# The Influence of Ionizing Radiation on Paclitaxel-Loaded Nanoparticles Based on PLGA
## 3 Results and Discussion
### The Influence of Ionizing Radiation on Thermal Properties of Polyesters  
The influence of ionizing radiation on the thermal properties of PLGA was assessed by DSC. During the first heating of the non-irradiated sample (Figure 2a), a decrease in heat flow was observed at temperatures above 30 \({}^{\circ}\)C, possibly connected to the glass transition. It was accompanied by an endothermal effect with the minimum of 77.3 \({}^{\circ}\)C. In the higher temperature range, the exothermal effect of cold crystallization (recrystallization of the polymer chains into the structure with lower energy; \(T_{\rm c}=103.7\)\({}^{\circ}\)C) was observed. It was followed by the endothermal effect with two minima (at 138.0 \({}^{\circ}\)C and 147.1 \({}^{\circ}\)C). The two minima of the endothermal effect can be related to a superposition of the melting of different supramolecular structures of the polymer and, precisely, to the melting of less- and better-ordered crystals [29]. [PERSON] et al. [30] attributed them to the melting of the more tightly packed linear polymer fraction (at higher temperature) and the more branched chains (at lower temperature). The genesis of the endotherm accompanying the glass transition (\(T_{\rm min}=77.3\)\({}^{\circ}\)C) is not entirely clear. It can be considered as related to the melting of the low-ordered fractions of PLGA, that are possibly characterized by the polymer of low molecular weight (oligomers). It follows the hypothesis given by [PERSON] et al. [31], who related the endothermal peak between 20 \({}^{\circ}\)C and 40 \({}^{\circ}\)C of PEG-PLGA samples to the melting of free PEG-PLGA blocks of the polymer that were formed during storage or as a result of radiation-induced degradation [31]. During the second heating (Figure 2b), the initial sample undergoes recrystallization followed by melting, with the enthalpies of both processes being similar (48-50 J g\({}^{-1}\)) (indicating that almost all of the melted material was formed during recrystallization occurring at a lower temperature during the same heating cycle). Moreover, these enthalpies were similar to the enthalpy of melting occurring on first heating. Based on the above data, the copolymer was found to reveal a semicrystalline structure (\(X_{c}=35.1\%\)). Additionally, single \(T_{\rm g}\) indicates good homogeneity of the copolymer. The influence of ionizing radiation on the thermal and crystalline properties of PLGA is shown in Table 4. No change in \(T_{\rm g}\) was observed after irradiation with EB, whereas a relatively small effect was observed after the gamma irradiation (decrease from 48.1 \({}^{\circ}\)C to 47.6 \({}^{\circ}\)C). However, the changes were noticed in the enthalpies and characteristic temperatures of the crystallization and melting processes (Table 4). This was particularly evident in the enthalpies of the processes. It was also noticed that the effect of gamma irradiation was higher than that of EB. irradiation process (crosslinking and/or chain scission). Importantly, the irradiation of the samples was conducted in air. The presence of atmospheric oxygen enables reactions of alkyl radicals with oxygen. In the case of prolonged exposure to radiation in the gamma chamber, this resulted in high participation in the degradation process. However, rapid irradiation with a high dose (as for EB in our case) acts similarly to the exposure of the material to irradiation in an inert gas atmosphere, which prevents the formation of peroxyl radicals within the interior of the material. This occurs because atmospheric oxygen does not have time to diffuse into the sample and cannot react with its interior. In that case, the outer layer of the material (exposed to oxygen), favors oxidative degradation, while some crosslinking may occur inside the material [32].