A Smart Supramolecular Hydrogel Exhibiting pH-Modulated Viscoelastic Properties

Authors

  • N. E. Shi,

    1. State Key Laboratory of Coordination Chemistry, National Laboratory of Solid State Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 (P.R. China)
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  • H. Dong,

    1. Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 (P.R. China)
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  • G. Yin,

    1. State Key Laboratory of Coordination Chemistry, National Laboratory of Solid State Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 (P.R. China)
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  • Z. Xu,

    1. State Key Laboratory of Coordination Chemistry, National Laboratory of Solid State Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 (P.R. China)
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  • S. H. Li

    1. Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 (P.R. China)
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  • Financial support from the National Natural Science Foundation of China project No. 90606005, No. 20490210, No. 20371026, and No. 20571040 is greatly appreciated. Supporting Information is available online from Wiley InterScience or from the author.

Abstract

The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen-bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi-pyridyl-based gelator, N, N′, N″-tris(3-pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from 7.0 to 5.0. At pH 7.0, the amide molecules are assembled into an ordered structure as a result of the hydrogen bonds between the amide N–H bond and the nitrogen on the pyridyl group (N–H…Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N–H…Py are partially broken because the pyridyl groups are partly protonated at pH 5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X-ray diffraction (XRD) measurements and field-emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel, thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol–gel transition are reversible and controllable in the material.

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