## 2 Materials and Methods
### Sample Preparation and Irradiation  
The sheets of PLA were cleaned with deionized water and cut into two different-sized pieces for sample treatment, the smaller being 52 mm \(\times\) 4 mm and the larger being 52 mm \(\times\) 16 mm. The 52 mm \(\times\) 4 mm sample was used for electron paramagnetic resonance (EPR) characterization, while the 52 mm \(\times\) 16 mm sample was divided between FTIR and water vapor transmission rate (WVTR) testing. FTIR is a non-destructive test, and it was performed before the WVTR tests. Each pair of small and large PLA strips was placed into an individual 40 mL septum vial and purged at 7 psi for 20 min with inert gas, which was either nitrogen or argon. These septum vials were then sealed with parafilm prior to irradiation.  
Irradiation was conducted at the Medical Industrial Radiation Facility at the National Institutes of Standards and Technology Physical Measurement Laboratory using a [PERSON] traveling-wave electron linear accelerator. The pulse width was maintained at 6 \(\upmu\)s with a pulse repetition of 100 pulses/s and an energy of 11.5 MeV. The samples were irradiated to a total dose of 5, 10, 15, and 20 kGy and at an average dose rate of 68 kGy/h. Dosimetry was conducted using strips of alanine placed in the path of the electron beam. Samples were placed on a foam block 100 cm from the exit beam. The electron beam was run at room temperature. Once irradiated, the samples were kept on dry ice in the absence of oxygen prior to transfer from septum vials to EPR tubes for subsequent EPR analysis. This sample transfer was conducted in a glove box purged with nitrogen at 7 psi.  
Radical Structure and Concentration Decay Characterization Using Electron Paramagnetic Resonance (EPR) Spectrometry  
2,2-diphenyl-1-picrylhydrazyl (DPPPH) was used to calibrate the frequencies from the microwave generator, and EPR measurements were taken for each of the irradiated samples. DPPH was chosen as it contains a stable free radical which is possible through stabilization provided by the three rings in its structure. The following instrument parameters were used: microwave power of 5 mW, frequency modulation of 100 kHz, modulation amplitude of 3.12 G, receiver gain of 6.32 \(\times\) 10\({}^{3}\), center field at 3500 G, sweep width of 1000 G, conversion time of 40.96 ms, and time constant of 20.48. The parameters were selected to avoid overmodulation or signal distortion, and four sweeps over the magnetic field spectrum were conducted for each sample to maintain a high level of signal resolution. The expected number of hyperfines for an EPR spectrum can be described by Equation (1):  
\[\textit{number of hyperfines}=\Pi 2n_{i}l_{i}+1 \tag{1}\]  
Figure 1: Characterization of unirradiated PLA. (**a**) Average DSC cycle 1; (**b**) average cycle 2; (**c**) XRD diffractogram; (**d**) FTIR spectra (labels: 2994.96, 1746.74, 1451.68, 1080.44, 866.86 cm\({}^{-1}\)).  
where \(n_{i}\) denotes the number of equivalent nuclei and \(I_{i}\) denotes the nuclear spin of the equivalent nuclei. The concentration of radicals in spins per gram was determined from Equation (2):  
\[[x]=\frac{[std]A_{x}R_{x}{(scan_{x})}^{2}G_{std}M_{std}{(g_{std})}^{2}{[s(s+1)]}_{ std}}{A_{std}R_{std}{(scan_{std})}^{2}G_{x}M_{x}g_{x}^{2}{[s(s+1)]}_{x}} \tag{2}\]  
where \([x]\) denotes radical concentration, \(A_{x}\) denotes the double integral of the EPR first derivative curve, \(scan_{x}\) denotes the magnetic field sweep width in gauss, \(G\) is the gain of the signal amplifier (Hz), \(M\) is the modulation amplitude (G), \(g\) is the Lande factor, \(s\) is the radical spin quantum number, and \(R\) is the Kramers degeneracy of the EPR spectrum. The subscript \(std\) refers to the parameters measured from the manganese sulfate monohydrate (MnSO\({}_{4}\cdot\)H\({}_{2}\)O) standard sample and the subscript \(x\) refers to the parameters measured from the experimental samples.