*3.6. SEM Images*

The microstructure of the fibers was observed using a scanning electron microscope (Figure 6). Cross-sections of individual fibers were also made using a focus ion beam (FIB) microscope (Figure 7). The surface morphology and fiber cross-sections were different for every fiber. The standard PAN fibers had a visible fibrous structure with characteristic furrows on their surface (Figure 6a). The cross-sections of these fibers had a characteristic bean-like shape (Figure 6b). The surface of the PAN/PANI blended fibers treated with hydrochloric acid was very smooth, and their cross-section was oval (Figures 6c,d and 7b). Deep furrows were clearly visible on the surface of the PAN/PANI in situ fibers. The cross-sections of these fibers showed pores and cracks (Figure 7c). The cross-section shape was also bean-like, but the surface structure was clearly different from the inside of the fibers (Figure 6e,f). The structure of the PAN/PANI in situ fibers was discontinuous and contained many defects. It was clearly more heterogeneous than in the PAN/PANI blended fibers (Figure 6e,f).

**Figure 6.** SEM images of fibers: (**<sup>a</sup>**,**b**) PAN; (**<sup>c</sup>**,**d**) PAN/PANI blended; and (**<sup>e</sup>**,**f**) PAN/PANI in situ.

**Figure 7.** SEM images of the cross-section of: (**a**) PAN; (**b**) PAN/PANI blended; and (**c**) PAN/PANI in situ.

The reason for such differences in the microstructure of fibers can be, above all, the difference in the size of the macromolecules that have been in the solution (see Section 3.1), and also the amount of tension during the spinning process (see Section 2.4). As previously mentioned, during the synthesis of the PANI in the DMF and PAN solution, transparent crystals were formed (Figure 1), which were then filtered off. Thus, only low molecular weight molecules were left in the solution, which hindered the fiber formation process. Moreover, according to FTIR results (Figure 5) (Table 2), during the synthesis of PANI in the DMF and PAN solution, new interactions appeared between the functional groups of both polymers. This could weaken the interaction between the polyacrylonitrile chains themselves.
