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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 differentEavalues of 273 ± 12 and 16 ± 14 kJ mol−1at 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 approximatelyTg.
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Jump-precursor state emerges below the crossover temperature in supercooled o -terphenyl
In a supercooled liquid, the crossover temperature Tc separates a high-temperature region of diffusive dynamics from a low-temperature region of activated dynamics. A molecular-dynamics simulation of all-atom, flexible o-terphenyl [J. Phys. Chem. B 117, 12898 (2013)] is analyzed with advanced statistical methods to reveal the molecular features associated with this crossover. The simulations extend to an α-relaxation time of 14 μs (272.5 K), two-orders of magnitude slower than at Tc (290 K). At Tc and below, a distinct state emerges that immediately precedes an orientational jump. Compared to the initial, tightly caged state, this jump-precursor state has a looser cage, with solid-angular excursions of 0.054–0.0125×4π sr. At Tc (290 K), rate heterogeneity is already the dominant cause of stretched relaxation. Exchange within the distribution of rates is faster than α-relaxation at Tc, but becomes equal to it at the lowest temperature simulated (272.5 K). The results trend toward a recent experimental observation near the glass transition (243 K) [Phys. Rev. E 98, 040603(R) (2018)], which saw exchange substantially slower than α-relaxation. Overall, the dynamic crossover comprises multiple phenomena: the development of heterogeneity, an increasing jump size, an emerging jump-precursor state, and a lengthening exchange time. The crossover is neither sharp, nor a simple superposition of the high- and low-temperature regimes; it is a broad region that contains unique and complex phenomena.
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- PAR ID:
- 10254016
- Publisher / Repository:
- journals.aps.org
- Date Published:
- Journal Name:
- Physical review E
- Volume:
- 103
- Issue:
- 5
- ISSN:
- 2470-0053
- Format(s):
- Medium: X Size: 1.3MB Other: pdf/A
- Size(s):
- 1.3MB
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1103/PhysRevE.103.L050601
