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Structure and Morphology Development in Syndiotactic Polypropylene During Isothermal Crystallization and Subsequent Melting

Published

Author(s)

Z G. Wang, X H. Wang, B S. Hsiao, R A. Phillips, Brian Rodriguez, S Srinivas, Haonan Wang, Charles C. Han

Abstract

Structure and morphology development during isothermal crystallization and subsequent melting of syndiotactic polypropylene (sPP) was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods using synchrotron radiation and thermal analysis (DSC). The morphology of sPP isothermally crystallized at 100 degrees C for 3 hrs ws obtained by transmission electron microscopy (TEM). The time and temperature dependent parameters such as long period, L, crystal lamellar thickness, lc, amorphous layer thickness, la. scattering invariant, Q, crystallinity, Xc, lateral crystal sizes. L200 and L020, and unit cell parameters a and b were extracted from SAXS and WAXD results during isothermal crystallization and subsequent melting processes. The decreasing long period and crystal thickness indicate that thinner secondary crystal lamellae are formed during the secondary crystallization process. The decreases in unit cell parameters a and b during the isothermal crystallization process suggest that crystal perfection takes place during secondary crystallization. The changes in the morphological parameters (the invariant, Q, crystallinity, Xc, long period, L, and the crystal thickness, lc) during subsequent melting followed a two-stage melting process, corresponding to the dual endothermic melting peaks of sPP in the DSC scan. We conclude that these dual melting peaks can be explained by the melting of secondary and primary lamellae (first peak) and the recrystallization-melting process (second peak), respectively. Additional minor endothermic peak located at the lowest temperature, was also visible and might be related to the melting of the secondary, thinner and defective lamellae. TEM results are consistent with the SAXS data, and support the assignment of the larger value (l1)(from the correlation function) as the crystal lamellar thickness. WAXD showed that during melting, thermal expansion was greater along the b axis than that along the a xis. The anisotropy in themal expansion may the result of larger thermal stress, lower stiffness, and a smaller Poisson ratio along the b axis.
Citation
Journal of Polymer Science Part B-Polymer Physics
Volume
39
Issue
No. 23

Keywords

isothermal crystallization and melting, small angle x-ray scattering, Structure and Morphology, syndiotactic polypropylene, wide angle x-ray diffraction

Citation

Wang, Z. , Wang, X. , Hsiao, B. , Phillips, R. , Rodriguez, B. , Srinivas, S. , Wang, H. and Han, C. (2001), Structure and Morphology Development in Syndiotactic Polypropylene During Isothermal Crystallization and Subsequent Melting, Journal of Polymer Science Part B-Polymer Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=851861 (Accessed February 26, 2024)
Created November 30, 2001, Updated October 12, 2021