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Enhancement of the durability in fatigue of semi-crystalline polymers by strengthening their molecular interactions.

Didier Long

Mercredi 10 décembre 2014

à 11h en salle C. Brot

N. G . Hussein, E. Mourglia-Seignobos1, D.R. Long1 ,L.-A. Fillot, L. Odoni1, P.Sotta1, L. Vanel1, C. Rochas2

1 : LPMA, UMR CNRS/Rhodia 5268, CRTL, 85 rue des frères Perret, BP62, 69192 SAINT-FONS Cedex, France

2 : CERMAV-CNRS, BP 53, 38041 Grenoble Cedex 9, France

We have studied recently fatigue mechanisms in PA 6,6 by a combination of mechanical experiments, X-ray scattering (SAXS and USAXS), electron microscopy and density measurements. X-ray experiments indicate that the damage is initiated between crystalline lamellae perpendicular to the direction of the fatigue stress. In a first stage, damaging corresponds to uncorrelated nucleations of essentially spherical cavities with sizes distributed between 10 and 100 nm. The number of these cavities increases progressively. At the end of this stage, the volume fraction of the damaged regions in the samples is of order 1%. Nucleation of cavities appear thus as the elementary mechanism which sets the time scale for damaging in the material. Based on this analysis, we have enhanced the cohesive energy of polyamide by the introduction of auto-associative functions, notably phenolic functions offering strong H-bonds interactions. We characterized our modified samples using various techniques. Crystallinity was characterized using DSC and X-ray experiments. The molecular weights of the modified polymers were measured by GPC. The chemical modifications resulted in an increase of the glass transition temperature of the modified samples and an increase of their surface tension. We compared the life-time in fatigue of the new samples to those of the un-modified PA samples at the same level of stress to modulus ratio. Some unwanted changes (e.g. too large decrease of crystalline ratio) resulted in a decrease of the mechanical properties. In some instances however, the life-time of the modified systems was increased by a factor 3 as compared to that of the non-modified samples. The effects of the different changes (crystalline structure and ratios, molecular weight, polydispersity,…) will be discussed. Our work may thus open the way to a better control of the durability in fatigue of polymer materials.