A novel supramolecular polymer fabricated via stronger hydrogen-bonding interactions between substituted amide groups: design, synthesis, properties and mechanism

Abstract

A novel difunctional monomer, 1,12-di(2-pyridine carbaldehyde-4′-hydrazinephenylate) dodecane (defined as DPCHP-DODE), was designed and synthesized through a two-step reaction process. The chemical structure of DPCHP-DODE was confirmed by a ¹H nuclear magnetic resonance (¹H-NMR) spectrum, Fourier-transform infrared (FT-IR) spectrum and elemental analysis (EA). Because the electron densities of both the proton donor (N–H) and proton acceptor (CO) were modulated by the substitutes, amide groups could form stronger hydrogen-bonding interactions between them. With such substituted amide groups at both ends, the molecules of DPCHP-DODE assembled into a supramolecular polymer during the cooling process of the melting DPCHP-DODE, and the resulting samples (defined as DPCHP-DODE₁₁₀ for the isothermal crystallized sample) exhibited a series of properties similar to conventional polymers, such as fibers dragged from the melt, adherence to the glass substrate, glass-transition, broad melting range, irreversible melting and crystallization process, and solid-like dynamic mechanical properties. Especially, DPCHP-DODE₁₁₀ could adhere to the glass substrate with a shear strength of 0.8 MPa, which is comparable to that of poly alpha-olefins/styrene–ethylene–butylene copolymer hot-melt adhesive (0.9–1.3 MPa). The effect of the electron density of the proton donors and proton acceptors could be confirmed by comparing the dynamic mechanical properties at an amorphous state between DPCHP-DODE_cooled and quenched di(2-pyridine carbaldehyde)-hexanediohydrazide (DPCH_cooled), a designed monomer with electron density modulation on the proton donor only. Moreover, temperature-dependent FT-IR spectra were used to identify the differences in supramolecular interactions between DPCHP-DODE and DPCHP-DODE₁₁₀. Although both hydrogen-bonded and free CO and N–H groups existed in them, DPCHP-DODE₁₁₀ had more hydrogen-bonded CO and N–H groups both at room temperature and during the heating process, which might be responsible for the polymer properties. Based on which, the formation mechanism of the supramolecular polymer was finally suggested.