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

Abstract

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-T g 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 T g dependencies following an approximate linear dependence with n −1 as T g cr = 249 ± 5 + (231 ± 18/n) and T g am = 240 ± 5 + (232 ± 17/n). The large T g 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 T g 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, α am , (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.

Broadband Dielectric Spectroscopy Study of Biobased Poly(alkylene 2,5-furanoate)s’ Molecular Dynamics

“…In fact, this result is quite interesting due to a number of reasons. First, previous reports on different poly(alkylene furanoate)s described the local relaxation as a single-component process, [27,[29][30][31] whose activation energies were similar to those found for our β 1 contributions. Since the presence of the β 2 component depends on the thermal history of the samples, the absence of this relaxation in previous works can be linked to the different thermal protocols used.…”

“…The four polyesters were successfully processed into films by compression molding, and such films were subjected to thermal, diffractometric, mechanical, and gas permeability characterization. The results indicated that glycol subunit length determined the final polymer properties, finding an odd-even trend [31,32]. In detail, the polymers with an odd number of carbon atoms in the glycol subunit showed a lower tendency to crystallize with consequent formation of a less perfect crystalline phase, i.e., they were characterized by a lower melting temperature than the polymers obtained from glycol with an even number of carbon atoms, and they exhibited superior gas barrier performances.…”

“…These new polyesters have been characterized from the thermal, mechanical, and dielectric point of view [17][18][19][20][21][22][23][24][25][26]. In fact, in the particular case of BDS, there have been several investigations focused on furan-based polymers, revealing complex molecular dynamics [27][28][29][30][31]. A recent work by Papamokos and collaborators presented a detailed study on the molecular dynamics of poly (alkylene 2,5-furanoates) using a combined approach of Density Functional Theory (DFT) calculations with BDS experiments [31].…”

“…In fact, in the particular case of BDS, there have been several investigations focused on furan-based polymers, revealing complex molecular dynamics [27][28][29][30][31]. A recent work by Papamokos and collaborators presented a detailed study on the molecular dynamics of poly (alkylene 2,5-furanoates) using a combined approach of Density Functional Theory (DFT) calculations with BDS experiments [31]. The authors' results revealed interesting conformational properties in these systems, driven by the packing of helical segments within the amorphous domains [31].…”

“…A recent work by Papamokos and collaborators presented a detailed study on the molecular dynamics of poly (alkylene 2,5-furanoates) using a combined approach of Density Functional Theory (DFT) calculations with BDS experiments [31]. The authors' results revealed interesting conformational properties in these systems, driven by the packing of helical segments within the amorphous domains [31]. In addition, Guidotti et al prepared by two-stage melt polycondensation four 100% bio-based polyesters of 2,5-furandicarboxylic acid, utilizing 1,3-propanediol, 1,4-butanediol, 1,5 pentanediol and 1,6-hexanediol (PTF, PBF, PPeF, and PHF, respectively) [24,26,28,32,33].…”