Origin of Glass Transition of Poly(2-vinylpyridine). A Temperature- and Pressure-Dependent Dielectric Spectroscopy Study

The dynamics of poly(2-vinylpyridine) (P2VP) have been studied as a function of temperature (in the range from 123 to 453 K), pressure (0.1−270 MPa), and molecular weight (1.1 × 103−3.0 × 104 g/mol), using dielectric spectroscopy (DS) within the frequency range from 10-2 to 106 Hz. Structural methods (wide-angle X-ray scattering) have been employed in parallel with thermodynamic methods (pressure−volume−temperature). Three relaxation processes were found: two above the glass temperature (T g) associated with the segmental (α-) process and a slower process with an apparent activation volume comparable to the monomer volume and another well below T g, with an Arrhenius T dependence (β-process). The results from the dynamic study combined with the thermodynamic results revealed that both decreasing thermal energy and insufficient volume lead to glass formation at lower temperatures, with the former having the stronger effect at temperatures near T g (i.e., values of the ratio of apparent activation energies at constant volume and constant pressure in the range Q V /Q P ∼ 0.6−0.85 for the different temperatures and pressures investigated).