# Mechanical and Conductive Behavior of Graphite Filled Polysulfone-Based Composites
## 3 Results and Discussion
### Thermal Conductivity  
The thermal characteristics of solid materials, such as their melting and degradation temperatures, glass transition temperature, melt and crystallization enthalpy, specific and latent heats, polymorphism, and purity, can be studied qualitatively and quantitatively using DSC. Because the heat capacity of the polysulfone brand Ultrason S 2010 does not appear to be temperature-dependent in the literature, we determined it, and the findings are displayed in Figure 7. Heat capacity increases in the 180-190 \({}^{\circ}\)C temperature range as a result of the progress of the glass transition process. Additionally, the heat capacity of graphite fillers described in the literature varies depending on the brand of graphite material, so we conducted measurements for our graphite materials. The results showed that our artificial and natural graphite had practically the same dependencies, with a discrepancy of more than 0.02-0.04 J/g K. Figure 8 illustrates the type of change in the heat capacity of the graphite we used. Based on the obtained dependences, according to the rule of mixtures, we determined the values of heat capacities, which were subsequently used in calculating the thermal conductivity of materials. Comparison of experimentally determined values of heat capacity for AG and NG determined experimentally and calculated theoretically showed their good convergence. Figure 8 shows the thermal conductivity of the polysulfone filled with varying amounts of graphite filler as a function of temperature. As we can see, the thermal conductivity of the composite decreased with increase of temperature up to 150 \({}^{\circ}\)C. The decrease in the thermal conductivity values of these composites may be caused by the phonon-phonon scattering rate, which is predicted to increase with temperature. As the establishment of a thermal pathway in the polymeric matrix leads to thermal conductivity, it reduces phonon scattering mechanisms and improves phonon transport. This analysis agrees with previous research that suggested that the observed relationship between thermal conductivity and temperature was caused by the fact that with increasing temperature, the intensity of thermal vibrations of atoms and molecules in the material increases, which seriously affected  
Figure 7: Specific heat capacity versus temperature for Gr (1) and PSU (2). the movement of electrons and phonons across the composite films, reducing the in-plane thermal conductivity values. Although using graphite as a filler generally increases thermal conductivity, each type of graphite has unique properties and effects on the thermal conductivity of the composite. This can be explained by the fact that increasing the mass fraction of graphite in the composites caused the development of compact graphite networks, which offered pathways for thermal conductivity and, as a result, increased the rate of heat transfer. From  
Figure 8: Thermal conductivity temperature dependencies of PSU-based composites filled with 0 (1), 30 (2), 40 (3), 50 (4), 60 (5) and 70 (6) wt.% of NG (**a**), AG (**b**) and EG (**c**). Figure 8b, we can see that using AG at low filler content, the filler particles separated by the PSU matrix started to form an effective thermal conduction path. However, after a filler proportion exceeded the thermal permeation threshold, the thermal conductivity of the composite increased more rapidly due to the increase in forming a thermal network among the fillers. Thus, using AG as a filler, the thermal conductivity seems to increase quickly at filler mass fraction 50 wt.%. and this is because a critical concentration arrives at this area, causing the filler molecules to be in close contact with one another. However, point interactions between the particles lead to lower thermal conductivity values than natural graphite.