Stabilization of Nuclei of Lamellar Polymer Crystals: Insights from a Comparison of the Hoffman–Weeks Line with the Crystallization Line

We have studied melting of poly­(butylene succinate), isothermally crystallized over a wide temperature range, employing a combination of the Hoffman–Weeks plot and the Gibbs–Thomson crystallization line, determined by small-angle X-ray scattering measurements. A change in the slope <i><b>α</b></i> of the Hoffman–Weeks (H–W) line, accompanied by a change of the slope of the crystallization line, was observed for crystallization temperatures higher than 110 °C. <i><b>α</b></i> was reaching a value of 1, implying that no intersection point between the H–W line and the <i>T</i>_<i>m</i> = <i>T</i>_<i>c</i> line could be obtained. (<i>T</i>_<i>m</i> is the measured melting temperature and <i>T</i>_<i>c</i> is the temperature at which the sample was crystallized). This observation was corroborated by the crystallization line, which was found to be parallel to the melting line for <i>T</i>_<i>c</i> > 110 °C. We relate these changes in slope to different stabilization mechanisms of the secondary nuclei at the growth front of polymer lamellar crystals. For <i>T</i>_<i>c</i> > 110 °C, secondary nuclei are proposed to be stabilized by coalescence of neighboring nuclei, all having a small width. By contrast, for <i>T</i>_<i>c</i> > 110 °C, the number density of secondary nuclei is low and thus their coalescence is rare. Accordingly, nuclei are stabilized by growing in size, mainly increasing their width