Coupling Mechanical and Electrical Properties in Spin Crossover Polymer Composites.

Spin crossover particles of formula [Fe{(Htrz)2 (trz)}0.9 (NH2 -trz)0.3 ](BF4 )1.1 and average size of 20 nm ± 8 nm are homogeneously dispersed in poly(vinylidene fluoride-co-trifluoro-ethylene), P(VDF-TrFE), and poly(vinylidene fluoride) (PVDF) matrices to form macroscopic (cm-scale), freestanding, and flexible nanocomposite materials. The composites exhibit concomitant thermal expansion and discharge current peaks on cycling around the spin transition temperatures, i.e., new "product properties" resulting from the synergy between the particles and the matrix. Poling the P(VDF-TrFE) (70-30 mol%) samples loaded with 25 wt% of particles in 18 MV m-1 electric field results in a piezoelectric coefficient d33 = -3.3 pC N-1 . The poled samples display substantially amplified discharges and altered spin transition properties. Analysis of mechanical and dielectric properties reveals that both strain (1%) and permittivity (40%) changes in the composite accompany the spin transition in the particles, giving direct evidence for strong electromechanical couplings between the components. These results provide a novel route for the deployment of molecular spin crossover materials as actuators in artificial muscles and generators in thermal energy harvesting devices.