**4. Deformation of isolated spherulites**

According to our previous studies [30, 31] concerning the yield behavior of typical spherulitic polymers such as PE and *α*-iPP, several lamellae tend to cluster into bundles with tie molecules, where these are separated from one another by the amorphous regions and the lamellar clusters constituting of spherulites act as deformation units. The lamellar clusters are bridged by the inter-cluster or intercrystalline links, as proposed by Keith-Padden et al. [32], thus acting as stress transmitters. The stacked lamellae or lamellar clusters are fragmented into cluster units or blocks at the yield point, resulting in a stress drop. Beyond the yield point, the plastic deformation involves the rotation of the cluster units and the sliding of stacked lamellae inside each cluster units, and the fragmented cluster units are rearranged into microfibrils in the necking region [33]. The continuous structural transformation corresponds to the neck propagation. In the case of *β*-spherulitic iPP showing a broad yield process, the lamellar clusters disintegrate accompanied by sliding crystalline stems and chain slip inside the crystalline lamellae. Consequently, the fragmentation of lamellae and/or lamellar clusters hardly occurs. These differences in yielding mechanism between *α*- and *β*-spherulitic iPPs are due to the differences not only in the cohesive force between crystalline chains but also in the spherulite morphology.

spherulite structure is converted into a highly oriented one. On the other hand, the *β*-iPP exhibits broader yield peaks without obvious necking formation and has a lower yield strength than that of *α*-iPP. Furthermore, the *β*-iPP specimen began to be whitened with further extension, whereas the *α*-iPP exhibited partial stress whitening beyond yielding. This stress whitening seems to be caused by the forma-

*Polypropylene*

Previously, we reported a method for preparing a thin film with huge isolated spherulites embedded in a soft (smectic) matrix [36]. The deformation mechanism of spherulites can be examined from the direct observation of the stretched film on the polarized optical microscope. Thus, we mounted the manual stretcher on the optical microscope to observe the deformation process of the isolated spherulites. **Figure 10a** shows the optical microscopic pictures of an isolated *α*-spherulite. A few arc-shaped cracks rapidly appeared in the polar zone in the initial stages of

*Polarized optical microscopic pictures of (a) uniaxial stretching of an isolated* α*-acicular spherulite, (b) uniaxial stretching perpendicular to the sheaf axis of an isolated* β*-sheaf spherulite, and (c) uniaxial*

*stretching parallel to the sheaf axis of an isolated* β*-sheaf spherulite.*

tion of numerous voids after yielding [35].

*β-Modified*

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

 *Isotactic* 

*Tensile Properties in*

*DOI:* 

**Figure 10.**

**81**

**Figure 8** shows an atomic force microscopy (AFM) micrograph of the morphology of the *β*-form iPP spherulites prepared in this work. The morphology is significantly different from the typical spherulite morphology of *α*-phase iPP. This spherulite seems to be type III according to Norton and Keller's classification [34] because there is no lamellar twisting within the spherulites. The embryo *β*-spherulites consist of parallel stacked lamellae. This type of spherulite is referred to as a sheaflike structure. Shi et al. [19] reported that the *β*-spherulites develop initially as rodlike structures and then by branching of the lamellae, finally evolving into sheaflike structures. In this case, the spherulite is formed from one crystal via a unidirectional growth mechanism. The spherical shape is attained through continuous branching and fanning via the intermediate stage of sheaves. However, the *α*-spherulites consist of an aggregate of chain-folded lamellae growing from a central point (nucleus). This is referred to as an acicular structure. Both structural models of *α*- and *β*-spherulites are shown in **Figure 9**.

It is likely that the spherulite morphology plays a central role in controlling the plastic deformation and tensile behavior of both PE and iPP materials. The mechanical responses to tensile yielding and the deformation process are considered to be fundamentally different between *α*- and *β*-spherulites. Tensile deformation of *α*-iPP materials is accompanied by necking process, in which the initial isotropic

**Figure 8.** *AFM pictures of a* β*- spherulite.*

**Figure 9.** *Illustrations of lamellar arrangement of* α*- and* β*-spherulites.*

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

showing a broad yield process, the lamellar clusters disintegrate accompanied by sliding crystalline stems and chain slip inside the crystalline lamellae. Consequently, the fragmentation of lamellae and/or lamellar clusters hardly occurs. These differences in yielding mechanism between *α*- and *β*-spherulitic iPPs are due to the differences not only in the cohesive force between crystalline chains but also in the

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

**Figure 8** shows an atomic force microscopy (AFM) micrograph of the morphology of the *β*-form iPP spherulites prepared in this work. The morphology is significantly different from the typical spherulite morphology of *α*-phase iPP. This spherulite seems to be type III according to Norton and Keller's classification [34] because there is no lamellar twisting within the spherulites. The embryo *β*-spherulites consist of parallel stacked lamellae. This type of spherulite is referred to as a sheaflike structure. Shi et al. [19] reported that the *β*-spherulites develop initially as rodlike structures and then by branching of the lamellae, finally evolving into sheaflike structures. In this case, the spherulite is formed from one crystal via a unidirectional growth mechanism. The spherical shape is attained through continuous branching and fanning via the intermediate stage of sheaves. However, the *α*-spherulites consist of an aggregate of chain-folded lamellae growing from a central point (nucleus). This is referred to as an acicular structure. Both structural

It is likely that the spherulite morphology plays a central role in controlling the

mechanical responses to tensile yielding and the deformation process are considered to be fundamentally different between *α*- and *β*-spherulites. Tensile deformation of *α*-iPP materials is accompanied by necking process, in which the initial isotropic

plastic deformation and tensile behavior of both PE and iPP materials. The

spherulite morphology.

**Figure 9.**

**80**

**Figure 8.**

*AFM pictures of a* β*- spherulite.*

*Illustrations of lamellar arrangement of* α*- and* β*-spherulites.*

models of *α*- and *β*-spherulites are shown in **Figure 9**.

spherulite structure is converted into a highly oriented one. On the other hand, the *β*-iPP exhibits broader yield peaks without obvious necking formation and has a lower yield strength than that of *α*-iPP. Furthermore, the *β*-iPP specimen began to be whitened with further extension, whereas the *α*-iPP exhibited partial stress whitening beyond yielding. This stress whitening seems to be caused by the formation of numerous voids after yielding [35].

Previously, we reported a method for preparing a thin film with huge isolated spherulites embedded in a soft (smectic) matrix [36]. The deformation mechanism of spherulites can be examined from the direct observation of the stretched film on the polarized optical microscope. Thus, we mounted the manual stretcher on the optical microscope to observe the deformation process of the isolated spherulites.

**Figure 10a** shows the optical microscopic pictures of an isolated *α*-spherulite. A few arc-shaped cracks rapidly appeared in the polar zone in the initial stages of

#### **Figure 10.**

*Polarized optical microscopic pictures of (a) uniaxial stretching of an isolated* α*-acicular spherulite, (b) uniaxial stretching perpendicular to the sheaf axis of an isolated* β*-sheaf spherulite, and (c) uniaxial stretching parallel to the sheaf axis of an isolated* β*-sheaf spherulite.*

stretching. With increasing strain, the arc-shaped cracks developed in the polar zone and proceed from the outer to the inner portions of the spherulite. Subsequently, radial craze-like fractures began to form in the equatorial region perpendicular to the stretching axis, and then the radial crazing progressed along with the spherulite radius, resulting in the evolution of large dark bands in the equatorial region. The evolution of the dark bands is related to the yield process as demonstrated previously by Nitta et al. [36]. It should be noted here that the deformation mechanism of *α*-spherulites is isotropic because crystalline lamellae within *α*spherulites radiate from a common center and the crystalline lamellae aggregate with spherical symmetry. Unlike the *α*-spherulite, *β*-spherulites are sheaflike type of spherulites with a spherical asymmetry (see **Figure 9**). As shown in **Figure 10b** and **c**, the deformation behavior of the *β*-spherulite depends largely on the stretching direction with respect to the sheaf axis. In the case of the *β*-spherulites, when drawn perpendicular to the sheaf direction, the radial crazing preferentially appeared in the equatorial zone along the sheaf axis, and then the dark crazing zone developed further with increasing strain. Finally, a hole appeared in the center of the deformed spherulite, indicating that the deformation is concentrated perpendicular to the stacked lamellae located in the center of the spherulite. On the other hand, when drawing along the sheaf axis, the spherulite was deformed into an ellipsoid accompanied by the formation of crazed cracks, and there is no clear strain concentration.

It was found that the strength of the *β*-spherulite is anisotropic and depends on the direction of the embryo or parallel stacked lamellae in the center of the spherulite. When the *β*-spherulites were subjected to stress perpendicular to the sheaf axis (see **Figure 10b**), obvious deformation bands generated preferentially near the equatorial zone within the uniaxially deformed spherulites. According to previous theoretical [37, 38] and experimental results [39], the equatorial region, particularly the center of the spherulites, is subjected to higher strains and stresses as compared to the polar region. Consequently, interlamellar separation is likely to occur near the equatorial plane of the stacked sheaflike lamellae because the sheaf direction is perpendicular to the loading direction. As the strain increased, separation of the sheaf-lamellae continued, and more deformation bands and crazes generated preferentially near the equatorial zone of the deformed spherulites. In the final stage, holes or local disintegration appeared near the center of the deformed spherulites. This lamellar separation was accompanied by massive voiding at the onset of the formation of a microporous structure, which is preferential for the applications of *β*-phase iPP [29, 40]. On the other hand, when the *β*-spherulite was stretched in the growth direction of the embryo sheaf (see **Figure 10c**), there was no obvious deformation bands around the equatorial zone. Thus, intralamellar deformation is likely to take place for the sheaf-lamellae under uniaxial tension because the sheaflamellae are parallel to the loading direction. Considering that the intralamellar stretching of the sheaf-lamellae involves the unfolding of chains, leading to local necking or sliding, the intralamellar stretching of sheaf-lamellae strongly resists deformation compared to the interlamellar separation; thus, no localized deformation bands appeared near the equatorial zone.

As well-known, the *β* ! *α* transformation occurs on heat treatment. The film having isolated *β*-spherulites was heated up to 433 K at a rate of 2 K/min and then quenched in an ice-water bath. This treatment allows the recrystallization into *α*-modification within the isolated *β*-spherulites. The arrangement of the crystalline lamellae in the α-spherulites prepared by the *β* ! *α* transformation is a sheaflike structure, which is different from the usual *α*-spherulites showing an acicular structure. Thus, the sheaflike spherulite prepared by the *β* ! *α* transformation process is a new type of *α*-spherulite. As shown in **Figure 11**, the sheaflike

*α*-spherulite is optically negative, indicating that there are no traces of a cross-

*Polarized optical microscopic pictures of (a) uniaxial stretching perpendicular to sheaf axis of an isolated* α*sheaf spherulite and (b) uniaxial stretching parallel to sheaf axis of an isolated* α*-sheaf spherulite.*

The deformation behavior of the sheaflike *α*-spherulite was also anisotropic and significantly different from that of the acicular type of *α*-spherulite as shown in **Figure 10**. When the axis of the sheaf was transverse to the loading direction, the deformation bands appeared obviously along the sheaf axis, and then the further deformation extended the highly oriented and deformed zone in the equatorial region of the deformed spherulite. The uniaxially deformed spherulite is clearly divided into two parts: one being nearly undeformed and another being considerably deformed. The nearly undeformed sections are jointed by the transition zone that propagates in the stretching direction. The interlamellar separation of stacked

hatched structure within the *β*-spherulites.

**Figure 11.**

*Tensile Properties in*

*DOI:*  *β-Modified*

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

 *Isotactic* 

*Polypropylene*

**83**

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

#### **Figure 11.**

stretching. With increasing strain, the arc-shaped cracks developed in the polar zone and proceed from the outer to the inner portions of the spherulite. Subsequently, radial craze-like fractures began to form in the equatorial region perpendicular to the stretching axis, and then the radial crazing progressed along with the spherulite radius, resulting in the evolution of large dark bands in the equatorial region. The evolution of the dark bands is related to the yield process as demonstrated previously by Nitta et al. [36]. It should be noted here that the deformation mechanism of *α*-spherulites is isotropic because crystalline lamellae within *α*spherulites radiate from a common center and the crystalline lamellae aggregate with spherical symmetry. Unlike the *α*-spherulite, *β*-spherulites are sheaflike type of spherulites with a spherical asymmetry (see **Figure 9**). As shown in **Figure 10b**

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

and **c**, the deformation behavior of the *β*-spherulite depends largely on the stretching direction with respect to the sheaf axis. In the case of the

clear strain concentration.

tion bands appeared near the equatorial zone.

**82**

*β*-spherulites, when drawn perpendicular to the sheaf direction, the radial crazing preferentially appeared in the equatorial zone along the sheaf axis, and then the dark crazing zone developed further with increasing strain. Finally, a hole appeared in the center of the deformed spherulite, indicating that the deformation is concentrated perpendicular to the stacked lamellae located in the center of the spherulite. On the other hand, when drawing along the sheaf axis, the spherulite was deformed into an ellipsoid accompanied by the formation of crazed cracks, and there is no

It was found that the strength of the *β*-spherulite is anisotropic and depends on the direction of the embryo or parallel stacked lamellae in the center of the spherulite. When the *β*-spherulites were subjected to stress perpendicular to the sheaf axis (see **Figure 10b**), obvious deformation bands generated preferentially near the equatorial zone within the uniaxially deformed spherulites. According to previous theoretical [37, 38] and experimental results [39], the equatorial region, particularly the center of the spherulites, is subjected to higher strains and stresses as compared to the polar region. Consequently, interlamellar separation is likely to occur near the equatorial plane of the stacked sheaflike lamellae because the sheaf direction is perpendicular to the loading direction. As the strain increased, separation of the sheaf-lamellae continued, and more deformation bands and crazes generated preferentially near the equatorial zone of the deformed spherulites. In the final stage, holes or local disintegration appeared near the center of the deformed spherulites. This lamellar separation was accompanied by massive voiding at the onset of the formation of a microporous structure, which is preferential for the applications of *β*-phase iPP [29, 40]. On the other hand, when the *β*-spherulite was stretched in the growth direction of the embryo sheaf (see **Figure 10c**), there was no obvious deformation bands around the equatorial zone. Thus, intralamellar deformation is likely to take place for the sheaf-lamellae under uniaxial tension because the sheaflamellae are parallel to the loading direction. Considering that the intralamellar stretching of the sheaf-lamellae involves the unfolding of chains, leading to local necking or sliding, the intralamellar stretching of sheaf-lamellae strongly resists deformation compared to the interlamellar separation; thus, no localized deforma-

As well-known, the *β* ! *α* transformation occurs on heat treatment. The film having isolated *β*-spherulites was heated up to 433 K at a rate of 2 K/min and then quenched in an ice-water bath. This treatment allows the recrystallization into *α*-modification within the isolated *β*-spherulites. The arrangement of the crystalline lamellae in the α-spherulites prepared by the *β* ! *α* transformation is a sheaflike structure, which is different from the usual *α*-spherulites showing an acicular structure. Thus, the sheaflike spherulite prepared by the *β* ! *α* transformation process is a new type of *α*-spherulite. As shown in **Figure 11**, the sheaflike

*Polarized optical microscopic pictures of (a) uniaxial stretching perpendicular to sheaf axis of an isolated* α*sheaf spherulite and (b) uniaxial stretching parallel to sheaf axis of an isolated* α*-sheaf spherulite.*

*α*-spherulite is optically negative, indicating that there are no traces of a crosshatched structure within the *β*-spherulites.

The deformation behavior of the sheaflike *α*-spherulite was also anisotropic and significantly different from that of the acicular type of *α*-spherulite as shown in **Figure 10**. When the axis of the sheaf was transverse to the loading direction, the deformation bands appeared obviously along the sheaf axis, and then the further deformation extended the highly oriented and deformed zone in the equatorial region of the deformed spherulite. The uniaxially deformed spherulite is clearly divided into two parts: one being nearly undeformed and another being considerably deformed. The nearly undeformed sections are jointed by the transition zone that propagates in the stretching direction. The interlamellar separation of stacked

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 PE spherulite is sheaflike.

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,

*Polypropylene*

**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).*

and *β*-sheaf.

*Tensile Properties in*

*DOI:*  *β-Modified*

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

 *Isotactic* 

**Figure 13.**

**85**

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 within spherulites.
