TIT# The role of the amorphous phase in the recrystallization process
     of cold-crystallized poly(ethylene terephthalate)
AUT# Pieruccini, M.; Flores, A.; N”chel, U.;
     Di Marco, G.; Stribeck, N.; Balt  Calleja, F. J.;
SOU# Eur. Phys. J. E (2008), submitted June 9
LOC# xv116
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ABS# The process of recrystallization in poly(ethylene terephthalate) (PET)
     is studied by means of X-ray scattering (SAXS and WAXS) and dynamical
     mechanical thermal analysis. Samples cold-crystallized for 9 h at
     temperatures $T_c$ = 100øC and $T_c$ = 160øC, i.e. in the middle of the
     $\alpha$ dispersion region and close to its upper bound respectively,
     are analyzed. During heating from room temperature, a structural
     rearrangement of the stacks is always found at
     $T_r \simeq T_c + 20$~$^{\rm{o}}$C. This process is characterized by a
     decrease of the linear crystallinity, irrespective of $T_c$; on the other
     hand, the WAXS crystallinity never increases with $T$ below
     $T_c + 30$~$^{\rm{o}}$C. The lamellar thickness in the low-$T_c$ sample
     decreases significantly after the structural transition, whereas in the
     high-$T_c$ sample the lamellar thickness remains almost unchanged. In
     both, high- and low-$T_c$, the interlamellar thickness increases above
     $T_r$. Moreover, the high-$T_c$ sample shows a lower rate of decrease of
     the mechanical performance with increasing $T$ as the threshold $T_r$ is
     crossed. This result is interpreted in terms of the formation of rigid
     amorphous domains where the chains are partially oriented. The presence
     of these domains would determine \emph{i}) the stabilization of the
     crystalline lamellae from the thermodynamic point of view and \emph{ii})
     the increase of the elastic modulus of the amorphous interlamellar
     regions. This idea is discussed by resorting to a phase diagram. An
     estimation of the chemical potential increase of the interlamellar
     amorphous regions, due to the enhancement of the structural constraints
     hindering segmental mobility, is offered. Finally, previous calculations
     developed within the framework of the gaussian chain model
     [F. J. Balt$\acute{\textrm{a}}$ Calleja \emph{et al.}
     Phys. Rev. B {\bf 75}, 224201 (2007)], are used here to estimate the
     degree of chain orientation induced by the structural transition of the
     stacks.