Thickening-Induced Faceting Habit Change in Solution-Grown Poly(l-lactic acid) Crystals
Jrjeng Ruan
Hsing-Yuan Huang
Yi-Fang Huang
Chian Lin
Annette Thierry
Bernard Lotz
An-Chung Su
10.1021/ma902052a.s001
https://acs.figshare.com/articles/journal_contribution/Thickening_Induced_Faceting_Habit_Change_in_Solution_Grown_Poly_l_lactic_acid_Crystals/2786038
Morphological evolution of poly(l-lactic acid) (PLLA) crystals obtained via crystallization at <i>T</i><sub>c</sub> = 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 (<i>t</i><sub>i</sub> = 10 h at <i>T</i><sub>i</sub> = 90 °C), there emerged “<i>a</i> 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 <i>T</i><sub>c</sub>, 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 reeling-in 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 <i>T</i><sub>c</sub> = 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 <i>T</i><sub>c</sub>. Therefore, the {100} facets of truncated lozenge develop wider at higher <i>T</i><sub>c</sub>.
2010-03-09 00:00:00
PLLA
CrystalsMorphological evolution
incubation scheme
10 h
nucleation procedure
lenticular crystal
lozenge stage
transmission electron microscopy
faceting habit
rhombic lozenge crystals
Tc
rhombic lozenges
supercooling dependence
α form