Thermodynamic Interactions in Isotope Blends:  Experiment and Theory

Small-angle neutron scattering (SANS) studies of binary mixtures provide χNS, a measure of thermodynamic interactions between dissimilar polymer chains, one of which is usually labeled with deuterium. For polymers differing only in isotopic substitution (isotope blends), χNS is seen to diverge strongly upward (or sometimes downward) at low concentrations of either blend component. This concentration dependence seems to vanish in the limit of large degree of polymerization N. Experimental results can be described by χNS(φ,N) = β + γ/Nφ(1 − φ), where φ is the volume fraction of deuterated polymer. For SANS from a series of blends with different φ it is shown that systematic errors in N and/or the static structure factor S(0) lead to precisely the same χNS(φ,N) when the Flory−Huggins interaction parameter χ is constant. While results for some isotope blend systems can be accounted for with reasonable error estimates, others appear to have a real dependence of χNS on φ and N. It is suggested that these “non-Flory−Huggins” effects stem from a modified entropy of mixing that is most evident in dilute blends. The concentration dependence of χNS(φ) has no practical effect on macroscopic phase behavior.