In Situ Analysis of Local Strain Distribution of Amorphous Polyrotaxane Adhesives Constrained by Metal Substrates

Abstract

In situ structural analysis under loads of amorphous adhesives buried between metal substrates was conducted by using synchrotron X-ray nanobeam diffraction and an embedded local strain probe. Thermoplastic resins composed of polyrotaxanes, featuring bulky cyclic molecules threaded with a linear polymer, were sandwiched between stainless-steel substrates with diffraction data attributed to the correlation distance between their cyclic components being collected. The nanobeam was irradiated parallel to the substrates and scanned, while distances from the substrate were changed to obtain depth profiles of correlation distance. Then, tensile loads were applied stepwise to the sandwiched sample to reveal local strain distributions of the resins under different loads. Different strain distributions and their changes with loads were observed in resins with different adhesive strengths. The sample with strong adhesion showed pronounced strain localization near the interface between resin and substrate, whereas the weak sample displayed moderate strain further from the interface, with negligible strain near the interface. When the load increased, localized strain saturation occurred, followed by strain propagation into adjacent areas, indicative of a typical strain delocalization process associated with strain hardening near a highly constrained interface by substrates. This analysis elucidated the deformation and fracture processes of buried polymers, offering insight into those adhesive strength differences phenomenologically. This in situ analysis of local strain distribution contributes to comprehending adhesion mechanisms required for advancing multimaterialization technology and holds applicability to materials with buried interfaces such as nanocomposites.