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Molecular scale understanding of the embrittlement in polyethylene ocean debris.

The fate of plastic waste is a pressing issue since it forms a visible and long-lived reminder of the environmental impact of consumer habits. In this study we examine the structural changes in the lamellar arrangements of semi-crystalline polyethylene (PE) packaging waste with an aim to understand the physical mechanisms of embrittlement in PE exposed to the marine environment. Microplastics from PE packaging were collected in the Atlantic Ocean and are compared to new and weathered commercial PE boxes. Several experimental techniques interrogate the effects of environmental exposure on their bulk and surface properties. Size exclusion chromatography (SEC) determines the molecular weight distribution of the PE polymer chains and differential scanning calorimetry (DSC) gives the crystallinity. Small and wide angles X-ray scattering examines the packing of PE chains into semi-crystalline lamellae. Longitudinal acoustic mode Raman spectroscopy provides a complementary measurement on the length of PE polymer chains extending through the crystalline lamellar domains. The overall picture at the molecular scale is that although PE becomes more crystalline with environmental exposure, the lamellar order present in new packing boxes is disrupted by the weathering process. This process has important implications for embrittlement and subsequent degradation.