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 α 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. α was reaching a value of 1, implying that no intersection point between the H–W line and the Tm = Tc line could be obtained. (Tm is the measured melting temperature and Tc 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 Tc > 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 Tc > 110 °C, secondary nuclei are proposed to be stabilized by coalescence of neighboring nuclei, all having a small width. By contrast, for Tc > 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.