**Author details**

larger than that of the *β*-sheaf iPP. This indicates that the yield strength is much more sensitive to crystal phase modification than to the lamellar arrangement of the spherulites. The reduced yield stress of *β*-sheaf iPP compared to those of the *α*-acicular and *α*-sheaf iPPs is associated with the greater chain mobility in the *β*-phase crystals. The lower packing density of the *β*-phase is accompanied by the

*Comparison of stress-strain curves of* α*-spherulitic iPP samples with different contents of sheaflike spherulites.*

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

To confirm these conclusions, we compared the stress-strain behaviors measured from 320 to 380 K for *α*-spherulitic iPP sheets with different amounts of sheaflike spherulites which were prepared by tempering the iPP samples with the different amount of *β*-spherulites. Note here that these iPP sheets have a fixed crystallinity of about 74%. As a result, *α*-spherulitic iPP sheets having various sheaflike spherulite contents with constant crystallinity were prepared. **Figure 14** also shows that the ductility enhances as the content of sheaf spherulites increased, whereas the yield strengths of all sheets are the same and almost insensitive to the

Based on our investigation of the tensile properties of *α*-iPP and *β*-modified iPP, in which all the other structural parameters, such as overall crystallinity and spher-

ulite size, were controlled, the following conclusions can be drawn:

reduced stem interactions in the *β*-crystalline lamellae as compared to the *α*-lamellae, leading to the lower yield stress of the *β*-phase compared to that of the *α*-phase. An additional factor reducing the lamellar strength is the crystallographic symmetry of the hexagonal *β*-phase, which provides three equivalent

glide planes.

**Figure 14.**

**6. Summary**

**86**

lamellar arrangement of the spherulites.

Koh-hei Nitta\* and Tsutomu Takashima Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan

\*Address all correspondence to: nitta@se.kanazawa-u.ac.jp

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
