Multiple crystal forms and complex melting behavior of syndiotactic polystyrene (sPS) were thoroughly examined using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). This study has provided clear evidence for interpretation of relationships between the multiple melting peaks and polymorphism crystals in sPS under isothermal melt-crystallization, dynamic cooling, or other thermal histories. For the first time, assignment of different crystal types to the numerous melting peaks in sPS was accomplished. Effects on the mechanism of chain packing were examined for correlating the relationships between the polymorphism and multiple melting peaks. The R-crystal packing can be favored and becomes an alternative route in sPS crystallization under three conditions: (1) slow cooling from molten state, (2) melt crystallization at low temperatures, or (3) cold crystallization from quenched amorphous glass. By comparison, melt-crystallization at most accessible temperatures produces solidified sPS containing both R-type and β-type crystals of various fractions, but a higher temperature tends to favor greater fractions of β-type crystal. The β-type crystal becomes the only discernible species if sPS is melt-crystallized at temperatures equal to or higher than 260 °C, which suggests that in conditions approaching equilibrium the β-crystal lamella is the favored packing in sPS.
Polymorphic behavior (i.e., the development of α and β forms) of melt-crystallized syndiotactic
polystyrene, sPS, has been studied by structure analysis of FTIR (Fourier transform infrared spectroscopy),
WAXD (wide-angle X-ray diffraction), and ED (electron diffraction) as well as thermal analysis of DSC
(differential scanning calorimetry). Significant factors that influence the formation of polymorphism were
examined and described. Isolated α crystals and β crystals of sPS crystallized at different crystallization
temperatures from the melt have been obtained. The equilibrium melting temperatures, T
m
o, of both
forms were determined by using linear Hoffman−Weeks (H−W) extrapolation and nonlinear H−W
treatment. The T
m
o (i.e., structural metastability) of β form in sPS was found to be higher than that of
α form. The occurrence of phase stability inversion with lamellar size (i.e., morphological metastability)
in sPS was recognized. Transformations from α to β phase in the stage of crystal growth or in the heating
scanning process have been evidenced. The interlinkage of structural metastability and morphological
metastability in sPS polymorphism was examined. The behavior of phase transformation has been
successfully interpreted in terms of the stability inversion phase diagram. The formation of the α form
is indeed a kinetic result of crystallization.
Three-dimensional dissecting onto interior lamellar structures of spherulites in poly(ethylene adipate) (PEA) in bulk forms crystallized at 28 ± 3°C were studied in correlation with thin-film forms. Interior morphology in bulk PEA samples revealed onion-like alternating shells. Interior spherulites clearly exposes that the lamellae within the shell interior are composed of stacked plate lamellae in radial direction while the shell surface region are arranged by tangential lamellae. Shell interior and surface correspond to the valley and ridge of ring-banded on the top surface, respectively. Morphology characterization on fracturing patterns further evidence that the shells could be cleaved apart along the shell surface, indicating weak physical connection between these two perpendicularly oriented shells. Further thermal analysis and X-ray results also supported existence of highly ordered lamellar packing into two mutually perpendicular chain orientations in the 28°C-crystallized PEA bulks, which differed significantly from the randomly oriented lamellae in either 2 or 33°C-crystallized PEA bulks.
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