Morphology evolution of immiscible polymer blends as directed by nanoparticle self-agglomeration

This work aims to clarify the effect of nanoparticle self-agglomeration structure on the morphology of polymer blends. The morphology development of polystyrene (PS)/polyamide (PA6) blends with titanium dioxide (TiO₂) nanoparticles preferentially localized in the PA6 domains was investigated by means of electron microscopy observation, viscoelastic analysis and selective extraction tests. It was shown that the preferential dispersion of TiO₂ leads to a significant reduction of the PA6 continuity in the PS/PA6 blend. The size of the PA6 domain increases gradually with further increasing the nanoparticle loading whereas the co-continuity of the PS/PA6/TiO₂ mixture is destroyed by isothermal post-treatments. These experimental results are completely different from those in carbon black, nanoclay or nano-silica-filled immiscible polymer blends. To elucidate the progression to the uneven morphology change, the dynamic process of microfibril break-up and droplet coalescence in the molten PS/PA6/TiO₂ mixture was traced in real-time through optical microscopy. It was confirmed that the self-agglomerating pattern of the nanoparticle in the polymer melts plays a key role in directing the morphology evolution of the immiscible polymer blend: unlike the self-agglomeration of carbon black to form three-dimensionally continuous network structure, the TiO₂ nanoparticles tend to form separate clusters in the PA6 phase. This prevents PA6 droplets from fusing together to form a continuous network during the coalescence and producing larger PA6 domains at higher TiO₂ loads.