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.

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Stress relaxation experiments were performed on two grades of polycarbonate at room temperature. The tests were performed in uniaxial extension and compression at deformations from the small strain, linear viscoelastic regime to the highly nonlinear viscoelastic regime just below yield. Simultaneous to the control of axial strain, both stress and lateral strain were measured, the latter providing the volume change for the samples. The volume change measurements in tension show initial dilatation of the samples followed by a volume relaxation that at the largest strains in one grade of polycarbonate leads to densities greater than those of the undeformed sample. In the case of the compression measurements, the volume decreases upon deformation, but, rather than relaxing back towards the undeformed volume, the samples continuously densify. The differences in the tensile dilatation for the two grades of polycarbonate suggest that the volume behavior may be related to the propensity to yield vs. fracture.

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The effect of polymer structure on fragility was determined by relating the apparent fragility to the relaxation response, heat capacity, and thermal expansion. For the 14 polymers studied, the fragility estimates based on the relaxation behavior (log aT) correlated well with the thermodynamic estimates of ΔCp/Mo, and Δα. In general, polymers with less sterically hindered repeat unit structures exhibited strong behavior. Polymers with sterically hindered backbones containing oxygen or ringed structures in the backbone were consistently fragile using log aT, ΔCp/Mo, and Δα as measures of fragility. On the other hand, using Cp1/Cpg as a fragility criterion resulted in very different fragility classifications.