Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

We investigated the interplay between the thermomechanical and spin-crossover (SCO) properties of a series of stimuli-responsive polymer composites consisting of [Fe(NH2trz)3]SO4 and [Fe(Htrz)(trz)2]BF4 particles (trz = 1,2,4-triazolato) embedded in thermoplastic polyurethane, TPU, and poly(vinylidene fluoride–trifluoroethylene), P(VDF-TrFE), matrices. The effective thermoelastic coefficients and transformation stress and strain in the composites were assessed utilizing dynamical mechanical analysis (DMA), differential scanning calorimetry (DSC), thermal expansion, and thermal stress measurements. Remarkably, the composites display a characteristic elastic softening and increased mechanical damping around the spin transition temperature, which arise from the significant spin state–volume strain coupling in the particles and scale semiquantitatively with the pressure derivative of the low spin fraction (∂n∂P)T. Crucially, for a given particle volume fraction, the transformation strain (respectively stress) substantially increases (respectively decreases) in soft matrices, which was rationalized by micromechanical simulations. The results provide a fundamental understanding and a quantitative guideline for the design of SCO@polymer composites for applications in mechanical transducers.