ABSTRACT

By means of synchrotron-based and simultaneous small/wide-angle X-ray scattering and differential scanning calorimetry, we adopted our previously developed method to determine the equilibrium melting temperature (Tmº) and the fold surface energy (σe) of poly(L-lactic acid)(PLLA).With gradually increased temperature, the crystalline lamellae approached solid-melt equilibrium for reliable construction of the Gibbs-Thomson (GT) melting line. In addition, the
lamellar thicknesses were more confidently determined by the arrayed-disks model fitting instead of the previous method of one-dimensional correlation function or Kratky-Porod approximation. From the GT line thus determined, we have obtained Tmº ≈ 199 ºC and σe ≈ 46 mJ m−2. Comparison of the present Tmº and σe values with those previously reported using different approaches are critically
made in terms of consistency between the experimental design and inherent assumptions within each theoretical model. We proposed that the fold surface energy depends on intricacies in the fold structure, which are kinetically determined, generally path-dependent, and subjected to
reorganization before reaching the equilibrium melting line. This explains the wide-range of reported σe values for a given polymer from nucleation, growth, or melting studies and the poor correlation to chain rigidity among different polymers.

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