## 3 Results and Discussion
### Morphology  
The cross-section morphology of the thick section of PS/CB 5 wt % microparts is shown in Figure 5. Figure 5a,b showed that the CB particles have a relatively uniform distribution in the core layer of the thick section, in a form of small aggregates or a chain-like structure, suggesting that the high structure CB particles could form a random conductive network via self-assembly even under very high shearing conditions, which is beneficial to the enhancement of \(\sigma\). A similar phenomenon was observed by [PERSON] and co-workers [50] in CIM PP/HDPE/CB macroparts. However, despite the uniform distribution of CB aggregates in the shear layer, there are some regions devoid of CB particles and the shear-induced depletion of CB particles could be the contributing factor. For example, [PERSON] [51] reported that the loss of \(\sigma\) in CPCs could be attributed to shear-induced migration of conductive fillers during the mold filling process in injection molding. In addition, [PERSON] et al. [52] reported that the migration of conductive fillers became more severe with an increase of shearing conditions. [PERSON] et al. [29] pointed out that shear rates as high as \(10^{6}\)/s are not rare in \(\upmu\)IM, and the shear layer exhibits a higher shear rate relative to that of the core layer [25]. Therefore, the shear-induced migration of CB particles would become more significant in the shear layer, as shown in Figure 5d. The cross-section morphology of the thick section of PS/CNT 5 wt % microparts is shown in Figure 6. Figure 6 showed that CNT has a relatively uniform distribution across the TD. Moreover, CNT agglomerates seem to be absent from the thick section across the TD and no obvious shear-induced depletion effect on nanotubes-containing counterparts is observed in the shear layer. This suggests that there exists a good wettability between PS macromolecular chains and the outer surface of CNT, which leads to their good dispersion in the host matrix [25; 53]. In addition, the particle size could play a role when it comes to the shear-induced migration phenomenon. For example, unlike CB particles, the adopted CNT has a very high aspect ratio (>1000). Thus, the shear-induced depletion effect might be insignificant in terms of the particle size of nanotubes and the rapid mold filling process in \(\upmu\)IM. Figure 4: The TD \(\sigma\) for the thick section of different combinations of CB/CNT fillers loaded PS microparts. Results were collected from samples before and after the annealing treatment at 100 \({}^{\circ}\)C for 2 h.  
The cross-section microstructure for the thick section of 5 wt %-PS/CNT50/CB50 microparts, prior to annealing treatment, is displayed in Figure 7, indicating that both the CNT and CB have a relatively uniform distribution across the TD. The existence of CNT agglomerates could be detected in the core layer of microparts, which have been labeled in black contours, as shown in Figure 7a. However, the discretely dispersed CB and CNT would be unfavorable for the effective construction of conductive pathways within subsequent microparts. For example, [PERSON] et al. [54] proposed that the co-existence of two independent phases, namely the particle phase (i.e., CB aggregates) and the bridge phase (i.e., individually dispersed CNT or CNT agglomerates) is unfavorable for the enhancement of \(\sigma\). Consequently, the \(\sigma\) for 5 wt %-PS/CNT50/CB50 microparts is somewhat lower than that of only CNT-containing counterparts, as displayed in Figure 3.  
Figure 5: The morphology (TD) taken from the **(a,b)** core and (**c,d**) shear layers of the thick section of PS/CB 5 wt % microparts. Figure 6: The morphology (TD) taken from the **(a,b)** core and (**c,d**) shear layers of the thick section of PS/CNT 5 wt % microparts. The cross-section microstructure for the thick section of 5 wt %-PS/CNT50/CB50 microparts after annealing treatment is given in Figure 8. Similar to Figure 7, both the CB and CNT have a relatively uniform distribution in PS.