## 3 Results and Discussion
### Electrical Conductivity
On the other hand, the interaction between polymer matrix and carbon nanofillers or the state of dispersion of conductive particles may play a role in constructing co-supportive conductive network since a synergistic enhancement of \(\sigma\) was typically observed for the semi-crystalline polymer, e.g., PP ([PERSON], 2006) and epoxy-based composites ([PERSON], 2006; [PERSON], 2006; [PERSON], 2006). In a previous study ([PERSON], 2006), we have demonstrated that the intrinsic properties of polymer matrix would determine the state of dispersion of incorporated CNT, thereby affecting the \(\sigma\) of subsequent moldings. Thus, a thorough investigation on the influence of intrinsic properties of the host polymers on the properties of hybrid carbon fillers loaded systems would be helpful to elucidate this phenomenon. Figure 3: The \(\sigma\) for both the thick and middle sections of microparts which were molded from different carbon filled PS nanocomposites. The measurements were conducted with respect to the TD and FD, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample ID** & **Thick Section-TD (\(\times 10^{-5}\) S/cm)** & **Thick Section-FD (\(\times 10^{-5}\) S/cm)** \\ \hline PS/CNT 10 wt \% & 6.59 & 127 \\
10 wt \%-PS/CNT70/CB30 & 20.2 & 155 \\
10 wt \%-PS/CNT50/CB50 & 14.1 & 137 \\
10 wt \%-PS/CNT30/CB70 & 28.2 & 172 \\ PS/CB 10 wt \% & 62.9 & 201 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The average \(\sigma\) for the thick section of 10 wt % carbon fillers loaded PS microparts. Overall, the synergistic enhancement of \(\sigma\) could be achieved in PS microparts when the total carbon fillers concentration is 3 wt % wherein the CB and CNT have equivalent weight fractions, i.e., 3 wt %-PS/CNT50/CB50. Interestingly, the middle section of 3 wt %-PS/CNT30/CB70 microparts showed strong synergy in terms of the enhancement of \(\sigma\) when compared with other filler-containing counterparts. However, such effect was absent at higher filler concentrations and the \(\sigma\) of subsequent microparts tends to increase with increasing fractions of CNT and CB when the total filler concentration is 5 and 10 wt %, respectively. To investigate the effect of annealing on the \(\sigma\) of micromoldings, microparts molded from the nanocomposites with 10 wt % total carbon fillers were subject to a thermal treatment at 100 \({}^{\circ}\)C for 2 h under vacuum. Figure 4 indicated that the TD \(\sigma\) for the thick section of PS/CB 10 wt % microparts decreased remarkably after the annealing treatment. Herein, two possible mechanisms are proposed to explain the annealing-induced decrease of \(\sigma\) for only CB-containing PS microparts: (1) the very high shear rate and large thermal gradients during \(\upmu\)IM could largely induce a preferred orientation of polymer chains and the added fillers in flow direction. Thus, the rapid solidification of generated structure has little chance returning to a random orientation [44]. The mobility of polymer chains could be greatly improved during the annealing treatment, which leads to a relaxation of the orientated polymer chains to random coils [45]. In addition, since the adopted CB has a relatively higher surface area (\(\sim\)1400 m\({}^{2}\)/g) compared with the CNT, the dispersed CB particles have a greater tendency to form segregated agglomerates, which could disrupt the conductive pathways within the polymer matrix. (2) Since the mobility of the polymer chains can be greatly improved at higher temperatures, more macromolecular chains are likely to migrate to the outer surface of CB particles due to the good wettability of PS with CB. Moreover, the high surface area of CB could be a contributing factor because a certain amount of polymer chains is required to wet the surface of CB particles. A similar downward trend of \(\sigma\) with temperature was reported by [PERSON] and [PERSON] [45] in a carbon nanofiber filled PS system.