No abstract

We address the synthesis of novel poly(ester amide)s containing 2,5-furandicarboxylic acid (2,5-FDCA). A dimethyl 4,4′-((furan-2,5-dicarbonyl)bis(azanediyl))dibenzoate (2,5-FDCAn) model compound is synthesized to protect the thermally instable carboxylic acid groups of the 2,5-FDCA moiety. The properties of 2,5-FDCAn are compared with its terephthalic acid (TA), 2,5-thiophenedicarboxylic acid (2,5-TDCA), and isophthalic acid (IA) analogues. Furthermore, the feasibility of the melt polycondensation reaction of these model compounds with different aliphatic diols is investigated. The polymers thus synthesized are analyzed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarization optical microscopy (POM), nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). Solution NMR analysis indicates that the oxygen heteroatom in the furan ring functions as a hydrogen bond acceptor, resulting in the formation of intramolecular hydrogen bonds with the amide hydrogens in 2,5-FDCAn. It is anticipated that such intramolecular hydrogen bonds compete with the formation of intermolecular hydrogen bonds, which is generally considered to be the driving force for polyamide crystallization. As a consequence, 2,5-FDCAn exhibits a suppressed melting temperature, decreased crystallinity, and an increased solubility. The FTIR and WAXD data confirm the perturbed hydrogen bonding nature of 2,5-FDCAn and its limited ability to crystallize. The application of this low melting compound allows for the synthesis of novel poly(ester amide)s via a melt polycondensation reaction. In contrast, to obtain high molar mass polymers containing the TA, 2,5-TDCA, and IA analogue, solvent is required for polymerization. Similar to the properties of 2,5-FDCAn, the obtained polymers exhibit increased solubility, decreased melting temperatures, and low degrees of crystallinity compared to their TA, 2,5-TDCA, and IA based analogues.

Natural nacre is well-known by its unique properties due to the well-recognized "bricks-and-mortar" structure. Inspired by the natural nacre, graphene oxide (GO) was reduced by dopamine with simultaneous coating by polydopamine (PDA) in aqueous solution to yield polydopamine-capped reduce GO (PDG). The artificial nacre nanocomposite materials of poly(vinyl alcohol) (PVA) and PDG presenting layered structure had been successfully constructed via a vacuum-assisted assembly process, in which PDG and PVA served as "bricks" and "mortar", respectively. A combination of hydrogen bonding, strong adhesion and friction between PDG nanosheets and PVA chains resulted in enhancements for mechanical properties. The tensile strength, elongation at break, and toughness of PDG-PVA nanocomposite reached to 327 ± 19.3 MPa, 8 ± 0.2%, and 13.0 ± 0.7 MJ m, which is simultaneously 2.4, 8, and 7 times higher than that of nature nacre with 80-135 MPa, ∼1%, and ∼1.8 MJ m, respectively. More interestingly, the obtained nanocomposites demonstrated a high anisotropy of thermal conductivity (k/k ≈ 380). Combined with superior mechanical properties and high anisotropy of thermal conductivity make these biomimetic materials promising candidates in aerospace, tissue engineering, and thermal management applications.

In synthetic as well as natural polyamides, hydrogen bonding and conformations of amide motifs are strongly influenced by the presence of ions and their concentration, water molecules, and their structure, as well as the pH of the solution. This concept combined with solubility of synthetic aliphatic polyamides, in particular nylons, in water at elevated temperature and corresponding vapor pressure is evaluated as a new reversible shielding route in the processing of these polymers. So far, reversible shielding has not been feasible due to a lack in controlling desired activation and deactivation of hydrogen bonding at the judicious moments. Here we show that in the presence of large halogen anions, crystallization from the random coil state is suppressed by hydrophobic hydration, where the amorphous state of the fast crystallizing nylons can be maintained even at 20 °C. Small hydrating lithium cations are favored since they strengthen the hydrophobic nature of the anions. Complete deshielding of hydrogen bonding, after processing, is facilitated by simple migration of ions in water that allows recovery of the desired conformation and structure.