Formation of Interlinked Shish-Kebabs in Injection-Molded

Aug 10, 2012 - Polyethylene under the Coexistence of Lightly Cross-Linked Chain. Network ... linked polyethylene (LCPE), which can be considered as st...
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Formation of Interlinked Shish-Kebabs in Injection-Molded Polyethylene under the Coexistence of Lightly Cross-Linked Chain Network and Oscillation Shear Flow Hao-Ran Yang,† Jun Lei,*,† Liangbin Li,‡ Qiang Fu,† and Zhong-Ming Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: It has been well established that the entangled molecular network facilitates the formation of shish-kebabs under flow field, however, the entangled network, usually formed by long chains, tends to disentangle due to molecular relaxation. In the present work, a small amount of lightly crosslinked polyethylene (LCPE), which can be considered as stable molecular chain networks, was added to short-chain polyethylene and then injectionmolded using a modified injection molding technology-oscillation shear injection molding (OSIM), which can exert a successive shear field on the melt in the mold cavity during packing stage. The hierarchic structure of the OSIM samples was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). It was found that the oscillation flow field promoted the formation of interlinked shish-kebabs in the intermediate layer of OSIM samples, while there are still typical spherulites in the core layer of OSIM neat polyethylene (PE). The interlinked shish-kebab structure led to remarkable mechanical enhancement from 27.6 and 810.2 MPa of conventional injection molding (CIM) samples to 42.7 and 1091.9 MPa of OSIM samples for tensile strength and modulus, respectively. More importantly, under the same flow condition, the samples containing LCPE networks (termed PEX) exhibit rich shish-kebab structure both in the intermediate and core layers. Moreover, the addition of LCPE also generated stronger interlinked shish-kebabs, in which kebabs and shishes are connected by covalent bonds, rather than topological entanglement points. This special structure leads to further reinforcement from 29.6 and 879.5 MPa of CIM PEX samples to 57.5 and 1311.7 MPa of OSIM PEX samples for tensile strength and modulus, respectively. The results demonstrated that the networks with stable entanglement points are more helpful to induce the formation of shishes under flow than those with topological entanglement points. Our results set up a new method to reinforce polymer parts by tailoring the structure and morphology.

1. INTRODUCTION

kebab structure in them because for almost all processing methods of polymers, molecular chains are subjected to strong elongational or shear flow during processing.13 However, the industrial products prepared by conventional processing methods cannot get reinforced by formation of shish-kebabs as expected. Take injection molding as an example: polymer chains are subjected to intensive shear flow during mold filling stage, and it could lead to high orientation of the molecular chains. But there are only a small amount of oriented crystals rather than shish-kebab structures in the final injection molding parts. And the region containing oriented crystals is limited to a very thin layer close to the surface of product due to the sufficient relaxation of chains caused by slow cooling of melt in

The control of crystallization behavior in semicrystalline polymers is of crucial importance because the crystallinity and crystalline morphology of semicrystalline materials strongly affect their physical properties, from mechanical to optical.1−3 Typically, polymer products consist of two distinct crystalline morphologies: spherulites and shish-kebab crystals.4,5 The former is formed in quiescent crystallization condition while the latter crystallizes from the highly oriented and stretched polymer chains under shear or elongational flow. During the past few decades, researches on the flow induced shish-kebab have attracted much attention because the transition from a relatively isotropic, spherulitic morphology to a highly oriented, shish-kebab morphology can bring out notable reinforcement on polymer products.6−14 Since shish-kebab can be created under flow, it seems that reinforced polymer products can be easily obtained with shish© 2012 American Chemical Society

Received: May 15, 2012 Revised: August 2, 2012 Published: August 10, 2012 6600

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the inner layers.15,16 This heterogeneous structure not only goes against the reinforcement of injection molding parts but also leads to the deterioration of mechanical properties due to the residual stress caused by different levels of crystal orientation in the thickness direction.16,17 Therefore, in practical processing conditions, the relaxation of the oriented molecular chains must be suppressed in order to obtain shishkebab crystals.15 For this purpose, some new processing techniques such as dynamic packing injection molding (DPIM),18−20 vibration-assisted injection molding,21−23 shearcontrolled orientation injection molding (SCORIM),24−26 and push−pull processing27,28 have been developed. Recently, in our group, a type of injection molding method called oscillation shear injection molding (OSIM) has been successfully used to suppress the relaxation of molecular chains and fabricate injection-molded parts with shish-kebabs.29−31 Introducing external flow into conventional practical processing does give us the chance to suppress the relaxation of oriented molecular chains, and obtain shish-kebabs, but it is just the first step to get polymer products reinforced. Apart from the emergence of shish-kebab structure, there is another major factor responsible for the mechanical properties of industrial products, which is reasonably the relative proportions of shish-kebab structures in polymer artifacts.5 Thus, another challenge is to find a way to increase the content of shishkebabs in polymer parts under the same processing condition. This requires us to learn about the formation mechanism of shish-kebab and try to know what kind of molecules tend to form shish under flow. On the conventional view of point, the coil−stretch transition was considered as the necessary pathway that single molecular chain must undergo in order to form shish. And the formation of shish-kebab is enhanced by long polymer chains because they are more easily deformed and undergo coil−stretch transition than short chains under flow field due to the long relaxation time of them.4,32,33 However, for the necessity of coil−stretch transition, some recent experimental results demonstrated that in entangled polymer melt, without stretching long chains out from the entangled network to undergo coil−stretch transition, the stretched network is sufficient to induce the shish-kebab.34,35 And regarding the role of long chains in the formation of shish-kebab, recently, Kornfield and co-workers claimed that the oriented crystallization was promoted by long chain-long chain overlap, rather than a single chain effect.36 Moreover, Kanaya et al. showed that the reason that the long chains facilitate the formation of shishes is the existence of entanglement points formed by them.37 In order to utilize the effect of long chains on shishkebabs in the practical processing, some researches were carried out to reinforce injection-molded products of polymers by adding a small amount of long chains.18,38 The results all verified long chain component did promote more shish-kebabs under flow field condition, and enhance the mechanical properties of the polymer parts. However, a common question in these works arose in that it was not easy to generate shishkebabs in the core layer of the injection-molded samples due to the relaxation of oriented molecular chains or chain segments. It is usually difficult to suppress the relaxation of oriented molecular chains after cessation of shear since the entanglement points formed by long chains are not so stable that they may slip or disentangle as soon as the shear flow ceases, which allows the oriented molecules to coil back to their random state. Nevertheless, these instructive findings indicate that the content of shish-kebabs can be increased through suppressing

the molecular chain relaxation, thus further reinforcing the polymer parts. Unfortunately, the preservation of molecular orientation is a practical challenge because the residence time of the polymer melt frequently exceeds the relaxation time of the molecular chains in the practical polymer processing. Since the oriented molecular network formed by physical entanglement of long chains cannot be well retained after shear ceases, they are less effective to induce shish-kebabs. We speculate the existence of molecular networks consisting of chemically cross-linked points (i.e., permanent entangled networks) probably further facilitates the formation of shishkebab because compared to the topological entanglement points, molecular cross-linked points alone cannot slip and disentangle during and after shear cessation and hence the relaxation of the oriented molecular network can be effectively slowed down. For this purpose, in the current work, polyethylene (PE) is lightly cross-linked to obtain the entangled network with permanent entanglement points via light electron-beam irradiation cross-linking.39,40 The use of electron-beam irradiation cross-linking can exclude the disturbance of impurity probably imported by other crosslinking methods as far as possible. The lightly cross-linked polyethylene (LCPE) is melt blended with short-chain PE together, and then the blend is injection-molded into samples for later measurement through a modified injection molding machineOSIM technology, which provides successive shear on melt in cavity during solidification.29 The crystalline structure and crystalline morphology of the injection-molded samples were characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The combined influence of successive shear and LCPE on the shish-kebabs and thus mechanical properties is thoroughly elucidated.

2. EXPERIMENTAL SECTION Material. HDPE (trade marked as 2911) was purchased from Fushun Petroleum Chemical Co., China, with a melt flow rate (MFR) of 20 g/10 min (190 °C, 2.16 kg), Mw = 6 × 104 g/mol, and Mw/Mn = 3.3, where Mw and Mn are the weight-average molecular weight and the number-average molecular weight, respectively. Sample Preparation. The irradiation of the HDPE pellets was carried out at room temperature by using an electron accelerator JJ-2, (Institute of Nuclear Science and Technology of Sichuan University), with an energy of 1.7 MeV. The irradiation dose is 5kGy. The characteristics of electron-irradiation cross-linked PE (including rheological properties, gel content and gel permeation chromatography) were shown in Supporting Information. 20 wt % LCPE pellets were mixed with pure HDPE by a solution blending procedure to ensure that the two species were intimately mixed at the molecular level. First, LCPE and HDPE were dissolved in xylene at 140 °C by continuous stirring in an oil bath to obtain uniform dispersion. Then, the solution was extracted by adding alcohol. After that, the extracted solid mixture was filtered and dried in a vacuum oven at 60 °C. The above dried HDPE/LCPE blend was used as a master batch to fabricate the blend containing 3 wt % LCPE for next injection molding by melt mixing in a twin-screw extruder. The processing temperature profile was limited within 160−180 °C from hopper to die, and the screw speed was fixed at 120 rpm. Finally, the dumbbell samples were molded by OSIM in a temperature profile of 160−180 °C from hopper to nozzle. The detailed description of this technology has been reported in the Supporting Information. The OSIM apparatus can provide intensive shear field with shear rate ranging from several s−1 up to hundreds of s−1. The shear imposed on melt in the mold cavity is stronger and stronger with time during cooling because the melt channel becomes smaller and smaller. The initial maximal shear rate is set to be about 220 s−1 in this work. 6601

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min−1 under a nitrogen atmosphere. The data were collected from 40 to 200 °C, and the sample weight was about 5−10 mg.

However, shear will cease while mold gates freeze; even the melt in cavity does not totally solidify. It is the most common case that the melt in core region solidifies after cessation of shear because the gates usually freeze earlier than the melt of core layer, so the OSIM samples include a core layer with less oriented lamellae. The main feature of OSIM is that polymer structure can be modulated and well controlled by the particular “melt manipulation” processing, which does not exist in conventional processing methods. The OSIM samples with and without LCPE are designated as OSIM PE and OSIM PEX, respectively, in this work. Conventional injection molding (CIM) samples were also prepared under the same processing conditions (only without oscillation shear) for comparison, designated as CIM PE and CIM PEX respectively. Mechanical Properties Tests. Tensile properties of the dumbbell samples were measured at 23 °C using the Instron Instrument Model 5576 according to ASTM D-638 at a cross-head speed of 50 mm/min. Two Dimensional WAXD Measurement. To characterize the crystalline structure in the width direction of samples using 2DWAXD, we started with a 6 mm wide and 4 mm thick dumbbell tensile bar and machined away all of the tensile bar except for a 1 mm thick piece (the 6 mm width remains unchanged), as shown in Figure 1. The

3. RESULTS Crystalline Structure and Molecular Orientation. Figure 2 shows the selected 2D-WAXD patterns of CIM and

Figure 2. 2D-WAXD patterns: (a) CIM PE; (b) CIM PEX; (c) OSIM PE; (d) OSIM PEX. The numbers at top of the patterns indicate the positions away from surface of specimens, where X-rays irradiate.

OSIM samples of PE and PEX from different layers, viz. surface, intermediate (locations of 500, 1000, and 2000 μm away from surface) and core layers. The two clear diffraction reflections in the WAXD patterns from inner to outer represent the (110) and (200) crystal planes of PE, respectively. The arclike diffractions in Figure 2, parts c and d, demonstrate the orientation of lamellae. In order to reveal the orientation degree of the molecular chains, the (110) intensity distribution along the azimuthal angle between 0 and 360° was integrated and shown in parts a−d of Figure 3. The orientation of lamellae can be directly reflected by the azimuthal width. Meanwhile, the orientation parameters were also estimated from the intensity of the (110) along azimuthal angle and listed in Table 1. All patterns in Figure 2, parts a and b, show two isotropic diffraction circles, which indicates that there is no appreciable oriented structure in both CIM samples from skin (surface) to core regions, as shown in Figure 3, parts a and b, as well as Table 1. The responding orientation parameters are zero. One can easily understand why the crystals in the inner layers of CIM samples are isotropic since the molecular chains therein have enough time to relax at the solidification stage. However, the skin layer is different from the general fact that there always exists a thin layer of highly oriented structure in the conventional injection-molded parts due to the quick solidification of melt close to mold cavity walls.16 This phenomenon may be because the mold temperature (60 °C) in the current work is slightly higher than usual, and thus the relaxation of polymer chains is fast. Additionally, the relatively short HDPE chains (low molecular weight HDPE) used in this experiment tend to relax before solidified. Similar to the CIM PE sample, the OSIM PE still does not exhibit obvious oriented feature in the skin region (Figure 2c) due to the same reason mentioned above. However, in the intermediate layer the arc like diffractions of the (110) plane are clearly seen in the patterns, which indicates the existence of

Figure 1. Schematic diagram of the positions of the samples for WAXD measurement: MD, the molding direction (i.e., flow direction); TD, the transverse direction; ND, the direction normal to the MD−TD plane.

position of the sample obtained is located in the middle of the bar. The direction normal to MD−TD (the molding direction-transverse direction) plane was defined as ND. The X-ray beam with 0.5 mm width was perpendicular to the MD−TD plane, moved from exterior to inner, took a picture interval of 0.5 mm, and ceased in the position of 3 mm far from the edge because the remaining part was symmetrical, setting the first signal picture as surface layer which located in the range of 0−0.5 mm. The measurements were carried on the synchrotron light source (wavelength λ = 0.15498 nm) with the MarCCD as the detector at National Synchrotron Radiation Laboratory, Hefei, China, and done in transmission geometry; meanwhile, the incident X-ray beam was set perpendicular to the flow direction. For evaluation of molecular orientation, the orientation parameter was calculated mathematically using Picken’s method from the (110) reflection of WAXD for PE.41 Scanning Electron Microscopy (SEM). The test specimens were cryogenically fractured in liquid nitrogen, and etched by 1% solution of potassium permanganate in a mixture of sulphuric acid, 85% orthophosphoric acid and water. Then the etched surface was covered with a thin layer of gold and observed by an SEM instrument (model JSM-5900LV) operating at 20 kV. Differential Scanning Calorimetry (DSC). It was performed using a TA-DSC 910s system with a constant heating rate of 10 °C 6602

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Figure 3. Intensity distribution of crystal planes: (a) CIM PE; (b) CIM PEX; (c) OSIM PE; (d) OSIM PEX along the azimuthal angle from 0 to 360°. The numbers represent the positions sampled.

become weak again and the azimuthal width is also broader than that of the intermediate layer due to the cessation of shear after the mold gates freeze. The low orientation degree of lamellae (orientation parameter is 0.36) in core layer demonstrates that no shear and short chains of PE both are disadvantageous to the formation of oriented crystals because they cannot effectively suppress the relaxation of oriented molecular chains. This exactly provides a chance to reveal the effect of adding LCPE into PE matrix on the crystalline structure upon shearing. To our surprise, the molecular chains in the skin region of OSIM PEX show orientation information along the flow direction and their orientation parameter reaches 0.5 (Table 1). However, the orientation parameters of skin layers of OSIM PE

Table 1. Orientation Parameter of Four Samples Calculated from the Azimuth Diffraction Curves of the (110) Plane positions (distance away from surface, μm) samples

surface

500

1000

2000

3000

CIM PE CIM PEX OSIM PE OSIM PEX

0 0 0 0.50

0 0 0.88 0.98

0 0 0.98 0.96

0 0 0.91 0.96

0 0 0.36 0.95

orientation structure. It can also be confirmed by the curves of Figure 3c and the data of Table 1. The oriented crystals should undoubtedly be ascribed to the intense shear provided by OSIM. In the core region, the anisotropic diffraction turns to

Figure 4. SEM images of etched skin regions of (a) CIM PE and (b) CIM PEX. 6603

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Figure 5. SEM images of etched OSIM PE and PEX samples. a1, b1, c1 and a2, b2, c2 represent the SEM images of three regions of OSIM PE and OSIM PEX from skin to core, respectively. b1′ and b2′ are the images with higher magnification in the position of intermediate layer of OSIM PE and PEX than b, respectively. The flow direction is horizontal for all images.

results indicate that the superstructure of the OSIM PEX becomes homogeneous compared to the OSIM PE. However, the existence of oriented crystals does not mean the formation of shish-kebab structure, so the crystalline morphology was further observed by SEM. Crystalline Morphology. Figure 4 shows the SEM micrographs of CIM samples. For both CIM samples, the typical spherulites exist throughout the whole sample without

and both CIM samples are all zero. More importantly, the molecular chains are highly oriented in all the regions of OSIM PEX sample except the skin region. As depicted in Figure 2d, the diffraction reflections of (110) plane are all anisotropic for regions from 500 to 3000 μm away from surface. The azimuthal width of each layers (except the skin layer) is very narrow and their intensity difference seems not clear (Figure 3d). All the orientation parameters are higher than 0.95 (Table 1). The 6604

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Figure 6. DSC melting curves of (a) CIM PE and (b) CIM PEX.

of gyration of a molecule (about 20−30 nm).35 It reflects that the shish was probably generated by stretched network or multiple molecular chains rather than single molecular chain. From the SEM images of shear layers of OSIM samples, an exciting structure, i.e., interlinked shish-kebabs could be found, which has never been reported before. Different from other common shish-kebabs such as those formed in polymer solutions45 and polymer melts,43 and the interlocked shishkebabs46 obtained through other processing methods, we can see from the shish-kebab structures in the red square of Figure 5b2′ that the adjacent shishes are closely connected by numbers of kebabs, in other words, they share the same kebabs. It seems that one kebab which originates from one shish grows over the vicinal shishes and thus the two shishes are linked by this kebab to form an interlinked shish-kebab structure. We speculate that this structure would bring about more significant enhancement effect on mechanical properties than those normal shish-kebabs because the linkage between shishes can prevent the slippage of molecular chains and bear bigger stress when external force is exerted. As to the reason for generating interlinked shishkebabs, we think it should be ascribed to the processing condition. Hobbs43 carried out the study on crystallization of PE in quiescent condition after cessation of shear and obtained interlocked and interlinked shish-kebabs at the same time. He has also explained the process of the formation of interlocked shish-kebab, which are in brief the slightly oriented zones in front of growing crystal due to the volume reduction, disentanglement or entanglement’s migration away from the crystal growth front, metastable regions, and impurities or other noncrystallizable material in front of the growing crystal. The four factors are dominated by crystallization not exterior conditions in his work and determine that the interlocked shish-kebab is the major part of shish-kebab. In Hobbs’s work, after shear cessation, the crystallization condition is quiescent. But our system crystallizes under reciprocal shear flow in the whole packing stage. In this stage, continuous and reciprocal shear were exerted to melt in the cavity during solidification and crystallization, and led to different growth mode of crystal. First, packing during crystallization can compensate the volume reduction caused by crystallization in time. Second, the reciprocal shear flow can certainly influence formation of metastable regions, entanglement’s migration, and the aggregation of impurities in front of growing crystal. Moreover, the shear can decrease the entropy between the melt and crystal and promote the crystallization process. The molecular chains in melt between two approaching lamellae easily fold into the two existing crystalline lattice at the same time. So two opposing lamellae tend to join together, and form interlinked shish-kebab.

any traces of oriented crystals. The absence of the oriented structure can be ascribed to the short molecular chains, relatively high mold temperature, and sufficient time to relax for those oriented molecules formed during mold filling. This result implies the lightly cross-linked chains alone cannot induce the formation of shish-kebabs under quiescent crystallization conditions. Because there is no obvious difference in the crystalline morphology from intermediate to core regions of both CIM samples, only the SEM micrographs of the skin layers for both CIM samples are shown. Figure 5 shows the crystalline morphology of OSIM samples. As shown in Figures 5, parts a1 and b1, the regular and irregular ring-banded spherulites are present in the skin regions of OSIM PE and PEX samples, respectively. The above WAXD result shows slight molecular orientation in this region for OSIM PEX, whereas the oriented lamellae cannot be clearly seen. This deviation is probably caused by the different detection scale of SEM and WAXD measurement, i.e., the SEM image exhibits larger scale aggregate state structure of crystals while the 2DWAXD indicates the oriented molecular structure, even the molecular chain segments.29 Another factor is the degree of orientation is too small. For the formation of ring-banded spherulites, Zhang et al. reported that they can be created under flow fields with certain intensity.42 The intensity of the flow must be higher than the lower critical value in the initial crystallization stage which is strong enough to induce the eligible lamellar twisting, and less than the upper critical value, under which the oriented lamellae perpendicular to the shear direction can form. It is glad to see that the oriented crystals detected by WAXD in the intermediate layers of both OSIM samples are indeed shish-kebabs as shown in Figure 5, parts b1 and b2. In this layer, the shishes are aligned along the flow direction on which the oriented lamellae (kebabs) epitaxially grow.4 Similar to the shish-kebabs observed by Hobbs, et al. using atomic force microscopic (AFM), some kebabs in Figure 5, parts b1 and b2 are also tortuous rather than straight.43,44 Besides, although the effect of LCPE cannot be revealed in this layer (because shishkebabs are formed in both OSIM samples), there are still some differences between shish-kebabs of two OSIM samples. Compared to the OSIM PE sample, the shishes in the OSIM PEX are more obvious and wider; in the meantime, the kebabs are also thicker. These differences were probably caused by the different entanglement states of the two types of molecular chains networks, i.e. topological entangled network and the lightly cross-linked network. Parts b1′ and b2′ of Figure 5 are high magnification images of shish-kebab structure, from which we can clearly recognize shish and kebab crystals. The width of the shishes in part b2′ is ca. 100 nm, much larger than the radius 6605

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Figure 7. DSC curves of (a) OSIM PE and (b) OSIM PEX from surface to core layer.

OSIM PEX tends to generate shishes with big dimension in width and length due to the permanent entanglement points and slow relaxation process led by the addition of LCPE. Different from the intermediate layer, there is only one melting peak in the core region of OSIM PE and PEX. For OSIM PE sample, it is the correct reflection of true condition because shish is absent according to the SEM observation. Nevertheless, it does not mean the absence of shishes in the core layer of OSIM PEX; instead, the shishes do exist based on the SEM results. The disappearance of the second melting peak may be due to the relatively low content of shishes in this region, after all it is very difficult to generate shish-kebabs in core layer because of sufficient relaxation led by the slow cooling and cessation of shear. Mechanical Properties. Since different crystalline structures have been created in CIM and OSIM samples, we are curious whether such superstructure can lead to the difference in mechanical properties. Figure 8a represents the tensile strength and tensile modulus of as-prepared samples. As for CIM PE and PEX samples, their tensile strength and Young’s modulus are almost at the same level, where the addition of LCPE only brings out an increase of 2 and 70 MPa for tensile strength and Young’s modulus, respectively. This means a small amount of LCPE has no obvious reinforcement effect on the HDPE under conventional injection molding condition. It is because LCPE cannot induce formation of new structure, such as shish-kebab, under quiescent condition according to the morphology of CIM PE and PEX. After applying OSIM technique, the tensile properties of the OSIM samples have a significant increase compared to the CIM samples. As can be seen from Figure 8, the tensile strength of OSIM PE increases notably from 27.6 MPa of CIM PE to 42.7 MPa; meanwhile the Young’s modulus is enhanced from 810.2 to 1091.9 MPa. Obviously, the reinforcement effect of OSIM method on the neat PE should be attributed to the transition of crystalline structure in intermediate region from isotropic spherulitic to oriented shish-kebab. However, the most attractive is the OSIM PEX sample obtained more remarkable reinforcement. The tensile strength and modulus of OSIM PEX reach 57.5 MPa and 1311.7 MPa, respectively, which are 14.8 and 219.8 MPa higher than those of OSIM PE sample. The enhancement is caused by the addition of LCPE when OSIM technique is utilized. In contrast, adding LCPE into HDPE matrix contributed 2 and 80 MPa to the tensile strength and modulus of CIM sample, respectively, under quiescent condition. The further reinforcement of mechanical properties must be due to the structural change of the OSIM PEX sample, viz. the core layer of OSIM PEX samples containing shish-

The crystalline structures of the core layers of OSIM PE and PEX samples are shown in Figure 5, parts c1 and c2. Since only a low level of molecular orientation exists in the core layer of OSIM PE sample (Table 1), one can hardly see any oriented lamellae, not to mention the shish-kebabs (Figure 5c1) because the formation of shish-kebab is relatively difficult compared to the oriented lamellae due to the relaxation of molecular chains. On the contrary, the oriented lamellae containing shish-kebabs are clearly seen in the core layer of the OSIM PEX sample (Figure 5c2). This is a very interesting observation which signifies flow induced shish nuclei can survive in the core region of OSIM PEX and crystallize into shish-kebab even under the slow solidification condition after cessation of shear. The result has never been reported before and it is undoubtedly a good signal for us to enhance the mechanical properties of polymer products through modulating the structure in them. By contrasting the morphologies of OSIM PE and PEX samples, one can see the LCPE certainly took effect in the flow-induced formation of shish-kebab in this region, and it will be discussed in detail hereinafter. Thermal Behavior (DSC). The DSC melting curves of two CIM PE and PEX samples are shown in Figure 6. Their melting temperature is between 134 and 135 °C. Nearly no change in the melting temperature indicates there is no other crystalline structure in them, and the thickness of the lamellae is almost the same in both samples from skin to core layers. Figure 7 shows the DSC melting curves of OSIM PE and OSIM PEX. For the OSIM PE sample, the melting temperature is 132.46, 135.31, and 134.86 °C in skin, intermediate, and core layers, respectively. For the OSIM PEX, the melting temperature from skin to core layer is enhanced to 134.71, 137.36, and 135.75 °C, respectively. The higher melting temperature indicates that the lamellae in the OSIM PEX sample are thicker than those in the OSIM PE sample which is consistent with the SEM observation. Interestingly, a second melting peak exists in the DSC curves of intermediate layer of both OSIM samples, which can be attributed to the melting of shish crystals. Their melting temperature is 137.82 and 139.50 °C for OSIM PE and PEX, respectively, which is about 2 °C higher than melting temperature of lamellae. However, the melting temperature of traditional shish crystals composed of extended chains is about 5−10 °C higher than the first melting temperature.46 This is possibly because the molecular chains in the shish crystals do not fully extend under shear flow. Moreover, the temperature of the second melting peak in the intermediate layer of OSIM PEX is ca. 2.0 °C higher than that of OSIM PE. We speculate that this difference is because the shishes originate from the LCPE under flow. In flow field, 6606

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cost can be reduced; on the other hand, in this system one need not consider the interfacial adhesion between LCPE and HDPE matrix because with the same molecular chain structure, they are thermodynamically compatible. Besides that, the effect of LCPE in HDPE matrix is to induce more shish-kebabs especially in the core region of injection molding bars so that the inner structures of them are more homogeneous, thus polymer products with high performance can be obtained.

4. DISCUSSION Although the molecular origin of the formation of shish-kebab structure is far away from clarity, our current study is one good example for shear-induced crystallization applying to factual polymer processing. The experimental results demonstrate that the LCPE network did facilitate the formation of shish-kebabs under shear flow. Additionally, the simultaneous application of LCPE and strong shear flow imparted the injection molding parts (OSIM PEX) with a homogeneous orientation distribution across the thickness direction, in which the shish-kebab structure existed in the injection molding bars from intermediate to core region except for the skin layer (from the surface to 100 μm away from the surface). With the shishkebabs and their homogeneous distribution, the mechanical properties were significantly improved compared to the neat PE sample under the same processing condition. In this part, we will try to explain the role of LCPE in the formation of shishkebab in the OSIM samples. We wish the current work would be helpful for the profound understanding of shear-induced shish-kebab. According to recent research findings, shish-kebabs can be generated from the stretched networks of entangled chains under shear or elongational flow.34,35 Usually, the entangled networks in polymer concentrated solutions and polymer melts are built up by the topological interwinding of reptation chains. All entanglement points in the network are in dynamic balance as physical cross-links,49 which easily change their positions ceaselessly or disappear and are rebuilt simultaneously somewhere else due to the slippage between molecular chains. Here, we call the type of entanglement as unstable one. As reported by Hsiao et al.49 and Han et al.,34,35 when suffered from shear flow, some entangled networks will be deformed and thus part of chain segments in them is oriented along flow direction to crystallize into shishes, which serve as the nucleation sites for the epitaxial growth of kebabs. However, shish cannot be formed if the molecular chains or chain segments oriented under flow relax sufficiently before crystallization, so the essential prerequisites to induce shish are generating molecular orientation and suppressing relaxation. On the basis of the point of view, short molecular chains are unfavorable for the formation of shish if there is no successive shear. As shown in Figure 9a, under this circumstance without successive shear, stretched chain segments tend to relax after cessation of shear. Moreover, the relaxation is more sufficient if the temperature is high. During the relaxation process, slippage between molecular chains and rearrangement of entanglement points occur. As a result, the whole network would relax back to the random conformation and no shish-kebabs can be formed (seeing Figure 9a). This evolution mode of entanglement networks could explain the crystallization of the intermediate and core regions of CIM PE and PEX samples. Generally, oriented lamellae can be formed in the skin layer of CIM samples, because oriented molecular chains in the melt close mold walls led by fast mold filling can be quickly frozen and

Figure 8. Tensile properties of four samples: (a) tensile strength and modulus; (b) stress−strain curves.

kebab structures while there are only spherulites and some lightly oriented lamellae in the core layer of OSIM PE samples, as suggested by WAXD and SEM results. The phenomenon that shish-kebabs exist in the core layer of OSIM PEX sample is evidently due to the cooperative effect of adding LCPE into HDPE matrix and exerting shear on melt during packing stage. The stress−strain curve is shown in Figure 8b. The CIM samples show normal stress−strain curve with ductile failure and the elongation at break is 325% and 230% for CIM PE and CIM PEX, respectively. With the crystalline structures changing from isotropic to anisotropic, the elongation at break dramatically decreases to around 20% for both two OSIM samples. In the direction of orientation, because of the covalent bonds, the yield stress becomes stronger and accordingly elongation decreases to a large extent, exhibiting a ductile to brittle behavior depending on the degree of orientation.47,48 It is a characteristic of the extreme molecular orientation for semicrystalline polymers. And this change can also be observed in our previous work on UHMWPE/HDPE blends31 and isotactic polypropylene (iPP).29 Although the decrease of elongation at break is large, it is still enough for the application of polymer products under general conditions. And it is worth noting that under OSIM processing condition, with the addition of such a small amount of LCPE (the concentration of LCPE is only 3 wt % and the actual content of permanent entangled network in it is even smaller) the mechanical properties can be dramatically improved compared to neat HDPE produced under the same processing condition. In contrast with other reinforcing materials such as glass fiber, on one hand, the additive amount of LCPE is relatively low, so the 6607

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so the crystallization in core layer is strongly dependent on the melt temperature and molecular chains’ structure. The intensive shear heating, slow cooling of the melt in core region, and short molecular chains of HDPE used here together make it very difficult to form shish, so what happened to molecules in the core layer of OSIM PE sample is just as depicted in Figure 9a. The corresponding structure is isotropic spherulites. As mentioned in the Introduction, in order to create more shishkebabs, LCPE was added into short-chain PE matrix. The very stable (in fact permanent) entanglement points in LCPE can certainly slow down the relaxation of molecular chains or entanglement networks when oriented (the relaxation time is calculated from rheological results and shown in Supporting Information). In skin layer of OSIM PEX, the WAXD measurement shows the lamellae are lightly oriented and the degree of orientation is ca. 0.5. The SEM images also show irregular ring-banded spherulites owing to the light orientation although the oriented lamellae are not so obvious. Compared to the orientation degree of skin layer of CIM PEX samples (0), it demonstrates that shear exerted after mold filling finished induced some oriented lamellae formed in the skin layer of OSIM PEX sample. As to the ring-banded spherulites in OSIM PEX as well as PE samples, it has been mentioned above that they are easily formed when the intensity of shear falls into between the lower and upper critical values. According to this, it is understandable why ring-banded spherulites can be generated in the skin region of both OSIM samples. After mold filling, the polymer melt close to mold cavity walls started to crystallize, but due to the relatively high mold temperature, the solidification is relatively slow. Meanwhile, the successive shear supplied by OSIM was imposed on the melt. That is, the molecular chains suffered from shear flow while solidifying. The intensity of the shear flow in this region just falls in the range helpful of the formation of ring-banded spherulites so that it could induce the lamellar twisting without orienting them along the flow direction. For the intermediate layer of OSIM PEX sample, the crystallization process may be illustrated by Figure 9, parts b and c, because both two kinds of entangled networks are able to form shish. It is interesting to note the stable entanglement networks in OSIM PEX still induced the formation of bigger shishes, which were proved by DSC result and SEM images. When stretched and oriented under shear flow, the oriented chain segments in entanglement networks would serve as the primary shish nuclei to induce the formation of shishes and finally each LCPE network would become the backbone of shish. The stability of shish nuclei is very important for generating shish, which is mainly decided by the entanglement networks. Obviously, entanglement networks with slow relaxation process, such as those in LCPE, are more helpful to form shish nuclei and thus shishes. Nevertheless, the most prominent contribution of LCPE is to promote the formation of shishes in core region of OSIM PEX, as shown by the SEM images and WAXD results. The crystallization can also be explained by Figure 9(c). After cessation of shear, the stretched LCPE networks in core region also tend to relax. However, the cross-linking points in the networks would never slip or disentangle. The stretched chain segments between them are confined tightly and are kept in their stretched state despite of the high melt temperature. As the cooling continues, the oriented chain segments in the stretched networks more easily become shish nuclei when the melt temperature falls to temperature easy to nucleate. And

Figure 9. Evolution of traditional entangled network (a and b) and the permanent entangled network (c) under flow.

crystallize. However the lightly higher mold temperature set in this experiment as well as the short-chain PE matrix caused sufficient relaxation of oriented molecular chains, thus formation of isotropic crystals in the skin layer even for CIM PEX sample containing very stable entanglement networks. In fact, the most possible is that a very thin lamellae layer was formed in the skin region of CIM samples because the mold temperature was much lower than the crystallization temperature. The oriented lamellae layer is so thin that they cannot be detected by WAXD measurement, so the orientation parameters in skin layer of CIM samples are all zero. Another possible reason is the skin layer is a region from surface to 0.1 mm far away from the surface of sample, and the measurement position is not right in the oriented lamellae region whose thickness is much thinner than 0.1 mm. For other regions except for skin, more sufficient relaxation could take place during slow solidification after mold filling finishes because the melt temperature is much higher than crystallization temperature. It resulted in the isotropic spherulites from skin to core regions of both CIM PE and PEX samples as the SEM and WAXD results show. For the skin layer of OSIM PE, the solidification process was almost the same as CIM samples. However, normal spherulites and ring-banded spherulites were induced for CIM and OSIM PE samples, respectively, depending on the existence of shear flow or not. The formation of ring-banded spherulites will be discussed hereinafter with those of OSIM PEX. In the intermediate layer of OSIM PE, the shear was intensive enough and lasted in the whole crystallization period, so the deformed networks composed of unstable entanglement points could also stay in their oriented and stretched conformation and ultimately formed shishes, as illustrated by Figure 9b. Although the material used here is short-chain PE, successive strong shear exerted on melt during solidification is able to guarantee entangled networks’ orientation until solidified or crystallizing into shishes. In the evolution process, slippage between molecular chains and rearrangement of entanglement points are of no importance because the molecular chains can be always kept to be oriented due to the successive shear. Hence, large amount of shish-kebabs formed in the intermediate layer of OSIM PE sample. However, the crystallization process of core layer of OSIM PE sample is not the same. In core region, there is no shear during crystallization because the mold gates have been frozen and shear cannot be exerted on melt in cavity, 6608

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then, the shishes form based on the stable nuclei. Finally, the lamellae epitaxially grow on the shishes to generate the shishkebabs structure. According to the above analysis, we wonder whether the essential reason that entanglements of long chains and the entanglement points are important for the formation of shishkebab is due to the existence of the “relatively stable” entanglement points in the long chain-short chain system. We speculate that by adding long-chain component such as ultra high molecular weight polyethylene (UHMWPE) into short-chain PE matrix, the “relatively stable” entanglement points can be obtained in deformed network when the concentration of long chains is above a threshold and the “relatively stable” entanglement points can also suppress the relaxation of the oriented network because the entanglement state of long chains is much more complex and the disentanglement is more difficult due to the long relaxation time of long chains. But by using the long chain-short chain system, it is still difficult to illustrate that either the “relatively stable” entanglement points formed by long chains or the long chains themselves with long relaxation time is the dominant factor in suppressing the relaxation of the whole stretched network. Maybe the two factors are both significant. However, in this work, we excluded the disturbance of long chains by using LCPE and short-chain PE matrix (seeing GPC results in Supporting Information), thus the significant effect of stable entanglement points on the formation of shish was directly revealed. Its existence was actually conducive for the formation of shish under shear flow, which has been substantially supported by the current experiment results. More significantly, this finding is meaningful and instructive for obtaining shishkebab structure in those polymers whose molecular weight available for general use is relatively low so far as a result of relatively hysteretic synthesis technology such as poly(L-lactide) (PLLA) under practical processing condition. About the enhancement of mechanical properties of OSIM samples compared to the CIM samples, first, it results from the formation of shish-kebab structure. Second, the existence of shish-kebabs in the core region of OSIM PEX sample further improved its mechanical properties in contrast with OSIM PE. Finally, there is another reason for the improvement of mechanical properties that we would like to mention. As shown in Figure 5, with the help of interlinked shish-kebabs in which shishes are connected by kebabs, external stress could be undertaken partially by the highly oriented shishes which are considered very strong. Furthermore, when LCPE are added into PE matrix, the microkebab structure, shown in Figure 10b, is more helpful for reinforcement of mechanical properties of sample because partial microkebab (red lines) and shish (blue lines) are linked by covalent bond and correspondingly the linking strength between shishes would be stronger. However, the microkebab (red lines in Figure 10a) in interlinked shishkebab of OSIM PE sample is connected with shish by topological entanglement point. As a result, the mechanical properties of OSIM PE are lower than that of OSIM PEX. The current work demonstrates that addition of stable entanglement networks could greatly promote the formation of shish-kebabs which even exist in the core region of OSIM PEX sample where the shish-kebab can hardly be generated due to the long relaxation time led by slow cooling. The presence of LCPE can also strengthen the connection of shishes and kebabs in the form of the interlinked shish-kebabs, in which microkebabs are covalently linked with shishes. The mechanical

Figure 10. Sketch of microkebabs of traditional shish-kebab (a) and shish-kebab induced by permanently entangled network (b). The black dots denote the covalent linkage between shish and microkebab.

properties of OSIM PEX are reasonably enhanced remarkably. The addition of a small amount of LCPE does not cause the complexity of the polymeric material, even almost without the increase of cost. This set up a new method to reinforce polymer parts by tailoring the structure and morphology, which is particularly feasible to those polymers whose high molecular weight cannot be achieved as a result of relatively hysteretic synthesis technology such as PLLA.

5. CONCLUSION We have successfully got the objective to tune the tensile properties of PE by controlling the structure and morphology in it and to reveal the effect of entangled network on the formation of shish-kebabs under flow by addition of permanently entangled network of PE. As we anticipated, dynamic samples with permanently entangled network in them have the highest performance. The rheological results of LCPE indicate that the degree of cross-linking is low. It is just our intent in this work. The major achievement of this work is that addition of LCPE led to more shish-kebabs appearing in the OSIM PEX sample. Especially, shish-kebabs also formed in the core region of OSIM PEX. It has not been reported before and enlightens us to put forward the formation mechanism of shishes induced by flow. We speculate that adequate entanglement points which help the chains retain their orientation during and after shear are necessary to form shish-kebabs. The regions with LCPE have more permanent entanglement points, so the oriented structures can survive after the networks are stretched under shear flow. The oriented structures are apt to form shishes in the following steps, as primary nucleating sites for kebabs to overgrowing on them. The permanently entangled network also causes the formation of covalently interlinked shish-kebabs, thus the enhancement of mechanical properties of OSIM blend sample.

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ASSOCIATED CONTENT

S Supporting Information *

Characterizations of electron-irradiation cross-linked PE, including figures showing dynamic rheological properties, results of SAOS measurements, and molecular weights and molecular weight distributions, and an illustrated discussion of oscillation shear injection molding. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.L.) [email protected]; (Z.M.-L.) [email protected].. 6609

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support to this subject by the National Natural Science Foundation of China (Nos. 51033004, 50925311, 51121001).

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