Enhancing the molecular cooperativity of polyvinyl butyral using liquid additives

ABSTRACT: The specific role of acetonitrile and methoxypropionitrile, as accelerators of the relaxation dynamics of polyvinylbutyral (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 a-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 b-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 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.