Entropic surface segregation from athermal polymer blends: Polymer flexibility vs bulkiness.

We examine athermal binary blends composed of conformationally asymmetric polymers of equal molecular volume next to a surface of width ξ. The self-consistent field theory (SCFT) of Gaussian chains predicts that the more compact polymer with the shorter average end-to-end length, R, is entropically favored at the surface. Here, we extend the SCFT to worm-like chains with small persistence lengths, ℓ, relative to their contour lengths, ℓ, for which R≈2ℓℓ. In the limit of ℓ ≪ ξ, we recover the Gaussian-chain prediction where the segregation depends only on the product ℓℓ, but for realistic polymer/air surfaces with ξ ∼ ℓ, the segregation depends separately on the two quantities. Although the surface continues to favor flexible polymers with smaller ℓ and bulky polymers with shorter ℓ, the effect of bulkiness is more pronounced. This imbalance can, under specific conditions, lead to anomalous surface segregation of the more extended polymer. For this to happen, the polymer must be bulkier and stiffer, with a stiffness that is sufficient to produce a larger R yet not so rigid as to reverse the surface affinity that favors bulky polymers.