Fate of Magnetic Nanoparticles during Stimulated Healing of Thermoplastic Elastomers

We have investigated the heating mechanism in industriallyrelevant,multi-block copolymers filled with Fe nanoparticles and subjectedto an oscillatory magnetic field that enables polymer healing in acontactless manner. While this procedure aims to extend the lifetimeof a wide range of thermoplastic polymers, repeated or prolonged stimulushealing is likely to modify their structure, mechanics, and abilityto heat, which must therefore be characterized in depth. In particular,our work sheds light on the physical origin of the secondary heatingmechanism detected in soft systems subjected to magnetic hyperthermiaand triggered by copolymer chain dissociation. In spite of earlierobservations, the origin of this additional heating remained unclear.By using both static and dynamic X-ray scattering methods (small-angleX-ray scattering and X-ray photon correlation spectroscopy, respectively),we demonstrate that beyond magnetic hysteresis losses, the enormousdrop of viscosity at the polymer melting temperature enables motionof nanoparticles that generates additional heat through friction.Additionally, we show that applying induction heating for a few minutesis found to magnetize the nanoparticles, which causes them to alignin dipolar chains and leads to nonmonotonic translational dynamics.By extrapolating these observations to rotational dynamics and thecorresponding amount of heat generated through friction, we not onlyclarify the origin of the secondary heating mechanism but also rationalizethe presence of a possible temperature maximum observed during inductionheating.