Efficient Utilization of Atactic Polypropylene in Its Isotactic

May 23, 2014 - Atactic polypropylene (aPP) and isotactic polypropylene (iPP) were ... polyethylene by the introduction of crosslink network and period...
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Efficient Utilization of Atactic Polypropylene in Its Isotactic Polypropylene Blends via “Structuring” Processing Zhengchi Zhang,† Rui Zhang,† Yanfei Huang,† Jun Lei,*,† Yan-Hui Chen,† Jian-hua Tang,‡ and Zhong-Ming Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, and ‡College of Chemical Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: Atactic polypropylene (aPP) and isotactic polypropylene (iPP) were incorporated into a new blending material with tailored microstructure. Improved mechanical properties were realized through the application of a shear flow field to injection molding aimed at making use of aPP on a large scale. The hierarchic structure of the oscillation shear injection molding (OSIM) parts was characterized through wide-angle X-ray diffraction, small-angle X-ray scattering, and scanning electron microscopy. It was found that a high and homogeneous orientation inner structure, that is, intense shear flow induced shishkebabs, is successfully obtained in aPP/iPP blends, which can markedly improve the mechanical performance of blends and offset the mechanical properties decline due to the addition of aPP with poor properties. Owing to the tailored microstructure, as 10 wt % aPP is added, the tensile strength and impact toughness of OSIM samples climb from 30.6 MPa and 4.8 KJ/m2 for normal injection molded samples to 57.8 MPa and 16.8 KJ/m2, respectively. Even when the aPP content reaches 30 wt %, the OSIM samples retain tensile strength of 52.8 MPa and impact strength of 13.1 KJ/m2 compared to only 20.2 MPa and 7.3 kJ/m2 for normal samples. This shows the potential for practical applications because of the satisfactory properties of blends and efficient utilization of aPP. In addition, we found that the practical tensile strength of OSIM samples is significantly higher than the theoretical value calculated through mixing principle. For normal samples, an opposite behavior is observed. Although the addition of aPP usually will lead to a remarkable degradation of mechanical properties, it plays a positive role in modifying the inner structure of injection-molded blend samples when structuring processing is used to provide continuous shear flow during processing. Through OSIM technique, we successfully turn “waste” into wealth, which opens a new field for efficient usage of aPP and oil resources.

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INTRODUCTION Atactic polypropylene (aPP) is an inevitable noncrystalline byproduct with very low molecular weight obtained during polymerizing propylene into isotactic polypropylene (iPP). It is generally regarded as an industrial waste and only involves very limited usages (mainly adhesives, coatings, and compatibilizer after chemical modification process1,2) because of its poor heat resistance and mechanical properties.3 Although the catalyst efficiency exceeds 95% in industrial production, taking the high iPP output and oil resource depletion into consideration, it is necessary to open up a new application field to effectively make use of aPP. Because of a similar molecular structure and good affinity between iPP and aPP,4−7 it is reasonable to consider blending aPP with iPP. However, the addition of aPP usually leads to a serious degradation of mechanical properties of the iPP products. Friedrich reported that the yield strength of iPP/ aPP blends decreased from about 40 to 20 MPa when aPP content increased from 5 to 20 wt %, although aPP could promote impact strength to some extent at ambient temperature.8 Hence, it is a crucial issue to suppress the decline in mechanical properties caused by incorporation of aPP. It has been long well established that self-reinforcement through the generation of oriented structure is an effective way to reinforce polymers and their blends, as this does not introduce any additional component and is favorably recycled after used. For an example of semicrystalline polymers, an oriented crystalline © XXXX American Chemical Society

structure, such as shish-kebab, has been proved to considerably enhance mechanical properties. It is logical that mechanical properties of aPP/iPP blends can be enhanced once the shishkebabs are created. It is well-known that shish-kebab structure can be induced by an external shear field.9 By contrast with spherulites formed under quiescent crystallization condition, shish-kebab crystals possess specific anisotropic microstructures with aligned texture whose longitudinal axis (shish) is parallel to the extension/ shearing flow direction.10−12 During the past few decades, flow induced shish-kebabs have attracted much attention because the transition from a relatively isotropic, spherulitic morphology to a highly oriented shish-kebab morphology in polymer can markedly improve the tensile strength,13 modulus and stiffness,14 decrease thepermeability15 and increase the thermal stability of the polymer,16 and the higher the fraction of shishkebab is, the better the reinforcement effect of the properties of the polymer is.17 In our previous work,18 a strategy to use a hot runner mold with big-sized gates, high melting temperature, and long packing time during conventional injection-molding was adopted to prepare aPP/iPP blends. The hot runner and long Received: March 26, 2014 Revised: May 8, 2014 Accepted: May 23, 2014

A

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scattering (SAXS), and scanning electronic microscopy (SEM), etc.

packing time meant higher temperature and longer time for iPP crystallization, and also facilitated the formation of shishkebabs, thereby offsetting the adverse effect of aPP. The tensile strength of the resultant aPP/iPP blend with 20 wt % aPP only decreased to 40.5 MPa compared to 45.9 MPa of neat iPP. However, when the aPP content exceeded 20 wt %, the mechanical properties of the aPP/iPP blends showed a dramatic decline. This indicates our previous strategy cannot yet ensure efficient utilization of aPP. Nevertheless, this work gave us some inspiration that the shear flow, even local flow, caused by long packing time and big-sized gates, is helpful to generate oriented crystals and enhance the crystallinity. Unfortunately, for conventional injection molding, it is hard to prepare samples with a high content of shish-kebab structure because of the weak shear flow, and slow cooling of inner melt which leaves sufficient time for stretched chains to relax.19,20 Some theoretical analysis also implied that it is difficult to induce the formation of fine shishkebab structure under realistic processing conditions.21,22 Therefore, in order to promote the formation of shish-kebabs, some unconventional processing techniques have been developed to control the structure evolution during processing by utilizing special device and setting special processing parameters, which was termed “structuring” processing.19 In our group, a type of injection molding method called oscillation shear injection molding (OSIM), which imposes oscillation shear flow on polymer melts during solidification at the packing stage of injection molding, has been successfully used to process melt into samples with tunable inner structure, especially oriented crystals, and in turn good mechanical performance.17,20,23,24 Through the OSIM technique, Zhong et al. reported that a homogeneous orientation distribution across the thickness direction was obtained with the presence of a three-dimensional polyester microfibrillar network in iPP.20 According to the work of Chen et al., the injection-molded iPP parts with bamboo-like bionic structure, i.e. reinforced and fibrillar outer layer as well as toughened inner layer, were successfully prepared by OSIM, which is the result of the competition of shear-induced crystallization and β-nucleator, and accounts for its superb mechanical properties.23 Meanwhile, the intense shear flow in mold cavity provided by the OSIM technique suppresses the relaxation of oriented molecular chains, which can introduce a large amount of shish-kebabs even in biodegradable poly(L-lactic acid) (PLLA) parts with short chain length and semirigid chain backbone, leading to an obvious increase in mechanical properties as reported by Xu et al.24 In a word, through the OSIM technique, the inner structure of an injection-molded part can be modified by application of a shear flow. In the current work, injection-molded parts of aPP/iPP blends with different aPP contents up to 30 wt % were fabricated via the OSIM technique with a guiding ideology of “structuring processing”. The oscillation shear flow during the packing stage of OSIM induced a high content of shish-kebab structure from the surface to core of the injection-molded parts. A significant improvement of the mechanical properties was obtained, herein; as 30 wt % of aPP was added, the tensile strength and impact strength of aPP/iPP samples could reach 52.8 MPa and 13.1 KJ/m2, respectively, which are values much higher than those of conventional injection-molded neat iPP samples. To reveal the reinforcing mechanism, the inner structures of the OSIM samples were thoroughly characterized by wide-angle X-ray diffraction (WAXD), small-angle X-ray

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EXPERIMENTAL SECTION Materials. aPP was purchased from Exxon Mobil with a trademark of 399, Mw = 5.2 × 104g/mol, and Mw/Mn = 4.1. iPP (S1003) was supplied by Dushanzi Petroleum Chemical Co. (China) with a melt flow rate (MFR) of 3 g/10 min (230 °C, 21.6 N), Mw = 39.9 × 104 g/mol, and Mw/Mn = 4.6. The tacticities of aPP and iPP were characterized by using solvent extraction for 6 h and are 0 and about 98%, respectively. Sample Preparation. The iPP/aPP blends with different aPP contents were melt mixed by employing a twin-screw extruder to produce pellets for subsequent injection molding. The screw speed of the twin-screw extruder was kept constant at 250 rpm, and the processing temperature profile was 165− 200 °C from hopper to die. The blended pellets were injectionmolded into dumbbell bars by utilizing the OSIM technique, the details of which will be described in the next section. Four kinds of samples were prepared and denoted as OSIM-A0, OSIM-A10, OSIM-A20, and OSIM-A30, in which the numbers mean the percent content of aPP by weight. OSIM Technology. Compared to conventional injection molding (CIM), the major difference of oscillation shear injection molding (OSIM) mainly exists in the packing stage because of use of a unique mold as shown in Figure 1. The

Figure 1. Schematic illustration of OSIM mold.

continuous oscillatory shear was provided at the packing stage of the injection molding through the homodromous movement of two pistons, and the shear flow of the melt stops until the gates are solidified. The details of this technology had been described elsewhere before.17,20,23 The temperature profile for OSIM was 180, 190, 190, 180, and 180 °C from hopper to nozzle, respectively. To accommodate the viscosities of blends melt with different aPP contents, the temperatures of the hot runner were 210, 200, 200, and 190 °C for OSIM-A0, OSIM-A10, OSIM-A20, and OSIM-A30, respectively. The mold temperature, packing time, and packing pressure during injection molding was the same for all blends at 60 °C, 180 s, and 60 MPa, respectively. The CIM samples, just for comparison purposes, were made with same molding parameters (only without the oscillatory shear). Mechanical Properties Test. The tensile properties of all samples were tested with an Instron Instrument (model 5576) B

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at 23 °C with a crosshead speed of 50 mm/min according to ASTMD-638. The impact test with notch was carried out at 23 °C in terms of the standard of GB/T 1843-96. The dimension of the testing sample was 60 × 6 × 4 mm3 with a V-notch of 1.2 mm depth. X-ray Diffraction Measurement. Two-dimensional (2D) wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were conducted at the Advanced Polymers Beamline (X27C, λ = 1.371 Å) with a Mar CCD (with a resolution of 512 × 512 pixels and pixel size =158 μm) as a detector in the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The distances between sample and detector were 132.6 and 1930 mm, respectively, for WAXD and SAXS. The samples for X-ray measurements were obtained from the standard tensile dumbbell through machining as shown in Figure 2 with a dimension of 6 × 6 × 1 mm3. The details of

Xc =

ΣAcryst ΣAcryst + ΣA amorp

(3)

where Acryst and Aamorp are the fitted areas of crystalline and amorphous phases, respectively. Scanning Electronic Microscopy (SEM). For morphological observation of the crystalline phase, the injectionmolded parts were first cryo-fractured in liquid nitrogen. Then, the surface for observation was treated by permanganic etching according to the method reported by Bassett et al.26 After that, surfaces to be observed were sputter-coated with a thin layer of gold in order to enhance electrical conductivity. Finally, a field emission SEM (Inspect-F, Fei, Finland), operating at 20 kV, was employed to observe the morphology of samples.

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RESULTS Mechanical Properties. The mechanical properties of OSIM and CIM samples are listed in Figure 3. Figure 3a

Figure 2. Diagrammatic drawing of the positions of the sample for Xray measurements.

sample preparation can be found in ref 23. The incident direction of X-ray beam is parallel with ND that is normal to the MD−TD plane. Seven positions: 0, 500 (skin layer), 1000, 1500, 2000 (intermediate layer), 2500, and 3000 (core layer) μm down from MD-TD surface, were scanned, respectively. WAXD Data Analysis. To obtain the degree of orientation along flow direction, the orientation parameter, that is, Hermans’ orientation function, was calculated according to Picken’s method25 from 2θ = 14.7° (i.e., 040 reflections of PP α form) of WAXD. The Hermans’ orientation function defined as 1 F = (3 cos2 φ − 1) (1) 2 2 where cos φ is the orientation factor defined as 2

cos φ =

∫0

π /2

Figure 3. Mechanical properties with errors of iPP/aPP blends with different aPP contents: (a) selective stress−strain curves, (b) tensile strength, (c) impact strength, (d) tensile modulus, and (e) elongation at break.

I(θ ) cos2 θ sin θ dθ

∫0

π /2

I(θ ) sin θ dθ

illustrates the typical stress−strain curves of CIM and OSIM samples. All samples display an obvious yield and ductile tensile behavior. As aPP component increases, the elongation at break increases along with a decline of tensile strength. In addition, the tensile strength of OSIM samples is obviously higher than that of the CIM sample with the decline in elongation at break. The specific values of tensile strength, impact strength, tensile modulus, and elongation at break of all samples are listed in Figure 3 panels b, c, d, and e, respectively. In comparison with the CIM samples, the OSIM samples exhibit a remarkable increase in tensile strength and impact strength. The tensile strength (Figure 3b) rises from 35.6 MPa for CIM neat iPP to 58.5 MPa for OSIM neat iPP. In the meantime, the

(2)

where φ is the angle between the flow direction and the direction perpendicular to a given (hkl) crystal plane, and I(θ) represents the diffraction intensity. The values of the orientation parameter F, −0.5, and 1.0, mean perfectly parallel and perpendicular orientation related to flow direction, respectively. For an unoriented sample, F equals 0. The profiles of 1D-WAXD were gained on the basis of the circularly integrated intensities of 2D-WAXD patterns. Then, by using the deconvoluting−peak technique, the total crystallinity Xc was calculated by the following equation: C

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Figure 4. SEM micrographs of different layers of OSIM samples after etched. The a, b, c, and d in the micrographs represent the samples A0, A10, A20, and A30, respectively. The subscripts 1, 2, and 3 represent the skin, intermediate, and core layers of samples, respectively.

enhanced compared to CIM technology. Hence, possible conclusions can be obtained that, the continuous intense shear flow in mold cavity provided by OSIM technique during the package stage of injection-molding processing may induce oriented molecular chains and suppresses their relaxation, which could introduce a large amount of shish-kebabs in the injection-molded parts. When it comes to ductility, the CIM samples fracture in a ductile way, the elongation at break ranging within 140−630% with a strong dependence of aPP content, while the OSIM samples show a reduced but basically invariable elongation at break of ca. 50% (the values of elongation at break are summarized in Figure 3e). In spite of this, the elongation at break of OSIM samples still falls into the region between 50% and 60%, which is quite enough for general application. For stiffness, an increased tensile modulus (Figure 3d) is also observed from 1411 MPa for CIM neat iPP up to 1826 MPa for

impact strength (Figure 3c) of OSIM neat iPP increases by 341% from 4.4 KJ/m2 of CIM sample up to 19.4 KJ/m2. These dramatical increases in mechanical properties certainly resulted from the specific inner structure caused by OSIM. Most importantly, with the addition of aPP, the tensile strength and impact strength of OSIM samples apparently outperform those of CIM samples. For example, as 10 wt % aPP is added, the tensile strength and impact toughness of OSIM samples climb from 30.6 MPa and 4.8 KJ/m2 for CIM samples to 57.8 MPa and 16.8 KJ/m2, respectively. Even when the aPP content reaches 30 wt %, the OSIM samples retain a tensile strength of 52.8 MPa and impact strength of 13.1 KJ/m2, in comparison to 20.2 MPa and 7.3 kJ/m2 for CIM ones. It is interesting that the difference in tensile strength gets more obvious as the aPP content rises. This issue will be discussed in detail in the following parts. In a word, the tensile and impact strength of aPP/iPP blends through OSIM technology are dramatically D

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However, there are indeed some delicate differences in crystalline morphologies between OSIM-A0 and OSIM-A10. As shown in parts b1 and b2 in Figure 4, some obvious void can be observed between two adjacent kebabs in sample OSIM-A10, while the kebabs in neat iPP stack much more tightly (parts a1 and a2 of Figure 4). Because chain segments of aPP cannot pack into the iPP nuclei or lamellae, aPP component will be excluded into the amorphous region when iPP chains start to arrange into lattice. Consequently, the amorphous aPP component existing between crystalline regions is quite easily oxidized and decomposed by strong acid and oxidant during etching due to its poor chemical stability and dissolution resistibility. Hence apparent voids are left on the surface of the sample between kebabs after etching. Even so, as illustrated in Figure 4b, the continuity of shish in the direction of flow is not disturbed by the aPP component. In other words, iPP crystalline regions together with chain entanglements still can contact or interlink with each other and form an iPP crystal network, as mentioned by Bicakci et al. in their paper about PEEK and PEI.29 The iPP crystalline network is the main stress transfer mesh. The crystalline morphologies of aPP/iPP blends with 20 and 30 wt % aPP contents are shown in the third and fourth rows of Figure 4. Similar morphologies, that is, well aligned shishkebabs in the skin and intermediate layers and a little disorder of shish-kebabs in the core layer, are observed. It is reasonable to find that the voids between two neighboring kebabs become more and more obvious because the contents of noncrystalline aPP in the samples of OSIM-A20 and OSIM-A30 increase further in contrast with OSIM-A10. However, it is glad to see that the integrity of the shish-kebab network, always regarded as a strong and stiff structure, is still well maintained in the flow direction, and can bear the most stress in that direction even whenthe aPP component reaches a high proportion, that is, 20 and 30 wt %. In this case, the mechanical properties of aPP/iPP blends with 20 and 30 wt % aPP contents just decrease slightly compared to neat iPP sample. Crystalline Structure. The crystal structure was indirectly verified by 2D-WAXD patterns (Figure 5) of OSIM samples

OSIM samples, and the tensile modulus of OSIM samples with different aPP contents through the whole range of our experiment is higher than that of CIM samples. However, because of the poor properties of aPP, tensile modulus decreases with the increase of aPP component, which is the weak spot under the out stress and results in that the OSIM samples are prone to deformation. So the stiffness of samples with the addition of aPP component shows a considerably decrease even with OSIM processing. With the aid of OSIM technology, the mechanical performance of aPP/iPP blends with aPP contents up to 30 wt % can be dramatically improved through self-reinforcement compared to CIM technology at the same aPP content. Above all, the OSIM aPP/iPP blend with 30 wt % aPP has the potential for practical applications because of their satisfactory properties and relatively high-amount usage of aPP, which could thus potentially achieve efficient utilization of aPP. In comparison with the results of Wenig et al., the yield stress of the predrawn aPP/iPP blend film (flow gradient = 104 s−1) was only about 30 MPa with 10 wt % aPP content.27 To our best knowledge, the best mechanical properties of aPP/iPP blends with aPP content up to 30 wt % were achieved by the OSIM technique as shown in this essay. These superior properties inspire our interest to elucidate the structure−property relationship of aPP/iPP blends prepared by the OSIM technique. Hereinafter, we carefully examined its microstructure by employing high resolution SEM and detailed X-ray characterization (WAXD and SAXS). Crystalline Morphology. To obtain a clear understanding of the crystalline morphology of OSIM samples, direct observation was performed by a high resolution SEM, as shown in Figure 4. The first row of Figure 4 (i.e., a1, a2, and a3) shows the crystalline structure of OSIM neat iPP. A large amount of oriented crystals, i.e., typical shish-kebab superstructure can be easily identified. In particular, many interlocking shish-kebabs, with adjacent kebabs penetrating into each other and forming an interlocking state, are observed in OSIM parts. These results are similar to the crystalline morphology in polyethylene/ ultrahigh molecular weight polyethylene blends obtained through the OSIM technology as well.28 Unexpectedly, shishkebabs with a little disorder appear in the core layer of the OSIM neat iPP where, generally, an isotropic structure, such as spherulite, is expected in this region of injection molded parts because slow cooling of melt leaves sufficient time for stretched chains to relax. Different from the transient shear flow in CIM, the continuous shear flow in OSIM is continuously imposed on the melt until the gates become frozen. Therefore, most oriented melts could induce the formation of shish-kebabs from the skin to core regions of OSIM parts prior to relaxation, which accounts for the excellent mechanical properties of the OSIM samples. With the addition of 10 wt % aPP component, the crystal phase morphology of iPP (see in Figure 4, parts b1, b2, and b3) in the OSIM-A10 sample is similar to the neat iPP sample (OSIM-A0). Shish-kebab superstructures can be observed in every layer of the bulk. Because of the interlocking shish-kebab superstructure existing in every layer of injection-molded parts, the decline of mechanical properties of blends caused by the addition of aPP can be certainly offset by manipulating the superstructure of the injection-molded parts by employing structuring processing.

Figure 5. 2D-WAXD patterns of (a) OSIM-A0, (b) OSIM-A10, (c) OSIM-A20, and (d) OSIM-A30. The numbers at the top of the patterns denote the positions away from the surfaces of specimens, where X-rays irradiate. E

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by the fact that the orientation parameters vary little when aPP content increases from 0 to 30 wt % as listed in Table 1, especially for both skin and intermediate layers. It is just the oriented shish-kebab superstructure that offset or blanketed the negative influence of aPP component on the mechanical properties of samples. Although Carlson et al.31 reported that the oriented chains of iPP under flow tend to relax back to their random state after cessation of shear in aPP/iPP blends because aPP molecular chains are much shorter than iPP and are beneficial for the mobility of iPP molecules resulting in difficulty in generating oriented crystals like shish-kebab in aPP/iPP blends, the situation is opposite when successive and intense shear are provided by OSIM during the nucleation and crystallization process of iPP. aPP will play an active role in the process under shear flow because it is noncrystalline and has a low molecular weight. When iPP component starts to crystallize in the mold cavity, aPP component still maintains in melt. Hence, the presence of aPP will not only not disturb the crystallization process of iPP, but also promote it. Oriented Structure. To further clarify the inner structure, especially crystalline morphology and shish-kebab structures, of OSIM samples, 2D-SAXS measurements were further performed. Figure 6 depicts the representative 2D-SAXS patterns.

with different aPP contents from different layers, viz. surface, intermediate (locations of 500, 1000, and 2000 μm away from surface), and core layers (3000 μm away from surface). Only the crystal information on iPP in blends can be shown in the diffraction patterns of WAXD, because aPP is amorphous, and its chain segments are not likely to be incorporated into the crystal stem.30 The 2D-WAXD patterns of all samples basically show five diffraction reflections associated with different lattice planes of iPP, from inner to outer circles, corresponding to (110), (040), (130), (111), and (1̅31) crystal planes, respectively, which are characteristic of α-crystals. In addition, (300) and (311) lattice planes appear in the core layer of OSIM-A30, corresponding to the reflection of β-phase, as shown in Figure 5d. The arclike diffractions in Figure 5, except for that of the core layers, should be attributed to the orientation of lamellae. In the core regions of the samples, 3000 μm away from surface, the diffraction patterns suddenly turn into isotropic diffraction circles, as shown in Figure 5, indicating a random distribution of crystalline lamellae. The orientation parameters were calculated on the basis of the intensity of the (040) along the azimuthal angle to estimate the degree of orientation of molecular chains as listed in Table 1. For samples with different aPP contents, the orientation Table 1. Orientation Parameters of Four Samples Calculated from the Azimuth Diffraction Curves of the (040) Plane locations (distance away from surface, μm) samples

surface

500

1000

2000

3000

OSIM-A0 OSIM-A10 OSIM-A20 OSIM-A30

0.98 0.99 0.99 0.99

0.98 0.97 0.99 0.99

0.98 0.98 0.98 0.99

0.98 0.98 0.98 0.98

0.10 0.21 0.15 0.15

parameters are extraordinarily close to 1, that is, 0.97, 0.98, or 0.99, in the regions from the surface to 2000 μm depth, indicating the orientation axes are perfectly parallel to the flow direction. There is no doubt that the oriented iPP crystals should be ascribed to the intense shear which is applied through the moving of two pistons during the packing stage of OSIM. Under the shear flow, molecular chains of iPP and aPP start to transfer from coil to stretched bundles, that is, the precursors of shish-kebabs, which will induce the formation of shish-kebabs when iPP chains start to pack into crystal. In comparison, the orientation parameters of core layers of samples are much lower than other layers, within the range of 0.10−0.21, as a result of cessation of shear flow and relaxation of the oriented nucleus. It is worth saying that the WAXD results are not in contradiction to the morphology obtained by SEM. Although shish-kebabs are observed in the core layers of injection-molded parts, they are not well aligned in the flow direction. The difference in observation scale between X-ray (statistic lamella orientation) and SEM (crystalline morphologies) leads to the difference in results. According to the previous results obtained by our group,18 the formation of oriented structure in the aPP/iPP blends was suppressed by the addition of aPP component. When the aPP component exceeds 20 wt %, phase inversion and percolation phenomenon in mechanical properties are obviously observed. However, in this experiment, we found that, even when aPP content reaches 30 wt %, the starting content of phase inversion, the formation and morphology of oriented structure seem not to be disturbed by aPP component. It can be proven

Figure 6. 2D-SAXS patterns of (a) OSIM-A0, (b) OSIM-A10, (c) OSIM-A20, and (d) OSIM-A30. The numbers at top of the patterns indicate the locations away from the surface of specimens, where Xrays irradiate.

For all samples (parts a, b, c and d of Figure 6) in the layers except for core, equatorial streaks and vertical lobes are clearly seen, which confirms the existence of shish-kebab structure with shish aligned along the flow direction and kebabs perpendicularly growing from it through epitaxial growth, respectively. From the skin to intermediate layers, that is, within the regions from surface to 2000 μm away from surface, the 2D-SAXS patterns show few changes for all the samples with different aPP contents, indicating that the inner structure of both skin and intermediate regions for all OSIM samples is almost similar in a highly oriented shish-kebab structure. However, the SAXS patterns of core layers are different from those of outer regions and become isotropic, especially in the core layers of OSIM-A10 and 30 (Figure 6, parts b and d). This is consistent with the WAXD results. F

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Figure 7. 1D-WAXD curves of samples of (a) OSIM-A0, (b) OSIM-A10, (c) OSIM-A20, (d) and OSIM-A30 gained from circularly integrated intensities of 2D-WAXD patterns. The wavelength of X-ray is 1.371 Å.

the chains. Overall, the 1D-WAXD profiles demonstrate that the crystal form of iPP under the shear flow provided by OSIM processing has been barely impacted by aPP component. This result is in line with that reported by Lohse et al.; that is, the lattice parameters of iPP crystals were not changed by adding aPP.33 In other words, the crystallization process of iPP is dominated by the shear field during injection molding, and the effect of aPP on the crystallization of iPP can be ignored while employing OSIM technique. The estimated crystallinity of iPP obtained by iterative peakfit procedure (see the description in Experimental Section) is shown in Table 2. The results of Table 2 show that for neat

Comparing to the scattering signals of OSIM-A0, those of OSIM-A10 and 20 (parts b and c of Figure 6) illustrate a more intensive streak normal to the flow direction, which was maybe resulted from relatively big electron density difference between amorphous aPP region and oriented iPP crystalline region. When the aPP component further reaches 30 wt %, although the 2D-SAXS scattering patterns (Figure 6d) become fuzzy because of phase inversion, they still can show clear shish-kebab structures in OSIM-A30. As a whole, even aPP is usually regarded as an adverse factor to the formation of oriented crystalline structure under weak or short-lasting shear flow, a large amount of shish-kebabs still emerge in all samples with different aPP contents. These good results should be attributed to the successive intense shear flow provided by OSIM during the packing stage. Crystal Forms and Crystallinity. To further understand the crystallinity of different samples as a function of position and aPP content, 1D-WAXD profiles, obtained by circularly integrating intensities of 2D-WAXD patterns and shown in Figure 7, were analyzed. In Figure 7a, 1D-WAXD curves of OSIM-A0, it is seen that the main reflection peaks correspond to the usual α-crystal. The most intensive diffraction peaks of the α-crystal occur at 2θ = 12.4°, 14.6°, 16.2°, 18.6° and 19.2°, corresponding to (110)α, (040)α, (130)α, (111)α, (1̅31)α/ (311)α reflections, respectively. Small amount of β-crystals that can be induced by the surface of α-row nuclei,32 whose most characteristic diffraction peak occurs at 2θ = 14.0° and can be assigned to (300)β reflections, appear in the each layers of all the OSIM samples. However, the formation of γ-modification, whose most characteristic is (117)γ with the diffraction peaks occurring at 2θ = 18.9° in 1D-WAXD profiles, is totally unexpected because iPP employed in our work is a kind of commercial product manufactured through Ziegler−Natta catalyst. This phenomenon may be related to the type and distribution of insertion mistakes during polymerization along

Table 2. Crystallinity (Xc) of Four Samples Obtained by Deconvoluting the Peaks of 1D-WAXD Profiles positions (distance away from surface, μm) samples

surface

500

1000

2000

3000

average

OSIM-A0 OSIM-A10 OSIM-A20 OSIM-A30

0.49 0.49 0.46 0.47

0.49 0.50 0.50 0.46

0.50 0.49 0.47 0.45

0.44 0.48 0.46 0.45

0.46 0.46 0.46 0.46

0.48 0.48 0.47 0.46

iPP, i.e. OSIM-A0, the crystallinity varies only a little just from 0.44 to 0.50 in different regions, suggesting that the crystallinity of iPP is all most the same in the bulk of OSIM samples. Similarly, for aPP/iPP blends with different aPP contents, the crystallinity also changes little from the surface to the core layer. According to the results of our previous work,18 the onset crystallization temperature shifted by nearly 5 °C toward lower temperature for the blend with 30 wt % aPP, demonstrating without any doubt that aPP suppressed the crystallization rate of iPP. Nevertheless, the average crystallinity only decreases from 0.48 to 0.46 with the aPP content increasing to 30 wt %. Therefore, it is reasonable to conclude that aPP component has G

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However, in most cases, shear will cease before the melt in the core region totally solidifies. So, in the center of OSIM parts, melt have enough time to relax and induce less oriented structure resulting in isotropic WAXD and SAXS patterns and low orientation parameters. Through SEM pictures, well aligned shish-kebabs are observed in the skin and intermediate layers of OSIM parts (parts a1, a2, b1, b2, c1, c2, and d1, d2 of Figure 4), indicating that the highly oriented structure implied by the X-ray results is shish-kebabs definitely. However, not well aligned shish-kebabs are shown in the core layers of OSIM parts (parts a3, b3, c3 and d3 of Figure 4). That is why the X-ray results imply isotropic structure in this region. These partly oriented shish-kebabs could stem from the partial relaxation of oriented nuclei. In other words, the oriented direction of the nuclei changes due to the relaxation and diffusion of the melt but the nuclei with oriented chains does not disappear because of its high thermal stability and triggers the following crystallization. So the shish-kebabs in the core layers of OSIM parts are aligned in different directions and are statistically homogeneous, resulting in isotropic WAXD and SAXS patterns and low orientation parameter. In summary, through OSIM processing, a large amount of shish-kebabs are induced in the injection-molded parts from the surface to core. These highly oriented structures make great contributions to the mechanical properties of aPP/iPP blends. As compared to that of CIM parts, the mechanical properties of OSIM parts have a significant improvement as shown in the section of mechanical properties. The tensile strength of OSIMA30 declines only less than 10% and reaches 52.8 MPa. In the case of such high content of aPP, it was inconceivable before to achieve such good mechanical properties. Moreover, it is worth emphasizing that the interconnecting shish-kebab networks (formed by iPP) from the surface to core induced by the intense shear flow of the OSIM processing are the major reason for the enhancement of mechanical properties of aPP/iPP blends. In addition, for CIM processing, during the injection stage, the interaction between hot polymer melt and cold mold walls triggers high strain, high stress, and large cooling rate, which result in the formation of highly oriented structure, even shishkebabs, in skin layer. As discussed above, slow cooling rate guarantees sufficient relaxation of chains in the core layer, thus oriented crystals can only be induced in a very limited region close to the surface of the product. Therefore, CIM parts usually exhibit an inhomogeneous structure, namely a skin-core structure, which is the origin of the residual stress caused by different levels of crystal orientation in the thickness direction,34 which leads to the deterioration of mechanical properties of injection molding parts. From a practical point of view, elimination of the skin-core structure is expected to improve mechanical properties. For OSIM samples, the highly oriented shish-kebabs in every layer of injection-molded parts induced by intense shear flow lead to a homogeneous orientation distribution across the thickness direction, which has been definitely confirmed by the X-ray results and morphologies. This high and homogeneous orientation inner structure achieved by OSIM technology also dramatically improves the mechanical properties of aPP/iPP blends. External Plasticization of aPP during Continuous Reciprocating Shear Flow. On the basis of the mixing principle, the tensile strength of blends composed of two miscible polymers can be roughly predicted using the formula:

not remarkable impaction on the crystallinity of iPP with the existence of intense shear flow. The decline of mechanical properties of aPP/iPP blends caused by adding aPP component can be compensated to some extent due to the relatively stable crystallinity of iPP. As well-known, oriented, and stretched polymer chains can be induced under flow field, resulting in a big reduction of entropy and energy barrier to form crystals, which can offset the suppression of aPP component to the crystallization of iPP. In other words, although aPP has negative effect to the crystallization of iPP in aPP/iPP blends, the intense shear flow provided by OSIM accelerates the crystallization kinetics in our case.

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DISCUSSION The above results indicate that aPP/iPP blends with 10 to 30 wt % aPP show excellent mechanical properties by utilizing OSIM technique that is a kind of “structuring processing” to modify the inner structure of injection-molded parts through continuous reciprocating shear flow during the packing stage. The improved mechanical properties result from the inner structure of OSIM parts, that is, the existing shish-kebabs in every layer.17,19,24 Thus, it is reasonable to believe that the OSIM aPP/iPP blends have great potential to be used as practical materials. Homogeneous Orientation Inner Structure through the OSIM Technique. As is well-known, anisotropically oriented structures, such as shish-kebabs, make immense contribution to the mechanical properties of polymers in aligned direction. Considering its significantly theoretical values and practical applications, shish-kebab structures have aroused the interest of researchers in the past few decades. However, it is fairly difficult to induce a large amount of shish-kebab structure under realistic processing conditions. Specially for aPP/iPP blends, in situ optical with shear cell was applied to study the crystallization of iPP in the blends under shear flow in our previous work.18 Unfortunately, the results reveal that aPP is an adverse factor in the formation of shish-kebabs, which makes it more challenging to induce a large amount of shishkebabs in aPP/iPP blends through CIM processing. During OSIM processing, melt is first injected into the mold cavity after the preplasticizing stage. Then the two pistons move out of phase during the packing stage resulting in intense shear flow in the mold cavity during solidification with a shear rate ranging in the region between several s−1 and hundreds of s−1. During cooling the channel of melt flow in the mold cavity becomes narrower and narrower, leading to a more intense shear flow imposed on melt. Under the shear flow, molecular chains of iPP and aPP start to transfer from coil to stretched bundles, that is, the precursors of shish-kebabs. Although aPP is an adverse factor for the formation of shish-kebabs and crystallization of iPP, intense shear flow provided by OSIM technology results in a big reduction of the entropy and energy barrier to form crystals, which dominates the crystallization of iPP. Once the melt is subjected to shear flow emerging on the mold wall or frozen layer due to velocity gradient, it starts to solidify immediately before oriented chains relax back to coils. Therefore, shish-kebabs can be induced in almost every layer of OSIM parts as shown by the WAXD and SAXS results and SEM morphologies. From the WAXD and SAXS results shown here, anisotropic patterns indicate highly oriented structure in every layer from the skin to core, except for exactly center (as shown in Figures 5 and 6 and Table 1), which results from the successive shear. H

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P = P1φ1 + P2φ2 + Iφ1φ2

samples, the decline of mechanical properties should be ascribed to both the increase of aPP content and the decrease of iPP crystallinity. It is just the latter that resulted in the fact that the practical tensile strength is lower to the theoretical value. However, as discussed above, the intense shear flow provided by OSIM technique dominates the crystallization of iPP during processing and leads to the formation of a highly oriented structure in injection-molded parts. During OSIM processing, besides the oriented nuclei that can induce the formation of shish-kebabs, point nuclei that will induce isotropic crystals may emerge in neat iPP melt especially in the regions rich of low molecular weight chains. These oriented and isotropic nuclei with entangled molecular chains attached on them will increase the viscosity of melt dramatically, which may weaken shear flow in mold cavity, even lead to untimely solidification of melt. On the other hand, the ordered segments that can be aligned into crystals are limited in the melt. So the growth of shish-kebabs and isotropic crystals has competition in the sample. Without enough ordered segments in the melt, even new oriented nuclei will be triggered continuously; no new shish-kebabs will emerge in the sample. For the reasons listed above, the continuous intense shear flow provided by OSIM technique does not give full play to talents in neat iPP melt. For melt with an aPP component, as shown in part II of Figure 9, aPP chains with low molecular weight and high mobility perform as external plasticization after the formation of oriented nuclei which reduces the viscosity of melt and make the continuous intense shear flow become fully effective during OSIM processing. Meanwhile, aPP is beneficial for the mobility of iPP molecular chains, which inhibits the formation of stable isotropic nuclei in the melt. Only highly oriented nuclei can overcome the energy barrier and trigger the following crystallization. Therefore, almost all ordered segments could participate in the formation of shish-kebabs. Consequently, more and more impeccable shish-kebab superstructures may be induced under the continuous shear flow. Additionally, long packing time is in favor of forming a dense inner structure, which is also helpful for obtaining good mechanical properties of aPP/iPP blend. Overall, although the addition of aPP usually will lead to a remarkable degradation of mechanical properties, in the meantime it also plays a positive part in modifying the

where P1, P2 and φ1, φ2 are the tensile strength and the concentration of two components, respectively, and I is the interaction factor. We assume that the contribution of aPP component to tensile strength is almost 0 due to its poor properties. In this instance, the tensile strength of aPP/iPP blends obtained by both CIM and OSIM techniques should be proportional to the iPP contents in the blends. However, as shown in Figure 8, the theoretical and practical values of tensile

Figure 8. Comparison between theoretical and practical tensile strength of OSIM and CIM samples, where (a) is the practical tensile strength of OSIM samples, (b) is the theoretical tensile strength calculated from mixing principle of OSIM samples, (c) is the theoretical tensile strength calculated from mixing principle of CIM samples and (d) is the practical tensile strength of CIM samples.

strength of aPP/iPP blends with different aPP contents show an obvious split with different aPP contents. The practical tensile strength of OSIM samples is significantly higher than the theoretical value. For CIM samples, an opposite behavior is observed. This phenomenon reminds us that the influence of aPP component on the mechanical properties of blends might be different in CIM and OSIM samples. As reported in our previous work, the aPP component is an adverse factor to the crystallization of iPP. Therefore, for CIM

Figure 9. Schematic representation for the formation mechanisms of shish-kebabs induced by continuous intense shear flow in (I) neat iPP melt and (II) aPP/iPP blends. I

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Notes

inner structure of injection-molded blend samples when structuring processing is used to provide continuous shear flow during processing. Hence, the practical mechanical properties of OSIM samples are much higher than the usually expected value. Furthermore, the difference between OSIM and CIM only exists in the packing stage, so most injection molding machines can be transformed into an OSIM device just through installing the OSIM mold and connecting it with the hydraulic system of an injection molding machine. Here, it must be emphasized that the OSIM mold is most important for the OSIM technique, and it is more suitable to product moldings with a slightly simple shape. When it comes to industrial applicability, cost is important too. One of the extra costs of OSIM technique is determined by the unique mold that is actually a hot runner mold that is not expensive. Another extra cost may be caused by the prolonged cycle of processing. However, these costs are offset because of the efficient usage of aPP and the satisfactory properties of products; the OSIM technique is capable of turning “waste” into wealth and opens a new field for efficient usage of aPP and oil resources. In the end, we have to declare that although injectionmolded parts of aPP/iPP blends with aPP content lower than 30 wt % have satisfying mechanical properties because of their high and homogeneous oriented inner structure, that is, shishkebab from the surface to core of sample, exhibiting the potential to make good use of industrial waste aPP, it is still a big challenge to obtain aPP/iPP blends with higher aPP content and considerable mechanical properties due to the phase inversion of these two miscible polymers. More detailed works and ingenious structure-tailoring methods are needed in later research.

The authors declare no competing financial interest.

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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. This work was supported by National Natural Science Foundation of China (51121001, 51273114 and U1162131) and the Program of Introducing Talents of Discipline to Universities (B13040).

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CONCLUSION High and homogeneous orientation inner structure, that is, shish-kebabs induced by intense shear flow from the surface to the core of OSIM samples, is successfully obtained by OSIM processing in aPP/iPP blends with different aPP contents. The OSIM samples with this structure have a dramatic improvement both in tensile strength and in impact strength, compared with the CIM samples. Besides, aPP content generally impacting the mechanical properties of aPP/iPP blends turned out to have small effect when OSIM technique was used. Thanks to the positive reinforcing and toughening effects of interconnecting shish-kebabs networks even when aPP content reaches 30 wt %, the tensile strength and impact strength is 52.8 MPa and 13.1 KJ/m2, respectively, which is much higher than the CIM neat iPP samples. Through OSIM technology practical injection-molded aPP/iPP blends with satisfying mechanical properties are obtained. In addition, we found that the practical mechanical properties of OSIM samples are much higher than the usually expected value calculated through the mixing principle. Through the OSIM technique, we successfully turned “waste”, the byproduct of iPP production, into wealth, which opens a new field for efficient usage of aPP and oil resources.

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