## 4 Discussion
The unoccupied space in between polymer electronic clouds is considered as free volume. [PERSON] et al. evaluated the effect of the thermoforming process, preservation in artificial saliva, and their combination on the mechanical properties of three thermoplastic polymers [19]. Considering the clinical aspects, the absorption of liquids could also affect the size of the aligner by modifying the adaptation to the patient's arch. This aspect combined with the change in the mechanical properties of thermoplastic materials could lead to a potential loss of effectiveness [19; 20]. Ideally, an aligner should apply a gentle, consistent force over time. To be effective, the ideal material should therefore be stiff enough with a high yield strength. None of the four materials tested in the [PERSON] et al. demonstrated all these characteristics. The thickness of the aligner material has a great influence on the force it develops; however, the tested materials are not all of the same thickness and therefore not the same [21]. Through the bending test, it was shown that the single-layer materials (F22 Aligner and Duran) were more than four times stiffer than their double-layer counterparts (Erkoloc Pro and Durasoft). This feature should be calculated as some aligners may not be able to exert enough force to drive the teeth into their programmed positions. Stress decay tests showed in all samples that this was very rapid during the first 8 h of application, subsequently tending to decrease to a plateau. This indicates that the performance of an aligner should be evaluated not only upon insertion, but also after 15-20 h. There were, in fact, notable differences in how the materials performed over the 24-h test period [21; 24; 39; 40; 41]. [PERSON] et al. 's research work has some limitations. It is an in vitro analysis that evaluates the behavior of different materials before the thermoforming phase where the samples are not the same. Furthermore, these characteristics can change during the treatment period due to the physical, chemical, and masticatory stress that can occur in the oral cavity [21; 23]. [PERSON] et al. demonstrated rapid stress decay in all materials during the first hours of application before reaching a plateau phase (Figure 5). The limitations are congruent with [PERSON] et al. 's [22; 41; 42; 43; 44]. Other properties such as roughness and surface energy have been described in the [PERSON] et al. study using PETG-based materials. The materials showed low surface roughness values as these interfere with the wetting phenomena due to saliva. The roughness results after thermoforming were much higher [26]. The total bond work provides the retention capacity of the intra-oral biofilm. Thus, differences in this parameter can be hypothesized to have a clinical impact for pellicle formation. The tested materials showed the following differences: CAM showed the highest values, followed by COP and the DUR and ERK group [26]. To conclude, in the absence of statistically significant differences in the roughness parameters of the received and thermoformed materials, the differences recorded in the surface energy parameters can control biofilm formation and plaque development on the aligners [26; 27; 28; 29; 30; 31; 32; 45; 46]. [PERSON] et al. dealt with the study of color, physicochemical properties, and water absorption of PU- and PETG-based materials [28]. Investigations on water absorption provide indications of the interaction with the external aqueous environment and of their ability to maintain their characteristics during use, preventing mechanical degradation phenomena. The penetration of water molecules into the oral environment can lead to  
Figure 5: Stress relaxation. modifications of the aligner from a physio-chemical point of view, causing phenomena of mechanical degradation and swelling. For these reasons, an ideal orthodontic clear aligner must have low water absorption properties, the worst performance being given by the PU-based aligner [28; 29; 30]. [PERSON] et al.