Phase behavior of amorphous binary mixtures of perdeuterated and normal 1,4-polybutadienes.

A series of amorphous binary mixtures of normal and perdeuterated 1,4-polybutadienes has been examined by small-angle neutron scattering (SANS). Contrary to the generally held assumption that isotopic polymer mixtures form ideal solutions, we conclusively demonstrate that these mixtures are characterized by an upper critical solution temperature (UCST). This isotope effect derives from a small, measurable difference in segment volume between normal and perdeuterated species. Above the UCST, the SANS results are quantitatively predicted by the mean-field theory of deGennes for homogeneous binary polymer mixtures. Increasing the degree of polymerization raises the critical temperature, resulting in phase separation.