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Related Concept Videos

Polymer Classification: Stereospecificity01:26

Polymer Classification: Stereospecificity

Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and...
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Crystalline structure changes in preoriented metallocene-based isotactic polypropylene upon annealing.

Yan Wang1, Jia-Zhuang Xu, Yan-Hui Chen

  • 1SINOPEC Fushun Research Institute of Petroleum & Petrochemicals, Liaoning Fushun 113001, China.

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Annealing metallocene-based isotactic polypropylene (m-iPP) influences its crystal structure. Lower temperatures favor alpha-crystals, while higher temperatures promote gamma-crystal formation due to molecular dynamics.

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Area of Science:

  • Polymer Science
  • Materials Science
  • Crystallography

Background:

  • Metallocene-based isotactic polypropylene (m-iPP) exhibits a shish-kebab structure after preorientation.
  • Understanding the annealing behavior of m-iPP is crucial for controlling its morphology and properties.

Purpose of the Study:

  • To investigate the effect of annealing temperature on the melting and crystallization behavior of partially melted m-iPP.
  • To elucidate the structural evolution and crystal phase transitions during annealing.

Main Methods:

  • Synchrotron wide-angle X-ray diffraction (WAXD)
  • Small-angle X-ray scattering (SAXS)
  • Differential scanning calorimetry (DSC)

Main Results:

  • Low-temperature annealing led to lamellar thickening, lateral growth, and a decrease in gamma-crystal fraction, favoring alpha-crystals due to topological constraints.
  • Medium-temperature annealing increased the crystallizable melt, promoting gamma-crystal formation alongside shish-kebab and macro-unoriented structures.
  • High-temperature annealing, where no crystals survived, greatly favored gamma-crystal formation due to molecular dynamics, resulting in randomly oriented lamellae.

Conclusions:

  • The annealing temperature critically dictates the crystal structure and morphology of m-iPP.
  • Topological constraints and molecular dynamics play key roles in crystal evolution and phase selection.
  • A comprehensive understanding of structural changes at different scales allows for predicting morphological features based on annealing conditions.