# Coaxial Fibres Incorporated with Phase Change Materials for Thermoregulation Applications
## 3 Results
### Chemical Composition (ATR-FTIR)  
ATR-FTIR was used to investigate the chemical composition of the hollow and uniaxial fibres derived from CA [44; 45]. The obtained spectra revealed characteristic peaks, mainly at 1745, 1657, 1376, 1238, 1047, and 901 cm\({}^{-1}\), indicating the prevalent bonds and functional groups within these fibres [46]. The 1745 cm\({}^{-1}\) peak was attributed to the stretching vibration of the carbonyl group (C=O) from acetate units, signalling the successful acetylation [47]. A peak near 1657 cm\({}^{-1}\), approximating the expected band for C=O stretching in DMF around 1670 cm\({}^{-1}\), refers to a potential residual solvent within the resultant fibres [44]. Furthermore, peaks at 1376 and 1238 cm\({}^{-1}\) were ascribed to the symmetric deformation vibrations of CH3 in the acetate units and the ester linkage C-O-C stretching in the CA molecule, respectively [48]. A band at 1047 cm\({}^{-1}\) confirmed the presence of typical C-O stretching vibrations found in cellulose derivatives, further affirming the structural integrity post fibre fabrication [44]. The 901 cm\({}^{-1}\) peak is characteristic of the glycosidic ring's out-of-plane vibration, cementing the fibre's polysaccharide character [49].  
ATR-FTIR also provided detailed chemical structure analyses of the coaxial fibres comprising a CA sheath enveloping a PEG2000 core (Figure 10a,b). The analysis exhibited noticeable peaks, offering insights into molecular interactions and functional groups within the fibre. The 2885 cm\({}^{-1}\) peak resonated with C-H stretching vibrations of methyl (-CH\({}_{3}\)) and methylene (-CH\({}_{2}\)-) groups, aligning with PEG2000 repetitive methylene units and cellulose acetate's structural attributes [50]. A 1745 cm\({}^{-1}\) peak corresponded to the C=O stretching vibration characteristic of the acetate groups bonded with the cellulose polymer, underlining the seamless incorporation of the acetate structure [47]. The peak at 1657 cm\({}^{-1}\) is notable; while it aligns with C=O stretching vibrations, potentially emanating from residual DMF, it also alludes to potential contributions from PEG (more specifically, any traces of water or -OH group deformations) [51]. Peaks observed at 1465, 1376, and 1368 cm\({}^{-1}\) match with the structural vibrations of PEG2000 and acetate groups, signifying methylene deformation and methyl symmetric deformations, respectively [52]. Additionally, peaks at 1278, 1238, and 1105 cm\({}^{-1}\) can be tied to varied C-O and C-O-C stretching, denoting the ester connections in cellulose acetate and PEG2000 recurrent ether linkages [53]. Peaks at 1047, 939, and 901 cm\({}^{-1}\) further cement the polysaccharide nature of the fibre, with the latter denoting the glycosidic ring's out-of-plane vibration in cellulose structures [53]. The 835 cm\({}^{-1}\) peak might have origins in the PEG framework [50].  
An FTIR analysis of the recycled CAt and its respective spectra are shown in Figure 10c. As can be seen, the spectrum for the recycled CAt is very similar to the spectrum of the commercial CA. There is a pronounced band at 1748 cm\({}^{-1}\), which may be due to the stretching of the ester's carbonyl, a characteristic of acetyl groups. Based on the literature, this peak is one of the main indications that cellulose has been acetylated [54]. It is also possible to see the presence of a band between 1300 and 1200 cm\({}^{-1}\) (peak at 1238 cm\({}^{-1}\)), which represents the C-O stretching of acetate. The appearance of the band at 1376 cm\({}^{-1}\) corresponds to the vibration of the C-H bond, which is also a characteristic band of acetyl groups. Therefore, the ATR-FTIR spectra indicate that acetylation of the cellulose molecule has been successfully carried out [55].  
Figure 10: ATR-FTIR spectra of (**a**) Fh_30, PCF_30_10, PCF_30_20 and PCF_30_30 fibres; (**b**) Fh_50_10 and PCF_50_10 fibres; (**c**) CAt, Fht and PCFt fibres.