# Doped Epoxy Resins as an Alternative to Luminescent Optical Sensors
## 3 Results and Discussion
### Thermal and Spectroscopic Analysis
The same situation is observed for samples No.1-30 and No.1-60 but in both cases from the temperature of 150 \({}^{\circ}\)C, which is consistent with the first DTG values (Table 4). In the case of DSC curves, all emerging effects are endothermic and their first maxima are slightly shifted towards lower temperatures compared to the DTG maxima. The addition of photosensitive substances (samples No.2 and No.3) shifts the first DTG maximum towards lower temperatures when comparing it to the pure matrix, and this should be carefully monitored, because each subsequent inclusion of additional substances usually disturbs the polymerization process, causing a deterioration in the quality of the obtained structures. The first maxima recorded by DSC, as before, remain around 120 \({}^{\circ}\)C, and are related to the endothermic thermal decomposition of the smallest molecules released from the structure, i.e., the active solvent. This is confirmed by mass spectroscopy and the presence of \(m/z\) ions: 27, 28, 29, 42, 43, 44 and \(m/z=14\) and 15 (methylene and methyl groups) which appear around the temperature of 130 \({}^{\circ}\)C. The exception are samples No.3-15, where the mentioned ions appear already around 100 \({}^{\circ}\)C, which indicates poor polymerization, confirmed by the double-bond conversion result (38.6%, Table 3). It is worth adding at this point that the temperature of 200 \({}^{\circ}\)C is the limit temperature in polymer optical fiber technology, and it is directly related to the technological process itself, because it is around 200 \({}^{\circ}\)C that polymer fibers are generally drawn out. In fact, it may shift towards higher temperatures, especially for cross-linked compositions, and due to the glass transition temperature of the matrix. But, during thermal tests, when we compare the processing possibilities of materials, we always assume a temperature of 200 \({}^{\circ}\)C as the temperature limiting the possibility of using the material. Therefore, when analyzing the data, the results are presented as those above and below the mentioned temperature. One more maximum (or two in the case of samples No.1-15, No.2-30 and No.2-60) is observed in the DTG and DSC curves before the final decomposition occurs. It appears within the temperature range of 230-240 \({}^{\circ}\)C. It is related (and read at the maximum visible on the ion current curve Figure 2a) to the presence of ions \(m/z=\) 14,15, 17-19 and ions \(m/z=\) 26-28 related to the vinyl group most likely from NVP (there are no other ions indicating the decomposition of the resin). There are also ions coming from the breakdown of the lactam ring \(m/z=\) 40-44 and 53-56: C\({}_{2}\)H\({}_{2}\)N\({}^{+}\), C\({}_{3}\)H\({}_{5}\)\({}^{+}\), C\({}_{2}\)H\({}_{2}\)O\({}^{+}\), C\({}_{2}\)H\({}_{3}\)O\({}^{+}\), H\({}_{2}\)NCO\({}^{+}\), C\({}_{3}\)H\({}_{3}\)N\({}^{+}\), C\({}_{3}\)H\({}_{4}\)N\({}^{+}\), C\({}_{3}\)H\({}_{3}\)O\({}^{+}\) and C\({}_{3}\)H\({}_{4}\)O\({}^{+}\). Their presence confirms the further release of active solvent molecules and their decomposition. Based on the DTG curves, it was also possible to determine the temperature at the maximum decomposition and for all the samples discussed (i.e., containing 25 wt.% NVP addition) it is approximately 437 \({}^{\circ}\)C. In the case of DSC curves, it is worth noting that several maxima appeared in the temperature range where proper sample decomposition occurs. This indicates the complexity of the process related to the heterogeneity of the system, but the fact that these peaks occur at similar temperatures most likely indicates the same decomposition mechanism. In the case of the addition of fluorescein, these maxima are slightly shifted towards lower temperatures (sample series No.3). In addition to the ions mentioned so far, in the maximum of the ion current (at a temperature of approximately 440 \({}^{\circ}\)C, Figure 2), there are ions indicating the complete decomposition of the substances included in the tested materials.