Estimation of the Size of Critical Secondary Nuclei of Melt-Grown Poly(l-lactide) Lamellar Crystals

Recently, we proposed a simple method to derive the size of critical secondary nuclei formed on the growth front of lamellar crystals via the nucleation kinetics of random copolymers and polymer blends. [Zhang, S.; et al. Macromolecules, 2019, 52, 7439–7447] The present study focuses on revealing the size of the critical secondary nucleus for poly(l-lactide) (PLLA) α-form spherulites crystallized from the melt. The number of crystalline stems within a critical secondary nucleus was estimated at different crystallization temperatures using a series of random copolymers. The number of polymer chains involved in a critical secondary nucleus was determined by introducing poly(d,l-lactide) (PDLLA) as a diluent in crystalline/amorphous miscible blends. At crystallization temperatures ranging from 120 to 140 °C, a critical secondary nucleus of a PLLA α-form crystal contained around 32 to 52 l-lactide units. From the final lamellar thickness, the lower limit of the number of stems in a critical nucleus was estimated to be 1.6–2.1. Considering the lamellar thickening coefficient, the upper limit of the number of stems in a nucleus ranged from 4.1 to 5.4. On average, the stems within a critical secondary nucleus were contributed by about 1.9 different polymer chains at 120 °C, indicating the existence of intermolecular nucleation.