TIT# Recrystallization studies in isotropic cold crystallized PET:
     influence of heating rate
AUT# Flores, A.; Pieruccini, M.; N”chel, U.; Stribeck, N.; Balt  Calleja, F. J.;
SOU# Polymer (2008), 49(4), 965-973
LOC# xv109
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COM#
APP#
MAT#
ABS# The nanostructural changes associated to the multiple melting behaviour
     of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have
     been investigated by means of simultaneous wide- and small-angle X-ray
     scattering, using a synchrotron radiation source. Variations in the
     degree of crystallinity, coherent lateral crystal size and long period
     values, as a function of temperature, for two different heating rates are
     reported for cold-crystallized samples in the 100 - 190 øC range. The
     Interface Distribution Function analysis is also employed to provide the
     crystalline and amorphous layer thickness values at various temperatures
     of interest. Results suggest that samples crystallized at both low
     ($T_a$ = 100 - 120 øC) and high ($T_a$ = 160 - 190 øC) temperatures are
     subjected to a nearly continuous nanostructural reorganization process
     upon heating, starting immediately above $T_g$ (÷80 øC) and giving rise to
     complete melting at ÷260 øC. For all the Ta investigated, a
     melting-recrystallization mechanism seems to take place once $T_a$ is
     exceeded, concurrently to the low-temperature endotherm observed in the
     DSC scans. For low-$T_a$ and slow heating rates (2 øC/min), a conspicuous
     recrystallization process is predominant within $T_a$ + 30 øC ó T ó 200 øC.
     In contrast, for high-$T_a$, an increasingly strong melting process is
     observed. For both, high- and low-$T_a$, an extensive structural
     reorganization takes place above 200 øC, involving the appearance of new
     lamellar stacks simultaneously to the final melting process. The two
     mechanisms should contribute to the high-temperature endotherm in the DSC
     scan. Finally, the use of a high heating rate is found to hinder the
     material's overall recrystallization process during the heating run and
     suggests that the high-temperature endotherm is ascribed to the melting
     of lamellae generated or thickened during the heating run.