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摘要
The main features of an original crystal modification of isotactic polypropylene (iPP) are described. This form is obtained with a stereodefective or an-isotactic iPP produced with a zirconene catalyst by Rieger et al. [Macromolecules 1990, 23, 3559-3568]. In thin films, most of the material crystallizes as composite crystals made of alpha phase backbones with large amounts of gamma phase overgrowths. The new form is nucleated by a crystalcrystal growth transition on parent alpha phase crystals. It is probably produced with only the most stereodefective part of the material. The single crystals formed in thin films are very similar to the aiPP elongated laths but do not display any of their overgrowths (alpha alpha lamellar branching or alpha gamma epitaxial relationship). The electron diffraction evidence (hk0 and hk1 reflections accessed from flat-on and tilted single crystals, respectively) indicates an orthorhombic unit cell with parameters alpha = 12.50 angstrom, beta = 24.60 angstrom, and c = 6.5 angstrom that houses eight stems (space group Pccn). The chain conformation is the archetypical 3(1) helix of the alpha, beta, and gamma forms of iPP but arranged in a near-double-tetragonal packing. As in alpha iPP, four distinct layers are packed along the b-axis (that is however approximate to 4 angstrom longer), but the a-axis repeat distance is nearly doubled. Stereodefects (rr defects in all m sequences) induce abnormal locations of the methyl groups as exist in polyisobutylene. These defects generate a near-irrational helix projection that translates in an unusual tetragonal-like packing (for 3-fold helices) and results in a low-density metastable crystal structure.
- 出版日期2014-11-11
