## 3 Results and Discussion
### Thermal Behavior
#### 3.3.1 Thermal-Gravimetric Analyses  
Thermal-gravimetric analyses (TGA) were performed on the starting filament and the printed samples in order to investigate their thermal stability in a temperature range from 25 \({}^{\circ}\)C to 800 \({}^{\circ}\)C. However, no differences were observed between TGA curves of the filament and the specimens when they were independently tested in the air and argon atmospheres. This suggests that the FDM process did not influence the material thermal and thermo-oxidative degradation processes.  
Figure 3 compares the TGA and derivative-TGA curves for Ultem 9085 under oxidizing (Figure 3a) and inert (Figure 3b) atmospheres.  
Both the curves collected in air and argon atmospheres show an initial degradation temperature (conventionally, it corresponds to the temperature at which there is a weight loss equals to 5%) of about 447.5 \({}^{\circ}\)C.  
However, a significant difference in terms of solid residue could be observed--the thermo-oxidative degradation of Ultem 9085 was almost complete in air, leaving a residue of about 1%; on the contrary, a residue of about 44% with respect to the initial weight was observed after heating the material in inert atmosphere up to 800 \({}^{\circ}\)C. These results are comparable to those obtained by [PERSON] et al. [31] who investigated the thermal and thermo-oxidative stability of some polyetherimide; these authors reported a lower residue quantity when heating PEI in air (14-22 wt% at maximum temperature of 700 \({}^{\circ}\)C) with respect to inert atmosphere (43-54 wt%).  
XRD analysis was performed on the solid residue obtained after degradation in inert atmosphere to investigate its composition. The XRD spectrum (Figure 4) shows the presence of a broad hump in 2 theta range from 15\({}^{\circ}\) to 30\({}^{\circ}\) only; additional crystalline phases were not observed. This implies that the residual fraction is mainly constituted by amorphous carbonaceous species.  
The degradation of the material under investigation always occurred in two steps; the D-TGA curves show two peaks which correspond to the maximum rates of weight loss detected at 484.2 \({}^{\circ}\)C and 582.5 \({}^{\circ}\)C in air and at 495.8 \({}^{\circ}\)C and 592.5 \({}^{\circ}\)C in argon, respectively. The two-step degradation mechanism can be explained considering that Ultem 9085 is a mixture of PEI and polycarbonate (PC) copolymer blend; the latter is added in order to improve the material flow [13].  
According to [PERSON] et al. [32] the degradation of polycarbonate in inert atmosphere shows only one degradation step; the maximum degradation temperature is observed at 504.8 \({}^{\circ}\)C. On the basis of this outcome, the first degradation step of Ultem 9085 can be attributed to PC, and the second one to the degradation of PEI. A comparison between the intensity of the peak at higher temperature in  
Figure 4: XRD pattern of Ultem 9085 solid residue obtained after thermal degradation.  
Figure 3: TGA (continuous lines) and D-TGA (dotted lines) curves for Ultem 9085 under (**a**) air and (**b**) argon atmosphere.  
D-TGA curves is evidence of a different degradation mechanism of PEI component in air as compared to argon atmosphere.