Effect of blending and copolymerization of nitrile-butadiene polymer on rubber network structure and their properties

Synthetic high-performance elastomers take on a fundamental role in industrial applications, because of their ability to withstand rather extreme conditions, regarding temperature, solvents, chemicals. In particular, NBR has merited special attention over the years, because of its oil and fuel resistance, required for instance in sealing or roller belts. To further increase its properties, especially mechanically, blending with other polymers was adopted as solution. The use of ENR, which results compatible with NBR thanks to the epoxide groups, was proved beneficial for increased tensile and fatigue strength, higher moduli and higher tanδ, due to the presence of the NR units[1]. However, researches focused mainly on the variation of properties[2], without investigating the network structure evolution compared to the single polymers. In fact, butadiene and isoprene, the two reactive units in these polymers, show[3] completely different distributions of crosslinks in sulphur-cured systems and different evolution of network structure with time, that is also expected in NBR and ENR. Additional heterogeneity is present because of sub- or nanoscale domains separations[4]. Moreover, a polymer blend can be subjected to intrinsic inhomogeneities, which are dependent not only on the polymers and their relative ratio, but also on the mixing process, creating undesired variabilities. A possible alternative is the use of copolymers that contain the monomer units under consideration. In this framework, we can find NBIR which is a random copolymer of nitrile, butadiene and isoprene units. It shows properties combining similar swelling resistance of base NBR and comparable mechanical properties to 50/50 blend of NBR/ENR. These observations need to be addressed not only accounting for its chemical structure but also considering the resulting network structure. Therefore, we prepared different model compounds based on NBR, ENR, NBR/ENR blends with different compositions and NBIR. Compounds curing was performed at different times and temperatures, analysing each network structure with LF-TD-NMR, that provides information on the crosslinks density and their distribution, and chemically cleaving sulphur-bonds of different degrees. The results where then correlated with mechanical (DMA, stress-strain), thermal (TGA, DSC) and swelling properties, elucidating the interaction between the two polymers, the reactivity of the different monomer units and the origin of the improvement that can be achieved with copolymerization rather than blending.