In this paper, we present results of morphological
studies during long time melt crystallization and subsequent melting in poly(aryl ether ether ketone)
(PEEK). Morphological changes were
monitored via small angle X-ray scattering (SAXS). SAXS data were
analyzed via a combination of the
correlation and interface distribution functions. Our analysis
indicates the following: (1) The semicrystalline morphology is best described by a three-phase, dual lamellar
stack model. Stacks of a finite number
of lamellae and interlamellar amorphous layers are separated from each
other by interstack regions of
amorphous material (liquid pockets). (2) Secondary crystallization
occurs via the formation of secondary
lamellar stacks within the liquid pockets. Secondary lamellae are
thinner than primary lamellae (70 Å
vs 120 Å), and the amorphous layer thicknesses are about 47 Å in both
stacks. (3) The low endotherm
observed during a heating scan is associated with the melting of the
secondary lamellae. (4) At room
temperature, the semicrystalline PEEK material is in a state of
dilational stress (negative hydrostatic
pressure) which may originate from the secondary crystallization
process in constrained liquid pockets.
In-situ synchtrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction
(WAXD) are used to follow the isothermal crystallization (lamellar thickness, crystallinity, orientation,
and parent-to-daughter ratio) of a polydisperse isotactic polypropylene subjected to “short term shearing”
as a function of imposed shear stress, shearing duration, and crystallization temperature. The X-ray
data are interpreted in view of the real-space morphological information from ex-situ microscopy. Under
“mild” shearing conditions (shear stress less than a critical value and shearing duration less than a critical
time), needlelike nuclei are induced during shear but are so far apart that crystallites splay substantially
as they grow to form somewhat distorted spherulites; the X-ray results show weakly oriented growth on
a time scale that is rapid compared to quiescent crystallization and show that the orientation distribution
broadens as crystallization progresses. Stronger shearing leads to the elaboration of these nuclei into
threadlike structures that template the formation of highly oriented crystals with fiberlike orientation.
The parent-to-daughter ratio is influenced by both temperature and flow. As expected, increasing the
crystallization temperature leads to fewer daughter crystals relative to the parents. Shear also enhances
the formation of parents relative to daughters: as parent crystals form with their chain axis along the
flow direction, the epitaxial daughter crystals have their chain axis in an unfavorable direction,
perpendicular to the flow.
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