Excellent Toughening of 2,6-Diaminopyridine Derived Poly (Urethane Urea) via Dynamic Cross-Linkages and Interfering with Hydrogen Bonding of Urea Groups from Partially Coordinated Ligands

Abstract

Conventional approaches to synthesize thermoplastic polyurethane (TPU) with excellent robustness are limited by a competing relationship between soft and hard segments for tuning mechanical properties in terms of chain flexibility and micro-phase separation. Herein, we present a facile and effective way of simultaneously improving the tensile strength, elongation, and toughness by constructing dynamic cross-linkages from metal-ligand interaction between Zn2+ and pyridine moiety in backbone of poly(urethane urea) (PUU) derived from 2,6-diaminopyridine and poly(propylene glycol). It was found that a Zn2+/pyridine ratio of 1:4 is the most effective for improving robustness. Specifically, tensile strength, elongation, and toughness could be remarkably increased to 16.0 MPa, 1286%, and 89.3 MJ/m3 with 226%, 29%, and 185% increments compared to uncomplexed PUU, respectively. Results from UV-vis, Fourier transform infrared spectroscopy (FTIR), cyclic tensile tests, and stress relaxation reveal that metal-ligand interaction significantly interferes with the hydrogen bonding of urea groups, thus leading to weakening of stiffness. Furthermore, half of vacant ligands enable dynamic complexation during stretching, which consequently ensures constant noncovalent cross-linkages for constraining mutual chain sliding, contributing to simultaneous improvement of tensile strength, elongation, and toughness. This work provides a promising approach for designing TPU with excellent robustness.

Mechanically Robust and Self-Healable Polyureas Based on Multiple Dynamic Bonds

“…In addition, compared with PU-L (without the addition of zinc ions), the absorption peaks of amide I, amide II, and amide III bands in the PU-L/Zn curve show significant changes, indicating that the O and N atoms in the urea bond participate in coordination with Zn­(II) and also that the formation of coordination bonds has an effect on the formation of hydrogen bonds between urea groups, which is consistent with the results of the related literature. , As shown in the UV–vis spectrum of Figure g, the PU-L solution shows two absorption peaks centered at ∼244 and 306 nm, assigning to the n → π* transition of urea groups and the π → π* transition of pyridine groups, respectively . By contrast, the addition of Zn­(II) resulted in a new feature peak in the UV spectrum of PU-L/Zn-3 at ∼276 nm, which is attributed to the characteristic absorbance of the Zn-urea charge-transfer electronic transition. , In addition, a bathochromic shift of the pyridine ligand-centered π–π* electronic transition is observed in Figure g . All the above results can prove the formation of coordination bonds, and the nitrogen heterocycle as well as N and O atoms in the urea bond are able to coordinate with Zn­(II), thus forming strong physical cross-links in the polyurea network.…”

Toward Robust, Tough, Self-Healable Supramolecular Elastomers for Potential Application in Flexible Substrates

“…The ATR-FTIR spectra of PDMS-DAP@Fe- x shown in Figure b reflect that the amide I band at 1636 cm –1 and amide II band at 1572 cm –1 shifted to 1626 and 1576 cm –1 , respectively, suggesting the coordination between Fe­(III) and urea group. Besides, the disappearance of peak at 1684 cm –1 validates the coordination between Fe­(III) and O in the urea group. , The UV–vis spectra of the prepared elastomers were characterized to further verify the existence of coordination bonding interactions. As shown in Figure c, upon introduction of Fe­(III) ions, a new absorption at 336 nm appeared, verifying the coordination between Fe­(III) and N pyridine . , However, the decreasing solubility of hybrid networks caused by the increased coordination networks led to the decrease of the band strength.…”

“…Besides, the disappearance of peak at 1684 cm −1 validates the coordination between Fe(III) and O in the urea group. 36,37 The UV−vis spectra of the prepared elastomers were characterized to further verify the existence of coordination bonding interactions. As shown in Figure 1c, upon introduction of Fe(III) ions, a new absorption at 336 nm appeared, verifying the coordination between Fe(III) and N pyridine .…”

Ultra‐Tough Elastomers from Stereochemistry‐Directed Hydrogen Bonding in Isosorbide‐Based Polymers

“…It should be noted that all materials exhibit a single T g , which indicates that the materials are not phase separated, which could be beneficial to chain mobility. [20] ISPU and IMPU were uniaxially strained at a strain rate of 10 mm min À 1 to initially assess their mechanical properties. This revealed that the ISPU behaved comparably to IMPU, [18] both as tough elastomers with high tensile strength and extensibility at failure.…”