THERMODYNAMIC INTERACTIONS IN ISOTOPE BLENDS - EXPERIMENT AND THEORY

Small-angle neutron scattering (SANS) studies of binary mixtures provi de chi(NS), a measure of thermodynamic interactions between dissimilar polymer chains, one of which is usually labeled with deuterium. For p olymers differing only in isotopic substitution (isotope blends), chi( NS) is Seen to diverge strongly upward (or sometimes downward) at low concentrations of either blend component. This concentration dependenc e seems to vanish in the limit of large degree of polymerization N. Ex perimental results can be described by chi(NS)(phi,N) = beta + gamma/N phi(1 - phi), where phi is the volume fraction of deuterated polymer. For SANS from a series of blends with different phi it is shown that systematic errors in N and/or the static structure factor S(0) lead to precisely the same chi(NS)(phi,N) when the Flory-Huggins interaction parameter chi is constant. While results for some isotope blend system s can be accounted for with reasonable error estimates, others appear to have a real dependence of chi(NS) on phi and N. It is suggested tha t these ''non-Flory-Huggins'' effects stem from a modified entropy of mixing that is most evident in dilute blends. The concentration depend ence of chi(NS)(phi) has no practical effect on macroscopic phase beha vior.