Both an experimental and a theoretical investigation of the fracture propagation mechanisms acting at the process zone scale in glassy polymers are presented. The main aim is to establish a common modeling for different kinds of glassy polymers presenting either steady-state fracture propagation or stick-slip fracture propagation or both, depending on loading conditions and sample shape. On the experimental point of view, new insights are provided by in-situ AFM measurements of the viscoplastic strain fields acting within the micrometric process zone in a brittle epoxy resin, which highlight an extremely slow unexpected steadystate regime with finite plastic strains of about 30% around a blunt crack tip, and accompanied by propagating shear lips. On the theoretical point of view, we apply to glassy polymers some recently developed models for describing soft dissipative fracture that are pertinent with the observed finite strains. We propose a unified modeling of the fracture energy for both the steady-state and stick-slip fracture propagation based on the evaluation of the energy dissipation density at a characteristic strain rate induced in the process zone by the competition between the crack propagation velocity and the macroscopic sample loading rate.

The thin film of life that inhabits all plastics in the oceans, so-called “plastisphere,” has multiple effects on the fate and impacts of plastic in the marine environment. Here, we aimed to evaluate the relative influence of the plastic size, shape, chemical composition, and environmental changes such as a phytoplankton bloom in shaping the plastisphere abundance, diversity and activity. Polyethylene (PE) and polylactide acid (PLA) together with glass controls in the forms of meso-debris (18 mm diameter) and large-microplastics (LMP; 3 mm diameter), as well as small-microplastics (SMP) of 100 μm diameter with spherical or irregular shapes were immerged in seawater during 2 months. Results of bacterial abundance (confocal microscopy) and diversity (16S rRNA Illumina sequencing) indicated that the three classical biofilm colonization phases (primo-colonization after 3 days; growing phase after 10 days; maturation phase after 30 days) were not influenced by the size and the shape of the materials, even when a diatom bloom (Pseudo-nitzschia sp.) occurred after the first month of incubation. However, plastic size and shape had an effect on bacterial activity (3H leucine incorporation). Bacterial communities associated with the material of 100 μm size fraction showed the highest activity compared to all other material sizes. A mature biofilm developed within 30 days on all material types, with higher bacterial abundance on the plastics compared to glass, and distinct bacterial assemblages were detected on each material type. The diatom bloom event had a great impact on the plastisphere of all materials, resulting in a drastic change in diversity and activity. Our results showed that the plastic size and shape had relatively low influence on the plastisphere abundance, diversity, and activity, as compared to the plastic composition or the presence of a phytoplankton bloom.

Plastic pollution has become a worldwide concern. It was demonstrated that plastic breaks down to nanoscale particles in the environment, forming so-called nanoplastics. It is important to understand their ecological impact, but their structure is not elucidated. In this original work, we characterize the microstructure of oceanic polyethylene debris and compare them to the nonweathered objects. Cross-sections are analysed by several emergent mapping techniques. We highlight deep modifications of the debris within a layer a few hundred microns thick. The most 2 intense modifications are macromolecule oxidation and a considerable decrease in the molecular weight. The adsorption of organic pollutants and trace metals is also confined to this outer layer. Fragmentation of the oxidized layer of the plastic debris is the most likely source of nanoplastics. Consequently nanoplastic chemical nature differ greatly from plastics.

We analyze stress distributions in a two-dimensional bidisperse cemented granular packing for a broad range of the values of particle-size ratio, the volumes of large and small particles, and the amount of cementing matrix. In such textured porous materials, the stress concentration, which controls the fracture and fragmentation of the material under tensile loading or in grinding processes, reflects not only the porosity but also the contact network of the particle phase and the resulting stress chains. By means of peridynamic simulations under tensile loading, we show how both the texture and stress distribution depend on size ratio, volume ratio, and the amount of the cementing matrix. In particular, the volume fraction of the class of small particles plays a key role in homogenizing stresses across the system by reducing porosity. Interestingly, the texture controls not only the porosity but also the distribution of pores inside the system with its statistical variability, found to be strongly correlated with the homogeneity of stresses inside the large particles. The most homogeneous stress distribution occurs for the largest size ratio and largest volume fraction of small particles, corresponding to the lowest pore size dispersion and the cushioning effect of small particles and its similar role to the binding matrix for stress redistribution across the packing. At higher porosity, the tensile stresses above the mean stress fall off exponentially in all phases with an exponent that strongly depends on the texture. The exponential part broadens with decreasing matrix volume fraction and particle-size ratio. These correlations reveal the strong interplay between size polydispersity and the cohesive action of the binding matrix for stress distribution, which is significant for the behavior of textured materials in grinding operations.