Aggregation of nitrile-butadiene copolymers in solutions

The nonuniformity of nitrile-butadiene rubber solutions in toluene and chloroform has been studied. Dynamic-light-scattering measurements show that these solutions contain two kinds of scattering particles: small particles 10–16 nm in radius corresponding to the macromolecular coil and large particles 400–500 nm in radius corresponding to aggregates. The aggregates are stable in solutions for at least several days. Such stability may be interpreted in terms of topological factors supposing that the aggregates are formed via noncovalent interactions of tens of thousands of highly branched entangled chains. The chains become disentangled even during slight shear. Thus, the solution of nitrile-butadiene rubber subjected to soft rolling mostly contains small particles. However, in the absence of external fields, such a structure is very stable in solution and even more so in the rubber block. During storage of nitrile-butadiene rubber, formation of a relatively small quantity of covalent crosslinks suffices for fixing the structure of aggregates and prevents the disentanglement of chains. Thus, the crosslinking of the aggregates may be primarily responsible for the natural aging of nitrile-butadiene rubber during storage.