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, ⟨M w⟩ = 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 (T c) of 90, 50, or ∼0 °C. The crystal habits drastically changed from a facet lozenge shape at T c = 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 T c 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.

Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR

“…This fact indicates that the 13 C selective isotopic labeling approach is applicable to chain-folding analysis in both melt- and solution-grown crystals over a wide T c range. The same conclusions were obtained in i PP [53] and PLLA systems [58].…”

“…The important findings falsify the well-established LH theory [3,4], and point out importance of primary nucleation in polymer crystallization [6,17]. The folded chain structures accessible by ssNMR [52,53,55,56,57,58,62] are consistent with recent views on primary nucleation in amino acids [87], metal ions [88,89], and proteins [90], among others. Combining atomic scale information by ssNMR with direct observations of morphology, as well as visualization of chains by computations, will significantly contribute to understanding in the research field of crystallization.…”

“…The simulated DQ curve based on the shortest 13 C– 13 C interstem distance of 4.0 Å and relaxation value of T 2 = 8.3 ms can reproduce the experimental data (red curves in Figure 1e,f). Similar packing analysis was conducted on i PP [53,54] and PLLA crystals [58,63]. In all systems, the internuclear distances determined by NMR are consistent with those by WAXD and electron diffraction (ED) within 5% errors.…”

“…Very recently, Wang et al investigated morphology, chain-packing structure, and chain-folding patterns of 13 C 30% CH 3 labeled PLLA in solution-grown crystals at T c = 90, 50, and ~0 °C [58]. At the highest T c , lozenge shape single crystals with well-defined growth faces are obtained while the lowest T c induces dendrite morphology.…”

“…Recently, 13 C– 13 C double quantum (DQ) NMR spectroscopy has been applied to elucidate chain trajectory and packing structures for 13 C selectively isotopic enriched isotactic -Poly(1-butene) ( i PB1), isotactic -poly(propylene) ( i PP), and poly(L-lactic acid) (PLLA) in the melt- and solution-grown crystals [52,53,54,55,56,57,58,59,60,61,62,63]. 13 C– 13 C dipolar interaction is governed by internuclear distance, spin topology, and spin number.…”