4D spin crossover–polymer composite materials were printed using a stereolithographic method.

Spin crossover particles dispersed in a piezo/ferroelectric poly(vinylidene fluoride-co-trifluoro-ethylene), P(VDF–TrFE), matrix give rise to inspiring mechano-electric phenomena, with possible applications for energy harvesting and sensing. Particles of different chemical compositions, morphologies and concentrations were loaded into copolymers with different TrFE molar contents to investigate the effect of these parameters on the mechano-electric coupling between the polymer and the spin crossover material. The samples were characterized by elemental analysis, powder X-ray diffraction, Raman spectroscopy, optical reflectivity, differential scanning calorimetry, and electron microscopy and were studied also for their piezo/pyroelectric properties. We show that it is possible to tune simultaneously the spin transition temperature of the particles and the Curie temperature of the matrix in such a way that the piezoelectric effect from the spin transition and the pyroelectric response of the polymer can be concomitantly observed. This result provides prospects for the development of smart, multifunctional electroactive polymer composites and for increasing their thermal–electrical harvester output.

A series of spin-crossover (SCO) coordination nanoparticles (ca. 60 nm) with the general formulae [Fe(Htrz)_1+y−x(trz)_2−y(NH₂trz)_x](BF₄)_y·nH₂O (x = 0, 0.1, 0.2 and 0.3) were synthesized in concentrated solutions without using any surfactant or polymer. The nanoparticle powders were investigated by transmission electron microscopy, powder X-ray diffraction, magnetometry, calorimetry, Raman/IR spectroscopies, elemental analysis and ⁵⁷Fe Mössbauer spectrometry. Remarkably, the latter revealed a large decrease of the lattice stiffness when incorporating a small amount of amino-triazole ligand, reflected by the drop of the Debye temperature from 285 K (x = 0) to 205 K (x = 0.3). This collapse of the lattice cohesion was attributed to a reorganization of the supramolecular interactions between the Fe-triazole chains. This effect on the SCO properties is also discussed.

Nature suggests concepts for materials with efficient mechanical energy storage and release, i.e., resilience, involving small energy dissipation upon mechanical loading and unloading, such as in resilin and elastin. These materials facilitate burst-like movements involving high stiffness and low strain and high reversibility. Synthetic hydrogels that allow highly reversible mechanical energy storage have remained a challenge, despite mimicking biological soft tissues. Here we show a synthetic concept using fixed hydrogel polymer compositions based on sequentially swollen and sequentially photopolymerized gelation steps for hierarchical networks. The sequential swellings facilitate the balance of properties between resilience and dissipation upon controlling of the chain extension. At low hierarchical levels, we show resilience with small hysteresis with increased stiffness and resilient energy storage, whereas at high hierarchical levels, a transition is shown to a dissipative and considerably reinforced state. The generality of this approach is shown using several photopolymerizable monomers.