STRUCTURE AND CONFORMATION OF POLY(ETHYLENE OXIDE), PEO, IN THE TRIGONAL FORM OF THE PEO UREA COMPLEX AT 173-K

The formation of addition complexes between poly(ethylene oxide) (PEO) and urea has been investigated for PEO of various molecular weights. Scanning electron micrographs show the crystals to be made up of "pseudohexagonal" prisms. These in turn consist in stacks of thick prisms that become platelets as MBAR(w) increases. The evolution of the unit-cell dimensions of these adducts as a function of the molecular weight is reported. A single crystal of the PEO-urea complex was obtained, and its crystal structure established by using standard X-ray diffraction techniques with intensity data measured at 173 K. The crystal belongs to the trigonal system, P3(1)21 space group, and has a unit cell of dimensions a = b = 10.540 (2) angstrom, c = 9.094 (2) angstrom, gamma = 120-degrees. The crystal structure was established by direct methods. The refinement of the atomic coordinates and the temperature factors converged to the value of 0.052 for the agreement index R. The stoichiometry of the adduct is (-CH2CH2O-)4.(urea)9. In this crystal, one finds that the urea molecules from pseudohexagonal channels in a honeycomblike pattern. The walls of the channels are built by the hydrogen-bonded urea molecules (2/3). The channels are filled with the poly(ethylene oxide) chains and extra urea molecules (1/3). The extra urea molecules within the channels are hydrogen bonded to the poly(ethylene oxide) chain as well as to the channel urea molecules. The poly(ethylene oxide) chain in a given channel may have one of six possible orientations. In the asymmetric unit, poly(ethylene oxide) has the (ttg)n conformation. It can also be described as having an approximate 4(1) helix. This helix is slightly more squashed than it is in pure PEO. There the advance per chemical unit measured along the chain axis is 2.770 angstrom, whereas in the PEO-urea complex the advance per chemical unit is only 2.313 angstrom.