Thermal Patterning of a Critical Polymer Blend

Utilizing the Soret effect, we have employed a moderately focused laser beam ($30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, 20 mW) to write spatial composition patterns into layers of the critical polymer blend poly(dimethyl siloxane)/poly(ethyl-methyl siloxane) (PDMS/PEMS, ${M}_{w}=16.4/22.8\text{ }\text{ }\mathrm{kg}/\mathrm{mol}$) both in the one- and in the two-phase region a few degrees above and below the critical temperature ${T}_{c}=37.7\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Because of the critical divergence of the Soret coefficient, moderate temperature gradients are sufficient to induce composition modulations of large amplitude. In the two-phase regime the spinodal demixing pattern can be locally manipulated in a controlled way. 2D simulations based on a modified Cahn-Hilliard equation are able to reproduce the essential spatial and temporal features observed in the experiments.