Stem Tilt in the Contact Plane of Epitaxially Grown Polylactide Lamellae and Its Direct Correlation with Lamellar Bending

Via transmission electron microscopy, it was revealed that epitaxial crystallization on hexamethylbenzene substrate led to oriented arrays of individual long-chain poly(L-lactide) edge-on lamellae. Selected-area electron diffraction patterns indicated orientational deviations of packed stems from lamellar normal, i.e., stem tilt on the bc contact plane as a result of stairlike stem packing. Upon growth beyond edges of underneath substrates, correlated bending of lamellae in a counter-clockwise manner was developed. The direct relationship between the tilted stem orientation and the bending sense of lamellar growth supports the notion of more severe accumulation of fold surface stresses around acute corners of growth fronts created by the stairlike packing scheme. Selected molecular weights and growth temperatures caused the changes in the magnitude of stem tilt, further manifesting the role of fold-loop repulsion on lamellar basal planes in creating surface stresses. In contrast, the type of chiral center on polylactide backbone appears to have played an important role in tilted orientation of stem packing during lamellar growth.