Isothermal Thickening and Thinning Processes in Low Molecular Weight Poly(ethylene oxide) Fractions Crystallized from the Melt. 5. Effect of Chain Defects

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Abstract

Two-arm poly(ethylene oxide) (PEO) fractions with different molecular weights (MWs) have been prepared. For each fraction, both arms have equal lengths of MW = 2300 or 5500. The MWs and molecular weight distributions of two-arm PEOs after fractionation are determined from vapor pressure osmometry, gel permeation chromatography, and light scattering. Compared to linear PEO fractions with similar molecular lengths, the two-arm PEOs can be viewed as linear chains with a well-defined defect at the center of the molecule. Self-diffusion coefficients of the two-arm PEOs are measured and compared with linear PEOs having molecular lengths equivalent to both a single arm and a whole molecule. The crystallization behavior of the PEOs is monitored via wide angle X-ray diffraction, small angle X-ray scattering, and differential scanning calorimetry. Over a wide undercooling range, the two-arm PEO molecules do not appear to recognize the defects at the center of the chains during the initial stage of crystallization. During this stage of crystallization, they form nonintegral folding chain crystals having a fold length longer than the arm length. The defects are recognized only after the initial crystallization and gradually migrate to the lamellar surface through an apparent thinning process. The crystallization kinetics of the two-arm PEOs are significantly slower than those of the linear PEO molecules having a length equivalent to a single arm as well as a combined length of two arms. The melting behavior of the two-arm PEOs is, however, similar to that of linear PEOs which have a length of a single arm. The single lamellar crystal morphology of the two-arm PEOs observed via polarized light microscopy shows the faceting−rounding−refaceting effect with decreasing undercooling. Nevertheless, refaceted single crystals at very low undercoolings have a rectangular shape rather than the hexagonal one generally observed in the linear PEOs. This difference in the single crystal morphology may be caused by a change in the folding directions due to the large defects on the lamellar surfaces.

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