Crystallization of Ultrathin Films of Polylactides:  From Chain Chirality to Lamella Curvature and Twisting

The crystallization of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) in ultrathin films (15 nm) has been followed between 125 and 160 °C using in situ atomic force microscopy. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which is related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behavior was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions, without any external nucleation. As shown by electron diffraction patterns, the crystalline unit cells of the two enantiomers are identical. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behavior. Moreover, the direction of curvature of the lamellae can be linked with the sense of twisting of the poly(lactide) lamellae in banded spherulites, and the temperature dependence of the radius of curvature can be correlated with the distance between the extinction rings. Those observations are consistent with Keith and Padden's model since the curved crystals in ultrathin films can be considered as “half-lamellae”, which give, when associated together, twisted complete lamellae.