*3.9. Thermal Properties*

Thermogravimetric analysis of the pure PAN fiber and PANI-containing fibers showed some similarity between the samples (Figure 11). Three mass losses were observed for all materials. The first between 25 and 100 ◦C (Table 6—T1), the second between 200 and 300 ◦C (Table 6—T2), and the third at around 420 ◦C (Table 6—T3).

**Figure 11.** TG (**a**) and DTG (**b**) curves of polymer fibers.

**Table 6.** Data obtained from the thermogravimetric curve, T1,2,3—temperature of subsequent losses in mass.


The first mass loss was caused by the loss of water bound in the material, which was due to polyaniline adsorbing large amounts of water. Water can be associated with polyaniline macromolecules in two ways: strong and weak. Poorly bound water molecules are connected to the PANI chain with one hydrogen bond (Figure 12a). In contrast, strongly bound water is connected by two hydrogen bonds (Figure 12b), most often between two adjacent PANI chains [25].

**Figure 12.** Polyaniline with weak (**a**) and strong (**b**) bound water [30].

Poorly adsorbed water can be removed while drying with nitrogen at room temperature, while the strongly bound one is mostly removed at a temperature in the range of 60–140 ◦C. Removal of this water is reversible and it can be re-absorbed. However, further removal of water from the polymer at 150–220 ◦C is an irreversible process and leads to polymer degradation [34,35].

The first derivatives of the TG curve showed that the most moisture was adsorbed in the PAN/PANI in situ fiber. Less water was present in the pure PAN fiber, and the smallest amount was found in the PAN/PANI blended fiber. As pure polyaniline can bind up to 40% by weight of water [36], it is possible to determine its presence in fibers based on the analysis of the water-related peak. An intense peak, much larger than in the reference fiber, was associated with the presence of water in the PAN/PANI in situ fiber. This demonstrated the presence of polyaniline in fibers, which was also confirmed by the FTIR results (Table 5) and XRD (Table 6). A very small amount of adsorbed water in the PAN/PANI blended fiber resulted from the fact that the polyaniline, after the synthesis process, was dried before its dissolution in dimethylformamide.

Further fiber degradation was associated primarily with the phenomenon of polyacrylonitrile cyclization occurring under nitrogen [37]. Analysis of the first derivative of TG curve (i.e., DTG curve) showed that in the case of the pure PAN and PAN/PANI blended samples, the degradation can be described as one-step and starts at around 280 ◦C. The PAN/PANI blended fiber was characterized by a very rapid decomposition in this temperature range, which persisted up to 350 ◦C.

The PAN/PANI in situ fiber was decomposed in two stages. It had a wide weight loss of about 10% in the temperature range of 200–290 ◦C. The initial weight loss in this range was associated with the removal of the rest of the strongly bound water. On the other hand, according to the obtained results and literature data, the weight loss was due to the presence of low-molecular polyaniline fractions in the material, which underwent gradual decomposition [38]. This process is characteristic for PANI degradation, which is why it indicated its presence in the fiber. Another loss beginning at 290 ◦C and ending at 380 ◦C was associated with PAN cyclization. Comparing the results obtained in all the fibers showed that the in situ synthesis of polyaniline was a difficult cyclization. This was closely correlated with the FTIR conclusions (Table 5), which indicated electrostatic interactions between the macromolecules of polyacrylonitrile and polyaniline in the PAN/PANI in situ fibers.

The third loss of fiber mass corresponded to the carbonation process of the PAN, which consisted of dehydrogenation and denitrogenation processes [39]. As a result of these processes, a cyclic carbon structure was created. However, when combining the maximum transformation temperature, it can be seen that the lowest value of this parameter was for the PAN/PANI blended fiber. This clearly indicated that the addition of polyaniline introduced by the blended method facilitated the carbonization process.

The total weight loss (Δmtot) after measurement was about 15% higher for the pure PAN fiber than for the modified fibers (Table 6). Considering the high thermal resistance of PANI, this proved the effectiveness of both methods of introducing a polymer additive to the fiber.

To verify the phase composition of the fibers, DSC measurements were conducted (Figure 13). The obtained results indicated that the pure PAN fiber showed a glass transition at a temperature of about 100 ◦C both for heating and cooling runs, which was characteristic for polyacrylonitrile [40,41].

**Figure 13.** Differential scanning calorimetry (DSC) curve of polymer fibers: (**a**) heating curves; (**b**) cooling curves.

In the case of the PAN/PANI in situ fibers' heating curve, a wide endothermic peak was seen with a maximum at 50 ◦C, associated with the removal of poorly bound water. An endothermic peak was seen at 102 ◦C, associated with the desorption of strongly bound water to the PANI [38]. Subsequently, the glass transition in the PAN was observed at about 110 ◦C, but its exact value was difficult to accurately determine because of the overlap of both processes. At 140 ◦C, one more endothermic peak was visible, which may be the result of the presence of low molecular weight fractions in the material remaining after the PANI polymerization process (Figure 13a). In the cooling curve, two exothermic peaks were observed, which could be connected with crystallization of previous melted low molecular fractions during heating. Also, a glass transition of PAN at about 100 ◦C was present (Figure 13b). The obtained results indicated a large heterogeneity of the system, which was also confirmed by previous SEM, mechanical strength, and TG measurements.

In the case of the PAN/PANI blended fibers, only the PAN glass transition (100 ◦C) was visible in the heating curve, with an additional transition at around 150 ◦C. Because this temperature was not identical to the glass transition of polyaniline, and was not observed in the front samples (Figure 13a), the authors believe that it was the result of the presence of small amounts of low molecular weight PANI fractions in the material structure, which was well mixed with pure polyacrylonitrile; therefore, the obtained fibers had good mechanical parameters (Table 3) and a smooth structure (Figure 6b). This fraction was homogenized with the rest of the material because during the cooling run, no crystallization peaks were present. Only Tg of PAN was observed (Figure 13b). This phenomenon also explains the problem in observing peaks from PANI to XRD, and the presence of only partial peaks in the FTIR.
