Molecular dynamic in binary mixtures and polymer blends with large difference in glass transition temperatures of the two components: A critical review

The dynamics in highly asymmetric mixtures with large difference in Tg of the two components are complex. At low values of the lower-Tg2 component only the α1-relaxation and its Tgα1 were found together with a fast relaxation. Three different interpretations of the fast relaxation were given. The first one interpreted the fast relaxation as the JG β-relaxation, and the α2-relaxation is present but not resolved. The second one interpreted it as localized and noncooperative α′-relaxation of the low-Tg2 component totally confined by the frozen high-Tg1 component. The third one interpreted it also as the confined α2-relaxation, but it is liquid-like and cooperative. We review exhaustively the experimental data of many highly asymmetric mixtures and polymer blends to show unequivocally that the observed fast relaxation is the JG β-relaxation. Notwithstanding, the confined-but-cooperative α2-relaxation exists, albeit not resolved. Experimental data are exclusively considered and objectively utilized to arrive at the conclusions.