Role of polyoctahedral oligomeric silsesquioxane nanostructures with varied side groups in modulating nanocomposite rheological and mechanical behavior

Abstract Polyoctahedral oligomeric silsesquioxane (POSS) represents a unique class of hybrid materials with cage-like structures that have gained attention as reinforcing fillers in polymer and rubber composites. These materials enhance mechanical and thermal properties, making them valuable in nanocomposite applications. This study explores the role of POSS nanoparticles with different functional side groups on the mechanical and rheological properties of (Polyamide/Ethylene–vinyl acetate) PA/EVA and (polyamide/Acrylonitrile Butadiene Rubber) PA/NBR composites. Four types of POSS with varied functional groups (TriSilanolPhenyl, TriSilanolIsoButyl, TriSilanolIsoOctyl, and OctaIsoButyl) were synthesized through controlled hydrolysis and condensation reactions and incorporated into the composites. Following synthesis, their structural integrity and functionalization were verified using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). Tensile and impact tests revealed a functional group-dependent improvement in mechanical properties, following the order: TriSilanolPhenyl > TriSilanolIsoButyl > TriSilanolIsoOctyl > OctaIsoButyl. POSS nanoparticles significantly enhanced the impact strength and elongation at break of PA/EVA and PA/NBR blends. Rheological characterization substantiated these findings. In PA/NBR blends containing POSS, both storage modulus and complex viscosity increased following the same order observed in mechanical testing. TriSilanolPhenyl POSS showed greater influence on the rheological properties of PA/NBR compared to PA/EVA, aligning with mechanical results. For instance, the impact strength of the PA/NBR/TriSilanolPhenyl composite increased by 196% compared to neat PA/NBR, while that of PA/EVA/TriSilanolPhenyl improved by 99% compared to neat PA/EVA. Finally, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) showed a broadly distributed silicon signal at the microscale but could not resolve the phase-specific localization of POSS between the PA matrix and the elastomer phases. However, the rheological response of PA/EVA—specifically the modest increase in zero-shear viscosity and absence of a damping-peak shift—was consistent with the hypothesis that a substantial fraction of POSS may reside within the dispersed EVA droplets rather than in the PA continuous phase. In contrast, for PA/NBR, the dramatic rise in zero-shear viscosity and significant damping-peak suppression supported the hypothesis of a greater POSS contribution from the PA-rich continuous phase and/or the PA/NBR interfacial region.