## 3 Results and Discussion
### Electrical Conductivity
When the total filler concentration is increased to 5 wt %, it is expected that enough conductive pathways can be constructed by CNT in PS because the values of \(\sigma\) for PS/CNT 5 wt % microparts is always higher than that of PS/CB 5 wt % counterparts, as shown in Figure 3. However, no synergistic effect of CNT and CB on the \(\sigma\) of subsequent microparts is detected since the \(\sigma\) for the thick section of hybrid filler-containing microparts is normally within the range of PS/CB 5 wt % (lower bound) and PS/CNT 5 wt % (upper bound) counterparts whereas the \(\sigma\) for the middle section is invariably lower than that of PS/CB 5 wt % or PS/CNT 5 wt % counterparts, regardless of measurement directions. Moreover, the \(\sigma\) for either section of PS/CNT/CB microparts increases with an incremental loading fraction of CNT, revealing that the presence of CNT is advantageous to the formation of conductive pathways. For example, the FD \(\sigma\) and TD \(\sigma\) for the thick section increase from 5.81 \(\times\) 10\({}^{-9}\) (5 wt %-PS/CNT30/CB70) to 1.16 \(\times\) 10\({}^{-6}\) S/cm (5 wt %-PS/CNT70/CB30) and from 3.66 \(\times\) 10\({}^{-10}\) (5 wt %-PS/CNT30/CB70) to 4.05 \(\times\) 10\({}^{-8}\) S/cm (5 wt %-PS/CNT70/CB30), respectively. A monotonic increase of FD \(\sigma\) for the thick section is attributed to the preferential alignment of CNT along FD since more conductive pathways can be constructed with an increasing weight fraction of nanotubes. Also, a concurrent increase of TD \(\sigma\) for the thick section is ascribed to the spatial dispersion of nanotubes across TD, thereby facilitating the formation of conductive pathways across TD, albeit the favored orientation of CNT along the flow direction. Furthermore, more filler orientation in the middle section is expected with an increase of shear rates, arising from the sharp decrease of the mold cavity thickness, from 0.85 (thick section) to 0.50 mm (middle section), along the predominant melt flow direction. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\gamma\) & \(\mathbf{fi}_{i}^{d}\) & \(\mathbf{fi}_{i}^{p}\) & \(\mathbf{fi}_{12}\) \\ \cline{2-5}  & **mN/m** & **mN/m** & **mN/m** & **mN/m** \\ \hline PS 1  & 23.44 & 23.272 & 0.168 & \\ \hline CB 2  & 21.77 & 19.59 & 2.18 & \\ \hline CNT 3  & 27.8 & 17.6 & 10.2 & \\ \hline PS/CB & & & & 3.29 \\ \hline PS/CNT & & & & 11.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface tension of pure components and the interfacial tension of PS-filler at 260 \({}^{\circ}\)C.  
Consequently, the FD \(\sigma\) for the middle section is always higher than that of its thick section counterpart whereas the TD \(\sigma\) for the thick section is higher than that of the middle section. Similarly, such trend also applies to microparts which have a total filler concentration of 10 wt %, in which case sufficient conductive pathways can be formed. In addition, Table 3 indicated that the presence of CB is generally beneficial to the enhancement of \(\sigma\) for subsequent microparts, which can be attributed to the uniform distribution of CB within the host matrix. Based on the range of total carbon filler concentrations studied, no obvious synergistic effect of hybrid loading of CNT and CB on the enhancement of \(\sigma\) for PS microparts was detected. According to a review by [PERSON] et al. (2014), hybrid carbon fillers loading of CNT and CB does not always guarantee a synergy in enhancing the \(\sigma\) of subsequent polymer composites. On the one hand, the ratio of carbon nanofillers in the hybrid mixture is crucial to determining the properties of polymer composites, such as the mechanical properties, thermal and electrical conductivity ([PERSON], 1999; [PERSON], 2000).