Cross-Link Density and Strain-Induced Molecular Chain Orientation Relationship in Hydrogenated Nitrile Butadiene Rubbers

The relationship between cross-link density and molecular orientation achieved upon stretching by sulfur-vulcanized hydrogenated nitrile butadiene rubbers (HNBR) with varying acrylonitrile (ACN) and residual double-bond (iRDB) contents is investigated. Cross-link density was measured using equilibrium swelling tests and double-quantum proton NMR (DQ-NMR), and the results were correlated to the orientation parameter extracted from wide-angle X-ray scattering (WAXS) measurements of stretched samples at various deformations. The values of cross-link density νe extracted from swelling experiments using affine and phantom models showed a nearly linear increase as the available sulfur content in the curative package increases above a threshold, but with nonzero intercepts due to the presence of trapped entanglements. DQ-NMR measurements yielded values of the residual dipolar coupling constant Dres, proportional to total cross-link density, including temporary entanglements. The fraction of defects (dangling ends, sol fraction) was also estimated from DQ-NMR tests, finding that this quantity tends to decrease as the Dres values increase. A linear correlation between Dres and νe was found, confirming the complementarity of these techniques. The values of the order parameter extracted from the WAXS analysis, after normalization by Dres, revealed a linear increase as the strain increases until reaching a plateau, associated with strain-induced crystallization (SIC) in high-ACN samples, and achievement of the maximum extension of the network strands for the amorphous samples, with low ACN content. Determination of the slope of the straight lines interpolating these data enabled calculating the amorphous segment form factor (K), equal to ≈0.056 using the affine model and 0.036 using the phantom model. Overall, the study demonstrates that both the density and homogeneity of the cross-linked network critically affect molecular orientation mechanisms and mechanical performance of HNBR, providing key insights for designing high-performance elastomers for demanding engineering applications.