An ocean of microplastics tracked at the nanoscale

What is the fate of polymer debris in the ocean? Aging of polyethylene packaging after prolonged marine exposure has been tracked at the nanoscale. A recent work demonstrates that the semi-crystalline polymer is deeply modified, with an increase in crystallinity along with a disruption of the lamellar order. This process has important implications for embrittlement, breaking of plastics in ever smaller fragments, and subsequent chemical degradation, ultimately determining the lifetime of this packaging in the environment.

Despite only becoming visible in recent decades, the waste from plastic wrapping and packaging forms a visually obvious impact of consumer habits in the Anthropocene era. Melt processed low density polyethylene (PE) is one of the key culprits, which fills a demand for a low-cost and transparent barrier material for transport and retail display. A recent publication in the journal Environment Science & Technology has brought the perspectives of X-ray scattering and polymer physics to the question of the lifetime of polyethylene waste in the environment by investigating the nanostructure of PE in ocean microplastics collected in the Atlantic Ocean. The authors drew some very simple, but important, conclusions as to the processes determining the reintegration of PE into the natural carbon cycle.

Left: Sketch of the structural evolution of polyethylene -PE- during marine exposure. Right: Typical Wide Angle X-ray Scattering image obtained from a commercial PE new packaging and measured on the MORPHEUS platform at LPS.

First of all, prolonged exposure to a marine environment induces a very strong reduction in the length of the polymer chains, by a chain scission mechanism photo-induced by the sun UV light. This releases the kinetic frustration of entangled PE chains to crystallization and leads to an evolution of the semi-crystalline nanostructure of PE. The ratio between the number of crystalline and amorphous PE monomers increases, leading to a marked increase in crystallinity. This original result is obtained by two completely independent methods, calorimetry (DSC) on the one hand, and wide-angle X-ray scattering (see figure) on the other. The crystalline lamellae remain present, without any noticeable increase in thickness, as shown by Raman spectroscopy. But the characteristic stacking of the crystalline lamellae, separated by amorphous regions, fades with exposure to the marine environment. Indeed, small angle X-ray scattering shows that the peak associated with the period of this stacking, which is present in new PE packaging, is very weak or even absent in most micro-debris. This shows a significant rearrangement of the crystalline lamellae in the polymeric material. A sketch (see figure) summarizes the evolution of the nanostructure, with an increase in crystallinity and a strong disruption of the lamellar order.

From this ensemble of measurement techniques, it was possible to conclude that the original nanoscale organization of PE in a layered lamellae structure, which is responsible for the excellent barrier properties of PE in packaging, is disrupted by environmental exposure. The change is triggered by the release of kinetic frustration of entangled PE chains to crystallization by the cutting of the PE chains due to environmental UV exposure. This structural degradation should lead to faster environmental oxidation of PE than would be expected on the basis of previous work on terrestrial weathering.

This collaborative work between different French laboratories (*) has been initiated by Christopher Garvey during his sabbatical at the Laboratoire de Physique des Solides (LPS) and supported by the CNRS and the Université Paris-Saclay.

(*) Teams involved :
- IMRCP, Université de Toulouse, CNRS UMR 5623, Université Paul Sabatier, Toulouse
- LPS, Matrix team, MORPHEUS platform
- Laboratoire MONARIS, Sorbonne Université, CNRS, Paris
- C2P2 – LCPP group, UMR CNRS 5265, Université de Lyon, ESCPE Lyon, Villeurbanne

Molecular scale understanding of the embrittlement in polyethylene ocean debris
Christopher Garvey, Marianne Impéror-Clerc, Stéphan Rouzière, Gwenael Gouadec, Olivier Boyron, Laura Rowenczyk, Anne-François Mingotaud & Alexandra ter Halle.
Environ. Sci. Technol. 2020, 54, 11173-11181

Christopher Garvey, Stéphan Rouzière, Marianne Impéror-Clerc