Simultaneous SAXS-WAXS Experiments on Semi-Crystalline Polymers: Example of PA11 and Its Brill Transition

This manuscript of the special issue Microstructural Evolution and Mechanical Behavior of Semi-Crystalline Polymers aims to show that Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) experiments performed simultaneously constitute a unique tool to obtain valuable information on the hierarchical structure of semi-crystalline polymers. These structural quantitative data are needed to model macroscopic properties of polymeric materials, for example their mechanical properties. To illustrate our point, we focus our study on the structure and morphology of polyamide 11. Through a simultaneous SAXS-WAXS experiment, we show that the absence of enthalpic signal in Differential Scanning Calorimetry (DSC) is not synonymous with the absence of structural and morphological evolution with temperature. The case of a thermally activated crystal-crystal transition, the Brill transition, is particularly detailed. Through this SAXS-WAXS study, we show, among other points, and for the first time, that the periodicity of crystalline lamellae (L-P) changes at the transition, probably due to a modification of the amorphous phase's free volume at the Brill transition. We also explain the crucial role of annealing to stabilize polymeric materials that may experience temperature changes over their lifetime. The influence of the annealing on the perfection of crystalline structure, morphology and mechanical behavior is more particularly studied.