Water-driven segmental cooperativity in polyvinyl butyral

Measurements of thermogravimetry, dynamic mechanical and dielectric analysis, and infrared absorption have been performed in polyvinyl butyral (PVB) to investigate the effect of absorbed water on relaxation dynamics. Increasing water content plasticizes PVB, also causing a growing intermolecular cooperativity mainly for the secondary relaxation motions. Mechanical and dielectric β-relaxations for dry- and wet-PVB fit onto the same Arrhenius plots showing the very close nature of underlying mechanisms. In particular, the sub-glass mechanical β-relaxation in wet-PVB exhibits a frequency factor which increases up to 1016 s−1, disclosing an activation entropy as high as about 59 J/Kmol. It is suggested the existence of cooperative transitions of relaxing side units in adjacent chains, mainly driven by bridges formed through water molecules between the hydroxyl groups of vinyl alcohol segments. Below the glass transition temperature Tg, the bridges are mechanically stable and give rise to an increase of the elastic modulus by more than a factor 2 when compared to that of a fully dry-PVB.