Pollution of the ocean by plastic litter has become a major environmental problem resulting from its accumulation in terrestrial and marine environments. When mismanaged, plastics enter the aquatic environment where they undergo degradation and fragmentation into microplastics that are now ubiquitous in all aquatic compartments. In addition to the fact that microplastics are impossible to remove from the marine environment, they are even more damaging than the macro waste. Various studies have shown that microplastics are ingested by many types of marine organisms leading to adverse effects at several levels of the food chain and of the marine ecosystems. It is also suspected that microplastics, that constitute a new habitat for micro-organisms, are vectors for potentially pathogenic bacteria.The fate of polymers in the aquatic environment depends both on abiotic phenomena (UV, mechanical stress), and on biotic ones, due to the colonization of plastics by marine micro-organisms (bacteria, phytoplankton, fungi, etc.). A primary step for bio-degradation is the constitution of a biofilm and reduction of the polymer chain length via exo-enzymes produced by bacteria from the biofilm. Once polymer chains are short enough, they can be assimilated by bacteria. While abiotic phenomena lead to the damage and fragmentation of a polymer by oxidation and hydrolysis mechanisms, creation of structural defects and fracture propagation, it is generally admitted that only biotic phenomena will result into the complete bio-degradation of a polymer, i.e. its conversion into biomass, water and CO2. In the marine environment, many questions remain about the relative kinetics of abiotic and biotic degradation and their respective impact in terms of fragmentation. For instance, several papers have recently reported that the size distribution of particles collected in the ocean between 5mm and a few hundreds of microns, does not seem to correspond to a single-kinetic fragmentation process.The erosion patterns of semi-crystalline polymers have been extensively studied in laboratory under enzymatic or bacterial conditions and various degradation patterns have been observed whose occurrence is mainly linked to the difference in the erosion kinetics between crystalline and amorphous regions. To date, there are much less studies addressing how the evolution of these surface patterns will in turn influence the erosion process, lead to fracture and potential fragments generation.In order to study the enzymatic erosion process, we used the well-known model system PDLLA/proteinase K. Being specifically interested in the role of heterogeneities at the scale of a few nanometers to a few micrometers, we used a polymer of a given chemical composition (PDLLA, 1.7% of D-mer, Mn = 95 kg/mol, polydispersity index I=1.63) and monitored its morphology through its change in crystallinity ratio, everything else remaining constant.Three types of samples were studied: 100% amorphous (A), semi-crystalline with 5% (SC5) and 35% (SC35) crystallinity.The samples morphologies were characterized through DSC, polarized optical microscopy (POM) and SEM. Enzymatic erosion kinetics were measured through weight loss experiments for the 3 polymers and the erosion patterns were observed over time through atomic force microscopy (AFM) and SEM. In order to interpret the results, we combined a simple two-phase geometric erosion model with the well-known Michaelis-Menten model for enzymatic kinetics. Our geometric erosion model is based on the evolution of the erosion front with time induced by the erosion rate difference between crystalline and amorphous regions. This new model accounts very well for the experimental results. Moreover, we observed a morphology-dependent release of fragments, which the model is also able to predict. In particular, one observes the release of spherulites as long as they are smaller than a critical size determined in the model. Some important consequences relevant for the understanding of the formation of micro-plastics in the ocean can be drawn from these experiments.

The increase of plastics and microplastics in the environment is a major environmental challenge. Still, little is known about the degradation kinetics of macroplastics into smaller particles, under the joint actions of micro-organisms and physico-chemical factors, like UV or mechanical constraints. In order to gain insight into (bio)-degradation in various media, we perform accelerated erosion experiments by using a well-known enzymatic system. We show that the microstructure of semi-crystalline polymers plays a crucial role in the pattern formation at their surface. For the first time, the release of fragments of micrometric size is evidenced, through a mechanism that does not involve fracture propagation. A geometric erosion model allows a quantitative understanding of erosion rates and surface patterns, and provides a critical heterogeneity size, parting two types of behavior: spherulites either released, or eroded in situ. This new geometric approach could constitute a useful tool to predict the erosion kinetics and micro-particle generation in various media.

An original setup combining a very stable loading stage, an atomic force microscopy and an environmental chamber, allows to obtain very stable sub-critical fracture propagation in oxide glasses under controlled environment, and subsequently to finely characterize the nanometric roughness properties of the crack surfaces. In this presentation, I will focus, on one hand, on the special experimental care that has to be devoted to measure the metrological properties of such nanometric rough surface by Atomic Force Microscopy (AFM) and, on the other hand, on the interest of analyses in terms of physical indicators related to the self-affine nature of the fracture surfaces. Due to the comparable nanometric amplitude of the surface roughness, the AFM tip size and the instrumental noise, it seems interesting to discuss about potential experimental artefacts highlighted by Lechenault et al. The roughness amplitude of several oxide glasses was shown to decrease as a function of the stress intensity factor, to be quite insensitive to the relative humidity and to increase with the content of network modifiers . These results will be discussed in terms on several modellings concerning the coupling between crack propagation, material heterogeneity, crack tip plastic deformation and water diffusion at the crack tip taking into account the self-affine nature of the fracture surfaces.

Fracture propagation is inherently a multiscale problem, involving the coupling of many length scales from sample dimension to molecular level. Fracture mechanics provides a valuable link between the macroscopic scale of the structural loading of the samples and the scale of the process zone for brittle materials. Modeling the toughness of materials requires yet an investigation at scales smaller than this process zone, which is nanometric in oxide glasses and micrometric in polymer glasses.We present here the important insights that have been obtained through an in situ experimental investigation of the strain fields in the micrometric neighborhood of a propagating crack. We show the richness of atomic force microscopy combined with digital image correlation although it limits the observations to the external surface of the sample and to very slow crack propagation (below nm/s). For oxide glasses, this novel technique provided enlightening information on the nanoscale mechanisms of stress corrosion during subcritical crack propagation (Ciccotti, J Phys D Appl Phys 42:214006, 2009; Pallares et al., Corros Rev 33(6):501–514, 2015), including the relevance of crack tip plasticity (Han et al., EPL 89:66003, 2010), stress-induced ion exchange processes (Célarié et al., J Non-Cryst Solids 353:51–68, 2007), and capillary condensation in the crack tip cavity (Grimaldi et al., Phys Rev Lett 100:165505, 2008; Pallares et al., J Am Ceram Soc 94:2613–2618, 2011). An extension of this technique has recently been developed for glassy polymers (George et al., J Mech Phys Solids 112:109–125, 2018), leading to novel insights on the transition between crazing and shear yielding mechanisms and to promising new ways to link the toughness properties to the time-dependent large strain material properties of these nominally brittle materials.

Light transmission through wheat (T. aestivum L.) grain longitudinal cross sections of different thickness was used to study the endosperm microstructure and was shown to strictly follow a Beer-Lambert law allowing a non ambiguous quantification of the endosperm vitreousness. Therefore similar samples obtained from near-isogenic lines differing by hardness and grown in two distinct environments affecting their vitreousness were analyzed and confirmed the relationship between light transmission and the endosperm microstructure. In each sample, moreover analysis of light transmission within the different grain parts highlighted the greater compactness of the central endosperm cheeks in comparison with the distal and the proximal regions. These results helps a better understanding of the endosperm microstructure.