  The plastic deformation of Polybutylene terephthalate (PBT) is investigated
at three temperatures (120$^\circ$C, 150$^\circ$C, 180$^\circ$C) representative
of the span between glass transition, $T_g \approx 55^\circ$C and melting $T_m
\approx 225^\circ$C temperatures. The mechanisms of tensile deformation are
analyzed as a function of the true strain using a combination of
temperature-controlled tensile experiments coupled with 2D digital image
correlation and synchrotron sourced wide angle (WAXS) and small angle (SAXS)
X-ray scattering the amorphous and crystalline regions causing the transition
between polymorphs, respectively from the isotropic amorphous phase to a
smectic phase and from the $\alpha$ crystalline phase to the $\beta$ phase.
