## 3 Results and Discussion
### FTIR Spectroscopy  
In order to analyze the influence of the fused deposition modeling process on the PLA material due to various thermal processes with a high temperature gradient (melting and extrusion followed by cooling and solidification), FTIR spectra of both the neat filament and the 100% infill FDM samples were recorded (Figure 8 and Table 3). Spectra were acquired from the lower layer. The neat filament shows characteristic peaks of PLA. The vibrational bands of the PLA filament and their assignations are given in Table 2. Strong carbonyl (-C=O) stretches are observed at a wave number of 1746 cm\({}^{-1}\). Additionally, there are symmetric and asymmetric vibrations of the alkyl groups -CH and -CH\({}_{2}\) between 2800 and  
Figure 6: SEM micrograph of 3D print made with 100% infill; (**a**) side view under a magnification of 50\(\times\), (**b**) side view under a magnification of 200\(\times\), and (**c**) cross-section view under a magnification of 50\(\times\).  
Figure 7: SEM micrograph of 3D print made with 100% infill (cross section view under magnification of 200\(\times\)).  
3000 cm\({}^{-1}\). Furthermore, there is an asymmetric tension band of the aliphatic C-O-C group at 1182 cm\({}^{-1}\) and -C-O-C- asymmetric stretch at \(\sim\)1078 cm\({}^{-1}\).  
The FTIR spectra of the fused deposition modeling (FDM)-printed samples display identical vibrational bands, indicating that there are no substantial structural changes in the surface resulting from the cooling and solidification processes. This consistency in the spectra implies that the molecular composition and arrangement of the printed material remain largely unchanged during the transition from the melted to the solid state. The absence of significant alterations in the surface structure underscores the stability of the FDM printing process in preserving the material's chemical integrity and overall composition.  
Figure 8: FTIR spectra of PLA FDM models made with 100% infill and neat PLA filament.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Peak Position/cm\({}^{-1}\)** & **Assignation According to Refs. [26; 27; 31; 32; 33]** \\ \hline
2922 & C-H stretching \\
2850 & C-H stretching \\
1746 & C-O stretching \\
1454 & -CH\({}_{3}\) asymmetric bending \\
1384 & -C–H symmetric bending \\
1359 & -C–H symmetric bending \\
1267 & -C–H bending \\
1182 & -C-O-C stretching \\
1078 & CH\({}_{3}\) deformation \\
1078 & C-O-C stretching \\
1037 & C-CH\({}_{3}\) stretching \\
868 & C-COO vibration \\
754 & C=O stretching \\ \hline \hline \end{tabular}
\end{table}
Table 3: Vibrational bands of PLA filament and their assignations.  
PLA is a semi-crystalline polymer; thus, it is important to monitor the change in crystallinity during a processing method utilizing heat, since the crystallization speed of PLA is slower than the extruding process [34]. [PERSON] et al. [34] showed the anisotropy of 3D-printed objects due to the occurrence of orientation phenomena during the filament deposition and the formation both of ordered and disordered crystalline forms. The determination of polymer crystallinity is of major concern since it influences many polymer properties (hardness, modulus, tensile strength, stiffness, crease, and melting points) [35]. The crystallinity index of PLA could be calculated as the ratio between the two band areas A754/A868 [36]. The ATR-FTIR results indicate that there are no significant changes in the surface chemistry of PLA filaments during FDM printing, except for molecular reorientations and a decrease in crystallinity, which can be attributed to a combination of thermal, mechanical, and structural changes that occur during the printing process (Table 4), which is in agreement with previous research [36].