Effects of solution status on single-crystal growth habit of poly(l-lactide)

Crystal nucleation in a quiescent 0.3 wt % poly(l-lactide) (PLLA)/p-xylene solution is sluggish at 90 °C. Instead of straightforward crystallization, there is a sigmoid decrease in solution viscosity, reflecting reduced hydrodynamic volume (V_H), and therefore the collapse of dissolved PLLA chains during this incubation stage. With enhanced nucleation via gentle stirring, kinetically favored "a-axis" lenticular crystals start to emerge; the acute apex angle of these lenticular crystals decreases with the collapse of PLLA coils, indicating an altered preference in the step deposition of molecular stems on the growth front. In subsequent growth of transformed truncated lozenge crystals, the decreased V_H results in suppressed crystal growth and a higher L₁₀₀/L₁₁₀ ratio between (100) and (110) facet widths, implying decreased driving force toward crystallization or increased barriers of surface nucleation, both are consistent with the notion of collapsed chains of lowered free energy via preordering in the solution state. We conclude that these preordered chains in the solution state are responsible for (1) easier nucleation upon shear, (2) the formation of lenticular crystals with modified growth habit, and (3) significantly suppressed surface nucleation in the later development of truncated lozenge crystals. The preordered coils are more responsive to stirring-induced nucleation yet of suppressed surface nucleation at the crystal growth front; this bears relevance and provides better insights to the precursor concept recently proposed in the literature.