STRUCTURES OF 2 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 A ̊°. 1 A ̊=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 A ̊, 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 (-t2gt2gt- g ̄-)2 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 (-tg2t g ̄2-) 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. © 1990.