Impact of Entanglement on Folding of Semicrystalline Polymer during Crystallization

Upon cooling, semicrystalline polymers experience crystallizationand form alternatively stacked layers consisting of thin crystal lamellaeand amorphous ones. The unique morphology, crystallinity, and crystallizationkinetics highly depend on the molecular weight. Therefore, it is deducedthat entanglement impacts crystallization kinetics, as well as hierarchicallycrystalline structures. However, the impact of entanglement on foldedcrystalline chains has not been well understood due to experimentaldifficulties. In this work, chain-folding structures for seven C-13 CH3 labeled poly(l-lactic acid)s withvarious molecular weights (M (w)s) were investigatedby C-13-C-13 double quantum NMR spectroscopy.As a result, chain-folding events were categorized into three different M (w) regimes: (i) The lowest M (w) sample (2K g/mol) adopts an extended chain conformation(folding number, n = 0) (regime I); (ii) Intermediate M (w) ones possess mixtures of non- and once-foldedstructures, and the once-folded fraction suddenly increases abovethe entanglement length (M (e)), up to M (w) = 45K g/mol (regime II); (iii) The high M (w) ones (M (w) >45Kg/mol) adopt the highest chance for an adjacent re-entry structurewith n = 1.0 in the well-developed entangled network(regime III). It was suggested that entanglement induces folding ofthe semicrystalline polymer.