Effect of covalent cross-links on the network structure of thermo-reversible ionic elastomers

The effect of forming additional covalent cross-links on the thermo-reversible network structure of ionic elastomers was evaluated. Ionic elastomers are characterized by a strong physically cross-linked network resulting from a phase separation of ionic-rich nano-domains. The ionic domains are formed by the association of ionic groups that act as cross-links, promoting the elastic behaviour of these polymers. In addition, the trapped glassy rubber around the ionic associations improves the physical properties of these materials acting as reinforcing points of the soft rubbery matrix. These materials are considered as thermoplastic elastomers because of the thermo-labile nature of the ionic associations; however, this property limits their potential applications at elevated temperatures. In order to overcome this disadvantage some permanent covalent cross-links were formed in the ionic structure, improving their properties at high temperature but without altering their reversible thermoplastic nature. The formation of covalent crosslinks increases the number of ionic nano-domains that present a smaller and more homogeneous size distribution. This variation in the network structure is closely related to the enhanced properties shown by these materials. Therefore the proper combination of covalent and ionic cross-links allows the control of the network structure of ionic elastomers in order to obtain tunable properties for these materials.