Characterization of the Slow Molecular Dynamics of Poly(l‐Lactic Acid) in α and α′ Phases, in a Glassy State, and in a Complex with Poly(d‐Lactic Acid) by Solid‐State NMR

Abstract

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.

Insertion‐Crystallization‐Induced Low‐Temperature Annealing Peaks in Melt‐Crystallized Poly(l‐Lactic Acid)

“…[ 57 ] The formation of RAF, probably requires fixation of the polymer chains in the crystal and is therefore only observed at temperatures where the PLLA crystals are in the fixed state. [ 58 ] This is in agreement with the observation that for PLLA crystallized at high temperature, significant RAF is detected at T g , thus being formed during slow cooling. Possible reasons previously discussed are low‐temperature vitrification of rigid amorphous structure developed during high‐temperature crystallization, in line with the above said, or nondetectable crystallization during cooling connected with RAF formation.…”

Bulk Enthalpy of Melting of Poly (l‐lactic acid) (PLLA) Determined by Fast Scanning Chip Calorimetry

“…The dynamics of these defects in PLLA was previously characterized. [ 65 ] Specifically, the increase of the lamellar thickness is also indicative for the absence of a glassy amorphous layer at the fold‐surface of the lamellae, that is, absence of a vitrified RAF at the crystallization temperature, as in such case longitudinal crystal growth is assumed restricted. The increase of the crystal thickness during isothermal annealing at the temperature of primary crystallization is confirmed in an independent study, reporting values of the lamellar thickness of 13.9 and 16 nm after melt‐crystallization of PLLA at 140 °C for 90 and 600 min, respectively, [ 66 ] suggesting lamellar thickening as a valid secondary‐crystallization process for PLLA.…”

Poly(glycolic acid) Nanofibers via Sea‐Island Melt‐Spinning

“…Following a small exothermic peak emerges a major endothermic peak at ≈176 °C corresponding to T m of PLLA. The small exothermic peak was associated with the transition of α ′‐ to α‐crystals with PLLA . Finally, an endothermic peak becomes visible at ≈216 °C with the high temperature region enlarged, corresponding to T m of PGA …”