Intramolecular and Intermolecular Packing in Polymer Crystallization

Recently, the chain-folding structure of ¹³C-labeled poly­(l-lactic acid) (PLLA) with different molecular weights (M) of 46K (small, s), 90K (middle, m), and 320K g/mol (large, l) in the solution-grown crystals [Macromolecules 2018, 51, 8729–8737] was studied by solid-state nuclear magnetic resonance (ssNMR). In this work, the chain-folding and packing structures, long period, and morphology of ¹³C-labeled s-, m-, and l-PLLA in the melt-grown crystals crystallized at T_c = 90 and 150 °C are investigated by ssNMR, small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM), respectively. All the s-, m-, and l-PLLA adopt thermodynamically stable α crystals and large spherulites with a diameter of ∼2 mm at T_c = 150 °C. A large supercooling induces kinetically favored disordered α′ crystals and nodular structures for all three samples. By comparing the experimental ¹³C–¹³C double quantum (DQ) buildup data with the simulated ones, we found that all three PLLA samples with different M_ws adopt adjacent re-entry structure with the same mean value of successive adjacent re-entry number, n, of 1.5–2 in both α and α′ crystals under the assumption of full adjacent re-entry, F = 100%. Through systematic studies on the chain-folding structure as functions of T_c and M_w, it is concluded that the chain-folding process is a local event and is independent of the overall crystallization process. On the basis of the chain-folding and packing structures in both solution- and melt-grown crystals, contributions of the intramolecular and intermolecular packing to the order–disorder phenomenon of the overall packing structure will be discussed.