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. (C) 2015 Elsevier Ltd. All rights reserved.