Size Effects in the Oscillatory Rotation Dynamics of Ni Nanorods in Poly(ethylene oxide) Solutions

The rotation of Ni nanorods, dispersed in dilute and semidilute poly­(ethylene oxide) solutions, is investigated. Ni nanorods with similar diameter but different lengths are synthesized using the anodic aluminum oxide template method and characterized by transmission electron microscopy and static magnetic field-dependent optical transmission (SFOT) of linearly polarized light. The rotational motion of nanorods, determined by oscillating magnetic field-dependent optical transmission (OFOT) measurements, is analyzed to retrieve the local dynamic modulus of the polymer solution. The effect of probe size relative to intrinsic length scales of the polymer solution is systematically investigated by variation of the nanorod size, polymer molar mass, and concentration. A significant decrease in the zero-shear rate viscosity is observed in the semidilute entangled regime, which depends on the hydrodynamic length of the nanorods, L_h, and polymer radius of gyration, R_g, but not on PEO concentration. The relative viscosity can be approximated by η₀^OFOT/η₀^macro = exp ( – 5.6R_g/L_h). The macroscopic dynamic modulus of the entangled polymer solutions is measured by small-amplitude oscillatory shear. The local dynamic modulus, obtained from nanorod oscillation measurements, exhibits systematic changes with decreasing size of the probe particles, which indicate entanglement reduction as the physical origin of the size effect in this particular particle/polymer system.