Molecular weight scaling in critical polymer mixtures

Symmetric binary mixtures of partially deuterated polymers were prepared at the critical composition. The segment-segment interaction energy parameter χeff was varied by adjusting the difference in deuterium content {Δ}XD between species. χeff was measured as a function of temperature, {Δ}χD, and molecular weight by small-angle neutron scattering. The critical point for demixing was determined to scale as χeff,c{~}N^-δ with {δ}=1.01±{}0.05, where N is the degree of polymerization. This result confirms the mean-field prediction of Flory and Huggins.