This poster was presented by Astrid Delorme at the 32nd annual SETAC Europe meeting in Copenhagen 15th-19th of May 2022, under the “8.04 – Late Breaking Science Extended Submission” session. Abstract: SEALIVE (Strategies of circular Economy and Advanced bio-based solutions to keep our Lands and seas alIVE from plastics contamination) is a H2020 innovation project, which vision is to reduce plastic waste and contamination on land and in seas by boosting the use of biomaterials and contributing to the circular economy with cohesive bio-plastic strategies. As part of WP7, polymer degradation is investigated in several real service life and end-of-life conditions, in order to assess the life cycle of polymers and to eventually develop international biodegradation and ecotoxicity standards for bioplastics. Particularly, urgent concerns of polymers’ end-of-life are focused on the potential hazards and risks associated with microplastics. A prevailing source of microplastics is the fragmentation of larger plastics or product wear, however studies on the rates of fragmentation of polymers under various conditions are scarce [1, 2]. Photodegradation of polymers leads to structural changes of the polymer backbone, such as oxidation with formation of carbonyl functional groups, chain scission and cross-linking [3, 4]. These transformations at molecular level impact the overall mechanical properties of the plastic and favour embrittlement and fragmentation, leading to the generation of micro- and nano-plastics. Understanding the fragmentation rates and behaviour of photo-aged macroplastic generating secondary microplastics will provide vital information to enable a full risk-assessment of plastics needed for international biodegradation and ecotoxicity standards. Here we present a comparative study between a photoaged conventional polymer, polytethylene (PE), and an emerging biopolymer, polylactic acid (PLA), with a focus on the interrelation between photoaging and brittleness. Changes in morphological and physico-chemical properties of the photoaged polymer films are followed through the use of melt rheology, DSC, FTIR, UV-VIS, SEM and durometry with the aim to identify most relevant polymer properties, such as molecular weight, crystallinity and tensile properties that could explain the brittleness and fragmentation behaviour of aged polymers. In this work, we demonstrate the relationship between the brittleness behaviour of the aged polymers with the crosslinking and chain scission mechanisms which occur during the photoaging. [1] A. A. Koelmans, N. H. Mohamed Nor, E. Hermsen, M. Kooi, S. M. Mintenig, J. De France. Water Res., 2019, 155, 410-422. [2] M. Kooi, A. A. Koelmans. Environ. Sci. Technol., 2019, 6 (9), 551–557. [3] S. Commereuc, H. Askanian, V. Verney, A. Celli. Macromol. Symp. 2010, 296, 378–387. [4] S. Commereuc, H. Askanian, V. Verney, A. Celli, P. Marchese, C. Berti. Polym. Degrad. Stab. 2013, 98, 1321–1328.