Comparative Study on the Molecular Dynamics of a Series of Polypropylene Glycols

Broadband dielectric spectroscopy (BDS) is employed to study the molecular dynamics of hydroxyl- (OH-) and amino- (NH2-) terminated polypropylene glycols (PPGs) of varying molecular weight. Besides the dynamic glass transition (α relaxation), a normal mode process and a secondary γ-relaxation is observed, the latter being assigned to librational fluctuations of the polar COC moiety. Additionally a further process is found and proven in two independent experiments (annealing PPGs at high temperatures and physical aging) to originate from residual H2O impurities in the sample and is therefore not related to the dynamics of PPG. It is occasionally, albeit incorrectly, discussed in the literature as Johari−Goldsten (JG) relaxation. Furthermore, aging in the systems under study is analyzed and found to follow the Nagel−Leheny equation.