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