## 3 Results and Discussion
### Electrical Conductivity
Besides, the trend of decreasing ratio of (FD/TD)\({}_{\sigma}\) with an increase of filler concentration is applicable to either only CNT or hybrid carbon fillers (regardless of the weight ratio of CNT/CB) loaded PS micropparts. Similar to PS/CNT 3 wt % microparts, the values of \(\sigma\) for both the thick and middle sections of 3 wt %-PS/CNT70/CB30 microparts cannot be determined using the Keithley 6514 electrometer due to insufficient conductive pathways; however, corresponding values could be detected for the other combinations of hybrid carbon fillers loaded PS systems (at the same total filler concentration, i.e., 3 wt %) where CB occupies a relatively higher weight fraction, suggesting that high structure CB is more effective than CNT in enhancing the \(\sigma\) of subsequent moldings. In this scenario, the polymer-filler interfacial interaction might be an influencing factor. According to [PERSON] et al. [34], a lower interfacial tension between polymer matrix and fillers promotes a better wettability of polymer chains with the added fillers, thereby leading to an improved dispersion of added fillers in the host matrix. As a result, the improved dispersion of conductive fillers would, in turn, increase the \(p_{\mathrm{c}}\) of CPCs [35]. The values of surface tension for PS, CB and CNT, and the interfacial tension between PS and different carbon fillers at 260 \({}^{\circ}\)C are listed in Table 2. The interfacial tension between polymer/filler pair was calculated using the [PERSON]'s harmonic mean average equation [36]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**(FD/TD)\({}_{\sigma}\)**} \\ \cline{2-3}  & **Thick Section** & **Middle Section** \\ \hline PS/CB 3 wt \% & 12.6 & N/A \\ PS/CB 5 wt \% & 5.6 & \(4.3\times 10^{5}\) \\ PS/CB 10 wt \% & 3.2 & 527 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The ratio of (FD/TD)\({}_{\sigma}\) for both the thick and middle sections of CB filled PS micropparts. \[\gamma_{12}=\gamma_{1}+\gamma_{2}-4\Bigg{(}\frac{\gamma_{1}^{d}\gamma_{2}^{d}}{ \gamma_{1}^{d}+\gamma_{2}^{d}}-\frac{\gamma_{1}^{p}\gamma_{2}^{p}}{\gamma_{1}^{p} +\gamma_{2}^{p}}\Bigg{)} \tag{2}\]  
where \(\gamma_{12}\) is the interfacial tension between component 1 and component 2; \(\gamma_{i}\) is the surface tension of component \(i\), which equals to \(\gamma_{i}^{d}\) and \(\gamma_{i}^{p}\). In addition, \(\gamma_{i}^{d}\) and \(\gamma_{i}^{p}\) are the dispersion part and the polar part of surface tension of the \(i^{\text{th}}\) component, respectively. Table 2 reveals that the interfacial tension of PS/CB pair is lower than that of the PS/CNT pair, indicating that polymer chains could easily wet outer surface of CB particles, which leads to an improved dispersion of CB in PS. The improved dispersion of CB particles explains the fact that although a direct contact of conductive fillers is unlikely at a lower total filler concentration (i.e., 3 wt %) [31], it allows free passage of electrons though the 'tunneling' or 'hopping' mechanism in the host matrix [39]. It is thus not surprising that the values of \(\sigma\) for both the thick and middle sections of microparts molded from 3 wt %-PS/CNT70/CB30 and PS/CNT 3 wt % nanocomposites are not detectable due to a lack of sufficient conductive pathways, regardless of the measurement directions. In addition, a typical synergistic effect of hybrid carbon fillers on the \(\sigma\) for both the thick and middle sections of 3 wt %-PS/CNT50/CB50 microparts is discernible since the values of \(\sigma\) for the 3 wt %-PS/CNT50/CB50 microparts are generally superior to those of PS/CNT 3 wt % or PS/CB 3 wt % counterparts. In this scenario, it could be deduced that the co-existence of CB particles with CNT facilitates the formation of conductive pathways within the host polymer.