SURFACE-INDUCED ASYMMETRIES DURING SPINODAL DECOMPOSITION IN OFF-CRITICAL POLYMER MIXTURES

We have studied the growth of the wetting layer formed at the surfaces of symmetric and nonsymmetric mixtures of poly(ethylenepropylene) (PE P) and perdeuterated poly(ethylenepropylene) (dPEP) during spinodal de composition. For off-critical quenches, the growth rate is found to be strongly dependent on the bulk composition of the mixtures. If the mi nority phase wets the surface, the wetting layer is found to grow slow er than in the reverse situation, where the majority phase wets the su rface. In the latter case, hydrodynamic effects may play a role, incre asing the rate of wetting layer growth. The surface spinodal waves are shallower for the case of off-critical compositions as compared to cr itical compositions. However, independent of bulk composition, all sur face composition profiles exhibit universal scaling behavior in the ne ar-surface region. Our conclusions are strengthened by numerical simul ations of a diffusively coarsening mixture near a surface that attract s one of the phases, No appreciable acceleration of growth of the surf ace layer is observed in simulations where the wetting phase is the ma jority phase, suggesting that the acceleration observed experimentally is due to fluid flow. The simulations also show that there can be ''s urface-induced nucleation'' in the case that the majority phase is att racted by the substrate: the expulsion of minority phase can lower the nucleation barrier near the substrate, causing more droplets to nucle ate there than in the bulk.