## 3 Results and Discussion  
### Identification of the PLA Radiolytically Produced Free Radicals and Their Kinetics Decay  
Representative EPR spectra from each of the four doses were taken for comparison and plotted in Figure 2. Upon examination of each of the spectra, it is apparent that the dominant splitting pattern is four peaks with relative intensity magnitudes of 1:3:3:1. Assuming interactions between atoms less than three bond-lengths away from the radical contribute to the hyperfine structure, this splitting pattern matches the predicted result from Equation (1). H-abstraction from PLA results in three equivalent hydrogen atoms located two bond-lengths away from the carbon-centered radical. Substitution of \(n=3\) for the number of equivalent hydrogen atoms and \(I=\
icefrac{{1}}{{2}}\) for the nuclear spin of hydrogen yields four hyperfine peaks, which is in agreement with experimental results.  
This dose dependence can be further expanded upon by calculating the number of radicals present in the sample over time, with concentration expressed in spins per gram. Figure 3 illustrates this calculation. Even on day 2, in the absence of oxygen, relatively stable free radicals are still present. We believe it is a mixture of alkoxyl radicals, including radical (b) in the proposed mechanism of Figure 4. After one week, the radical concentration had decreased below the effective detection limit of the spectrometer and all yields were approximately zero spins per gram. For the reactions that occur after the irradiation of PLA, the rate-determining step is the abstraction of hydrogen by alkoxyl radicals. The decay fits neither first nor second order reaction kinetics due to complexity of the free radicals present.  
Figure 2: (**a–c**) Time series of EPR spectra for PLA irradiated at 4 different doses: 5 kGy (red), 10 kGy (blue), 15 kGy (pink), and 20 kGy (green). There were three time points used: (**a**) same day as irradiation (day 0); (**b**) day following irradiation (day 1); (**c**) second day following irradiation (day 2). (**d–f**) Dose-yield plots over the first three days. These measurements were taken twice at 5 kGy, twice at 10 kGy, four times at 15 kGy, and twice at 20 kGy. (**d**) Day 0; (**e**) day 1; (**f**) day 2.  
Figure 4 shows the overall proposed reaction mechanisms of the radiolysis of PLA in the absence of oxygen. The EPR results demonstrate the radiation-induced C-O-C bond scissions that lead to the formation of the oxidative alkoxyl radicals and the reductive C-centered radicals. The alkoxyl radicals abstract H atoms from the neighboring molecule, giving rise to the formation of more C-centered radicals. The carbon-centered radicals on nearby PLA chains are not expected to undergo crosslinking reactions (reaction pathway i) because of steric hindrance. After chain scission (reaction pathways ii and iii), the alkoxyl radical abstracts a hydrogen from an adjacent polymer chain, giving rise to the formation of C-centered radicals. In addition, alkoxyl radicals undergo \(\beta\)-fragmentation and 1,2-H-shift reactions [44]. Since oxygen is not present during the EPR measurements only, the peroxyl radicals are not expected to be present, thus limiting the potential for chain scission [37]. In this investigation, dry ice was used to slow radical concentration decay kinetics until they could be characterized by EPR, and the inert atmosphere was used to prevent the formation of undesired radical species, such as the peroxyl radical, that could impact the resultant PLA structure. As expected, the presence of the L and D stereoregular forms of PLA shown in Figure 5 did not impact the shape of the EPR spectra.  
Figure 4: Proposed mechanism for chain scission of PLA after electron beam irradiation.  
Figure 3: Time dependence radical concentration decay for PLA samples after irradiation at 5 kGy, 10 kGy, 15 kGy, and 20 kGy. The values at day 0 on the x-axis correspond to spectra that were taken a couple of hours after irradiation and kept in dry ice.