The bulk enthalpy of melting of 𝜶-crystals of poly (L-lactic acid) (PLLA) is evaluated by fast scanning calorimetry (FSC), by correlating the melting enthalpy of samples of different crystallinity with the corresponding heat capacity at 90 °C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140 °C towards the heat capacity of fully solid PLLA yields a value of 104.5±6 J g −1 when melting occurs at 180-200 °C. The analysis relies on a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis the solid fraction (90 °C). Formation and vitrification of an RAF are suppressed by avoiding continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140 °C to 90 °C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other.

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Based on a limited set of model samples it has been recently shown that semicrystalline polymers exhibit different morphological characteristics depending on the existence and timescale of intracrystalline chain diffusion (ICD) relative to the kinetics of crystal growth. Here, the generality of these reports for the case of so-called crystal-fixed polymers without or very slow ICD is tested by providing a detailed nuclear magnetic resonance (NMR) and small-angle X-ray scattering analysis of polybutylene succinate (PBS), a biodegradable polyester. By using a combination of NMR techniques covering different timescales it is shown that there is no intracrystalline dynamics on a time scale faster than 1 s, that is, PBS is crystal-fixed. An expected crystallinity below 50% and the typical morphology consisting of lamellar crystals with a well-defined crystal thickness and a broad thickness distribution of the amorphous interlayers are confirmed. By combining these results with differential scanning calorimetry measurements, a more precise value could be provided for the enthalpy of melting than previously available. The mechanical properties at room temperature are furthermore influenced by additional insertion crystallization taking place during cooling, which leads to an increase of the mechanical modulus by a factor of ≈2.5 as compared to the state at the end of isothermal crystallization.

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