*3.8. XRD*

The fiber crystal structure was determined based on XRD studies (Figure 8). On all diffractograms, peaks characteristic of polyacrylonitrile were visible. Particularly clear was the peak at 2θ~18◦, which was characteristic of the hexagonal structure of PAN. In the PAN/PANI blended fiber spectrum there were no peaks characteristic of PANI, most probably due to its small amount in the fibers. However, they were clearly visible in the PAN/PANI in situ fibers, and corresponded to the pseudo-orthorhombic phase of PANI [28].

**Figure 8.** Diffractogram of the fibers.

The degree of fiber crystallinity (Xc) was calculated using the Hinrichen method (3):

$$\chi\_{\mathbf{c}} = \frac{\mathbf{I\_c}}{\mathbf{I\_c + I\_a}},\tag{3}$$

where Ic is the integral under the peaks corresponding to the crystalline phase of the polymer, and Ia is the integral under the peaks corresponding to the amorphous phase of the polymer.

Crystal and amorphic peaks were determined based on the optimization carried out in the WAXFIT program. Curve fitting was performed using the Rosenbrock method. The degree of crystallinity for the PAN and PAN/PANI blended fibers was the same at 0.53, while for the PAN/PANI in situ fibers it was significantly lower at 0.32 (Table 4). The significantly lower crystallinity degree of the PAN/PANI in situ fibers was caused by a lack of stretching, and was correlated with their mechanical strength.



The higher degree of crystallinity, in the case of fibers, was associated primarily with a larger ordering of polymer chains along the fiber axis [29]. This parameter depended on the type and degree of interactions between macromolecules.

The polyacrylonitrile macromolecules contained positively charged CN groups (Figure 9a), and caused the chains to twist into a helix, from which the crystallites are made [30]. In the PANI, due to the presence of amine groups, hydrogen bonds and dipole-dipole interactions were formed between the macromolecules (Figure 9b). H-bonding interactions between adjacent chains caused the stiffness of the PANI chain [31], and an easier orientation of the chains in the material containing only polyaniline.

**Figure 9.** Chemical structure of: (**a**) PAN; and (**b**) PANI.

However, in the material containing both of these polymers, the crystallization process was difficult due to the interactions between nitrile (PAN) and amine (PANI) groups (Figure 10). In addition, numerous cracks and voids appeared in the structure of the fiber (Figure 6c). Analysis of the literature data [32] indicated that the nitrile group, in some cases, may have interfered with or acted as a barrier when ordering polymer chains. This can take place in the PAN/PANI in situ fibers, where the strong interaction between the polymer chains blocks their movement, thereby limiting the crystallization process.

The average size of PAN crystallites was determined based on the half-width of the peak 2θ~17◦, characteristic of the plane (001) in a hexagonal structured PAN. The size of the PANI crystallites was determined based on the peaks 2θ~23◦ and 26◦, characteristic of the planes (005) and (111). The calculations were made using the Debye–Scherrer Equation (4):

$$\mathcal{L}\_{\text{hlk}} = \frac{\mathbf{K} \times \lambda}{\boldsymbol{\beta} \times \cos \theta\_{\text{max}}},\tag{4}$$

where L(hlk) is the average size of crystallites, K is the Scherrer constants (0.89), θmax is the angle for the maximum peak (rad), λ is the length of the radiation beam (Å), and β is the half-width of the peak (rad).

The inter-chain separation length (R) was determined based on the analysis of the most intense crystalline peak from the following Equation (5) [31]:

$$R = \frac{5\lambda}{8\sin\theta\_{\text{max}}}.\tag{5}$$

The distance between planes (the d-spacing) was determined from the Bragg Equation (6):

$$\mathbf{n}\lambda = 2\mathbf{d}\sin\theta,\tag{6}$$

where n is the deflection integer.

> Lattice strain ε was determined from Formula (7):

$$
\beta = 4\varepsilon \tan \theta \tag{7}
$$

Data on the crystal structure of the obtained fibers was collected in Table 5. The crystal phase of the polyacrylonitrile in all the fibers had a similar structure. Crystallite size, d-spacing, inter-chain separation, and lattice strain all had very similar values. The size of the polyaniline crystallites was larger and amounted to 15.8 (111) and 17.9 (005) nm. Differences in the structure of both chemical and crystalline polyacrylonitrile and polyaniline present in fibers were reflected in their macroscopic structure and mechanical properties.

**Figure 10.** Diagram of interaction between the PAN and PANI chains.

**Table 5.** Crystallinity properties of fibers.


Inter-chain separation length is a parameter affecting the electrical conductivity of fibers. This value represents the distance between the electron jump between the chains. The smaller it is, the higher the probability of charge hopping. The distance of the order of ~4 Å corresponded to the literature concerning polyaniline, and allowed hopping to be charged between the chains [24,33].
