Quantification of the Effect of Shish-Kebab Structure on the

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Quantification of the Effect of Shish-Kebab Structure on the Mechanical Properties of Polypropylene Samples by Controlling Shear Layer Thickness Dashan Mi, Chao Xia, Ming Jin, Feifei Wang, Kaizhi Shen, and Jie Zhang* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, China ABSTRACT: Oriented “shish-kebab” structure can enable remarkable mechanical enhancement in polymers. Therefore, the formation mechanism and practical application of this structure have been extensively studied. However, the effect of shish-kebab content on mechanical properties is still uncertain. Knowledge of this effect is crucial in the academic and industrial fields but remains elusive because shish-kebab content is difficult to control. In this work, a self-developed multiflow vibrateinjection molding was used to produce samples with different shear layer thicknesses. The content of shish-kebab was represented by R, i.e., the thickness ratio of shear layer (composed by shish-kebab) to the whole sample. Results showed that with increased R impact/tensile strength exponentially increased, whereas elongation at break exponentially decreased. Based on the results, a modified model was proposed to interpret the strengthening and toughening mechanism. This study established a new method of predicting and controlling the mechanical properties of samples with shish-kebab and spherulite structures.

1. INTRODUCTION Isotactic polypropylene (iPP), one of the most important semicrystalline polymers, has many good properties, such as low cost, good stiffness, and high heat resistance.1−3 However, under conventional processing conditions, iPP tends to crystallize into spherulites. With such supermolecule structure, iPP exhibits low impact toughness, particularly at low temperatures, which restricts its extensive application.4,5 Therefore, iPP toughening is a research hotspot. The methods to enhance impact properties can be summarized as follows: (1) Adding elastomers into the matrix, such as styrene− ethylene−butylene−styrene and ethylene−propylene−diene− monomer.6,7 Nevertheless, this method usually decreases both modulus and strength.8 (2) Introducing β-crystal to iPP. βcrystal has several advantages over α-crystal, such as improved elongation at break and impact strength.9−11 However, the βcrystal also decreases the corresponding tensile strength and modulus.12 (3) Forming structures with molecular orientation (such as shish-kebab), which is regarded a kind of selfreinforced structure. This structure attracts increased attention because it can simultaneously improve impact strength and other properties, such as remarkably improved tensile strength, modulus, stiffness, and thermal stability, as well as decreased permeability.13−15 Crystallization during elongation or shear flow is required to produce shish-kebabs. Standard injection molding processes have shear flow, but only a few shish-kebabs can be produced on the molding surface because slow cooling of the melt in the interior of the molding allows oriented chains to relax before crystallization.16,17 Some unusual injection molding technolo© 2016 American Chemical Society

gies that impose a strong shear on the melt have been developed. These technologies include shear-controlled orientation in injection molding,18 oscillatory shear injection molding technology,19 and vibrate-injection molding (VIM).20 These methods provide a relatively thick sheared layer (or high sample orientation) for enhanced performance. To date, the role of shish-kebab content in an injectionmolded part, which commonly has a typical skin−core structure, remains unknown. In other words, the existence of a quantitative relationship between shish-kebab content and part performance is not yet confirmed. The reason is the absence of an efficient way to control shear layer thickness that could vary within a wide range. This issue has been addressed by our recently developed technology named multiflow VIM (MFVIM). MFVIM is a new process of pressure VIM (PVIM), whose mechanism has been described in our previous papers.20,21 Different from PVIM, a mold with a flash groove that can form multiflow in the sample during the packing stage is used. The schematic of the process is shown in Figure 1. The mold is initially filled with melt at a certain injection pressure, and no melt could spill through the flash groove. The oscillatory pressure is subsequently introduced to form a second/third flow during the packing stage, and a part of the melt could be pushed out of the cavity through the flash groove. Notably, to express the principle simply, only three melt flows are drawn in Figure 1. Six flows could be achieved in the Received: April 25, 2016 Revised: June 12, 2016 Published: June 20, 2016 4571

DOI: 10.1021/acs.macromol.6b00822 Macromolecules 2016, 49, 4571−4578

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where Iα(110), Iα(040), and Iα(130) are the intensities for α-form peaks (110), (040), and (130) planes, respectively, while Iβ(300) is the intensity of β-form peak (300) in linear WAXD pattern. Additionally, the orientation of lamellar crystals in the injectionmolded parts was calculated using the Hermans orientation function.23 In this method, the crystal orientation was characterized by the average orientation of the normal to the crystal plane with respect to an external reference frame. Accordingly, the flow direction was taken as the reference direction. For a set of hkl planes, the average orientation, expressed as ⟨cos2 φ⟩hkl, was calculated mathematically using the equation 2

⟨cos φ⟩hkl =

f=

I(φ) cos2 φ sin φ dφ

∫0

π /2

I(φ) sin φ dφ

(2)

3⟨cos2 φ⟩hkl − 1 2

(3)

with f having a value of −0.5 with the normal of the reflection plane being perpendicular to the reference direction (φ = 90°), a value of 1 with the normal of the refection plane parallel being the reference direction (φ = 0°), and a value of 0 with the orientation being random. The (040) diffraction rings in the two-dimensional patterns were chosen for calculating the degree of orientation in this study.24 2.3. Polarized Light Microscopy (PLM). Thin slices cut by microtome were used for optical morphology observations. The observation zones were located in the middle of samples along the flow direction. Morphology observations were conducted using a DX-1 (Jiang Xi Phoenix Optical Co., China) microscope connected to a Nikon 500D digital camera. 2.4. Scanning Electron Microscope (SEM). A JEOL field emission scanning electron microscope (model JSM-7500F, Japan) was used for SEM observations. The specimens were gold sputtered after being etched in acid solution. 2.5. Measurement of Mechanical Properties. The tensile test was conducted at room temperature (20 °C) on an electro-universal testing machine (Instron 5569) with a cross-head speed of 30 mm/ min. The notched Izod impact strength of the specimens was measured with a VJ-40 Izod machine at room temperature. Before the test, a 45° V-shaped notch (depth 2 mm) was made. The flexural test was conducted on an AGS-J universal materials testing machine according to ASTM D 790.25 All of these tests are conducted along the injection direction, and the values of all the mechanical properties were calculated as averages of over five samples.

experiment via adjusting the vibration frequency. Samples with different shear layer thicknesses can be prepared using these techniques, and shish-kebab content can thus be controlled. In the present work, the relationship between the thickness of shish-kebab layer and mechanical property was revealed for the first time. Results indicated that the flexural modulus and the thickness ratio of shear layer to the whole sample R presented a quadratic function relation, but other properties (including tensile strength, impact strength, and elongation at break) exponentially increased or decreased with increased R. To reveal the reinforcing mechanism, the structures of MFVIM samples were carefully characterized by using wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and polarized light microscopy (PLM). A schematic of morphology and the fracture process was proposed accordingly.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. iPP (T30S) was purchased from Dushanzi Petroleum Chemical Co., China, with a melt flow index rate of 3 g/10 min (230 °C/2.16 kg), Mw = 39.9 × 104 g/ mol, and Mw/Mn = 4.6. Conventional injection molding (CIM) and MFVIM were used to prepare iPP samples, which were rectangular and had dimension of 59 mm × 60 mm × 3 mm. These samples were cut into certain shapes (dumbbell or strip shaped) along the flow direction for different mechanical tests. Shear layer thickness was controlled by changing the MFVIM parameters. The injection and holding pressures were both 50 MPa, and the vibration pressure was 90 MPa. The samples were labeled according to melt-flow times. For example, S1 represents the sample that experienced only one melt flow, whereas S6 represents the one that experienced six melt-flow times. 2.2. X-ray Measurements. The synchrotron 2D-WAXD experiment was carried out on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The dimension of the rectangle-shaped beam was 0.5 × 0.8 mm2, and the wavelength of light was 0.124 nm. The sample-to-detector distance was 140 and 1850 mm for WAXD and SAXS, respectively. Linear WAXD profiles were obtained from circular integration of intensities from 2D-WAXD images, and the relative amount of β-phase (Kβ) was calculated by the widely accepted formula proposed by Turner-Jones et al.:22

3. RESULTS AND DISCUSSION 3.1. Crystalline Structure and Orientation. PLM was used to observe the shear-induced multilayer structure of MFVIM and CIM samples, and results are shown in Figure 2.

Figure 2. PLM observation of the multilayer structure of injectionmolded samples. R is the thickness ratio of the shear layer to the whole sample.

Iβ(300) Iβ(300) + Iα(110) + Iα(040) + Iα(130)

π /2

with φ being the azimuthal angle and I(φ) being the scattered intensity along the angle φ. Herman’s orientation function, f, was defined as

Figure 1. Schematic of multiflow vibrate-injection molding: (a, b) first flow; (c, d) second flow; and (e, f) third flow.

Kβ =

∫0

(1) 4572

DOI: 10.1021/acs.macromol.6b00822 Macromolecules 2016, 49, 4571−4578

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The orientation degrees calculated from the 2D-WAXD results are presented in Figure 5a and Table 1. The orientation of CIM was lower than that of MFVIM samples. High orientation areas existed in S2, and this area increased from S2 to S6, specifically for S6, from skin to core. All layers had a high orientation degree. Unlike the degree of orientation, Kβ did not simply increase from S1 to S6, as shown in Figure 5b and Table 2. Except for S1 and S6, the Kβ of all samples reached its highest value when the layer orientation degree sharply decreased, as shown in Figure 5a. For S1 and S6, the orientation degree also sharply decreased near the skin or core layer. However, the size of WAXD beam was too large to focus on the thinner shear layer in S1 or thinner core layer in S6; thus, a sharp decrease in orientation cannot be detected. The formation mechanism of β-crystals can be explained by shear flow, which can induce α-iPP row structures. The surface of these row structures provided nucleation sites for β-crystal growth.21 The portion of β-crystals remarkably depended on shear rate. β-Crystals can be formed at a low shear rate and reach a plateau value when shear rate exceeds 57 s−1.26 At a high shear rate, strong shear flow induces a highly oriented structure, i.e., shish-kebab, which is not favorable to β-crystal formation.27 Therefore, β-crystals tended to form at the edge of the shear area. In addition, as shown in Figure 5b, Kβ in S1 was higher than that in other samples because the first flow contacted with the cold mold wall, and the subsequent flows contacted with the solidified polymer. Therefore, they had different crystallized conditions. The S1 case probably had the favorable temperature field for the growth of β crystal.28,29 In our case, the β crystal growth should be restricted at high temperatures, so the β fraction in S2, S3, S4, S5, and S6 never exceeded that in S1. Figure 6 shows the 2D-SAXS patterns and SEM images taken at different distances from the surface of S3 and S6. The flow direction is horizontal for SEM but vertical for SAXS. In the case of S3 (Figure 6a), a visible and narrow meridional maximum was observed near the skin, whereas a weak and broad meridional maximum appeared in the core. The weak oriented signal in the core should be caused by the size of SAXS beam (0.5 × 0.8 mm2) that was larger than the layer thickness (0.3 mm). Furthermore, the equatorial streak was clear near the skin, and the equatorial streak was totally invisible in the core. The meridional maxima in the SAXS pattern can be attributed to the formation of kebab-like lamellar structures and some well oriented lamella that were perpendicular to the flow direction, whereas the equatorial streak can be attributed to the formation of shish or microfibrils and/or a*-axis oriented “daughter” lamellae growing epitaxially. These conclusions were confirmed by SEM images, as shown in Figure 6 (b1, c1, and d1). Clearly, highly oriented shish-kebabs were formed in the shear layers, whereas isotropic spherulites were observed in the core layer. Figure 6 (a2, b2, c2, and d2) shows that S6 had a highly oriented shish-kebab structure in each layer except for the core. As shown in Figure 6 (d2), a column crystal was formed in the core layer of S6. 3.2. Mechanical Properties. Figure 7a illustrates the flexural modulus as a function of different R values, which are listed in Table 3. As the core had a mean distance from the central plane of 3(1/4 − R/4) and the oriented layers were 3(1/2 − R/4) away from the central plane. Thus, the flexural modulus (M) was calculated as follows:

The thickness of the core layers was plotted in red arrows, and the thickness ratio of the shear layer to the whole sample R was marked in the bottom left corner. As expected, the sample prepared via CIM showed a typical skin−core structure (Figure 2a). The skin (or shear layer) was thin and consisted of shishkebabs (cannot be seen clearly), whereas the core was thick and consisted of spherulites. Nevertheless, when multiflow was introduced, the shear layer progressively thickened, as shown in Figures 2b−f. For S6, the core layer almost disappeared and reached a R of 97%. To the best of our knowledge, such thick shear layer for injection-molded iPP has never been reported before. Figure 3 shows a series of WAXD patterns from different layers, which can illustrate the crystal information on iPP

Figure 3. 2D-WAXD patterns of samples with different shear layer thicknesses.

samples. The diffraction intensity distribution consisted of five diffraction rings associated with different lattice planes of iPP, including (110), (040), (130), (111), and (−131) from inner to outer circles, which were typically of α-crystals. An additional (300) lattice plane appeared in the skin layer of S1 and shear layers of S2−S6 corresponding to the reflection of β-crystals. A γ(117) diffraction ring was also observed in the MFVIM samples. This ring was associated with the instant high pressure and shear during multiflow. After circularly integrating the intensities of 2D-WAXD patterns, linear WAXD curves were obtained and are presented in Figure 4. S1 had mainly α-crystals, except for a small little amount of β-crystals near the skin (Figure 4a). This finding was caused by the fact that during the injection process, when iPP melt flowed into the mold cavity, shear flow emerged because of the velocity gradient, which caused the iPP molecules to be oriented. These oriented molecules then induced the generation of a few α-row nuclei. Therefore, the β-crystals subsequently grew on the formed α-row nuclei. For S2, βcrystals appeared in the first and second layers, For S3, they were formed in the outer three layers. When flow times reached six, (300) peaks were found in all layers. The formation mechanism of β-crystals in these samples was similar to that for S1. The only difference was that the oriented melt made contact with the solidified polymer instead of the mold surface. The relationship between the oriented structure and β-crystals in MFVIM samples (S2−S6) is discussed in detail in the subsequent parts of this paper.

M = MR = 0(1 − R )2 + MR = 1R(2 − R ) 4573

(4)

DOI: 10.1021/acs.macromol.6b00822 Macromolecules 2016, 49, 4571−4578

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Figure 4. 1D-WAXD curves of samples of (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5 obtained from circularly integrated intensities of 2D-WAXD patterns.

Table 1. Orientation Degree of Different Samples at Various Distances Away from the Sample Surface orientation degree

Figure 5. (a) Degree of orientation and (b) β-crystal fraction of different samples from skin to core.

distance from the edge (mm)

S1

S2

S3

S4

S5

S6

0.3 0.6 0.9 1.2 1.5

0.59 0.52 0.5 0.47 0.02

0.8904 0.62 0.5 0.46 0.0219

0.9123 0.92 0.53 0.49 0.03

0.92 0.93 0.87 0.62 0.04

0.91 0.9 0.9 0.82 0.5

0.93 0.92 0.91 0.9 0.89

quently, the flexural modulus increase significantly at low R because the flexural modulus of shish-kebabs is higher than that of spherulites. In addition, the shish-kebab structure on the

where MR=0 and MR=1 are the flexural modulus of sample that only spherulite or shish-kebab exists, respectively. Conse4574

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Macromolecules Table 2. Relative Content of β Crystals of Various Samples at Various Distances Away from the Sample Surface

Table 3. Different Mechanical Properties for Various R R

relative content of β crystals (%) distance from the edge (mm)

S1

S2

S3

S4

S5

S6

0.3 0.6 0.9 1.2 1.5

8.18 3.37 0 0 0

4.49 4.22 0 0 0

3.7 2.3 6.2 0 0

3.2 1.2 1.3 3.6 0

2.3 2.5 3.2 4.2 0

2.1 2.4 0 1.2 5.3

flexual modulus (MPa) impact strength (kJ/m2) elongation at break (%) tensile strength (MPa)

0.03

0.24

0.5

0.68

0.76

0.97

1384

1886

2615

2825

2839

3061

4.17

6.00

7.19

8.76

11.93

18.01

618

200

125

78

50

54

35.8

39.7

42.0

46.6

50.3

65.00

the impact strength can be tremendously improved. Accordingly, the impact strength can be expressed as follows: y = A exp(R /t ) + y0

(5)

where A = 0.7176, t = 0.3214, y0 = 3.5445, and R = S/W; S is the shear layer thickness, and W is the thickness of the whole sample. Notably, this function is only one possible arbitrary function that can be used to interpolate values. Similar statements also apply to elongation and tensile strength. To further study the fracture mechanism, the fracture surfaces of impacted samples were investigated by using SEM, and the images are shown in Figure 8. In S1 (Figure 8a−d), many “patches” appeared as a result of microvoiding (accompanied by crazing), which occurred during fracture. The fracture surface of S3 can be easily divided into two areas, as indicated by the dotted line in Figure 8e: one was a smooth area (shear layer), and the other area was rough (core layer). Shish-kebabs were supposed to fracture rapidly and thus formed a smooth surface, as shown in Figure 8f. The failure of S6 completely differed from those of S1 and S3. SEM images taken from the shear layer area (Figure 8j,k) showed a coarse surface formed by plastic fracture. As shown in Figure 8l, the fracture surfaces of shear (shish-kebab) produced more patches than those of core (spherulite) layers, which tremendously increased the impact strength. These results indicated that the brittle−ductile transition of shish-kebab was related to the shish-kebab content. The relationship between fracture mechanism and impact strength is discussed in further detail in Figure 11.

Figure 6. 2D-SAXS patterns and SEM images taken at different distances from the surfaces of S3 and S6.

surfaces of the sample has a more significant effect because they are further away from the central plane. Figure 7b shows the impact strength as a function of different R values. S6 had a 350% improvement in impact strength compared with S1. Moreover, at low R, the impact strength increased slowly with increased R because failure was controlled by the weakest phase. Statistically, the reduced volume of the weaker phase strengthened the sample. Eventually the material did not fail even when the weaker component did because the stronger phase dominated at high R. Therefore, when R > 0.5,

Figure 7. (a) Flexural modulus and (b) impact strength vs R (thickness ratio of shear layer to whole sample). 4575

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Figure 8. SEM microphotographs taken of the fracture surface of S1 (a)−(d), S3 (e)−(h), and S6 (i)−(l). The left column is the full view of the surface fracture.

Figure 9. Mechanical properties of different samples: (a) selective stress−strain curves and (b) elongation at break and tensile strength vs R.

Figure 10. Digital photos of yield samples are shown in the left. SEM image of the broken surface of (a) S1 and (b) S5. Local magnification photos are shown in (a1), (a2), (b1), and (b2).

function of R are illustrated in Figure 9b. Equation 5 was also used to fit the changes in elongation and tensile strength. The parameters are shown in Figure 9. The results showed that elongation at break exponentially decreased with R, where S1

Figure 9a illustrates the stress−strain curves of different injection-molded samples. With increased shish-kebab content, elongation at break decreased and the tensile strength increased. Elongation at break and tensile strength as a 4576

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yield at a faster speed to absorb more impact energy.30 The shish-kebab can transmit the impact force to a deeper area, but the ductility of shish-kebab was much lower than that of spherulite. Figure 11b shows the impact fracture morphology of S3 and S6, which can be used as representative samples. A smooth fracture surface formed in the shish-kebab area of S3, but a rough fracture formed in that of S6. As aforementioned, the low shish-kebab content had little effect on increasing impact strength because when the spherulite yielded to absorb the energy, the shish-kebab had already broken, meaning that shish-kebab had to sustain almost all of the impact energy at the initial stage of impact test. Thus, the less content of shish-kebab (S3) can be easily fractured at that stage. The fast and brittle fracture of shish-kebab caused the formation of a very smooth fracture surface in the shear layer, as shown in Figure 8f. However, when shish-kebab composed the largest part (S6), it sustained the impact energy together in the initial stage of the test. Thus, the shish-kebab can absorb more energy and greatly improve the impact strength. Figures 11c, 11d, and 11e show the tensile test of S1, S3, and S6, respectively. For S1, spherulites deformed by stretching in the initial stage and stretched into fibers in the next stage. When shish-kebab composed the small part of the sample, shish-kebabs and spherulite deformed together in the initial stage. However, the shish-kebab fracture before the spherulite reached its highest stress, meaning that spherulite and shish-kebab cannot bear the tensile forces at the same time. This lack of synchronization can decrease the tensile strength. With increased shear layer thickness, the declining factor did not play a major role in tensile strength. Therefore, the low shish-kebab content had little effect to increase the tensile strength, but greater shishkebab content can greatly enhance the tensile strength.

showed the highest value (623%), which was about 10 times that of S6. However, tensile strength exponentially increased as a function of R, where S6 showed the highest tensile strength (63 MPa), which was about 75% higher than that of S1. These results indicated that shish-kebab can increase tensile strength at the cost of decreasing ductility. In general, thinner shear layers caused more rapid decrease in elongation at break (R < 0.2). In addition, tensile strength significantly increased only in the case of sufficient shear layer thickness (R > 0.5). Digital photos of yield samples are shown in the left of Figure 10, and the broken surfaces are shown in Figure 10a,b. As aforementioned, elongation at break drastically decreased with increased shish-kebab content because the molecular chains in isotropic spherulites and the amorphous region can be easily stretched. After stretching, many fibrillar structures formed on the surface, as shown in Figure 10 (a1 and a2). Nevertheless, the oriented shish-kebab in S5 cannot be further stretched, so this structure broke at a low elongation. As shown in Figure 10 (b1), the break surface of shear layer was smoother than that of S1. In the core layer of S5 (Figure 10 (b2)), some fibrillar structures were also formed, but their elongation was lower than that of S1. Thus, the spherulites in the core layer cannot absorb a stress as high as they did in S1. This phenomenon can explain the slow growth rate of tensile strength when the shishkebab content was low. Schematics of the fracture process and morphology are shown in Figure 11. Figure 11a is a three-dimensional model,

4. CONCLUSION PP injection-molded parts with various fractions of shear layer were successfully prepared by using the MFVIM technique. The major achievement of this work was the identification of the relationship between shish-kebab content and mechanical properties, which can be used to control the part performance. Results indicated that with increased shish-kebab fraction, the flexural modulus considerably increased at low R, and the other properties (including tensile strength, impact strength, and elongation at break) exponentially increased or decreased.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We are indebted to the Shanghai Synchrotron Radiation Facility (SSRF) in Shanghai, China, for WAXD experiments.

Figure 11. Schematic of the fracture process and morphology of the composites: (a) three-dimensional model, (b) impact test, and (c−e) tensile test.

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DOI: 10.1021/acs.macromol.6b00822 Macromolecules 2016, 49, 4571−4578