Search

Upon cooling, semicrystalline polymers experience crystallization and form alternatively stacked layers consisting of thin crystal lamellae and amorphous ones. The unique morphology, crystallinity, and crystallization kinetics highly depend on the molecular weight. Therefore, it is deduced that entanglement impacts crystallization kinetics, as well as hierarchically crystalline structures. However, the impact of entanglement on folded crystalline chains has not been well understood due to experimental difficulties. In this work, chain-folding structures for seven 13 C CH 3 labeled poly(l-lactic acid)s with various molecular weights ( M w s) were investigated by 13 C- 13 C 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-folded structures, and the once-folded fraction suddenly increases above the entanglement length ( M e ), up to M w = 45K g/mol (regime II); (iii) The high M w ones ( M w > 45K g/mol) adopt the highest chance for an adjacent re-entry structure with n = 1.0 in the well-developed entangled network (regime III). It was suggested that entanglement induces folding of the semicrystalline polymer.

Semicrystalline polymers are categorized as either mobile or fixed crystals, depending on chain mobility in the crystalline region. In this work, we investigate molecular dynamics and phase structure in the cocrystal consisting of fixed and mobile polymer crystals by solid-state (ss) nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that (i) the mobile component begins large amplitude motions associated with crystal-crystal transition, while fixed ones keep their rigidity in the cocrystal, and (ii) asymmetric molecular dynamics leads to nanosegregations into mobile- and fixed-rich domains in the cocrystal below the melting temperature ( T m ). The observed phase separation induced by asymmetric molecular dynamics is similar to the phase separation of the miscible amorphous polymer blend; however, it is limited to two dimensions due to the parallel packing of the stems inside the cocrystal, as well as chain connectivity at the crystalline-amorphous boundary.