Investigation of water diffusion mechanisms in relation to polymer relaxations in polyamides

et al.Diffusion in semicrystalline polymers is a complex phenomenon because of the existence of specific interactions (nonpolar or polar), dynamic heterogeneities, and crystalline phases. The diffusion of water in two semicrystalline polyamides (PA6,6 and PA6,10) was investigated in order to determine the diffusion mechanisms and the influence of polymer relaxations on this process. Liquid water diffusion follows a Fickian mechanism in PA6,10 and a non-Fickian or anomalous mechanism in PA6,6. Through a quasi-equilibrium experiment in dynamic vapor sorption, it is shown that this difference results from the dependence of the diffusion coefficients on water concentration. Moreover, the influence of the polymer relaxations was assessed by broadband dielectric spectroscopy. The dielectric characteristic relaxation times of the α relaxation, associated with the glass transition, and of the β relaxation, related to more local dynamics, have been measured. A simple comparison with the time scale of diffusion suggests that diffusion and polyamide α relaxation should not be directly correlated. However, diffusion is correlated to the secondary β relaxation, which encompasses the local chain dynamics of hydrogen-bonded amide groups in the presence of water. A mechanism of diffusion based on the trapping of water molecules between neighboring sorption sites (amide groups) is proposed in these strongly interacting polymers. It is suggested that diffusion is limited by the relaxation time of hydrogen bonds between water molecules and amide groups and the change in conformation of these amide groups present in polyamides.Funded by the European Soft Matter Infrastructure (ESMI) program (grant no. 262348).Peer Reviewe

Connectivity of the Hexagonal, Cubic, and Isotropic Phases of the C12EO6/H2O Lyotropic Mixture Investigated by Tracer Diffusion and X-ray Scattering

The connectivity of the hydrophobic medium in the nonionic binary system C12EO6/H2O is studied by monitoring the diffusion constants of tracer molecules at the transition between the hexagonal mesophase and the fluid isotropic phase. The increase in the transverse diffusion coefficient on approaching the isotropic phase reveals the proliferation of bridgelike defects connecting the surfactant cylinders. This suggests that the isotropic phase has a highly connected structure. Indeed, we find similar diffusion coefficients in the isotropic and cubic bicontinuous phases. The temperature dependence of the lattice parameter in the hexagonal phase confirms the change in connectivity close to the hexagonal-isotropic transition. Finally, an X-ray investigation of the isotropic phase shows that its structure is locally similar to that of the hexagonal phase

Dynamique moléculaire et plasticité d'un polymère amorphe sous traction

L'amélioration des performances des matériaux polymères passent notamment par une meilleure compression du lien entre dynamiques moléculaires à l'échelle microscopique et propriétés mécaniques macroscopiques. Dans ce but, la mobilité des chaînes polymères a été sondée par une mesure de spectroscopie diélectrique lors de la déformation plastique d'un polymère vitreux

A chain mechanism for flagellum growth.

Bacteria swim by means of long flagella extending from the cell surface. These are assembled from thousands of protein subunits translocated across the cell membrane by an export machinery at the base of each flagellum. Unfolded subunits then transit through a narrow channel at the core of the growing flagellum to the tip, where they crystallize into the nascent structure. As the flagellum lengthens outside the cell, the rate of flagellum growth does not change. The mystery is how subunit transit is maintained at a constant rate without a discernible energy source in the channel of the external flagellum. We present evidence for a simple physical mechanism for flagellum growth that harnesses the entropic force of the unfolded subunits themselves. We show that a subunit docked at the export machinery can be captured by a free subunit through head-to-tail linkage of juxtaposed amino (N)- and carboxy (C)-terminal helices. We propose that sequential rounds of linkage would generate a multisubunit chain that pulls successive subunits into and through the channel to the flagellum tip, and by isolating filaments growing on bacterial cells we reveal the predicted chain of head-to-tail linked subunits in the transit channel of flagella. Thermodynamic analysis confirms that links in the subunit chain can withstand the pulling force generated by rounds of subunit crystallization at the flagellum tip, and polymer theory predicts that as the N terminus of each unfolded subunit crystallizes, the entropic force at the subunit C terminus would increase, rapidly overcoming the threshold required to pull the next subunit from the export machinery. This pulling force would adjust automatically over the increasing length of the growing flagellum, maintaining a constant rate of subunit delivery to the tip

Etude par resonance magnetique nucleaire du deuterium de l'ordre orientationnel local dans un reseau polymere sous contrainte uniaxiale

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Facettage des phases cubiques lyotropes. Elastomères étudiés par RMN. Transition vitreuse dans les polymères en volume et en films minces. Fluage dans un système élastique désordonné

My research activity has been devoted to the physical properties of crosslinked polymers, confined polymer chains (block copolymers), reinforced elastomers. I have been primarily using an NMR approach to measure the elastic energy stored at the scale of polymer chains, using NMR concepts which I strongly contributed to develop. On the other hand, I have developed Monte Carlo numerical simulations to correlate the NMR behaviour to chain statistical properties in elastomers and other confined polymer systems. I also worked on various aspects of the physis of liquid crystals: the relationship between structure and molecular dynamics, directional growth, phase transitions, faceting of cubic phases. I was the first author to observe and interpret 3D faceting in lyotropic cubic giant monocrystals. I actually take a prominent part in the projects and development of the polymer group in Laboratoire de Physique des Solides. Within this context, I am developing a research activity devoted to glass transition in polymers in the bulk and in thin films and to the physical properties of reinforced elastomers : non linear effects, plasticity, fatigue. I am developing both experimental (NMR, mechanical measurements, small angle scattering) and numerical (percolation in 2D systems in relation to glass transition in thin films, simulations of reinforced elastomers by dissipative particle dynamics) activities. My projects are on the ultimate properties of nanostructured polymer systems, both in the experimental and numerical sides.Mon travail de recherche a été consacré aux propriétés physiques des polymères réticulés, des chaînes polymères confinées (copolymères blocs) et des élastomères chargés. J'ai utilisé principalement la RMN pour étudier l'énergie élastique stockée à l'échelle des chaînes polymères, en utilisant des concepts RMN que j'ai largement contribué à développer. D'autre part, j'ai développé des simulations numériques de Monte Carlo pour relier les mesures RMN aux propriétés statistiques des chaînes dans les élastomères et autres systèmes polymères confinés. Je me suis aussi intéressé à différents aspects de la physique des cristaux liquides : liens entre la dynamique locale et la structure, changements de phases, croissance directionnelle, facettage tridimensionnel des phases cubiques, que j'ai été le premier à mettre en évidence. Je prends actuellement une part essentielle dans le développement et les projets du « pôle polymères » du Laboratoire de Physique des Solides. Dans ce cadre, mes activités de recherche portent sur la transition vitreuse dans les polymères en volume et en films minces et sur les propriétés physiques des élastomères renforcés : effets non linéaires, plasticité, fatigue. Mes activité sont à la fois expérimentales (RMN, diffusion aux petits angles, mesures mécaniques) et numériques (percolation dans les systèmes bidimensionnels en lien avec la transition vitreuse en film mince, simulations des élastomères renforcés par dynamique particulaire dissipative). Mes projets concernent les propriétés ultimes des systèmes polymériques nanostructurés