Highlighting the interdependence between volumetric contribution of fragility and cooperativity for polymeric segmental relaxation

The blurring around the link between the isobaric fragility and the characteristic size of cooperative rearranging region for glass-forming liquids has been cleared up by considering volumetric and thermal contributions of the structural relaxation. The measurement of these contributions is carried out for three amorphous thermoplastic polymers using broadband dielectric spectroscopy under pressure, providing an understanding of the link between isobaric fragilities, glass transition temperatures, and microstructures. The cooperative rearranging region (CRR) volume is calculated as a function of pressure using the extended Donth's approach, and the values are compared with the activation volume at the glass transition under different isobaric conditions. By combining these different results, a link between the chemical structure and the influence of pressure/temperature on the molecular mobility can be established. Furthermore, this study shows also a strong correlation between the activation volume, leading to the volumetric contribution of the isobaric fragility, and the CRR volume. Finally, this work highlights the influence of inter- and intra-molecular interactions on thermal and volumetric contributions of the isobaric fragility as a function of pressure.