Recently, slow molecular dynamics of poly(l‐lactic acid) (PLLA) by using 1D and 2D exchange NMR are investigated. In this work, slow molecular dynamics of PLLA chains in the α′, a stereocomplex (SC) with poly(d‐lactic acid), and glassy states are investigated in terms of centerband‐only detection of exchange (CODEX) NMR. The mixing‐time dependence of the CODEX data demonstrates that the molecular dynamics of stems become slower in the order of α′, α, and SC. The temperature dependence of the correlation time 〈τc〉 of the helical jump motions in the α and SC phases simply exhibits Arrhenius behaviors, with activation energy, Ea, values of 91 ± 1 and 97 ± 1 kJ mol−1, respectively. In contrast, the temperature dependence of 〈τc〉 in the α′ sample exhibits two Arrhenius lines with substantially different Ea values of 273 ± 12 and 16 ± 14 kJ mol−1 at temperatures below and above 84 °C. The obtained kinetics of molecular dynamics not only establish the relationship between packing structure and dynamics in PLLA polymorphs and in the SC, but also allow for an understanding of the coupled dynamics between the crystalline and amorphous regions at approximately Tg.
Slow molecular dynamics of poly(l‐lactic acid) chains in the α′, a stereocomplex with poly(d‐lactic acid), and glassy states are investigated in terms of solid‐state nuclear magnetic resonance (NMR) spectroscopy.