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, R₀, is entropically favored at the surface. Here, we extend the SCFT to worm-like chains with small persistence lengths, ℓ_p, relative to their contour lengths, ℓ_c, for which R₀≈2ℓ_pℓ_c. In the limit of ℓ_p ≪ ξ, we recover the Gaussian-chain prediction where the segregation depends only on the product ℓ_pℓ_c, but for realistic polymer/air surfaces with ξ ∼ ℓ_p, the segregation depends separately on the two quantities. Although the surface continues to favor flexible polymers with smaller ℓ_p and bulky polymers with shorter ℓ_c, 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 R₀ yet not so rigid as to reverse the surface affinity that favors bulky polymers.