## 3 Results and Discussion
### DSC Results  
The thermal behavior of the unirradiated and irradiated samples from the thermal analysis is given in Table 2. The glass transition of unirradiated samples was 60.8 \(\pm\) 1.2 \({}^{\circ}\)C on average. Therefore, the samples were in a glassy phase during irradiation. Since the total dose used in this study was comparatively low, as a result, the degradation was minimal and there was no effect on \(T_{g}\).  
\begin{table}
\begin{tabular}{c c c} \hline
**Sample** & **Oxidation Index of C-O** & **Oxidation Index of C=O** \\ \hline Unirradiated & 6.27 \(\pm\) 0.13 & 15.4 \(\pm\) 0.31 \\
5 kGy & 6.64 \(\pm\) 0.13 & 18.5 \(\pm\) 0.37 \\
10 kGy & 7.57 \(\pm\) 0.15 & 20.1 \(\pm\) 0.40 \\
15 kGy & 6.97 \(\pm\) 0.14 & 18.4 \(\pm\) 0.37 \\
20 kGy & 7.24 \(\pm\) 0.14 & 18.0 \(\pm\) 0.37 \\ \hline \end{tabular}
\end{table}
Table 1: The OIs of the C-O bond, using the peak at approximately 1100 cm\({}^{-1}\), and the C=O bond, using the peak at approximately 1750 cm\({}^{-1}\). Calculated using the CH\({}_{3}\) peak at approximately 1450 cm\({}^{-1}\) as a reference [49].  
Figure 5: Stereoregular forms of PLA: (**a**) poly-D-lactic acid; (**b**) poly-L-lactic acid; (**c**) copolymer of P(DL)LA with equal proportions L and D subunits.  
The cold crystallization and softening points were found to be on average 122.1 \(\pm\) 1.7 \({}^{\circ}\)C and 153.3 \(\pm\) 2.2 \({}^{\circ}\)C, respectively. The average transition temperatures for irradiated samples slightly decreased with increasing dose due to chain scissions resulting in shorter chains. A negligible ratio of crystallinity was found to be 1.2% on average in unirradiated samples. With increasing radiation dose, an increase in crystallinity was observed, with crystallinity of 2.4 \(\pm\) 0.12% at 5 kGy and 7.3 \(\pm\) 0.4% at 20 kGy. A larger increase in crystallinity was expected [35]. However, the steric hindrance on the backbone of the PLA chains impeded crystallization of these broken chains.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Glass Transition (\({}^{\circ}\)C)** & **Softening Point (\({}^{\circ}\)C)** & **Percent Crystallinity (\%)** \\ \hline Unirradiated & 60.8 \(\pm\) 1.2 & 153.3 \(\pm\) 2.2 & 1.2 (negligible) \\
5 kGy & 59.6 \(\pm\) 0.7 & 152.3 \(\pm\) 1.5 & 2.4 \(\pm\) 0.12 \\
20 kGy & 59.5 \(\pm\) 0.9 & 151.2 \(\pm\) 0.8 & 7.3 \(\pm\) 0.4 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Average values of key thermal properties and crystallinity changes as a function of irradiation dose, calculated using DSC spectra obtained from the second heating cycle.  
Figure 6: FTIR spectra from samples irradiated at 0, 5, 10, 15, and 20 kGy in the absence of oxygen, followed by exposure to air after irradiation. (**a**) Comparison of spectra within the range of the C=O peak; (**b**) comparison of spectra within the range of the C-O peak; (**c**) full spectra.  
As shown in Figure 7, a pronounced double melting peak was observed in the first heating cycle, with the two peaks occurring at approximately 155 \({}^{\circ}\)C and 161 \({}^{\circ}\)C. The stereocomplex consisting of both PLLA and PDLA has a higher melting point than tactically pure PLA due to interlocking of oriented chains [50]. A double peak due to stereocomplexation would be found around 220 \({}^{\circ}\)C, much higher than the second peak we observed [50; 51; 52]. Therefore, it is more likely that the double peak is due to perfected spherulites. The melt-recrystallization model has been used to describe similar behavior in PLA and semicrystalline polymers in the literature [53; 54]. Small imperfect crystalline spherulites realign and become more stable perfected spherulites, which creates an exothermic peak that divides the melting curve into two [53; 55]. This process is dependent on heating rate, as recrystallization is a slow process. The shoulder behavior seen in our second heating curve agrees with the behavior seen at 10 \({}^{\circ}\)C as outlined by [PERSON] et al. [53].