A unique morphology of freeze-dried poly(ethylene oxide) and its transformation

Freeze-dried poly(ethylene oxide) (PEO) samples were prepared from a 1 x 10 (-2) wt.% solution in benzene. After isothermal crystallization at 318.2 +/ - 0.1 K, a unique spherulite-like morphology with special inner structure w as observed by transmission electron microscope. The crystal structure was identified to be the triclinic form. The molecular chains in it adopt the p lanar zigzag conformation, which was only found in the sample under tension . Howe-ver, it was revealed in this paper that the triclinic crystals forme d in the freeze-drying process can exist at room temperature for a long per iod. After being heated at a temperature far above the equilibrium melting point of PEO and cooled down to 318.2 +/- 0.1 K, they transformed to the co mmon monoclinic form. The resulting crystals show a great variety of regula r polygonal shapes with multiple twin patterns. (C) 2000 Elsevier Science L td. All rights reserved.