A New 蔚 Crystal Modification Found in Stereodefective Isotactic Polypropylene Samples
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- 作者：Bernard Lotz
- 刊名：Macromolecules
- 出版年：2014
- 出版时间：November 11, 2014
- 年：2014
- 卷：47
- 期：21
- 页码：7612-7624
- 全文大小：482K
- ISSN：1520-5835
文摘
The main features of an original crystal modification of isotactic polypropylene (iPP) are described. This form is obtained with a stereodefective or 鈥渁n-isotactic鈥?iPP produced with a zirconene catalyst by Rieger et al. [ Macromolecules 1990, 23, 3559鈭?568]. In thin films, most of the material crystallizes as composite crystals made of 伪 phase backbones with large amounts of 纬 phase overgrowths. The new form is nucleated by a crystal鈥揷rystal growth transition on parent 伪 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 伪iPP elongated laths but do not display any of their overgrowths (伪鈥撐?lamellar branching or 伪鈥撐?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 a = 12.50 脜, b = 24.60 脜, and c = 6.5 脜 that houses eight stems (space group Pccn). The chain conformation is the archetypical 31 helix of the 伪, 尾, and 纬 forms of iPP but arranged in a near-double-tetragonal packing. As in 伪iPP, four distinct layers are packed along the b-axis (that is however 鈮? 脜 longer), but the a-axis repeat distance is nearly doubled. Stereodefects (rr defects in all m sequences) induce 鈥渁bnormal鈥?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.