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, M_w = 16.4 kg mol⁻¹) and poly(ethyl-methyl siloxane) (PEMS, M_w = 22.8 kg mol⁻¹) for 6 × 10⁻⁴ ≤ ε ≤ 0.2. Ising-like critical scaling is observed close to T_c and mean field behavior for large ε. The thermal diffusion coefficient D_T 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 D_T. The Soret coefficient S_T = D_T/D diverges with the Ising-like critical scaling S_T ∼ ε^−0.67 close to T_c. In the mean field regime S_T ∼ S(0) ∼ ε^−1.0 scales like the structure factor S(0). Thermally activated friction effects cancel in S_T, similar to the situation in binary glass formers.

PACS numbers: 61.25.Hq, 61.41.+e, 64.60.Fr, 66.10.Cb