Rheo-Optical FTIR Studies and Iminium Carboxylate Complex-Driven Ionic Networks

Abstract Conventional sulfur network of epoxidized natural rubber (ENR) system is not stable as it is becoming more dense resulting in fast hardening of the matrix and therefore, practical use of sulfur-cured ENR is rather limited. Here, we report the stable and reversible ionic network formation of ENR using dicarboxylic acid and a suitable heterocyclic aromatic amine. This study investigates the effects of various aliphatic and aromatic heterocyclic amines on the cross-linking behavior of dicarboxylic acid, aiming to enhance the stability of ionic intermediates to establish reversible networks rather than covalent cross-links. Rheo-optical FTIR monitoring of the orientation function of carboxylate ions reveals reversible changes during cyclic uniaxial mechanical deformation, indicating that the carboxylate groups are grafted onto the backbone of the ENR macromolecules. Compared to sulfur-cured ENR, dodecanedioic acid in combination with 4-dimethylaminopyridine (DMAP) offers similar mechanical strength but improved aging resistance which can be attributed to the ionic carboxylate-based cross-linking structure. This combination leads to impart a considerable stable ionic network with self-healing nature of the rubber composites. In this system after self-healing, the tensile strength is found to be as much as ∼11 MPa, a value which is rare in the literature.