**5. Effects of spherulite morphology on tensile properties**

To investigate the effects of the lamellar organization within the spherulites on the tensile properties of bulk iPP sheets, *α*-modified iPP samples were prepared by

**Figure 12.** *Dependence of* β*-contents of* β*-nucleated iPP on the heat-treatment temperature.*

#### *Tensile Properties in β-Modified Isotactic Polypropylene DOI: http://dx.doi.org/10.5772/intechopen.83348*

sheaf-lamellae in the equatorial zone was initiated in the first stage of deformation, and then the separation of the sheaf-lamellae continues, and more deformation bands proceeded as the strain was further increased. On the other hand, when the sheaf axis was in the draw direction (see **Figure 11b**), the spherulite was initially deformed to an ellipse of similar shape to that expected for affine deformation. This is because its equatorial region is tougher because the lamellae parallel to the stretching direction more strongly resist deformation than the lamellae perpendicular to the loading direction as mentioned in the discussion of *β*-spherulite deformation. Subsequent deformation caused micro-necking in such a way that the traces of the sheaf structure remain in the center portion of the spherulite. The sheaf-lamellae located perpendicular to the loading direction are brittle, whereas the sheaf-lamellae located parallel to the loading direction are tougher or ductile. This anisotropic deformation behavior is quite different from the isotropic deformation of acicular *α*-spherulites, but it is similar to those of sheaf *β*-spherulites as well as isolated PE spherulites, as shown by Lee et al. [41]. This is plausible because

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

It has been long recognized that the deformation of crystalline polymers must be considered in terms of various structural parameters such as crystallinity, lamellar thickness or long period, and spherulite size. However, the present results imply that deformation behavior and mechanical response of bulk iPP materials are affected not only by these structural factors but also by the morphological texture

To investigate the effects of the lamellar organization within the spherulites on the tensile properties of bulk iPP sheets, *α*-modified iPP samples were prepared by

**5. Effects of spherulite morphology on tensile properties**

*Dependence of* β*-contents of* β*-nucleated iPP on the heat-treatment temperature.*

PE spherulite is sheaflike.

within spherulites.

**Figure 12.**

**84**

the heat treatment of *β*-iPP (PP98) sheets. The PP98 sheets were heated at a 2 K/min and kept for 300 min at a fixed temperature. The *β*-phase contents are plotted against the fixed temperature in **Figure 12**. The *β* ! *α* transformation occurs at around 413 K, and the *β*-iPP was completely transformed into the *α*-phase above 427 K. It should be noted here that the thus-prepared iPP sheets contain sheaf type of spherulites. Consequently, we obtained three types of iPP sheets having a fixed crystallinity of around 73%, for example, the *α*-iPP sheets showing acicular spherulites, the *α*-iPP sheets showing sheaflike spherulites, and the *β*-iPP sheets showing sheaflike spherulites. Here, we have referred to these samples as *α*-acicular, *α*-sheaf, and *β*-sheaf.

**Figure 13** shows the stress-strain curves measured at various temperatures for *α*-acicular, *α*-sheaf, and *β*-sheaf sheets. At all temperatures, the stress–strain curves in the initial elastic strain domain were almost the same for these three samples. This is plausible because the crystallinities of these samples are almost equal. This also indicates that Young's modulus is dominated by the bulk crystallinity and is almost independent of the lamellar morphology of the spherulites and of the crystal modification. In addition, the *α*-acicular iPP sample is in more brittle manner than the *α*-sheaf and the *β*-sheaf iPP samples and broke around the yield peak except at 380 K. This indicates that the plastic deformation is much more sensitive to the change of the spherulite texture than to crystalline modification. This corroborates the previous results that the deformation behavior of isolated *β*-sheaf and *α*-sheaf spherulites is similar and significantly different from that of the *α*-acicular spherulites. Moreover, *β*-spherulites show a greater resistance to break when the strain direction is almost parallel to the sheaf axis. Interestingly, the yield strengths in *α*-acicular and *α*-sheaf iPPs are almost the same, although

*Comparison of stress-strain curves of spherulitic iPP sheets with a fixed crystallinity: α-acicular spherulites (blue), α-sheaf spherulites (green), and β-sheaf spherulites (red).*

(1) The stress-strain curves in the initial elastic strain region are dominated by the effects of crystallinity but are almost insensitive to changes in the crystal

*Polypropylene*

(2) The yield strength is more sensitive to the crystal modification than the lamellar arrangement of spherulites, and *α*-iPP shows a higher yield stress

(3) The plastic deformation process of the spherulites is sensitive to the lamellar arrangement of spherulites. The sheaflike spherulites are more ductile than the acicular spherulites and exhibit anisotropy in their plastic properties. The improved drawability and ductility of *β*-iPP compared with *α*-iPP is thus associated with the enhanced toughness resulting from multiple deformation

phases, as well as to the lamellar arrangements of spherulites.

than *β*-modified iPP.

*β-Modified*

*http://dx.doi.org/10.5772/intechopen.83348*

 *Isotactic* 

*Tensile Properties in*

*DOI:* 

**Author details**

**87**

Koh-hei Nitta\* and Tsutomu Takashima

provided the original work is properly cited.

Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan

© 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,

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

processes in the sheaflike spherulites.

**Figure 14.** *Comparison of stress-strain curves of* α*-spherulitic iPP samples with different contents of sheaflike spherulites.*

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 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.

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 lamellar arrangement of the spherulites.

## **6. Summary**

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

