Nanoscale heterogeneous structure of polyacrylonitrile-co-butadiene with different molecular mobilities analyzed by spin–spin relaxation time

Abstract

To identify components with different spin–spin relaxation times, T2, in the solid-echo pulse proton nuclear magnetic resonance (1H-NMR) spectra of crude acrylonitrile (AN)–butadiene rubbers (NBRs) with five different AN contents, we tried to understand the inhomogeneity in the crude NBRs in terms of their microstructures and molecular mobilities. The results of small-angle X-ray scattering, differential scanning calorimetry and dynamic mechanical analysis showed that crude NBRs have a single-phase and homogeneous morphology on the nanoscale. The microstructure of the crude NBRs shows alternately copolymerized AN–butadiene (BU) and BU block sequences, as indicated by 1H-NMR spectra. The T2 of the crude NBRs revealed three components with different molecular mobilities, even in homogeneous samples. The content of the highest-mobility component with T2l is negligible. Judging from the AN content dependence of the 1H ratio of these components, the low-mobility component with T2s and high-mobility component with T2m were assigned to the alternately copolymerized AN–BU sequences and BU block sequences, respectively. Rubber materials are often used in fluid sealing. Molecular mobility of rubber is one of the important parameter for designing suitable material for rubber seals. Pulsed proton nuclear magnetic resonance (1H-NMR) results showed that crude acrylonitrile–butadiene rubber has two components with long and short spin–spin relaxation time assigned to the altenatively copolymerized acrylonitrile–butadiene and butadiene block sequences, respectively. We conclude that these components with different mobilities in crude rubber originated from a inhomogeneity in the first-order structure of molecular chains.