TIT# SAXS Study of Oriented Crystallization of Polypropylene from a
     Quiescent Melt
AUT# Stribeck, N.; Nöchel, U.; Almendárez Camarillo, A.; Roth, S. V.;
     Dommach, M.; Bösecke, P.;
SOU# xv 103
LOC# Macromolecules (2007), ~, submitted
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ABS# Uniaxially oriented polypropylene (PP) is molten in the synchrotron beam
     and crystallized from the quiescent melt keeping its orientation in order
     to study the mechanisms of its crystallization. We document the different
     nanostructures observed as a function of melt-annealing temperature,
     undercooling and time.
     In order to obtain a melt that crystallizes with high preferential
     orientation again, a melt-annealing temperature between 170°C and 176°C
     is chosen. Isothermal crystallization at 155°C results in slow formation
     of (primary) lamellae placed at random. As the crystallization
     temperature is decreased (150, 145, and 140°C), more and more secondary
     crystallites are observed which develop from a block mesostructure
     according to Strobl's mechanism. During the isothermal phase the blocks
     are fusing more or less to form imperfect lamellae. The structure
     evolution observed in the time-resolved small-angle X-ray scattering
     (SAXS) data during crystallization and re-melting facilitates
     discrimination between the block structure and another frequently
     discussed morphology, Keller's cross-hatched structure. While after all
     our quiescent crystallization experiments most of the crystallites are
     blocks or incompletely fused lamellae, the hard-elastic precursor
     material which has been made under extreme gradients of temperature and
     pressure exhibits the melting of homogeneous and extended lamellae. As
     we apply a steep temperature gradient (-100°C/min) to our melt in a
     nonisothermal crystallization experiment, we initially observe the
     formation of homogeneous and extended lamellae, as well.