The specific role of acetonitrile and methoxypropionitrile, as accelerators of the relaxation dynamics of polyvinyl butyral (PVB), was investigated in polymer/additive mixtures with a saturation liquid content. The aim was to improve the ionic mobility of PVB-based solid electrolytes to be used in solid dye-sensitized solar cells. Mechanical and dielectric relaxation measurements between 120 K and 380 K revealed that the -relaxation observed above 330 K in dry-PVB is shifted quite below room temperature in PVB/additives. Both the additives cause a growing intermolecular cooperativity, the sub-glass -relaxation exhibiting a strength enhanced by a factor 3 and a frequency factor which increases from 10(15) s(-1) to 10(21) s(-1). This discloses an activation entropy as high as 165.7 J/K mol in comparison to 40.8 J/K mol in dry-PVB. It is suggested the existence of cooperative transitions, mainly driven by bridges formed through additive molecules, which influence both short- and long-scale segmental motions and also favor the ion dynamics in PVB/additive/electrolyte systems. The room temperature ionic conductivity sigma(rt) exhibits large changes from 6.4*10(-14) S/m in dry PVB, through 1.5*10(-8) S/m in PVB/LiI, to 2.45*10(-5) S/m in PVB/MPN/LiI. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 340-346