Solid-State NMR Study of the Chain Trajectory and Crystallization Mechanism of Poly(l-lactic acid) in Dilute Solution

Abstract

The nucleation and growth mechanisms of semicrystalline polymers are a controversial topic in polymer science. In this work, we investigate the chain-folding pattern, packing structure, and crystal habits of poly(l-lactic acid) (PLLA) with a relatively low molecular weight, ⟨Mw⟩ = 46K g/mol, and PDI = 1.4 in single crystals formed from dilute amyl acetate (AA) solution (0.05 or 0.005 wt %) at a crystallization temperature (Tc) of 90, 50, or ∼0 °C. The crystal habits drastically changed from a facet lozenge shape at Tc = 90 °C to dendrites at ∼0 °C, whereas the chains adopt a thermodynamically stable α packing structure at both 90 and 0 °C. Comparing the experimental and simulated 13C–13C double quantum (DQ) buildup curves of 13C-labeled PLLA chains in crystals blended with nonlabeled chains at a mixing ratio of 1:9 indicates that the PLLA chains fold adjacently in multiple rows when the Tc ranges from 90 to ∼0 °C. The results at different length scales suggest that (i) a majority of the chains self-fold in dilute solution and form baby nuclei (intramolecular nucleation) and (ii) the intermolecular aggregation process (secondary nucleation), which is dominated by kinetics, results in morphological differences.