| Isotactic polypropylene (iPP) is a kind of semi-crystalline polymer, the crystalline structure and perfection of which depends largely on the crystallization and annealing conditions. Under different thermal histories, crystalline polymer can form different crystalline structure and supermolecular structure. Recently, much research on the effect of annealing under relatively low temperature on structure of crystalline polymer has been reported.This paper researched the effect of annealing nearby melting point on structure of iPP sheets. Fistly, differential scanning calorimeter (DSC)、 small angle X-ray scattering (SAXS)、 wide angle X-ray diffraction (WAXD) and polarized optical microscopy (POM) have been used to study the effect of annealing on the crystal morphology and structure of iPP sheet at different temperatures for30min in constant temperature oven. Besides, in situ SAXS and in situ WAXD have been used to research the crystalline structure of iPP sheets in annealing process. The work and relevant results are summarized as follows:1. iPP sheets, obtained from Hakke Polylab System, were annealed in constant temperature oven for30min, annealing temperature was set at80℃,100℃,140℃,155℃,160℃,165℃,170℃and180℃. The results indicated that, because of stretching flow fields, β-crystal was formed in the processing. When iPP sheets were annealed after140℃, the relative content of β-iPP increased. With the increasing of annealing temperature from140℃to165℃, melting point, crystallinity and long period improved, Moreover, when annealing temperature was up to170℃and180℃, melting point, long period and crystallinity of iPP sheet decreased.2. In situ SAXS and in situ WAXD were used to study evolution of crystal morphology and structure in annealing processing at different temperatures. The experient results indicated that, when annealing temperature was145℃and155℃, a small part of unperfect crystalline melted and the crystallinity decreased a little while long period increased during heating process. The long period and crystallinity of iPP increased because of recrystallization in isothermal processing for60min. Some crystal with defect formed during cooling process, so long period of iPP decreased; When the iPP sample was heated from25℃to167℃, crystallinity decreased obviousely and long period increased, but in isothermal process for60min the part of crystal with defect melted and the melted part crystaled on the basis of unmelted crystal, resulting in more perfect crystal and during cooling process, crystallinity improved quickly and long period decreased; Total crystal melted under180℃and190℃, lossing crystal nucleus. Crystal with defect formed in iPP sample in cooling pcocess, and the crystallinity and long period decreased. |
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