Orientation of polymer chains in spherulites of poly(ethylene oxide)-urea inclusion compounds

Abstract

Orientation of polymer chains in the spherulites of polymer-small molecule inclusion compounds (IC) is an interesting issue. Two kinds of spherulite morphologies were observed in the metastable β phase of poly(ethylene oxide) (PEO)/urea IC under the same isothermal crystallization temperature. Via polarized micro-Fourier transform infrared spectroscopy and two-dimensional wide angle X-ray diffraction technique, we found that PEO chains in the two morphologies oriented differently: one along the radius and the other along the tangential direction of the spherulites. The β IC spherulites with polymer chains along the radial direction crystallized directly from melt. In contrast, the metastable spherulites with polymer chain oriented along the tangential direction among the preformed urea crystals. The phase transition from the metastable β spherulites to the stable α IC during heating was examined as well. For the both types of spherulites of the β IC, the conversion rate along polymer chain direction was lower than that perpendicular to the polymer chain. After phase transition, the orientations of PEO chains almost did not change. The two types of morphologies with different orientations of β IC crystals in the spherulites can be attributed to the competitive nucleation behavior of the β PEO-urea IC and pure urea crystals, which varies with the EO/urea molar ratio in the mixture melt. With the increasing EO/urea molar ratio, the fastest growth direction of β IC will change from the direction along the PEO chain to that normal to the PEO chain. Namely, the polymer/small molecule molar ratio has profound effect on the growth behavior of the β phase IC crystals.