Thermal diffusion in a critical polymer blend

Abstract

Collective and thermal diffusion (Ludwig–Soret effect) has been studied in a polymer blend of poly(dimethyl siloxane) (PDMS, Mw = 16.4 kg mol−1) and poly(ethyl-methyl siloxane) (PEMS, Mw = 22.8 kg mol−1) for 6 × 10−4 ≤ ε ≤ 0.2. Ising-like critical scaling is observed close to Tc and mean field behavior for large ε. The thermal diffusion coefficient DT shows Arrhenius-like thermal activation but no critical behavior. The temperature dependence of the collective diffusion coefficient D in the mean field regime is explained by the mean field critical exponent γ = 1.0 and a thermal activation of the background contribution with the same activation energy as DT. The Soret coefficient ST = DT/D diverges with the Ising-like critical scaling ST ∼ ε−0.67 close to Tc. In the mean field regime ST ∼ S(0) ∼ ε−1.0 scales like the structure factor S(0). Thermally activated friction effects cancel in ST, similar to the situation in binary glass formers.