Triazolinedione-functionalized isoprene rubber composites with self-adhesion via cross-linking with zinc dimethacrylate†

We report a novel system for the direct adhesion of cross-linked rubbers based on the introduction of triazolinedione (TAD)-derived urazole moieties into cis-1,4-polyisoprene (PI) followed by the addition of zinc dimethacrylate (ZDMA) with the aim of forming dissociative ionic cross-links. The modification of PI is achieved by a TAD-based click reaction using 4-phenyl-1,2,4-triazoline-3,5-dione (PhTAD). The formation of cross-linking via the TAD units and ZDMA is demonstrated by the increase in the elastic-torque curves of the resulting rubber composites at elevated temperature. The addition of the radical-trapping agent N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine to the cross-linking system suppresses the increase in its elastic torque, indicating that cross-linking between the TAD units and ZDMA proceeds by a radical mechanism. This mechanism is supported by the fact that the use of either zinc chloride or zinc acetate instead of ZDMA did not show an increase in the elastic torque, excluding the possibility of coordination cross-linking between TAD units and zinc centres. The obtained TAD–ZDMA cross-linked rubbers show unique temperature dependence in dynamic mechanical analysis (DMA), reflecting the dissociation of the ionic cross-linking at elevated temperatures. Strain–sweep DMA tests showed a typical Payne effect with an increasing amount of TAD units in the PI, supporting the formation of ZDMA–ZDMA filler interactions. Tensile tests revealed that the fracture energies of the TAD-PI/ZDMA composites are comparable to those of samples prepared using a peroxide-based curing system, and a tensile strength of up to 18.2 MPa at 525% elongation was achieved when 3.1 mol% of TAD was incorporated into the PI with 40 phr of ZDMA. A direct adhesion is demonstrated using T-peel tests, in which the adhesion-peeling force reached up to 6.55 N mm⁻¹ when 4.3 mol% of TAD was incorporated into the PI with 40 phr of ZDMA. The maximum peeling force shows a good correlation with the difference between the E′ values at 25 °C and 145 °C in the DMA tests, except in the case of the sample with a too-high degree of cross-linking, indicating that the degree of dissociative cross-linking is the main factor determining the adhesion strength in the TAD–ZDMA cross-linking system.