Hierarchically oriented crystalline structures of HDPE induced by strong second melt penetration

Abstract

Recently, a melt-penetrating process in which the first melt suffered from only one direction penetrating action was achieved by our home-made multi-melt multi-injection molding (MMMIM). In this work, a high-density polyethylene (HDPE) melt was penetrated by a high-speed second HDPE melt via an MMMIM instrument. It was found that hierarchically oriented crystalline structures were generated in the melt-penetrating sample along the thickness, investigated by SEM, synchrotron 2D-WAXD and 2D-SAXS; however, only isotropic spherulites were formed in non-melt-penetrating samples. 2D-WAXD/2D-SAXS results demonstrated that in the melt-penetrating sample, the degree of orientation in the subskin layer was larger than that in other layers, and confirmed the existence of the shish–kebab structures.

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1. Introduction

It is well known that the crystalline morphologies of the products of semi-crystalline polymers are the direct consequence of the thermo-mechanical fields during processing. Various supermolecular structures, such as spherulites,¹ cylindrites,² and shish–kebabs,³ have been observed in different molding technologies, especially in injection molding, due to complex thermo-mechanical environments from skin to core along the sample thickness.⁴ The morphologies of semi-crystalline polymers intensely affect their final properties.^5,6 Therefore, in order to control the morphologies and meet the requirements of various applications, novel molding technologies, including shear-controlled orientation in injection molding (SCORIM),⁷ oscillatory shear injection molding (OSIM)⁸ and vibration-assisted injection molding (VAIM),^9,10 have been developed. The key principle of these innovations is imposing a strong shear field on the polymer melts¹¹ to realize notable changes in the final morphologies associated with the flow-induced transition from isotropic structures, for example, spherulites, to highly oriented structures such as shish–kebab structures.

According to the same principle, our group put forward the concept of multi-fluid multi-injection molding (MFMIM),¹² an innovative molding process compared with conventional injection molding (CIM). The key point of this process is that the cavity is partially filled with the first polymer melt, and then this melt is penetrated by a second fluid with high speed, finally the cavity is completely occupied, and then the sample is cooled down gradually. A schematic representation of MFMIM is shown in Fig. 1. Gas-assisted injection molding (GAIM), as a typical kind of MFMIM in which an inert gas acts as the second fluid, is universally known for its excellent advantages,^13–16 and some interesting microstructures, such as banded spherulites¹⁷ and shish–kebabs¹⁸ owing to the strong shear field brought about by gas penetration, have been revealed.¹⁹

Fig. 1 Schematicrepresentation of MFMIM: (a) partial filling of the first melt; (b) penetration of the second fluid; (c) melt packing stage.

In the multi-melt multi-injection molding (MMMIM) process, a polymer melt instead of inert nitrogen gas acts as the second fluid to penetrate the first polymer melt, i.e. in other words, a melt-penetrating process was achieved by MMMIM. Compared with the melt penetrating with other fluids, the melt penetrating in MMMIM has two characteristics: (1) higher friction coefficient between the two melts than that between polymer melt and gas or water; therefore, such a high coefficient might lead to a stronger shear field, which benefits the formation of highly oriented crystalline structures; however, (2) the high temperature provided by the second melt will give rise to quick relaxation behavior, which is not beneficial for the formation and retention of oriented crystalline structures.

In fact, these two competing characteristics in the formation of oriented crystals have drawn much attention over several decades. Li²⁰ observed that the scattering intensity of shish increases with an increase in strain by in situ SAXS measurements. Based on the classical concept of coil-stretch transition, increasing strain will enhance the quantities of chains over critical molecular weight.²¹ Somani et al.²² pointed out that a high shear rate is more effective in enhancing the molecular orientation, i.e. in other words, for a given molecular weight, only when the shear rate exceeds the critical value can the longer chains be extended. According to Janeschitz-Kriegl²³ a transition to thread-like precursors emerged at a high enough mechanical load and oriented structures (like shish–kebab) are formed. Thus, a strong shear field is helpful to form the oriented crystalline structures. On the other hand, Hsiao et al.^24,25 have revealed that the lifetime of long-chain precursors decreased with increasing temperature, and the orientation and alignment of polymer chains relax quickly at high temperature compared with the case at low temperature. Similarly, according to Matsuba,²⁶ some unstable precursors dissolved gradually, and others stabilized to crystal, which demonstrated that the result of competition between chain relaxation and crystallization is responsible for the formation of oriented structures. The stabilized precursors induced the crystallization into “shish–kebab” morphology via the mechanism proposed by Ogino et al.²⁷ Thus, there is the possibility that the oriented chains have sufficient time to relax and the oriented crystals cannot be formed due to the high temperature of the second melt in the penetrating process.

The melt penetrating process has high temperature and strong shear two competitive factors for the formation of oriented crystals at the same time. Thus, a question aroused: whether oriented crystal structures (especially shish–kebabs) can be formed in the sample experiencing melt penetrating with the above two contradictory characteristics? In this work, we focused on the crystalline morphologies of HDPE samples experiencing melt penetration in MMMIM, which will be helpful for us to increase the understanding of the competition of these two factors during melt penetration. For comparison, the crystalline microstructures in a non-melt-penetrating sample produced by CIM were also studied.

2. Experimental

2.1. Materials

The high-density polyethylene resin (HDPE, Model: DGDA-6098) is obtained from Qilu Petroleum Chemical Co. Ltd, P.R. China, having a melt flow rate (MFR) of 0.08 g/10 min (190°C/2.16 kg, ASTM D1238) with a weight-average molecular weight (Mw) of 5.63 × 10⁵ g mol. (MDI = 14.7).

2.2. Preparation

The melt penetration process is performed in MMMIM, and the specimens obtained are named melt-penetrating samples; moreover, the non-melt-penetrating samples represent the parts molded by CIM. A schematic diagram of the MMMIM instrument used is shown in Fig. 2. It consists of two injection units: an SM60HC injection molding machine by which the first polymer is plasticized and injected into the cavity, and an SHJ20 micro-pneumatic injection molding machine (the injection pressure, air pressure, is supplied by an air compressor) by which the penetrating material is plasticized and injected to penetrate the first polymer melt. The two injection units are controlled independently; thus, the delay time between the onset of the two units can be adjusted. The processing parameters of MMMIM and CIM are listed in Table 1. The CIM process was conducted with the same conditions but without using the SHJ20 injection unit (the second unit).

Fig. 2 Schematic representation of the MMMIM setup utilized in this work.

Table 1 Processing parameters applied in this study for both MMMIM and CIM

Parameters	CIM	MMMIM
		Injection unit 1	Injection unit 2
Injection pressure (MPa)	24	24	48
Melt temperature (°C)	240	240	240
Mold temperature (°C)	100	100	100
Delay time (s)	—	1
Cool time (s)	150	150
Injection volume (%)	—	50	50

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The specimens for SEM observation were cut in the direction parallel to the flow direction (FD), as illustrated in Fig. 3. First, samples with a thickness of 5 mm were cut at a position 30 mm away from the gate of the MMMIM and CIM parts, and then the slices (50 μm in thickness) were cut from the central part of the sample along the flow direction by using a microtome. The samples for 2D-WAXD and 2D-SAXS measurement were cut from the 70 mm-wide and 5 mm-thick specimen to a 20 mm-wide and 1 mm-thick piece, and the samples were obtained from 30 mm away from the gate. The X-ray beam with 0.4 mm width was normal to the FD-ND plane, and it moved from the exterior to the interior. These samples consist of four locations: (a) (skin layer), (b) (subskin layer), (c) (intermediate layer), and (d) (core layer), representing the distance from the surface to the center, respectively.

Fig. 3 Schematic representation of obtaining a sample for SEM and 2D-WAXD/SAXS characterizations. The letters represent different layers from the sample surface to the center, (a) skin layer; (b) subskin layer; (c) intermediate layer; (d) core layer.

2.3. Characterizations

Scanning electron microscopy (SEM). These slices were etched by a permanganic etching technique.²⁸ After the surfaces were washed, dried and coated with gold, the crystalline morphologies from the skin to the core in the MMMIM and CIM were observed by an SEM instrument, FEI (INSPECT-F), operated at 20 kV.

Two-dimensional wide-angle X-ray diffraction (2D-WAXD). The 2D-WAXD measurements were conducted on a Rigaku Denki RAD-8 diffractometer at the National Synchrotron Radiation Laboratory of the University of Science and Technology of China. The wavelength used is 0.154 nm. The sample-to-detector distance was 420 mm, and the size of the X-ray beam was 0.4 × 0.4 mm. The (110) plane of HDPE was analyzed by azimuthal scans (0°–360°).

The degree of crystal orientation of different regions could be calculated by Hermans orientation function f,²⁹ which is described as follows:

where (cos² ϕ) was defined as:where ϕ is the angle between the orientation direction and the normal vector of a given (hk0) lattice plane, and I(ϕ) is the intensity diffracted from the (hk0) planes, which are normal to the crystallographic direction. The limiting values of the orientation function, f, taking ϕ = 0 as the shear flow direction are −0.5 for a perfectly perpendicular orientation and 1.0 for a perfectly parallel orientation.

Two-dimensional small-angle X-ray scattering (2D-SAXS). Two-dimensional small-angle X-ray scattering (2D-SAXS) was conducted at the beamline BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). For 2D-SAXS measurements, the X-ray had a wavelength of 0.124 nm and the beam size was 0.4 × 1.0 mm. The 2D-SAXS signals were collected by an X-ray CCD detector (Model Mar165, 2048 × 2048 pixels of 80.56 × 80.56 μm). The sample-to-detector distance was 2000 mm. One-dimensional scattering intensity distributions have been obtained by integrating the two-dimensional scattering patterns. For the isotropic samples a 360° integration of intensity at each scattering vector q (q = 4π(sin θ)/λ, where 2θ is the scattering angle and λ is the wavelength) has been performed. The SAXS data were calibrated for background scattering and normalized with respect to the primary beam intensity. Changes in scattering intensity due to different sample thicknesses have been corrected using ionization chambers before and after the sample.

3. Results and discussion

3.1. Hierarchically crystalline structures

The crystal morphologies in different layers of the melt-penetrating sample molded by MMMIM were shown in Fig. 4. The fine and oriented lamella stacks, perpendicular to the melt flow direction, dominated the skin region of the melt-penetrating sample, as shown in Fig. 4a. The thickness of such morphology evolution was about 100 μm and treated as the skin layer. However, SEM observations do not show any evidence of the extended chain structures (shish) in this region under our experimental conditions. This observation indicated that some kind of structural orientation occurs at moderate flow conditions prior to the conditions under which the shish–kebab morphology is formed.^30,31

Fig. 4 SEM micrographs of crystal morphologies at different locations of the melt-penetrating part of HDPE 6098 molded by MMMIM. (a) skin layer; (b) subskin layer; (c) intermediate layer; (d) core layer. (a), (b) and (c) are of a magnification of 20 000 while (d) is of a magnification of 10 000. Flow direction is vertical.

Crystals composed of thread-like cores encircled by plate-like lamellar crystals in the subskin layer were shown in Fig. 4b. Such structures were well in line with the features of the so-called shish–kebab structure first observed in the mid-1960s.^32–34 It seemed that the shish-like structure was not a continuous homogeneous substance, and its estimated length varies from 3 μm to 10 μm. The shish–kebab structure has been found in the special stress field induced by some newly developed injection molding technologies, such as SCORIM⁷ and push-pull processing,³⁵ but seldom observed in CIM.³⁶ Less compact and ordered oriented lamella were arranged loosely normal to the flow direction in the intermediate layer, as shown in Fig. 4c. It seems that the oriented lamella stacks had been formed but relaxed partly after experiencing the higher temperature because they were close to the center of the cavity. The spherulites were dominant in the core layer of the melt-penetrating specimen and the shish–kebab structure disappears, as shown in Fig. 4d.

One can see the common spherulites appear from the skin to the core layer of the whole layers in a non-melt-penetrating specimen molded by CIM; moreover, from Fig. 5a–d, only the spherulites in the core layer were larger than those in the skin layer. In summary, isotropic crystal structures, spherulites, composed of randomly distributed lamella were formed both in the non-melt-penetrating part and in the core layer of the melt-penetrating sample.

Fig. 5 SEM photomicrographs of a non-melt-penetrating sample molded by CIM at a magnification of 10 000; the left is the skin and the right is the core layer.

3.2. Oriented crystalline structure

Fig. 6 (top) shows a series of 2D-WAXD patterns of different layers, including skin, subskin, intermediate and core layers from the melt-penetrating sample. The diffraction rings were associated with (110) (inner circle) and (200) (outer circle) lattice planes of orthorhombic HDPE lamellae, and the arc-like diffractions were sharp and had narrow azimuthal width, demonstrating that the preferentially oriented crystals existed along the flow direction^25,37,38 in the skin, subskin and intermediate layers, due to the intense shear flow caused by the strong melt penetrating. However, only circle diffraction was shown in the core layer, illustrating that the crystals were randomly oriented. Moreover, (see Fig. 6 (bottom)) the whole layers of the non-melt-penetrating sample show a diffuse ring with no orientation. The ring patterns were expressive of the formation of isotropic spherulites, which are in agreement with SEM observations. As shown in Fig. 6 (top), from subskin layer to core layer, the intensity distribution of the (110) plane changed from arc-like diffractions to diffuse ring patterns, which suggested that the orientation decreased gradually from the subskin layer to the core layer in the melt-penetrating sample. These results will be further proved by the orientation factor f quantitatively (shown in Table 2).

Fig. 6 2D-WAXD patterns of specimens with different layers experiencing melt-penetrating (top) and non-melt-penetrating (bottom) processes. M represents melt-penetrating and C represents non-melt-penetrating. Flow direction is vertical.

Table 2 The orientation factor f calculated by 2D-WAXD of specimens in different layers

Sample	Melt-penetrating				Non-melt-penetrating
Layer	Skin	Subskin	Intermediate	Core	Skin	Subskin	Intermediate	Core
f	0.67	0.69	0.54	0.43	0.46	0.33	0.27	—

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To compare the orientation behavior among different layers quantitatively, the Hermans orientation factor, f, which is widely used to characterize the degree of crystal orientation quantitatively, was calculated and listed in Table 2. As shown in Table 2, the orientation factor f increased first, and reached a maximum value 0.69 in the subskin layer, and then decreased gradually from the intermediate layer to the core layer and reached a minimum value of 0.43. In the subskin layer, the orientation factor is 0.69, slightly higher than that (0.67) in the skin layer, because of the shishes existing in the subskin layer rather than in the skin layer. Moreover, as seen in the SEM observations (see Fig. 4), the shish–kebabs existed in the subskin layer, while only oriented lamella (kebab) were formed in the skin layer, which agrees well with the previous work in which the existence of shishes enhanced orientation.¹⁸ Because the orientation factor f is used to demonstrate the degree of crystal orientation, it is noteworthy that the orientation factor f in the skin layer is 0.67, higher than that (0.54) in the intermediate layer, which resulted from the lamella in the skin layer being more compact and oriented than those in the intermediate layer. Even if the less compact and oriented lamella were formed in the intermediate layer, the orientation factor was 0.54, still higher than that in other layers consisting of isotropic spherulites. All the orientation factors of the layers dominated by isotropic spherulites were less than 0.5, especially the f in the core layer of the non-melt-penetrating sample, which was too low to be calculated.

The crystalline structures of melt-penetrating and non-melt-penetrating samples were further studied by 2D-SAXS, as shown in Fig. 7. It can be seen that only two straight meridional segments, concentrated on both sides of the beam, stop at the skin layer, as shown in Fig. 7 (top), illustrating the presence of a highly oriented lamellar structure aligned with its lamella normal to the flow direction.³⁹ The equatorial streak first appeared at the subskin, suggesting the existence of shish-like structures containing fibrillar crystals, which may be formed from bundles of elongated molecular chains between entangled networks along the flow direction.³⁹ From the intermediate layer to the core layer, the scattering maxima change from along the flow direction to the isotropic signal, which suggests a gradual decrease in the orientation to the random spherulites finally. The orientation factors in these layers decreased from 0.69 in the subskin layer to 0.43 in the core layer, as shown in Table 2. The layers of the non-melt-penetrating sample also showed a diffuse ring signal with no orientation (see Fig. 7 (bottom)), which illustrated randomly lamellar structures consistent with the 2D-WAXD and SEM results.

Fig. 7 2D-SAXS scattering patterns of melt-penetrating (top) and non-melt-penetrating (bottom) samples from different layers. Flow direction is vertical.

In order to elucidate this phenomenon clearly, the variation in the intensity as a function of azimuthal angle was drawn in Fig. 8. Only one-half of the data was presented for the sake of clarity and the azimuthal profile was obtained from 2D-SAXS quantitatively. Note that two main features can be identified. First, the meridional direction signals represented kebabs. This 1D azimuthal profile was equivalent to the orientation distribution.⁴⁰ Note that the same is not true for equatorial reflections. As shown in Fig. 8, the half peak width decreased from 45° for the skin layer to 42.5° for the subskin layer, and then increased gradually to 72° for the core layer, indicating that orientation distribution first increased, and then decreased, from the skin to the core layer, and the same trend was shown in orientation factors calculated from 2D-WAXD. These observations demonstrated that shishes existed in the subskin layer enhancing the orientation, and the less compact and oriented lamella were formed in the intermediate layer, of which the orientation was still higher than that in other layers consisting of isotropic spherulites, which is in good agreement with the 2D-WAXD results (see Table 2). Second, the normalized scattering intensity peak appearing in the equatorial direction at the subskin layer indicated that the shish structure parallel to the flow direction was large enough to be detected by SAXS. Moreover, no equatorial SAXS patterns were detected in other layers, which indicated that the shish were very small and farther apart, or showed no electron density contrast against the surroundings.

Fig. 8 Scattering intensity distribution along azimuthal angle of melt-penetrating and non-melt-penetrating samples from different layers.

To elucidate the evolution of the lamellar long spacing (L), the one-dimensional scattering intensity distribution along the meridional direction is plotted in Fig. 9. The peak position q is related to the value of the long spacing (L), corresponding to the thickness of lamellar and amorphous regions between two lamellae via the Bragg equation:

Fig. 9 Lorentz-corrected 1D SAXS intensity profiles (Iq² versus q) along the meridian in different layers. Peak positions were used to calculate the long periods (L).

As shown in Fig. 10, it is seen that the long spacing of HDPE lamellae in the melt-penetrating sample is about 25 nm and in the core layer the long spacing shows only a slight increase of about 1.5 nm, reaching 26.5 nm. The specific values are shown in Table 3. In the non-melt-penetrating sample, the long spacing values appeared slightly larger than those in the melt-penetrating sample, reaching about 27.9 nm. This probably arises from the ordering structures of spherulites being much weaker than those of shish–kebab. However, we accepted the idea^25,41 that the change is not significant when the crystalline morphologies change from oriented structures to isotropic structures. This result agrees well with previous research⁴² on the trend of long spacings in different layers from the mold wall to the center in the aPP/iPP blends.

Fig. 10 Long periods of different layers in melt-penetrating and non-melt-penetrating samples.

Table 3 The long spacing in different layers of melt-penetrating and non-melt-penetrating samples

Sample	Melt-penetrating				Non-melt-penetrating
Layer	Skin	Subskin	Intermediate	Core	Skin	Subskin	Intermediate	Core
L	25.7	24.9	25.0	26.5	26.5	27.1	27.9	26.9

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Combining the 2D-SAXS/WAXD with SEM data with the equatorial streak and meridional segment patterns shown at the same time, the shish–kebab structures were confirmed in the subskin layer; thus, the half peak width of the azimuthal profile reached the minimum, 42.5°, and the orientation factor reached the maximum value, 0.69. In the skin layer, only meridional segments without equatorial segments appeared, and oriented crystals (kebabs) were observed and their orientation factor was 0.67, slightly less than that in the subskin layer. Compared with the oriented crystals (kebabs), less compact and oriented crystals were formed in the intermediate layer; however, the orientation factor was 0.54, still higher than that in the regions consisting of spherulites, in which scattering patterns were a diffused ring with orientation factor less than 0.5.

3.3. The formation process and mechanism of hierarchical crystalline structures

It is well known that the final hierarchical crystalline structures are dependent on the strain rate and the rate of orientation relaxation.⁴³ At low strain rates, flow-induced oriented molecular conformations can relax; thus, the crystal morphology is expected to be spherulites. However, at high strain rates, the molecular chain segments are stretched to form oriented crystals at the crystal growth stage. For the non-melt-penetrating sample, one can see the spherulites were dominant in not only the core layer but also in the skin zone molded by CIM. The shear rate in the non-melt-penetrating process is less than the critical value according to Keller⁴⁴ and the molecular segments might orient along the flow direction, but stretching was not caused by such a moderate shear rate; thus, the lamella grew randomly during the cooling stage and developed into spherulites, as shown in Fig. 11(I). In other words, the polymer melt was still in the random coil state; therefore, the 2D-WAXD/SAXS patterns of non-melt-penetrating samples molded by CIM were a diffused ring with orientation factor less than 0.5, as shown in Fig. 6 and 7.

Fig. 11 Schematic diagram of various oriented crystalline structure formation mechanisms. (I) spherulites in non-melt-penetrating sample; (II) hierarchical crystal structures of skin and subskin zones in melt-penetrating specimen molded by MMMIM. (III) Intermediate layer in melt-penetrating sample.

For the melt-penetrating sample, the melt was injected into the cavity first, and flow-induced precursor structures similar to the scaffold^45–47 were not formed as in the non-melt-penetrating process. However, after a delay time, the second penetrating melt was injected into the cavity, and the penetrated melt underwent a stronger shear field caused by the melt penetrating (see Fig. 11(II)). One can distinctly see shish–kebab structures in the subskin layer of the melt-penetrating sample, as shown in Fig. 4b. It was considered that due to the melt penetrating, the melts in the subskin underwent quite a high shear rate to the extent that might reach the critical value for shish–kebab formation for a given molecular weight.^48,49 Although the mechanism of shish–kebab formation is still unclear and openly debated, both the classical coil-stretch transition theory²³ and stretched network²⁰ theory involve a critical shear rate or strain. It is important for the shish–kebab formation that the shear rate exceeds the critical value because stretching is a necessary condition for the formation of shish–kebab morphology. The molecules are stretched and aligned to form fibrillar nuclei then the shish nuclei act as nucleating agents for the rest of the polymer melt, resulting in the crystallization of kebabs. The shish signal appears only when the strain is larger than a particular value;²⁰ thus, the content of shish exceeds the critical content ϕ_c as can be observed in the subskin from the SEM and 2D-SAXS signals.²³ Moreover, equatorial streak and meridional segment patterns were shown at the same time, and their orientation factor reaches the maximum value 0.69 (see Table 2).

In the skin layer, a shear field introduced by melt penetrating might not reach the critical value for shish formation; therefore, only kebabs were formed and aligned normal to the flow direction, as shown in Fig. 4a, or it is possible that the concentration of shish-induced flow in the surface area was very low compared with the subskin region, and thus scanning electron microscopy cannot detect any signal.²³ For these regions, the cooling rate was so fast that the oriented crystal structure can be retained and monitored by SEM and 2D-SAXS characterization, in which only meridional segments without equatorial segments were observed; therefore, the orientation factor in the region was 0.67, only slightly less than that (0.69) in the subskin layer, due to the absence of shishes. It was deemed that melt penetrating of the second melt brought about oriented crystal morphologies, as shown in Fig. 11(II), rather than the relaxation effect of the second melt with high temperature, making the oriented crystal structure relaxed in these regions.

Compared with the crystalline structures in the skin layer, the less compact and ordered lamella were formed in the intermediate layer of the melt-penetrating sample because the melt in this region was much closer to the center of the cavity with higher temperature,⁵⁰ resulting in these oriented lamella being relaxed partly. One can see a number of less compact and ordered oriented crystalline structures, and these structures were arranged more loosely than those in the skin region (see Fig. 4c), of which the orientation factor was 0.54, less than that in the skin layer, while the orientation factor was still higher than that in the areas dominated by spherulites, in which scattering patterns were a diffused ring with no orientation. Thus, it was demonstrated that these oriented crystal structures, consisting of less compact and ordered oriented lamella between the subskin region and core-material region, were partly relaxed during the slow cooling rate to form the transition region, as shown in Fig. 11(III).

4. Conclusions

There are hierarchical crystalline structures along the thickness existing in the melt-penetrating sample molded by MMMIM due to the strong shear field induced by the second melt penetrating, rather than the negative factor of oriented structures brought about by high temperature. Shish–kebab-like morphologies emerged in the subskin layer, oriented lamella and less compact oriented lamella appeared in the skin and intermediate layer, respectively, and only spherulites appeared in the non-melt-penetrating part molded by CIM. The 2D-WAXD results demonstrated that the degree of orientation of the subskin layer was larger than that of other layers in the melt-penetrating sample. In conclusion, all the results, 2D-WAXD/2D-SAXS and SEM, were closely related to each other to confirm the existence of hierarchically oriented crystalline structures in the melt-penetrating sample molded by MMMIM. Since the shish–kebab structure may obviously enhance mechanical properties; therefore, it will be our further work to study its mechanical properties and the mechanism of shish–kebab-like formation in such a process.

Acknowledgements

We would like to express our sincere thanks to the National Natural Science Foundation of China for financial support (21174092) (51121001). In particular, the authors gratefully acknowledge Mr Chao-liang Zhang for his assistance in SEM observations. The 2D-SAXS experiments were performed at the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China).

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