CRYSTALLINE-STRUCTURE OF THE POLY(ETHYLENE OXIDE) P-NITROPHENOL COMPLEX .2. FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

We have previously reported the existence of a crystalline compound made of poly(ethylene oxide) (PEO) and p-nitrophenol (pnp). From X-ray fiber patterns on stretched and spherulitic samples of this complex, a triclinic unit cell (a = 1.172 nm, b = 0.555 nm, c = 1.557 nm, alpha = 90.7-degrees, beta = 87.1-degrees, and gamma = 104.0-degrees) was deduced; the unit cell contains 6 PEO monomeric units and 4 pnp molecules, in agreement with the stoichiometry deduced from the phase diagram. The aim of this paper is to elucidate the conformation of the PEO chains and the mutual arrangement of the PEO and pnp molecules in the unit cell. To carry out this, we studied the dichroism of the IR bands of pnp in two differently oriented samples, namely, in stretched samples and in spherulites. The benzene rings are found to be perpendicular to the c crystallographic parameter (chain axis), and the 1-4 axis of pnp is found to be parallel to the a* reciprocal parameter. These observations completely determine the orientation of the pnp molecules in the unit cell. The conformation of the polymeric chains in the complex is not helical as in pure PEO. From the C2h factor group of the PEO molecules, deduced from the FTIR observations, and the normal mode analysis of hydrogenated and deuterated PEO, we propose the (t2gt2gt3t2g't2g't3) glide type conformation. In conclusion, it appears that in the PEO-pnp complex a stack of pnp molecules stabilizes the surrounding PEO molecules in this new conformation.