PhD defense: Benjamin Poggi

Title:

Mechanofluorochromic polymers as mechanical stress probes

Resume:

Among smart materials, mechanochromic (MC) or mechanofluorochromic (MFC) ones respond to mechanical stimuli by changes in their UV-Visible absorption or fluorescence emission properties respectively. Thus, the possibility to follow fluorescence modifications remotely, in real time, and by common and non-invasive spectroscopic measurements makes them good candidates for mechanical stress probes. However, these materials are most often qualitatively studied, and the lack of quantification of their MC or MFC response limits the growth of applications. Therefore, this thesis work focused not only on the development of new MC or MFC materials but also on the design of experimental setups capable of applying a controlled mechanical stress while following the emission modifications. A comparative study conducted on powders of MFC compounds evidenced their higher sensitivity to shearing stress than to pure compression. Difluoroboron β-diketonates complexes substituted by an alkyl side chain of adjustable length were then dispersed into polymer matrices. The application of controlled traction and friction forces to the resulting composites, as well as their observation by microscopy, showed that the MFC response intensity depends on the size of the fluorophore aggregates dispersed in the polymer and thus on the alkyl chain length. Additionally, it was established that the fluorescence signal detected depends on the nature of the applied stress. Finally, naphthopyrans mechanophores were synthesized and functionalized in order to incorporate them covalently into polyurethane and silicon elastomers. Their photochromic properties have been studied both in solution and in polymer. The influence of the latter on the MC properties have been evidenced: qualitatively, a more or less pronounced color change is observed upon traction and/or friction depending on the nature of the polymer.

This work is available online.