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A Saiter, et al.. Segmental mobility and glass transition of poly(ethylene-vinyl acetate) copolymers : Is there a continuum in the dynamic glass transitions from PVAc to PE?. Polymer, Elsevier, 2015, 76, pp.213-219. 10.1016/j.polymer.2015 The segmental dynamics of amorphous poly(ethylene-vinyl acetate) copolymers (from PVAc to EVA50) were studied. In that sample set with similar backbone stiffness and different amount of dipoles, the dynamic glass transition was investigated by Modulated Temperature Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy measurements. A decrease of the cooperativity length scale was obtained with the vinyl acetate (VAc) content decreasing. On the other hand, there was no modification of the temperature dependence of the relaxation time. Thus, the fragility value is quite constant whatever the VAc content. These results show that fragility and cooperativity have two different origins. An extrapolation to nonconstrained polyethylene amorphous phase was proposed and new glass transition temperature and fragility values were determined. Highlights 1.Amorphous phase mobility is studied for poly(ethylene-vinyl acetate) copolymers 2.Fragility is constant from PVAc to EVA50 while cooperativity length decreases 3.New extrapolated T g and fragility values are proposed for PE 4.Fragility and cooperativity are not governed by the same macromolecular properties Keywords Amorphous phase, dynamic glass transition, poly(ethylene-vinyl acetate) . Therefore, structural relaxation temperature dependences can be defined as Super-Arrhenius, due to a possible behavior between two extreme limits: "strong" glass forming liquids for which the viscosity variations (or relaxation time) are very slow and follow an Arrhenius law, and the "fragile" glass forming liquids for which a very abrupt and steep non-Arrhenian variations can be observed.The fragility index (m) quantifies the steepness of the temperature dependence of the relaxation time () close to T g (defined at τ = 100s) and can be calculated as follows:The fragility values for different materials such as polymers, metallic glasses, organic and inorganic ionic glasses, and for small organic molecules were summarized Several approaches have been proposed to explain the correlation between the fragility and the molecular mobility near T g [10,16,17]. According to the theory proposed by Adam and Gibbs [18], it is well accepted that the α relaxation process is cooperative in nature: a structural unit can move only if a certain number of neighboring structural units move also. Besides, the molecular motions are mainly governed by the intermolecular interactions having important effects in the viscous slowing down of molecular dynamics when the glass-forming liquid is cooled-down close to T g . Thus, the notion of Cooperative Rearranging Region (CRR) was introduced, and the CRR size can be estimated according to different models and theories in terms of characteristic length scale or in terms of structural unit number [16,19-22]. Accordi...