EFFECT OF DEUTERIUM SUBSTITUTION ON THE SURFACE INTERACTIONS IN BINARY POLYMER MIXTURES

We have examined the effect of deuterium labeling on surface interacti ons in mixtures of random olefinic copolymers [C4H8](1-x)[C2H3(C2H5)]( x). Based on surface segregation data we have determined a surface ene rgy difference chi(s) between pure blend constituents. In each binary mixture components have different fractions x(1), x(2) of the group C2 H3(C2H5), and one component is labeled by deuterium (dx) while the oth er is hydrogenous (hx). The mixtures are grouped in four pairs of stru cturally identical blends with swapped labeled constituent (dx(1)/hx(2 ), hx(1)/dx(2)). For each pair the surface energy parameter chi(s) inc reases when the component with higher fraction x is deuterated, i.e., chi(s)(dx(1)/hx(2)) > chi(s)(hx(1)/dx(2)) for x(1) > x(2). A similar p attern has been found previously for the bulk interaction parameter ch i. This is explained by the solubility parameter formalism aided by th e lattice theory relating the surface excess to missing-neighbor effec t. chi(s) has also an additional contribution, insensitive to deuteriu m swapping effect, and related to entropically driven surface enrichme nt in a more stiff blend component with a lower fraction x. Both entha lpic and entropic contributions to chi(s) seem to depend on the extent of chemical mismatch between blend components. (C) 1998 John Wiley & Sons, Inc.