La pollution des océans par les déchets plastiques est devenue un problème environnemental majeur résultant de son accumulation dans les environnements terrestre et marin. Lorsqu'ils sont mal gérés, les plastiques pénètrent dans le milieu aquatique où ils subissent une dégradation et une fragmentation en microplastiques désormais omniprésents dans tous les milieux aquatiques (Law, 2017). Outre le fait qu'il est impossible d'éliminer les microplastiques du milieu marin, leur impact sur l’environnement est plus important. Diverses études ont montré que de nombreux types d'organismes marins ingéraient des microplastiques, ce qui entraînait des effets néfastes à plusieurs niveaux de la chaîne alimentaire et des écosystèmes marins (Rochman et al., 2016 ; Chae et al., 2017). On soupçonne également que les microplastiques, qui constituent un nouvel habitat pour les micro-organismes, sont des vecteurs de bactéries potentiellement pathogènes (Kirstein et al.,2016 ; Dussud et al.,2018).Le devenir des polymères dans le milieu aquatique dépend à la fois de phénomènes abiotiques (UV, stress mécanique) et biotiques, dus à la colonisation du plastique par des micro-organismes marins (bactéries, phytoplancton, champignons, etc.). Une des principales étapes de la biodégradation est la constitution d'un biofilm et la réduction de la longueur des chaînes de polymère via des exo-enzymes produites par des bactéries issues du biofilm. Une fois que les chaînes de polymère sont suffisamment courtes, elles peuvent être assimilées par les bactéries (Ennouri et al., 2017). Alors que les phénomènes abiotiques entraînent l’endommagement et la fragmentation d’un polymère par des mécanismes d’oxydation et d’hydrolyse, la création de défauts structurels et la propagation de fractures, il est généralement admis que seuls les phénomènes biotiques conduiront à la biodégradation complète d’un polymère, c’est-à-dire à sa conversion en biomasse, eau et CO2. En milieu marin, de nombreuses questions demeurent quant à la cinétique relative de la dégradation abiotique et biotique et à leur impact respectif en termes de fragmentation (Shah et al., 2008). Par exemple, plusieurs articles (Ter Halle et al., 2016 ; Cozar et al., 2018) ont récemment rapporté que la distribution en taille des particules collectées dans l'océan entre 5 mm et quelques centaines de microns ne semble pas correspondre à un processus de fragmentation monocinétique.Les profils d'érosion des polymères semi-cristallins ont fait l'objet d'études approfondies en laboratoire dans des conditions enzymatiques ou bactériennes et divers profils de dégradation ont été observés, leur apparition est principalement liée à la différence de cinétique d'érosion entre les régions cristallines et amorphes (Morse et al., 2011 ; Martinez-Tobon et al., 2018). À ce jour, il y a beaucoup moins d'études sur la manière dont l'évolution des patterns de surface influencera à son tour le processus d'érosion, et pourra conduire à la fracture ou à la génération de fragments.Afin d'étudier le processus d'érosion enzymatique, nous avons utilisé le système bien connu PDLLA / protéinase K (Yamashita et al., 2005). Etant particulièrement intéressés par le rôle des hétérogénéités à l’échelle de quelques nanomètres à quelques micromètres, nous avons utilisé un polymère de composition chimique donnée (PDLLA, 1,7% de D-mer, Mn = 95 kg / mol, indice de polydispersité I = 1,63) et de morphologie contrôlée par traitement thermique.

Pollution of the ocean by plastic litter has become a major environmental problem : when mismanaged, plastics enter the 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 macroscopic waste.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 micro-organisms. 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.We studied the enzymatic erosion process in semi-crystalline polymersto understand the potential fracture and fragments generation in relation to the formation of erosion patterns . 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 and monitored its morphology through its change in crystallinity ratio, everything else remaining constant. We used the well-known model system PDLLA/proteinase KEnzymatic erosion kinetics were measured through weight loss experiments and 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 and unexpectedly predicts that after a lag time, the final erosion rate will be the one of the fastest eroding phase. 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 results.

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 macroscopic waste.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. 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.In order toWe studiedy the enzymatic erosion process in semi-crystalline polymersand to understand the potential fracture and fragments generation in relation to the formation of erosion patterns in semi-crystalline polymers, 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 and monitored its morphology through its change in crystallinity ratio, everything else remaining constant. We used the well-known model system PDLLA/proteinase KEnzymatic erosion kinetics were measured through weight loss experiments and 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 and unexpectedly predicts that after a lag time, the final erosion rate will be the one of the fastest eroding phase. 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 resultsexperiments.

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.

Hard and soft proteins at fluid interfaces

We have investigated the wetting and surface diffusion of mesoporous colloidal silica particles at the water surface and the adsorption of cationic cetyltrimethylammonium (CTA+) surfactant on these particles. Porous silica colloids diffuse at the surface of water and in the volume, interacting with cationic surfactants that can adsorb inside the pores of the particles. We observed that surfactant adsorption on mesoporous silica depends dramatically not only on the particle pore size but also on specific counterion effects. We measured striking differences both on a macroscopic property of the interface, i.e., surface tension, and also at a single particle level by evaluating the translational diffusion of partially wetted particles at the fluid interface. We varied the pore size from 2 to 7 nm and explored the effects of ions possessing different hydration number and kosmotropic/chaotropic character. At concentrations lower than the critical micellar concentration, we evidence that cationic surfactants adsorb on silica as surface micelles and surfactant adsorption inside the pores occurs only if the pore diameter is larger than the size of surface micelles. With a view to understand the surprising different adsorption behavior of CTA+OH– and CTA+Br– on porous silica particles, we investigated the effect of counterions on the surfactant adsorption on porous silica colloids by tuning the pH and the counterion properties.

Heliconal nematic twist bend (NTB) phases are usually not well oriented in planar commercial cells, even in thin (~1 µm) ones where a micron-sized periodic texture is usually observed [1,2,3] (see Fig.1-a). The origin of the stripe patterns is attributed to the rapid shrinking of the pseudo-layers below the N-NTB phase transition temperature, which yields an undulation instability. The observed periodicity is found to be twice the cell gap [1].We have explored the effect of the strength of the anchorings of the substrates on the resulting patterns. We used polymer aligning layers of tunable azimuthal and zenithal strengths to design liquid crystal cells with surface anchorings ranging from ultraweak to strong. We then have studied the patterns formed by the NTB phase of 1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB). We will show how the period of the stripes can be varied in a large range in cells of the same thickness but with different anchoring preparations. With adequate anchoring conditions (see Fig.1-b-c), an excellent and spontaneous planar alignment of the NTB phase of CB7CB can be obtained much below the phase transition temperature.
References:
[1] Panov V.P. et al, Phys. Rev. Lett., 2010, 105; 167801.
[2] Challa P. K. et al, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2014, 89, 060501.
[3] Vaupotič N. et al., Chem. Phys. Chem., 2018, 19,2566.