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Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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Tailored Nanostructured HDPE Wax/UHMWPE Reactor Blends as Additives for Melt-Processable All-Polyethylene Composites and in Situ UHMWPE Fiber Reinforcement Daniel Hofmann, Alexander Kurek, Ralf Thomann, Jeremia Schwabe, Stefan Mark, Markus Enders, Timo Hees, and Rolf Mülhaupt* Freiburg Materials Research Center FMF and Institute for Macromolecular Chemistry, Albert-Ludwigs-University Freiburg, Stefan-Meier-Str. 21, Freiburg D-79104, Germany S Supporting Information *

ABSTRACT: Tailored polyethylene reactor blend additives (RB) with ultrabroad bimodal molar mass distributions comprise nanophase-separated ultrahigh molar mass polyethylene (UHMWPE) uniformly dispersed in polyethylene wax. During injection molding of high-density polyethylene (HDPE) together with variable amounts of the nanophase-separated HDPE wax/ UHMWPE (70/30) additive (RB30) flow-induced oriented crystallization affords shish-kebab fiber-like UHMWPE nanostructures accounting for efficient HDPE self-reinforcement. RB additives are readily prepared by ethylene polymerization on silica-supported two-site chromium catalysts which simultaneously produce HDPE wax together with disentangled nanoplateletlike UHMWPE. The presence of HDPE wax is essential for lowering melt viscosity at high UHMWPE content. Since HDPE wax crystallizes onto extended-chain UHMWPE shish to form kebab structures, high HDPE wax content is tolerated without encountering emission problems and impairing mechanical properties as observed in the absence of UHMWPE. This in situ reinforcement substantially improves HDPE toughness/stiffness/strength balance as reflected by simultaneously increased Young’s modulus (+365%), tensile strength (+392%), and impact resistance (+197%). The performance of self-reinforced polyethylene (PE-SRC) is far superior to that of melt-blended UHMWPE/HDPE and the majority of PE nanocomposites. Neither hazardous UHMWPE nanoparticles nor alien inorganic nanofillers are required.

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INTRODUCTION Among polymeric materials, ultrahigh molecular weight polyethylene (UHMWPE) with average molecular mass exceeding 106 g mol−1 is well-known to exhibit extraordinary toughness combined with abrasion and fatigue resistance, lubrication, strength, and biocompatibility.1−3 However, massive entanglement of UHMWPE accounts for high melt viscosity far beyond the range tolerated by classical injection molding of high-density polyethylene (HDPE). Hence, special processing such as gel spinning, sintering, ram extrusion, and compression molding is required to fabricate UHMWPE fibers, coatings, moldings, and components of artificial hips.2−9 In the past numerous attempts have failed to produce HDPE/ UHMWPE blends combining UHMWPE performance with facile HDPE processing by classical injection molding or extrusion, respectively.10−17 In conventional melt-compounding micrometer-sized UHMWPE particles do not fully melt during short holdup times typical for HDPE injection molding and extrusion. Hence, in melt blending UHMWPE is dispersed as micrometer-sized filler within the HDPE matrix. Moreover, conventional mixing of UHMWPE and HDPE is accompanied by massive increase of melt viscosity owing to UHMWPE entanglement. According to Boscoletto and co-workers, less © XXXX American Chemical Society

than 3 wt % UHMWPE is dissolved within the HDPE matrix. Albeit small amounts of UHMWPE serve as tie molecules improving fatigue resistance of HDPE pipes, such low UHMWPE content is insufficient to significantly improve toughness, tensile strength, and abrasion resistance.10−17 Going well beyond the scope of classical HDPE injection molding, special processing technologies have emerged enabling fabrication of self-reinforced all-polyethylene composites (PE-SRC) in which the HDPE matrix is reinforced with fiber-like UHMWPE nano- and microphases.18−20 Typically, the in situ formation of fiber-like extended-chain UHMWPE is governed by the polyethylene molar mass distribution. Upon melt processing near the HDPE melting temperature extensional-flow- and shear-induced crystallization afford extendedchain high molar mass polyethylene as shish which nucleates PE crystallization to form shish-kebab fiber-like structures.19,21,22 In the past, PE-SRC fabrication required special compounding techniques for enabling nanophase separation and dispersion of aligned UHMWPE chains by applying Received: September 2, 2017 Revised: October 5, 2017

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DOI: 10.1021/acs.macromol.7b01891 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Preparation of the Nanostructured HDPE Wax/UHMWPE (70/30) Reactor Blend (RB30) by Coimmobilization of CrBIP and CrQCp on Mesoporous MAO-Activated Silica NF70 Prior to Ethylene Polymerization

oscillating shear fields and extensional flow by means of extrusion using convergent dies or by shear-assisted injection molding, among them shear controlled orientation injection molding (SCORIM), oscillation shear injection molding (OSIM), and dynamic packing injection molding (DPIM).23−29 In addition to high shear rates and straining, especially UHMWPE addition was reported to promote shish formation of extended-chain UHMWPE nanostructures, thus markedly improving PE-SCR performance.30−43 For instance, Xu et al. reported on enhancing mechanical properties by means of OSIM processing of HDPE in the presence of 40 wt % UHMWPE.42,43 Improvements of tensile strength (+184%), Young’s modulus (+39%), and impact resistance (+46%) was achieved in parallel and vertical sample orientation, though property improvements perpendicular to injection direction appeared less pronounced with respect to compression-molded UHMWPE.42 Furthermore, Shen and co-workers added low molar mass PE (HDPE wax) as a third component to HDPE and UHMWPE processed by means of DPIM.37 Owing to improved melt flow obtained in the presence of 9 wt % PE wax, the tensile strength of the corresponding polyethylene blends increased up to +385% with respect to HDPE. Obviously, while the flow-induced alignment of UHMWPE promotes shish formation of extended-chain UHMWPE, HDPE wax addition lowers melt viscosity and facilitates processing which is the prerequisite for improving mechanical properties via selfreinforcement in the presence of high UHMWPE content. Hence, nanophase-separated UHMWPE dispersed in HDPE wax holds great promise as high-performance additive for HDPE and other polymer materials reinforced with in situ formed extended-chain UHMWPE. Owing to problematic melt compounding of entangled UHMWPE in conjunction with health and safety hazards associated with handling of nanometer-scaled UHMWPE powders, it is imperative to explore new approaches toward tailoring nanophase-separated HDPE wax/UHMWPE blends. In state-of-the-art manufacturing of bimodal HDPE resins for pipe extrusion, less than 3 wt % UHMWPE is incorporated into HDPE by means of reactor blend formation using cascade reactors.44−51 Today blending together different single site catalysts on the same support produces melt-processable nanophase-separated HDPE/ UHMWPE and even HDPE/UHMWPE/PE wax reactor blends with high UHMWPE content in a single reactor.50−57 For instance, unprecedented control of bi- and trimodal ultrabroad polyethylene molar mass distributions is achieved by means of ethylene polymerization on multisite catalysts in which quinolylsilylcyclopentadienylchromium (CrQCp) produces UHMWPE unaffected by the presence of other single site catalysts producing HDPE or HDPE wax, respectively. Hence,

the UHMWPE content of reactor blends (RB) is readily controlled by varying the CrQCp content of supported multisite catalyst systems without affecting the average molar masses of individual polyethylene fractions produced on different sites. Moreover, nanophase separation of UHMWPE during ethylene polymerization affords nanometer-scaled UHMWPE uniformly dispersed within HDPE or HDPE/ HDPE wax, respectively.50−58 Since nanophase-separated UHMWPE is an integral part of the entire polyethylene molecular architecture, no UHMWPE nanoparticles are formed. Unlike micrometer-sized UHMWPE, however, nanometer-scaled UHMWPE readily melts during injection molding. Opposite to conventional reactor blends with high UHMWPE content, owing to drastically reduced entanglement of nanophase-separated UHMWPE, much higher UHMWPE content is tolerated in classical HDPE injection molding producing PE-SRC with in situ shish-kebab extended-chain UHMWPE nanostructures resulting from flow-induced crystallization.52 While large amounts of HDPE wax causes severe embrittlement and emissions in the absence of UHMWPE, the incorporation of HDPE wax into RB lowers melt viscosity, thus enabling to increase UHMWPE content paralleled by improved toughness/stiffness/strength balance of the corresponding PESRCs.53 Since HDPE wax is incorporated into shish-kebab fiber-like structures via cocrystallization, considerably less emission problems are encountered in the presence of nanometer-scaled UHMWPE. However, to date tailoring PESRCs requires designing of multisite catalysts for ethylene polymerization in which the reactor blend properties are governed by catalyst compositions. In view of sustainable PESRC development with in situ formation of extended-chain UHMWPE nanostructures as reinforcing phases, it is highly desirable to explore melt compounding of new polymer reactor blend additives with ultrabroad molar mass distribution comprising UHMWPE nanostructures embedded in HDPE wax. Herein we report on tailored nanophase-separated HDPE wax/UHMWPE reactor blends (RB30) containing 30 wt % UHMWPE which are used as additives for producing PE-SRC by means of HDPE injection molding without requiring variation of HDPE polymerization processes. The influence of RB30 additive content on PE-SRC morphology development, especially in situ formation of UHMWPE fiber-like structures, as well as thermal and mechanical properties is elucidated.

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EXPERIMENTAL SECTION

Materials. HDPE was provided by LyondellBasell (Hostalen GC7260; MFR 23 g 10 min−1, 190 °C, 2.16 kg). Commercial UHMWPE was supplied by Celanese (GUR4120; MFR no flow, 190 °C, 2.16 kg). Toluene, n-heptane, and triisobutylaluminum (TiBAl, 1.0 B


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Macromolecules Table 1. Composition and Mw of PE Components Applied for PE-SRC and Reference Blend Fabrication material a,b

RB30 PEWa,c HDPEd GURe,g

HDPE wax (wt %)

Mwf (kg mol−1)

70 100

1.4 2.0

HDPE bulk (wt %)

100

Mwf (kg mol−1)

UHWMPE (wt %)

Mwf (kg mol−1)

30

3500

100

5000

77.0

Polymerization conditions: n-heptane = 0.6 L; TiBAl = 3.0 mL; TPol = 40 °C; tPol = 2 h; pethylene = 5 bar. bNanostructured HDPE wax/UHMWPE reactor blend, Σ(CrBIP + CrQCp) = 22.8 μmol L−1, average activity = 4 440 kg(PE) mol(cat)−1 h−1, average productivity = 450 g(PE) g(kat+support)−1, average polydispersity (PD) = 1200. cHDPE wax, Σ(CrBIP) = 9.5 μmol L−1, activity = 24 900 kg(PE) mol(cat)−1 h−1; productivity = 1890 g(PE) g(kat+support)−1. dCommercial HDPE, type Hostalen GC7260. eCommercial UHMWPE, type GUR4120. fWeight-averaged molecular mass, determined by HT-SEC in trichlorobenzene. gTechnical data sheet GUR4120. a

Scheme 2. PE-SRC Fabrication by Means of in Situ Shish-Kebab Nanofiber Formation during Melt Compounding of HDPE Together with Nanostructured RB30 Exploiting Extensional-Flow Mediated UHMWPE Alignment

M in hexane) were purchased by Sigma-Aldrich. Toluene and nheptane were further purified using a Vacuum Atmospheres Co. purifier prior to use. Methylaluminoxane (MAO, Crompton, 10 wt % in toluene) and ethylene (3.0, Air Liquide) were used without further purification. RB30 Synthesis. The synthetic route to the HDPE wax/ UHMWPE reactor blend (RB30) containing 30 wt % UHMWPE is illustrated in Scheme 1. All experiments were carried out under an argon atmosphere by standard Schlenk, vacuum, and glovebox techniques. Supported twosite chromium catalyst were prepared by immobilizing a blend of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine chromium(III) chloride (CrBIP) and dichloro-η5-[3,4,5trimethyl-1-(8-quinolyl)-2-trimethylsilylcyclopentadienyl]chromium(III) (CrQCp) on methylaluminoxane (MAO)-activated mesoporous silica support (NF70), as reported previously.55−57 The silica nanofoam NF70 (average pore diameter 70 nm; BET surface area 787 m2 g−1) and CrBIP were obtained according to procedures as reported elsewhere in detail.57,59,60 CrQCp was provided by the group of M. Enders by Dr. S. Mark, University of Heidelberg.61 Typically, 270 mg of NF70 was in situ activated by adding MAO (10 wt % in toluene, 7 × 10−3 L g−1) and subsequent stirring (45 min) at room temperature. After sedimentation, the activated catalyst support was washed twice (toluene, 10 mL) prior to catalyst immobilization using CrBIP (6.0 mg, 11.4 μmol) and CrQCp (1.0 mg, 2.3 μmol) solutions in toluene (10 mL). For this purpose, toluene solutions of CrBIP and CrQCp were added by syringe consecutively and stirred for 5 min. The supernatant liquid was removed. Then the immobilized catalyst

system was dispersed in n-heptane (10 mL) and transferred into the polymerization reactor by means of a syringe. In order to ensure a complete addition of the entire catalyst particles, this procedure was repeated once. Polymerization was carried out in a 2 L double jacket steel reactor (multipurpose polymerization reactor MPPR II), connected with a thermostat (PT 100, Julabo FP 50), mass flow meters (Bronckhorst), and electromagnetic valves (Bürkert) and equipped with mechanical stirring. MPPR II was filled with n-heptane (580 mL), and TiBAl (3 mL) was added as a scavenger prior to injection of the activated catalyst system. Polymerization was carried out for 2 h at 5 bar ethylene pressure at 40 °C and a stirring speed of 450 rpm. The product was filtered and dried to constant weight (65 °C, 20 mbar). Finally, reactor blend masterbatch fabrication was achieved by combining 12 reactor blend batches, each containing 30 wt % nanoplatelet-like UHMWPE embedded within 70 wt % HDPE wax (total yield of RB30 = 1.46 kg). Regarding the preparation of merely physically mixed reference blends, HDPE wax (PEW) was synthesized as described by immobilizing only CrBIP (3.0 mg, 5.7 μmol) on NF70 (150 mg) prior to ethylene polymerization (yield of PEW was 283.5 g). Table 1 summarizes compositions and molar masses (Mw) of all PE components. Fabrication oF PE-SRCs. As is illustrated in Scheme 2, PE-SRC fabrication was achieved by HDPE/RB30 melt extrusion and subsequent injection molding. Typically, HDPE was milled (Retsch ZM 200, Retsch GmbH, Haan, Germany; grit size ≤1 mm) prior to physical premixing RB30 together with a blend of stabilizers (0.1 wt %, Irganox1010, Irgafos168, 1:1; BASF Schweiz AG). Melt compounding C


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fractions were removed by cautious dipping in hot xylene (≤130 °C), followed by coating the exposed structures with Ag prior to imaging (Polaron sputter coater SC 7640, layer thickness ≤30 nm). The melt flow index (MFI) of extruded granules was determined with a Meltflixer LT (Coesfeld Materialtest, Dortmund, Germany; 190 °C, 11.66 kg). The thermal degradation of blends was examined by thermogravimetric analysis (TGA) using a STA 409 (Netzsch, Selb, Germany; N2 atmosphere, 10 K min−1). Stress/strain measurements were performed on a Zwick Z-005 (Zwick Roell, Ulm, Germany (specimen type 5A)). All tensile measurements were carried out according to DIN EN ISO 527-1/-3, yet at a clamping chucks distance of 40 mm in order to prevent a frequently occurring pull-out of the entire samples. Notched Izod impact strength was investigated on a Zwick pendulum in accordance with DIN EN ISO 180. Average particle size and particle size distribution were determined with a LA-950 laser particle size analyzer (Horiba) equipped with a He−Ne laser (λ= 632.8 nm) using EtOH as solvent.

of the individual samples was conducted on a corotating twin-screw microcompounder Xplore (DSM, Heerlen, Netherlands) at Tbarrel = 200 °C, a screw speed of 120 rpm, and an average hold-up time of 3 min. The subsequent microinjection molding (Xplore, DSM) was carried out at Ttransfer = 200 °C, Tmold = 60 °C, and pmax = 9 bar in order to prepare tensile test and impact test specimens. Table 2

Table 2. Preparation of PE-SRC Together with HDPE/ UHMWPE Reference Blends by Means of Melt Compounding compound HDPE 10-RB30 20-RB30 30-RB30 40-RB30 21-PEW 9-GUR 21/9-PEW/ GUR

composition Hostalen GC7260 HDPE + RB30 HDPE + RB30 HDPE + RB30 HDPE + RB30 PEW + HDPE HDPE + GUR PEW + HDPE + GUR

HDPE wax (wt %)

HDPE (wt %)

UHMWPE (wt %)

100 7 14 21 28 21 21

90 80 70 60 79 91 70

3 6 9 12

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RESULTS AND DISCUSSION The key intermediate for producing self-reinforced all-polyethylene composites (PE-SRC) by injection molding of highdensity polyethylene (HDPE) is the nanostructured reactor blend additive (RB30) having ultrabroad bimodal molar mass distribution and containing 30 wt % nanophase-separated UHMWPE. As illustrated in Scheme 1, micrometer-sized RB30 powder is readily available by ethylene polymerization on twosite chromium catalysts supported on mesoporous silica tethered with methylalumoxane (MAO) prior to immobilization of the two different chromium complexes. While MAOactivated 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine chromium(III) chloride (CrBIP) produces HDPE wax, nanophase-separated disentangled UHMWPE dispersed within the HDPE wax matrix is formed on MAO-activated dichloro-η5[3,4,5-trimethyl-1-(8-quinolyl)-2-trimethylsilylcyclopentadienyl]chromium(III) (CrQCp). In accordance with previous reports in the literature and verified by the HT-SEC trace displayed in Figure 1 (left), RB30 exhibits an ultrabroad bimodal molar mass distribution with polydispersity (Mw/Mn) around 200 combining HDPE wax (1.4 × 103 g mol−1) with

9 9

summarizes the individual sample compositions and the corresponding denotations whereas the Supporting Information provides a detailed overview of applied processing parameters (Tables S1 and S2). Material Characterization. The molecular weight distribution of PE components was determined by high-temperature size exclusion chromatography (HT-SEC) on a PL-220 chromatograph (Polymer Laboratories) equipped with a differential refractive index detector (DRI) and a differential viscometer 210 R (Viscotek). PE was referred to as UHMWPE at Mw ≥ 106 g mol−1. Measurements were performed at 150 °C with three PLGel Olexis columns in 1,2,4-trichlorobenzene (Merck), stabilized with 0.2 wt % of 2,6-di-tert-butyl-(4-methylphenol) (Aldrich) as solvent at constant flow rate (1.0 mL min−1). Columns were calibrated using 12 PS standards of a narrow MWD. The morphology of RB30 particles as well as shish-kebab orientation and fracture surfaces within PE-SRCs was investigated by scanning electron microscopy (SEM) on a Quanta FEG 250 (FEI, Eindhoven, Netherlands, accelerating voltage 20 kV). Soluble PE

Figure 1. Nanostructured UHMWPE/HDPE wax reactor blend RB30: Ultrabroad bimodal molar mass distribution (left) as determined by size exclusion chromatography, particle morphology as determined by photographic imaging (above), and particle size distribution (right) as determined by means of laser diffraction. D


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Figure 2. SEM images of compacted polyethylene powders after extracting low molar mass PE with hot xylene: RB30 (above) showing UHMWPE nanoplatelet-like morphology (a, b, c) in comparison to polyethylene with monomodal molar mass distribution (below) such as commercial UHMWPE (d, e) and HDPE (f, g).

Figure 3. Melt flow index (MFI) of HDPE/RB30 blends as a function of the RB30 content (left) and thermogravimetric analysis of melt extruded HDPE and 30-RB30 as compared to neat RB30 (right); MFI determination was performed by applying 11.66 kg load at 190 °C; TGA was carried out at 10 K min−1 under a N2 atmosphere.

UHMWPE (3.5 × 106 g mol−1).52,53,55−57,60,62 At the CrBIP/ CrQCp molar ratio of 4.9, 30 wt % UHMWPE is uniformly dispersed in HDPE wax. The particle size distribution of virgin RB30 powder was examined by laser diffraction. As is apparent from Figure 1 (right), virgin RB30 powder exhibits a bimodal particle size distribution with an average particle size of 95 μm. According to the particle size analysis, no particles smaller than 10 μm were detected. In order to investigate RB30 morphology by means of scanning electron microscopy (SEM), the micrometer-sized RB30 particles were compacted at room temperature followed by extraction of HDPE wax with hot xylene. After removing HDPE wax, the remaining UHMWPE exhibited a porous assembly of nanoplatelet-like UHMWPE structures (see Figure 2a−c) similar to those reported by Rastogi and co-workers for disentangled UHMWPE.63−65 Albeit UHMWPE nanostructures resemble those of nanoplatelets, it should be noted that no UHMWPE nanoparticles are formed during polymerization. This is beneficial to melt processing because no safety precautions related to handling of

hazardous organic nanoparticles are required. Owing to the danger of electrostatic discharges, the handling electrically insulating UHMWPE nanoparticles poses severe explosion hazards. Interestingly, no UHMWPE nanostructures were detected after exposing compacted powders of commercial UHMWPE (Figure 2d,e) and HDPE (Figure 2f,g) to hot xylene. Melt Compounding RB30 with HDPE. Various amounts of the RB30 additive were melt compounded with HDPE using a twin-screw microextruder prior to injection molding. The influence of the RB30 content on the melt flow index (MFI) is displayed in Figure 3 (see also Table S3 in the Supporting Information). Because of their high melt viscosities, neither neat RB30 nor UHMWPE enables MFI measurement. In sharp contrast, up to 40 wt % RB30 (equivalent to 12 wt % UHMWPE of the resulting blend) can be added to HDPE without drastically impairing melt processing by extrusion and injection molding. The addition of 40 wt % RB30 to HDPE reduced the MFI of 96.0 g 10 min−1 for neat HDPE to 6.8 g 10 E


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Figure 4. SEM images of injection-molded HDPE/RB30 (30-RB30) blend containing 30 wt % RB30 (above) as compared to neat HDPE in the absence of UHMWPE (below); after removing low molar mass HDPE with hot xylene only RB30 compounds show shish-kebab fiber-like superstructures (a, b, c) whereas the surface of neat HDPE (d, e, f) was rough without any indication shish-kebab formation.

Table 3. Mechanical Properties of PE-SRCs Based on HDPE/RB30 Blends and Merely Physically Mixed PE/UHMWPE Reference Samples as a Function of HDPE Wax and UHMWPE Content compound

HDPE wax content (wt %)

UHMWPE content (wt %)

YMa (GPa)

Fmaxb (MPa)

HDPE 10-RB30 20-RB30 30-RB30 40-RB30 21-PEW 9-GUR 21/9-PEW/GUR

0 7 14 21 28 21 0 21

0 3 6 9 12 0 9 9

0.81 ± 0.04 1.16 ± 0.04 1.83 ± 0.11 2.67 ± 0.30 3.77 ± 0.73 0.69 ± 0.08 0.95 ± 0.0.1 0.76 ± 0.02

27.3 42.7 76.5 113.5 134.2 25.3 32.4 29.6

± ± ± ± ± ± ± ±

0.4 1.8 3.5 6.8 3.9 1.3 0.5 0.3

εc (%) 610 14 11 11 12 880 37 63

± ± ± ± ± ± ± ±

120 1 1 1 1 30 4 10

Izodd (kJ m−2) 3.6 6.6 9.5 10.7 10.0 1.4 7.4 3.4

± ± ± ± ± ± ± ±

0.1 0.2 0.4 0.9 0.6 0.1 0.1 0.6

a

Young’s modulus. bMaximum tensile strength. cElongation at break. dNotched Izod impact strength; please note that stress/strain data detection was performed at a clamping chucks distance of 40 mm due to a frequent pull-out of the entire samples at 50 mm though applying maximum contact pressure.

min−1. In order to assess emission and odor problems related to PE wax addition, the blends and individual components were examined by means of thermogravimetric analysis (TGA). The low molar mass HDPE wax is reported to decompose thermally at markedly lower temperatures with respect to HDPE, thus accounting for processing and emission problems.66 As is apparent from Figure 3 (right), the addition of 30 wt % RB30 (equivalent to HDPE wax content of around 20 wt %) did not adversely affect the thermal stability of HDPE. Moreover, also virgin RB30 containing 70 wt % HDPE wax did not cause emissions at processing temperatures up to 200 °C. Morphology. The morphologies of HDPE/RB30 compounds, particularly with respect to the PE-SRC superstructure formation, were examined by means of SEM imaging. It is wellknown that microfibrillar polyethylene shish fragments nucleate shish-kebab polyethylene crystallization in the melt prior to perpendicular epitaxial growth of kebabs.18−20,30,67,68 Moreover, adding UHMWPE promotes shish evolution by long chain alignment, in particular within oscillating shear field process-

ing.32−44 The influence of RB30 addition on morphology development was elucidated by analyzing the cross sections of injection-molded HDPE/RB30 tensile test bars and compared with neat HDPE. Both samples were etched with hot xylene in order to remove soluble HDPE wax as well as low molar mass HDPE fractions prior to SEM investigation of exposed surfaces along the most oriented region (see Figure 2), closely following procedures reported by Fu and co-workers.18 The resulting morphologies illustrated in Figure 4 display random sections located between sample skin and the core of 30-RB30 and HDPE, respectively (see also Figures S2 and S3). As can be seen from SEM images, injection molding of 30RB30 accounted for highly anisotropic in situ formation of shish-kebab fiber-like UHMWPE structures by orientation of aligned UHMWPE chains in the extensional flow field. The observed fiber structures exhibited broad structural varieties having up to 500 μm length (Figure 4a), while the diameters range from few hundred nanometers up to several micrometers (Figure 4b,c). The array of shish-kebab dimensions is thereby F


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Figure 5. Fracture behavior (above/left), exemplary stress/strain development (above/right), relative Young’s modulus (below/left), and relative tensile strength (below/right) of injection-molded PE-SRC samples as a function of the RB30 content.

Figure 6. Relative notched Izod impact strength of HDPE/RB30 samples as a function of RB30 content (above/left), 30-RB30 ductility, and top view of fracture surfaces illustrating HDPE and 30-RB30 (above/right); SEM images revealing the hierarchical fracture surface of 30-RB30 (below/ left) as compared to neat HDPE (below/right).

attributed to multimodal PE composition and locally different shear stress conditions during injection molding, which are

known to affect uniform coil−stretch chain transition and superlattice crystallization.28,30,35,36 In sharp contrast, injection G


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uniform surface of increased roughness without any indications of such structure formation. Hence, the increased impact resistance by RB30 addition was attributed to the coexistence of interconnected shish-kebab fiber reinforcement (intermediate layer) in addition to energy dissipating tie molecules (core). However, since SEM examination at higher resolution (cf. Figure S4) could not clarify the explicit mechanism in detail, its origin is still a matter of current investigation. As compared to recently published results, the increase in stiffness and strength of RB30-based PE-SRCs considerably surpassed that of OSIM-processed UMWPE blends reported by Xu and co-workers while exhibiting somewhat lower impact resistance.42,43 Moreover, 40-RB30 afforded slightly enhanced strength with respect to trimodal PE processed by means of DPIM, yet at higher UHMWPE content increased by +25%.37 However, as oscillating shear flow processes require auxiliary equipment in conjunction with larger noneconomic molding cycles, this RB-30-based PE-SRC fabrication by conventional melt extrusion and injection molding represents a significant progress. Moreover, the RB30 blends are far superior to binary and ternary melt blends of HDPE, HDPE wax (PEW), and UHMWPE (GUR) when using identical conditions and parameters with respect to processing of 30-RB30. The compositions and properties of reference blends are listed in Tables 1 and 2. As is apparent from Table 3 and Figure 7, the

molding of neat HDPE affords rough surfaces without any indications of such substructures or preferred texture orientations (Figure 4d−f). Mechanical Properties. All melt-processable PE-SRCs exhibit outstanding mechanical properties attributed to the enormous intrinsic load capacity of in situ formed shish-kebab fiber-like UHMWPE structures having excellent interfacial adhesion via cocrystallization with the HDPE matrix.18−20 While HDPE/UHMWPE sample orientation triggered by oscillating shear flow is well-known to yield extraordinary stiffness or ultimate strength, common injection molding of HDPE blends with micrometer-sized UHMWPE particles blends failed to produce PE-SRCs.10−17 The impact of RB30 addition on HDPE modulus, tensile strength, and elongation at break was investigated by means of stress/strain measurements. The results are summarized in Table 3, while Figure 5 displays fracture behavior, stress/strain curves, and the relative trends of Young’s modulus and tensile strength development as a function of the RB30 content. Obviously, the RB30 addition during melt-processing accounted for a striking increase of HDPE mechanical properties, as reflected by high PE-SRC upper load limits governed by the RB30 content (see Figure 5, right above). The incorporation of 40 wt % RB30 (40-RB30) required nearly 5-fold tensile stress in order to achieve identical strain with respect to HDPE. Moreover, Young’s modulus and maximum tensile strength simultaneously increased up to 3.77 GPa (+365%) and 134.2 MPa (+392%), respectively. The individual fracture behavior of 30-RB30 and 40-RB30 (Figure 5, left on top) revealed further stiffening at higher RB30 content, as sample fracture generally occurred well beyond the lower necking, and thus, the markedly larger cross section area of dumbbelled test specimens remained unconsidered within tensile test characterization. Yet, it should be noted that this enormous PE-SRC reinforcement is at the expense of elongation at break, in accord with observations for many other fiber-reinforced polymer composites (Figure 5, right above, and Table 3). However, despite the loss of elongation at break, HDPE/ RB30 samples were not rendered brittle but remained ductile, as demonstrated for 30-RB30 (see Figure 6). The influence of RB30 on PE-SRC toughness, as measured by notched Izod impact strength, was determined. Common UHMWPE/HDPE blends are well-known to enhance the fracture resistance of HDPE due to interconnecting crystalline matrix domains via UHMWPE tie molecules.13,17,44 Furthermore, PE-SRCs containg shish-kebab fiber-like UHMWPE structures were likewise proclaimed to surpass matrix toughness. Particularly, according to Hsiao and co-workers, interlocking multiple shish appears to be a convenient strategy toward producing lightweight engineering architectures and simultaneously improving stiffness and impact resistance.18−20,30,32,42,43 As can be seen from toughness measurements summarized in Table 3 and displayed in Figure 6, the addition of 30 wt % RB30 likewise affords remarkable toughening with notched Izod impact strength increasing up to 10.7 kJ m−2 (+197%). Obviously, higher RB30 content did not further improve tougheness which appeared to reach a plateau value. The obtained fracture surfaces were analyzed by means of SEM in order to gain more insight into RB30 toughening mechansims. The morphological comparison revealed PE-SRC-typical structural hierarchies for 30-RB30, which can be subdivided into a sample skin, a highly oriented intermediate layer, and the less anisotropic interior core (Figure 6, below/left).18,28 In sharp contrast, HDPE exhibited merely a

Figure 7. Mechanical properties of 30-RB30 as compared to binary and ternary reference blend samples containing identical amounts of HDPE, HDPE wax (PEW), and UHMWPE (GUR) but without nanometer-scaled RB30 predispersion.

addition of HDPE wax to HDPE (21-PEW) accounted for a drastic collapse of impact resistance whereas melt blending of HDPE with UHMWPE (9-GUR) caused higher toughness (+105%), in agreement with reports in the literature.12,13,17 However, in sharp contrast to 30-RB30, the ternary mixture containing identical amounts of HDPE wax, HDPE, and UHMWPE (21/9-PEW/GUR) entirely failed to improve mechanical properties. Moreover, no shish-kebab fiber structures and self-reinforcement by UHMWPE alignment were observed. In fact, mircrometer-sized UHMWPE failed to melt. Clearly, this experimental result emphatically confirmed that conventional physical mixing of micrometer-sized PEW wax, UHMWPE, and HDPE in an extruder fails to produce in situ shish-kebab fiber-like UHMWPE structure formation by flow-induced oriented polyethylene crystallization. Additional experimental evidence resulted by comparing DSC analyses of H


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Macromolecules

Figure 8. Comparison of mechanical property profiles (left) and stiffness/toughness balance (right) of PE-SCRs composed of HDPE and RB30 as compared to neat HDPE.

components. This outstanding PE-SRC performance is attributed to the coexistence of energy dissipating tie molecules and shish-kebab fiber-like superstructures of ultrastrong extended-chain UHMWPE formed by flow-induced crystallization during injection molding. The PE-SRC properties are readily tailored by varying the blend ratio of HDPE and RB, whereas comparable state-of-the-art PE-SRCs require designing multisite catalyst systems in which the ratio of different catalytic sites governs morphology and mechanical properies. As a consequence, melt compounding of HDPE with RB additives represents an attractive route to convert commodity HDPE into high performance all-PE composites. Moreover, RB-based all-polymer-composite technology goes well beyond current limitations in polyolefin development and holds attractive new prospects for tailoring next-generation sustainable lightweight engineering plastics exhibiting an attractive combination of cost-, energy-, eco-, and resource efficiency together with low carbon footprint and facile recycling. The sustainable development of nanostructured RB additives may enable the incorporation of in situ formed UHMWPE fiber-like nanostructures as reinforcing phases into a variety of other thermoplastic and elastomeric polymer matrices.

HDPE, 30-RB30, and 21/9-PEW/GUR (cf. Table S4 and Figure S5). Under identical conditions only 30-RB30, produced by melt compounding 30 wt % RB30 with HDPE, accounted for the nanophase separation and efficient dispersion of extended chain UHMWPE as reinforcing phase within the HDPE matrix.

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CONCLUSION All-polyethylene composites reinforced with in situ formed extended-chain UHMWPE fiber structures and simultaneously improved toughness/stiffness/strength balance are readily tailored by melt blending commodity HDPE with nanostructured polyethylene reactor blends (RB) having ultrabroad bimodal molar mass distributions. Opposite to conventional self-reinforced polyethylene (PE-SRC), no special processing methods, uneconomic processing conditions, prolonged cycle times, or three-site polymerization catalysis are required. In a single reactor ethylene polymerization on highly active supported two-site chromium catalysts yields melt-processable nanostructured PE reactor blends (RB30) as new polymer additive and masterbatch containing 70 wt % HDPE wax (1.4 × 103 g mol−1) together with 30 wt % uniformly dispersed nanophase-separated UHMWPE (3.5 × 106 g mol−1). Albeit UHMWPE superstructures, as evidenced by SEM imaging of compacted RB30 powder after extracting PE wax with hot xylene, resemble nanoplatelets, no hazardous UHMWPE nanoparticles are formed since UHMWPE is part of the entire PE structural hierarchies. Such nanometer-scaled UHMWPE dispersions embedded in HDPE wax were unfeasible by stateof-the-art compounding processes. While HDPE wax serves as lubricant and processing aid, nanoplatelet-like disentangled UHMWPE is uniformly dispersed, readily melts during injection molding, and forms shish-kebab fiber-like extendedchain UHMWPE reinforcing phases via shear- and extensionalflow induced crystallization. High amounts of HDPE wax lubricant are tolerated since HDPE wax cocrystallizes and is incorporated into the shish-kebab structures. Hence, as it is obvious from HDPE/RB30 property profiles displayed in Figure 8, the addition of up to 40 wt % RB30 affords significantly improved properties as reflected by the simultaneous increase of Young’s modulus (+365%), tensile strength (+392%), and toughness (+197%) with respect to HDPE. The PE-SRCs performance is far superior to that of conventional binary and ternary HDPE/UHMWE blends having the identical compositions but using micrometer-sized blend

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01891. Additional data on processing parameters, SEM analysis, MFI and TGA investigation as well as DSC examination results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax (+49) 761 203 6319; e-mail [email protected] (R.M.). ORCID

Markus Enders: 0000-0003-0415-1992 Timo Hees: 0000-0003-1238-0349 Author Contributions

D.H. and A.K. contributed equally. Notes

The authors declare no competing financial interest. I


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ACKNOWLEDGMENTS The authors kindly thank Dr. Shahram Mihan and LyondellBasell Polyolefine GmbH for material support. Further thanks are dedicated to the members of Freiburg Materials Research Center, in particular Andreas Warmbold and Marina Hagios for assistance in polymerization and polymer characterization. This work was funded by the German Federal Ministry of Education and Research (BMBF, project no. 03X3565C, “multiKAT”, and project no. 03XP0054C (“CATEFF”).

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