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.
Highlights
In this study emphasis was put on substances with spin crossover occurring somewhat above room temperature, as such compounds can potentially be used as active components in temperature-initiated switchable devices [18,24,25,26,27]
A formulation with a mass ratio of 10:1 between ultrahigh molecular weight polyethylene (UHMWPE) and [Fe(NH2 trz)3 ](2ns)2 consists in proportions of 1 g UHMWPE, 0.1 g [Fe(NH2 trz)3 ](2ns)2 and 9 g stearic acid.] The required amounts of UHMWPE, [Fe(NH2 trz)3 ](2ns)2 and stearic acid were added to a 25 mL round-bottomed flask
Homogeneous ribbons of [Fe(NH2 trz)3 ](2ns)2 –UHMWPE blends were prepared by extrusion of the two components in stearic acid at 160 ◦ C, i.e., well above the melting temperature of the acid (71 ◦ C [41])
Summary
In the case of iron(II), a d6 ion, spin crossover originates in a change of the low spin state with S = 0 to the high spin state with S = 2 upon increase in temperature and vice versa. In this study emphasis was put on substances with spin crossover occurring somewhat above room temperature, as such compounds can potentially be used as active components in temperature-initiated switchable devices [18,24,25,26,27] This is the case, e.g., for X = 2-naphthalene sulfonate (2ns), which has been studied extensively in solution and in the solid state [4,10,11,12]
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