Structural Unit of Polymer Crystallization in Dilute Solution As Studied by Solid-State NMR and 13C Isotope Labeling

Abstract

The nucleation and growth mechanisms of semicrystalline polymers during crystallization in dilute solutions have been debated in the past decades. Wang et al. recently reported that 13C CH3 labeled poly(l-lactic acid) (PLLA), with small weight-average molecular weight ⟨Mw⟩ = 46K g/mol (s), adopted three-dimensional nanoclusters via adjacent re-entry folding in the solution-grown crystals, and its mean size was independent of crystallization temperature (Tc) [Macromolecules 2017, 50, 6404–6414]. In this work, 13C CH3 labeled PLLA chains with middle and large ⟨Mw⟩ of 90K g/mol (m) and 326 K g/mol (l), respectively, were synthesized. Both ⟨Mw⟩ and kinetics effects on the chain-folding structures and crystal habits of the 13C CH3 labeled PLLA were systematically investigated by solid-state (ss) NMR and atomic force microscopy (AFM). s-, m-, and l-PLLAs commonly adopted faceted lozenge single crystals in dilute amyl acetate (AA) solution (0.05 wt %) under a very low supercooling, ΔT = 40 °C, but dendrites with multiple layers under a rapidly quenched condition, ΔT = ∼130 °C. By comparison of the experimental and simulated 13C–13C DQ buildup curves, it was indicated that (i) all s-, m-, l-PLLA adopted the same mean structure of the three-dimensional nanoclusters in multiple rows via adjacent re-entry structure in the lozenge shape and dendrite crystals and (ii) the mean size of the nanocluster was smaller than the nanocluster size expected from the smallest ⟨Mw⟩ used in this study. On the basis of the crystal habits as well as dimension and size of the nanoclusters, it was suggested that (i) the PLLA chains with different ⟨Mw⟩s initially adopted the same nanoclusters via folding (stage I) and (ii) the aggregation process of the nanoclusters, which was dominated by kinetics, resultantly led to the different morphological features as a function of ΔT (stage II).