Effect of deuterium substitution on the surface interactions in binary polymer mixtures

We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C 4 H 8 ] 1 x [C 2 H 3 (C 2 H 5 )] x . Based on surface segregation data we have determined a surface energy difference X s between pure blend constituents. In each binary mixture components have different fractions x 1 , x 2 of the group C 2 H 3 (C 2 H 5 ), and one component is labeled by deuterium (dx) while the other is hydrogenous (hx). The mixtures are grouped in four pairs of structurally identical blends with swapped labeled constituent (dx 1 /hx 2 , hx 1 /dx 2 ). For each pair the surface energy parameter x s increases when the component with higher fraction x is deuterated, i.e., X s (dx 1 /hx 2 ) > X s (hx 1 /dx 2 ) for x 1 > x 2 . A similar pattern has been found previously for the bulk interaction parameter X. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. X s has also an additional contribution, insensitive to deuterium swapping effect, and related to entropically driven surface enrichment in a more stiff blend component with a lower fraction x. Both enthalpic and entropic contributions to X s seem to depend on the extent of chemical mismatch between blend components.