Elucidating the Role of Alkoxy Radicals in Polyethylene Radio-Oxidation Kinetics

Abstract

Polyethylene is a widely used polymer and a prototypical polyolefin. Understanding degradation mechanisms at the atomic scale leading to oxidation under the effect of temperature and/or irradiation is crucial for many long-term applications. Here, we focus on a subset of reactions belonging to the generally assumed radio-oxidation reaction scheme and involving alkoxy radicals. We follow a theoretical approach based on density functional theory, in order to assess the role of these species, which are short lived and thus hard to detect experimentally. For every considered reaction, we calculate the reaction enthalpy and the energy barrier, and we evaluate the influence of the local atomic environment, taking advantage of a model surface of a crystalline lamella which mimics the interface between crystalline and amorphous zones. Our results suggest that in certain conditions, the kinetic pathway can bypass the formation of hydroperoxides. The concentrations of alkoxy and peroxy radicals during radio-oxidation and their ratio are important parameters controlling the predominance of chain scission or crosslinking in the polymer. The data presented here constitute part of a database that can be used to set up kinetic simulations based on homogeneous chemical kinetics or Monte Carlo algorithms.

Combining Experimental and Theoretical Tools to Probe Radio-Oxidation Products in Polyethylene

“…If, however, for some reason, some oxygen happens to be trapped in a crystalline environment or able to penetrate it, our results do not support the idea that inside the crystal the frequency values are less dispersed than in the amorphous region and thus distiguishable in any way. It is true that the lamella in our model is much thinner than real crystalline lamellae in polyethylene, but we stress also the importance of being able to simulate phenomena at the interface between the amorphous and crystal phases, which constitutes a relevant fraction of the material [ 44 , 45 ]; moreover, at lamellae surfaces, at least some of the relevant reactions involved in PE aging and carbonyl production may take place more easily than elsewhere [ 37 ].…”

“…A last note on the inclusion in the list of carbonyl species of ketone+alcohol pairs, for which we do not have an experimental reference. In a recent paper assessing the viability of various reaction paths through energy barrier calculations [ 37 ], we suggest that a relevant ketone production mechanism leads to such pairs. Our calculations of vibrational modes show a slight decrease in the average frequency and, thus, we think it is interesting to include them in the deconvolution procedure.…”

“…[ 23 ]. This is not without consequences in assessing the aging mechanisms in polyethylene, because the presence of aldehydes is directly related to the occurrence of -scission reactions [ 49 ], possibly triggered by alkoxy radicals [ 37 ]. Esters, on their side, require beforehand the presence of ketone groups that are attacked by alkoxy radicals [ 50 ].…”

“…We relied on a slab supercell including a thin crystalline lamella separated by its periodic images, in the direction perpendicular to the lamella surface, by a region that is empty space except for a single chain connecting the two lamella surfaces. This model, containing 150 atoms, is a variant of the model described in detail in [ 37 ]; the modification consists of the connecting chain, which was branched on each surface by removing a hydrogen atom. A view of this model can be seen in Figure 1 .…”

“…A view of this model can be seen in Figure 1 . The model was fully relaxed by keeping fixed the in-plane lattice parameter of the orthorhombic crystal and, for the out-of-plane lattice parameter, we kept the value used in [ 37 ]. We note that this gives an overall density (0.88 g/cm ) that is close to typical experimental values for LLDPE.…”