**Abstract**

Spherulitic isotactic polypropylenes (iPPs) having a wide range of *β-phase* contents were prepared by adding *β*-nucleators, and the effects of the *β*-phase modification on the mechanical properties of the iPP were investigated. This chapter described the tensile properties of *β*-nucleated iPP, while key structural parameters, such as spherulite size and crystallinity, were controlled. The increase in the *β*-phase content led to broader yield peaks and an enhancement in the yield toughness but to a reduction in the yield strength. On the other hand, the initial elastic modulus was found to be independent of the *β*-contents. Furthermore, the deformation of the *β*-spherulites, which have a sheaflike structure, was anisotropic and depended on the stretching direction with respect to the sheaf axis. Consequently, the improved drawability and ductility of *β*-iPP compared to *α*-iPP are thus associated with the enhanced toughness resulting from the multiple deformation processes in the sheaflike spherulites.

**Keywords:** *β*-phase crystal, mechanical properties, tensile deformation, spherulite, crystalline morphology

## **1. Introduction**

As well-known, isotactic polypropylene (iPP) is a polymorphic material with various crystal forms [1], such as monoclinic (α), hexagonal (β), triclinic (γ), and smectic, of which the *α*-phase is the most typical crystalline form. Commercial grades of iPP usually crystallize into the α-phase with sporadical occurrence of the *β*-phase under higher supercooling [2]. Crystallization under a temperature gradient [3] or flow-induced crystallization [4, 5] encourages the formation of the *β*-phase. To prepare of *β*-modified iPP samples, the introduction of selective *β*-nucleators is the most reliable method [6]. However, unless using specific *β*-nucleating agents, the *β*-phase cannot be obtained at high levels and is always accompanied by *α*-phase crystals. The *α/β* ratio is very sensitive to the crystallization temperature and the cooling rate because of the different nucleation rates of the two crystalline species. Varga et al. [7] found that pure *β*-phase can be achieved in the presence of some selective *β*-nucleators by the selection of appropriate thermal conditions for crystallization. Furthermore, the *β*-phase was found to be transformed to the *α*-phase by heat treatment [8]. This demonstrates that the monoclinic structure is thermodynamically stable, whereas the hexagonal *β*-phase is metastable and difficult to obtain under normal processing conditions.

Recently the number of practical studies has increased [9] because the impact strength and toughness of *β*-nucleated iPP exceed those of *α*-iPP. Although many studies have compared the mechanical properties of *α*-iPP and *β*-iPP, the morphological origin of the differences in the mechanical properties has not been clarified yet.

The mechanical properties of semicrystalline polymers such as iPP and polyethylene (PE) are governed by their morphological features which are specified by several structural variables such as the degree of crystallinity, spherulite size, crystalline thickness, and structural organization of the supermolecular structure [10]. These diversity and independencies of these structural variables make it difficult to provide a molecular or structural interpretation for the mechanical properties and deformation behavior of semicrystalline polymers [10]. Indeed, changing the thermal or processing conditions involves the concomitant modification of several structural parameters; thus, it is difficult to determine the structural origin of the change in mechanical properties as reported by Labour et al. [11]. Consequently, it is necessary to keep all the other structural parameters to be fixed to elucidate the effects of a given structural parameter on the mechanical properties. Very few studies have dealt with the mechanical properties of *β*-nucleated iPP with a wide range of *β*-phase contents, while all the other structural parameters, such as supermolecular organization and crystallinity, are controlled. The aim of this chapter is to elucidate the influence of the *β*-phase modification on the tensile properties of iPP. For this purpose, crystallization procedures, for the production of iPP sheets having a wide range of *β*-phase contents with fixed crystallinity and spherulite size, were developed. In addition, the effects of spherulitic morphology on tensile properties were investigated by comparing the differences in deformation mechanism of isolated α- and *β*-spherulites.

fraction of the iPP. In this chapter, PP0 is denoted by *α*-iPP and PP98 is denoted

*Polypropylene*

The crystalline *β*-phase content (the volume fraction of the *β*-phase in the crystalline portion) was determined from WAXD patterns. The WAXD measurements were carried out at room temperature with a Rigaku RU-200 diffractometer with Ni-filtered Cu-Kα radiation from a generator operated at 40 kV and 100 mA. The *β*-phase fraction in the crystalline part of the specimens was assessed from the ratio of the area of the main (300) *β*-phase to the sum of the areas of the four main crystalline reflections: (110), (040), and (130) from the *α*-phase plus (300) from

Here, we modified the analysis method proposed by Somani et al. [13] to obtain the volume fraction of *β*-phase in the crystalline fraction quantitatively. The reflection peaks in the WAXD profiles were deconvoluted. In the WAXD profile, (110) at 14.1°, (040) at 16.9°, and (130) at 18.5° are the principal reflections (in 2*θ*) of the *α*-phase crystals of iPP, whereas (300) at 16.1° is the principal reflection of *β*-phase crystals, and are considered as the markers for *α*-phase and *β*-phase crystals, respectively. The various reflection areas were computed after subtraction of the

The volume fraction of *β*-phase crystals was calculated using the following

*ϕβ* <sup>¼</sup> *<sup>S</sup>β*ð Þ <sup>300</sup> *<sup>ρ</sup>*�<sup>1</sup>

*<sup>S</sup>β*ð Þ <sup>300</sup> *<sup>ρ</sup>*�<sup>1</sup>

of the (300) reflection peak, and *S<sup>α</sup>* is the sum of the areas of (110), (040), and (130) peaks of *α*-phase crystals, respectively. The calibration constant *k* was estimated to be 1.11 from the difference in the sensitivity of WAXD reflections with respect to the thickness of the sheets for the *α*-phase and *β*-phase reflections.

Crystallinity can be precisely determined from density data. The densities of the specimens were determined by the flotation method. A binary medium prepared from various ratios of distilled water and ethanol was used. The volume crystallinity

> *<sup>χ</sup><sup>V</sup>* <sup>¼</sup> *<sup>ρ</sup>* � *<sup>ρ</sup><sup>a</sup> ρ<sup>c</sup>* � *ρ<sup>a</sup>*

*βc*

*<sup>β</sup><sup>c</sup>* þ *kSαρ*�<sup>1</sup> *αc*

*S<sup>α</sup>* ¼ *S<sup>α</sup>*ð Þ <sup>110</sup> þ *S<sup>α</sup>*ð Þ <sup>040</sup> þ *S<sup>α</sup>*ð Þ <sup>130</sup> (2)

) is the density of *α*-phase crystal [17], *S<sup>β</sup>* is the area

) is the density of the *β*-phase

(1)

(3)

the *β*-phase using Turner-Jones method [12].

*β-Modified*

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

 *Isotactic* 

Here *k* is the calibration factor, *ρβ<sup>c</sup>* (=921 kgm�<sup>3</sup>

crystal [14–16], *ρα<sup>c</sup> (*=936 kgm�<sup>3</sup>

can be obtained using

**73**

by *β*-iPP.

*Characteristics of iPP sheets.*

*Tensile Properties in*

*DOI:* 

**Table 1.**

amorphous halo.

equations:

and
