Hyperdiffusive dynamics in conjugated polymer blends and fullerene absorbing solutions

Abstract

We have observed the hyperdiffusive dynamics of light scatterers accompanied by their ballistic motion by a dynamic light scattering (DLS) technique in blends of semiconducting polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV) with a low-molecular-weight organic acceptor 2,4,7-trinitrofluorenone, TNF) and in fullerene C₆₀ solution. The DLS autocorrelation function has been found to follow the Kohlrausch-Williams-Watt function with the exponent β in the range of 0.5–2, depending on the laser power, scattering vector, optical absorption and type of sample. We show that the hyperdiffusive dynamics (β > 1) results from laser-induced convection caused by optical absorption at the DLS probing wavelength. The convection appears in the DLS data as a ballistic motion with velocities in the range of 5–60 μm s⁻¹ depending on the laser power, type of sample and their absorption. The convection is driven by the local temperature difference, ΔT ∼ 0.1–0.2 K, as evaluated from thermal lens effect measurements. We demonstrate that by decreasing the absorbed laser power, laser-induced hyperdiffusion can be avoided in both the polymer and fullerene samples so that the inherent thermal molecular motion can be probed. Specifically, in the polymer solutions, we have found a slow relaxation mode with the characteristic spatial spectrum of the inverse relaxation time Γ_s α q⁴. This mode has been assigned to the coupled motion of entangled conjugated polymer chains.