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Polypropylene Injection Molded Part with Novel Macroscopic Bamboo-like Bionic Structure Run Su, Zeqi Zhang, Xiang Gao, Yao Ge, Ke Wang,* and Qiang Fu* Department of Polymer Science and Materials, State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, Chengdu, 610065, P.R. China ReceiVed: March 8, 2010; ReVised Manuscript ReceiVed: May 13, 2010
A macroscopic bamboo-like bionic structure was fabricated in the injection-molded bar of isotactic polypropylene via the combined effect of “melt manipulation” and β-nucleator. Such structure consists of strengthened shell zone with high orientation and low β-phase amount, and toughened core part with isotropic texture and dominant β-modification. The influences of shear intensity on structural hierarchies, polymorphism, and crystalline morphology were estimated. Both toughness and strength can be significantly improved with increasing shear intensity on the bamboo-like structure. Our study suggested an alternative approach to achieve excellent comprehensive mechanics in polypropylene via macroscopic structural design during the practical molding process. 1. Introduction Isotactic polypropylene (iPP), as one of the most important commodity polymers, has good comprehensive properties. However, the toughness of iPP is usually low and limits its application. Thus, the toughening of iPP has attracted the interest of researchers and the attention of industry. As a common consideration, the low impact toughness of iPP is caused by the formation of sizable spherulites and the dominant crystallographic phase, monoclinic R-crystal, with high rigidity. So addition of a nucleator, especially β-nucleator, is a simple but effective way to realize the toughening of iPP.1-5 Compared to the method of blending with rubber or thermoplastic elastomer,6-9 the crystallographic modification of adding nucleator can preserve strength, modulus, and thermal stability in a relatively high level. Molding processing is a necessary stage linking pristine polymer materials and specific practical application. Obviously, the hierarchical structure of polymer material parts can be controlled by adjusting thermal and/or shear conditions during molding processing, and the structural design plays an important role for optimizing the ultimate performance of polymer application. For example, recent studies indicated that simple thermo-treatment process, such as variation of molten temperature3 and post annealing,4 can manipulate the crystalline morphology and polymorphic composition in the molding parts of β-nucleated iPP, and thus cause significant increment in toughness upon a fixed amount of β-nucleator. The good toughness of β-PP is mainly attributed to easy slipping and/or elongating the lamellae during impact deformation. However, the “soften” behavior of β-PP also brings an obvious decrease in strength and stiffness. Receiving orientation of the crystalline structure via shear field can effectively compensate this shortcoming of mechanics. On the other hand, many studies have proved that shear can largely restrain the formation of β-phase in the melt with β-nucleating agent.5,10,11 The higher the shear rate is, the lower the β-phase crystallinity will be. It is well-known that there exists a skin-core structure in the injection molded bar of a polymer due to the thermal and * To whom correspondence should be addressed. Tel: 0086-28-85461795. E-mail:
[email protected] (K.W.),
[email protected] shear gradient. In the skin zone, a highly oriented structure is obtained and a isotropic texture exists in the core zone. Can we obtain, by combination of shear field and addition of β-nucleator, a 2-fold shell-core structure in the injectionmolding part of iPP: the outer regions, including the skin zone and the intermediate zone, are with shish-kebab superstructure and high orientation degree, and exhibit a “strengthening” feature; while the inner region (the core zone) possesses random crystalline structure and large amount of β-crystals, and correspondingly presents a “toughening” attribute. From a biomimetic view of point, the injection-molded part with strengthened shell and toughened core can be regarded as a bamboo-like structure. In this paper, we developed a facile way to obtain the bamboo-like bionic structure during injectionmolding process by adjusting the shear intensity imposed on the iPP melt with β-nucleator. In order to realize variation of shear intensity, one specific molding technique of “melt manipulation” methods, namely dynamic packing injection molding (DPIM), was utilized. The main feature of DPIM is to introduce the oscillatory shear field on the melt/solid interface of the cooling melt during the packing stage by two hydraulically actuated pistons that moved reversibly at a constant rate. The shear intensity can be modulated by varying the shear duration (number of repeated movements of pistons). The effect of shear in the shell zone was apparent, which induced high degree of orientation, shish-kebab superstructure, and depression of β-phase, whereas it was weak in the core zone, resulting in isotropic crystalline structure and high content of β-phase. Consequently, the structural hierarchies, such as the area ratio of shell/core, orientation level, polymorphic composition, and crystalline morphology, were impacted substantially with variation of shear intensity, and ultimately related to the improvement of mechanical properties. 2. Experimental Section 2.1. Materials. A commercially available isotactic polypropylene (trademarked as T30S), Mw ) 3.9 × 105 g/mol and Mw/ Mn ) 4.6, was supplied by Dushanzi Co. Ltd., China. The β-nucleating agent (TMB-5, based on aryl amide) was kindly provided by Shanxi Provincial Institute of Chemical Industry (China).
10.1021/jp1020802 2010 American Chemical Society Published on Web 07/19/2010
Macroscopic Bamboo-like Bionic Structure SCHEME 1: Illustration of the Number of Repeated Movements of Pistons (n)
2.2. Samples Preparation. The master batch of PP with β-nucleating agent (NA) was first prepared by premixing polypropylene with 3 wt % NA in a twin-screw extruder (TSSJ25 corotating twin-screw extruder, the L/D ratio of the screws is 32, and D ) 25 mm). The barrel temperatures were 160-200 °C and a screw speed of 120 rpm was used. Then, the master batch and pure iPP were extruded to prepare iPP specimens containing 0.3 wt % NA. The pelletized granules were subsequently injected into a mold with the aid of an SZ100g injection-
Figure 1. Photographs of the cross section of various samples.
J. Phys. Chem. B, Vol. 114, No. 31, 2010 9995 molding machine with the barrel temperature of 200 °C, and then dynamic packing injection molding technology was applied. The frequency of repeated movement of pistons was 0.3 Hz, and the shear rate of melt was 10 s-1, calculated from the mold geometry. Some parameters of injection molding and the schematic representation of DIPM can be found in our previous literature.12 For clarity, the number of repeated movements of pistons (n) is defined in Scheme 1. The larger the value of n, the stronger the shear field that is imposed on the melt during the packing stage. The as-prepared parts (3.6 mm in thickness) are labeled according to the number of repeated movements of pistons. For example, DPPB5 represents the sample that was subjected to oscillatory shearing for 5 times. 2.3. Synchrotron Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). The synchrotron 2D-WAXD experiments were carried out on the U7B beamline in the National Synchrotron Radiation Laboratory (NSRL), Hefei, China. The wavelength used was 0.1409 nm. The twodimensional diffraction patterns were recorded every 180 s by a Mar CCD 165 X-ray detector system in transmission mode at room temperature. The backgrounds of all the 2DWAXD patterns had been extracted, thus allowing a qualitative comparison between various samples. Azimuthal scans (0-360°) of 2D-WAXD were made for the corresponding
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Figure 2. 2D-WAXD patterns at the layer 400 µm from the skin for various samples. The red arrow indicates the flow direction and the same below.
lattice planes of the R-form polypropylene (R-PP) and the β modification (β-PP) at a step of 1°. The orientation level of various planes could be calculated by the orientation parameter f, which was calculated as follows:
f)
〈cos φ〉 ) 2
3〈cos2 φ〉 - 1 2
∫0π/2 I(φ) sin φ cos2 φ dφ ∫0π/2 I(φ) sin φ dφ
(1)
(2)
where φ is the angle between the normal of a given (hkl) crystal plane and the shear flow direction and I is intensity. Additionally, to calculate the relative amount of β-phase (Kβ), first the 1D-WAXD patterns were obtained by transforming from the 2D-WAXD patterns; then the widely accepted formula proposed by Turner-Jones et al. was used,13 which is as follows:
Kβ ) Iβ /(IR1 + IR2 + IR3 + Iβ) where Iβ is the intensity of the (300) reflection of the β-modification, and IR1, IR2, and IR3 are the intensities for the (110), (040), and (130) planes of the R-phase, respectively. 2.4. Scanning Electron Microscope (SEM). The thin slices were cut at different zones from the injection molded bars and first etched chemically by a mixed acid.14 Then the surface was coated with gold and subsequently examined by an FEI Inspect F scanning electron microscope instrument at 20 KV.
2.5. Polarized Light Microscopy. The morphological observation of the cross section of various samples was investigated on a Leica DMIP polarizing light microscopy (PLM) at room temperature. The 50 µm thick slices were cut from the cross section of the injection-molded parts. 2.6. Mechanical Properties Measurement. A SANS universal testing machine (Shenzhen, China) was used to measure the tensile properties, with a moving speed of 50 mm/min. The measurement temperature was about 20 °C. For impact toughness, the injection-molded parts were tested with an I200XJU2.75 Izod impact tester at room temperature. The values of all the mechanical properties were calculated as averages of over six specimens. 3. Results and Discussion 3.1. Effect of Shear on the Macroscopic Shell-Core Structure. To examine the shear-induced change of macroscopic structure, a series of polarizing light photographs related to the cross sections of the injection-molded parts are shown in Figure 1 with the increase of shear intensity (value of n). In all the PLM images, two zones with distinctly different optical characters can be observed: the high-light white zone, isotropic core zone, is surround by the dark-light multicolor zone, anisotropic shell zone, which agreed well with our previous works.15,16 As shear intensity increased, the area of the shell zone improved gradually and correspondingly the core zone reduced. Thus, the area ratios of shell/core can be well controlled by adjusting the shear intensity. The 2D-WAXD measurement is utilized to investigate the effect of shear intensity on the orientation level of the DPIM parts. A series of 2D-WAXD patterns obtained at the fixed depth of 400 µm for various samples are shown in Figure 2. From inner to outer, the reflection circles or arcs originate from the
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TABLE 1: Order Parameters at the Depth of 400 µm for Various Molded Bars sample
R-PP
β-PP
DPPB0 DPPB2 DPPB5 DPPB10 DPPB20 DPPB30
0.15 0.25 0.42 0.59 0.74 0.79
0.11 0.55 0.34 s s s
(110), (040), (130), (111), and (1j31) planes of iPP R-modification, and the characteristic reflection of β-form PP, the (300) plane, is located between the (110) and (040) planes of the R-phase. These characteristic reflections are indicated clearly in the patterns for DPPB5 and DPPB30. One can observe the isotropic circles for DPPB0, indicating almost no orientation of molecular chain at the depth of 400 µm for this sample. With the increase of shear intensity (value of n), the reflection circles of R-form PP change to strong arcs, indicating the c-axes of R-PP lamellae are preferentially oriented along the shearing direction. It is well-known that the polymer melt will be cooled and solidified from the wall of the mold cavity to the core part. When shear is applied during packing stage, the molecular chains can be preferentially oriented along the shear direction. However, the orientation will relax more or less in the melt after shear is terminated, which depends on the shear intensity. Thus, the larger the value of n the more the melt is sheared, resulting in higher degree of orientation in a certain depth of the injection-molded part. As for the (300) plane of the β-crystal,
Figure 3. 2D-WAXD patterns at different layers of DPPB5.
however, the reflection is first strengthened after oscillatory shearing for two and five times, and then disappears when the shear intensity is further increased (more than five times). Obviously, the formation of β-phase of iPP is restrained in strong shear field, which is in accord with the previous literature.10 In order to quantitatively estimate the orientation degree in various DPIM parts, the order parameters (f) for the (040) plane of R-crystal and the (300) plane of β-crystal have been calculated, according to eq 1, and the values of f are listed in Table 1. The orientation degree of R-PP is improved monotonously from 0.15 for DPPB0 to 0.79 for DPPB30. However, as for the orientation of β-PP, the situation is somewhat complicated. The orientation degree of β-phase in DPPB0 is very low, only 0.11. The reason is due to no further shear and serious relaxation during packing. The order parameter of β-PP is 0.55 in DPPB2, but that of R-PP is just only 0.25. It seems that the β-crystal is easy to be oriented with such shear intensity. When the sample is repeatedly sheared five times, the orientation of β-PP decreases again, which is 0.34. By calculating the content of β-phase from the 1D WAXD spectra (not shown here), we find that there is less amount of β-crystal at the depth of 400 µm in DPPB5 than in DPPB2. This result means that the β-phase is very sensitive to shear, and weak shear may favor the orientation of β-crystal, but stronger shear leads to depression of the formation of β-crystal. Many works have proved that shear can largely restrain the formation of β-phase in the melt with the β-nucleating agent. Huo et al. investigated the combined effect of shear and nucleating agent on the formation of β-phase10 They have proposed that shear accelerates the
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TABLE 2: Order Parameters (f) and Relative Amounts of β-Phase (Kβ) at Different Layers of DPPB5 sample DPPB5 DPPB5 DPPB5 DPPB5
0 µm 400 µm 800 µm 1800 µm
f
Kβ
0.76 0.42 0.24 0.19
0.14 0.29 0.36 0.52
nucleation and the growth of the induced R-phase, and thus the β-nucleating agent has little chance to take effect. Thus, the favorable formation of R-phase under the effect of shear is probably due to the reduced nucleation barrier of R-phase and also due to a transition of β-phase to R-phase. 3.2. Hierarchical Structure of the Injection-Molded Parts. Subsequently, the hierarchical structure from the outer region to the inner region will be characterized in detail on two representative specimens, DPPB5 and DPPB30, which are related to modest shear intensity and strong shear intensity, respectively. The 2D-WAXD patterns for different depths in DPPB5 are presented in Figure 3. These characteristic reflections are indicated clearly in the patterns for DPPB5 and DPPB30. One can observe strongly focused arcs for the outermost zone (0 µm), because of strong shear and fast cooling near the wall of the mold cavity. The trend is changed from the strong arcs to obscure ones or even circles from the outermost zone to the core zone (from 0 to 1800 µm). It should be noted that there are two reflection arcs of β-form PP at the skin, while there are six at the depth of 800 µm. One possible explanation is the difference in the orientation degree. When there is nearly no orientation, a perfect reflection circle can be seen; with the increase of the orientation degree, the circle degenerates into
six arcs and then two arcs. The order parameters of the (040) plane of R-crystal at various depths can be seen in Table 2. The order parameter of R-crystal decreases continuously from 0.76 (0 µm) to 0.19 (1800 µm). The gradual decrease of orientation degree of R-PP with the depth is reasonable, because of shear gradient and the difference in relaxation rate at different depths. To identify the polymorphic composition in DPPB5, the relative β-form content (Kβ) has been calculated according to eq 2 and is listed in Table 2. The β-phase can be found in the whole part, and the value of Kβ increases gradually with increment of depth, from 0.14 for 0 µm to 0.52 for 1800 µm. This indicates that the modest shear intensity of oscillatory shearing for five times is not enough for greatly depressing the formation of β-phase, although the relative amount of β-phase is not as high as that in the sample obtained via conventional injection molding (usually more than 0.9), which is consistent with the results in the literature.5,10 The experiment was repeated several times, and the result is nearly the same. The lower amount of β-phase is caused by shear. In the DPIM experiment, an oscillatory shear field on the melt/solid interface of the cooling melt was introduced during the packing stage although it is comparatively weak at the core part. Moreover, the crystalline morphologies at different depths of DPPB5 were exactly observed by SEM, as shown in Figure 4. For the depths of 0 and 400 µm, the highly oriented shish-kebab superstructure is dominant and the lamellae grow perpendicularly to the flow direction. However, the isotropic spherulite structure is found in the depths of 800 and 1600 µm. It should be noted that the β-phase can be easily distinguished at these two depths, which is characterized by flower-like crystals and loosely packed lamellae, as indicated by the arrows in Figure 4, c and d.
Figure 4. SEM photographs at different layers of DPPB5: (a, b (upper panels)) ×40 000; (c, d (lower panels)) ×5000. The orange arrows indicate typical β-modification of PP.
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Figure 5. 2D-WAXD patterns at different layers of DPPB30.
TABLE 3: Order Parameters (f) and Relative Amounts of β-Phase (Kβ) at Different Layers of DPPB30 sample DPPB30 DPPB30 DPPB30 DPPB30
0 µm 400 µm 800 µm 1800 µm
f
Kβ
0.75 0.79 0.68 0.37
0.13 s 0.08 0.32
Combining the results of 2D-WAXD and SEM, it can be concluded that a bamboo-like shell-core structure is obtained in DPPB5. The shell zone possesses high orientation level, anisotropic shish-kebab superstructure, and low content of β-phase, which exhibits a “strengthening” feature. In contrast, the core zone has isotropic crystalline morphology and large amounts of β-crystal, resulting in a “toughening” character. In contrast to DPPB5, the orientation level, polymorphic composition, and crystalline morphology in DPPB30 were also investigated. The corresponding 2D-WAXD patterns for different depths are presented in Figure 5 and the calculated values of order parameter of R-crystal and Kβ are listed in Table 3. For the depth range from 0 to 800 µm, the 2D-WAXD patterns show typical orientation character, and only at the most deep depth, 1600 µm, the orientation character becomes weak relatively. Correspondingly, even at the depth of 800 µm, the order parameter of R-crystal is high, which is approximately 0.7; but it decreases substantially when the depth is 1600 µm. On the other hand, at the outermost zone, the value of Kβ is 0.13 for DPPB30, which is close to that for DPPB5. For both of DPPB5 and DPPB30, the outermost zone (skin) is formed immediately after the melt is injected into the mold; that is, the
thermal and shear condition are the same at this zone between these two specimens, so the orientation level and polymorphic composition at this zone are almost the same between these two specimens, whereas the relative content of β-phase is very low at the intermediate depth, only 0.08 detected at 800 µm. This result implies that the shear intensity of repeatedly shearing for 30 times is strong enough for restraining the formation of β-phase. While the core zone (1600 µm) is formed after cessation of oscillatory shear, the structural relaxation occurs substantially, which is favorable for the appearance of β-crystal with the aid of β-nucleator. Therefore, the Kβ is highest (0.32) at this depth. The SEM micrographs of crystalline morphologies at different depths of DPPB30 are presented in Figure 6. For the depth range from 0 to 800 µm, the crystalline morphology is highly oriented and the shish-kebab superstructure can be found. Even at the depth of 1600 µm, the shish-kebab superstructure still exists, although the ordering degree of lamellae is obviously lower than at other depths. The β-phase, however, is difficult to be distinguished in the SEM micrograph. Obviously, the bamboo-like shell-core structure can also be perceived in DPPB30 though the whole orientation level of the DPIM part is relatively high. It should be noted that the β-crystal will be also oriented under the effect of shear; however, since this crystal form is mainly located in the core zone and its ordering parameter is relatively low, it is ignored. 3.3. Structure-Property Relation of Bamboo-like Part. As demonstrated above, the bamboo-like shell-core structure has been obtained in the DPIM parts of iPP containing β-nucleator; and the characters of the hierarchical structure of
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Figure 6. SEM photographs at different layers of DPPB30 with a magnification of 40 000.
like bionic structure brings a simultaneously toughened and strengthened effect on the molding parts of iPP material, which is mainly due to the synergetic influence of the “strengthened” shell zone and the “toughened” core zone. 4. Conclusions
Figure 7. Tensile and impact strengths of various injection-molded bars.
such shell-core parts were elucidated in detail through a combination of PLM, 2D-WAXD, and SEM. Finally, we would like to explore the potential of such bamboo-like bionic structure on the improvement of mechanical performance. The area ratio of shell/core is calculated, based on the PLM images of Figure 1, which is used to quantitatively estimate the structural variation with increase of shear intensity. Both impact toughness and tensile strength are plotted as a function of area ratio of shell/ core, as shown in Figure 7. Most interestingly, both impact toughness and tensile strength are significantly improved as the area ratio of shell/core increases from 0 to 80%. One can see that the tensile strength is nearly proportional to the ratio of shell/core; for impact strength, however, there is a little discordance. Obviously, it can be concluded that the bamboo-
A simple approach has been developed to facilely achieve the macroscopic bamboo-like bionic structure by imposing postshearing on the iPP melt with β-nucleator during the packing stage of injection molding. The bamboo-like structure consists of strengthened shell zone (high level of orientation and low β-phase amount) and toughened core zone (isotropic texture and dominant β-modification). Because both toughness and strength improved prominently with increase of shear intensity upon the existence of such bamboo-like shell-core structure, there is a huge potential for this novel strategy in macroscopic structural design during the practical molding process. Acknowledgment. We express our sincere thanks to the National Natural Science Foundation of China for Financial Support (50533050, 50903048, 50873063). Thanks also go to Prof. Liangbin Li and Guoqiang Pan of National Synchrotron Radiation Laboratory (NSRL) in the University of Science and Technology of China for their help in synchrotron WAXD experiments. References and Notes (1) Garbarczyk, J.; Paukszta, D. Polymer 1981, 22, 562. (2) Varga, J.; Menyha´rd, A. Macromolecules 2007, 40, 2422. (3) Luo, F.; Geng, C. Z.; Wang, K.; Deng, H.; Chen, F.; Fu, Q.; Na, B. Macromolecules 2009, 42, 9325. (4) Bai, H. W.; Wang, Y.; Zhang, Z. J.; Han, L.; Li, Y. L.; Liu, L.; Zhou, Z. W.; Men, Y. F. Macromolecules 2009, 42, 6647.
Macroscopic Bamboo-like Bionic Structure (5) Chen, Y. H.; Zhong, G. J.; Wang, Y.; Li, Z. M.; Li, L. B. Macromolecules 2009, 42, 4343. (6) Kotter, I.; Grellmann, W.; Koch, T.; Seidler, S. J. Appl. Polym. Sci. 2006, 100, 3364. (7) Stricker, F.; Thomann, Y.; Muelhaupt, R. J. Appl. Polym. Sci. 1998, 68, 1891. (8) Yokoyama, Y.; Ricco, T. Polymer 1998, 39, 3675. (9) Thio, Y. S.; Argon, A. S.; Cohen, R. E.; Weinberg, M. Polymer 2002, 43, 3661. (10) Huo, H.; Jiang, S. C.; An, L. J. Macromolecules 2004, 37, 2478. (11) Varga, J. J. Therm. Anal. 1989, 35, 1891.
J. Phys. Chem. B, Vol. 114, No. 31, 2010 10001 (12) Wang, K.; Liang, S.; Du, R. N.; Zhang, Q.; Fu, Q. Polymer 2004, 45, 7953. (13) Turner-Jones, A.; Aizelwood, J. M.; Beckett, D. R. Macromol. Chem. 1964, 75, 134. (14) Olley, R. H.; Bassett, D. C. Polymer 1982, 3, 1707. (15) Wang, Y.; Zou, H.; Fu, Q.; Zhang, G.; Shen, K. Z. J. Appl. Polym. Sci. 2002, 85, 236. (16) Na, B.; Zhang, Q.; Fu, Q.; Zhang, G.; Shen, K. Z. Polymer 2002, 43, 7367.
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