In situ polymerization of ethylene with bis(imino)pyridine iron(II) catalysts supported on clay: The synthesis and characterization of polyethylene–clay nanocomposites

In Situ Polymerization of Ethylene with Bis(imino)pyridineIron(II) Catalysts Supported on Clay: The Synthesis andCharacterization of Polyethylene–Clay Nanocomposites

SAPTARSHI RAY, GIRISH GALGALI, ASHISH LELE, S. SIVARAM

Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India

Received 10 August 2004; accepted 7 September 2004DOI: 10.1002/pola.20502Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Polyethylene–clay nanocomposites were synthesized by in situ polymeriza-tion with 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl] pyridine iron(II) dichloride sup-ported on a modified montmorillonite clay pretreated with methylaluminoxane (MAO).The catalysts and the obtained nanocomposites were examined with wide-angle X-rayscattering. The exfoliation of the clay was further established by transmission electronmicroscopy. Upon the treatment of the clay with MAO, there was an increase in thed-spacing of the clay galleries. No further increase in the d-spacing of the galleries wasobserved with the iron catalyst supported on the MAO-treated clay. The catalystactivity for ethylene polymerization was independent of the Al/Fe ratio. The exfoliationof the clay inside the polymer matrix depended on various parameters, such as the claycontent, catalyst content, and Al/Fe ratio. The crystallinity percentage and crystallitesize of the nanocomposites were affected by the degree of exfoliation of the clay.Moreover, when ethylene was polymerized with a mixture of the homogeneous iron(II)catalyst and clay, the degree of exfoliation was significantly lower than when thepolymerization was performed with a preformed clay-supported catalyst. This obser-vation suggested that in the supported catalyst, at least some of the active centersresided within the galleries of the clay. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A:Polym Chem 43: 304–318, 2005Keywords: clay; in situ polymerization; nanocomposites; polyethylene (PE); TEM;WAXS

INTRODUCTION

There is currently considerable interest in theliterature related to the preparation and proper-ties of polymer-layered silicate (clay) nanocom-posites.1–5 Polymer-layered silicate (PLS) nano-composite generally exhibit improved mechanicalproperties because reinforcement in the compos-ites occurs in two dimensions rather than in one

dimension. The nanocomposites further exhibit aremarkable increase in the thermal stability aswell as self-extinguishing characteristics. Theyalso show a several-fold reduction in the perme-ability of gases (e.g., O2, H2O, He, and CO2) be-cause of the introduction of tortuosity in the dif-fusion path of the permeate gases.4,6–12 In a poly-mer–clay nanocomposite, the clays are dispersedin the polymer matrix at the nanoscale; that is, atleast one dimension of the clay particles is in thenanometer scale. Such delaminated nanocompos-ites exhibit improved physical properties, in com-parison with conventional filled polymer compos-ites, at much lower silicate concentrations (�5 wt

Correspondence to: S. Sivaram (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 304–318 (2005)© 2004 Wiley Periodicals, Inc.

304

%). Consequently, they are far lighter in weightthan conventional filled polymers, and this makesthem competitive for specific applications.

The synthesis of polymer–clay nanocompositescan be performed by several methods. One of themost successful methods is in situ polymerization,in which the monomers are first introduced intothe clay galleries and expanded by a suitablemodifier; this is followed by polymerization of themonomers inside the gallery.13–16 The formationof a polymer–clay nanocomposite depends mainlyon the polymer/silicate compatibility, that is, thenature of the polymer (polar/nonpolar), the sur-face energy of the silicate surface, and the struc-tures of the modifiers. As the polymer intercalatesinto the silicate layers, it becomes confined insidethe layers, and this results in a decrease in theoverall entropy of the polymer chain. This has tobe compensated by a favorable enthalpy change,which arises because of favorable polymer–clayinteractions. This is readily achieved with high-surface-energy polymers such as polyamides, forwhich the polarity and hydrogen-bonding abilitiesgenerate the required amount of energy neces-sary for the hybrid formation. For low-surface-energy polymers such as polypropylene and poly-ethylene (PE), such intercalation may not be pos-sible. However, the use of a suitable modifier(compatibilizing agent) can make an incompatiblepolymer intercalate into the silicate galleries.17,18

There are some reports of the in situ polymer-ization of olefins with clay-supported early- andlate-transition-metal catalysts. For example, Tu-dor et al.19 synthesized polypropylene–clay nano-composites of synthetic hectorite treated withmethylaluminoxane (MAO) followed by the met-allocene Cp2Zr(Me)(THF)�. Recently Jin et al.20

reported heterogenizing TiCl4 into galleries ofmontmorillonite modified with methyl tallowbis(2-hydroxyethyl) quaternary ammonium moi-ety. Alexandre et al.21 synthesized PE–silicatenanocomposites by the in situ polymerization ofethylene with a constrained geometry catalyst(CGC)[(tert-butylamido)dimethyl(tetramethyl-�5-cyclopentadienyl) silane titanium(IV) dimethyl]supported on montmorillonite, hectorite, and ka-olin for comparative studies. Heinemann et al.22

reported the in situ polymerization of ethylene toproduce PE nanocomposites with metalloceneMe2Si(2-methylbenz[e]indenyl)2ZrCl2, nickel cata-lyst N,N�-bis(2, 6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadienenickel, and palladium cat-alyst {[ArNAC(Me)OC(Me)ANAr]Pd(CH3)(NCOCH3)}�BAr4�. The layered silicates used were

bentonites modified with dimethyldistearylam-monium or dimethylbenzylstearylammoniumcations. Bergman et al.23 intercalated a Brookhart-type palladium catalyst {[2,6-Pr2

i C6H3NOC(Me)-C(Me)ONC6H3Pr2

i -2,6]Pd(CH2)3CO2Me}{B[C6H3-(CF3)2]4} into synthetic fluorohectorite.

In this article, we describe studies aimed to-ward the preparation of PE nanocomposites withlate-transition-metal catalysts by in situ polymer-ization. The catalyst chosen for this purpose was2,6-bis[1-(2,6-diisopropylphenylimino)ethyl] pyri-dine iron(II) dichloride. A modified montmorillon-ite clay was used in this study. The objective ofthis study was to evaluate the efficacy of the cata-lyst in generating intercalated/exfoliated clay–PEnanocomposites via an in situ polymerization tech-nique. The PE–clay nanocomposites were studiedwith wide-angle X-ray scattering (WAXS), trans-mission electron microscopy (TEM), and rheologicalanalysis.

EXPERIMENTAL

Materials

All operations were performed under an argonatmosphere with standard Schlenk techniques.The solvents were dried with standard proce-dures. The catalysts were stored and transferredinside a Braun Labmaster 100m inert-atmo-sphere glovebox. Ethylene was obtained from theGas Cracker Complex of Indian Petrochemicals,Ltd. (Nagothane). MAO was procured from WitcoGmbH (Germany). The aluminum content inMAO, estimated by a back-titration method withstandard zinc sulfate and ethylenediaminetet-raacetic acid solutions and dithizone as a indica-tor, was approximately 4.5 mmol of Al/mL ofMAO. The methyl content of MAO was estimatedby the hydrolysis of MAO with 2 N H2SO4 and themeasurement of the evolution of methane in a gasburette; the approximate value of Me/Al in MAOwas 1.5–1.6. The ligand precursors, 2,6-di-acetylpyridine and 2,6-diisopropylaniline, wereprocured from Aldrich (United States) and usedwithout purification. Ferrous chloride was pro-cured from Sigma (United States). The catalyst2,6-bis[1-(2,6-diisopropylphenylimino)ethyl] pyri-dine iron(II) dichloride was synthesized accordingto a reported method.24 The clay was Cloisite 20Afrom Southern Clay Products (United States).This clay is basically montmorillonite modified bydimethyl–ditallow ammonium cations; it con-

IN SITU POLYMERIZATION OF ETHYLENE 305

tains approximately 65% C18, 30% C16, and 5%C14 chains. The modifier concentration was 95mequiv/100 g of clay. The average intergalleryspacing was approximately 24 Å.

Synthesis of Clay-Supported Catalysts: 2,6-Bis[1-(2,6-diisopropylphenylimino)ethyl] Pyridine Iron(II)Dichloride Catalyst Supported on Modified Clay

The modified clay used in the synthesis was pre-heated at 100 °C in vacuo for 4 h. The clay (1 g)was placed in a 250-mL, round-bottom flask, andto it, 30–50 mL of dry toluene was added. AnMAO solution (1 mL; 4.2 mmol of Al/mL) wasadded to the slurry of the clay in toluene. Themixture was stirred for 1 h. Afterwards, 80 mg ofthe iron catalyst suspended in 10 mL in dry tol-uene was added slowly to the mixture. The colorof the catalyst changed immediately from deepblue to brick red. The mixture was further stirredfor 2 h. Finally, the solvent was removed underreduced pressure, and the solid was dried.

Ethylene Polymerization with the Clay-SupportedCatalysts

Ethylene polymerizations were performed withthe clay-supported catalysts at 6 bar of pressurefor 1 h at 30 °C with a Buchi Miniclave 200-mLglass reactor equipped with a magnetic needle.The reactors were baked in an oven at 150 °C forat least 2 h, were then taken out and quicklyfitted to the ethylene supply line, and were sub-sequently cooled under ethylene pressure. Thesolid catalysts were taken out from the gloveboxand added to the reactor via a glass vial. Thesoluble catalysts were added to the reactorthrough a hypodermic syringe. Toluene (50 mL;dried over sodium and freshly distilled) wasadded to the reactor and saturated with ethylene.Finally, the required amount of MAO was in-jected to begin the polymerization. The pressurewas maintained at the required level. At the endof 1 h, the reaction was terminated by the addi-tion of acidified methanol. The polymer was fil-tered, washed, and dried at 40–50 °C in vacuo.

Ethylene Polymerization with a Homogeneous IronCatalyst in the Presence of Modified Clay (C-20A)

The clay (1 g), preheated at 100 °C in vacuo for4 h, was placed in a 250-mL, round-bottom flaskand slurried with 30 mL of dry toluene added. AnMAO solution (1 mL; 4.2 mmol of Al/mL) was

added to the slurry of the clay in toluene. Themixture was stirred for 1 h. The solvent was re-moved, and the clay was dried in vacuo. Approx-imately 100 mg of this clay was added to a glassreactor under an ethylene atmosphere. Dry tolu-ene (50 mL) was added to the clay, and the mix-ture was stirred for 20 min under an ethyleneatmosphere. The catalyst was added to the reac-tor as a suspension in toluene, and this was fol-lowed by the addition of MAO with a hypodermicsyringe. Ethylene polymerization was performedat 5 bar of pressure for 1 h at 30 °C. The reactionwas terminated by the addition of acidified meth-anol. The polymer was filtered, washed, and driedin vacuo.

Characterization

WAXS of the catalysts and the polymer sampleswas performed and compared with that of Cloisite20A. The clay peaks were scanned between 2 and10°, whereas the polymer peaks were scannedbetween 20 and 25°. The X-ray diffraction (XRD)experiments were performed with a Rigaku Dmax2500 diffractometer with Cu K� radiation (�� 0.15418 nm). The system consisted of a rotat-ing-anode generator with a copper target and awide-angle powder goniometer fitted with a high-temperature attachment. The generator was op-erated at 40 kV and 150 mA. All the experimentswere performed in the reflection mode. The sam-ple holder was a copper block, and powder sam-ples were attached to it for analysis. The scanningrate was 1°/min for 2� � 2–10° and 2°/min for 2�� 20–25°. TEM of the composite samples wasperformed with a JEOL model 1200EX instru-ment operated at an accelerated voltage of 80 kV.Samples for TEM were prepared by the disper-sion of the powder composites in toluene and theplacement of a drop of the dispersion onto a car-bon-coated TEM copper grid. The drop wasallowed to dry for 1 min, and the extra solu-tion was removed. The metal content of thesolid-supported catalyst was estimated with aPerkinElmer P100 inductively coupled plasma(ICP) ionization spectrometer. The molecularweights and distributions of the polymers weremeasured with gel permeation chromatography(GPC; PL-GPC 220) at 160 °C with 1,2,4-trichlo-robenzene (TCB) as the solvent and with polysty-rene as the standard. A 0.3–0.4% (w/v) solutionwas used at a flow rate of 1.0 mL/min. Before GPCof the polymers was conducted, the samples wereextracted in TCB at 140 °C. Approximately 250

306 RAY ET AL.

mg of the PE–clay composite was placed in apacket made of two filter papers (Whatman-1-grade) and submerged in TCB at 140 °C. The clayinside the packet agglomerated. After 30 min, thepacket was removed, hot TCB was poured intomethanol, and the precipitates were filtered anddried. This entire process was repeated a secondtime to ensure complete separation of PE fromclay. Typically, the final weight of the polymerwas approximately 150–200 mg.

Rheological Experiments

The samples for rheological measurements wereprepared by the compression molding of 25-mm-diameter disks at 150 °C. The viscoelastic prop-erties of the samples slowly evolved with timeupon annealing. In particular, the zero shear vis-cosity, as measured in creep experiments, in-creased slowly for about 45 min and reached asteady value after about 60 min. The effect ofannealing on the microstructure and rheologicalproperties of PLS nanocomposites has been welldocumented.25,26 All samples were therefore an-nealed in the rheometer at 170 °C for 1 h beforethe rheological measurements were performed.Dynamic oscillatory measurements were per-formed on a strain-controlled rheometer (modelARES, Rheometric Scientific) with 25-mm dispos-able aluminum parallel-plate fixtures. Heatingwas achieved with a forced convection oven sup-plied with N2, which also ensured an inert atmo-sphere. The rheological tests included oscillatorystrain sweep and frequency sweep tests at varioustemperatures in the range of 150–230 °C. Thestrain sweep tests were performed at a 10 rad/sfrequency over a strain range of 0.1–100% andwere used to differentiate the linear and nonlin-ear viscoelastic regimes. Frequency sweeps wereperformed over the frequency range of 0.1–100

rad/s at strain amplitudes that were chosen wellwithin the linear viscoelastic regime.

RESULTS AND DISCUSSION

Preparation and Characterization of the Clay-Supported Catalysts

The clay samples were pretreated with MAO be-fore they were treated with the catalyst. The en-tire quantity of MAO and the catalysts used re-mained in the solid catalysts after drying. Thiswas also confirmed by metal estimation with ICPand by Al estimation with titration analysis. Foursupported catalysts were made with different claycontents (Table 1). In all cases, the clays werepretreated with different quantities of MAObased on the amount of clay used. In one of theclay-supported iron catalysts, the quantity of thecatalyst was doubled to check the effect of thecatalyst concentration.

WAXS of the clay before and after treatmentwith MAO is shown in Figure 1. WAXS showsthat after the clay was treated with MAO, therewas a reduction in 2�, which corresponded to anincrease in the d-spacing of the clay galleries from24 to approximately 30 Å. This shows that MAOoccupied the intergallery space of the clay. Tudoret al.19 did not observe any change in the inter-gallery distance of hectorite upon treatment withMAO. However, their sample of hectorite had nohydrophobic modifier. No further change in 2�was observed when the MAO-treated clay wasreacted with the catalyst.

Ethylene Polymerization with the Clay-SupportedCatalysts

The polymerization of ethylene was performedwith catalysts Fe-1 to Fe-4 as well as the corre-

Table 1. Description of the Catalysts

CatalystCatalyst

(mol � 10�3) ClayAl

(mol � 10�3)Yield of

Clay Support Fe (%)a Al (%)b Al/Fe

Fe-1 0.13 1 g 4.2 1.4 g 0.53 8.2 32Fe-2 0.13 3 g 12.6 4.1 g 0.18 8.2 97Fe-3 0.13 5 g 21 6.5 g 0.11 8.7 160Fe-4 0.26 3 g 12.6 4.4 g 0.33 7.7 49

a Measured by ICP analysis.b Measured by a titration method.


sponding soluble catalyst. The results are shownin Table 2. As expected, all the catalysts producedcrystalline polymers with a sharp melting endo-therm in differential scanning calorimetry. Therewas no effect of the Al/Fe ratio on the activity ofthe catalyst.

In a separate study, the iron catalyst and theclay (previously treated with MAO) were individ-ually added to the reactor, and ethylene was po-lymerized (entry 2, Table 2). The catalyst activitywas similar to that of the homogeneous catalyst.

Molecular Weight and Molecular WeightDistribution

The obtained polymers exhibited a distinctly bi-modal distribution (Fig. 2) characteristic of theiron catalyst.24 A comparison of the polymers ob-tained from the homogeneous catalysts and cata-lyst Fe-1 showed an increase in the molecularweights of the polymers obtained from catalystFe-1 (Table 2). This showed the influence of het-erogenization of the catalyst on the polymer mo-lecular weight. However, there was no change inthe molecular weight distribution pattern for thecatalysts. Moreover, as the concentration of clayincreased from Fe-1 to Fe-2 and Fe-3, there was asteady reduction in the molecular weight of thepolymers (Fig. 2). This could be attributed to thefact that with an increase in the amount of clay,there was an increase in the concentration of sur-

face-attached MAO, and this led to a higher de-gree of chain transfer to aluminum. It was alsoobserved that the area under the GPC peak due tothe low-molecular-weight fraction of the polymerincreased with respect to the area under the high-molecular-weight peak with an increase in thequantity of clay (Fig. 2).

Structures and Properties of the PE–ClayNanocomposites

WAXS patterns of all the polymer–clay nanocom-posites were recorded in the 2� ranges of 2–10°and were compared with the diffraction patternsof the clay before and after the catalysts weresupported.

A comparison of the intensities of the WAXSpeaks of different samples showed that the extentof exfoliation was in the order Fe-1 � Fe-2 � Fe-3,which was the reverse of the order of the contentof clay in the polymer (Figs. 3–5), provided that,among other things, the amounts of the samplespacked on the sample holder during the WAXSexperiments were uniform. This claim may stillbe reasonable because samples Fe-2, Fe-3, andFe-4 showed hints of intercalated peaks, whereasFe-1 showed almost no trace of the peak. Also, acomparison of the polymers obtained from cata-lysts Fe-2 (Fig. 4) and Fe-4 (Fig. 6) showed that ahigher content of catalyst in the clay led to a

Figure 1. Comparison of the XRD peaks of (A) C-20A, (B) C-20A treated with MAO,and (C) C-20A/MAO treated with the iron(II) catalyst.

308 RAY ET AL.

greater degree of exfoliation. Moreover, the Al/Feratio affected the degree of exfoliation (Fig. 5).

The melting temperatures (Tm’s) of the PE–clay nanocomposites were lower than that of pris-tine PE prepared in the absence of clay (entries 1,4, 8, and 12, Table 2). The crystallite size andcrystallinity percentage of PEs obtained fromclay-supported Fe(II) pyridylamine catalysts(Fe-1 to Fe-3) are shown in Table 3. The crystal-lite size was estimated from the full width at halfmaxima (fwhm) for the PE peak at 2� � 21.3°(Fig. 7) with Scherer’s formula. Interestingly, onesample (entry 4, Table 2) showed a significantincrease in fwhm and a decrease in the crystallitesize in comparison with pristine PE. This samplealso showed clear exfoliation of clay (Fig. 8). An-other sample (entry 8, Table 2) showed a highercrystallite size and lower fwhm than sample 4.Sample 8 was only partially exfoliated, as shownby TEM micrographs (Fig. 9). Thus, a relation-

ship existed between the crystallite size and de-gree of exfoliation. Similar observations weremade for mica–poly(�-caprolactone) nanocompos-ites and mica–poly(ethylene terephthalate) (PET)nanocomposites.27,28 The crystallite sizes of bothpoly(�-caprolactone) and PET in completely exfo-liated samples were significantly lower thanthose of the pristine polymers. It could be con-cluded that the well-dispersed silicate layers rep-resented physical barriers that could restrict thecrystal growth of the confined PEs. Thus, exfoli-ated clays in these nanocomposites exhibited thecharacteristic of a nucleating agent.

The exfoliation of the clay in the polymer matrixwas further established by TEM. Figure 8 presentsTEM micrographs of the PE–clay nanocompositesobtained from iron catalyst Fe-1 (entry 4, Table 2).The exfoliated clay particles, approximately 10 nm� 100 nm and oriented randomly in all directions,can be seen in the micrographs.

Table 2. Ethylene Polymerization with the Clay-Supported 2,6-Bis[1-(2,6- diisopropylphenylimino)ethyl]Pyridine Iron(II) Dichloride Catalyst

SampleClay(mg)

[Fe](�106 mol)

[Al](�103 mol)a

Al/FeRatiob

Yield(g)

Activity(kg of PE/mol

of Fe/h)Clay(%)

Tm

(°C) Mp1 Mp2 � 10�3G�

(Pa)a

Homogeneous Fe1 — 10.0 10.0 1000 11.8 1186 — 136.0 1980 58.6 1150

Homogeneous Fe � Clay2 102 10.0 16.0 1600 11.4 1142 1.1 133.0 1180 46.3 800

Fe-13 104 13.1 21.0 1630 10.7 823 0.94 133.0 1080 48.2 24724 101 13 13.0 1030 6.5 500 1.5 133.0 3450 68.8 54495 106 13.1 7.9 630 10.0 769 1.0 135.0 6520 126.2 99806 104 13.1 2.6 230 8.0 615 1.2 135.3 6800 142.6 3395

Fe-27 305 13 21.0 1700 9.5 731 3.2 128.7 340 42.9 71558 302 13 13.0 1100 10.0 769 3.0 128.4 890 41.6 18249 300 13 7.8 700 11.0 846 2.7 129.5 1470 54.2 6222

10 301 13 2.6 300 11.8 908 2.5 135.3 1980 36.6 2780Fe-3

11 500 13 21.0 1760 9.0 692 5.9 129.5 650 54.2 2622212 502 13 13.0 1160 8.5 654 5.9 128.7 570 51.5 1076313 510 13.1 7.9 760 9.0 692 5.6 133.3 1400 56.1 378114 500 13 2.6 360 9.5 731 5.3 135.4 2170 56.6 5002

Fe-415 312 26.3 42.1 1650 11.8 454 2.5 127.5 740 28.2 —16 301 26 26.0 1050 11.0 423 2.7 128.9 1290 43.2 —17 304 26 15.6 650 12.0 462 2.5 131.8 3300 47.6 —18 305 26.1 5.2 250 12.0 461 2.5 133.5 3810 51.6 —

a Used in excess.b Total MAO, including MAO used during the catalyst synthesis and during polymerization.c At a frequency of 0.1 rad/s and at 150 °C.


Similar TEM images were reported by Sun andGarces:29 the clay particles were completely exfo-liated to a single platelet or a few tactoids whenpolypropylene–clay nanocomposite were obtainedby in situ polymerization with a metallocene cat-

alyst. Heinemann et al.22 also reported TEM evi-dence for the improved dispersion of the clay inPE–clay nanocomposites prepared by in situ po-lymerization in comparison with composites pre-pared by melt compounding. Figure 9(a,b) pre-

Figure 2. GPC results for PEs obtained with iron catalysts: (A) entry 4, (B) entry 8,(C) entry 12, and (D) entry 16 (Table 2).

Figure 3. WAXS results for polymers obtained with catalyst Fe-1: (a) entry 3, (b)entry 4, (c) entry 5, and (d) entry 6 (Table 2).

310 RAY ET AL.

sents TEM micrographs of partially exfoliatedsamples: Fe-2 (entry 8) and Fe-3 (entry 12). Themicrographs show partially exfoliated clay withmuch larger platelet sizes. At a higher magnifica-tion, the intercalation of the polymers inside theclay platelets is visible [Fig. 9(b)].

Polymerization of Ethylene with Bis(imino)pyridineIron(II) Catalyst in a Homogeneous Solution in thePresence of Clay

The modified clay, C-20A, was pretreated withMAO and dispersed in toluene. The polymeriza-




tion was started by the addition of the catalystfollowed by the cocatalyst to the reactor (entry 2,Table 2). The product thus obtained was com-pared with that obtained from a preformed clay-supported iron catalyst under similar conditions(Fe-1, entry 3, Table 2). The polymerization activ-ity, Tm, and peak molecular weights of the lowerand higher molecular fractions (Mp1 and Mp2, re-spectively) were similar in both cases. However,there was a significant difference in the WAXSpatterns of the two products (Fig. 10). The prod-uct obtained from the preformed clay-supportedcatalyst was predominantly exfoliated. However,the product obtained from a mixture of the Fe(II)catalyst and clay showed only intercalation.

Melt Rheology of the PE–Clay Nanocomposites

The viscoelastic material functions [melt elastic-ity (G�), loss modulus (G�), and complex viscosity(�*)] obtained from the frequency sweep datawere horizontally shifted with the principle oftime–temperature superposition to generate mas-ter curves. Nanocomposites of various parame-ters, such as the polymer molecular weight, claypercentage, and extent of dispersion, were chosento study the effects of these parameters on therheological properties of the nanocomposites.

Figure 11 shows a plot of G� versus the fre-quency for the polymers obtained from catalystFe-3 (entries 11–14, Table 2). These samples hap-pened to have similar Mp2 values but differentdegrees of exfoliation of the clay (Table 2 and Fig.5). Because long-chain molecules have a dominat-ing influence on the rheological properties, theseseries of samples could be expected to demon-strate the influence of clay dispersion on rheologi-cal properties. Except for entry 14 (Table 2), G�increased with an increase in the Al/Fe ratio atany given frequency. Furthermore, the slope ofthe plots at a low frequency decreased with anincrease in Al/Fe. Figure 5 suggests that the ex-tent of exfoliation increased with an increase inAl/Fe. At a loading of about 5 wt % clay, thenanocomposite was expected to consist of a perco-lating network of hydrodynamically interacting


Table 3. Effect of the Clay Concentration on theCrystalline Properties of the PE–ClayNanocomposites

SampleClay(%) fwhm

CrystalliteSize (nm)

Crystallinity(%)

1 0 0.3265 24.34 �954 1.5 0.4323 18.38 88.98 3.0 0.3628 21.90 88.5

12 5.9 0.3325 23.90 88.3

312 RAY ET AL.

clay platelets or tactoids. The strength of such anetwork was expected to grow with an increasingextent of exfoliation because more clay plateletscould participate in the network. Thus, the meltcould be expected to show solidlike rheologicalfeatures, that is, high values of G� and a weakerfrequency dependence in the low frequencyrange.30

The nanocomposites obtained from catalystFe-1 (entries 3–6, Table 2) also provided interest-

ing information. These samples seemed to havesimilar extents of clay exfoliation (Fig. 3),whereas the Mp2 values increased with a decreasein Al/Fe. This series of samples could be expectedto demonstrate the influence of the molecularweight distribution for similar extents of clay dis-persion. Figure 12 shows plots of G� versus thefrequency for samples Fe-1/230, Fe-1/630, Fe-1/1030, and Fe-1/1630. Except for Fe-1/230, G� in-creased with an increase in the Mp2 values at any

Figure 7. WAXS results for polymers obtained with the iron catalyst at 2� � 20–25°:(A) entry 1, (B) entry 4, (C) entry 8, and (D) entry 12 (Table 2).

Figure 8. TEM micrographs of fully exfoliated composite Fe-1 (entry 4 Table 2) at (a)a 200-nm scale and (b) at a 100-nm scale.


given frequency. This was obviously expected be-cause the long chains contributed significantly tothe elasticity of the melt. The reasons for whichsample Fe-1/230 did not fit into this trend are notclear.

Figure 13 presents plots of G� versus the fre-quency for the nanocomposites obtained by homo-

geneous polymerization in the presence of clay(entry 2, Table 2) and from catalyst Fe-1 (entry 3,Table 2). The latter showed significantly highervalues of G� at all frequencies than the former.The better dispersion of the nanoclay in the poly-mer obtained from catalyst Fe-1, compared withthat of the polymer obtained from the homoge-

Figure 9. TEM micrographs of partially exfoliated composites: (a) entry 8 and (b)entry 12 (Table 2).

Figure 10. Comparison of the WAXS peaks of C-20A and the composites obtainedfrom (a) a homogeneous catalyst in the presence of clay (entry 2, Table 2) and (b)catalyst Fe-1 (entry 3, Table 2).

314 RAY ET AL.

neous catalyst, as evidenced from the XRD data,suggested that for the same clay loading, theformer sample may have had a denser percolatingnetwork of clay platelets than the latter. Hence,the higher values of G� for the former could beexpected.

In another set of experiments, three samples(entries 3, 7, and 11, Table 2) were chosen thathad similar Mp2 values and similar extents of clayexfoliation but different clay loadings.25 Thus,they could be expected to show an effect of theclay loading on the rheological properties underotherwise similar states (of the molecular weightand clay dispersion). Figure 14 compares G�–fre-quency data for the polymer samples from cata-lysts Fe-1 (entry 3, Table 2), Fe-2 (entry 7, Table2), and Fe-3 (entry 11, Table 2), which had clayloadings of approximately 1, 3, and 6%, respec-tively. The G� values of the samples were in theincreasing order of Fe-1 � Fe-2 � Fe-3. Thehigher the clay loading was, the denser the per-colating network was that was formed by thedispersed platelets, and so G� could be expected to

be higher. The decreasing slope of the plots at alow frequency also supported this hypothesis.

CONCLUSIONS

In this article, we report a method for supportinga late-transition-metal catalyst, namely, 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl] pyridineiron(II) dichloride, on a modified montmorilloniteclay. The polymerization of ethylene was per-formed with clay-supported catalysts, with vari-ous amounts of clay and at various catalyst con-centrations and catalyst/cocatalyst ratios. The sa-lient conclusions from this study are as follows:

1. The amount of the clay with respect to thecatalyst is an important factor in determin-ing the extent of exfoliation and intercala-tion of the polymer into clay. A lesser degreeof exfoliation was observed when a largeramount of clay was used.

2. The degree exfoliation is greater when the

Figure 11. Comparison of the G� values for the Fe-3 series of samples, which con-tained similar clay loadings and similar molecular weights (Mp2) but different extentsof exfoliation: (�) entry 11, (�) entry 12, (E) entry 13, and (‚) entry 14 (Table 2).


Figure 12. Comparison of the G� values for the Fe-1 series of samples, which con-tained similar clay loadings and similar extents of exfoliation but different molecularweights (Mp2): (�) entry 3, (�) entry 4, (‚) entry 5, and (E) entry 6 (Table 2).

Figure 13. G� of homogeneously catalyzed nanocomposite samples versus heteroge-neously catalyzed nanocomposite samples: (�) entry 2 and (■) entry 3 (Table 2).

316 RAY ET AL.

polymerization is performed at a higherAl/Fe ratio.

3. Complete exfoliation leads to PE with alower crystallite size and lower crystallinity.

4. When ethylene is polymerized with a phys-ical mixture of a homogeneous iron(II) cat-alyst and clay, the degree of exfoliation issignificantly lower than when the polymer-ization is performed with a preformed clay-supported catalyst. This observation sug-gests that in the supported catalyst, at leastsome of the active centers reside within thegalleries of the clay.

5. For similar molecular weights (Mp2 values),G� increases with an increase in the extentof clay dispersion. This is in agreement withthe picture of hydrodynamically percolatedclay platelets in the melt. As the density ofthe network increases because of the greaterextent of exfoliation, the elasticity in-creases. Consequently, G� and �* also in-crease.

6. For a similar extent of exfoliation, G� (as

well as G� and �*) increases with an in-crease in the molecular weight (Mp2).

7. The elasticity of a heterogeneously cata-lyzed nanocomposite melt is higher thanthat of a homogeneously catalyzed sample.This is attributed to the better dispersion ofclay in the polymer.

8. The viscoelastic properties of the melt in-crease with the clay loading, and this isconsistent with the percolation picture.

S. Ray thanks the Council of Scientific and IndustrialResearch for junior and senior research fellowships. G.Galgali thanks the Council of Scientific and IndustrialResearch for a senior research fellowship.

REFERENCES AND NOTES

1. Giannelis, E. P.; Krishnamoorti, R.; Manias, E. AdvPolym Sci 1999, 138, 107–147.

2. Alexandre, M.; Dubois, P. Mater Sci Eng 2000, 28,1–63.

Figure 14. Comparison of the G� values for samples with similar Al/Fe ratios, similarmolecular weights, and similar extents of clay exfoliation but different clay loadings:(}) entry 3, (Œ) entry 7, and (■) entry 11 (Table 2).


3. Carrado, K. A. Appl Clay Sci 2000, 17, 1–23.4. LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J Appl Clay

Sci 1999, 15, 11–29.5. Ray, S. S.; Okamoto, M. Prog Polym Sci 2003, 28,

1529–1641.6. Giannelis, E. P. Adv Mater 1996, 8, 29–35.7. Wang, Z.; Pinnavaia, T. J Chem Mater 1998, 10,

3769–3771.7. Lemmon, J. P.; Lerner, M. M. Chem Mater 1994, 6,

207–210.9. Wang, Z.; Lan, T.; Pinnavaia, T. J Chem Mater

1996, 8, 2200–2204.10. Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J Chem

Mater 1994, 6, 573–575.11. Nicoud, J. F. Science 1994, 263, 636–637.12. Yano, K.; Usuki, A.; Okada, A.; Kuranchi, T.; Ka-

migaito, O. J Polym Sci Part A: Polym Chem 1993,31, 2493–2498.

13. Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.;Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J Ma-ter Res 1993, 8, 1179–1184.

14. Munzy, C. D.; Butler, B. D.; Hanley, H. J. M.;Tsvetkov, F.; Peiffer, D. G. Mater Lett 1996, 28,379–384.

15. Doh, J. G.; Cho, I. Polym Bull 1998, 41, 511–518.16. Akelah, A.; Moet, A. J Mater Sci 1996, 31, 3589–

3596.17. Kato, M.; Usuki, A.; Okada, A. J Appl Polym Sci

1997, 66, 1781–1785.18. Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.;

Okada, A. Macromolecules 1997, 30, 6333–6338.

19. Tudor, J.; Willington, L.; O’Hare, D.; Royan, B.Chem Commun 1996, 17, 2031–2032.

20. Jin, Y. H.; Park, H. J.; Im, S. S.; Kwak, S. Y.; Kwak,S. Macromol Rapid Commun 2002, 23, 135–140.

21. Alexandre, M.; Dubois, P.; Sun, T.; Garces, J. M.;Jerome, R. Polymer 2002, 43, 2123–2132.

22. Heinemann, J.; Reichert, P.; Thomann, R.; Mul-haupt, R. Macromol Rapid Commun 1999, 20, 423–430.

23. Bergman, J. S.; Coates, G. W.; Chen, H.; Giannelis,E. P.; Thomas, M. G. Chem Commun (Cambridge)1999, 21, 2179–2180.

24. Britovsek, G. J.; Bruce, M.; Gibson, V. C.; Kimber-ley, B. S.; Maddox, P. J.; Mastroianni, S.; McTav-ish, S. J.; Redshaw, C.; Solan, G. A.; Stromberg, S.;White, A. J.; Williams, D. J. J Am Chem Soc 1998,121, 8728–8740.

25. Solomon, M. J.; Almusallam, A. S.; Seefeldt, K. F.;Somwangthanaroj, A.; Varadan, P. Macromole-cules 2001, 34, 1864–1872.

26. Galgali, G. S.; Ramesh C.; Lele, A. K. Macromole-cules 2001, 34, 852–858.

27. Messersmith, P. B.; Giannelis, E. P. J Polym SciPart A: Polym Chem 1995, 33, 1047–1057.

28. Saujanya, C.; Imai, Y.; Tateyama, H. Polym Bull2002, 49, 69–76.

29. Sun, T.; Garces, J. M. Adv Mater 2002, 14, 128–130.

30. Ren, J.; Casanueva, B. F.; Mitchell, C. A.; Krish-namoorti, R. Macromolecules 2003, 36, 4188 –4194.

318 RAY ET AL.

Documents

In situ polymerization of ethylene with bis(imino)pyridine iron(II) catalysts supported on clay: The synthesis and characterization of polyethylene–clay nanocomposites