Co-Processing of [Fe(NH2trz)3](2ns)2 and UHMWPE into Materials Combining Spin Crossover and High Mechanical Strength

Abstract

The coordination polymer [Fe(NH2trz)3](2ns)2 (NH2trz = 4-amino-1,2,4-triazole and 2ns− = counterion 2-naphthalene sulfonate) exhibits the rare phenomenon of spin crossover in an attractive temperature range, i.e., somewhat above room temperature. Spin crossover in [Fe(NH2trz)3](2ns)2 is manifested by thermochromism, which is accompanied by a magnetic transition from diamagnetism to paramagnetism. However, [Fe(NH2trz)3](2ns)2 is brittle and difficult to process, which limits its use. In this study, we show that [Fe(NH2trz)3](2ns)2 can be co-processed with ultrahigh molecular weight polyethylene (UHMWPE), which possesses outstanding mechanical properties, particularly when tensile-drawn. Therefore, [Fe(NH2trz)3](2ns)2–UHMWPE blends were gel-processed by extrusion, employing a relatively poor solvent, which has recently been shown to offer advantages compared to good solvents. Uniform and flexible films, ribbons and fibers with [Fe(NH2trz)3](2ns)2 fractions as high as 33.3% m/m were obtained that could be readily drawn. Spin crossover in the coordination polymer is retained in these materials, as evident from their thermochromism. The tensile strength and Young’s modulus of the blends exceed those of typical commodity polymers. Thus, the films, ribbons and fibers constitute a special class of multifunctional materials that combine the flexibility and excellent mechanical properties of drawn UHMWPE with the spin crossover behavior of [Fe(NH2trz)3](2ns)2.