Dynamic fragility in polymers: A comparison in isobaric and isochoric conditions

The dynamic fragilities of six polymeric materials have been compared for isobaric (constant pressure) and isochoric (constant volume) conditions. Data were taken from the published literature for dynamic experiments performed at elevated pressures. Published pressure–volume–temperature (PVT) data were then used to determine the dynamic response as a function of temperature for both the isochoric and isobaric conditions. It is found that the pressure and volume dependences of the dynamic fragility vary greatly among the polymers for which data were available. For poly(vinyl acetate) (PVAc) and poly(ethyl acrylate) (PEA), the dynamic fragility is independent of the pressure and specific volume and the isochoric and isobaric fragilities are almost the same. On the other hand for poly(vinylchloride) (PVC), polystyrene (PS) and poly(methyl acrylate) (PMA), the dynamic fragility is sensitive to changes of pressure and volume and the isobaric behavior is more fragile than the isochoric behavior when referenced to the same glass formation points (Pg,Vg,Tg). It is also remarked that the signature Vogel–Fulcher temperature dependence of the isobaric dynamics is lost for the isochoric behavior of both PMA and PVC.