## 4 Results
### Cycling Stability
#### Chemical Characterization  
In order to simplify the analysis for chemical degradation, two peaks of each material were considered as characteristic peaks. The first peak considered was contained in the region known as the fingerprint region (1500 to 500 cm\({}^{-1}\)), and the second peak was contained in the high-wavenumber region (4000 to 1500 cm\({}^{-1}\)).  
Figure 1: Phase change temperature: (**a**) melting; (**b**) solidification.  
The chemical characterization showed that the results obtained could be classified into three different groups and, for a better comprehension, two examples of each are shown in Figure 3. The first group contained the PCMs which presented no chemical degradation over cycles since the FT-IR spectrums were practically equal and the two considered peaks remained identical. Among the sixteen PCM evaluated, it was found that adipic acid (see Figure 3a) and benzanilide (see Figure 3b) showed this behaviour. The second group contained the PCMs which experienced a certain level of degradation since differences between initial and cycled FT-IR spectra existed. It was observed that HDPE (see Figure 3c), dimethyl terephthalate (see Figure 3d), hydroquinone, potassium  
Figure 2: Phase change enthalpy: (**a**) melting; (**b**) solidification.  
thiocyanate, benzamide, phtalic anhydride, and sebacic acid, presented slight variations in their spectrum. The high-wavenumber region-peak of dimethyl terephthalate (2853 cm\({}^{-1}\)), hydroquinone (3015 cm\({}^{-1}\)), and potassium thiocyanate (3432 cm\({}^{-1}\)) did not appear after thermal cycling. Although benzamide basically did not present differences in its spectrum after 10 thermal cycles, several chemical degradation processes took place after 100 cycles. In this case, peaks from both regions were moved to higher transmittance values (768 and 3363 cm\({}^{-1}\)). Phtalic anhydride peaks remained at the same wavenumber after cycling (903 and 1758 cm\({}^{-1}\)), although transmittance values shifted during thermal cycling. Sebacic acid showed a similar behaviour and both considered peaks (923 and 1687 cm\({}^{-1}\)) did not change, but the spectrum shifted to lower transmittance values. The HDPE spectrum presented two sharp peaks in the high wavenumber region, corresponding to CH\({}_{2}\) asymmetric stretching (approx. 2919 and 2851 cm\({}^{-1}\)). The characteristic HDPE doublet observed in the fingerprint region (1473 and 1463 cm\({}^{-1}\), bending deformation) appeared slightly displaced after thermal cycling. In the HDPE spectrum, it is also important to notice that after 100 cycles, a new peak appeared at 1019 cm\({}^{-1}\), indicating the formation of new bonds between polymer chains (attributed to C-O bond stretching). Finally, the third group contained the PCMs which underwent serious chemical degradation since huge differences in the FT-IR spectra of cycled materials could be noticed. It was found that dulcitol (see Figure 3e), urea (see Figure 3f), salicylic acid, D-mannitol, maleic acid, and benzoic acid showed this behaviour. New peaks appeared in the spectra of urea (1334, 1411 and 1069 cm\({}^{-1}\)) and salicylic acid (1739 cm\({}^{-1}\)). D-mannitol presented displaced peaks in the high wavenumber region, and several peaks at low wavenumbers disappeared (i.e., 575 cm\({}^{-1}\)). After 100 cycles, the fingerprint region peaks of maleic acid and benzoic acid presented profound differences (861 and 931 cm\({}^{-1}\) respectively). Dulcitol showed that in the fingerprint region, few peaks disappeared (500 to 800 cm\({}^{-1}\) region), while in the high-wavenumber region, large differences were observed. In addition, new peaks appeared in the cycled samples in the 1640 and 1740 cm\({}^{-1}\) region (CO-double bonds), which was an indication of oxidation during cycling.  
Figure 3: _Cont._