M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

4 8 H I G H L I G H T S 2 0 2 3 I

Structure and dynamics evolution of magnetic nanoparticles during healing of thermoplastic rubbers

Repairing plastics is one of the most promising routes to reducing their impact on the environment. A convenient way to achieve this goal consists of making them responsive to an oscillatory magnetic field, enabling their selective melting through induction heating. In-situ X-ray measurements provide a clear picture of the complex mechanisms that occur during the operation.

Healing materials is at the very heart of current societal issues regarding sustainable development. In particular, improving the lifetime of plastics and rubbers is one of the key challenges to drastically reduce their impact on the environment. While self-healing of soft materials has been a hot topic for the past 15 years, its applications at an industrial level are still limited because of the relatively weak properties and low chemical stability of the corresponding polymers. On the other hand, contactless stimulus-healing involving microwaves, laser or magnetic radiations can trigger phase transitions of structural (stronger) polymers, opening the way to a broad range of applications.

Following this idea, in this work responsive nanocomposites based on a semi-crystalline thermoplastic polyurethane (TPU) matrix and iron (Fe) nanoparticles were produced and investigated from structural and dynamical points of view in the presence of a high-frequency magnetic field, making it possible to first heal damaged materials, and then post-treat the rough surface of 3D printed parts (Figure 31) [1]. The temperature, structure and dynamics of the nanoparticles was monitored in real- time, respectively through infrared (IR) imaging, small- angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) at beamline ID02, making it possible to clarify the non-trivial thermal trajectory [T(t)] experienced by the samples under induction heating (see setup in Figure 32a). In fact, while a simple exponential rise function was expected ( pizza-in-oven scenario), the temperature measurements emphasised the presence

of a second heating mechanism appearing at the melting point of the TPU, followed by a maximum a few minutes later (see green-yellow and yellow-red transitions in Figure 32b).

The time-resolved experiments revealed that beyond the first increase of the temperature, generated by magnetic hysteresis losses of the Fe nanoparticles (green zone in Figure 32b), the second heating source originated from

Fig. 31: a) Thermal imaging photograph taken during dumbbell specimen healing illustrated in (b).

c) Smoothing post-treatment of 3D-printed parts.