Introduction

Polypropylene (PP) is one of the most versatile commodities, which can be used in many fields due to its excellent properties. However, the applications of PP are greatly restricted because of its poor toughness, especially because of its poor notched toughness. Blending with elastomers was thought to be the most effective method to achieve PP materials with enhanced impact toughness. It is well-known that several toughening mechanisms were proposed to explain the enhanced toughness of elastomer-toughened polymer blends, such as cavitation [1], multiple crazing [2], shearing yielding [3], and percolation [4]. In fact, many factors have influence on the toughening effect of elastomer-toughened polymer blends. For an elastomer-toughened PP system, most studies pay attention to the properties of the matrix [5, 6], the structure [7–9], and the rheology of polymer blends [10, 11]; the interfacial adhesion and compatibility between the matrix and elastomer fillers [12, 13]; and the shape [14], size [14, 15], and distribution [16] of dispersed elastomer particles in the matrix.

As a kind of typical, semi-crystalline polymer, the crystallization behavior of the PP matrix, such as the size and crystallization kinetics of spherulite, was also a significant factor affecting properties of the matrix. Moreover, the flexible backbone and side chain of elastomers show high affinity with the PP molecular chain. When certain molecular chain segments of elastomers move into the PP molecular chain, entanglement between PP molecules and elastomer molecules could occur in the interface between the PP matrix and dispersed elastomer phase [17], which will have a great influence on the crystallization behavior of the PP matrix. Therefore, much research has been conducted to explore the effect of elastomers on the crystallization behavior of the PP matrix [18–24]. However, there is still an unanswered question: what is the special role of the elastomer phase during the crystallization process of the PP matrix? Some studies [18] regarded the elastomer phase as an obstacle, which can prevent and disturb the regular arrangement of PP molecules. However, some other studies [19, 22, 23] considered the dispersed elastomer phase as a nucleation agent, which can induce the crystallization of the PP matrix at higher temperatures. However, the effect of molecular interaction, resulting from entanglement, on the molecular conformation and crystallization behavior of the PP matrix was hardly reported, despite its profound importance for understanding the role of the elastomer phase during the crystallization process of the PP matrix.

On the other hand, some research shows that the elastomer content [25] has an important influence on the deformation and fracture behavior of polymer blends. Nonetheless, there are not many reports about the evolution of plastic deformation of impact fractures along with the variation of elastomer content. Moreover, it is well-known that elastomer particles can initiate crazes and shear bands in blends as stress concentration factors during the impact process [26]. However, the impact-induced morphological evolution of elastomer particles during the impact process was seldom discussed, which should be of great benefit to understand the toughening mechanism deeply. In our previous work [27], impact-induced morphological evolution of POE particles was found in response to the external impact force. But the effect of impact speed and impact energy on the impact-induced morphological evolution of POE particles was not discussed in detail.

In this present work, special attention was paid to the crystallization kinetics behavior, molecular interaction, and impact-induced morphological evolution in PP/POE blends. The effect of the molecular interaction between PP molecules and POE molecules on the molecular conformation of the PP matrix is discussed qualitatively. Moreover, the special role of POE particle on the PP crystallization phase and PP amorphous phase are discussed by investigating the crystallization kinetics behavior of PP/POE blends with different POE contents. Furthermore, impact-induced morphological evolution of POE particles during the impact process under different impact speeds and impact energies is examined for the first time, which can help us gain insight into the role of POE during the impact process. And the evolution of the plastic deformation stage of impact fracture along with the variation of elastomer content is discussed in detail. The study will provide a systematic and deep toughening mechanism from microscopic molecular interaction to macroscopic deformation.

Experimental

Materials

The PP was of grade 1040 with an MFI of 5.0 g/10 min at 230 °C, 2.16 kg, supplied by Formosa Plastics Co. The POE was Engage 8200 manufactured by Dow Chemical, with an MFI of 5.0 g/10 min, at 190 °C, 2.16 kg. Their densities were 0.900 × 103 and 0.870 × 103 kg/m3, respectively.

Sample preparation

Firstly, PP/POE blends with varied weight ratios (100/0, 95/5, 90/10, 85/15, 80/20, and 75/25) were extruded through a twin-screw extruder. The temperature of the mixing section was 175–195–210–205 °C from the hopper to the die. The PP/POE blends sheet was prepared through a single-screw extruder, and the temperature of extrusion was 175–195–210–205 °C from the hopper to the die.

Notched Izod impact test

The notched Izod impact strength was tested following GB/ 1943–2007 with a XJU-22 impact test machine. The sample, whose size was 80 × 10 × 4 mm3, was prepared though compression molding of the PP/POE sheets at 200 °C under a pressure of 10 MPa. The depth of the notch was 2.0 mm. Each impact test included five parallel experiments, and the results were averaged.

Crystallization behavior analysis

Differential scanning calorimeter (DSC, TA-Q20) was performed to investigate the thermal and crystallization behavior of PP/POE blends with different POE contents. The program for DSC measurement was run from 40 to 200 °C, which was the first heating scan. After the sample was equilibrated at 200 °C for 3 min to erase previous thermal and stress history of blends, it was cooled to 40 °C, and then reheated to 200 °C for the second heating scan. The cooling and heating rates were 10 °C/min.

Polarized optical microscopy (POM, BX51, Olympus) was used to observe the crystalline morphology of PP/POE blends. A 15-μm slice sample was cut from the impact samples vertical to the flow direction using a rotary microtome (YD-2508B). In order to observe the crystallization kinetics in PP/POE blends, the specimen was melt-pressed between two cover glasses on a hot stage (INSTEC HCS302) at 210 °C. The melted specimen was then cooled down to 135 °C. Structural development during the isothermal annealing at 135 °C was observed under the optical microscope equipped with a video recording system and exposure control unit.

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM, JSM-5900LV, Japan) were performed to examine the dispersed POE phase morphology. Samples were quenched in liquid nitrogen and cryofractured perpendicular to the melt flow direction. The as-obtained samples were then chemically etched on their surfaces with heptane in an oil bath at 100 °C for 10 h, preferentially dissolving the POE phase. The impact-fractured surface after the impact test was also observed with SEM.

In order to understand the toughening mechanism deeply, the morphological evolution of the dispersed POE phase during impact process under different impact speeds and impact energies were examined using a part-impact test, which was performed with the XJU-22 impact test machine. During the impact process, the pendulum was raised to angles of 180, 150, 120, 90, and 60° from the vertically fixed specimen (seen in Fig. 1), and then released to hit the specimen. The as-obtained samples were then chemically etched on their surfaces with heptane in an oil bath at 100 °C for 10 h, preferentially dissolving the POE phase, and then the phase morphology of POE particles was observed with SEM (JSM-5900LV, Japan).

Fig. 1
figure 1

The schematic representation of part-impact test

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FTIR and polarized-FTIR test

The FTIR spectroscopy measurement was conducted with the Nicolet-IS10 (Thermo Electron Co., USA) spectrometer. The IR source was provided by a Thermo Nicolet FTIR spectrometer with a resolution of 2 cm−1 and an accumulation of 32 scans. The test slice sample with a thickness of 15 μm was cut by a rotary microtome (YD-2508B) along the melt flowing direction directly from the impact sample. On the other hand, polarized infrared spectra were collected by rotating a ZnSe polarizer. Samples were installed perpendicular to the incident beam direction. When the polarizer was rotated, FTIR spectra were recorded as a function of the angle of rotation in the transmittance mode.

On the other hand, an important parameter used to describe optically measured macromolecular orientation conformation over the past 50 years is the dichroic ratio (R), defined as [28]:

$$ R=\frac{A_{\Big\Vert }}{A_{\perp }} $$
(1)
$$ f=\frac{R-1}{R+2} $$
(2)

Where A║and A┴ are the parallel and perpendicular absorbance at the same positions, respectively, and f is the orientation function. To determine the dichroic ratio (R) of polypropylene, as is well-known, bands at 998 cm−1 are usually used to evaluate the PP crystalline phase (fc) and bands at 973 cm−1 are used to estimate both crystalline and amorphous phases (fav). Thus, the orientation function for the amorphous phase (fam) was derived on the basis of the two phase models according to Eq. (3) [28, 29]:

$$ {f}_{av}={X}_c\times {f}_c+\left(1-{X}_c\right)\times {f}_{am} $$
(3)

Where Xc is the degree of crystallinity obtained from DSC measurement. Therefore, the dichroic ratio of the PP amorphous phase (Ram) can be deduced though Eqs. (1), (2) and (3).

Results and discussions

Morphological analysis

It is known that elastomer content has a great influence on the dispersed phase morphology [25]. Therefore, SEM images of heptane-etched PP/POE blends, average particle diameter (d) and average inter-particle distance (h) of POE particles were investigated, respectively. As indicated in Fig. 2, the average particle diameter of POE particles increased as POE content increased, from 0.24 μm in the PP/POE blend with 5 wt.% POE to 0.80 μm in the PP/POE blend with 20 wt.% POE. Moreover, the average inter-particle distance tended to decrease as POE content increased.

Fig. 2
figure 2

SEM images of heptane-etched PP/POE blends, (represented with subscripts 1), statistic of average particle diameter (represented with subscripts 2), and average inter-particle distance (represented with subscripts 3). The POE content of a, b, c, and d were 5, 10, 15, 20 wt.%, respectively

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On the other hand, a cube box model with unit volume, 1 μm3, was set up to evaluate the surface area of POE particles in PP/POE blends, as seen in Fig. 3. The results showed that the surface area of POE particles in the unit volume of PP/POE blends with 0, 5, 10, 15, 20 wt.% POE content were 0, 1.71, 1.68, 1.91, 2.05 μm2, respectively. Interestingly, the surface area of POE particles in PP/POE blends with 5, 10 wt.% POE content was very similar, but when the POE content reached 15 wt.%, the surface area of POE particles significantly increased, which is considered to have a great influence on the molecular interaction in the interface and the toughening effect of blends, and will be explored in detail later.

Fig. 3
figure 3

The schematic representations of the cube box model of a PP/POE blend with unit volume

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Molecular interaction analysis

It is well-known that FTIR spectroscopy, especially polarized FTIR, is an important tool for investigating polymer blends structure and qualitatively characterizing the molecular chain conformation [30–32]. Fig. 4a and b were the FTIR spectrum obtained from neat PP and neat POE, respectively. In the FTIR spectrum of PP, the peak at 998 cm−1 is attributed to the vibration of the crystalline phase of the PP component [27], and the peak at 973 cm−1 is always considered the average vibration of both the crystalline and amorphous phases of the PP component. Furthermore, the absorbance peak at 1,167 cm−1 and 840 cm−1 in the FTIR spectrum of PP are considered to be associated with the conformation of long PP helical molecular chains in the crystalline phase [33] and the conformation of short sequential chain segments in the crystalline phase [34], respectively. And the peak at 899 cm−1, which can also reflect the conformation change in the crystalline phase, always has the opposite polarization compared with that of 1,167 cm−1 and 840 cm−1 [35]. In the FTIR spectrum of POE, the peak at 720 cm−1, derived from the (CH2 ) 5 deformation vibration of the hexyl side in the POE molecular chain, can be considered the characteristic absorbance peak of POE.

Fig. 4
figure 4

FTIR spectra absorbance of PP (a) and POE (b) measured in transmission mode

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During processing, some small tensile stress was retained in the sample. Interestingly, a dichroism signal would respond to a small strain along the melt flowing direction, which can be used to evaluate qualitatively the effect of POE particles on the PP matrix from a molecular level. Therefore, IR spectroscopic measurements with a linearly polarized IR light source were conducted on neat PP and a PP/POE blend with 20 wt.% POE as a function of the rotation angle of the specimen. The measured peak intensities of selected IR bands are plotted with respect to the rotation angle of the specimen, as a polar diagram. Figure 5 is the polar diagram of IR peaks at 998, 973, and 720 cm−1 of neat PP and PP/POE blends, respectively. As seen in Fig. 5, maximum absorbance intensity at 998 and 973 cm−1 occurred along the direction 0↔180°, which is parallel to the melt flowing direction, but minimum absorbance intensity occurred along the direction 90↔270°, which is perpendicular to the melt flowing direction. This phenomenon indicates that the PP molecular chains in both the crystalline phase and the amorphous phase tend to orient along the stress direction. In contrast, maximum absorbance intensity at 720 cm−1 occurred along the direction 90↔270° and minimum absorbance intensity occurred along the direction 0↔180°, which was thought to be because the (CH2 ) 5 deformation vibration of the hexyl side chain is perpendicular to the backbone chain of POE. However, interestingly, compared to the polar diagram of IR peaks of neat PP with that of PP/POE blends, it can be observed that the addition of POE can affect the dichroism signal response of PP molecular chains in both the crystalline phase and amorphous phase. Table 1 presents the dichroic ratio (Rc) in the PP crystalline phase (998 cm−1), average dichroic ratio (Rav) of both the crystalline phase and amorphous phase (973 cm−1), and dichroic ratio (Ram) in the PP amorphous phase in PP/POE blends with different POE contents. The dichroic ratio in the PP crystalline phase, which presented significant change due to the addition of a POE phase, decreased as the surface area of POE particles increased. However, the dichroic ratio in the PP amorphous phase changed little. Therefore, it is reasonable to believe that the effect of POE particles is mainly on the PP crystalline phase.

Fig. 5
figure 5

Polar diagrams of some specific vibration absorbance peaks of neat PP (subscripts 1) and a PP/POE blend (subscripts 2) with 20 wt.% content, measured by linearly polarized IR spectroscopy as a function of the angle of rotation of the film: (a), (b), and (c) are vibrations at 998, 973, and 720 cm−1, respectively

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Table 1 Total surface area of POE particle in the unit volume (S), dichroic ratio (R) at absorbances of 998, 973, and 720 cm−1, and notched impact strength of PP/POE blends with different POE contents
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Then, molecular chain conformation in the PP crystalline phase in neat PP and PP/POE blends were investigated, respectively. Polar diagrams of IR peaks at 1167, 899, and 840 cm−1 of neat PP and PP/POE blends were presented in Fig. 6. It is clearly observed that, for both neat PP and PP/POE blends, maximum absorbance intensities at 1167 and 840 cm−1 occurred along the direction 0↔180°, which is parallel to the melt flowing direction, and minimum absorbance intensity occurred along the direction 90↔270°, which is perpendicular to the melt flowing direction. This indicates that both the long PP helical molecular chain and short sequential chain segment in the crystalline phase tend to arrange along the melt flowing direction. However, the dispersed POE phase could weaken the influence of the external stress field on the conformation of the PP molecular chain, which is ascribed to the existence of entanglement between POE molecules and PP molecules in the interface.

Fig. 6
figure 6

Polar diagrams of some specific vibration absorbance peaks of neat PP (subscripts 1) and a PP/POE blend (subscripts 2) with 20 wt.% POE content, measured by linearly polarized IR spectroscopy as a function of the angle of rotation of the film: (a), (b), and (c) are vibrations at 1167, 899, and 840 cm−1, respectively

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Combining the above result, one can image that the dispersed POE phase can affect the PP molecular chain conformation, especially the molecular chain in the crystalline phase, due to the entanglement between POE molecules and PP molecules in the interface. Therefore, it is logical to ask how the POE phase influences the crystallization behavior of the PP matrix. Thus, the crystallization behavior of neat PP and PP/POE blends are discussed in detail later.

Crystallization behavior analysis

As indicated in Table 2, the crystallinity of the PP matrix decreased as POE content increased. Moreover, the crystallization temperature of the PP/POE blend changed little with the addition of POE, indicating that the role of heterogeneous nucleation of the POE phase was not obvious. So how does the POE phase affect the crystallization process of the PP matrix? This needs to be studied deeply and systematically.

Table 2 DSC data of the PP/POE blends
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The spherulite size of PP was explored at first. As seen in Fig. 7, The spherulite size of the PP matrix decreased with the increase of POE content. Then, the effect of the POE phase on the spherulite growth rate of PP was examined by observing the crystallization process of spherulites at a constant elevated temperature using optical microscopy equipped with a precise temperature-controlled hot stage. Figure 8 shows the spherulite growth in PP/POE blends with different POE contents for isothermal crystallization at 135 °C. One can observe that the number of nucleation sites changed little with the variation of POE content, indicating that the POE phase did not play the role of heterogeneous nucleation once again. Then, the radii of spherulites were measured with four to six spherulites for each sample and plotted as a function of time, as shown in Fig. 9. The slope of the line gives us the spherulite growth rate. With the addition of POE, the spherulite growth rate decreased compared with that of pure PP. As the POE content increased, the spherulite growth rate decreased gradually.

Fig. 7
figure 7

POM images of PP/POE blends with different POE contents. The subscripts 1 and 2 represent bright field OM and cross-polarized field OM, respectively. The POE content of a, b, c, d, and e were 0, 5, 10, 15, and 20 wt.%, respectively

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Fig. 8
figure 8

Selected POM micrographs for PP/POE blends with different POE contents during isothermal crystallization at 135 °C after 1 min pre-heating at 210 °C. The POE content of a, b, c, d, and e were 0, 5, 10, 15, and 20 wt.%, respectively

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Fig. 9
figure 9

Evaluation of spherulite growth rate from the plot of the radii of spherulites vs. time for PP/POE blends with different POE contents

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It is known that the spherulite structure of PP can be disturbed and become imperfect with the addition of elastomers [18]. In a POE-toughened system, in fact, the POE particle can be regarded as an obstacle, which will prevent and disturb the regular arrangement of PP molecules. The PP lamellae or the ordered PP molecules would find a bridge or way around the POE obstacle to keep crystallization continuous. Thus, the size and inter-particle distance of POE obstacles have a great influence on the arrangement of PP lamellae or PP molecules. Therefore, a crystallization model was established to explain the influence of POE on the crystallization kinetics of the PP matrix, as shown in Fig. 10. As a hypothesis, the particle diameter (d) and inter-particle distance (h) of POE particles in PP/POE blends distribute equally in three dimensions. If the length (L) of the straight line in pure PP (seen in Fig. 10a) measured 10 μm, then the length of PP lamellae or molecules through the PP/POE blend (seen in Fig. 10b) can be measured using the following equation:

Fig. 10
figure 10

Model of crystallization of pure PP (a) and PP/POE blends (b)

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$$ L={L}_1+n\times \left(h+\frac{\pi }{2}d\right) $$
(4)
$$ n=\frac{10}{h+d} $$
(5)
$$ {L}_1=10-n\times \left(h+d\right) $$
(6)

Where n is the number of the unit length (containing d and h) in the model. Then, the length of the line would measure 11.08, 11.43, 11.76, and 12.74 μm in PP/POE blends with 5, 10, 15, and 20 wt.% POE content, respectively. The longer the length is, the longer time it takes for PP lamellae to go around, and the slower crystallization is observed. The degree of variation of crystallization kinetics data obtained from the model is larger than the result obtained from POM observation. This could be because the POE particles are not all dispersed in the PP crystalline phase. And a small number of POE particles dispersed in PP amorphous phase, which can prevent PP spherulites from growing outward and reduce the spherulite size. Therefore, the crystallization of PP is confined due to the existence of POE in both crystalline phase and amorphous phases. Moreover, the larger the POE content is, the more obvious the role of confined crystallization is.

Furthermore, in order to understand clearly the role of the POE phase during the crystallization process of the PP matrix, schematic representation of the growth of spherulites in PP/POE blends is presented in Fig. 11a. During the crystallization process, PP lamellae would find a way along the edge of POE particles to keep crystallization continuous. Moreover, POE particles themselves can be regarded as aggregation of random coils during the crystallization process of PP. Therefore, one can imagine the molecular morphology in the PP phase, POE phase, and interface, as indicated in Fig. 11b. A coated structure was formed in the interface. PP lamellae or PP molecules were arranged regularly outside the coated structure while POE molecules were distributed randomly inside the coated structure. Interestingly, the mutually entangled molecules of PP and POE were distributed in the middle of the coated structure, which can connect the crystalline phase of PP and the random POE phase. The as-formed coated structure in the interface is considered to play a significant role on the macro properties of blends. The relationship between micro-morphology and macro properties will be discussed in detail later.

Fig. 11
figure 11

The schematic representations of the growth of spherulites in PP/POE blends (a) and the coated structure and molecular morphology around POE particles (b)

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Toughening mechanism

As seen in Table 1, the notched impact strength of PP/POE blends with 0, 5, 10, 15, and 20 wt.% POE were 6.0, 10.6, 20.2, 38.0, and 44.8 KJ/m2, respectively. The notched impact strength of PP/POE blends was improved as expected with the increase of POE content, especially when the POE content was above 10 wt.%. Then, in order to understand the toughening mechanism more deeply, the fracture surface of blends with different POE contents and the morphological evolution of POE particles during the impact process under different impact speeds and impact energies were observed using SEM images, respectively. As shown in Fig. 12, the fracture surface of neat PP was very smooth and had some small crazes, which was a typical characteristic of a brittle fracture. For the blend with 5 wt.% POE, cavitation and multiple crazes occurred on the fracture surface of the sample. For the blend with 10 wt.% POE, the number of cavitations on the surface decreased while multiple crazes become more obvious. For the blend with 15 wt.% POE, the cavitations almost disappeared and a few small shear bands occurred at the same time. Moreover, a lot of large shear bands occurred on the fracture surface of the blend with 20 wt.% POE. The fracture surface of blends with 15 and 20 wt.% POE are a typical characteristic of a ductile fracture. Therefore, it is reasonably considered that the micro-deformation of POE-toughened semi-crystalline PP is a multi-stage mechanism: cavitation and multiple crazes played a leading role when the POE content was relatively low (<15 wt.%), and shear bands played the leading role when the POE content relatively high (>15 wt.%). Moreover, the extent of cavitation, multiple crazes, and shear bands varied as POE content changed. The influence of cavitation, crazes, and shear bands on absorbing impact energy is also different. A shear band is a larger plastic deformation than cavitation and multiple crazes, which will absorb much more impact energy during the impact process.

Fig. 12
figure 12

Fracture surface of PP/POE blends with different POE contents. The POE content of a, b, c, d, and e were 0, 5, 10, 15, and 20 wt.%, respectively

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However, how does the POE content affect the type of toughening mechanism and fracture behavior? Our previous work [27] had found that the POE particles in PP/POE blends showed morphological evolution during the impact process. But a quantitative relationship between morphological evolution and absorbed impact energy was not discussed in detail, which was important for understanding deeply the role of POE during the impact process. Figure 13 shows SEM images of the hetpane-etched fracture surface of the blend with 20 wt.% POE after the impact with different impact speed and impact energy. The particle diameter and inter-particle distance after the impact from different angles were calculated statistically. All the data are presented in Table 3. It can be observed that the dispersed POE particle size during the impact process become small compared with that before the impact. Moreover, the degree of morphological evolution of POE particles tended to increase as the impact speed and impact energy increased. In fact, the morphological evolution of POE particles was a process of energy absorption [27]. Moreover, it is reasonably considered that the morphological evolution of POE particles during the impact process is related to the as-formed coated structure around POE particles, but there are not many reports on this relationship.

Fig. 13
figure 13

SEM images of heptane-etched fracture surfaces of PP/POE blends with 20 wt.% content. a, b, c, d, e, and f were impacted from 0, 180, 150, 120, 90, and 60°, respectively

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Table 3 Impact speed, impact energy, average particle diameter (d), and inter-particle distance (h) of PP/POE blends impacted from different angles
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Figure 14 shows a schematic representation of macro- and micro-evolution of PP/POE blends during the impact process. It is known that the microunits of the deformation region deform in two methods during the impact process: high speed tensile deformation and shear deformation due to the tensile speed gradients in the deformation region [36]. Therefore, the impact force can be decomposed into two forces during the impact fracture process: tensile force (F”) and shear force (F’) due to tensile speed gradients, as shown in Fig. 14c. Thus, the plastic deformation of POE particles consists of two methods: tensile deformation and shear deformation, as shown in Fig. 14a and b, respectively. Moreover, the as-formed coated structure will come into effect to resist the tensile force. Firstly, the PP lamellae or ordered PP molecules outside the coated structure were stretched, which can absorb a little impact energy. Then, the mutually entangled PP molecules and POE molecules will be stretched under the effect of tensile force, which will dissipate some impact energy and play an important role during the fracture process. What is more, PP molecules and POE molecules were broken during the fracture process. Part of the POE molecules were left on the other side of the fracture surface, which was the main cause of the induced morphological evolution of the dispersed POE phase.

Fig. 14
figure 14

The schematic representation of macro- and micro-evolution during the impact process

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On the other hand, shear force will induce the PP matrix to generate crazes, which can then be terminated when they encounter POE particles. Moreover, the POE particles can also induce the PP matrix to generate massive crazes to absorb more impact energy [26], avoiding the further development of crazes into cracks. Therefore, either the effect of terminating crazes or re-inducing crazes of PP/POE blends increased as POE content increased. When the crack occurs, and then the crack tip touches the interface, the interface is damaged due to high stresses. With the interaction of tensile force and shear force, the POE particles are fully debonded. Therefore, the as-formed coated structure was damaged, accompanied by the formation of cavitation. As a consequence, the crack is blunted, the singular stress field is changed to a regular one and the crack stays arrested near the particle, as seen in Fig. 14b. The blunting of micro-cracks in connection with cavitation can enhance the fracture toughness of the blends. Moreover, only particles located close to the crack tip significantly influence micro-crack behavior. So if the POE content increases and inter-particle distance decreases, the length that the micro-cracks need to be blunted from one POE particle to another POE particle become small. Also, as POE content increases, the area of blunting of the micro-crack develops from a point into a line, and then into an area, resulting in the plastic deformation of blends from cavitation into multiple crazes, and then into large shear bands. Therefore, the toughness of PP/POE blends is enhanced gradually as the POE content increases.

Conclusions

The influence of the dispersed POE phase on the PP matrix was discussed by investigating the crystallization kinetics behavior, molecular interaction, and impact-induced morphological evolution in PP/POE blends. The existence of POE particles is considered to play the role of an obstacle during the crystallization process of PP, which could interrupt the regular arrangement of PP lamellae or PP molecules, increase the growth path of PP lamellae, and reduce the growth space of spherulites, resulting in a slower spherulite growth rate and smaller spherulite size. Moreover, the spherulite growth rate, spherulite size, and crystallinity of the PP matrix tend to decrease with the increase of POE content.

On the other hand, a coated structure was formed around POE particles, which is a transition state structure of molecules with different morphologies. The as-formed coated structure was considered the origin of the cavitation effect and impact-induced morphology evolution of POE particles during the impact process. Also, the micro-deformation in PP/POE blends is a multi-stage mechanism, in which the POE content plays a decisive role. When the POE content is low (<15 wt.%), the plastic deformation during the fracture process is mainly attributed to cavitation and multiple crazes. And when the POE content is high (>15 wt.%), the plastic deformation during the fracture process is mainly attributed to multiple crazes and shear bands. This work provides us a deep insight into the toughening mechanism of an elastomer-toughened PP system from the molecular level.