Polymer Layered Silicate/Carbon Nanotube Nanocomposites: The Catalyzed Polymerization Approach
S. Peeterbroeck, B. Lepoittevin, E. Pollet, S. Benali, C. Broekaert, M. Alexandre, D. Bonduel, P. Viville, R. Lazzaroni, P. Dubois Materia Nova Research Center, University of Mons-Hainaut, Place du Parc 20, 7000 Mons, Belgium
A two-step route to polymer/layered silicate nanocomposites characterized by a large extent of nanoplatelet delamination is presented. It consists first in the preparation of poly(-caprolactone) (PCL)-grafted clay masterbatches (containing ⬃30 wt% clay) by in situ intercalative polymerization of -caprolactone (CL) in bulk. The CL polymerization is promoted at the surface of organomodified clays bearing targeted amounts of alcohol functionalities used as initiating species. The influence of alcohol concentration on clay platelets coverage by PCL has been studied by atomic force microscopy. In a second step, these masterbatches are readily dispersed in commercial matrices (PCL, PVC, chlorinated polyethylene) via rather conventional melt blending. The morphology of the resulting nanocomposites has been characterized as well as their thermal and mechanical properties. Interestingly enough, this “masterbatch” process can be extrapolated to another type of nanocomposites, i.e., polyolefin carbon nanotube nanocomposites. Accordingly, homogeneous surface coating of multi-walled carbon nanotubes (MWNTs) is first achieved by in situ polymerization of ethylene as catalyzed directly from the nanotube previously surfacetreated by a highly active metallocene-based complex. This polyolefin coating allows for the destructuration of the native nanotube bundles leading upon further melt blending with, e.g., ethylene-co-vinyl acetate copolymers (EVA), to high performance nanocomposites with finely dispersed MWNTs. POLYM. ENG. SCI., 46:1022–1030, 2006. © 2006 Society of Plastics Engineers
INTRODUCTION Homogeneously dispersing rigid particles in polymer matrices represents a very common and widely used method that
Correspondence to: Philippe Dubois; e-mail:
[email protected] Contract grant sponsors: Re´gion Wallonne Programmes WDU: TECMAVER, WINNOMAT: PROCOMO and NANOTECHNOLOGIES: BINANOCO; Contract grant sponsor: Belgian Federal Government Office of Science Policy; contract grant number: SSTC-PAI 5/3. DOI 10.1002/pen.20560 Published online in Wiley InterScience (www.interscience.wiley. com). © 2006 Society of Plastics Engineers
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allows for readily increasing the stiffness of the so-obtained composite materials. Depending on the intrinsic nature of the added filler, other properties can also be enhanced such as fire resistance, electrical, and thermal properties. However, such improvements of composite performances usually require high filling levels, detrimental to the ultimate mechanical properties of the resulting materials. Worldwide, and for at least 20 years, there has been an intense desire to tailor the structure and composition of materials on sizes in the order of the nanometer. Indeed, in addition to the classical macro- and microcomposites, where the filler has dimensions in the order of several microns or more, one can distinguish composites for which at least one dimension of the filler is in the nanometer range. In fact, the terms of “nanomaterials” or “nanostructured materials” appeared at the end of the eighties, but their practical use is clearly much older. Indeed, Romans and craftsmen of the middle ages used nanoprecipitates (colloı¨dal systems) originated from the reductive treatment of metal salts (gold, silver) to produce stained glasses. The use of nanoparticles has also been largely exploited in the case of the pneumatic elastomer reinforcement, which consists in the dispersion of carbon and silica nanometric particles [1]. Among these nanomaterials, the introduction of layered nanofillers (such as layered silicates, with one dimension in the order of the nanometer range) or fibrous nanofillers (such as carbon nanotubes, with two dimensions in the order of the nanometer scale) has opened a new area of research [1– 4]. The improvement of composite materials properties with such nanofillers proves really important, even if not well completely understood. Polymer-based nanocomposites, and especially polymer-layered silicate and carbon nanotube nanocomposites, represent a radical alternative to conventionally filled polymers. Because of the dispersion of nanometer-size sheets or tubes, these nanocomposites exhibit markedly improved properties when compared with the unfilled polymers or conventional microcomposites. Moreover, the characteristic properties of nanocomposites are usually observed at nanofiller contents as low as 1–5 wt%. Among these improvements, material stiffness is in-
creased while maintaining high toughness, permeability to oxygen and other fluids is decreased, and thermal stability and fire retardancy are enhanced [1–3]. Undoubtedly, polymer nanocomposites represent a new class of composite materials with remarkable thermo-mechanical performances that have been recorded using layered silicates (nano-clays) or carbon nanotubes as nanofiller precursors. This contribution aims at reviewing very recent developments in syntheses, characterization, and properties of polymer-based nanocomposites filled with either nano-clays or carbon nanotubes. Undoubtedly, the key-challenge remains reaching a high level of nanoparticle dissociation (i.e., either to delaminate the silicate nanoplatelets or to break down the bundles of aggregated carbon nanotubes) ultimately leading to their fine individual dispersion upon melt blending within the selected polymer matrix. In that context, this report will show the exceptional efficiency of the in situ polymerization/grafting process as catalyzed directly from the nanofiller (clay or carbon nanotube) surface allowing for the complete destructuration of the native filler aggregates. Dissociated nanoparticles are accordingly recovered, their surface is homogeneously coated/grafted by the in situ grown polymer chains as generated by this so-called “polymerization-filling technique” (PFT) [5]. Interestingly enough, such surface-coated nano-clays or carbon nanotubes can be added as “masterbatch” in commercial polymeric matrices. As a result of the predestructuration of the nanofillers by PFT, it comes out that the resulting polymer nanocomposites display much higher thermo-mechanical properties even at very low nanofiller loading. After the experimental section, both types of polymer-based nanocomposites filled with either clays or carbon nanotubes will be discussed separately and the key-role of in situ catalyzed polymerization as promoted from the nanofiller surface will be highlighted. EXPERIMENTAL Materials Clay-based nanocomposites. Two commercially available montmorillonites have been used in this study and were supplied by Southern Clay Products: Cloisite® 30B, a montmorillonite organo-modified by 21 wt% of methyl bis(2hydroxyethyl)tallowalkyl ammonium and Cloisite® Na, a Na⫹ montmorillonite. To vary the density of alcohol groups at the surface of the clay, it has been also organomodified by mixtures of 2-hydroxyethyl(hexadecyl)-dimethylammonium iodide and hexadecyltrimethylammonium iodide. Their synthesis as well as the process for the Cloisite® Na organomodification is described elsewhere and resulted in organomodified clays covered by 10 or 50% of monohydroxylated ammonium cations [6]. -caprolactone (CL) and trimethylaluminum were provided by Fluka and tin(II) octoate by Aldrich. L, L-lactide (LA) was purchased from Boehringer Ingelheim and recrystallized three times in dried toluene (20 wt%/vol) before use. Tyrin®3652 (chloriDOI 10.1002/pen
nated polyethylene or CPE with 36 wt% chlorine) used in this study was obtained from Dupont Dow Elastomers. The CPE was systematically stabilized by 4 phr Lankroflex® E2307 (epoxidized soybean oil, Brenntag). Commercial poly(-caprolactone) PCL (CAPA®650) was supplied by Solvay Chemicals sector-SBU caprolactones and was characterized by a number-average molecular weight (Mn) of 49 kg/mol. Carbon nanotube-based nanocomposites. Multi-walled nanotubes (MWNTs) were kindly provided by Nanocyl SA (Sambreville, Belgium) and were produced by catalytic decomposition of acetylene on transition metal particles (Co, Fe) supported on Al2O3. Ethylene (99.95%) was purchased from Air Liquid and used as received. n-Heptane is furnished by ChemLab Wallonie and was dried over molecular sieve (4 Å). Toluene was received from Labscan and was used freshly distilled over calcium hydride. Modified methylaluminoxane (MMAO) was received from AkzoNobel Polymer Chemicals as a 30 wt% solution in toluene. This solution was further diluted in dried toluene (1.62 M) for safe handling. MMAO corresponds to methylaluminoxane (MAO) where about 3 methyl units per 100 are substituted by longer alkyl chains like isobutyl for better solubilization. Bis(pentamethyl-5-cyclopentadienyl)zirconium(IV) dichloride was purchased from Aldrich and dissolved in dried toluene to reach a 1.7 ⫻ 10⫺3 M concentration. The ethylene-co-vinyl acetate copolymer (EVA) used was Escorene UL00328 from Exxon, characterized by a vinyl acetate content of 27 wt% and a melt flow index at 190°C under 2.16 kg ⫽ 3 g/10 min. Polymerization/Grafting onto Organo-Clays The preparation of the PCL-based masterbatches has been described elsewhere [6] as well as the preparation of clay surface-grafted poly(CL-b-LA) diblock copolyester by sequential and controlled ring-opening polymerization of CL and LA [6, 7]. Preparation of Nanocomposite by Clay (or Masterbatch) Dispersion Nanocomposites have been prepared by melt-blending the masterbatches or the organo-modified clay in a Brabender OHG internal mixer at a given temperature and for a given time, depending on the polymer: CPE or PCL. Polyethylene-Coating of Carbon Nanotubes Carbon Nanotube Activation. One gram of carbon nanotubes (CNTs), previously dried overnight at 105°C under reduced pressure (10⫺2 mm Hg) were added, under nitrogen, with 40 ml of dried and deoxygenated n-heptane and 3 ml of MMAO (1.62 M). The CNTs in contact with MMAO were stirred for 1 h at 40°C. Solvents were then POLYMER ENGINEERING AND SCIENCE—2006
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distilled off at 40°C under reduced pressure. Treated CNTs were heated up to 150°C under reduced pressure for 90 min. Polymerization Step. The treated CNTs, ⬃1 g, were dispersed in 175 ml dried n-heptane and the whole was transferred into 250 ml glass-reactor in a glove-box. Then, 9.6 ml of Cp*2ZrCl2 (1.7 ⫻10⫺3 M) was added to the suspension. The stirred mixture was then heated up to 50°C for 15 min. The reactor was purged by ethylene (0.5 min) in order to remove nitrogen. The synthesis was carried out under a constant pressure of 2.7 bars of ethylene at 50°C and vigorous stirring for 1 h. The final material was precipitated in 600 ml methanol acidified with hydrochloric acid and was dried at 70°C for ⬃7 h under reduced pressure. The recovered MWNTs were coated by 40 wt% of PE as determined by TGA. Preparation of EVA Carbon Nanotube Nanocomposites Pristine MWNTs (pMWNTs) or PE-coated MWNTs (cMWNTs) were ‘dried’ mixed with the EVA matrix (total mass of mixture: 6 g) in order to reach 3 wt% of MWNTs in the final material. The mixture was introduced in the addition funnel of a Minilab twin-screw mini-extruder (ThermoHaake) equipped with co-rotating screws and a closed loop for recirculation. The temperature of the mini-extruder was set at 140°C for EVA and pMWNTs and 170°C for cMWNTs. The mixture was introduced in the mini-extruder for 4 min at 30 rpm screw speed. Then the speed of the screws was increased to 45 rpm for 6 min, allowing the materials to circulate in the closed loop. The nanocomposites were recovered by opening the miniextruder, in order to avoid any preferential orientation of the nanotubes during the extrusion process. The recovered material was shaped as a square plate (30 ⫻ 30 ⫻ 0.3 mm3) by compression molding at 140°C. Neat EVA has been processed under the same conditions.
creasing deformations (amplitudes). Young’s modulus was determined as the slope of the stress–strain curve obtained. Transmission electron microphotographs (TEM) of claybased nanocomposites were obtained with a Philips CM200 apparatus, using an accelerator voltage of 120 kV. The clay-based nanocomposites samples were 70-nm thick and prepared from 3-mm hot-pressed plates with a LEICA Ultracut UCT ultracryomicrotome cutting at ⫺100°C. TEM analyses of pMWNTs and cMWNTs have been realized using dilute suspension of the nanofillers in n-heptane. One drop of this suspension was deposited onto a TEM grid and the solvent was let evaporated. Samples for TEM analyses of the EVA-based nanocomposites were prepared with a Reichert Jung Ultracut 3E, FC4E ultracryomicrotome cutting at ⫺130°C to obtain 70 – 80-nm thick slices. Their TEM pictures were obtained with a Philips CM100 apparatus using an accelerator voltage of 100 kV. For AFM imaging, the as-synthesized nanohybrids were dissolved in toluene to reach a final concentration of 0.1 wt%, and the solutions were deposited by casting on mica substrates [8]. The images were recorded in tapping mode (TM) in ambient atmosphere at room temperature with a Nanoscope IIIa microscope (Veeco Inst., Santa Barbara, CA). The probes were commercially available silicon tips, with a spring constant of 24 –52 N/m, a resonance frequency in the 264 –339 kHz range, and a typical radius of curvature in the 10 –15 nm range. Tensile properties of unfilled CPE and CPE clay nanocomposites were measured with a Lloyd LR 10 K tensile testing apparatus, at 20°C, with a constant deformation rate of 50 mm/min, with dumbbell-shaped specimens prepared from compression molded samples according to the 638 type V ASTM norm. Five specimens were tested for each nanocomposite sample and the mean values and standard deviation were calculated and reported. RESULTS AND DISCUSSION
Characterizations The thermal transitions of the materials were determined by differential scanning calorimetry (DSC), using either (for clay-based nanocomposites) a DSC 2920 from TA Instrument at a heating rate of 10 K/min from ⫺100 to 200°C, or (for carbon nanotube-based nanocompositions) a Q100 differential scanning calorimeter from TA Instruments at a heating rate of 10 K/min from ⫺75°C to 150°C followed by a controlled cooling from 150°C to ⫺75°C at 10 K/min and a second heating scan from ⫺75°C to 150°C at 10 K/min. The reported values were recorded during the second heating scan. TGA analyses were carried out using a thermogravimetric analyzer Q50 from TA instruments, under helium from room temperature to 800°C at 20 K/min. DMTA analyses were performed at 25°C on compression molded (0.5-mm thick and 5-mm wide) sheets of either unfilled EVA or EVA carbon nanotube nanocomposites using a DMA2980 from TA Instrument. Measurements were carried out in tensile mode under the sweep amplitude mode (at 1 Hz) allowing to evaluate tensile stress for in1024
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Polymer Organoclay Nanocomposites Melt blending of organo-clays in polymer matrices is usually not enough efficient to promote a high extent of exfoliation, actually needed for optimal nanocomposite properties. On another hand, in situ polymerization, coupled to grafting of the polymer chains on the clay platelets allows forming exfoliated nanocomposites at high clay dilution, but precludes the use of commercial polymer matrices. An alternative route for the formation of highly exfoliated nanocomposites in commercial matrices is presented. It consists in preparing poly(-caprolactone) (PCL)-grafted masterbatches (⬃30 wt% clay) by controlled intercalative polymerization of -caprolactone (CL), starting from organomodified clays bearing targeted amounts of alcohol functionalities as initiating species. In a first part, the control of the polymerization will be attested by the ability to prepare block copolymers of aliphatic polyesters. Then the morphology adopted by PCL at the surface of the clay platelets DOI 10.1002/pen
FIG. 1.
Synthetic pathway for the preparation of clay-grafted aliphatic (co)polyesters (1) and (2).
in function of the density of initiating species will be probed by atomic force microscopy (AFM). Finally, the use of such highly filled “PCL-grafted organoclay ” masterbatch for the preparation of nanocomposites in a matrix known to be miscible with PCL, i.e., chlorinated polyethylene (CPE), will be demonstrated. Preparation of Clay Surface-Grafted by PCL Or poly(CL-b-LA) (co)polymers by Controlled Ring-Opening Polymerization. Figure 1 presents the synthetic pathway followed to prepare PCL masterbatches (1) and organo-clay surface-grafted by poly(CL-b-LA) diblock copolyesters (2). Aluminum alkoxides are well known for their efficiency to promote the controlled ring-opening polymerization (ROP) of CL [9]. Accordingly, triethylaluminum has been reacted with the surface OH functions of the organo-clay swollen in toluene to form the alkoxide bond. CL polymerization has then been carried out by adding the desired amount of lactone monomer. Two organo-clays have been tested with relative OH function concentration of 10 and 50 mol%. In all cases, polymerization occurred smoothly and reached a PCL content close to 30 wt%, i.e., a good composition for allowing the visualization of numerous individual clay nanoplatelets and the polymer grafting onto their surface. Interestingly, an SEC analysis has been performed on the extracted PCL chains that were polymerized from the clay surface modified with 10% in OH functions. A numberaverage molecular weight (Mn) of 2900 g/mol has been determined; such a value is in very good agreement with the Mn (3000 g/mol) expected on the basis of the monomer-tohydroxyl molar ratio and the CL monomer conversion. Another piece of evidence for the control achievable over the polymerization/grafting reaction is the sequential block DOI 10.1002/pen
copolymerization of CL and LA monomers. As expected for a controlled process, poly(CL-b-LA) diblock copolymers were surface-grafted onto the organo-clay (2). Their relative comonomer composition agrees with the starting composition, and each polyester block crystallizes, with melting temperatures at 53 and 150°C for PCL and PLA blocks, respectively [7]. Clearly, the possibility to graft such diblock copolyesters onto organo-clays is a strong indication that the (co)polymerization grafting reaction is indeed controlled. Influence of Density of Alcohol Functions on the Morphology Adopted by PCL Chains Grafted on Single Nanoplatelets. AFM in tapping mode has been used to determine the morphology adopted by PCL chains grafted onto clay nanoplatelets, depending on the amount of alcohol used to modify the organo-clay surface [8]. Figure 2 shows the AFM image obtained on a clay nanoplatelet modified with 10% of alcohol-bearing ammonium cations grafted with 24 wt% PCL as well as the resulting cross-sectional analysis of the image following the dotted line (Fig. 2). For comparison, the same cross-sectional analysis has been carried out on clay nanoplatelets modified with 50% of alcohol-bearing ammonium cations grafted with 32 wt% PCL (Fig. 3). At low density of alcohol (initiating species) functions, PCL appears as dots (1.3 nm high) at the surface of the nanoplatelets. At higher density of alcohol functions, the PCL forms a uniform coating (3.8 nm thick) on the surface of the nanoplatelets. The coexistence of PCL domains and uncovered areas that is observed in the 10% OH system (instead of a thinner homogeneous polymer coating) probably means that the OH-containing ions, which initially were homogeneously dispersed on the platelet surface, gather during the polymerPOLYMER ENGINEERING AND SCIENCE—2006
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FIG. 3. Topographic (550 ⫻ 225 nm2) AFM images (top : 2D and 3D views) and cross-sectional analysis (bottom) of individual clay nanoplatelets modified with 50% of alcohol-bearing ammonium cations grafted with 32 wt% PCL.
FIG. 2. Topographic (550 ⫻ 550 nm2) AFM image (top) and crosssectional analysis (bottom) of a single clay nanoplatelet modified with 10% of alcohol-bearing ammonium cations grafted with 24 wt% PCL (some PCL “buds” are shown by black arrows).
ization process via lateral mobility. Most probably, the driving force for such an assembly is the aggregation of the growing PCL chains to form polymer domains. This lateral mobility is made possible because the ammonium ions interact electrostatically with the clay surface; positional interchange of ammonium ions is therefore relatively easy and is promoted by the affinity between neighboring grafted PCL chains. The polymer deposit is not simply a continuous film growing in thickness with increased OH content. Instead, separate polymer islands are formed in the low-OHcontent systems, probably as a result of a phase separation process between the ammonium ions induced by the polymerization reaction. Homogeneous coverage and subsequent thickening only take place at 50% OH content. When this situation is achieved, adjacent platelets become fully independent of each other (because they are fully covered by the polymer), which greatly favors exfoliation [8]. Use of PCL-Grafted/clay as Masterbatch for Nanocomposite Preparation. A PCL/clay masterbatch (25 wt% clay) has been prepared in bulk by in situ intercalative polymerization of CL initiated by the hydroxyl groups borne by the Cloisite® 30B clay, using tin octoate as ROP catalyst [10]. This masterbatch has been melt blended 1026
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with CPE at 175°C for 10 min to reach 3 wt% clay content and compared with direct melt-blending of Cloisite® 30B and CPE under the same conditions. TEM micrographs of these two composite materials show a much better dispersion of the clay nanoplatelets with extensive exfoliation when the PCL masterbatch is used (Fig. 4b, see arrows) in comparison with the simple melt blend with the commercial organo-clay (Fig. 4a, see arrows). When comparing the mechanical properties (Table 1), a large increase in the Young’s modulus with only a slight decrease of tensile stress are observed for the exfoliated nanocomposite compared to both the unfilled matrix and the intercalated sample, indicating a large effect of exfoliation using the PCL masterbatch. The same effect can be observed when PCL or PVC commercial matrices are used [6]. For instance, the thermal stability of PCL nanocomposites as a function of clay content was investigated by thermogravimetry (TGA). A significant enhancement in thermal stability has been recorded for the thermoplastic matrices filled with the “PCL-grafted organo-clay” nanohybrids, thus
FIG. 4. TEM images of nanocomposites based on 3 wt% clay dispersed in CPE : (a) using Cloisite 30B; (b) using the PCL-grafted masterbatch. The large “round” particles are additives of the commercial CPE matrix. The arrows show the aggregated clay (image a) and delaminated nanoplatelets (image b).
DOI 10.1002/pen
TABLE 1.
Tensile properties of CPE, CPE ⫹ Cloisite 30B, and CPE ⫹ masterbatch, both with 3 wt% in inorganics (ASTM D 638 typeV).
Blends CPE CPE ⫹ Cloisite 30B CPE ⫹ Masterbatch
Stress at break (MPa)
Elongation at break (%)
Young’s modulus (MPa)
16.0 ⫾ 0.7 14.7 ⫾ 0.9 8.8 ⫾ 0.6
1302 ⫾ 29 1219 ⫾ 38 1111 ⫾ 63
4.3 ⫾ 0.3 8.1 ⫾ 0.8 14.3 ⫾ 3.0
added as masterbatch by melt blending. Polymer Carbon Nanotube Nanocomposites Carbon nanotubes (CNTs) constitute another family of potential nanofillers. This new allotropic form of carbon is built up of carbon atoms arranged in hexagons and pentagons, forming very long cylinders. They are flexible and resistant to an applied stress, they have potential applications in field emission devices with high electrical and thermal conductivity [11, 12]. CNTs have rapidly been tested as an advanced multi-functional filler in polymerbased nanocomposites. However, the homogeneous dispersion of native CNTs is relatively difficult to achieve, especially in apolar polymer matrices such as polyolefins. Indeed, carbon nanotubes tend to form long bundles that are thermodynamically stabilized by numerous – electronic interactions between the tubes [13–15]. Most of the techniques that have been used to disperse/dissociate these bundles, e.g., ultrasonication, chemical oxidation or reduction followed by chemical modification of the nanotube surface, are susceptible to break down or at least to perturb the extended delocalized system responsible for the unique properties displayed by CNTs [16 –18]. Furthermore, the use of high quantities of carbon nanotubes in nanocomposite production raises the question of their toxicity during handling. Indeed, owing to their very low bulk densities and their very high aspect ratio, small aggregates may enter the breathing system and promote inflammation in the lungs [19, 20]. Manual handling without gloves may also promote skin inflammation (dermatitis, hyperkeratosis) [21]. Interestingly enough, the aforementioned “masterbatch ” process has been extrapolated to the production of such a new type of nanocomposites, i.e., polyolefin carbon nanotube nanocomposites [22]. Accordingly, the surface coating of long multi-wall carbon nanotubes (MWNTs) has been achieved by in situ polymerization of ethylene as catalyzed directly from the nanotube previously surface-treated by a highly active metallocene-based complex. This so-called polymerization-filling technique (PFT) applied to carbon nanotubes consists in anchoring modified methylaluminoxane (MMAO), a well-known co-catalyst used in metallocene-based olefin polymerization process onto the surface of carbon nanotubes. After high temperature treatment and toluene washings, most of the MMAO remains anchored onto the carbon nanotube surface. A metallocene catalyst, i.e., bis(pentamethyl-5-cyclopentadienyl)zirconium (IV) dichloride (Cp2*ZrCl2) in this study, is then reacted with the DOI 10.1002/pen
surface-activated carbon nanotubes. A methylated cationic species (Cp2*ZrMe⫹) is formed upon reaction and is immobilized at the vicinity of the nanotubes surface by electrostatic interactions with simultaneously formed MMAO counteranions fixed at the nanotube surface. Addition of ethylene leads to the synthesis of polyethylene (PE) exclusively at the surface of the carbon nanotubes. As a result, a homogeneous PE coating develops around every individual carbon nanotube and totally disrupts the bundles. Such a coating limits the formation of airborne carbon nanotube aggregates, making handling much safer. It allows preparing a masterbatch of predispersed carbon nanotubes that can be melt-blended by very conventional processing tools with polyolefins (polyethylene) but also any types of polymeric matrices, even nonmiscible with the polyethylene coating, e.g., an ethylene-co-vinyl acetate copolymer (EVA), to form nanocomposites with largely improved properties. Sketch for the PE Growth at the Surface of MWNTs. Figure 5 presents the idealized sketch for PE growth onto CNT surface that ultimately leads to complete and homogeneous coating of the nanotubes. Upon treatment with MMAO and Cp2*ZrCl2, catalyst spots are created at the carbon nanotube surface as attested by surface chemical analysis (not shown here) [22a]. Upon ethylene addition, these spots start to produce PE chains that precipitate in the medium as soon as their molecular masses reach their solubility limit, forming PE patches all along the CNT surface. Upon continuous feeding of ethylene, the PE patches grow and slide along the nanotube to form PE sleeves. Eventually, the PE sleeves join together to form a continuous and homogeneous PE coating around the carbon nanotubes. Such a mechanism is supported by the kinetics of ethylene consumption, reported elsewhere [22] and which does not show any rate reduction with time, attesting for a continuous production of PE at a constant pace that is coherent with the PE sliding process. Such a polyolefin growth mechanism has been confirmed by TEM analysis as recorded on samples picked out all along the ethylene polymerization catalyzed by the metallocene/MMAO surface-treated MWNTs [22b]. The quantity of PE coating has been determined by thermogravimetric analysis. Figure 6a– 6c show TEM pictures for, respectively, neat MWNTs and MWNTs coated by 43 and 75 wt% of in situ grown polyethylene. When coated by 43 wt% of PE (Fig. 6b), MWNTs appear much less densely packed. At this PE content, each nanotube is covered by numerous PE patches most probably indicating the localization of anchored active catalyst species POLYMER ENGINEERING AND SCIENCE—2006
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FIG. 5. Sketch for the homogeneous polyethylene coating of carbon nanotubes via PFT.
from which PE chains are produced and precipitate at the nanotube surface. A closer observation of Figure 6b (inset) shows that besides the PE patches, one can observe PE sleeves that originates from the growth and possible junctions of PE patches. Figure 6c displays the TEM image of MWNTs totally covered by a homogeneous coating of PE. In this case, all the polyolefinic sleeves have joined together in order to produce this regular PE coating. The macroscopic morphology of the recovered powder of PE-coated MWNTs is much more compact and dense than pristine MWNTs. Such an increase in bulk density can be explained by the fact that the individually coated MWNTs tend to slightly stick to the others through the PE coating. There is therefore a much less production of airborne particles, which eases MWNTs handling during melt-blending processes. Homogeneously Dispersion of MWNTs in EVA Matrix via the “Masterbatch ” Process. Homogeneously PEcoated CNTs (cMWNTs) have been used as a predispersed masterbatch for the preparation of highly dispersed MWNTs/polymer nanocomposites. Following the experimental procedure aforementioned, a masterbatch (actually, cMWNTs coated by 40 wt% of PE) has been prepared by PFT and dispersed in a commercially available model polymer matrix, i.e., ethylene-co-vinyl acetate copolymer (EVA with 27 wt% VA unit), using a co-rotating twin-screw mini-extruder (see experimental). The amount of the nanofiller in the final materials has been fixed to 3 wt%. For the sake of comparison, unfilled EVA and EVA filled with 3 wt% of pristine CNTs (pMWNTs) have been processed under the same conditions. Even though EVA (at least with VA content higher than 10 wt%) and PE are known to be nonmiscible polymers [23, 24], TEM analysis clearly indicates that the cMWNTs are individually and homogeneously dispersed in the studied EVA matrix (Fig. 7b). Such morphology appears in sharp contrast with the simple melt blend of EVA and nonsurface treated pMWNTs where large carbon nanotubes aggregates can be observed (Fig. 7a). As assessed by DSC, two melting 1028
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endotherms at 72°C and 133°C, typical for EVA and PE, respectively, are measured, indicating that the two polymers in the blend are effectively immiscible and preserve their crystallinity. The mechanical properties of the resulting materials, i.e., unfilled EVA, EVA/pMWNTs and EVA/cMWNTs nanocomposites, have been evaluated using dynamic mechanical thermo-analysis (DMTA) in tensile mode, by measuring the tensile stress for increasing tensile strain applied on thick (0.5 mm) films processed by compression molding. The addition of 3 wt% of pMWNTs to EVA allows increasing the Young’s modulus from 12 to 19 MPa but the very high dispersion of the nanotubes in the nanocomposite based on the dispersion of the cMWNTs in the same EVA matrix allows reaching a very high Young’s modulus of 29 MPa, thus more than a threefold increase in stiffness. Since the crystallinity of the matrix is not influenced by the addition of the MWNTs, the main cause for this very high modulus
FIG. 6. TEM images of MWNTs surface-covered by in-situ grown polyethylene: (a) Initial bundles of long MWNTs; (b) MWNTs coated by 43 wt% PE featuring PE patches and PE sleeves (insets); (c) MWNTs coated by 75 wt% PE showing homogeneous coating as a result of sliding HDPE sleeves.
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resents the key-step for producing high performance polymer nanocomposites. As illustrated within this review, in situ catalyzed polymerization/grafting reactions performed from the nanofiller precursor surface can offer huge opportunities for reaching that target and pave the way to larger scale production of masterbatches, actually highly filled in nano-clays/carbon nanotubes, and readily dispersible within selected polymer matrices. ACKNOWLEDGMENTS FIG. 7. TEM images of EVA-based nanocomposites filled with MWNTs (3 wt%): (a) neat MWNTs (with bundle-like aggregations – see circles); (b) PE-coated MWNTs (40 wt% PE) (no aggregate remains, rather single segments of CNTs are visible and shown by the arrows).
enhancement has to find its origin in the tremendous improvement of the cMWNTs dispersion throughout the polymeric matrix. CONCLUSIONS Whatever the nature and geometry of the nanofillers, the main target to reach in polymer-based nanocomposites relies upon the control of the nanofiller dispersion in the matrix. Such fine and homogeneous nano-dispersion can be effective through more defined chemical interface compatibilization, in order to enhance always more the performance of the polymer matrix. In this frame, the masterbatch process, where a highly filled polymer-grafted/coated nanofiller “nanohybrid ” is dispersed in the molten matrix, appears to be a future intensively used pathway for the large scale preparation of nanocomposites presenting improved mechanical and thermal properties. This review has highlighted that ring-opening polymerization of -caprolactone starting from controlled amount of alcohol functions anchored at the surface of montmorillonite allows for producing such “polyester-grafted organoclay ” nanohybrids with tunable morphologies. The polymerization process is controlled as attested by the ability to synthesize poly(CL-b-LA) diblock copolymers grafted onto the organo-clay surface. The obtained nanoclay grafted with PCL chains can be used as efficient masterbatches for the preparation of clay-based nanocomposites based on commercial matrices miscible with PCL. As a result of the extensive clay exfoliation, mechanical properties of the materials are enhanced, compared to conventional organoclay/polymer melt blends. Similar trends have been registered for polymer carbon nanotube nanocomposites. The target remains exactly the same: Surface modification of nanotubes allowing for their de-aggregation and fine dispersion within the selected polymer matrix. For that purpose, the polymerization-filling technique applied to metallocene-based complexes anchored at carbon nanotube surface proved high efficiency. Undoubtedly, destruction of the nanofiller aggregates repDOI 10.1002/pen
The authors are much indebted to the Re´gion Wallonne and the Fonds Social Europe´en for support in the frame of Phasing out—Hainaut: Materia Nova. The authors wish to thank Nanocyl S.A. (Namur, Belgium) for kindly providing the carbon nanotubes. ABBREVIATIONS AFM: cMWNTs: CL: CNT: CPE: DMTA: DPn: EVA: MAO: MMAO: Mn: Mw/Mn: MWNTs: PCL: PLA: pMWNTs: Poly(CL-b-LA): PVC: SEC: TEM: TGA: XRD:
Atomic force microscopy polyethylene-coated multi-wall carbon nanotubes -caprolactone carbon nanotube chlorinated polyethylene Dynamic mechanical thermo-analysis degree of polymerization ethylene-co-vinyl acetate copolymer methylaluminoxane modified methylaluminoxane number average molecular weight polydispersity multi-wall carbon nanotubes poly(-caprolactone) polylactide pristine multi-wall carbon nanotubes block copolymer between -caprolactone and lactide poly(vinyl chloride) size exclusion chromatography transmission electron microscopy thermogravimetric analysis X-ray diffraction
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