Thickening-induced faceting habit change in solution-grown poly(L-lactic acid) crystals

Morphological evolution of poly(L-lactic acid) (PLLA) crystals obtained via crystallization at T_c = 85 to 97 °C from 0.1% solutions in mixed xylene was closely followed via transmission electron microscopy. Upon stirring-enhanced nucleation after the adopted incubation scheme (t_i = 10 h at T_i 90 °C), there emerged "a axis" lenticular crystals of the imperfectly packed α form, as indicated by the presence of the presumably forbidden (010) spot in the selected-area electron diffraction along the [001] zone. Subsequently observed during further growth was a gradual change from lenticular to truncated lozenge with {100} and {110} facets, driven by the outward propagation of the thickened interior (truncated lozenge in shape) within the lenticular crystal. The width of the {100} facets clearly decreased with decreasing T_c, resulting in rhombic lozenges with (110} edges at 87 °C. This change in faceting habit is interpreted in terms of the competition between primary growth (controlled by surface-nucleation) and interior thickening (controlled by the reelingin of chains from the primary thin rim via slip diffusion on the {110} surfaces). Eliminating the stirring-enhanced nucleation procedure, we have observed direct transition from lenticular to rhombic lozenge crystals at T_c = 85 °C without interior thickening, that is, bypassing entirely the truncated lozenge stage. It is hence proposed that the lamellar thickening is intrinsically a nucleation-and-growth process, initiated by the (presumably thick) nucleus from stirring, followed by the reeling-in process of stems in the rim. The supercooling dependence of the reeling-in rate is anisotropic and decreases most significantly along [100] directions with increasing T_c. Therefore, the {100} facets of truncated lozenge develop wider at higher T_c.