Solution electrospinning was used for the first time to prepare nanofibers of the stable (α) and metastable (β) complexes between poly(ethylene oxide) (PEO) and urea. Both types of fibers were highly crystalline and presented a large level of molecular orientation. Detailed characterization of the ill‐studied β complex was performed using wide angle X‐ray diffraction (WAXD), infrared spectroscopy, and differential scanning calorimetry (DSC). Results reveal that it possesses a 3:2 PEO:urea stoichiometry and suggest that it belongs to the orthorhombic system with a = 1.907 nm, b = 0.862 nm, and c = 0.773 nm. The PEO chains are oriented along the fiber axis and present a conformation significantly affected by strong hydrogen bonding with urea when compared with the pure polymer and the stable complex. A layered structure model is suggested for the metastable complex, in which the urea molecules would be arranged into a ribbon‐like structure intercalated between two PEO layers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1903–1913, 2008

Electrospinning was used for the first time to prepare nanofibers of the host/guest complex between poly(ethylene oxide) (PEO) and thiourea. It is shown by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) that the stoichiometry of the complex is (EO)(12)-(thiourea)(8), settling a series of conflicting values in literature reports. The complex crystallizes in a monoclinic unit cell with a = 9.15 A, b = 18.88 A, c = 8.25 A, and beta = 92.35 degrees. On the basis of WAXD, infrared spectroscopy, and polarized Raman scattering measurements, it is proposed that the complex adopts a layered structure in which alternating PEO and thiourea layers are stabilized by intermolecular hydrogen bonds. This structure is highly reminiscent of that of the beta complex between PEO and urea. A phase diagram was determined and shows that the complex melts incongruently at 110 degrees C to form a peritectic liquid and crystals of pure thiourea. The nanofibers of the PEO-thiourea present a very large molecular orientation with a (c) value of 0.76, among the largest reported for electrospun materials.

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We demonstrate that electrospinning is a simple and efficient method of preparing pure polymer complexes. Such complexes are often difficult to prepare by conventional methods because of the kinetically favored crystallization of small molecules. As proof-of-concept examples, nanofibers of the complexes of poly(ethylene oxide) with hydroquinone and sodium thiocyanate (NaSCN) were produced. Infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry were used to confirm the purity of the electrospun complexes and to establish the general applicability of this new strategy. It is concluded that the fast solvent evaporation is the critical factor enabling pure complex formation. This work should enable a better study of host/guest polymer complexes, for instance in the context of ionic conduction in the solid state.

Solution electrospinning was used to prepare fibers of both the stable – trigonal α and “metastable” – orthorhombic β complexes between poly(ethylene oxide) (PEO) and urea. The 300-800 nm fibers were highly crystalline and both types presented a relatively large level of molecular orientation. Characterization of the poorly-studied β complex was performed using wide-angle X-ray diffraction, infrared spectroscopy, optical microscopy and differential scanning calorimetry. It was shown that β complex possesses a 3:2 PEO:urea stoichiometry, in contrast with a previously suggested 1:1 molar ratio, and that the α inclusion complex keeps the 4:9 molar ratio as when prepared by the conventional co-crystallization method. A new structural model was suggested for the β complex, in which the unit cell would contain 12 PEO repeat units (4 chains in the ab plane with 3 repeat units along the c axis) and 8 urea molecules arranged in a ribbon-like structure and intercalated between the two PEO layers. This layer-structured β complex is quite different from the usual channel-like α inclusion complex and results in different phase transitional properties.