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Chain Conformation, Molecular Dynamics, and Thermal Properties of Poly(n‑methylene 2,5-furanoates) as a Function of Methylene Unit Sequence Length

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journal contribution
posted on 22.08.2019, 17:36 by George Papamokos, Theodoros Dimitriadis, Dimitrios N. Bikiaris, George Z. Papageorgiou, George Floudas
Poly­(n-methylene 2,5-furanoates) is a family of biobased polymers with outstanding gas barrier and mechanical properties and with the potential to frame the future in certain applications (e.g., food packaging, fibers, and engineering thermoplastics). Herein, we used combined efforts by density functional theory calculations and experiments to explore in detail the conformational properties, the thermodynamics, and the molecular dynamics in the poly­(n-methylene 2,5-furanoate) series as a function of n in the range from 2 poly­(ethylene furanoate) (PEF) to 12 poly­(dodecylene furanoate). The computational study employed the conformers suggested earlier [Macromolecules 2018, 51, 3515–3526] but used additional functionals and investigated, in addition to the monomer and trimer, the PEF nonamer with respect to conformations pertinent to the amorphous state. Depending on the conformer, variable dipole moments were obtained in the range from 2.1 to 6.1, 3.0 to 8.2, and 1.8 to 7.1 debye, respectively, for the monomer, the trimer, and the nonamer. Strikingly, both the trimer and more importantly, the nonamer exhibited very compact helical structures stabilized by π–π interactions of the furan rings. We suggest that the helical motifs within the amorphous state contribute to the barrier improvement for carbon dioxide in PEF as compared to PET. The distinct structural motifs of poly­(n-methylene 2,5-furanoate)­s exerted an influence on the sub-Tg and the segmental dynamics (average relaxation times and distribution of relaxation times, fragility, and dielectric strength). The segmental process shows Vogel–Fulcher–Tammann temperature dependence with distinctly different behaviors in the amorphous and crystalline states with Tg dependencies following an approximate linear dependence with n–1 as Tgcr = 249 ± 5 + (231 ± 18/n) and Tgam = 240 ± 5 + (232 ± 17/n). The large Tg reduction is compared with another homologous series, namely, poly­(n-alkyl methacrylates), where the internal plasticization takes place at the side group. Internal plasticization is more efficient in the latter because of the mobile free end. Apart from Tg reduction, they show (i) subglass dynamics with activation energies that decrease with increasing alkyl length [from 57.8 kJ/mol in PEF (n = 2) to 47 kJ/mol in PNF (n = 9)], revealing the unlocking of local dipolar motions by the flexible spaces, (ii) a narrow distribution within the segmental process, αam, (corresponding Kohlrausch–Williams–Watts stretching exponent of 0.48, i.e., among the narrower for amorphous polymers), (iii) segments with locally nearly antiparallel dipolar orientation correlations, and (iv) a constant fragility in the amorphous state independent of alkyl chain length. We suggest that pertinent to these dynamic features is the local packing of chains composed of compact helical segments.