A New ε Crystal Modification Found in Stereodefective Isotactic Polypropylene Samples

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 α phase backbones with large amounts of γ phase overgrowths. The new form is nucleated by a crystal–crystal growth transition on parent α 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 αiPP elongated laths but do not display any of their overgrowths (α–α lamellar branching or α–γ epitaxial relationship). The electron diffraction evidence (<i>hk</i>0 and <i>hk</i>1 reflections accessed from flat-on and tilted single crystals, respectively) indicates an orthorhombic unit cell with parameters <i>a</i> = 12.50 Å, <i>b</i> = 24.60 Å, and <i>c</i> = 6.5 Å that houses eight stems (space group <i>Pccn</i>). The chain conformation is the archetypical 3₁ helix of the α, β, and γ forms of iPP but arranged in a near-double-tetragonal packing. As in αiPP, four distinct layers are packed along the <i>b</i>-axis (that is however ≈4 Å longer), but the <i>a</i>-axis repeat distance is nearly doubled. Stereodefects (<i>rr</i> defects in all <i>m</i> 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