Novel sustainable, smart, and multifunctional 4D-printed nanocomposites with reprocessing and shape memory capabilities

The present paper explores the development of novel reprocessable nanocomposites with enhanced shape memory (SM) capability by Digital Light Processing 3D printing technology. A Covalent Adaptable Network was developed through a solvent-free strategy, based on polyurethane containing Diels Alder bonds, reinforced with carbon nanotubes (CNTs). The CNT addition allows for obtaining electrically conductive nanocomposites (up to 1.2 ± 0.7·10⁻¹ S m⁻¹). This enables Joule heating capability (average temperature over 100 °C by applying 100 V to the 0.3 wt.%CNT reinforced specimens), which was used as a heating method to trigger the SM cycle. First, the CNT content and thermal treatment were optimized to enhance the SM capabilities in a conventional oven (shape fixity ratio around 100%). Then, the SM capabilities triggered by Joule heating were characterized. Here, the optimized nanocomposites showed excellent shape fixity and recovery ratios (both above 95%). This heating method was proven to be low energy-consuming (approximately 1 W compared to around 750 W for a conventional oven), while also allowing for a fast, remote, and selective activation, which was demonstrated with a hand-like proof-of-concept by selectively recovering the permanent shape of each finger individually. On the other hand, the dynamic covalent bonds enable reprocessability. Here, the 3D-printed specimens were turned into powder and reprocessed using a powder processing tool to manufacture samples with a different geometry which were still electrically conductive, given the DA adduct formations. Results prove the multifunctional and smart capabilities of the developed nanocomposites, which make them suitable for applications such as soft robotics or actuators with an extended useful life, thus promoting sustainability.