Study of the properties and mechanical behavior laws of polyesters at different scales using in situ WAXS and Raman spectroscopy

The study of the thermal behavior of both polymers was carried out using differential scanning calorimetry (DSC) while simultaneously performing Raman acquisitions. The analysis of certain spectral zones made it possible to precisely determine the crystallinity ratio. These results were then compared to those obtained by DSC and WAXS. In addition, these analyses made it possible to monitor the conformational changes of the polymers during thermal cycles, particularly those associated with microstructural transitions, such as the glass transition, crystallization, and melting. The Raman crystallinity criteria obtained were subsequently applied during tensile tests. By correlating their evolution with the macromolecular orientation, it was possible to detect and quantify the mesophases. These latter constitute an essential intermediate state between the amorphous and crystalline states of polymers during the process of strain-induced crystallization, and their presence directly influences the mechanical behavior of the polymer.

Through the comprehensive WAXS and Raman data obtained in situ during a uniaxial tensile test, the micromechanisms of deformation were identified and measured in real-time with local strain and stress, and the contribution of each phase of the polymer (amorphous, crystalline) was decoupled considering the role of tie molecules and finally correlated with the macroscopic behavior. Then, two protocols identified and found in the literature will be applied. The first is based on 2D WAXS patterns and leads to the strain state in the crystalline phase. The second is based on Grüneisen's theory and uses Raman spectra. Starting from the measurements of the shift in position of particular scattering bands, it provides access to the strain tensor in both the amorphous and crystalline phases of the polymer.

PhD defended by Mouad Bouita