Fast Nanorod Diffusion through Entangled Polymer Melts

Nanorod diffusion in polymer melts is faster than predicted by the continuum model (CM). Rutherford backscattering spectrometry is used to measure the concentration profile of titanium dioxide (TiO₂) nanorods (L = 43 nm, d = 5 nm) in a polystyrene (PS) matrix having molecular weights (M) from 9 to 2000 kDa. In the entangled regime, the tracer diffusion coefficients (D) of TiO₂ decrease as the M^–1.4, whereas the CM predicts D_CM ∼ M^–3.0 using the measured zero-shear viscosity of TiO₂(1 vol %): PS­(M) blends. By plotting D/D_CM versus M/M_e, where M_e is the entanglement molecular weight, diffusion is enhanced by a factor of 10–10³ as M/M_e increases. The faster diffusion is attributed to decoupling of nanorod diffusion from polymer relaxations in the surrounding matrix, which is facilitated by the nanorod dimensions (i.e., L greater than and d less than the entanglement mesh size, 8 nm).