Relationship between the Relative Dielectric Constant and the Monomer Sequence of Acrylonitrile in Rubber

Acrylonitrile-butadiene rubbers (NBRs) have a lower glass transition temperature (Tg) and a higher dielectric constant than other rubbers. To understand how a low Tg and a high dielectric constant are compatible, we focused on the acrylonitrile (AN) monomer sequence in rubber and synthesized random and alternating copolymers to evaluate the effect of the sequence. The AN monomer sequence dependence of the relative dielectric constant was investigated by the C-N stretching vibration of the nitrile group through Fourier transform infrared spectroscopy and internal rotation potential energy measurements around the C-C bond within the nitrile group based on dimer model calculations. The alternating copolymers, including NBR, showed a higher dielectric constant than random copolymers. The alternating copolymer shifted from ∼2243 cm-1 for polyAN to ∼2236 cm-1 for NBRs, while the random copolymer only shifted to ∼2239 cm-1. The peak of the C-N stretching vibration was correlated with the AN sequence. The sequence dependence of the shift can be explained by the C-N bond length calculation. The internal rotation potential energy between gauche and trans of the NBR model was the lowest, indicating that the NBR main chain is flexible and that AN in the main chain rotates easily. Therefore, NBR has a high dielectric constant and a low Tg because of the presence of an alternating sequence and the flexibility of the NBR main chain.