The Role of Covalent and Ionic Cross-Links in Phase Separation and Mechanical Properties of Carboxylated Nitrile Rubbers (XNBR)

High Resolution Image Download MS PowerPoint Slide Carboxylated nitrile butadiene rubber (XNBR), comprising acrylonitrile, butadiene, and acrylic acid, can be cross-linked ionically (zinc oxide) and covalently (sulfur). Ionic cross-links form rigid domains that soften at elevated temperatures, while covalent cross-links enhance thermal stability. Understanding XNBR’s microstructure is essential to developing advanced rubbers with enhanced properties. This study uses atomic force microscopy (AFM) PeakForce quantitative nanomechanics (PF-QNM), and dynamic mechanical analysis (DMA) to investigate XNBR’s morphology and phase behavior. DMA identified three thermal transitions: the glass transition, an ionic transition, and a weak low-temperature transition. AFM revealed distinct nanophase separation: a soft butadiene-rich phase and a hard acrylonitrile-rich phase, for the first time in the literature, along with high-modulus acrylic acid clusters. Nonlinear multi-Gaussian peak fitting of AFM modulus histograms allowed quantitative analysis of phase-specific moduli and volume fractions, demonstrating their dependence on cross-link type and density. This approach deepens understanding of XNBR’s phase behavior, providing insights to optimize its performance in high-performance rubber applications.