| Field-induced surface segregation was investigated using spectroscopic and interfacial tension measurements to study migration of copolymer additives in polystyrene and polyethylene host. Experiments were performed under various thermodynamic and flow conditions. The research is responsible to answer fundamental questions ranging from the effect of molecular weight to the role played by stress gradients on migration.; In PS-b-PDMS/PS blends, selective DMS component enrichment at the air/polymer interface was observed. The instantaneous surface excess concentration of DMS groups, Δ&phis;DMS, was also found to depend on host polymer molecular weight and annealing conditions. The results can be summarized using the approximate scaling relation, Δ&phis;DMS ∼ MwPSα, where the scaling exponent α is a function of annealing conditions. For non-annealed PS-b-PDMS/PS blend samples, α ≈ −0.5, whereas for samples annealed at elevated temperatures α values in the range of 0.25 to 0.3 were found. Surprising observations of enhanced levels of PS-b-PDMS near aluminum/polymer interfaces also were reported. The experimental observations are discussed in terms of the molecular weight dependence of diffusivity, surface tension, and configurational entropy of the host polystyrene.; In PS-b-PMMA/PS, surface enrichment of low molecular weight PS-b-PMMA additives was found with increasing Mw,PS as host materials. Quantitatively, surface MMA concentration is proportional to Mw,PS−0.76 for thermodynamically equilibrated blends. This result is inconsistent with expectations based on the lower interfacial energy of PS. The surface migration appears to be driven by configurational entropic penalty of higher molecules at the surface due to the molecular weight disparity in the A/A-B blends and the migration induced by chain-end was not observed. Therefore, the observation can be explained by the theoretical result for pure entropic effect on the surface segregation in binary blends for which Φs ∼ 1/Mw, matrix.; In flow-induced migration experiments, Surface concentration of copolymer linearly increases as a function of wall shear rate at enough shearing time. Both decreasing die radius and increasing extrusion rate increase the effectiveness of stress-induced transport of radial copolymer. It was also observed that reducing radius is more effective than increasing extrusion rate on the migration. These findings provide a new approach for manipulating polymer surface. |
