Transformation of Epoxidized Natural Rubber into Ionomers by Grafting of 1<i>H</i>-Imidazolium Ion and Development of a Dynamic Reversible Network
Subhradeep Mandal, Sakrit Hait, Frank Simon, Anik Kumar Ghosh, Ulrich Scheler, Injamamul Arief, Toshio Tada, Tung X. Hoang, Sven Wießner, Gert Heinrich, Amit Das
Abstract
Epoxidized natural rubber (ENR) was grafted with imidazolium ions, giving the materials self-healing and reprocessing properties. These imidazolium ions in the form of neutral ionic repeating units transformed the polyisoprene-based polymer chains into ionomers. Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (NMR) analyses were carried out to confirm various interactions between ENR and 1H-imidazole. In particular, dynamic hydrogen bonding and the formation of ionic aggregates led to superior mechanical strength (tensile strength, ∼4.5 MPa; elongation at break, ∼700%), self-healing efficiency (90% in terms of elongation), and reprocessability of the resulting compound. Furthermore, the material exhibited excellent transparency with a normalized transparency of 90 ± 0.7% at a typical thickness of 0.5 mm. The visualization and the stability of the healing processes were confirmed by scanning electron microscopy (SEM) and confocal microscopy analyses. Finally, temperature scanning stress relaxation (TSSR) studies were carried out to identify the different types of stress relaxation behavior of the material and the effects of various noncovalent interactions on the high-temperature stability of the compounds. These studies could assist to gain significant insight into natural rubber-based advanced materials for sustainable development.