Laser-induced structures in a polymer blend in the vicinity of the phase boundary

We have determined the diffusion, thermal diffusion, and Soret coefficients of a poly(dimethyl siloxane)/poly(ethyl-methyl siloxane) (PDMS/PEMS) polymer blend as a function of composition and temperature within the homogeneous phase. The critical slowing down of the diffusion and the corresponding critical divergence of the Soret coefficient are described within the pseudospinodal concept both for critical and off-critical compositions. These data are used to model in detail the channel-like structures that form due to the Soret effect when a focused laser beam is scanned across a polymer film of 100microm thickness. A moderate vertical asymmetry is attributed to solutal convection. Although heat rapidly diffuses away from the laser focus, the composition distribution in the early stage resembles the sharp profile of the laser beam. PDMS accumulates within the center of the structures, whereas a thin PEMS-rich layer is formed that isolates the central core from the windows. Experimentally, the structures are analyzed by means of phase contrast microscopy. Possible applications as rewritable optical waveguides or tunable phase plates are briefly discussed.