Improved performance balance of polyethylene by simultaneously forming oriented crystals and blending ultrahigh-molecular-weight polyethylene

Abstract

Polyethylene as a versatile polymer is being increasingly used for parts whose surfaces are in contact with moving metallic components or solid particles. This needs polyethylene to be greatly improved in mechanical properties as well as wear resistance. To this end, in the current work, various contents of ultrahigh-molecular-weight polyethylene (UHMWPE) were added into high-density polyethylene (HDPE) for enhancement of wear resistance, while the oriented crystals, i.e., shish-kebabs, were induced by shear flow for mechanical reinforcement. With 30 wt% UHMWPE was added, highly improved performance balance was achieved. The tensile strength rose from 26.4 MPa for normal HDPE samples to 68.5 MPa for the modified HDPE blends. The same trend was observed for impact toughness, where the impact strength increased from 6.3 to 34.1 kJ m⁻². Moreover, addition of UHMWPE could reduce the wear rate from 22.1 to 7.6 mg MC⁻¹. A very interesting phenomenon was observed, in which the overall properties of the modified HDPE blends were constantly enhanced with the increase of UHMWPE content though UHMWPE itself does not have much better mechanical properties than the oriented HDPE. This was ascribed to the amplified shear effect as a result of UHMWPE addition. The exceptionally high melt viscosity of UHMWPE assumes a gel state even at high temperature, making it just deform and hardly flow under the shear field, which amplifies the flow velocity difference between UHMWPE phase and HDPE melt. The amplified shear effect resulted in more pronounced molecular orientation and thus formation of a higher content of shish-kebab microstructure. Our work indicated that the melt processing-structure control strategy can desirably manipulate polyethylene products with desired properties.

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1 Introduction

High-density polyethylene (HDPE) with a broad spectrum of physical properties is the most common plastic in our daily life, and the annual output of it is more than 10 million tons.¹ Due to its excellent low temperature flexibility, good moisture stability, and easy processability, HDPE is being increasingly used for parts whose surfaces are in contact with solid particles or moving metallic components, such as municipal drainage pipes and guide rail parts. Under these conditions, the HDPE parts have to endure stretch, impact as well as bend stress.^2,3 In spite of its above favorable properties, pitting, scratching, deformation, abrasion or even damage frequently happened to HDPE parts, which seriously affects the performance of the HDPE parts. Unfortunately, poor mechanical strength and wear resistance of HDPE further shorten its lifetime.⁴ Therefore, it is of high importance to improve the mechanical properties and concurrently promote the wear resistance of HDPE parts.

Polymer blending in which the reinforced component (generally an engineering polymers) is designedly introduced, is widely established as a versatile and economical strategy to manipulate a product with desired properties.^5–7 It is reasonable to assume that addition of a reinforced phase with better mechanical properties and good wear resistance will improve the properties of the HDPE matrix. However, poor adhesion between HDPE and the polymers as reinforced phase (e.g., polyamide) results in no substantial improvement in the mechanical properties unless an effective compatibilizer is incorporated.^8,9 Generally, interfacial bonding between the matrix and the reinforcing element is critical in determining the resultant mechanical properties of polymer blends.

As another member of polyethylene family, ultrahigh-molecular-weight polyethylene (UHMWPE) is featured by good autolubrication (good abrasion resistance), high impact resistance, high toughness, excellent fatigue resistance, outstanding resistance to environmental stress cracking and chemical resistance, and widely used as bearing materials.^10–12 Blending of HDPE and UHMWPE is considered an effective strategy to improve the performance of HDPE since their identical chemical structures can guarantee the mutual compatibility at the interface.¹³ Lucas et al. reported that the tensile strength and yield strength of the HDPE blend containing 30 wt% UHMWPE was increased by 30% and 20%, respectively.³ More interestingly, in the abrasion tests, the experimental value of volumetric loss of the blends was less than the reference value. They suggested it was the good interface between HDPE and UHMWPE phase that hindered pullout of the entire UHMWPE particles, contributing to the excellent abrasive resistance.

In order to further optimize the properties of HDPE/UHMWPE blends, properly tailoring microstructures especially inducing formation of self-reinforced structure should be warranted, e.g., formation of shish-kebab superstructure under flow field, which could properly endow the polymeric material with enhanced strength.^14–17 Also, many studies confirmed that the high-molecular-weight species (such as UHMWPE) facilitate the efficient formation of shish-kebabs in the entangled melt under a given flow condition.^15–17 The long chains of UHMWPE can be stretched under shear or elongational flow fields in melts and crystallize into shish and the coiled chains are subsequently adsorbed onto the shish and form kebabs.¹⁷ In this scenario, the key issue is to establish the controllable flow fields in the practical processing.^18,19 Obviously, quasi-static processing method such as compression molding, which was adopted in most aforementioned research, failed to attain this. Oscillatory shear injection molding (OSIM) has been found as an effective approach to modulate the morphology and structure of polymer products, the main feature of which is that during solidification the polymer melt in the mold is subjected to high oscillatory shear stress given by two pistons moved reversibly at the same frequency.^19,20 Extensive study has been carried out in the bulk polymer including polyethylene,²¹ isotactic polypropylene,^19,22 polyethylene terephthalate,²³ polylactic acid,^18,24 and polymer blends including isotactic polypropylene/high-density polyethylene,²⁵ high-density polyethylene/ethylene vinyl acetate,²⁶ isotactic polypropylene/polyethylene terephthalate.²⁷ The resultant mechanical properties were proved to be significantly enhanced by advent of the highly oriented morphologies.

In this work, we aimed at achieving HDPE/UHMWPE blends with improved mechanical properties and excellent abrasion resistance. Before melt blending, a very small amount of UHMWPE was initially added into HDPE through solution blending to act as the precursors of highly oriented crystalline structures.^28,29 The processability of the HDPE/UHMWPE blends was guaranteed by controlling the content of high molecular weight components through melt blending, then OSIM technology was used to manipulate the microstructures of molding parts. The results showed that a large amount of self-reinforced shish-kebab superstructure was formed in the OSIM PE blends. Compared to conventional injection molded (CIM) samples, tensile and impact strength of the OSIM PE blends were significantly enhanced, where the largest increase is 159% and 441%, respectively. In addition, wear resistance was also greatly improved due to the addition of UHMWPE, with wear rate reduced from 22.1 mg MC⁻¹ to 7.6 mg MC⁻¹. Moreover, mechanical properties of OSIM PE blends improved with increasing content of UHMWPE, which was attributed to the amplification effect of shear brought out by UHMWPE.

2 Materials and methods

2.1 Materials

The chosen HDPE was supplied by the Dow Chemical Company, with a melt flow rate of 20 g/10 min (190 °C, 21.6 N), M_w = 1.2 × 10⁵ g mol⁻¹. UHMWPE, M_v = 5.5–6.0 × 10⁶ g mol⁻¹, was provided by Second Auxiliary Factory, Beijing, China.

2.2 Sample preparation

OSIM PE blends. 2 wt% UHMWPE, which was significantly higher than the estimated overlap concentration of UHMWPE (c* ∼ 0.2 wt%),³⁰ was pre-mixed with 98 wt% neat HDPE by a solution blending procedure to ensure that the two species were mixed at the molecular level. In order to avoid confusion, as to the HDPE/UHMWPE blend containing 2 wt% UHMWPE, here we still sign it as HDPE, which was used as a blend matrix to fabricate HDPE/UHMWPE blends containing 10%, 20% and 30 wt% UHMWPE for subsequent injection molding by melt mixing in a twin-screw extruder. The processing temperature profile was limited within 160–190 °C from hopper to die, and the screw speed was fixed at 80 rpm. The obtained blend pellets were injection molded into a dumbbell mold of the OSIM machine in a temperature profile of 170–200 °C from hopper to nozzle. Generally, the OSIM can provide a peak shear rate in a single cycle from several s⁻¹ up to hundreds of s⁻¹. In this work, the shear rate is about 220 s⁻¹. The detailed description of this technology is available in our previous articles.^17,19 The main feature of OSIM is as follows: the continuous oscillatory shear was provided at the packing stage of the injection molding cycle. When the movements of two pistons were out of phase with each other during the holding period, the resulting oscillatory shear force would shape the PE melt reciprocatively along the length direction of the mold. Before solidifying, the PE melt continuously underwent repeated shear stress until the pistons were stopped.

CIM PE blends. The conventional injection molded PE blends were also carried out under the same processing conditions only without oscillation shear for comparison.

2.3 Mechanical testing

Tensile properties of CIM UHMWPE and OSIM PE blend samples were measured using the Instron Instrument Model 5576 according to ASTM D-638 at a cross-head speed of 10 mm min⁻¹. Notched Izod impact strengths of the above samples were carried out according to the standard GB/T 1843-96. Five specimens were tested, and the average value was reported.

2.4 Wear testing

Wear behavior was evaluated using MM-200 wear tester (Xuanhua Testing Machine Factory, Hebei, China). The specimens slide against GCr15 stainless steel, on which CoCr coating (R_a = 0.02 μm) was prepared, with a block-on-ring contact, providing a contact normal force of 200 ± 0.2 N. The block specimens are of size 30 mm × 7 mm × 4 mm. The diameter of the counterpart steel ring is 40 mm. The tests were carried out at a linear velocity of 0.43 m s⁻¹, ambient temperature around 25 °C and one revolution were regarded as a cycle. Wear rate was calculated as the linear regression of weight loss versus number of cycles from 0.5 MC to 1 MC. Three test specimens of each sample were tested.¹⁷

2.5 Two-dimensional synchrotron X-ray scattering

It is believed that the OSIM and CIM samples have a hierarchic structure, which gradually changes from outer to inner layer.^27,31 Accordingly, the sample was characterized layer by layer in the width direction. For the sake of brevity, the skin, intermediate, and core layers are referred to the layers from surface to ca. 0.5 mm deep, from 0.5 to ca. 1.5 mm, and from 1.5 to ca. 3.0 mm in the width direction (the total width is 6.0 mm), respectively.¹⁹ 2D-WAXD and 2D-SAXS measurements were carried out at room temperature at the Advanced Polymers Beamline (X27C, λ = 1.371 Å) in the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). A MAR CCD X-ray detector (MARUSA) was employed for detection of WXAD and SAXS images, having a resolution of 1024 × 1024 pixels (pixel size = 158.44 μm). The sample to detector distance was 112.6 and 2330 mm for WAXD (calibrated by an aluminum oxide (Al₂O₃) standard) and SAXS (calibrated by a silver behenate (AgBe) standard), respectively. The Fit-2D software package was used to analyze the WAXD and SAXS patterns. For evaluation of molecular orientation, the orientation parameter was calculated mathematically using Picken's method from the (110) reflection of WAXD for PE.³²

2.6 Scanning electron microscopy (SEM)

The test samples including CIM and OSIM samples were first cooled down in liquid nitrogen for approximately 30 min. They were then cryogenically fractured in the direction parallel to flow direction in liquid nitrogen, and etched by using the mixing acid solution.^16,33 Then the etched surface was covered with a thin layer of gold and observed under a JSM-9600 (JEOL, Japan) scanning electron microscopy (SEM) with an accelerating voltage of 20 kV.

2.7 Differential scanning calorimetry (DSC)

DSC studies were performed on Netzsch DSC 204 under the nitrogen atmosphere. The samples were heated from 40 °C to 180 °C at a heating rate of 10 °C min⁻¹ under nitrogen purge. Heat flow as a function of time and temperature was recorded. Crystallinity of the samples (n = 3 each) was determined by integrating the enthalpy peak from 40 °C to 160 °C, and normalizing it with the enthalpy of melting of 100% crystalline polyethylene, 291 J g⁻¹.

3 Results and discussion

3.1 Mechanical properties

Fig. 1a shows the tensile strength of PE blends with different UHMWPE contents obtained by OSIM and CIM technologies. It can be observed that the OSIM samples exhibit much higher tensile strength than the CIM ones at the same composition. Over the entire range, addition of UHMWPE brings out more significant improvement of tensile strength in the OSIM samples than the CIM samples. For instance, with 30 wt% of UHMWPE, the tensile strength rises by 61% for the CIM samples from 26.4 to 42.7 MPa, while 77% for the OSIM samples from 38.7 to 68.5 MPa. The tensile modulus displays the similar trend as the tensile strength for both the OSIM and CIM samples, as presented in Fig. 1b. Tensile modulus of the OSIM PE blends containing 30 wt% UHMWPE increased from 1.07 GPa for neat HDPE to 1.59 GPa. To our surprise, UHMWPE itself does not possess superior tensile strength and modulus to HDPE, but it does play a role for the remarkable increase in tensile strength and tensile modulus for the OSIM samples. Such mechanical reinforcement on HDPE by incorporation of UHMWPE has never been reported in the literature. Fig. 1c shows the elongation at break of the above samples, which decreases with increasing the UHMWPE content. It is worth pointing out that the elongation at break of the OSIM samples decreases less than the neat HDPE, compared to that of the corresponding CIM samples containing the same content of UHMWPE. Especially, when the UHMWPE content rises to 30 wt%, the ultimate elongation of the OSIM samples is 71.9%, almost the same as the CIM ones (70.1%). This may be due to the OSIM technology, which can mix polymer blends more adequately and accordingly reduce structure defects in the samples. As shown in Fig. 1d, the OSIM samples present an admirable performance in notched impact resistance compared with the CIM ones at the same blend composition. With the addition of 30 wt% UHMWPE, the impact strength for the OSIM samples increased from 15.8 to 34.1 kJ m⁻² and from 6.3 to 25.3 kJ m⁻² for the CIM ones. The superb properties of OSIM samples could be ascribed to strong interfacial adhesion between UHMWPE and HDPE, and formation of some unique structure induced by the applied shear flow, which will be discussed later. An issue needs to clarify here is that mechanical properties of neat HDPE samples in this work are relatively poorer than those in some published literature, which should be ascribed to its low molecular weight.^33,34 The reason for choosing a low molecular weight polyethylene is to ensure good liquidity of the PE blends for feasible processing. The abrasion tests reveal a significant improvement on the abrasion resistance of both the CIM and OSIM blends compared to the neat HDPE, as shown in Fig. 1e. For both CIM and OSIM samples the wear rate declines with increasing UHMWPE content. Adding 30 wt% UHMWPE reduced the wear rate of the CIM and OSIM samples from 22.1 and 17.3 mg MC⁻¹ to 8.5 and 7.6 mg MC⁻¹, respectively. This result revealed that the integrity of the microstructure of UHMWPE was retained during polymer blending, thus its outstanding wear resistance was fully preserved and substantially displayed in PE blend samples. Moreover, in comparison with CIM samples, the wear rate of OSIM samples is low at the same content of UHMWPE, also implying the microstructure changes through applying shear flow field. The improved wear resistance can prevent HDPE/UHMWPE blends from scratches, gouges, and scoring marks when rubbing with metallic component or solid particles.^35,36 In brief, the unique microstructure formed under the cooperative effect of shear flow and UHMWPE significantly enhanced the comprehensive performance of the OSIM samples. Hence, a conclusion can be safely drawn that the simultaneous reinforcement and toughening of HDPE/UHMWPE blends with effectively improved wear resistance have been successfully achieved as expected through manipulating microstructure.

Fig. 1 Mechanical properties and wear resistance of CIM and OSIM samples and compression molded (CM) UHMWPE: (a) tensile strength, (b) tensile modulus, (c) elongation at break, (d) impact strength and (e) wear rate.

3.2 WAXD and SAXS results

To deeply understand the superior properties of the OSIM PE blend samples, their microstructure was examined by WAXD and SAXS. Selected WAXD patterns of the CIM samples and OSIM samples from different layers are shown in Fig. 2, where the reflection signal from inner to outer circles are assigned to the (110) plane and (200) plane of polyethylene orthorhombic crystals, respectively.³⁰ For CIM samples, two arc-like strong diffraction reflections appear in the skin layer, which is the signal of oriented crystalline structure; while the isotropic circles are observed in the intermediate and core layers, indicating a random orientation. In contrast, the strong anisotropic diffraction arcs focused in the equatorial direction appear in both the skin and intermediate layers of OSIM samples. The phenomena accounts for the shear flow existing in OSIM processing which effectively facilitates formation of oriented crystalline superstructure, even in the intermediate layer where generally no molecular orientation exists in the CIM samples.¹⁹ The degree of orientation for CIM and OSIM samples is calculated as listed in Table 1. For CIM samples, there is moderate molecular orientation (degree of orientation is 0.66 to 0.75) in the skin layer. During the mold filling, the polymer melt which initially contacts the cool mold surface is quickly consolidated, resulting in the orientation of the skin layer. Whereas the orientation for OSIM samples is very high in the skin layer and reaches the highest in the intermediate layer, which should be ascribed to the effect of oscillation shear flow provided by OSIM. We speculate it is exactly this oriented structure which brings about enhanced mechanical properties.

Fig. 2 2D-WAXD patterns of neat HDPE (a and a′), 10 wt% HDPE/UHMWPE blends (b and b′), 20 wt% HDPE/UHMWPE blends (c and c′) and 30 wt% HDPE/UHMWPE blends (d and d′) of CIM samples (a, b, c and d) and OSIM samples (a′, b′, c′ and d′).

Table 1 Oriented degrees of various layers of CIM samples (a) and OSIM samples (b) obtained by 2D-WAXD

CIM samples
Depth from surface (μm)	UHMWPE content (wt%)
	0	10	20	30
100	0.712	0.669	0.752	0.709
1200	0	0	0	0
3000	0	0	0	0

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OSIM samples
Depth from surface (μm)	UHMWPE content (wt%)
	0	10	20	30
100	0.753	0.817	0.823	0.841
1200	0.923	0.935	0.957	0.963
3000	0.269	0.273	0.281	0.296

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Fig. 3 shows selected SAXS patterns of CIM and OSIM samples from skin to core layer. The skin layers of all CIM samples show meridional maximum without any signal of equatorial streak, intermediate and core layers only exhibited the diffused scattering feature. The skin and intermediate layers of OSIM neat HDPE also just present meridional maximum, while both equatorial streak and meridional scattering maxima appear in the skin layers of OSIM PE blend samples. The equatorial streak verifies the existence of shish structure parallel to the flow direction.³⁷ The meridional scattering maxima refers to the kebabs, which grow perpendicularly to the shish axis through the folded-chain crystallization process. The difference between SAXS patterns of the above samples suggests shear flow during OSIM processing and long-chain component, including UHMWPE chains and the high molecular weight fraction in HDPE, are essential for the formation of shish-kebab structure. As reported, when the concentration of long chains is above a threshold (overlapping concentration), entanglements of long chains and the “relatively stable” entanglement points would be obtained under a given shear rate.^37–40 The stable entanglement points formed by long chains play a crucial role in the formation of shish structure. In the present work, the content (2 wt%) of UHMWPE dispersed in HDPE matrix of the PE blends at molecular level is significantly higher than the overlapping concentration, c* (0.2 wt%), therefore, the “relatively stable” entanglement points could be formed which would suppress the relaxation of the whole stretched network after the shear flow provided by OSIM process. As to the intermediate layer, all the OSIM samples exhibit meridional scattering maxima, which is the signal of oriented arrangement of PE lamellae, while no trace of meridional maximum for CIM samples. This is also due to molecular orientation and the formation of oriented crystalline in the intermediate layer of OSIM PE blends induced by the applied oscillation shear. Finally, one can distinctly identify isotropic scattering in the core layer of both CIM and OSIM samples, indicating a random lamellae orientation. In the core region, the slow solidifying rate leaves a sufficient time for oriented molecules (or network) to relax, leading formation of spherulites instead of shish-kebab structure.¹⁹

Fig. 3 2D-SAXS patterns of neat HDPE (a and a′), 10 wt% HDPE/UHMWPE blends (b and b′), 20 wt% HDPE/UHMWPE blends (c and c′) and 30 wt% HDPE/UHMWPE blends (d and d′) of CIM samples (a, b, c and d) and OSIM samples (a′, b′, c′, and d′).

3.3 SEM results

To further ascertain the crystalline structure of these samples clearly, the amorphous phase was chemically etched and extracted out from cryo-fracture surface by using the mixing acid solution. Typical SEM images of crystalline structure from skin to core layer for the CIM and OSIM samples of neat HDPE and PE blends containing 30 wt% UHMWPE are present in Fig. 4. One observes the spherulite-like structure along the whole sample thickness of CIM neat HDPE. 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. For the CIM PE blends, randomly arranged lamellae in the skin layer and spherulite-like structure in intermediate and core layer (Fig. 4a and b) are apparent. This result implies the long molecular chains (UHMWPE chains) alone cannot induce the formation of shish-kebabs under quiescent crystallization conditions. It is interesting to find in the skin layer of CIM PE blends that PE lamellar and UHMWPE lamellar penetrate into each other (see the region circled by the rectangle in Fig. 4b), which provides a strong interfacial adhesion. In the intermediate and core layer of CIM samples, increased temperature and sufficient relax time of molecular chains cause random arranged lamellae.

Fig. 4 SEM micrographs of neat HDPE (a and a′) and 30 wt% HDPE/UHMWPE blends (b and b′) of CIM samples (a and b) and OSIM samples (a′ and b′). The shear flow direction is vertical.

Fig. 4a′ and 4b′ show the SEM images of the etched OSIM samples, respectively. As shown in Fig. 4a′, the loose lamellae assemble perpendicular to the flow direction and also some twisted growths of lamellae exists at the skin layer of OSIM neat HDPE, highly arranged lamellae along the flow direction in the intermediate layer and cluster-like crystalline with randomly distributed lamellae in the core. Compared with CIM neat HDPE, the strengthened orientation of OSIM neat HDPE should undoubtedly be attributed to the intense shear provided by OSIM. In the whole packing stage of OSIM, continuous and reciprocal shear was exerted to melt in the cavity during solidification and crystallization, which is more beneficial for profound molecular orientation.⁴¹ It is inspiring to see that the oriented crystals detected by SAXS in the skin layer of OSIM PE blends are indeed shish-kebabs as shown in Fig. 4b′. In this layer, the shishes are aligned along the flow direction on which the oriented lamellae (kebabs) epitaxially grow.⁴² Similar to the shish-kebabs observed by Hobbs, et al. using atomic force microscopic (AFM), some kebabs in this layer are also tortuous rather than straight.^43,44 Moreover, large amounts of typical shish-kebab structure are also distinctly seen in the intermediate layer (as examples, some are indicated by arrows in Fig. 4b′), whereas the above SAXS result shows no sign of equatorial streak in the intermediate layer of OSIM PE blends. This deviation is probably caused by the different detection scale of SEM and SAXS measurement.¹⁶ The above observation clearly indicates both high molecular weight species (UHMWPE) and shear flow play important roles in inducing the formation of shish-kebab structure.

3.4 DSC results

Fig. 5 shows DSC heating curves of CIM and OSIM samples. Only one obvious endothermic peak appears for all CIM samples (Fig. 5a), indicating only one crystal structure exists in the three layers for each CIM sample. It can be clearly seen that the melting peak occurs at a higher temperature for CIM PE blend samples (around 133.5 °C) than neat HDPE (131 °C). The addition of UHMWPE causes higher melting point for PE blends.

Fig. 5 DSC heating curves of (a) CIM samples and (b) OSIM samples.

For OSIM samples, an interesting finding is that with addition of UHMWPE, two peaks appear in the heating curves of PE blends. Considering only one melting peak in the heating curves of CIM samples, we can safely deduce that the existence of shish structure in OSIM PE blends gives rise to the higher temperature peak. Moreover, the intensity of the higher melting point peak becomes stronger with increasing the UHMWPE content (Fig. 5b), implying molecular orientation is increased. However, shear frequency remains the same in all the OSIM PE blends. Then, what caused variation tendency of molecular orientation? The only reasonable explanation seems to be that incorporation of UHMWPE amplified the effect of shear flow, which is more effective in enhancing the molecular orientation and facilitating the formation of more oriented crystals. Different from the OSIM PE blends, there is only one melting peak for OSIM neat HDPE, indicating the absence of shish structure, which is well consistent with the SEM observation. Furthermore, compared to the CIM samples, the melting temperatures of OSIM samples are higher at the same content of UHMWPE. Oscillatory shear inducing thicker lamellae is responsible for higher melting point of OSIM samples.

The crystallinity of these samples estimated from the DSC curves is listed in Table 2. The crystallinity increases with the UHMWPE content for both CIM and OSIM samples. Moreover, compared to the CIM samples, the crystallinity of OSIM samples at the same content of UHMWPE shows an increase. As reported, the shear flow is favorable for forming oriented nuclei in a lower free energy through reducing the entropy of polymer molecular chains by orienting polymer molecules, thus enhancing the crystallization process of OSIM samples.^45–47 As mentioned above, the incorporation of UHMWPE amplified the shear flow and the amplification effect of shear strengthens with the increase content of UHMWPE in PE blend. From the above results, it can be concluded that the shear flow applied in OSIM process and the existence of UHMWPE phase exhibited a synergistic interaction, which promote the kinetics of the crystallization process and thus increase the crystallinity of OSIM samples.

Table 2 Crystallinity of CIM and OSIM samples

Crystallinity (%)	UHMWPE content (wt%)
	0	10	20	30
CIM samples	56.3	62.0	68.6	71.5
OSIM samples	63.7	67.0	77.0	78.3

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3.5 UHMWPE-induced amplification effect of shear flow

The above results demonstrated that our approach can successfully achieve improved balance between strength and wear resistance of HDPE/UHMWPE blends. The formation of shish-kebab structure contributed to the enhancement of mechanical strength, while the addition of UHMWPE significantly increased the wear resistance of HDPE/UHMWPE blends.

Finally, we would like to clarify why mechanical properties of OSIM PE blend samples are more enhanced with increasing UHMWPE content. As far as we know, the mechanical properties of UHMWPE itself are not much better than those of oriented HDPE. However, in this work the mechanical properties of OSIM PE blends were constantly enhanced with the increase proportion of UHMWPE. To answer this question, it needs to elucidate the influence of UHMWPE on structure and morphology formation under shear flow.

There is no doubt that the existence of UHMWPE phase makes the shear-induced crystallization behavior much more complicated than that in neat HDPE. It is conceivable that the UHMWPE phase in the HDPE matrix amplifies the effect of shear flow, which would result in more pronounced molecular orientation.^48,49 First, the exceptionally high melt viscosity of UHMWPE assumes a gel state even at high temperature, making it just deform and hardly flow under the shear field, which causes flow velocity difference between UHMWPE phase and HDPE melt and finally results in the so called local shear amplification. Then, during flow of the OSIM PE blend, UHMWPE with the gel state occupies the flow channel which decreases the effective space for the hot melt to pass through. In the blend system containing larger content of UHMWPE, when subjected to high pulse shear stress at a set frequency, the hot melt is forced to cross over narrower channel in a fixed time, thus, the flow rate of the melt is speeded. Therefore, a great enhancement in local stress was evident in the interfacial region between HDPE and UHMWPE, resulting in more distinct molecular orientation. Also, the amplification effect increases with the amount of UHMWPE added into HDPE. The two factors exhibit a synergistic effect on the formation of shish-kebab self-reinforced structure, which further improve the properties of OSIM PE blends with higher content of UHMWPE. Shear amplification is an important concept that was firstly proposed by Cakmak et al. to explain the enhanced orientation of injection molded nylon 6/clay nanocomposite.⁴⁸ Considering much higher orientation for the composites than that for pure iPP, Fu et al. verified once again that shear amplification indeed existed in iPP/clay nanocomposites.⁵⁰ In our previous work, we observed an amplification effect of shear on the shear-induced row nuclei and orientation of isotactic polypropylene crystals under the existence of shear flow and nanoparticles.^49,51

The microstructure evolution of neat HDPE and HDPE/UHMWPE blends under the applied shear flow should be as follows. Under shear effect, orderly arranged but relatively sparse lamellae are induced in neat HDPE, because HDPE chains used in this study are short. In HDPE/UHMWPE blends, long chains in HDPE matrix are forced to align along the flow direction, acting as shishes, and the lamellae perpendicularly crystallize on shish upon an epitaxial growth mode, forming shish-kebab microstructure. As mentioned above, UHMWPE phase in HDPE/UHMWPE blends amplifies the effect of shear flow, resulting in more pronounced molecular orientation and eventually leading formation of higher content of shish-kebab microstructure.

4 Conclusions

In this work, OSIM PE blend with remarkably improved mechanical properties and wear resistance has been successfully achieved, which endows itself with prolonged life-span, especially in the occasion required high strength and wear resistance. The combined effect of UHMWPE and shear flow on the formation of crystal structure of HDPE/UHMWPE blends was carefully investigated. Rich self-reinforced shish-kebab structure was induced under the coexistence of shear flow and UHMWPE, which significantly facilitated enhancement of mechanical properties of OSIM PE blends. For example, the tensile strength rises for CIM neat HDPE from 26.4 to 68.5 MPa for OSIM PE blends containing 30 wt% UHMWPE. The same trend was observed for impact toughness, where the impact strength increased from 6.3 to 34.1 kJ m⁻². The addition of UHMWPE also efficiently increased wear resistance of PE blends, where the wear rate of OSIM PE blend containing 30 wt% UHMWPE was reduced to 7.6 mg MC⁻¹ compared to 22.1 mg MC⁻¹ for CIM neat HDPE. Moreover, comprehensive properties of the OSIM samples were constantly enhanced with the increase of UHMWPE content though UHMWPE itself does not own much better mechanical properties than the oriented HDPE, which should be ascribed to amplification effect on shear brought out by UHMWPE. More amount of UHMWPE would result in more pronounced molecular orientation, contributing to the formation of more stable oriented structure with higher quantity. It is worth mentioning that shear application effect is very interesting and important, which opens up a door for us toward achieving more highly oriented structure in polymer products. More importantly, our exploration suggests a promising routine to achieve PE blends with superior performance.

Acknowledgements

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (50925311, 51033004, 51273131). This work was subsidized by the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (KF201202).

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