Novel Graphene UHMWPE Nanocomposites ... - ACS Publications

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Novel Graphene UHMWPE Nanocomposites Prepared by Polymerization Filling Using Single-Site Catalysts Supported on Functionalized Graphene Nanosheet Dispersions Markus Stürzel,*,† Fabian Kempe,† Yi Thomann,† Stefan Mark,‡ Markus Enders,‡ and Rolf Mülhaupt*,† †

Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, University of Freiburg, Stefan-Meier-Str. 31, D-79098 Freiburg, Germany ‡ Anorganisch-Chemisches Institut, University of Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany ABSTRACT: Novel families of ultrahigh-molecular-weight polyethylene (UHMWPE) nanocomposites, containing uniformly dispersed, functionalized graphene (FG) nanosheets, were prepared by means of the polymerization filling technique (PFT). Unparalleled by any other carbon and boehmite nanocomposites, FG/UHMWPE exhibited an unusual simultaneous improvement in stiffness, elongation at break, and effective nucleation of polyethylene crystallization at only 1 wt % FG content. FG nanosheets are ultrathinwith a thickness of only one carbon atom and lateral dimensions of several micrometers. Owing to the presence of surface hydroxyl groups on the FG, single FG/methylaluminoxane (MAO) nanosheets can be effectively dispersed in n-heptane, thus enabling immobilization of an MAO-activated chromium (Cr1) single-site catalyst on FG. In contrast to nanometer-scale carbon black (CB), multiwall carbon nanotubes (CNT), graphite, and nanoboehmite, which failed to form stable dispersions, FG/MAO/Cr1 afforded the highest catalyst activities and excellent morphological control. In polymerization filling, the integration of a nanoparticle dispersion into the polymerization process eliminated the need for special safety and handling precautions typically required by conventional compounding of nanoparticles with ultralow bulk densities.

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UHMWPE/GNP nanocomposite films that exhibit substantially improved fracture toughness and tensile strength at a very low GNP content of only 0.1 wt %.6 In conventional approaches toward UHMWPE nanocomposites, graphite oxide (GO) and chemically reduced GO were dispersed in diluents such as ethanol and then spray-coated onto UHMWPE powder prior to compression molding.7 For optimizing the material properties it is of great interest to implement nanofiller dispersion directly into ethylene polymerization to benefit from the low viscosity of the liquid polymerization media, thus preventing emission of nanoparticles at the same time. Catalytic polymerization in the presence of fillers (“in-situ polymerization”) is used extensively to produce conventional thermoplastic polyolefin compounds with very high filler content. In polymerization-filling processes, the fillers are used as catalyst supports to enable matrix growth directly from the nanofiller surface. Progress in the polymerization-filling technique (PFT) for polyolefins has been reviewed by Dubois and Kaminsky.8−10 Single-site catalyst technology offers exciting new opportunities for tailoring olefin polymerization11 and PFT processes by designing both molecular polyolefin architectures and polyolefin particle morphologies. PFT affords polyolefin nanocomposites with significantly improved CNT dispersion and electrical conductivity, thus enabling effective electromagnetic shielding of

INTRODUCTION Ultrahigh-molecular-weight polyethylene (UHMWPE), with molar weights exceeding 1 million, is well-known for its ultrahigh toughness combined with high abrasion resistance, very low friction coefficient, low moisture uptake, and excellent chemical stability.1 Sheets, parts, and fibers are produced from UHMWPE. Applications range from wear-resistant transport belts and supertough engineering plastics to biomaterials, implants for use in hip, knee, and spine replacements, ultrastrength gel-spun UHMWPE fabrics for water-repellant ropes, cut-resistant gloves, and bullet-proof body armor.2 For many applications it is highly desirable to improve the UHMWPE crystallization rate, toughness/stiffness balance, and especially the electrical as well as thermal conductivity because UHMWPE is a thermal and electrical insulator. Provided that effective dispersion is achieved, anisotropic nanoparticles can significantly improve polyolefin performance.3 UHMWPE nano- and microcomposites with carbon black, graphite,4 and multiwall carbon nanotubes (MWCNT),5 processed by means of compression molding, have been developed to improve electrical conductivity. It should be noted that dry powder blending with nanoparticles requires special safety precautions and handling procedures to prevent emissions, dust explosions, and health hazards resulting from nanoparticle inhalation or adsorption. A recent advance introduced electrostatic spraying of graphene nanoplatelet (GNP) suspensions with an average thickness of 6−8 nm and a specific surface area of 120−150 m2/g. Such GNP stacks containing more than 60 graphene nanosheets produce © 2012 American Chemical Society

Received: July 4, 2012 Revised: August 13, 2012 Published: August 30, 2012 6878

dx.doi.org/10.1021/ma301376q | Macromolecules 2012, 45, 6878−6887

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conventional polyolefins.12 Yet, considerably less is known with respect to PFT-producing UHMWPE nanocomposites. Recently, Rastogi et al. reported on the formation of UHMWPE nanocomposites prepared by PFT using salicylaldimine catalysts supported on TiO2, ZrO2, and CNT. The resulting nanocomposites exhibited improved nanofiller dispersion and higher entanglement molar masses.13,14 In our research, we investigated PFT with FG nanosheets as nanofillers and catalyst support. Graphenes are carbon sheets with a thickness of one carbon atom. This single layer of sp2hybridized carbon atoms is arranged in a honeycomb-like lattice. With an atomic diameter of 0.14 nm and a sheet width of several micrometers, the aspect ratio of typical graphenes is larger than 10 000. They exhibit exceptional properties, including ultrahigh strength, ultrafast electron transport at room temperature, extraordinarily high stiffness, and abrasion resistance as well as strong UV and IR absorption.15 In addition to the highly perfect ideal graphenes, a variety of hydroxylfunctionalized graphenes are available.16−19 The preparation of FG was pioneered by Boehm and co-workers in 196921 by using graphite oxide (GO), which was first synthesized by Brodie at Oxford University over 160 years ago.22 Thermal or chemical reduction of GO provides FG nanosheets.23,24 The oxygen content of this layered compound is controlled by the temperature of the reduction process and decreases with increasing reduction temperature. The predominant oxygen functionality at temperatures above 400 °C corresponds to hydroxyl groups, whereas epoxy, carbonyl, and carboxyl groups undergo rapid thermolysis at such temperatures. The surface area of FG is generally in the range of 600−1200 m2/g. This is considerably smaller than that of ideal graphenes (2630 m2/ g).25,26 Upon shearing the micrometer-sized, accordion-like particle morphology of FG by means of sonication, the large FG particles completely disintegrate to afford single FG nanosheets. This is reflected by a massive increase in the specific surface area from 600 to around 1800 m2/g.27 Since FG with a high surface area has an ultralow bulk density, PFT is the process of choice for producing polyolefin/FG master batches that can be added to conventional polyolefin matrices. According to Yan et al., the addition of 1 wt % FG prior to compression molding significantly improves the wear resistance.28 Simultaneously with the improved wear resistance the biocompatibility of UHMWPE is not affected.29 Several groups have used graphite and FG in PFT. However, most catalysts failed to produce UHMWPE. Pretreatment of the filler with cocatalyst methylaluminoxane (MAO) is one of the most commonly used approaches for effective immobilization of metallocene and postmetallocene catalysts.11,30−35 Dubois successfully employed micrometer-sized graphite fillers in a PFT process to produce thermoplastic PE/graphite composites with an effective filler dispersion.36 FG nanocomposites with low-molecular-weight thermoplastic HDPE,37,38 LDPE,39 LLDPE,40 iPP,41 and other polymers such as polyaniline have been reported.42−46 To the best of our knowledge, there are no reports on PFT processes for producing UHMWPE/FG nanocomposites using FG nanosheets as the catalyst support. Here we report on single-site chromium(III) constrainedgeometry catalysts immobilized on emulsifier-free MAOimpregnated FG nanosheet dispersions in n-heptane. This novel catalyst generation is employed in PFT to produce new UHMWPE/FG nanocomposite families. FG is compared with other nanoparticles such as MWCNT, nanometer-scale

boehmites, carbon black, and conventional graphite. The influences of catalyst preparation on catalyst activity and the thermal, mechanical, and electrical properties of UHMWPE nanocomposites are investigated.

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EXPERIMENTAL SECTION

Materials and General Considerations. All reactions involving air- and moisture-sensitive compounds were carried out under a dry argon atmosphere using standard Schlenk techniques and a glovebox. Toluene (anhydrous), n-heptane (anhydrous), and triisobutylaluminum (TiBAl, 1 M in hexane) were purchased from Sigma-Aldrich. Toluene and heptanes were further purified using a Vacuum Atmospheres Co. solvent purifier. The catalyst, dichloro-η5-[3,4,5trimethyl-1-(8-quinolyl)-2-trimethylsilyl-cyclopentadienyl]chromium(III) (Cr1), was synthesized in the group of M. Enders by Dr. S. Mark, University of Heidelberg, following procedures previously reported.47 MAO, purchased from Crompton, had an Al content of 10 wt % in toluene and was stored under a dry argon atmosphere in a glovebox (MBraun MB 150B-G-II). Ethylene (3.0) was supplied by Air Liquide and was used without further purification. The following commercial, carbon-based catalyst supports were used: CNT (Baytubes C150P, Bayer AG), carbon black (Printex XE 2B, Evonik Industries), graphite (KFL 99.5, Kropfmühl AG), and boehmite (Disperal 40, kindly supplied by Sasol Germany). Mesoporous silica, NF 20, was synthesized by a modified method of Stucky et al.48,49 FG was synthesized from graphite using a modified Hummers method to obtain GO, which was thermally reduced by rapid heating (750 °C) under an N2 atmosphere to produce FG nanosheets.51 The properties of the nanoparticles employed in polymerization filling are summarized in Table 1.

Table 1. Materials Used as Catalyst Supports material FG carbon black graphite boehmite multiwall carbon nanotubes nanofoam NF20 a

specific surface areaa (m2/g) 600

elemental analysisb (%) 80.0 (C), 1.2 (H), 18.8 (O) 97.0 (C) 99.99 (C) n.d.c 97.0 (C)

1040 30 200 250

n.d.c

1200 b

Determined using BET N2 absorption. Determined by elemental analysis. cn.d. = not determined.

Catalyst Preparation and Ethylene Polymerization. The catalyst was prepared by heating the support for 2 h in vacuo at 110 °C. It was then dispersed in n-heptane (10 mg/mL) and sonicated for 40 min. The cocatalyst in toluene (10 wt %) was added, and the mixture was sonicated for 20 min. Catalyst Cr1 in toluene (0.1 mg/ mL) was added with a syringe, and the mixture was stirred for 20 min. The thus-activated catalyst was transferred into the reactor, and the polymerization was started. When inorganic supports were used, the temperature was maintained at 160 °C for 16 h in vacuo. Furthermore, after addition of MAO, the activated support was washed with fresh heptane to remove excess MAO. Ethylene polymerizations were carried out in a 200 mL double-jacket steel reactor equipped with a mechanical stirrer and connected to a thermostat. To the reactor were added dry n-heptane (80 mL) and triisobutylaluminum (TiBAl; 0.5 mL 1 M in hexane) as a scavenger. During the polymerization period, the ethylene pressure was kept constant at 5 bar, the temperature at 40 °C, and the stirring speed at 1000 rpm. Polymerization was stopped by injecting acidified methanol. The polymer was filtered off and dried for 16 h at 65 °C under reduced pressure to constant weight. Polymerizations for online kinetics measurements were carried out in a 600 mL Büchi steel autoclave equipped with a mechanical stirrer and a software interface. The reactor, previously charged with n6879


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heptane and TiBAl, was saturated three times with ethylene at 40 °C before polymerization was started by addition of the catalyst. Polymer Characterization. The Mw and MWD of the polymers were determined using a PL-220 chromatograph (Polymer Laboratories) equipped with a differential refractive index (DRI) detector, a differential viscometer 210 R (Viscotek), and a low-angle lightscattering (LASS) detector. Polyethylene was defined as UHMWPE when its Mw exceeded 106 g/mol. The measurements were performed at 150 °C with three PLGel Olexis columns, and 1,2,4trichlorobenzene (Merck) stabilized with 0.2 wt % 2,6-di-tert-butyl(4-methylphenol) (Aldrich) was used as a solvent at a flow rate of 1.0 mL/min. Columns were calibrated using 12 polystyrene samples with a narrow MWD. Melting points and the overall thermal behavior of the neat polymer were determined by differential scanning calorimetry using a DSC 6200 from Seiko Instruments. The polymer was heated from room temperature to 200 °C, kept at this temperature for 5 min, cooled to −70 °C, and then heated again to 200 °C. The heating rate was kept constant at 10 K/min. TEM microscopy was performed with a LEO EM 912 Omega device and SEM microscopy with a Quanta 250 FEG. Particle sizes were measured using a Camsizer from Retsch Technologies. The resistance of all nanocomposites was measured with a four-point probe. A correction factor that depends on the sample geometry was taken into account in the calculations.50 The tensile modulus of the nanocomposites was measured with a Zwick model Z005 (DIN EN ISO 527). Polymer Processing. The obtained UHMWPE/FG nanocomposite samples were compression-molded in a Collin 200P melt press (Dr. Collin GmbH, Germany). The tensile specimens were stamped out of 100 × 70 × 2 mm polymer plates. The compression-molding parameters are listed in Table 2.

react with MAO, thus bonding and coating MAO onto the FG surface. This oxygen functionality is essential for producing stable FG dispersions without requiring addition of emulsifiers.54−58 The resulting FG/MAO was used to support Cr1. The same catalyst preparation procedure was applied to other (nano)particles (cf. Table 1), such as multiwall CNT, conductive CB with an average size of 20 nm, and aluminum oxide hydroxide (boehmite) with average primary particle size of 40 nm as well as a conventional micrometer-sized graphite filler (80 μm) (cf. Table 1). In contrast to FG, all other carbon materials failed to produce stable dispersions in n-heptane. Both CNT and graphite contain few functional groups and are unable to effectively bond MAO and Cr1, which only loosely adhere to the carbon support. In contrast, CB has a beehive-like morphology59 resembling the structure of porous graphite and is able to adsorb MAO and Cr1 in its pores. Nanometer-scale boehmite was employed for comparison. In previous experiments, boehmite nanoparticles were used successfully in both PFT60 and melt extrusion61 to produce thermoplastic HDPE nanocomposites and iPP nanocomposites with effective boehmite dispersion and improved stiffness. The mesoporous silicate NF 20 (average pore diameter 20 nm) was synthesized according to methods proposed by Stucky et al.46,61 and by procedures reported earlier for HDPE formation.59 Ethylene was polymerized in n-heptane at 40 °C and 5 bar ethylene pressure for between 22 and 72 min in a stirred autoclave to produce nanocomposites with defined filler contents. At a constant Al/Cr molar ratio of 1400, the FG content in UHMWPE was varied (0.5, 1, 2.5, 5, and 10 wt %). The results of the ethylene polymerizations are summarized in Table 3. The online kinetics of the polymerizations (cf.

Table 2. Conditions for Processing UHMWPE Nanocomposites stages temp (°C) press. (bar) time (min)

1

2

3

4

5

210 15 10

210 30 30

210 40 10

210 50 5

25 60 30

Table 3. Polymerization Filling and Variation of the Filler Content entry

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1 2 3 4 5 6 7 8d 9 10e,f

RESULTS AND DISCUSSION Catalyst Preparation and Polymerization Filling. The synthetic strategy for immobilization of single-site chromium(III) catalysts on FG is shown in Scheme 1. In the first step, FG Scheme 1. Synthetic Route to a Single-Site Chromium Catalyst (Cr1) Supported on FG

filler FG FG FG FG FG CB CNT boehmite graphite NF 20

wt %a 0.5 1 2.5 5 10 5 5 5 5

tpol (min) 43 47 34 22 28 47 50 72 60 51

activityb (g/(mmol h))

Mwc (g/mol)

PDc

20 000 18 100 25 400 40 200 27 600 18 501 17 600 11 700 14 300 17 000

× × × × × × × × × ×

3.3 3.6 3.6 3.6 3.7 4.0 4.8 3.9 5.3 4.6

2.0 1.8 2.1 2.2 2.4 2.8 1.7 2.7 2.1 2.6

6

10 106 106 106 106 106 106 106 106 106

Wt % of filler in the product. bTime-averaged activity. cDetermined by HT-GPC. dWashing process between catalyst addition and addition to the polymerization vessel was included. eWashing process between catalyst addition and addition to the polymerization vessel was included. fFor the polymerization with NF 20 an amount of 50 mg of support was used to immobilize the catalyst. Polymerization conditions: Cr1 = 2.3 μmol/L; Al:Cr = 1400:1; pethylene = 5 bar; mpol = 10 g; Vheptane = 300 mL; TiBAl = 1 mL, 40 °C, n-heptane. a

(600 m2/g) was prepared from GO according to Hummers’ method51 and was subsequently thermally reduced by very rapid heating to 750 °C, following procedures reported by Aksay and co-workers.52,53 The resulting FG had an oxygen content of 18.8 wt % and a carbon content of 80.0 wt %. In the second step, the FG was dispersed in n-heptane by means of sonication prior to adding MAO and dichloro-η5-[3,4,5trimethyl-1-(8-quinolyl)-2-trimethylsilylcyclopentadienyl]chromium(III) (Cr1). The functional groups of FG (1.5 mmol OH/g) predominantly consisted of hydroxyl groups, which

Figure 1) imply that FG reaches the highest activities as compared to mesoporous silica, whereas other carbon-based fillers, except CB, show significant lower catalyst activities. Influence of Nanoparticles and Al/Cr1 on the Activity and Molecular Weight. UHMWPE nanocomposites were obtained by polymerizing ethylene at 5 bar and 40 °C in nheptane at different Al/Cr1 ratios in the presence of various fillers. Whereas homogeneous Cr1/MAO in toluene failed to 6880


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Figure 1. Ethylene polymerization kinetics using supported Cr1/MAO catalysts as a function of the support type.

Figure 2. Polymerization of ethylene with the Cr1/MAO system: (a) homogeneous polymerization in the absence of filler, (b) FG/Cr1/MAO, (c) CNT/Cr1/MAO, (d) CB/Cr1/MAO, (e) graphite/Cr1/MAO, and (f) boehmite/Cr1/MAO. Further conditions: catalyst support (100 mg), in the case of FG, 0.35 μmol of Cr1 was used instead of 7 μmol of Cr1, solvent heptane (100 mL), TiBAl (0.5 mmol), ethylene (5 bar), reaction time 30 min.

boehmite fillers afforded considerably higher catalyst activities. Furthermore, polyethylenes with monomodal molar mass distributions were produced in the presence of fillers. It should be noted that boehmite did not form stable dispersions. The

produce UHMWPE, polymerization in n-heptane gave polyethylene with a bimodal molecular weight distribution and less than 30 wt % UHMWPE. The low yields are attributed to low catalyst activities. In sharp contrast, PFT with carbon and 6881


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1 1 1 2 2.1 2.9 4.3 8.5 6.9 3.0

(g) activity

5700 8000 12000 24000 19400 8300 3.9 4.2 2.8 3.6 2.6 3.1 630 570 1670 2300 3000 2670

PD (g)

Mw

c

19600 23600 21800 19000 14300 12000

a

activity Al:Cr

2000:1 1500:1 1200:1 900:1 600:1 400:1

b

100 mg of support was used in each polymerization. bDetermined by weight [g/(mmol h)]. cDetermined by HT-GPC in 1,2,4-trichlorobenzene at 150 °C, [kg/mol]. Further conditions: [1] Cr1 3.5 μmol/L, TiBAl 0.5 mmol, Tpol = 30 min, Tpol = 40 °C, heptane (100 mL). dCr1 = 7.0 μmol/L was used instead of Cr1 = 3.5 μmol/L. eBimodal molecular weight distribution.

PD Mw

1160 1130 1280 2020 1820 830 3.3 4.6 5.3 5.8 6.0 5.2

(g) activity

9300 13 100 15 200 16500 17 200 14 900 3.3 2.9 3.0 3.1 4.2 2.9

PD Mw

2500 2670 2580 1970 2900 2880 2.5 2.2 2.4 2.3 2.6 2.5

(g) activity

6900 6000 6500 6300 7000 6900 4.7 4.9 4.6 5.7 6.1 6.7

PD Mw

1650 1600 1900 1800 1680 1340 3.9 6.2 6.8 6.4 6.2 4.3

(g) activity

10800 17400 19100 18000 17400 12000 2.7 3.6 3.1 2.0 3.5 3.2

PD Mw

1140 1650 1917 3100 3200 3500 3.0 3.2 3.7 5.2 6.3 7.6

(g) activity

8300 8800 10300 14600 17700 21400 3.6 3.4 3.2 9.2 4.2 4.1

PD Mw

485 978 200 700 900 000

no filler added boehmited graphited carbon blackd CNTd FGa

Table 4. Polymerization of Ethylene Using Homogenous Cr1/MAO and Nanoparticle-Supported Cr1 Catalysts 6882

3.5 4.2 3.9 3.4 2.6 2.2

Al/Cr ratio did not affect catalyst adsorption because the particle suspensions were washed with heptane to remove excess MAO prior to Cr1 addition. Such procedures were not possible for FG/Cr1/MAO because stable dispersions were obtained (no sedimentation over a period of 4 h). As illustrated in Figure 2, all carbon-supported Cr1/MAO catalysts afforded UHMWPE with molar masses close to or above 106 g/mol. The influence of filler type and Al/Cr play an important role (cf. Table 3). The highest activities of the FG/MAO/Cr1 catalyst were found at an Al/Cr molar ratio of 1500. Decreasing this ratio decreased the catalyst activity, whereas the UHMWPE molar mass substantially increased from 600 to 3000 kg/mol. Since it is well-known that MAO solutions contain small amounts of trimethylaluminum (TMA), it is very likely that simultaneously increasing the MAO and trimethylaluminum contents will decrease the polyethylene molar mass.63 At low Al/Cr ratios, the catalyst activity is lower because the content of MAO is not sufficient to scavenge catalyst poisons (e.g., hydroxyl groups) on the surface of FG. Very similar behavior and influence of the Al/Cr molar ratio on the catalyst activity were found for other carbon fillers, such as CNT and carbon black (cf. Figure 2 and Table 4), wherein optimum Al/Cr molar ratios are slightly different and most likely depend on the type of adhesion of MAO to the support. Typical polyethylene powders and PE particle size distributions are shown in Figure 3. The Cr1/MAO and the graphite/Cr1/ MAO catalysts generate fine powders. Such particle distributions with high proportions of small, dust-like particles are problematic with respect to health and safety and also make UHMWPE unsuitable for large-scale processing. Excellent morphological control was achieved in the presence of FG nanosheet dispersions. Much larger granule-like UHMWPE particles with a monomodal size distribution and average diameters of 4000 μm were obtained. No fouling and handling problems were encountered. The resulting particles were suitable for use in sintering and compression molding. Nanocomposite Properties. Thermal and mechanical properties of UHMWPE nanocomposites were determined as a function of filler type and content. The UHMWPE nanocomposite powders, produced by PFT, were compression-molded at 210 °C and then compressed further by increasing the pressure to 60 bar. The detailed conditions are given in Table 2. The mechanical properties are summarized in Table 5. To illustrate the trends more clearly, changes of properties are graphically displayed as a bar chart in Figure 4 using the commercially available unfilled UHMWPE (GUR 4120) as the 100% benchmark. Increasing the FG content from 0.5 to 10 wt % substantially increased the stiffness, expressed as Young’s modulus. In comparison to UHMWPE containing only 0.5 wt % FG, all other fillers with up to 5 wt % failed to achieve a stiffness of 630 MPa. Upon increasing the FG content above 1 wt %, the increase in Young’s modulus was accompanied by decreasing tensile strength and elongation at break. Fillers such as graphite and mesoporous silicate (NF20) failed to improve elongation and tensile strength at break. At a FG content of only 1 wt %, it was possible to simultaneously improve stiffness, strength, and elongation at break (+73% and 157%, respectively). Such an increase in both stiffness and elongation at break is unparalleled by all other nanocomposites. Thermal Properties. Melting and crystallization of UHMWPE nanocomposites was measured by means of thermal analysis (DSC) as a function of filler type and FG content. Commercial nonfilled UHMWPE (GUR 4120, Ticona) was

3.7e 7.8e 11.2e 5.1e 5.7e 5.2e

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Figure 3. Images of polyethylene particles, produced with Cr1/MAO (left), FG/MAO/Cr1 (center), and graphite/MAO/Cr1 (right). The UHMWPE samples (middle and left) each contain 1 wt % of filler. Particle size distributions were analyzed using an optical instrument (Camsizer).

Table 5. Mechanical and Thermal Properties of the UHMWPE Nanocomposites entry 1 2 3 4 5 6 7 8 9 10 11 a

sample

Young’s modulus (MPa) b

GUR 4120 FG 0.5 wt % FG1 wt % FG 2.5 wt % FG 5 wt % FG 10 wt % CNT 5 wt % CB 5 wt % graphite 5c boehmite 5c NF20

420 630 727 751 819 991 571 592 600 491 419

± ± ± ± ± ± ± ± ± ± ±

27 43 43 36 63 28 34 33 5 38 30

εmax (%) 100 120 257 161 153 39 206 179 77 225 81

± ± ± ± ± ± ± ± ± ± ±

23 9 13 22 16 23 20 4 5 16 12

Fmax (MPa)

Tma (°C)

ΔHm1 (J/g)

Tc (°C)

ΔHc (J/g)

± ± ± ± ± ± ± ± ± ± ±

140.9 143.4 143.2 144.3 142.8 143.0 140.9 142.4 139.7 143.8 144

208 254 238 233 233 228 257 215 210 258 220

109.9 119.5 115.6 116.0 116.4 116.8 115.5 114.1 112.3 115.3 109.8

182 152 143 156 150 133 154 131 145 162 165

38 44 96 61 58 14 78 68 34 85 30

6 2 13 14 9 3 16 3 1 13 4

First DSC heating cycle. bCommercial UHMWPE GUR 4120 supplied by Ticona. c5 wt % graphite and boehmite in each nanocomposite.

composites based on CNT, CB, and boehmite revealed significantly improved dispersion. The morphologies of FG nanocomposite fracture surfaces are displayed in Figure 7 as a function of the FG content. At a low FG content of 0.5 wt %, the fracture surfaces are very smooth and resemble that of neat UHMWPE, whereas at a FG content of 10 wt %, the surface roughness is significantly increased. It appears that larger structures are pulled out of the polyethylene matrix during fracture. For better evaluation of FG dispersion, thin sections were prepared and analyzed by TEM (cf. Figures 7c and 7d). At a low FG content of 1 wt %, the FG filler is uniformly dispersed in the polyethylene matrix, whereas at 10 wt %, the FG dispersion appears to be less effective with few larger assemblies visible (cf. Figure 7d). Electrical Conductivity. The electrical conductivity of melt-compressed UHMWPE nanocomposites containing different carbon allotropes was measured using the four-point method on cylindrical plates with 25 mm diameter and a height of 3 mm. The conductivity was calculated from the specific resistance, which was measured at three different voltages (0.1, 0.5, and 1 V) at four different positions on the test specimen. The results are graphically displayed in Figure 8. Even at a graphite content of 5 wt %, no conductivity was detected.

used as the 100% benchmark. The results are summarized in Table 5. Addition of FG nucleated the UHMWPE crystallization very effectively, as indicated by the increased crystallization temperature from 109.9 for unfilled UHMWPE to 119.5 °C at an FG content of 0.5 wt %. Only CB and CNT exhibited similar nucleation of UHMWPE crystallization. At FG contents larger than 0.5 wt %, nucleation was less effective, but it was still superior in comparison to the other carbon and boehmite fillers. Most likely the dispersion of 0.5 wt % is more effective compared to higher filler concentrations. Among the other carbon-based fillers, CNT and boehmite showed the highest nucleation effects (115.5 and 115.4 °C). Morphology. The morphology of UHMWPE nanocomposites and dispersion of the fillers were analyzed by means of scanning electron microscopy (SEM) imaging of fracture surfaces (Figures 6 and 7) and transmission electron microscopy (TEM) imaging of ultrathin sections (Figure 7). As illustrated in Figure 6c, large graphite flakes are dispersed in the UHMWPE matrix because graphite does not intercalate and does not form carbon nanoparticles during PFT. This is in agreement with earlier observations made by the group of Dubois.30 In comparison to graphite, UHMWPE nano6883


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Figure 5. DSC traces of UHMWPE crystallization as a function of FG content. The reference sample is unfilled commercial UHMWPE (GUR 4120). From left to right: GUR 4120; UHMWPE 1 wt % FG; UHMWPE 10 wt % FG; UHMWPE 0.5 wt % FG.

Figure 6. SEM images of fracture surfaces of UHMWPE nanocomposites: (a) CNT 5 wt %; (b) CB 5 wt %; (c) graphite 5 wt %; (d) boehmite 5 wt %.

graphite flakes. This is agreement with observations relating to the filler morphology, as reported above (Figure 3, right-hand sample containing 1 wt % graphite). In contrast, FG has a significantly lower percolation threshold of around 2.5 wt % FG. At 5 wt % FG content, the electrical conductivity was 1.5 × 10−5 S/cm, and with increasing FG content it reached a maximum for FG with 2 × 10−3 S/cm. This was surpassed only by CNT, which produced an electrical conductivity of 10−2 S/ cm at 5 wt % CNT content. The higher electrical conductivity of CNT results from the higher bulk conductivity of the CNT with respect to the somewhat lower electrical conductivity of FG, which still contained considerable defect structures that resulted from thermal reduction of GO. However, it should be noted that CNT agglomerates were not completely destroyed during polymerization filling, and large percolating CNT network structures are embedded in UHMWPE. The electrical conductivity of FG nanocomposites is sufficient for many applications and electromagnetic shielding.

Figure 4. Young’s modulus (above), elongation at break (center), and tensile strength at break (below) of UHMWPE nanocomposites as compared to those of commercial UHMWPE (GUR 4120), which was used as the 100% benchmark. Light gray bars indicate FG nanocomposites, and dark gray bars indicate reference nanocomposites based on CB, CNT, graphite, boehmite, and silicate nanofoam (NF20).

Obviously, graphite is very effectively encapsulated and coated by UHMWPE, which prevents percolation of the fairly large 6884


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shearing by sonication or high-pressure homogenization.51 All other carbon materials without functionalization failed to produce stable dispersions upon shearing. The resulting suspensions readily underwent sedimentation. Obviously, the FG hydroxyl groups enable covalent attachment of MAO, creating MAO-modified FG surfaces for effective immobilization of Cr1 and formation of highly active FG/MAO/Cr1 catalysts. In the absence of FG, MAO/Cr1 in nheptane is much less active and produces polyethylene with a bimodal molar mass distribution containing less than 30 wt % UHMWPE. Interestingly, the homogeneous MAO/Cr1 catalyst completely failed to produce UHMWPE in toluene. In conclusion, effective immobilization is the prime requirement to reduce chain termination by β-hydride elimination and chain transfer with aluminum alkyls. For all carbon nanofillers, the UHMWPE molar mass increased with decreasing Al/Cr molar ratio, most likely due to increasing chain transfer with aluminum alkyls at higher Al content. The catalyst activity with an optimum Al/Cr ratio is a function of the carbon filler type because, at low Al content, the ineffective scavenging of polar groups probably impairs catalyst activities. These novel type of catalysts are superior in their activity to those obtained for conventional mesoporous silica supports. Another striking feature of the FG/MAO/Cr1 catalyst is the very effective morphological control, producing millimetersized UHMWPE powder particles without a narrow particle size distribution. In the absence of FG, very fine dust-like polyethylene particles are formed, which leads to severe reactor fouling problems. Similar to the nonsupported MAO/Cr1, polymerization filling in the presence of graphite, CNT, and CB afforded rather broad UHMWPE particle size distributions with an unacceptably high content of fine UHMWPE particles. This could be due to ineffective immobilization of MAO/Cr1 on conventional nonfunctionalized carbon materials. More research is needed to clarify the role of catalyst fragmentation during ethylene polymerization. The polymerization filling technique has proven to be a highly effective route toward the production of UHMWPE nanocomposites. All carbon and boehmite fillers were dispersed very effectively in the UHMWPE matrix without impairing sintering and compression-molding processes. However, only FG/MAO/Cr1 produced effectively reinforced UHMWPE with ultrathin graphene nanosheets. Owing to the fact that UHMWPE growth occurs from the FG nanosheets, uniformly dispersed in n-heptane, the FG nanosheets are also very effectively dispersed within the UHMWPE matrix. In spite of the encapsulation of FG in UHMWPE, the electrical conductivity of such compression-molded UHMWPE nanocomposites indicates formation of a conductive graphene network with a percolation threshold of around 2.5 wt %. It should be noted that the percolation threshold value strongly depends upon the sample thickness and the chosen processing conditions, both of which can affect the alignment and percolation of FG nanosheets. Among all nanofillers, FG is quite exceptional with respect to unique UHMWPE nanocomposite property profiles. At a low FG content of 1 wt %, the very unusual simultaneous improvement of stiffness, strength, and elongation at break was achieved. In fact, the majority of polyolefin nanocomposites improved the stiffness only at the expense of embrittlement. Thermal analysis revealed that FG is a very effective nucleating agent for the crystallization of UHMWPE.

Figure 7. Morphology of FG/UHMWPE composites: (a) SEM of 0.5 wt % FG in UHMWPE, scale 50 μm; (b) SEM of 10 wt % FG in UHMWPE, scale 50 μm; (c) TEM section of 1 wt % FG in UHMWPE, scale 1 μm; (d) TEM ultrathin section of 10 wt % FG in UHMWPE, scale 1 μm.

Figure 8. Electrical conductivities of UHMWPE nanocomposites containing different carbon fillers.

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CONCLUSION In summary, among carbon allotropes such as graphite, nanometer-scaled CB, multiwall CNT, and nanoboehmites with a primary particle size of 40 nm, only FG nanosheets form very stable dispersions in n-heptane. Using the polymerization filling technique, FG/MAO/Cr1 afforded the highest catalyst activities in conjunction with formation of the highest UHMWPE molar masses well above 106 g/mol and excellent morphological control, as reflected by the formation of millimeter-sized UHMWPE particles and the absence of problematic fine particles typical of most other fillers. This unique behavior of FG/MAO/Cr1 catalysts is attributed to the presence of hydroxyl groups located at the edges of the FG nanosheets and the structural defects. During thermal reduction of graphite oxide, the hydroxyl content can be readily controlled between 40 and 1 wt % by means of the reduction temperature. As a consequence of regraphitization at elevated temperatures, the functional groups are exclusively located at the edges of FG graphene layers. The resulting large (micrometer-sized) particles of the hydroxyl-functionalized FG, which exhibit an accordion-like morphology and a specific surface area of 600 m2/g, disintegrate during mechanical 6885


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Ongoing research is aimed at improving our understanding of the role of UHMWPE growth and crystallization on FG surfaces. In conclusion, the polymerization-filling technique using FG/ MAO-supported single-site catalysts represents a very facile and versatile route toward novel UHMWPE/carbon hybrids and nanocomposites with simultaneous improvement of electrical conductivity, matrix reinforcement, thermooxidative stability, UV and IR absorption, barrier and abrasion resistance, lubrication, and thermal conductivity. Integrating carbon nanosheet dispersion into catalyst preparation and effectively embedding carbon nanosheets inside the large UHMWPE particles eliminates the need for special handling and safety precautions typically required by conventional processing using nanoparticles with extremely low bulk densities and ultrahigh aspect ratio.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.); rolf.muelhaupt@ makro.uni-freiburg.de (R.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this project by the German Federal Ministry of Education and Research (BMBF) within the “FUNgraphen” project (project 03X0111C). The authors also thank the members of the Freiburg Materials Research Center, in particular Andreas Warmbold, Julia Eckerle, and Mrs. Melissa Weingärtner, for assistance in polymerization catalysis and characterization of catalysts and polymers.

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