Morphological evolution and toughening mechanism of polypropylene and polypropylene/poly(ethylene-co-octene) alternating multilayered materials with enhanced low-temperature toughness

Abstract

In this paper, polypropylene (PP) and polypropylene/poly(ethylene-co-octene) blends (PP/POE) were fabricated into alternating multilayered materials to improve the low-temperature toughness of PP efficiently compared with conventional PP/POE blends. POM, SEM, micro-FTIR and part-impact test were performed to characterize and investigate the alternating multilayered microstructure and its relationship with mechanical properties. The results showed that the unique alternating multilayered microstructure could generate a distinctive distribution of POE, resulting in the great change in both macro- and micro-morphology of the materials. Most interestingly, the morphological evolution of the dispersed POE phase before and after the impact showed that a brittle–ductile transition (BDT) layer was formed at the interlayer interface between the adjacent PP layer and the PP/POE layer during the impact process, which was the main reason for the great improvement of the low-temperature toughness. Moreover, the rigidity of alternating multilayered materials was maintained very well because of the existence of the rigid PP layer, indicating that the alternating multilayered microstructure was very helpful to maintain a good balance between toughness and rigidity.

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1 Introduction

Polymer toughening is one of the most general and important topics in the research of polymer material science as the applications of many polymers are greatly restricted because of their poor toughness. Thus, polymer blending has always been considered a simple and feasible way to improve the toughness of the polymers. Previously, elastomers were extensively used as toughening agents because of their excellent elasticity and toughness. It is well known that many factors influence the toughening effect, such as the properties of the matrix,^1,2 the structure of polymer blends,^3–6 the interfacial adhesion and compatibility between the matrix and the elastomer fillers,^7,8 and the shape,⁹ size^9–11 and distribution¹² of elastomer particles in the matrix. Moreover, in order to obtain a material with greatly improved toughness, especially at a low temperature, large amounts of the elastomer must be added into the polymer.¹³ However, the rigidity of the material is significantly sacrificed because of the low modulus of the elastomer, and the cost is also increased because of the high price of the elastomer. To overcome the drawbacks resulted by addition of the elastomer, some studies^14–18 have been conducted on the polymer/elastomer/rigid filler ternary system. It has been found that the dispersed phase morphology and microstructure of ternary blends is very important for achieving the best combination of mechanical properties.

Therefore, many studies^19–22 have already been focused on how to achieve a material with balanced toughness and rigidity by adjusting the dispersed phase morphology and microstructure of the toughening system. Xu et al.²³ regulated the phase morphology and microstructure of the PP material successfully through the photocrosslinking under UV radiation, and both its toughness and tensile strength were enhanced simultaneously because of a more rigid mSEBS dispersed phase and strong interfacial bonding between the mSEBS dispersed phase and PP matrix. Yang et al.²⁴ achieved a core–shell structure, in which EPDM particles were closely surrounded with hydrophilic SiO₂ particles in the PP matrix through two-step processing. The results showed that the toughness and the modulus of PP were enhanced simultaneously because of the overlap of the “stress volume” between EPDM and SiO₂ particles caused by the unique microstructure mentioned above. Nonetheless, most studies paid attention to the influence of the microstructure and morphology on the performance of the materials. To the best of our knowledge, the effect of the impact process on the evolution of the microstructure and morphology was hardly reported, despite its profound importance for understanding the toughening mechanism.

It is well known that polypropylene (PP), one of the important commodity plastics, has been widely used in the construction, automobile and household appliance industries. However, the application of PP is limited greatly by its low impact strength, especially at temperatures below T_g as well as poor rigidity at an elevated temperature. Therefore, a lot of studies^{13,18,20,22–24} have been performed to achieve PP with balanced toughness and rigidity. In some recent studies, Zhang et al.^25–27 found that the layer-by-layer method can generate new polymer materials with much improved morphologies and physical properties as compared to the regular blends. Moreover, Gupta et al.²⁸ successfully fabricated an alternating poly(propylene-graft-maleic) (PP-g-MA) and poly(propylene-graft-maleic)/phosphate glass (PP-g-MA/Pglass) multilayered film with a reduction in gas permeation by two to three orders of magnitude compared with the neat PP-g-MA. More interestingly, the modulus of the alternating multilayered film was increased effectively without any considerable loss in ductility. However, the reason for the as-obtained balanced mechanical performance was not discussed in detail. In our opinion, it should be ascribed to the existence of a multilayered microstructure alternating with relatively ductile PP-g-MA and rigid PP-g-MA/Pglass. Moreover, the ductile PP-g-MA layer and the rigid PP-g-MA/Pglass layer can support each other in the mechanical tests.

In the present work, we try to introduce the alternating multilayered microstructure into the PP toughening system by alternating the PP matrix with another ductile PP/POE blends layer through multilayered co-extrusion technology, which was developed in our lab. The stratified composite sheets are prepared with alternating polymer layers, in which each layer is continuous along the extrusion direction.²⁹ Alternating multilayered materials with unique macro- and micro-morphologies will be investigated comparatively with conventional blends. Our main goal is to fabricate PP with an enhanced low-temperature toughness and balanced rigidity at the evaluated temperature. In addition, the relationship between the microstructure and the properties will be discussed, for the first time as far as we know, by investigating the morphological evolution of the dispersed POE phase during the impact process. This will provide not only a deep understanding of the toughening mechanism but also a new approach for obtaining PP with balanced mechanical properties.

2 Experimental

2.1 Materials

The PP was of grade 1300 with a MFI of 2.5 g per 10 min at 215 °C, 2.16 kg, supplied by Mao Ming Petro-chemistry Co. The POE was Engage 8150 manufactured by Du Pont-Dow Chemical, with an octane content of 25% and a MFI of 0.8 g per 10 min, at 215 °C, 2.16 kg. Their densities were 0.909 × 10³ and 0.863 × 10³ kg m⁻³, respectively.

2.2 Sample preparation

Firstly, PP and PP/POE blends with varied weight ratios (90/10, 80/20 and 70/30) were extruded through a twin-screw extruder. The temperature of the mixing section was 170–190–200–200–195 °C from the hopper to the die, and then the blends were dried in the vacuum drying chamber for 24 h after being cut with the granulator. The MFI of the PP/POE blends with 0, 10, 20, and 30 wt% POE, tested at 215 °C, 2.16 kg, are 2.5, 2.3, 2.0, and 1.7 g per 10 min, respectively. Then, PP and PP/POE blends were co-extruded as alternative multilayered materials by using the multilayered co-extrusion technology, and the temperature settings of the extruders for the PP section and the PP/POE blends section were 160–200–215–215–210 °C and 90–200–215–215–210 °C, respectively. A sketch of the co-extrusion technology is illustrated in Fig. 1. PP/POE blends and neat PP were extruded simultaneously from different extruders and merged into a two-layer melt in the co-extrusion block, and then the melt flowed through a series of layer multiplying elements (LMEs). In a LME, the melt was sliced into two left and right sections by a divider, and then recombined vertically as shown in Fig. 1. An assembly of nLMEs could produce the alternating multilayered material with 2⁽ⁿ⁺¹⁾ layers.

Fig. 1 Sketch of multilayered co-extrusion technology: A, B-single screw extruder; C-connector; D-co-extrusion block; E-layer multiplying element (LME).

In this study, alternating multilayered samples with 128 layers were extruded using six LMEs. The overall sheet thickness ranged from 1700 μm to 1800 μm. The nominal PP layer and PP/POE layer thickness was calculated from the number of layers, the sheet thickness and the total POE content. The total POE content can be changed by adjusting the extruder speed of the PP/POE section and the composition ratio of the PP/POE blends section. Then, the total POE content was calculated from the following formula:

where W_POE is the mass fraction of POE; ρ_b is the density of the blend; and ρ_PP and ρ_POE represent the densities of neat PP and POE, the values of which are 0.909 × 10³ and 0.863 × 10³ kg m⁻³, respectively. The densities of ρ_b, ρ_PP, and ρ_POE were measured through a high-precision density tester (MatsuHaku, GH-120M). The as-obtained materials are coded according to the abbreviation for alternating multilayered materials (AM) and the content of POE. For example, AM-18 represents the alternating multilayered material with about 18 wt% POE. Table 1 summarizes the characteristics of these layered samples. For comparison, conventional PP/POE blends and neat PP samples were prepared under the same heat and shearing force history, using one extruder of the multilayered co-extrusion technology with six LMEs.

Table 1 Sample code, total sheet thickness, nominal layer thickness, and total POE content in alternating multilayered materials and conventional blends

Sample code	Total sheet thickness (μm)	Nominal PP layer thickness (μm)	Nominal PP/POE layer thickness (μm)	POE content (wt%)^a
a POE content in the PP/POE layer in AM-4, AM-6, and AM-7 is 10 wt%; in AM-9, AM-12, and AM-14 is 20 wt%; and in AM-15, AM-17, and AM-18 is 30 wt%.
C-0	1700	—	—	0
C-10	1700	—	—	10
C-20	1800	—	—	20
C-30	1700	—	—	30
AM-4	1700	16.7	10.0	3.98
AM-6	1700	10.5	16.0	6.2
AM-7	1700	8.4	18.1	7.1
AM-19	1700	14.8	11.9	8.67
AM-12	1800	10.8	17.4	11.85
AM-14	1700	6.8	19.8	13.9
AM-15	1700	14.1	12.4	15.4
AM-17	1700	11.3	15.2	16.77
AM-18	1800	10.8	18.2	18.16

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2.3 Mechanical properties tests

2.3.1 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 × 10 mm³, was prepared though the compression molding of the alternating multilayered sheets at 180 °C under 10 MPa. The depth of the notch was 2.0 mm. All the samples were placed in the environmental simulation chambers (Binder, MKFT240) for 15 h at −40 °C and 40% relative humidity. Then, samples were taken out quickly for the notched Izod impact test from different directions as shown in Fig. 2. In order to understand the impact test clearly, we introduced a three-dimensional rectangular coordinate system (as seen in Fig. 2): x is the direction of the flow, y is the direction through the layer plane, and z is the direction parallel to the layer plane. Each impact test included five parallel experiments, and the results were averaged.

Fig. 2 Schematic representation of impact test through different directions of alternating multilayered materials and conventional blends: (a) and (b) are alternating multilayered materials; (c) and (d) are conventional blends.

2.3.2 Tensile yield strength, flexural modulus and strength tests. The standard tensile test was conducted at room temperature by using a tensile test machine (model CMT-4104) according to GB/T1040-92. The dumbbell-shaped sample, prepared directly with the sheets extruded by the multilayered co-extrusion technology, was tested through x direction at a tensile rate of 100 mm min⁻¹. The flexural tests were measured according to GB/9341-2000 using the CMT-4104 flexural test machine at room temperature. The sample, whose size was 80 × 10 × 4 mm³, was prepared though the compression molding of the alternating multilayered sheets at 180 °C under 10 MPa, and then, tested through y direction at a speed of 2 mm min⁻¹. At least five parallel specimens were tested in each test.

2.4 Polarized optical microscopy (POM) observation

Polarized optical microscopy (POM, Leica, DM2500P) was used to assess the layer integrity and continuity. The 20 μm slice sample was cut from the extruded sheet vertical to the flow direction by using a rotary microtome (YD-2508B).

In order to understand the fracture and toughening mechanism of conventional blends and alternating multilayered materials, the crack initiation and propagation stages were observed through a part-impact test, which was performed with the XJU-22 impact test machine. The specimen was also placed in the environmental simulation chambers (Binder, MKFT240) for 15 h at −40 °C, 40% relative humidity before the part impact. The pendulum was raised at an angle of 60° from the vertically fixed specimen, and then released to hit the specimen with an appropriate constant impact energy of about 0.5 J. The specimen was not broken into two halves as expected, and the propagating crack or craze stopped in the interior of the specimen. Similar to the method used to investigate the alternating multilayered microstructure, the initiation and propagation patterns of the crack or craze were collected by POM, with the 30 μm sample slices cut from the part-impact specimens along the crack propagation direction, but perpendicular to the impact direction. The pictures collected by POM were all recorded with a Pixelink camera (PL-A662).

2.5 Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM, JSM-5900LV, Japan) was performed to examine the impact-fractured surface, the morphological evolution of the dispersed POE phase in the conventional blends and the alternating multilayered materials during the impact process. The samples, used for investigating the morphological evolution, were prepared using an ultrathin freezing microtome (Leica, RM2265) at −100 °C. The as-obtained samples were then chemically etched with heptane in an oil bath at 100 °C for 10 h, preferentially dissolving the POE phase, for investigating the morphological evolution.

2.6 Micro-FTIR study

The molecular orientation of PP and POE along x direction in neat PP, conventional blend and alternating multilayered material were monitored by a Thermo Nicolet infrared microscope, which allows for a qualitative assessment of the POE content in the alternating multilayered materials. 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 30 μm was cut by the rotary microtome (YD-2508B) along the x direction directly from the extruded sheet. Polarized infrared spectra, both parallel and perpendicular to the sampling region, were collected by rotating a ZnSe polarizer.

The orientation function f, dichroic ratio R and structural absorbance A of the desired absorption band are deduced using the following relations:³⁰

where A_‖ and A_⊥ are the parallel and perpendicular absorbance at the same positions, respectively.

3 Results and discussion

3.1 Microstructure and morphology analysis

The most significant difference between the alternating multilayered material and the conventional blend with the same POE content could be understood reasonably as follows: the dispersed POE phase was layered together using co-extrusion technology to produce a multilayered microstructure alternating with the PP layer and the PP/POE layer. As expected, this unique multilayered microstructure alternating with the PP layer and the PP/POE layer was successfully fabricated, which can be well observed through the POM images presented in Fig. 3. The macroanisotropy in the alternating multilayered microstructure is very obvious, and the continuity of each layer is very good. This unique microstructure is considered to have a great influence on the toughening system; thus, investigations from both macroscopic and microscopic perspectives were conducted in detail later.

Fig. 3 POM images of neat PP (a), conventional blend (b) and alternating multilayered material (c).

The effect of the unique microstructure on the morphology of the dispersed POE phase was observed through the SEM images of the alternating multilayered material (AM-18) and the conventional blend (C-20) before the impact. As indicated in Fig. 4, the size and deformation of POE particles in AM-18 along both x and z directions become much larger than that in C-20. It has been known that the POE content in the PP/POE layer in AM-18 and in C-20 are 30 wt% and 20 wt%, respectively. Such a large difference in the POE content can be considered as the most important factor affecting the size and deformation of POE particles.¹³ Interestingly, the larger deformation of POE particles, resulting from this unique microstructure, would be able to improve the impact strength of the PP matrix when the external force is loaded perpendicular to the direction of deformation.³¹

Fig. 4 SEM images of heptane-etched surfaces before the impact of conventional blends and alternating multilayered materials: a and b are along the y direction; c and d are along the x direction.

3.2 Micro-FTIR analysis

Fig. 5 shows the FTIR absorbance spectra of neat PP, neat POE, conventional blend (C-20), interlayer interface in the PP/POE layer of the alternating multilayered material (AM-18) and the PP/POE center layer of AM-18. The vibration absorbance peak at 720 cm⁻¹, which should be attributed to the bending vibration of the hexyl side of the POE molecular chain, can be considered the characteristic absorbance peak of POE. The vibration absorbance peak at 973 cm⁻¹, due to the contribution of both the crystalline and the amorphous phase of the PP component,³⁰ can be regarded as the characteristic absorbance peak of PP. It is well known that both the height and the area of the absorption peak are positively related to the material content.^32,33 As shown in Fig. 5 and Table 2, the absorbance peak height and area at 720 cm⁻¹ of POE in the interlayer of AM-18 are stronger than those in C-20, but weaker than those in the center of the PP/POE layer of AM-18. The peak area ratio between 720 cm⁻¹ and 973 cm⁻¹ of the same sample, which can eliminate the effect of the sample thickness and qualitatively assess the difference in the POE content, presents a similar trend as that of the peak height and area. This is attributed to the existence of each PP/POE layer in the alternating multilayered microstructure, which can be regarded as a micro-pipe compared with the conventional blend. Radical migration³⁴ of dispersed deformable POE particles would take place in each PP/POE layer, resulting in the stratification of the POE content in the PP/POE layer. The POE content in the center of the PP/POE layer in AM-18 becomes higher than 30 wt%, and that at the interlayer interface between the PP layer and the PP/POE layer becomes lower than 30 wt%. Therefore, the interlayer interface between the adjacent layers can be regarded as a transition layer, due to the stratification of the POE content.

Fig. 5 FTIR absorbance spectra of neat PP, neat POE, conventional blend (C-20), the interlayer in the PP/POE layer of an alternating multilayered material (AM-18) and the PP/POE center layer.

Table 2 Orientation function (f) at 973 cm⁻¹ and 720 cm⁻¹, peak area at 973 cm⁻¹ and 720 cm⁻¹, and peak area ratio between 973 cm⁻¹ and 720 cm⁻¹ in conventional blends and alternating multilayered materials

Sample		f (973 cm^−1)	f (720 cm^−1)	Peak area (973 cm^−1)	Peak area (720 cm^−1)	Peak area ratio (720/973)
Neat PP		0.421 ± 0.018	—	1.386 ± 0.013	0	0
C-20		0.301 ± 0.006	0.187 ± 0.013	1.159 ± 0.002	0.260 ± 0.007	0.224
AM-18	PP layer	0.387 ± 0.010	—	1.353 ± 0.011	0	0
	BDT layer	0.298 ± 0.011	0.135 ± 0.011	0.745 ± 0.032	0.345 ± 0.023	0.463
	PP/POE layer	0.267 ± 0.008	0.108 ± 0.011	0.610 ± 009	0.387 ± 0.015	0.634

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Moreover, the viscosity of the PP/POE blends increases with increasing POE content. Stratification of viscosity will take place in the alternating multilayered microstructure, due to the stratification of the POE content. Furthermore, the stratification of viscosity is considered to generate great influence on the PP molecules and the POE molecules during the processing. Therefore, the orientations of the PP molecules and the POE molecules along the x-direction in AM-18 were monitored through the Micro-FTIR study compared with those in neat PP and C-20. To determine the orientation function of PP and POE, some characteristic bands were examined. As shown in Fig. 6, the 973 cm⁻¹ vibration is usually used to evaluate both the crystalline and amorphous phase of PP, namely, the average orientation function (f);³⁰ moreover, it is used to estimate the orientation function of POE according to the FTIR of neat POE. As shown in Table 2, the orientation functions of the PP molecules and the POE molecules in AM-18 present a gradient distribution phenomenon, reducing gradually from the PP layer to the transition layer and then to the PP/POE layer. This is consistent with the stratification of the POE content. Moreover, the orientation functions of the PP molecules and the POE molecules in the PP/POE layer in AM-18 decreased compared with those in the conventional blend. This also indicates that the difference in POE content has a great influence on the PP molecules and the POE molecules.

Fig. 6 A typical part of FTIR absorbance spectra in the parallel and vertical directions for PP/POE blends.

Combining the above results, one can observe that the unique alternating multilayered microstructure can generate a distinctive distribution of POE compared with conventional blends. This distinctive distribution of POE can lead to a great change in both the macro- and the micro-morphology of the blends, which would have a great effect on the performance of the material.

3.3 Toughness and rigidity

As indicated in Fig. 7(a)–(d), the mechanical properties of the alternating multilayered materials effectively improved compared with those of the conventional blends, which was as we had expected. The notched Izod impact strength of the alternating multilayered materials and the conventional blends at −40 °C in different directions increased with increasing POE content (as seen in Fig. 7(a)). It is interesting that the brittle–ductile transition (BDT) of the alternating multilayered materials through y direction occurs with about 15 wt% POE, while the conventional blends are still brittle under the same condition. The impact strength of an alternating multilayered material is considerably improved when the POE content is only 18 wt%, the value of which increases to 5.61 kJ m⁻². The impact strength of the conventional blends with 20 wt% and 30 wt% POE were 3.71 kJ m⁻² and 6.62 kJ m⁻², respectively. However, nearly no difference in the impact strength through the z direction between the alternating multilayered materials and the conventional blends can be found, indicating that there is anisotropy on the impact strength of alternating multilayered materials through different directions. On the other hand, the difference in the tensile yield strength between the alternating multilayered materials and the conventional blends was small, as shown in Fig. 7(b). However, the flexural modulus and the flexural strength of alternating multilayered materials improved greatly in contrast to those of the conventional blends, as shown in Fig. 7(c) and (d). The direction of external force in tensile tests is along the x direction while that in flexural tests is along the y direction, which results in the difference between the tensile properties and the flexural properties. This is consistent with the difference in impact strength in different directions.

Fig. 7 Mechanical properties of alternating multilayered materials and conventional blends with varying POE content (a): notched Izod impact strength at −40 °C through different directions; (b): tensile yield strength; (c): flexural modulus; (d): flexural strength.

Although the toughness and rigidity through the z direction of alternating multilayered materials are hardly enhanced, the toughness and rigidity through the y direction are greatly enhanced simultaneously. Therefore, it is logical to ask what leads to the simultaneous improvement of the toughness and the rigidity and what is the relationship between the microstructure and properties. Thus, the fracture mechanism and the toughening mechanism will be discussed in detail later.

3.4 Fracture and toughening mechanism

In order to ascertain the difference in the fracture and toughening mechanisms between conventional blends and alternating multilayered materials, the initiation and propagation patterns of the crack and craze produced through the part-impact test were studied, as presented in Fig. 8. Under almost the same impact speed and impact energy, the fracture degree and mechanism of these samples were entirely different. For an alternating multilayered material impacted through the y direction (AM-18(y)), there was no crack; however, a big yield region (dark zone) formed during the part-impact compared with the existence of an obvious crack in the alternating multilayered materials impacted through the z direction (AM-18(z)) as well as conventional blends impacted through both y and z directions (C-20(y) and C-20(z)). In addition, there was almost no difference in the crack initiation and propagation of AM-18(z), C-20(y) and C-20(z). In the above three samples, massive crazing around the crack tip was initiated and then propagated along the impact direction, and a fan-shaped craze zone was formed around the crack tip. At the same time, massive crazing was also initiated around the yield-region tip in AM-18(y), but when the craze in the PP layer came across the PP/POE layer, the stress dispersed quickly at the transition layer along the direction parallel to the layer. Therefore, the craze in AM-18(y) propagated not only along the impact direction but also along the direction at a small angle from the layer; thus, a rectangular craze zone was formed around the yield-region tip. Moreover, this special type of craze propagation, which was along the direction at a small angle from the layer, could absorb much impact energy and then prevent the development of a craze into a crack along the impact direction. The different phenomena observed between conventional blends and alternating multilayered materials through part-impact were consistent with the results of the notched Izod impact strength.

Fig. 8 Initiation and propagation patterns of crack and craze of conventional blends and alternating multilayered materials after the part-impact test performed through the y and z directions.

Furthermore, to gain more insight for understanding the fracture mechanism and toughening mechanism, it is important to investigate the impact fracture surface and morphological evolution of the dispersed POE phase in conventional blends and alternating multilayered materials during the impact process through different directions. As shown in Fig. 9 and 10, (a) and (b), (d) and (e) were the two impact fracture surfaces of the same sample, respectively, and the distances of A, B and C from the notch were 0, 3000 and 6000 μm, respectively. For an alternating multilayered material (AM-18) impacted through the z direction Fig. 10(e) and a conventional blend (C-20) impacted through the y and z directions (as seen in Fig. 9(a) and (e)), only the impact fracture surfaces near the notch are relatively rough. The impact fracture surfaces of the three samples mentioned above become smoother as the distance from the notch increases, and the materials break by brittle fracture as a whole. However, all the impact fracture surfaces of the alternating multilayered material (AM-18) impacted through the y direction are very rough, which are characterizations of a tough fracture (as seen in Fig. 10(a)). Moreover, for C-20 impacted through both the y and z directions, the size and deformation of POE particles decrease compared with those before the impact (as seen in Fig. 9(b)–(d)). It has been known that dispersed POE particles can initiate crazes and shear bands in blends as stress concentration factors,³⁵ and then absorb the impact energy to block the crack propagation. Moreover, the crazes can be terminated when the front end of a craze encounter POE particles, avoiding the further development of crazes into cracks and thereby improving the toughness of the material. Therefore, the morphological evolution of the POE particles during the impact process is in fact a process of impact energy absorption. Moreover, the notch plays the role of a strain concentrator and takes on most of the strain loading during the impact process.³⁶ Once the crack is initiated, the stress concentration will increase sharply because the width of the crack tip is much less than that of the notch tip, which makes the POE particles near the notch absorb much more energy to alleviate the stress concentrated on the cracks and thereby prevent the further development of cracks. However, farther the fracture area is from the notch tip, lower is the impact speed, less is the energy absorbed by the POE particles, and smaller is the change in the POE particles. With regard to AM-18 (as seen in Fig. 10(b)–(d)), the size and deformation of POE particles in both the y- and the z-directions decrease compared with those before the impact, which are similar to those of conventional blends. However, because of the unique alternating multilayered microstructure, an interesting phenomenon appears about the dispersed morphology of the POE phase in the transition layer between the adjacent layers: the size and deformation of the POE particles in the PP/POE layer are relatively uniform before the impact both in the center of PP/POE layer and in the transition layer; however, during the impact, the size of the POE particles in the transition layer becomes smaller than that in the center of the PP/POE layer. Moreover, it has been known that the conventional blend with 30 wt% POE is ductile and that with 20 wt% is brittle. Therefore, a multilayered microstructure alternating with a brittle PP layer and a ductile PP/POE layer occurs in alternating multilayered materials with 30 wt% POE in the PP/POE layer. Therefore, the transition layer, formed between the PP layer and the PP/POE layer in the alternating multilayered materials, would play the role of a brittle–ductile transition (BDT) layer during the impact process.

Fig. 9 SEM images of impact fracture surface of conventional blend (a and e); heptane-etched impact fracture surfaces (b and d); heptane-etched surface before the impact. (a) and (b) were the same sample impacted through the y direction, and (d) and (e) were the same sample impacted through the z direction.

Fig. 10 SEM images of impact fracture surface of alternating multilayered materials (a and e); heptane-etched impact fracture surfaces (b and d); heptane-etched surface before the impact. (a) and (b) were the same sample impacted through the y direction, (d) and (e) were the same sample impacted through the z direction.

However, the formations of the BDT layer through the y and z directions are very different. In the case of the alternating multilayered material impacted through the y direction, the force propagates perpendicular to the layers. When the force propagates from the brittle PP layer to the transition layer, the POE particles in the transition layer become stress concentrators quickly to absorb the impact energy, and then the particle size decreases dramatically compared with that before the impact. However, the number of stress concentrators in the center of the PP/POE layer became larger than that in the transition layer resulting from the stratification of the POE content, and the total energy absorbed by the POE particles in the center of the PP/POE layer decreased after the impact energy was absorbed by the POE particles in the transition layer. These two factors result in the particle size in this area changing lesser than that in the transition layer. Moreover, when the force propagates from the ductile layer to the brittle layer, due to the difference in the POE content, a response will be produced to concentrate more stress on the POE particles in the transition layer than on the POE particles in the center of the PP/POE layer. Therefore, it can successfully realize the transition from a ductile layer to a brittle layer, and the particle size thereby becomes smaller than that in the center of the PP/POE layer. Therefore, a BDT layer, like a bridge connecting the brittle layer and the ductile layer, is formed in the transition layer during the impact process. However, for an alternating multilayered material impacted through the z direction, the force propagates along the rigid and ductile layers. The POE particles in the transition layer concentrated much more stress than those in the PP/POE center, due to the stratification phenomenon of the POE content. Therefore, the POE particles in the transition layer absorb more energy than those in the center of the PP/POE layer to resist the external force, leading to the particle size at the layer interface becoming smaller than that in the center of the PP/POE layer. Furthermore, a BDT layer was formed in the transition layer during the impact process of AM-18(z).

More importantly, the impact direction plays a key role in determining the ultimate properties of the material. As shown in Fig. 11(a), when the force is loaded perpendicular to the layer, the force can propagate through an alternating circulation in the fracture process: brittle/brittle–ductile-transition/ductile; therefore, the BDT layer can successfully connect the brittle PP layer and the ductile PP/POE layer. Thus, the material breaks by a ductile fracture as a whole. In addition, the existence of the rigid PP layer can provide the ductile PP/POE layer with rigidity when the force propagates perpendicular to the layers in turn. Therefore, the rigidity of the material is maintained very well, which is helpful to obtain a material with balanced toughness and rigidity. However, as shown in Fig. 11(b), the formation of the BDT layer does not play the role of a bridge connecting the brittle PP layer and the ductile PP/POE layer when the alternating multilayered material is impacted through the z direction. Since the force propagates along the brittle PP layer, BDT layer and ductile PP/POE layer, the material breaks by brittle fracture as a whole, and the rigidity is not maintained very well.

Fig. 11 Schematic representation of the toughening mechanism of alternating multilayered materials: (a) is impacted through the y direction; (b) is impacted through the z direction.

On the other hand, a previous study indicates that the conventional blend with 20 wt% POE is brittle. Therefore, for conventional blends (C-20) impacted through either the y or z directions, there is only brittle fracture mechanism due to the absence of the alternating multilayered microstructure (as seen in Fig. 12).

Fig. 12 Schematic representation of the toughening mechanism for conventional blends: (a) is impacted through the y direction; (b) is impacted through the z direction.

4 Conclusions

PP and PP/POE alternating multilayered materials were prepared using the multilayered co-extrusion technology. Alternating multilayered materials became ductile at −40 °C at a lower POE content of about 15 wt%, due to the multilayered microstructure with alternating brittle PP and ductile PP/POE layers, while conventional blends with the same POE content were brittle. The notched impact strength through the y direction of the alternating multilayered materials with 18 wt% POE at −40 °C was greatly improved compared with that of conventional blends with the same POE content. At the same time, the flexural modulus and the flexural strength of the alternating multilayered materials were maintained very well. The great enhancement of toughness and balanced rigidity can be attributed to the transition layer, formed at the interlayer interface, which plays the role of the BDT layer during the impact process. This work provides us not only a deep insight for understanding the elastomer toughening mechanism but also a new train of thought for toughening PP, which can greatly broaden the applied range of PP as a high-performance structural material with balanced mechanical properties.

Acknowledgements

Financial supports of the National Natural Science Foundation of China (51273132, 51227802 and 51121001) and Program for New Century Excellent Talents in University (NCET-13-0392) are gratefully acknowledged.

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