Structures of two crystal forms of poly(ethylene oxide)-sodium thiocyanate complex with molar ratios of 3:1 and 1:1

The poly(ethylene oxide) (PEO)-sodium thiocyanate system was found to exhibit polymorphism: there exist at least three crystal modifications. Among them, the crystal structures of two kinds of PEO-NaSCN complex with molar ratios (EO:NaSCN) of 3:1 (form I) and 1:1 (form II), respectively, were determined by X-ray diffraction. Crystal data are as follows: form I, monoclinic P2₁/a, a = 16.83, b = 10.64, c(chain axis)=7.19 Å^°

1 Å=10⁻¹ nm, γ=125.5° (c-unique), N=12 EO units (2 chains) and 4 NaSCN ions; form II, monoclinic P2₁/c, a=7.55, b=12.10, c(chain axis)=5.83 Å, β=97.5° (b-unique), N=4 EO units (2 chains) and 4 NaSCN ions. Form I has a crystal structure resembling that of the PEO-NaI complex. The polymer chains have a twofold helical structure of conformation, the chain repeat comprising six EO units. The helical polymer chain coils around an array of Na ions and each Na ion is coordinated by four polymer O atoms and two N of the SCN ions (the coordination number is six). In form II, which exists only under high tension, the polymer chains have a glide structure of conformation, the chain repeat comprising two EO units. Since the PEO chain in form II takes a rather stretched conformation, the Na ions are not wrapped by the polymer chain. The coordination number is again six, but each Na ion is coordinated by two polymer O atoms, two N and two S of the SCN ions. Form II is transformed into form I when the tension is released.