Article pubs.acs.org/IECR
Tailored Crystalline Structure and Mechanical Properties of Isotactic Polypropylene/High Molecular Weight Polyethylene Blend Man Zhou, Dashan Mi, Fengyi Hou, and Jie Zhang* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: In this work, isotactic polypropylene and high density polyethylene blends with tailored crystalline structures were prepared through an accessible injection-molding method. Two hierarchic structures, i.e., shish-kebab structures and epitaxy structures, were both successfully obtained among the whole range of samples, which were carefully characterized through polarized light microscopy, scanning electron microscopy, differential scanning calorimetry and small-angle X-ray scattering. It was found that the special epitaxy crystalline structure showed better mechanical properties than the shishkebab one. The inclined polyethylene lamellae among the oriented polypropylene matrix not only enhances the interfacial adhesion but also facilitates the transmission of external force within matrix. Consequently, the tensile strength of the sample with epitaxy structures is around 46.58 MPa and its impact strength reaches up to 63.33 kJ/m2. These results provide a new method to industrially manufacture samples with tailored crystalline structures, making it possible for preparing general polymer materials with advanced properties.
■
INTRODUCTION Isotactic polypropylene (iPP) and high density polyethylene (HDPE) are two well-known important commercial polymer resins due to their relatively lower manufactural cost and practical properties. The iPP/HDPE blend, as a most common polymer blend system, has also been widely researched on the most primary polymer topics,1−4 i.e., the relationships between the processing conditions and the microstructure of sample as well as the mechanical properties. For semicrystalline polymers (e.g., iPP, HDPE and so on), processing methods generally play a crucial role in determining the final microstructure of the fabricated product.5−8 As a selfreinforced crystalline structure, shish-kebab structures were commonly used in improving the mechanical properties of polymers. Recently, our research group9 has made great accomplishments controlling the content of shish-kebab structures in iPP samples. Through the self-developed polymer processing method, i.e., multiflow vibrate injection molding (MFVIM), iPP samples with the whole range of shish-kebab structure can be successfully made, following by overall improvements in mechanical properties. However, for iPP/ HDPE blend, there is an alternative crystalline structure with much potential to possess more excellent properties, i.e., the epitaxy crystalline structure. This special surface-induced crystalline phenomenon results from the geometric lattice matching for (100) lattice plane of PE and (010) lattice plane of iPP, which has been demonstrated.10−12 Early studies10,13,14 postulated that this special structure was supposed to have great mechanical properties owing to the enhancement of interfacial adhesion, but it was not easy to largely obtain this structure among the common sample. How far the improvements can © 2017 American Chemical Society
really reach or whether the special epitaxy structure has better performance than the shish-kebab structure is still inconclusive. In recent years, works15−21 have tried to fabricate epitaxy crystalline structures in samples by accessible polymer processing methods. Fu’s research group15−19 first obtained this structure with the aid of dynamic packing injection molding, demonstrating the extensive shear field benefits the formation of epitaxy crystallization. However, the content of the epitaxy crystalline structure was very limited because it only existed in the shear layer, which only occupied less than the half of the whole sample. Niu20 successfully obtained a large scale of epitaxy structures in fabricating iPP/PE microfibrillar blends by the process of “extrusion−hot stretching−quenching”. It was demonstrated that the utilization of intensive shear field can induce epitaxially crystallization among the whole tape-like sample, leading to the improvement of interfacial adhesion as well as the tensile strength. Whereas the impact strength is not able to be tested due to the restrain of sample shape. How to effectively fabricate large scale of this special surface-induced crystalline structure by an accessible processing method is still a challenging question. There is also an valuable study by Deng,21 in which the mold temperature was proved to be of great significance to the formation of epitaxy crystallization. Reasons can be inferred that the higher mold temperature affect the relaxation of molecular chains, making the preoriented PE chains can Received: Revised: Accepted: Published: 8385
April 25, 2017 July 2, 2017 July 4, 2017 July 13, 2017 DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
Figure 1. Schematic drawing of sampling methods for each analyzes. MD, flow direction; TD, transverse direction; ND, normal direction.
Chemical Co, with a melt flow rate (MFR) of 1.18 g/10 min (190 °C, 2.16 kg) and density of 0.960 g/cm3. Sample Preparation. HDPE and iPP, with a weight ratio of 20/80, was melted together by a SHJ-25 corotating twinscrew extruder. The screw speed was 120 rpm and the temperatures from hopper to die were 150, 170, 180, 200, 200, 200, 200, 200, 190 °C. The pelletized and dried droplets were then molded by home-developed Multiflow vibrate-injection molding (MFVIM) method. Samples with almost full oriented structures are expected to be fabricated by adjusting the MFVIM parameters. The injection temperature was 180, 200, 205 and 205 °C from hopper to nozzle and the injection pressure was 30 MPa, the vibration and packing pressure were 50 MPa. Additionally, the mold temperature was set as 40 °C (sample A) and 135 °C (sample B) separately during the injection process to get two kinds of samples. When the injection process finished, the mold temperature was set as 40 °C to cool those samples immediately. Polarized Light Microscopy. To examine the crystalline structures of the samples obtained by different experimental methods, thin slices cut by microtome were used for optical morphology observations. There were three representative observation zones that were chosen from the whole range of samples along the flow direction. PLM observations were performed by using a DX-1 (Jiang Xi Phoenix Optical Co., China.) microscope connected to a Nikon 500D digital camera. Scanning Electron Microscopy. To carefully observe the crystalline morphology of the blends, the samples were chemically etched in a permanganate etching solution and then observed by scanning electronic microscopy. A solution of 8:4:1 volume of concentrated sulfuric acid, phosphoric acid and distilled water was prepared (1.5 g of potassium permanganate in 100 mL of mixture).22 The samples were cut parallel to the molding direction and then immersed in the solution at 60 °C
recrystallize onto to PP oriented crystalline and thus giving rise to this special surface-induced crystallization. Consequently, it is possible to transform the oriented crystalline structure into the desired epitaxial crystalline structure by adjusting the processing temperature. Based on all these pioneering studies, the aim of the present work is trying to fabricate iPP/HDPE samples with large scale of epitaxy crystalline structures. First, the MFVIM technology was proposed to persistently provide an extensive shear field on the melt and thus to fabricate samples with fully oriented structures. Then, by adjusting the mold temperature, the preoriented PE lamellae is expected to be relaxed while the iPP phase can remain oriented, thus serving as a template to induce the following epitaxial crystallization of the relaxed PE chains. Finally accomplishing the transformation from shish-kebab structure to epitaxy structure of PE phase. Polarized light microscopy (PLM) and scanning electron microscopy (SEM) were used to give direct information about the orientation morphology of samples, whereas small-angle X-ray scattering (SAXS) provided strong evidence on the existence of these special lamellae structures. Results indicated that the epitaxy structure exhibit excellent mechanical properties compared to the shish-kebab structure, especially for the impact strength. To reveal the reinforcing mechanism, the fracture surface was also carefully investigated under SEM. A schematic of morphology as well as the fracture mechanism were proposed accordingly.
■
EXPERIMENTS Materials. Isotactic polypropylene (iPP, trade name T30S) was purchased from Dushanzi Petroleum Chemical Co, with a melt flow rate (MFR) of 2.90 g/10 min (230 °C, 2.16 kg) and density of 0.910 g/cm3. High density polyethylene (HDPE, trade name 5000S) was purchased from Lanzhou Petroleum 8386
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
different positions were chosen to reflect the morphologies along the whole sample. It can be clearly seen that sample A shows oriented structures at these three representative positions, which is quite different from the typical “skin−core structures” in conventional injection molding. The colorful area, which refers to highly oriented morphology, occupied almost the whole range of samples along the thickness direction, whereas the core layer formed by isotropy structures only had very limited width. The sample with a unique fulloriented structure within the whole range has been successfully made by the MFVIM method. For sample B, one can also find that it has been occupied with highly oriented structures at different positions. In other words, although for sample B the mold temperature is much higher, similar results can also be obtained by adjusting the MFVIM parameters. However, more detailed differences in crystalline structure between these two samples need to be revealed by other characterizations. Figure 3 shows the SEM micrographs of middle position of samples. It displays the crystalline morphologies of samples in
for 7 h to dissolve away the amorphous phase of the samples. After that, the etched surface was covered with a thin layer of gold by sputtering and observed by a field emission SEM (Inspect-F, Fei, Finland) with an acceleration voltage of 15 kV. Two-Dimensional Small-angle X-ray Scattering (2DSAXS). Two-dimensional small-angle X-ray scattering (2DSAXS) measurements were carried out in the Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the X-ray was 0.124 nm and the rectangular beam had dimensions of 0.5 × 2 mm. A MAR CCD X-ray detector (MARUSA) was employed for detection of 2D-SAXS images, having a resolution of 3072 × 3072 pixels. The distance between sample and detector was 1910 mm. The sample was mounted on a three-dimensional elevator platform with its thickness direction is parallel to the Y-axis. From surface to the center of sample, four different positions: 300, 700, 1100 and 1500 μm down from surface, were scanned, respectively (shown in Figure 1). Differential Scanning Calorimetry (DSC). A TA-Q200 differential scanning calorimetry instrument was used to analyze the thermal behavior of crystallization and the crystallinity of each component. All measurements were conducted under the nitrogen atmosphere. As described in Figure 1, specimens (around 5 mg) were placed in a sealed aluminum pan and were heated to 200 °C at the heating rate of 10 °C/min. The crystallinity Xc of conmponent i in the blends was calculated by the following equation: Xc =
ΔHi ΔHimφi
where ΔHi is the enthalpy of fusion of component i, directly obtained from DSC, and φi is the mass fraction of component i in the blends. The enthalpy of fusion ΔHm i of 100% crystalline polymer is 293 and 207 J/g for HDPE and iPP, respectively.19 Measurement of Mechanical Properties. Tensile properties of dumbbell-shaped specimens were measured using a RGT-10 computer controlled electronic universal testing machine at a crosshead speed of 50 mm/min according to ASTM D-638. The Izod notched impact strength was measured with a UJ-40 Izod machine according to ASTM D256-04. Those measurements were both carried out at room temperature. For each composition, five samples were used and the values of all mechanical parameters were calculated as averages.
Figure 3. SEM micrographs of different layers of samples. The data on left represent the distance of the position from its edge. The flow direction is horizontal.
■
RESULTS AND DISCUSSION Crystalline Structure and Morphology. As shown in Figure 2, samples were carefully observed by PLM first, three
different layers, i.e., skin layer (200 μm), intermediate layer (800 μm) and core layer (1500 μm). As is known, PE phase is incompatible with the PP phase and they form as an immiscible two-phases system. Considering their different resistance ability to etching solution as well as their different content in the system (component ratio of PP to PE is 80:20), it is easy to distinguish them in the photographs. The dark area refers to PP phase, whereas the relatively white area represents dispersed PE phase. For sample A, a typical oriented crystalline phase morphology can be observed in three different layers. PE phase is separated phase and it is well dispersed in PP phase. And it can be seen clearly that the kebab lamellae of PE and PP are all arranged perpendicular to the flow direction. In other words, both PE and PP have formed their own shish-kebab structures. With the distance varying from the surface to the center of sample, this highly oriented crystalline morphology remains almost unchanged. The size of kebab lamellae seems to be
Figure 2. PLM pictures of different samples in three representative positions. 8387
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research increasing from the surface to core region of the sample, especially for that of PE in core region (Figure 3a3). This point will be discussed in detail in the following part. As is known, for conventional injection molding the conditions in core layer goes against the formation of oriented structure. But with the help of MFVIM, in which the polymer melt was forced to flow for several times depending on relevant parameters, the oriented structures can be largely obtained even in the center of the sample. Therefore, the sample was fabricated with a full oriented structure with the range of samples. Things changed dramatically when adjusting the mold temperature higher than the melting point of PE but lower than that of PP. For sample B, PP phase also exhibits highly oriented structures along the whole region with its lamellae aligned vertical to the flow direction. However, the dispersed PE phase mainly forms inclined lamellae, which is apart an angle to the flow direction. The peculiar lamellae arrangement also can be seen at the whole range of samples. This special phenomenon should be the typical epitaxy crystalline morphology, which has been fully illustrated in previous research.10−12,14 The epitaxy crystallization theory is established on the geometric lattice matching10,13,14 for the (100) lattice plane of PE and the (010) lattice plane of iPP because PE chains can exactly fit into the valleys formed by the methyl groups.11,12 As a result, the PE lamellae arranges apart from the oriented PP lamellae (kebab lamellae). It is also worth noting that the epitaxy crystallization morphology is quite similar to the TEM pictures in previous research.10,11,14 To our best knowledge, it is the first time to obtain an epitaxy crystalline structure for almost the whole range of samples by a conventional processing method. Although a few shish-kebab structures could also be seen in these three layers of sample B, their content decreases gradually whereas the epitaxy crystallization becomes more regular from the surface to the center of the sample. Moreover, the length of epitaxy crystalline seems to increase a little in the core layer. The formation mechanism of epitaxy morphology in the present work is a little different from the early work and can be explained as following. The multiflow behavior provided a strong shear field during the injection process and therefore induced large scale of oriented structures, even in the the core layer. When the mold temperature is higher, there is much possibility for the preoriented PE lamellae to be relaxed, but it has little influence on the oriented PP lamellae. Owing to the geometric lattice matching between them, the relaxed PE phase would recrystallize onto the oriented PP lamellae and then this peculiar crystalline morphology appears. In addition, with the distance varying from the surface to the center of sample, the actual mold temperature becomes higher and higher. The transformation from preoriented structure to epitaxy structure is much easier to achieve. So, the nearer to the center, the less shish-kebab structure is and the more regular epitaxy structure can be found. Lamellae Orientation and Arrangement. The lamellar orientation of sample was further characterized by 2D-SAXS, as shown in Figure 4. To reveal the detailed orientation relationship between PE lamellae and PP lamellae, the corresponding azimuthal scan of the 2D-SAXS patterns is also shown in Figure 5. From surface to center of the sample, four positions were detected. Normally, the scattering streaks at equator direction represent shish, whereas the scattering signals appearing at the meridian direction refer to the kebab lamellae. From the first line pictures in Figure 4, it can be observed that
Figure 4. 2D-SAXS patterns of samples. The data represent the distance of the position from its edge. The flow direction is vertical.
sample A exhibits typical shish-kebab structures among the whole range of its thickness direction. Correspondingly, there are four pecks in azimuthal scan curves (Figure 5a). Azimuthal angles equal 0° and 180° refer to the signal of the shish, whereas azimuthal angles −90° and +90° refer to the signal of the kebab. However, for sample B (the second line pictures in Figure 4), except for the strong scattering signals around the meridian, which mainly belong to the iPP oriented structures, there are other four scattering signals that are apart about ±30° from the equator direction separately. Compared with the similar studies15,20 on the crystalline structures of iPP/PE blends, as well as the clear lamellae structures in this work from SEM results (Figure 3), these scattering spots are indicative of the special epitaxially growth of PE lamellae on the oriented iPP lamellae, i.e., the signals of epitaxy structures. It should be pointed out that since PE content is only 20 wt %, thus the signals of epitaxial PE lamellae is relatively weak as compared with the strong signals from PP shish-kebab lamellae. Indeed, more distinct evidence on the appearance of epitaxy crystalline structures can be seen in Figure 5b. Compared to Figure 5a, small but clear, there are four extra peaks at −30°, +30°, 150° and 210° in the four curves. Undoubtedly, those results are quite in accord with the SEM morphologies and further confirm the unique structures of two samples. Sample A owns a full-oriented structure with both PE and PP formed typical shish-kebab crystalline, but sample B presents full-epitaxial structures with PE lamellae epitaxial growing onto oriented PP lamellae. In addition, much early research10−14 on epitaxy crystallization has been reported that the typical value of epitaxial angle between peculiar arranged PE lamellae and oriented PP lamellae is 50°, which results from the geometric lattice match for the (100) lattice plane of PE and the (010) lattice plane of iPP. However, in this case, the epitaxy angles are characterized as 60° according to azimuthal scan curves (Figure 5b), which seems to be a little inconsistent with the classical value of 50°. This interesting phenomenon has been discussed in detail in our previous work.23 Here, it is summarized as following. For early research, epitaxy crystallization is usually obtained under an ideal and static condition where the epitaxial growth of PE hardly follows the rule of geometric lattice match. But in current work, epitaxy crystallization is finished under extensive shear field, which would definitely affect the arrangement of epitaxial lamellae. In conclusion, the biggest differences between these two samples is the arrangement of PE lamellae. One is parallel to the kebab lamellae of PP and the other one is inclined alignment within the oriented PP matrix. These scattering patterns (Figure 4) were also qualitatively analyzed to further investigate the variation of long period of 8388
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
Figure 5. Azimuthal scan of the 2D-SAXS patterns of (a) sample A and (b) sample B from skin to core.
Figure 6. Variation of long period which was calculated by the equation L = 2π/q, where q is the peak position in the scattering curves (not shown here). (a) sample A, (b) sample B; green line refers to PP phase, whereas the blue line refers to PE phase.
Figure 7. DSC melting curves of samples. The melting points and crystallinities are shown in the picture.
crystalline lamellae, as shown in Figure 6. The long period (L) is calculated according to the Bragg equation, L = 2π/q, where q refers to the peak position in the corresponding 1D-SAXS curves (not shown here). Figure 6 illustrates the variation of long period from the surface to the center of sample. For
sample A, because PP and PE both formed shish-kebab structures, their signals were overlapped together. It is worth noting that the long period in Figure 6a should be explained as the average value of them, instead of simply judging that they share the same kebab long period. With the distance varying 8389
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
Figure 8. Tensile strength (a) and impact strength (b) of the samples.
high mold temperature is only set for the injection filling process. So, the higher mold temperature is less likely to have an obvious influence on the crystallization of PP. The higher mold temperature does not substantially change PP shish-kebab lamellae from thermodynamics. The differences of melting point and crystallinity of shish-kebab structures in these two samples can also be ignored. Mechanical Properties. Because these two samples possess two different crystalline structures and more importantly, those typical structures occupy almost the whole sample, it is imperative to verify which one exhibits better mechanical properties. Figure 8 shows the detailed comparison between them. Obviously, sample B, the one with epitaxy structures, exhibits excellent properties although the properties of the one with full-oriented structures is also not bad. The tensile strength of sample A is 40.34 MPa. Compared to normal pure PP parts, of which the tensile strength is about 35 MPa according to other works,24,25 it does not decrease as adding PE phase because of the existence of full-oriented structures. When the PE phase formed a special epitaxy structure, the tensile strength can further increase to 46.58 MPa. The improvement of impact strength is dramatic: sample A is about 39.17 kJ/m2, which is much higher than the normal pure PP parts as well as the pure PP parts with full-oriented structure (data can be found in reference 9). This good impact strength comes from the addition of a little PE phase and the full-oriented structures. More importantly, the impact strength of sample B, the one with epitaxy structure, further rises 61.7% to 63.33 kJ/m2. This huge improvement should be attributed to the special crystalline structures. First, the epitaxy crystalline structure exhibits similar crystallinity and a longer long period than the shish-kebab structure, suggesting the epitaxial lamellae is thicker. More importantly, because the biggest difference between these two samples lies on the arrangement of PE lamellae, it can be concluded that the inclined arrangement is much better than the vertical arrangement for PE lamellae and the former one is more favorable to the transmission of external force. Fracture Mechanism. To figure out why the impact strength can be greatly improved by the epitaxy structure, the fracture surface caused by notch impact test was carefully examined under SEM, as shown in Figure 9. From left to right,
from the surface to the center of sample, L of PP and PE in sample A gradually increased from 15.71 to 22.44 nm. A similar tendency can be found in sample B: L of PE increased from 23.27 to 27.32 nm, whereas L of PP increased from 22.05 to 25.65 nm. Possible reasons for the increasing long period from the skin to core could be ascribed to relatively higher temperature, and more work needs to be done to fully explain this seemingly abnormal phenomenon. In the current work, the differences between samples are the focus. It is presented that the epitaxial crystalline lamellae show a larger long period than the kebab lamellae for PE phase. On the one hand, higher mold temperature provides a relatively better environment for the growth of lamellae because the preoriented molecular chains can get relaxed and easy to arrange again. On the other hand, the epitaxy crystallization is a surface induced crystallization and it takes place at the interface between two phases. The oriented iPP lamellae serves as a good substrate for the growth of epitaxial PE lamellae based on the lattice match. So, it is reasonable that the epitaxy lamellae has a thicker arrangement than the shish-kebab one. Considering that they also share similar crystallinity, it is not surprising to observe that the epitaxy one has a longer long period. Thermal Behavior. The DSC melting results of samples are presented in Figure 7. There are two melting peaks in the curves. The lower one refers to PE phase and the higher one is PP phase. Comparing these samples, it is clear the melting point of PE keeps almost unchanged whereas its crystallinity increases slightly when it transforms from shish-kebab structure to epitaxy structure. A possible reason can be inferred that higher mold temperature is advantageous for the growth of crystalline. So, the epitaxy one shows a little larger crystallinity than that of shish-kebab one. It is worth noting that this difference between the crystallinity is too small to consider. For PP phase, in spite of the slight difference on long period of the two samples, there seems to be no obvious difference in melting point or crystallinity between them, indicating the shish-kebab structures in these two sample have no significant difference although their processing conditions were a bit different. This phenomenon proves that the high mold temperature has little influence on the crystallization of PP. There are two reasons. First, the high mold temperature (135 °C) is still lower than the melting point of PP (165 °C). The other one is that the 8390
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
Figure 9. Fracture surface of samples observed by SEM. Notch location is in the left and impact direction is horizontal.
these epitaxy crystalline lamellae are arranged at a certain titled angle, making it better able to withstand the external force from the perpendicular direction. Second, these epitaxy crystalline lamellae play the bridge role between the adjacent PP oriented lamellae and make a better connection within them, which further enhance the interfacial adhesion among the PP matrix. Owing to the existence of those inclined lamellae, the transmission and dissipation of external force will be more various. Theoretical supports can be easily obtained according to the fracture surface (Figure 9). The more various ways of the transmission of external force, the more energy that can be dissipated in matrix and the higher impact strength the sample can reach. Therefore, the impact strength is further improved when the oriented PE lamellae was tailored to be inclinedly arranged, i.e., epitaxially growing onto the oriented PP lamellae.
the two picture rows are the full view of fracture surface of samples. Sample A shows an unsmooth surface, especially in the crack propagation area (Figure 9a2). It is obvious to find the plastic deformation in the range of the whole cross section, which in turn proves the regularity of the structure. Sample B shows an uneven fracture surface. It is worth noting that the sample seems to be layered due to the multiflow injection behavior. That is the reason why the fracture surface is very rough even from a macroperspective. Owing to the existence of the inclined PE epitaxial-growth lamellae, which was dispersed uniformly in the PP matrix and served as an active bridge connecting the neighboring PP lamellae, the energy of external force can be easily dissipated into broader area of matrix. Therefore, the plastic deformation of sample B is much dramatical. Some ladder-shaped deformation can be found, which fully suggests the transmission and dispassion of the external force. Figure 10 illustrates the schematic illustration of the fracture mechanism. For sample A, both PP and PE formed their own
■
CONCLUSION In this work, iPP/HDPE samples with tailored crystalline structures and tailored mechanical properties were successfully prepared by using suitable processing conditions of the MFVIM technique. The major achievement of this work was to fabricate the sample with the whole range of epitaxy crystalline structures, making it possible for the commonly used iPP/ HDPE samples to be the advanced polymeric materials. Comparing the shish-kebab structure and epitaxy structure, the latter one shows great improvements of mechanical properties: its tensile strength is around 46.58 MPa and the impact strength reaches up to 63.33 kJ/m 2 . Results demonstrate that the epitaxial structure has similar crystallinity but a longer long period than the shish-kebab structure, suggesting a tighter lamellae arrangement. More importantly, the main reason for the huge improvement of mechanical properties should be attributed to the special epitaxy crystalline structure itself, which can further enhance the interfacial adhesion between HDPE and iPP compared to shish-kebab structure. It is the inclined epitaxy lamellae that facilitates the transmission and dissipation of external force within the matrix, showing a significant improvement in both strength and toughness. The present work establishes a good example to industrially achieve large scale of epitaxial structure in iPP/ HDPE samples with excellent properties.
Figure 10. Schematic illustration of the fracture mechanism.
shish-kebab structures. These vertically arranged kebab lamellaes have certain strength and can suffer considerable impact force. The applied external impact force can be transmitted straightly among the matrix due to the perpendicular arrangement of lamellae. Things become more complicated for sample B. There are many epitaxial-growing PE lamellae well dispersed among the oriented PP matrix. First, 8391
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
Article
Industrial & Engineering Chemistry Research
■
(15) Na, B.; Zhang, Q.; Wang, K.; Li, L. B.; Fu, Q. Origin of various lamellar orientations in high-density polyethylene/isotactic polypropylene blends achieved via dynamic packing injection molding: bulk crystallization vs. epitaxy. Polymer 2005, 46, 819−825. (16) Na, B.; Wang, K.; Zhao, P.; Zhang, Q.; Du, R. N.; Fu, Q.; Yu, Z. Q.; Chen, E. Q. Epitaxy growth and directed crystallization of highdensity polyethylene in the oriented blends with isotactic polypropylene. Polymer 2005, 46, 5258−5267. (17) Su, R.; Wang, K.; Zhao, P.; Zhang, Q.; Du, R.; Fu, Q.; Li, L.; Li, L. Shear-induced epitaxial crystallization in injection-molded bars of high-density polyethylene/isotactic polypropylene blends. Polymer 2007, 48, 4529−4536. (18) Su, R.; Wang, K.; Zhang, Q.; Chen, F.; Fu, Q.; Xu, W.; Na, B. Orientation and Epitaxy in the Injection-Molded Bars of Linear LowDensity Polyethylene/Isotactic Polypropylene Blends: An Infrared Dichroism Measurement. J. Phys. Chem. B 2009, 113, 7423−7429. (19) Su, R.; Wang, K.; Zhang, Q.; Chen, F.; Fu, Q. A.; Hu, N.; Chen, E. Q. Epitaxial crystallization and oriented structure of linear lowdensity polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding. Polym. Adv. Technol. 2011, 22, 225−231. (20) Niu, B.; Chen, J.-B.; Chen, J.; Ji, X.; Zhong, G.-J.; Li, Z.-M. Crystallization of linear low density polyethylene on an in situ oriented isotactic polypropylene substrate manipulated by an extensional flow field. CrystEngComm 2016, 18, 77−91. (21) Deng, P.; Whiteside, B.; Wang, F.; Norris, K.; Zhang, J. Epitaxial growth and morphological characteristics of isotactic polypropylene/ polyethylene blends: Scale effect and mold temperature. Polym. Test. 2014, 34, 192−201. (22) Olley, R. H.; Bassett, D. C. An improved permanganic etchant for polyolefines. Polymer 1982, 23, 1707−1710. (23) Zhou, M.; Mi, D.; Hou, F.; Zhang, J. Insight into understanding the evolution of the epitaxy crystallization in isotactic polypropylene and polyethylene blends. Polym. Adv. Technol. 2017. In press, DOI: 10.1002/pat.4052. (24) Zhou, M.; Li, X.-p.; Jin, M.; Xia, C.; Shen, K.-z.; Zhang, J. Simultaneously improving the tensile and impact properties of isotactic polypropylene with the cooperation of co. Chin. J. Polym. Sci. 2016, 34, 1001−1013. (25) Zhang, Z.; Zhang, R.; Huang, Y.; Lei, J.; Chen, Y.-H.; Tang, J.-h.; Li, Z.-M. Efficient Utilization of Atactic Polypropylene in Its Isotactic Polypropylene Blends via “Structuring” Processing. Ind. Eng. Chem. Res. 2014, 53, 10144−10154.
AUTHOR INFORMATION
Corresponding Author
*Prof. Jie Zhang. Tel: (+86) 130-8668-1699; E-mail: zhangjie@ scu.edu.cn. ORCID
Jie Zhang: 0000-0003-3800-697X Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful for the kind help and support of Shanghai Synchrotron Radiation Facility (SSRF) in X-ray measurement and the analysis of its results.
■
REFERENCES
(1) Lin, J.-H.; Pan, Y.-J.; Liu, C.-F.; Huang, C.-L.; Hsieh, C.-T.; Chen, C.-K.; Lin, Z.-I.; Lou, C.-W. Preparation and Compatibility Evaluation of Polypropylene/High Density Polyethylene Polyblends. Materials 2015, 8, 8850−8859. (2) Mofokeng, T. G.; Ojijo, V.; Ray, S. S. The Influence of Blend Ratio on the Morphology, Mechanical, Thermal, and Rheological Properties of PP/LDPE Blends. Macromol. Mater. Eng. 2016, 301, 1191−1201. (3) Yu, F.; Deng, H.; Bai, H.; Zhang, Q.; Wang, K.; Chen, F.; Fu, Q. Confine Clay in an Alternating Multi layered Structure through Injection Molding: A Simple and Efficient Route to Improve Barrier Performance of Polymeric Materials. ACS Appl. Mater. Interfaces 2015, 7, 10178−10189. (4) Zhang, S.; Deng, H.; Zhang, Q.; Fu, Q. Formation of Conductive Networks with Both Segregated and Double-Percolated Characteristic in Conductive Polymer Composites with Balanced Properties. ACS Appl. Mater. Interfaces 2014, 6, 6835−6844. (5) Yang, H.; Wang, Y.; Na, B.; Cao, W.; Li, J.; Zhang, Q.; Du, R. N.; Fu, Q. Spatial hierarchy and interfacial structure in injection-molded bars of polyolefin blends. Gaofenzi Xuebao 2007, 007, 209−218. (6) Wang, K.; Chen, F.; Li, Z. M.; Fu, Q. Control of the hierarchical structure of polymer articles via ″structuring″ processing. Prog. Polym. Sci. 2014, 39, 891−920. (7) Huang, L.; Wang, Z.; Zheng, G.; Guo, J. Z.; Dai, K.; Liu, C. Enhancing oriented crystals in injection-molded HDPE through introduction of pre-shear. Mater. Eng. 2015, 78, 12−18. (8) Wang, Z.; Zheng, G.; Wang, B.; Dai, K.; Guo, J. Z.; Liu, C.; Shen, C. Suppressing the skin−core structure in injection-molded HDPE parts via the combination of pre-shear and UHMWPE. RSC Adv. 2015, 5, 84483−84491. (9) Mi, D.; Xia, C.; Jin, M.; Wang, F.; Shen, K.; Zhang, J. Quantification of the Effect of Shish-Kebab Structure on the Mechanical Properties of Polypropylene Samples by Controlling Shear Layer Thickness. Macromolecules 2016, 49, 4571−4578. (10) Gross, B.; Peterman, J. SYNERGISMS OF MECHANICALPROPERTIES IN BLENDS OF SEMI-CRYSTALLINE POLYMERS. J. Mater. Sci. 1984, 19, 105−112. (11) Lotz, B.; Wittmann, J. C. POLYETHYLENE ISOTACTIC POLYPROPYLENE EPITAXY - ANALYSIS OF THE DIFFRACTION PATTERNS OF ORIENTED BIPHASIC BLENDS. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 1079−1087. (12) Petermann, J.; Xu, Y. THE ORIGIN OF HETEROEPITAXY IN THE SYSTEM OF UNIAXIALLY ORIENTED ISOTACTIC POLYPROPYLENE AND POLYETHYLENE. J. Mater. Sci. 1991, 26, 1211−1215. (13) Lovinger, A. J.; Williams, M. Tensile properties and morphology of blends of polyethylene and polypropylene. J. Appl. Polym. Sci. 1980, 25, 1703−1713. (14) Gohil, R. M. SYNERGISM IN MECHANICAL-PROPERTIES VIA EPITAXIAL-GROWTH IN POLYPROPYLENE POLYETHYLENE BLENDS. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 1713−1722. 8392
DOI: 10.1021/acs.iecr.7b01733 Ind. Eng. Chem. Res. 2017, 56, 8385−8392
