# The Influence of Ionizing Radiation on Paclitaxel-Loaded Nanoparticles Based on PLGA
## 3 Results and Discussion
### Synthesis and Characterization of Polymers
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Irradiation** & \(M_{\text{n}}\) **(kDa)a** & \(\Delta M_{\text{n}}\) **(\%)ab** & \(D\)a** & \(F_{\text{GG}}\) & \(I_{\text{LL}}\)e & \(I_{\text{GG}}\)e & \(T_{\text{II}\) **(LCL)** & \(T_{\text{II}\) **(GLG)** \\ \hline non-irr & 13.79 \(\pm\) 0.10 & n.a. & 1.83 \(\pm\) 0.01 & 0.15 & 4.33 & 1.23 & 0.15 & 1.14 \\ \hline irr-\(\gamma\) & 11.89 \(\pm\) 0.64 & \(-\)13.82 & 1.95 \(\pm\) 0.09 & 0.16 & 3.35 & 1.28 & 0.13 & 1.38 \\ \hline irr-e\({}^{-}\) & 11.85 \(\pm\) 0.15 & \(-\)14.07 & 1.96 \(\pm\) 0.03 & 0.16 & 4.61 & 1.22 & 0.15 & 0.79 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The influence of irradiation on the microstructure and physicochemical properties of PLGA. sequence (169.65 ppm, Table 3) dominated over other signals due to the high LA-to-GA ratio in the feed (LA:GA = 85:15). However, the presence of odd sequences, such as -LGL- and -GLG-, was also noticed, indicating the participation of the transesterification of the second mode accompanying the polymerization process. As a result, the obtained polymer was of a random structure and short average lengths of blocks (Table 2). As seen in Figure 1, no additional signals were found in the \({}^{13}\)C NMR spectra upon irradiation. Additionally, a detailed analysis of the polymer microstructure revealed only negligible changes in relation to the average length of glycolidyl units (\(l_{\text{GC}}\)\({}^{\text{e}}\)) and \(T_{\text{II[LGL]}}\) (Table 2). As seen from the value of \(T_{\text{II}}\) of lactyl units (L) (\(T_{\text{II[CLG]}}\) = 1.11), the synthesized polymer was of a random structure. After irradiation with \(\gamma\)-rays, \(T_{\text{II[CLG]}}\)\({}^{\gamma}\) increased to 1.38, and the average length of lactidyl units (\(l_{\text{LL}}\)) decreased from 4.33 to 3.35. These slight differences may suggest that the irradiation might cause some changes in the regions of the lactidyl units of the copolymer chain. It appears that the relative amount of -GLG- sequences in the copolymer increased after irradiation with \(\gamma\)-rays, possibly as a result of chain scission. However, after the irradiation of PLGA with EB, \(T_{\text{II[CLG]}}\) slightly decreased (\(T_{\text{II[CLG]}}\)\({}^{\text{EB}}\) = 0.79), and \(l_{\text{LL}}\) increased from 4.33 to 4.61. It has to be taken into consideration that irradiation with EB was conducted with a high dose rate in considerably lower time  
Figure 1: The influence of ionizing radiation on \({}^{13}\)C NMR spectra of PLGA non-irr (**a**), irr-\(\gamma\) (**b**) and irr-e\({}^{-}\) (**c**). \begin{table}
\begin{tabular}{c c} \hline \hline
**Chemical Shift (ppm)** & **Sequence** \\ \hline
169.65 & LLLL \\
169.58 & LLGG \\
169.50 & LGLL \\
169.47 & GGL \\
169.38 & GLG \\
166.55 & GGLG \\
166.49 & GCLL \\
166.47 & GGGG \\
166.44 & LGL \\
166.38 & LLGG \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical shift in \({}^{13}\)C NMR spectrum of PLGA. compared to irradiation in a gamma chamber. In fact, exposition in a gamma chamber needs a dozen hours to obtain a dose of 25 kGy, and there is enough time that free radicals reactions leading to chain scission may occur with high efficiency, especially in the presence of atmospheric oxygen. On the contrary, during exposition in EB, a considerable amount of radicals are formed in a short time. These (high molecular) radicals might recombine; thus, the processes of cross-linking become more probable. Additionally, due to limited contact with oxygen, degradation processes may also be limited, compared to irradiation with \(\gamma\) rays. As a result, these processes may lead to chain elongation. The overall results, i.e., decrease in molecular mass and changes in the polymer microstructure after irradiation, may be explained in terms of two concurring processes taking place during irradiation (chain scission and cross-linking). However, the overall decrease in the \(M_{\rm n}\) of PLGA suggests the predominance of chain scission over cross-linking.