  By taking advantage of the molecular weight dependence of the glass
transition of polymers and their ability to form perfectly miscible blends, we
propose a way to modify the fragility of a system, from fragile to strong,
keeping the same glass properties, i.e. vibrational density of states,
mean-square displacement and local structure. Both slow and fast dynamics are
investigated by calorimetry and neutron scattering in an athermal
polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene
considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer
have the same heat capacity in the glass and in the liquid, their fragilities
differ strongly. This difference in fragility is related to an extra
configurational entropy created by the mixing process and acting at a scale
much larger than the interchain distance, without affecting the fast dynamics
and the structure of the glass.
