Particle size-dependent viscoelastic Fe(ii)-triazole composites in the low-spin state

Abstract

A series of soft spin crossover (SCO) [Fe(Htrz)₂(trz)](BF₄)/NaPSS composites have been prepared by dispersing rod-shaped particles of different lengths (32 nm, 65 nm, 474 nm and 1.2 μm) in a concentrated sodium polystyrene sulfonate (NaPSS) aqueous solution. The magnetic investigation revealed hysteretic behaviour and significant widening of the hysteresis loop for all particle sizes, compared to the pristine particles, resulting essentially from the shift of the HS-to-LS transition temperature (T^↓_1/2) towards lower temperatures, attributed to the negative pressure effect exerted by the NaPSS polymer on the HS-to-LS transition of the particles. The rheological study of the composites, performed in the linear viscoelastic domain over a large time scale, revealed a critical gel like behaviour, indicative of scale-invariant relaxation behaviour. The strength of the gel increases as the particle size decreases or as the concentration increases, without changing the structural parameters, making it possible to tune the viscoelastic properties of the composites.