## 1 Introduction
Water radiolysis (Equation (1)) creates \({}^{\bullet}\)OH radicals (E\({}^{\circ}\) = 2.7 V) [34], one of the most powerful chemical oxidants according to: [32]  
\[\mathrm{H_{2}O}\leadsto[0.28]{{}^{\bullet}OH}+[0.06]\mathrm{H^{\bullet}}+[0.27 ]\mathrm{e_{aq}}^{-}+[0.05]\mathrm{H_{2}}+[0.07]\mathrm{H_{2}O_{2}}+[0.27] \mathrm{H_{aq}}^{+} \tag{1}\]  
where the numbers in brackets in Equation (1) are the gamma radiolytic yields (\(\mathrm{\mu mol\,J^{-1}}\)) of each produced species for neutral pH water. Further, dissolved O\({}_{2}\) is consumed by radiolytically generated reducing radicals from water radiolysis:  
\[\mathrm{O_{2}+e_{aq}}^{-}/\mathrm{H^{\bullet}}\rightarrow\mathrm{O_{2}}^{-}/ \mathrm{HO_{2}}^{\bullet} \tag{2}\]  
Cobalt-60 (\({}^{60}\)Co) gamma radiation sources generate \({}^{\bullet}\)OH radicals at rates of 10\({}^{-8}\) to 10\({}^{-5}\) M s\({}^{-1}\), many orders of magnitude faster than experienced in real-world waters, which allows laboratory experiments to be conducted over short times to replicate long-term environmental weathering. In water, polymeric materials have been shown to undergo radical reactions, consistent with other carbon-based compounds [35]. Reactions with \({}^{\bullet}\)OH typically afford hydrogen-atom abstraction from polyolefins such as PE (Equation (3)), creating carbon-centered radicals (\({}^{\bullet}\)[Polymer]):  
\[{{}^{\bullet}OH}+\mathrm{Polymer_{(aq)}}\rightarrow\mathrm{H_{2}O}+{{}^{ \bullet}[Polymer]_{(aq)}}. \tag{3}\]  
In aerated aqueous solutions (\(\sim\)0.25 mM O\({}_{2}\)), carbon-centered radicals are expected to rapidly react with dissolved O\({}_{2}\) to form peroxyl radicals (Polymer-O\({}_{2}\)\({}^{\bullet}\), Equation (4)), which can initiate polymer autooxidation through chain reactions. For the reactions of hydrocarbons in water under low O\({}_{2}\) to anaerobic conditions--potentially achieved by the radical consumption of O\({}_{2}\), (Equation (2))--the carbon-centered radical likely crosslinks with other carbon radicals (Equation (5)) or leads to scission products (Equation (6)) [36; 37]. For example, [PERSON] et al. reported the formation of unsaturated polymer segments from PE films submerged in water when O\({}_{2}\) was replaced by argon or nitrous oxide [37]. \[{}^{\bullet}[\text{Polymer}]_{\text{(aq)}}+\text{O}_{2}\rightarrow\text{Polymer-O}_{ 2}{}^{\bullet}{}_{\text{(aq)}}, \tag{4}\]  
\[{}^{\bullet}[\text{Polymer}]_{\text{(aq)}}+{}^{\bullet}[\text{Polymer}]_{ \text{(aq)}}\rightarrow\text{Polymer-Polymer}_{\text{(aq)}}, \tag{5}\]  
\[{}^{\bullet}[\text{Polymer}]_{\text{(aq)}}\rightarrow\text{scission products and smaller polymer species}. \tag{6}\]  
To date, weathering studies of microplastics have simulated oxygenated conditions in surface waters, a critical aspect for understanding the behavior of these pollutants [38]. However, microplastics that encounter low O\({}_{2}\) to anaerobic conditions have not been thoroughly studied, yet these low O\({}_{2}\) concentration regimes exist in stagnant waters, lower water column areas, and transiently in all waters. These conditions may be part of the reason for prolonged lifetimes in the environment and lead to varying morphologies and products of microplastics, some of which may remain adsorbed [39]. Here, we present a radiation chemistry investigation into the radical-induced weathering of PE microplastics that experience fluctuations in dissolved O\({}_{2}\) concentrations by utilizing initially aerated water mixtures where dissolved O\({}_{2}\) is depleted over time by radical consumption (Equation (2)).