG(0.0727 S·cm^-1),but less than that of RNG(0.5345 S·cm^-1),the oxygen contents of NanoG(15.53wt%) is more than that of RNG(13.67wt%),but less than that of ONG(21.91wt%).Carboxymethyl polyvinyl alcohol(CMPVA) was prepared by introducing carboxymethyl groups to polyvinyl alcohol(PVA) with chloroacetic acid.NanoG/CMPVA conducting composite film was prepared by dispersing NanoG in a CMPVA aqueous solution with the aid of sonication and then casted on plastic film to remove water.Carboxymethyl groups were proved to be linked to PVA by Fourier transform infrared(FTIR) spectra and the degree of carboxymethyl substitution on the PVA was 2.77%determined by conductometric titration. Experimental results show that carboxylate anions in introduced carboxymethyl groups faciliate the dispersal of NanoG with positive charges in the CMPVA solution.The CMPVA matrix was detected with XRD and FTIR spectra,and the results indicated that it was less crystal and suitable for the dispersion of NanoG.SEM was used to characterize the morphology of the composites to prove the homogeneous dispersion of NanoG in CMPVA.The percolation threshold of NanoG/CMPVA conducting composite film at room temperature is as low as 0.80 wt%.Catalyzed by silver sulfate,polystyrene sulfonic acid(PSS) was prepared by sulfonating polystyrene(PS),which was obtained by emulsion polymerization of styrene.In the sulfonation process,sulfonate groups were introduced on the phenyl groups of PS.NanoG/PSS conducting composite film was prepared by dispersing NanoG in PSS aqueous solution with the aid of sonication,and casting them on a plastic film.It is confirmed by the FTIR spectra that the sulfonic acid groups have been linked to PS.The introduced sulfonate anions on PS resulted from the ionization of sulfonic acid groups facilitate the dispersal process of NanoG with the positive charges.SEM was used to characterize the morphologies of NanoG/PSS conducting composite film and the results exhibit a homogeneous dispersion of NanoG in PSS matrix.A close observation of transmission electron micrographs(TEM) reveals that the PSS has been intercalated into the sub-layers of NanoG homogeneously.The electrical conductivity of NanoG/PSS conducting composite film reached 1.25x10^-4S·cm^-1 at concentration of 0.2 vol%and the electrical conduction phenomenon can be interpreted by classic percolation theory.Polyvinyl alcohol hydrosulfate(PVA-S) was obtained by esterification reaction of polyvinyl alcohol using sulfuric acid.The esterification degree of PVA is 1.60%obtained by calculation based on sulfur element content determined by Microcoulomb analyzer.The NanoG/PVA-S nanocomposite was fabricated by solution compounding in aqueous solution of PVA-S.SEM and optical micrographs were applied to confirm the formation of the NanoG/PVA-S nanocomposite.The tensile strength,elastic modulus and yield strength of the NanoG/PVA-S nanocomposite were measured and found to decrease with the increase of NanoG loading.The electrical conductivity rises with the increase in temperature,indicating that the temperature is an important parameter of governing the electrical conductivity.The obtained NanoG/PVA-S nanocomposite showes low percolation threshold value of about 0.2 wt%,and maximum of electrical conductivity of 5.4x10^-5 S·cm^-1.Cellulose nitrate was obtained via nitration of cellulose using the mixture of sulfuric acid and nitric acid.The molecular weight of the cellulose nitrate was calculated to be 1.3×10⁵ g·mol^-1 based on the intrinsic viscosity measured.The structure of cellulose nitrate was confirmed by its FT-IR spectra.Based on the nitrogen content measured vial Kjeldahl nitrogen detection equipment,the substituted degree of cellulose nitrate by nitric acid was calculated to be 2.75 per glucose unit.Three kind of cellulose nitrate conducting composite films respectively incorporating RNG,NanoG and ONG were prepared by mixing respective fillers with cellulose nitrate in acetone and subsequently removing the solvent.The electrical conductivites of three kinds of films increase with the weight fraction of the fillers.When the weight fraction of three kinds of graphite particles are low,electrical conductivites of the corresponding nanocomposites were close to each other and the percolation thresholds are equal to each other,indicating that the graphite particles have nearly the same aspect ratios. But when their weight fraction of them reached at 3wt%,the electrical conductivity were 0.0385,0.3333,0.0078S·cm^-1 corresponding to RNG,NanoG and ONG.Catalyzed by concentrated sulfuric acid,the acetyl cellulose was obtained via acetylation of cellulose using acetic anhydride,and the structure of acetyl cellulose was confirmed by its FT-IR spectra The molecular weight of the acetyl cellulose was calculated to be 2.2×10⁵g·mol^-1 based on the intrinsic viscosity measured,the substituted degree of acetyl cellulose by acetyl cellulose group was calculated to be 2.27-2.43 per glucose unit.The modification of cellulose by acetic anhydride increase its solubility in acetone and acetic acid. Three kind of acetyl cellulose conducting composite films respectively incorporating RNG, NanoG and ONG were prepared by mixing respective fillers with acetyl cellulose in acetic acid and subsequently removing the solventThe electrical conductivites of three kinds of films increase with the weight fraction of the fillers.When the weight fraction of three kinds of graphite particles are low,the electrical conductivites of the corresponding nanocomposites are close to each other and their percolation thresholds are ca.2wt%.the conducting sequence of three kinds of films are:NanoG＞RNG＞ONG.The SEM micrographs indicate that NanoG,RNG and ONG are homogeneously dispersed in acetyl cellulose matrix,and among them the dispersal effect of ONG is a little better than others.The thermogravimetric analysis (TGA) indicates that ONG enhances the thermal stability of acetyl cellulose slightly,and RNG shows the thermal stability of its own.