High-Pressure Studies on the Chain and Segmental Dynamics of a Series of Poly(propylene glycol) Derivatives

In this paper, we report comprehensive studies on the molecular dynamics of a series of poly(propylene glycol) (PPG) derivatives characterized by three different terminal groups and various molecular weight, Mn (below the entanglement point), at elevated pressure conditions (up to p = 1 GPa). Data collected at ambient pressure showed that as expected, the role of H-bonds in controlling the dynamics of studied polymers decreases with increasing chain length. It is well reflected in the glass-transition temperature, Tg, of modified PPGs, which tends to be similar with increasing Mn for all studied samples. However, high-pressure studies revealed quite interesting finding related to the variation in the pressure coefficient of the glass-transition temperature, dTg/dp, in PPG derivatives. Herein, a monotonic increase or decrease of dTg/dp is found for the native and modified polymers, respectively. This effect is most likely connected to the change in the H-bond strength, which could affect the conformations of the PPG backbone at different thermodynamic conditions. Moreover, from the activation volume, we have calculated the additional volume that is required for the cooperative segmental motions to occur at the Tg for each studied system herein. Interestingly, it is roughly equal to the volume of the two or three segments dependently on the definition of Tg (τα = 1 s or τα = 100 s). Finally, the evolution of the isobaric fragility, mp, has been analyzed for each sample. It was found that the mp's of amino- and methoxy-terminated PPGs of Mn = 400 g/mol decrease with increasing pressure, while for the other investigated materials, this parameter barely changes with the compression. Our high-pressure data obtained for polymers having the same molecular weight, dispersity, and polymer backbone but different ability in the formation of supramolecular structures may contribute to a better understanding of the behavior of such materials at varying thermodynamic conditions.