Microcrystals of FeII(phen)2(NCS)2 (phen = 1,10-phenanthroline), isolated by solvent-induced precipitation, were dispersed in glassy or semicrystalline matrices and subjected to thermal or chemical treatments. Interactions occurring between crystals and their macromolecular (or molecular) surroundings were revealed by drastic alteration of the first-order spin-transition afforded by this material. Depending on matrices and experimental treatments, the cooperativity in particles dispersion can be dampened, resulting in a more gradual transition or enhanced, providing a large hysteresis. The hysteresis broadening was also demonstrated by the incorporation of another first-order transition molecular material (bulk) into similar matrices. The different features associated with the in-polymer dispersions are accounted for in the framework of a mechanoelastic model based on a Monte-Carlo standard procedure.
