## 3 Results
### Thermal Analyses  
#### 3.4.1 Thermogravimetric Analysis (TGA)  
TGA was performed to evaluate the degradation temperature limits of pristine materials (CA, PEG 600 and 1000) and the PCFs produced to study and understand their thermal stability and suitability for applications in civil engineering construction materials. Although the temperatures analysed in this test exceed those typically found in built environments, it ensures that the PEG encapsulation process does not compromise the structural integrity of the PCFs at elevated temperatures. This is important for predicting the long-term stability of the materials. CA presented a single-step degradation behaviour, i.e., with an initial mass loss at \(\sim\)306 \({}^{\circ}\)C and finalised at \(\sim\)390 \({}^{\circ}\)C, resulting in a well-defined and homogeneous polymeric structure [40]. PEG 600 registered an initial mass loss at \(\sim\)272 \({}^{\circ}\)C, concluding its thermal degradation process at \(\sim\)408 \({}^{\circ}\)C [41], while PEG 1000 began degrading at \(\sim\)321 \({}^{\circ}\)C and ending it at \(\sim\)425 \({}^{\circ}\)C [42]. This difference between the PEGs (600 and 1000) may be related to the difference in molecular weight. PEG 600 has a lower molecular weight and, consequently, shorter molecular chains, while PEG 1000 has a higher molecular weight, with longer polymer chains and more excellent crystallinity, which requires more energy and higher temperature to be decomposed [41; 42]. The four PCFs (Figure 8a,b) exhibited a slight state of degradation around \(\sim\)60 \({}^{\circ}\)C, which can be attributed to the elimination of residual molecules of the solvents present in the composition of the fibre sheath [14; 43]. The two PCFs produced with PEG 600 in the core (PCF:50/600_90_140 and PCF:50/600_90_150) (Figure 8a) showed thermal degradation temperatures between \(\sim\)264 \({}^{\circ}\)C and \(\sim\)397 \({}^{\circ}\)C, values slightly lower than those presented for the pristine PEG 600 material (\(\sim\)272-408 \({}^{\circ}\)C). This slight can be explained by the partial interaction between the CA polymers and the PEGs and the possible partial hydrolysis of the ether bonds (C-O-C) [41]. Both molecular interaction and hydrolysis result in a lower energy required to initiate the thermal degradation process, as evidenced by the slightly lower temperatures at the onset of mass loss [44]. Despite this, the total degradation temperature range exceeded that of CA (\(\sim\)306-390 \({}^{\circ}\)C), evidencing that the CA sheath effectively protected the core containing PEG 600 during the thermal degradation. On the other hand, PCFs with PEG 1000 (PCF:50/1000_90_140 and PCF:50/1000_90_150) (Figure 8b) showed higher thermal degradation temperatures between \(\sim\)273 \({}^{\circ}\)C and \(\sim\)413 \({}^{\circ}\)C, indicating a greater thermal stability of PEG 1000 compared worth PEG 600. This greater stability may be related to the higher molecular weight and longer polymer chains of PEG 1000, which result in a more organized polymer structure. Furthermore, this structural organisation can reduce the probability of unwanted chemical interactions between the core and the sheath, highlighting how the thermal behaviour of PCFs is influenced by the structural and chemical properties of the PEGs chosen for the core [39]. The ejection speed used for the PEGs did not significantly impact the thermal degradation results due to the slight variations between the tested values. Figure 8: TGA curves were obtained from 30 \({}^{\circ}\)C to 500 \({}^{\circ}\)C under an N2 atmosphere, a calibrated flow rate of 200 mL/min and exposed to a heating rate of 10 \({}^{\circ}\)C/min. (**a**) Pristine materials (CA and PEG 600) and PCFs 600; (**b**) Pristine materials (CA and PEG 1000) and PCFs 1000. The thermal results obtained for the PCFs, especially the differences between PEG 600 and PEG 1000, show the relevance of these PCFs for practical applications in Civil Engineering construction materials.