## 3 Results
### Fourier Transform Infrared Spectroscopy (FTIR)  
Figure 8 shows the FTIR spectra demonstrating that Duran Plus, Imprelon S pd, Essix Plus, Tru-Tain DX, and Essix ACE were very similar in their structural composition, and they had the typical infrared bands of PETG, indicating that these materials are formed mainly from PETG polymer. The absorption bands in the range of 2950-2800 cm\({}^{-1}\) (at around 2926 and 2855 cm\({}^{-1}\)) were assigned to the asymmetric and symmetric stretching vibrations of C-H bonds (belonging to the methylene groups of the initial ethylene glycol/ethylene oxide monomers and to the methylene groups of the cyclohexane rings of the polymer main chain) and the bands located within 1715-1696 cm\({}^{-1}\) were associated with the stretching vibrations of C=O bonds of the ester groups that is related to the polymer backbone. At the same time, in the fingerprint region, the absorption peaks around 1244 cm\({}^{-1}\) were ascribed to the aromatic C-H in-plane bending vibrations, while those located at around 1097-956 cm\({}^{-1}\) were attributed to the cyclohexane ring vibrations combined with the aromatic C-H out-of-plane bending vibrations. The FTIR analysis of Zendura showed a spectrum that resembles semi-rigid polyurethane (PU). It exhibits a relatively broad peak at about 3306 cm\({}^{-1}\) attributed to the N-H stretching vibrations (with NH involved in hydrogen bonding) and two other absorption bands around 2930 and 2857 cm\({}^{-1}\) assigned to the asymmetric and symmetric C-H stretching vibrations of the methylene groups of the polyurethane main chain, respectively. The bands located around 1696-1697 cm\({}^{-1}\) and around 1595 and 1514 cm\({}^{-1}\), on the other hand, were assigned to the C=O stretching (with CO also implied in hydrogen bonding) and to the C=C-C aromatic ring stretching modes of vibrations, respectively. At the same time, the bands around 1200 cm\({}^{-1}\) were attributed to the aromatic C-H in-plane bending vibrations and those at 1067 cm\({}^{-1}\) were attributed to the aromatic C-H out-of-plane bending vibrations superimposed on the C-O-C stretching most likely bought by the presence of poly(ethylene glycol) segments in the structure of the polyurethane-based material.  
Figure 8: Superimposition of FTIR spectra of all investigated as received materials (Group 1).  
For all the retainer materials, the FTIR spectra of Groups 1, 2, and 3 were almost identical except for the Zendura material (Figure 9). It appears that the aging methods used had little effect on the surface molecular composition for all materials except Zendura. Zendura displayed a change in the intensity of absorbance bands, even though their positions in the spectrum practically have not significantly changed, indicating that the material remained the same throughout the different aging methods used. However, the decrease in the intensity of the absorbance bands following thermocycling denotes the beginning of chemical degradation. The results based on FTIR spectroscopy showed that Zendura had the least stable surface structural composition compared to all tested materials (Figure 9).  
Figure 9: Superimposition of FTIR spectra of Group 1: before thermoforming (green), Group 2: after thermoforming (blue), and Group 3: after thermoforming and subsequent thermocycling (red) of each investigated retainer material.