Broadband Dielectric Spectroscopy and Small- Angle Neutron Scattering Studies of Graphene-Chitosan Composites

Usually, graphite is used as electrode material in batteries. The electrical characteristic of the electrode is therefore fixed. It may be useful to have composites as electrodes the electrical characteristics of which can be controlled. In this regard in this research graphene – chitosan composites are studied. Also, usually the additives such as graphene are less in weight compared to the polymer matrix in which it is incorporated. In this research the weight of the graphene is greater than that of chitosan. The chitosan solution was prepared by adding 1% (w/v) of chitosan powder and 1.5% (w/v) of acetic acid to double distilled water. The solution was stirred at 60°C until a semitransparent thick chitosan solution was obtained. The solution was then cooled to room temperature. To this solution, appropriate amounts of graphene (Graphene Nano platelets—grade H25, XG sciences, USA) having an average thickness of approximately 15 nm, surface area of 60–80 m²/g and average particle diameter of 5 μm was added. The chitosan – graphene mixture with chitosan and graphene in the amounts (i) 20 mg chitosan and 100 mg graphene, (ii) 20 mg chitosan and 200 mg graphene and (iii) 20 mg chitosan and 300 mg graphene were dried in an oven at 100°C for 24 hours. The resultant powder was pelletized using a hot press in the form of discs. The dielectric and conductivity measurements were taken using a broadband dielectric spectrometer (BDS) Novo control technology (Germany) concept 80 at 1 V AC signal. The frequency range was from 0.1 Hz to 1 MHz. The measurements were taken at room temperature. It was observed that all the three samples had negative dielectric constant values in the frequency range 0.1Hz to 1MHz. The composites are therefore Meta composites. The dielectric constant at 0.1 Hz is -1.5 x 10⁹ for 20 mg chitosan - 100 mg graphene composite. It became more negative as the graphene concentration increased to 200 and 300 mg respectively. For all the three samples the value of dielectric constant decreased rapidly in the frequency range of 1MHz to 100 Hz, then remained plateau from 100 to 10 Hz and then decreased sharply from 10 to 0.1 Hz. The dielectric loss was nearly the same for all the three samples. The dielectric loss increased from ~ 20000 at 1MHz to ~ 10¹² at 0.1 Hz for the composites. This indicates that the graphene – chitosan pellets dissipate heat. For all the composites a broad shoulder peak could be observed at ~ 10⁵ Hz indicating the presence of a relaxation. The dielectric loss tangent had a minimum value at ~ 1 Hz. The conductivity of the composites is high. The conductivity for 20 mg chitosan - 100 mg graphene composite is ~ 0.2 S/cm. The conductivity increased as the graphene concentration increased. Maximum conductivity of 0.5 S/Cm could be observed. Small angle neutron scattering (SANS) is a useful technique to probe polymeric materials. The wavelength of the neutron beam used was 5.2 Å with a resolution (Δλ/λ) of about 15%. All of the data were collected in the accessible Q range of 0.017–0.35 Å−1. The measured samples were in the form of disks of thickness 1 mm. The scattered neutrons were detected using a 1-m-long He3 position-sensitive detector. SANS analysis showed that the graphene – chitosan composites had mass fractal structures with their fractal dimensions 0f ~ 2.95. The results show that (i) graphene – chitosan composite are Meta composites, have high heat dissipation, have high electrical conductivity and have graphene assembled as fractals in the composites. These materials can have potential applications as anti-static materials and as electrodes in fuel cells and batteries.