I. H. Syed, P. Stratmann, G. Hempel, M. Klüppel, K. Saalwächter.
Entanglements, Defects, and Inhomogeneities in Nitrile Butadiene
Rubbers: Macroscopic versus Microscopic Properties.
Macromolecules 49, 9004-9016 (2016).
Combined mechanical and NMR experiments were performed on bulk as well as swollen elastomers. Uniaxial stress–strain experiments in combination with current models of entangled rubber elasticity were used to disentangle the contributions from entanglements and chemical cross-links (including trapped entanglements) in a series of peroxide-cross-linked nitrile butadiene rubber (NBR) and its hydrogenated analogue (HNBR). The role of network defects was assessed by ultraslow stress–strain experiments and dynamic mechanical analysis. These macroscopic properties were compared with molecular-scale information obtained by advanced proton NMR spectroscopy techniques applied to bulk and swollen samples, which provide information on average cross-link density, its spatial heterogeneity, entanglement contributions, and the amount of inelastic defects. The emerging picture is that both NBR and HNBR elastomers feature significant cross-linking inhomogeneities, but the latter, related to on average much lower cross-linking efficiency and modified peroxide reactivity, contains significant (>20%) amounts of slowly relaxing, complex defect structures. For NBR, mechanical and NMR analyses are mutually consistent and allow for an absolute-value calibration of inter-cross-link and entanglement molecular weights and for a quantitative analysis of swelling experiments in a typical plasticizer (DBP), which turns out to be a good solvent. Data for HNBR suggest dominant entanglement effects, but likely due to the large defect content, mechanical and NMR results are not fully consistent. Highly inhomogeneous rubbers with significant defect contents remain to be a challenge for truly quantitative evaluation of different experimental results.