The (010)–(120) crystal growth face transformation in poly(ethylene oxide) spherulites

doi:10.1016/S0032-3861(97)00554-5

Polymer

Volume 39, Issues 6–7, 1998, Pages 1405-1414

https://doi.org/10.1016/S0032-3861(97)00554-5 Get rights and content

Abstract

The crystallization and melting of poly(ethylene oxide) (PEO) have been examined in detail using polarized light microscopy (PLM), polarized infrared microspectroscopy (PIRM) and differential scanning calorimetry (DSC). Examination of the orientation of the crystalline stems within the spherulites by PIRM supports the hypothesis that the dominant crystal growth face of PEO changes from the (010) crystallographic plane at crystallization temperatures (T_c) > 51°C to the (120) plane at T_c > 51°C. The cusp in the logarithmic growth rate—temperature plot appears to be the result of this phenomenon. Analysis of spherulitic growth rate data for the monodisperse sample (M_w = 1.8 × 10⁵) fails to provide evidence in support of the regime II/III transition proposed in a previous literature study and indicates that in the range 45–56°C crystallization occurs solely within regime III. The larger value for the product of the lateral and fold surface free energies in the case of (010) growth appears to arise from the higher fold surface free energ' of this growth face. The difference in the fold surface free energies of the different growth faces, 41 erg/cm² for the (010) face as compared with 29 erg/cm² for the (120) face, can be accounted for by the corresponding differences in the minimum chain length required for chain folding in the case of adjacent re-entry.

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As can be seen from the above, there is a lack of experiments showing how crystallization of partially entangled PEO occur. However, there is a lot information about the crystallization of fully entangled PEO [27–33], which can be useful for studies of the crystallization of disentangled PEO. Because PEO can be synthesized in a wide range of molecular masses, from 103 to 106 g/mol, with polydispersity between 1.2 and 2.0, the crystallization depends on this mass and dispersion.
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This study investigated the structure evolution of poly (ethylene oxide) (PEO) samples sheared at different temperatures above the equilibrium melting temperature followed by cooling to an isothermal temperature of 50 °C. We found that the higher shear temperature tended to decrease the number of entanglements to generate a larger network deformation, which was characterized by the deviation angle. It improved the orientation of chains and noticeably accelerated the crystallization process, however the final values of the long period, lamellae thickness and amorphous thickness remained almost similar. Based on the statistical theory of rubber elasticity, we proved that numerous entanglements, which were characterized by the longest relaxation time or the zero shear viscosity, indeed prevented the extension of chains when the concentration of the long chains was much higher than the long chain overlap concentration.
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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.
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Herein, we demonstrated that poly(ethylene oxide) (PEO), urea and thiourea can crystallize into novel ternary complex with the molar ratio of guest polymer and host small molecule as 3:2, and proved that the ternary complex behaves isomorphism phenomenon by varying the ratio between urea and thiourea for the first time. This observation gives a boost to prepare co-crystals of different small molecules that cannot be obtained by direct mixing without the aid of polymer chains.
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The PEO spherulite can be conveniently observed using a polarized light microscopy (PLM) [27,28]. So far, the crystal structure of POM, as well as PEO, has been studied in detail [2–20,23–28]. However, the crystalline/crystalline blends of POM/PEO were rarely reported.
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^†: Present address: Max Planck Institute for Polymer Research, Ackermannweg 10, Postfach 3148, D-55021 Mainz, Germany.

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Polymer paperThe (010)–(120) crystal growth face transformation in poly(ethylene oxide) spherulites

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The (010)–(120) crystal growth face transformation in poly(ethylene oxide) spherulites