Molecular weight scaling in critical polymer mixtures

Abstract

Symmetric binary mixtures of partially deuterated polymers were prepared at the critical composition. The segment-segment interaction energy parameter ${\mathrm{\ensuremath{\chi}}}_{\mathrm{eff}}$ was varied by adjusting the difference in deuterium content \ensuremath{\Delta}${\mathit{X}}_{\mathrm{D}}$ between species. ${\mathrm{\ensuremath{\chi}}}_{\mathrm{eff}}$ was measured as a function of temperature, \ensuremath{\Delta}${\mathrm{\ensuremath{\chi}}}_{\mathrm{D}}$, and molecular weight by small-angle neutron scattering. The critical point for demixing was determined to scale as ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}\mathrm{f}\mathrm{f},}$c\ensuremath{\sim}${\mathit{N}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with \ensuremath{\delta}=1.01\ifmmode\pm\else\textpm\fi{}0.05, where N is the degree of polymerization. This result confirms the mean-field prediction of Flory and Huggins.