  Fragility index and cooperativity length characterizing the molecular
mobility in the amorphous phase are for the first time calculated in drawn
polylactide (PLA). The microstructure of the samples is investigated from
wide-angle X-ray scattering (WAXS) whereas the amorphous phase dynamics are
revealed from broadband dielectric spectroscopy (BDS) and temperature-modulated
differential scanning calorimetry (TMDSC). The drawing processes induce the
decrease of both cooperativity and fragility with the orientation of the
macromolecules. Post-drawing annealing reveals an unusual absence of
correlation between the evolutions of cooperativity length and fragility. The
cooperativity length remains the same compared to the drawn sample while a huge
increase of the fragility index is recorded. By splitting the fragility index
in a volume contribution and an energetic contribution, it is revealed that the
amorphous phase in annealed samples exhibits a high energetic parameter, even
exceeding the amorphous matrix value. It is assumed that the relaxation process
is driven in such a way that the volume hindrance caused by the
thermomechanical constraint is compensated by the acceleration of segmental
motions linked to the increase of degrees of freedom. This result should also
contribute to the understanding of the constraint slackening in the amorphous
phase during annealing of drawn PLA, which causes among others the decrease of
its barrier properties.
