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Polymer 53 (2012) 4178e4186

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Polymer

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Dispersion of nano-clay at higher levels into polypropylene with carbon dioxidein the presence of maleated polypropylene

Chen Chen, Donald G. Baird*

Department of Chemical Engineering, Virginia Polytechnic Institute and State University, 133 Randolph Hall, Blacksburg, VA 24061, USA

a r t i c l e i n f o

Article history:Received 16 May 2012Received in revised form18 July 2012Accepted 21 July 2012Available online 27 July 2012

Keywords:Nano-clayMaleated polypropyleneSupercritical CO2

* Corresponding author. Tel.: þ1 540 231 5998; faxE-mail address: dbaird@vt.edu (D.G. Baird).

0032-3861/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2012.07.048

a b s t r a c t

The use of supercritical carbon dioxide (scCO2) has proven to be beneficial for surface modified mont-morillonite (MMT) nano-clay dispersion up to 6.6 wt% in a polypropylene (PP) matrix and lead toimproved material mechanical properties in our earlier research. Our further modifications of the pro-cessing procedure including a sequential mixing technique successfully extended the technique to PPcomposites with as much as 10 wt% of clays and continuously increasing mechanical properties. In orderto obtain additional enhancements of the composite properties at this clay level, polypropylene graftedwith maleic anhydride (PP-g-MA) is included in this work. The results from the studies of the mechanicalproperties, rheological properties, and transmission electron microscopy (TEM) show that PP-g-MA isgreatly beneficial in generating an exfoliated nano-clay morphology. Greater enhancements in themechanical properties and nano-clay dispersion in the polymer matrix are observed when PP-g-MA iscombined with the scCO2 and sequential mixing techniques. The PP-g-MA based nano-clay compositeshave a high degree of exfoliated structure even with the addition of up to approximately 10 wt% nano-clay when using this technique, with mechanical properties such as yield strength and Young’s modulusbeing increased by as much as 12 and 88%, respectively, relative to the polymer matrix. It is believed thatthe modulus reported here is the highest reported in the literature for conventional PP’s.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Since the successful preparation of nylon-6/nano-clay compos-ites by Toyota research group in 1993 [1], polymer-clay nano-composites have attracted tremendous interest because of theirpotential to exhibit enhanced thermal, barrier, physical, andmechanical properties compared to conventionally filled compos-ites, such as glass fiber composites [2e4]. The Toyota group founda 68% increase in tensile modulus, a 224% increase in flexuralmodulus, and an 87% increase in the heat distortion temperaturerelative to neat polymer material with the addition of 4.7 wt% clay[1]. However, the improvements of the composite mechanicalproperties in other polymer systems are not as significant as for thecase of nylon-6, especially for non-polar polymers, such as poly-propylene (PP) [5e8].

Among various methods that have been applied to compoundthe clay with the polymer matrix [9e16], melt blending pro-cessing is considered to be the most economical, flexible for

: þ1 540 231 2732.

All rights reserved.

formulation, and compatible with commercial practice [17,18].However, in the case of polymers such as PP, the melt com-pounding process itself is not sufficient to achieve well-dispersednano-clay structure and, therefore, a good reinforcement from theclay because of the incompatibility between the matrix and clayplatelets [19e21].

Maleic anhydride grafted polypropylene (PP-g-MA) has beenused to promote the interaction between PP and the nano-claysurface [22e26]. The anhydride group can coordinate with thesilicate tactoids which contains hydroxyl groups and provide theaffinity between the nano-clay and PP matrix [27e29]. It has beenshown that PP-g-MA is effective in facilitating the intercalation ofPP into nano-clay platelets and enhancing the physical propertiesof the composites [29,30]. Kim et al. [31] reported that meltblending nano-clay/PP using a twin screw extruder with the aid ofPP-g-MA provided about a 65% increase of Young’s modulus withthe addition of 7 wt% of nano-clay. However, the other mechanicalproperties of their composites such as % elongation at break weremuch lower than the pure PP matrix.

The use of supercritical carbon dioxide (scCO2) to exfoliatenano-clay coupled with the melt blending method has beenproved to be beneficial to create more exfoliated structure of

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e4186 4179

nano-clays [32e35]. Manke et al. [33] developed a process thatallows clay particles to be pre-treated with scCO2 in a pressurizedvessel and then catastrophically depressurized to atmosphericpressure so that the stacked nano-clay platelets were forced apart.The presence of exfoliated nano-clay particles was identified bywide angle X-ray diffraction (WAXD). However, they did notprovide any mechanism for assuring that the exfoliated particlesremained exfoliated once combined with the polymer at hightemperature. Nguyen et al. [35,36] developed a procedure thatalso involved this scCO2 exfoliation process, which relied on therapid expansion of nano-clay followed by direct injection intoa single-screw extruder. In the presence of PP-g-MA, the nano-clay/PP composite prepared by Nguyen et al. had an increase inYoung’s modulus as high as 69% for the nano-clay/PP composite ata clay loading of 6.6 wt% [36]. However, few researches havereached the clay loading over 7 wt% because of the increasingdifficulties of processing and significant aggregations of nano-clayas the clay loading increases.

In a previous study [21], we developed a semi-continuousprocess that can improve the nano-clay dispersion with the aid ofscCO2 based on Nguyen’s method [35], and extended it to a higherclay loadings as high as 10 wt% by introducing a sequential mixingtechnique. In the newly developed process, the nano-clay layerswere delaminated by catastrophically releasing the scCO2 treatedclay into the polymer pellets and reduced the collapse of nano-clayduring processing by sequentially mixing the nano-clay into poly-mer pellets. It was observed that for concentrations as high as 10wt%, intercalated nanocomposites were obtained, with a 63% increasein Young’s modulus and a 16% increase in tensile strength using PPas the matrix [21]. In this work, this procedure is extended furtherby incorporation of PP-g-MA. We want to ascertain whether or notadditional enhancements in the dispersion of nano-clays and themechanical properties of the nanocomposites could be achievedusing the newly developed processing method in the presence ofPP-g-MA. The effect of the PP-g-MA on the morphological,mechanical and linear viscoelastic properties of the nano-composites prepared with two different processing techniques isalso studied.

2. Experimental

2.1. Materials

The polymer matrix used in this work is polypropylene (Pro-Fax6523) which was obtained from Lyondell Basell (Houston, TX) andwas used as received. Themelt index of the polymer is 4 g/10min at230 �C and at a load of 2.16 kg. Maleic anhydride grafted poly-propylene (PP-g-MA, PB3150, MA content ¼ 0.5 wt%) was obtainedfrom Chemtura Corp. (Middlebury, CT) and was used as received.The melt index of the PP-g-MA is 52.2 g/10 min at 230 �C and ata load of 2.16 kg. The nano-clay (Cloisite 20A) was obtained fromSouthern Clay Products, Inc. (Gonzalez, TX) and was used as-received. Cloisite 20A is a surface modified montmorilloniteobtained through a cation exchange reaction, where the sodiumcation is replaced by dimethyl, dihydrogenated tallow, quaternaryammonium cation.

2.2. Clay concentration

Actual clay concentrations were determined by a burn-offtechnique in an ashing oven at 500 �C for 2 h. The reportedconcentrations are an average of three burn-off samples. The clayconcentrations reported here include the intercalants or theorganic modifiers.

2.3. Preparation of nano-clay/PP composites

Before mixing the nano-clay with the polymer pellets, the PPand PP-g-MA pellets were pre-mixed by melt compounding usinga single screw Killion KL-100 extruder at 200 �C and re-pelletizing.The weight amount of the PP-g-MA was determined to be threetimes the nano-clay weight, based on comparisons of thecomposite mechanical properties using different PP-g-MA/nano-clay ratios. For example, to make a composite that contains10 wt% of nano-clay, 30 wt% of PP-g-MA is needed. We found thatat a PP-g-MA/clay ratio of 1.5:1 does not provide enough interca-lation assistance and including too much of PP-g-MA (PP-g-MA/clay ratio ¼ 9:1) is detrimental to the matrix mechanical proper-ties, which agreed with what has been reported in the literature[31,37].

The pre-mixed PP/PP-g-MA polymer pellets and organic modi-fied nano-clays were dried separately at 80 �C under vacuumovernight. The nano-clays were then put in a pressurized chamberand allowed to be in direct contact with scCO2 at 3000 psi and at80 �C for 12 h. The dried polymer pellets were put into a 5 galpressure vessel. The nano-clay and polymer pellets were mixed asclay was released rapidly with the CO2 into the 5 gal pressurevessel. The nano-clay and pellet mixture was then collected and fedinto the extruder hopper. A pressure chamber of 660mlwas used tocontain nano-clays and was obtained from Parr InstrumentCompany (Moline, IL). The inlet/outlet of the chamber was sealedby a ball valve from High Pressure Equipment Company (Erie, PA).PP/clay mixture was then extruded at a melt temperature ofapproximately 200 �C and a screw speed of 20 rpm using a singlescrew Killion KL-100 extruder with a 25.4 mm (1 in) diameter, an L/D of 20:1, and variable channel depth from 12.80 mm at the feed to7.90 mm at the exit. A capillary die of 1.59 mm diameter and 20:1 L/D was attached to the end of the extruder. The extruded nano-composites were then chopped into pellets and dried at 80 �C ina vacuum oven overnight. The dried composite pellets wereinjection molded with an Arburg Allrounder (model 221-55-250)injection molding machine. The Arburg Allrounder operated witha 22-mm diameter barrel, L/D of 24:1, and a screw with a variableroot diameter from approximately 14.25mm at the feed to 19.3mmat the exit. A check ring non-return valve and an insulated nozzlethat was 2 mm in diameter were included in the apparatus. Thecomposites were injection molded with a melt temperature of200 �C, a mold temperature of 80 �C, a holding pressure of 5 bar,a screw speed of 200 rpm, and a rectangular end-gated mold withdimensions 80 mm � 76 mm � 1.5 mm. A sequential mixingprocedure that we developed earlier was used to prepare thesamples containing more than 5 wt% nano-clays in order to reducethe collapse of nano-clay during processing [21]. In this procedure,the 5% clay/polymer composite pellets were first obtained using theprocedure as described above. The dried 5% composite pellets werethen put back in the 5 gal pressure vessel to mix with more exfo-liated clays and injection molded into composite plaques yieldinga total clay concentration of 10 wt%. Composites with higher clayloadings than 10 wt% were not investigated in this work but will beconsidered in the future.

In addition to the procedure just described, the conventionaldirect blending method was used to prepare nanocomposites forcomparison purposes. In this approach, the organically modifiednano-clay (Cloisite 20A) was used as received. The clay and pre-mixed PP/PP-g-MA polymer pellets were mechanically mixed ina Kitchen Aid type mixer and dried together at 80 �C in a vacuumoven overnight. The mixture was then fed to an extruder and re-pelletized. The abbreviations used to refer to the individualsamples in the rest of this article as well as the actual clay loadingare listed in Table 1.

Table 1Actual clay concentration and abbreviation of individual samples.

Sample abbreviation Actual clayloading(wt%)

Sample description

PP e Pure PP5% MMT/PP DB 5.36 MMT/PP prepared by direct melt blending method5% MMT/PP CO2 5.44 MMT/PP prepared by scCO2 aided method10% MMT/PP DB 9.29 MMT/PP prepared by direct melt blending method10% MMT/PP CO2 9.53 MMT/PP prepared by scCO2 aided blending method and sequential mixingPP/PP-g-MA e PP/PP-g-MA (MA content ¼ 0.5 wt%) PP-g-MA to clay weight ratio ¼ 3:15% MMT/PP/PP-g-MA DB 4.50 MMT/PP/PP-g-MA composite prepared by direct melt blending method5% MMT/PP/PP-g-MA CO2 5.13 MMT/PP/PP-g-MA composite prepared by scCO2 aided method10% MMT/PP/PP-g-MA DB 9.82 MMT/PP/PP-g-MA composite prepared by direct melt blending method10% MMT/PP/PP-g-MA CO2 10.55 MMT/PP/PP-g-MA composite prepared by scCO2 aided blending method and sequential mixing

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e41864180

2.4. Tensile properties

The injection-molded plaques were cut into rectangular barslengthwise both along the flow direction and trans-flow direction,and the bars were approximately 6 mm wide, 1.5 mm thick, and80 mm long. Tensile tests on these bars were performed at roomtemperature with an Instron Model 4204 testing machine (Instron,Grove City, PA). An extensometer was used to accurately measureYoung’s modulus. The loadwasmeasured with a 5-kN load cell, andthe crosshead speed was kept at 1.27 mm/min during all tensiletests. For all tests, the average and the standard deviation werecalculated from at least six samples, and data points greater than 2standard deviations from the mean were omitted.

2.5. Rheological properties

Rheological measurements on the nanocomposites were per-formed using a Rheometrics mechanical spectrometer model 800(RMS-800). The injected plaques were stamped into 25 mmdiameter disks. Dynamic frequency sweep experiments were per-formed under a continuous nitrogen atmosphere using a 25 mmparallel plate fixture at 200 �C in the linear viscoelastic region of thematerials. The linear viscoelastic limit was determined using strainsweeps at a frequency of 10 rad/s and at the same temperature(200 �C). It was found that dynamic frequency sweep experimentscould be conducted at a strain of 5%. The elastic moduli (G0), lossmoduli (G00), and complex viscosities (h*) of the materials as func-tions of angular frequency (u) (ranging from 0.1 rad/s to 100 rad/s)were obtained at a temperature of 200 �C.

2.6. Structure and morphological characterization

Transmission electron microscopy (TEM) was used to characterizethe morphology of the nanocomposites. TEM measurements weregeneratedwithaPhilipsEM420Twithanacceleratingvoltageof100kV.The TEM samples, around 70 nm thick, were cut with a cryomicrotomequipped with a diamond knife at �55 �C. Injection molded sampleswere used for the TEM measurements. Wide-angle X-ray diffraction(WAXD) scans were not conducted because the TEM’s and increasedmechanical properties are sufficient to show that improved exfoliationof the clays occurred. Furthermore,WAXDcanbemisleading as the lackof sample or low concentration of clay in the region of the beam mayerroneously suggest there is an exfoliated structure [38,39].

3. Results and discussion

3.1. Transmission electron microscopy analysis

TEM analysis was carried out to evaluate the morphology of thenano-clay in the matrix. TEM images of the nano-clay composites

prepared with and without PP-g-MA using different processingmethods are compared in Figs. 1 and 2. Images with 5800�magnification are shown in Fig.1 to illustrate the general dispersionstate of the nano-clay, and images with higher magnification at34,000� are shown in Fig. 2 to reveal the detailed clay structure.Themorphologies of the 5 wt% samples preparedwith PP-g-MA aresignificantly different from those of the composites processedwithout PP-g-MA. As seen in Fig. 1(a, b), the dispersion of the nano-clay for the 5% MMT/PP both exhibited a mixture of phase sepa-rated and intercalated morphologies with tactoids in various sizes,with a slightly more intercalated structure for the one prepared bythe scCO2 aided method at high magnification (Fig. 2(b)). With theincorporation of PP-g-MA, the 5% MMT/PP/PP-g-MA samplesprepared by both the direct blending method and the scCO2 aidedmethod showmorphologies with a high degree of exfoliation, withonly a few silicate layers stacked together (Figs. 1 and 2(e, f)). Themorphologies of the two 5% MMT/PP/PP-g-MA samples are verysimilar to each other on both scales. This is probably because PP-g-MA is so effective in terms of improving the clay dispersion that thehelp from scCO2 technique is not obvious at this clay loading.

The 10 wt% nano-clay composites show a greater dependenceon the processing method in both scales as well as the presence ofPP-g-MA (shown in Figs. 1 and 2(c, d, g, h)). As can be seen fromFigs. 1 and 2(c), the 10% MMT/PP DB sample has very poor claydispersion, with severe aggregation and tactoids on the order ofhundreds of layers (estimated based on the aggregation size inFig. 2(c) and nano-clay thickness). This poor dispersion of clay inthe matrix is common because of the lack of the affinity betweenthe partially-polar clay surface and the highly non-polar polymermatrix, as well as the collapse of clay during processing when theclay concentration is high. Apparently conventional melt interca-lation is not effective in exfoliating/intercalating the nano-clay inpure PP matrix at this high loading. Using the scCO2 aided methodand sequential mixing, the MMT/PP nanocomposite (shown inFigs. 1 and 2(d)) has improved clay dispersion over the oneprepared by the direct blending method. The sample shows tac-toids on the order of tens of layers with a long and thin shape.However, a nano-claymorphology with a high degree of exfoliationwas not obtained. With the help of PP-g-MA, the 10% MMT/PP-g-MA DB sample (Figs. 1 and 2(g)) has a much higher degree of claydispersion and exfoliation than the samples processed with thesame procedure but without PP-g-MA. Although some large tac-toids are still visible, a significant amount of clay layers are clearlydispersed into the matrix. Although the silicate surface wasmodified with alkyl groups, part of the surface is still polar, espe-cially the edges of the surface. The ability of the polar functionalgroups of PP-g-MA to hydrogen bond and interact with the polaredges of the nano-clay layers helps promote the intercalation of thepolymer chains into the clay galleries [40]. Further improvement ofthe dispersion of nano-clay can be seen in the 10% MMT/PP/PP-g-

Fig. 1. Transmission electron micrographs at 5800� magnification of (a) 5 wt% MMT/PP nanocomposites processed by the direct blending method, (b) 5 wt% MMT/PP prepared bythe scCO2 aided melt blending, (c) 10 wt% MMT/PP prepared by the direct blending method, and (d) 10 wt% MMT/PP prepared by the scCO2 aided melt blending method withsequential mixing, (e) 5 wt% MMT/PP/PP-g-MA nanocomposites processed by the direct blending method, (f) 5 wt% MMT/PP/PP-g-MA prepared by the scCO2 aided melt blending,(g) 10 wt% MMT/PP/PP-g-MA prepared by the direct blending method, and (h) 10 wt% MMT/PP/PP-g-MA prepared by the scCO2 aided melt blending method with sequential mixing.

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e4186 4181

MA CO2 sample (Figs. 1 and 2(h)). The nano-clay platelets are fairlywell-dispersed in the matrix, even at high magnification, with anexfoliated structure dominated morphology and a few intercalatedsilicate layers. This observation suggests that the scCO2 aided meltprocessing method with sequential mixing step has a great benefitin terms of improved dispersion of nano-clay in the polymer meltwhen done in the presence of PP-g-MA.

3.2. Composite mechanical properties

In the previous study, we reported the mechanical properties ofvarious PP-clay nanocomposites prepared using different process-ing techniques without the incorporation of PP-g-MA at clay

loadings of 10 wt% [21]. We found that the scCO2 aided meltblending method with sequential mixing step is the only approachthat was able to provide a continuous increase in the mechanicalproperties of the injectionmolded samples with the increase of clayloading. As much as a 63% increase in Young’s modulus and a 16%increase in tensile strength were obtained at 10 wt% of clay loading.The other processing methods, including the conventional directmelt blending method, conventional direct melt blending methodwith sequentialmixing step, and scCO2 aidedmelt blendingmethodwithout sequential mixing step, did not lead to much improvementbecause of their inability to adequately exfoliate the nanoparticlesand keep the exfoliated nanoparticles from collapsing during pro-cessing at high temperature at this clay loading.

Fig. 2. Transmission electron micrographs at 34,000�magnification of (a) 5 wt% MMT/PP nanocomposites processed by the direct blending method, (b) 5 wt% MMT/PP prepared bythe scCO2 aided melt blending, (c) 10 wt% MMT/PP prepared by the direct blending method, and (d) 10 wt% MMT/PP prepared by the scCO2 aided melt blending method withsequential mixing, (e) 5 wt% MMT/PP/PP-g-MA nanocomposites processed by the direct blending method, (f) 5 wt% MMT/PP/PP-g-MA prepared by the scCO2 aided melt blending,(g) 10 wt% MMT/PP/PP-g-MA prepared by the direct blending method, and (h) 10 wt% MMT/PP/PP-g-MA prepared by the scCO2 aided melt blending method with sequential mixing.

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e41864182

In this section, the effect of the PP-g-MA on the mechanicalperformance of the nanocomposites prepared with two differentprocessing techniques, direct blending and CO2 aidedmethod (withsequential mixing step at 10 wt%), is examined. As shown in Table 2and compared in Fig. 3, the clay composites using PP-g-MA havea different response in the Young’s modulus than the PP-claynanocomposites without PP-g-MA. The PP/PP-g-MA matricesused in this study have the Young’s moduli of 1.530 � 0.072 GPaand 1.525 � 0.089 GPa corresponding to 5 wt% and 10 wt% of clayloadings, respectively, due to the slight loss of properties in thematrix due to PP-g-MA. By adding approximately 5 wt% of MMTusing the conventional melt blending method, the nanocompositeis found to have a Young’s modulus of 2.237 � 0.103 GPa, an

increase of 46% compared to the PP/PP-g-MA matrix. The 5% MMT/PP/PP-g-MA sample prepared by the scCO2 aided method hasa Young’s modulus of 2.469 � 0.149 GPa, which is only slightlyhigher than the 5% sample prepared by the direct blendingmethod.This is probably because the MA aided delamination of clay in thepolymer matrix is already sufficient to exfoliate the clay platelets assimilar morphologies were obtained for these two samples (dis-cussed in the previous section). Using the scCO2 aided methodcould not provide further improvement of the nano-clay dispersionat this concentration, and thus, the Young’s modulus does notincrease regardless of the processing approach. At the 10 wt% ofclay loading level, the direct blended MMT/PP/PP-g-MA sampleyielded a Young’s modulus of 2.657 � 0.110 GPa, an increase of 74%

Table 2Mechanical properties of MMT/PP and MMT/PP/PP-g-MA nanocomposite prepared using different processing methods.

Materials Young’s modulus (GPa) S.D. % increase Yield strength (MPa) S.D. % elongation S.D.

PP 1.548 0.065 e 28.80 0.69 >17 e

5% MMT/PP DB 1.968 0.033 27 30.98 0.29 >17 e

5% MMT/PP CO2 2.178 0.043 41 30.40 0.75 >17 e

10% MMT/PP DB 2.011 0.107 30 29.91 0.24 >17 e

10% MMT/PP CO2 2.524 0.108 63 33.42 0.52 >17 e

PP/PP-g-MA for 5% clay loading 1.530 0.072 e 28.10 0.76 >17 e

5% MMT/PP/PP-g-MA DB 2.237 0.103 46 31.09 0.78 >17 e

5% MMT/PP/PP-g-MA CO2 2.469 0.149 61 32.39 0.42 >17 e

PP/PP-g-MA for 10% clay loading 1.525 0.089 e 27.58 0.89 >17 e

10% MMT/PP/PP-g-MA DB 2.657 0.110 74 31.67 0.49 8.42 2.3410% MMT/PP/PP-g-MA CO2 2.866 0.111 88 30.81 0.48 >17 e

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e4186 4183

compared to the PP/PP-g-MAmatrix, much higher than the clay-PPcomposite processed without PP-g-MA using the same processingmethod. By using the newly developed procedure (scCO2 aidedmethod blending with sequential mixing), a Young’s modulus of2.866 � 0.111 GPa, which is an 88% increase in modulus relative tothat of the matrix, was obtained for the MMT/PP/PP-g-MAcomposites at 10 wt% of clay loading. About 20% of additionalenhancement of the Young’s modulus was obtained by the incor-poration of PP-g-MA when using the same techniques. The PP-g-MA has a positive effect in the composite reinforcement andprovide additional enhancement of the composite Young’smodulus, which is believed to be due to improved nano-claydispersion and increased bonding between the nanoparticles andthe matrix caused by PP-g-MA.

In addition to the improvement of the moduli of the clay-composites, all the MMT/PP/PP-g-MA samples are found to havea tensile strength which is about 10e15% higher than that of thematrix tensile strength (Table 2), which is similar to the observationwith the clay-PP composite without using PP-g-MA. The values of %elongation at break of the MMT/PP/PP-g-MA samples up to therecorded limit also show no significant difference than that of thepure PP matrix, except the 10% direct blended sample. It is encour-aging that by using the scCO2 aided melt blending method, theseother mechanical properties of the composites were able to retainthe same level along with the improvements in Young’s moduluswhen the composites were prepared in the presence of PP-g-MA.

0 2 4 6 8 10

1.5

2.0

2.5

3.0

3.5

Youn

g's

Mod

ulus

(GPa

)

clay concentration (wt %)

PP/PP-g-MA MMT/PP DB MMT/PP CO2 MMT/PP/PP-g-MA DB MMT/PP/PP-g-MA CO2 Halpin-Tsai

Fig. 3. Young’s moduli of MMT/PP nanocomposites and MMT/PP/PP-g-MA processedby different methods and the comparison with theoretical predictions (Note: The linesfor the experimental values are solely assisting to separate the data from processingmethods. These lines are not intended to show any trend of the modulus change).

The mechanical properties of the MMT/PP/PP-g-MA samplesmeasured in the transverse direction relative to the main flowdirection during mold filling are also reported in Table 3 and arecompared with their moduli in the flow direction in Fig. 4. There isan obvious anisotropic behavior of the injection molded plaque,even for the pure PP materials. The Young’s modulus of pure PPdecreased to 1.311 � 0.070 GPa from 1.548 � 0.065 GPa due to theorientation of the PP chains during crystallization. As can be seenfrom Fig. 4, the differences of composite moduli between flowdirection and transverse direction are larger than the difference ofthe pure polymer matrix, which suggests the possible orientationeffect of nano-clay platelets. The largest difference in the Young’smoduli between these two directions is seen in the 10% MMT/PP/PP-g-MA CO2 sample. This suggests that the clay particles may bemore likely to align at higher concentration and after treated withscCO2. The tensile strength and elongational properties of thecomposites in the transverse direction are also lower relative to theflow direction properties of the same samples. All of these obser-vations confirm that the anisotropic behavior of an injection mol-ded plaque exists in the nano-clay based composites, which isa common observation for glass fiber reinforced polymercomposites [41].

In order to determine to what degree we are realizing the fullpotential increase of the mechanical properties, it is necessary tocompare the observed property enhancements, such asmodulus, tothose predicted by composite theories, such as that of Halpin-Tsaimodel [42]. The Halpin-Tsai model, as shown in Eq. (1) is oftenused because of its simplicity despite the existence of more accu-rate theoretical predictions [43]. It assumes fully exfoliated andunidirectional clay platelets, as well as a high degree of adhesion ofthe filler particles to the surrounding polymer matrix. The model isgiven below:

Ec ¼ Em

"1þ xhff

1� hff

#(1)

where x ¼ 2(1/t) and

Table 3Mechanical properties of MMT/PP/PP-g-MA nanocomposite prepared usingdifferent processing methods on the transverse-flow direction.

Materials Young’smodulus(GPa)

S.D. Yieldstrength(MPa)

S.D. % elongation S.D.

PP 1.311 0.070 28.65 0.98 >17 e

5% MMT/PP/PP-g-MA DB 1.944 0.185 28.08 0.69 >17 e

5% MMT/PP/PP-g-MA CO2 2.048 0.097 29.16 0.42 12.04 4.3210% MMT/PP/PP-g-MA DB 2.374 0.164 26.69 0.56 3.79 1.0410% MMT/PP/PP-g-MA CO2 2.520 0.094 27.44 0.32 14.26 1.89

10-1 100 101 102

102

103

104

105

G' (

Pa)

frequency (rad/s)

Fig. 5. Storage modulus, G0 , of (:) PP, (,) 5 wt% MMT/PP/PP-g-MA compositeprepared by the direct blending method, (B) 5 wt% MMT/PP/PP-g-MA compositeprepared by the scCO2 aided method, (-)10 wt% MMT/PP/PP-g-MA compositeprepared by the direct blending method, (C)10 wt% MMT/PP/PP-g-MA compositeprepared by the scCO2 aided method and sequential mixing.

Fig. 4. The comparison of Young’s moduli on flow direction and transverse directionfor the PP matrix and MMT/PP/PP-g-MA composite.

10-1 100 101 102

103

104

G" (

Pa)

frequency (rad/s)

Fig. 6. Loss modulus, G00 , of (:) PP, (,) 5 wt% MMT/PP/PP-g-MA composite preparedby the direct blending method, (B) 5 wt% MMT/PP/PP-g-MA composite prepared bythe scCO2 aided method, (-)10 wt% MMT/PP/PP-g-MA composite prepared by thedirect blending method, (C)10 wt% MMT/PP/PP-g-MA composite prepared by thescCO2 aided method and sequential mixing.

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e41864184

h ¼hEf =Em � 1

ihEf =Em þ x

i (2)

Ef, Em and Ec are the filler, matrix, and composite modulus,respectively. ff is the filler volume fraction and 1/t is the aspectratio of the filler particles. The Young’s modulus of nano-clay Ef wastaken as 178 GPa [44] and the aspect ratio of the silicate platelets (L/D) was taken to be approximately 100 for fully exfoliated platelets[44]. With the density of the matrix, rm, equal to 0.9 g/cm3 and thedensity of nano-clays, rf , equal to 1.77 g/cm3, the volume fraction ofthe nano-clays ff can be found from the weight percentage wf .

ff ¼ rmwf =�rf � rf wf þ rmwf

�(3)

The theoretical and experimentally measured moduli of thecomposites versus weight percent of MMT are compared in Fig. 3.As can be seen, the experimental Young’s moduli are still belowthose predicted by the Halpin-Tsai model, even with the corpora-tionwith PP-g-MA at all concentrations. The PP-g-MA clearly raisedthe experimental data closer to the theoretical values. However, theactual modulus is still about 30% under the prediction at 5 wt% ofnano-clay and 50% lower than the theoretical value at the 10 wt%clay loading. The lack of agreement between the theory andexperimental values is due to the inability to satisfy the modelassumptions, including the lack of full exfoliation for the nano-clays, complete orientation of the filler particles in the flow direc-tion and significant bonding between MMT and polypropylene.

3.3. Nanocomposite rheological properties

In this section, we look at rheological behavior of the MMT/PP/PP-g-MA composites melts. The values of storage modulus, G0, lossmodulus, G00, and complex viscosity, jh*j, for the MMT/PP/PP-g-MAcomposites resulting from the dynamic frequency scan measure-ments are compared Figs. 5e7, respectively. Repeat rheologicalmeasurements on the same sample prepared multiple times revealthat G0, G00, and jh*j are accurate to about 2%.

As shown in Figs. 5e7, the G0, G00, and jh*j of the MMT/PP/PP-g-MA melt are greatly dependent on both the clay loading and pro-cessing method. The G0, G00, and jh*j values of the MMT/PP/PP-g-MAmelts greatly increased with the increase of clay loading. This could

be due to the influence of both of the increased clay loading and thehigher PP-g-MA amount. The 5% MMT/PP/PP-g-MA samplesprepared with different methods do not have significant differ-ences in the rheological properties, probably because they do nothave appreciable differences in morphology, as discussed in theprevious section. Both 10 wt% samples exhibit obvious “tail” in G0

and G00 and yield-like behavior in jh*j at the low frequencies.However, the 10% MMT/PP/PP-g-MA DB sample presents a higher“tail” than the 10% MMT/PP/PP-g-MA CO2 sample. It is generallyreported in the literature that a ‘‘tail’’ in the storage modulus, G0, orthe yield-like behavior in the complex viscosity, jh*j, versus angularfrequency at low frequencies can be observed when the nano-claysare exfoliated [45]. However, Nguyen et al. [35] found that the “tail”

10-1 100 101 102

103

104

105

Com

plex

Vis

cosi

ty (P

a S)

frequency (rad/s)

Fig. 7. Complex viscosity, jh*j versus frequency, u, of (:) PP, (,)5 wt% MMT/PP/PP-g-MA composite prepared by the direct blending method, (B) 5 wt% MMT/PP/PP-g-MAcomposite prepared by the scCO2 aided method, (-)10 wt% MMT/PP/PP-g-MAcomposite prepared by the direct blending method, (C)10 wt% MMT/PP/PP-g-MAcomposite prepared by the scCO2 aided method and sequential mixing.

C. Chen, D.G. Baird / Polymer 53 (2012) 4178e4186 4185

could be a result of network formed due to either interactions ofmatrix groups or large agglomerates of clay. Their samples withexfoliated structure did not create this “tail” in G0 while the samplethat had large agglomerates with the addition of 24 wt% of nano-clay exhibited a “tail”. In this work, the TEM image of the 10%MMT/PP-g-MA DB sample showed worse degree of clay dispersionand exfoliation than the 10% MMT/PP-g-MA CO2 sample. In thiscase, the higher tail is not definitely indicative for a more exfoliatedmorphology. The large aggregations of the nano-clays in the 10%MMT/PP-g-MADB sample combinedwith strongmatrix interactionbetween theMA groups could cause the higher tail of G0, G00, and themore obvious yield behavior of jh*j at the low frequencies. Ourobservation agrees with Nguyen’s observation. The “tail” of G0, G00,and yield behavior of jh*j at low frequencies does not suggest anexfoliated structure of nano-clay. The rheological characterizationis not sufficient for particle morphology conclusions. Conclusionsregarding the nano-clay morphology have to be made with thecombination of other characterization techniques.

4. Conclusions

Additional enhancements in composite mechanical propertieswere achieved using our method for exfoliating and dispersing thenano-clays into the PP matrix and continuous improvement of theproperties of the nano-clay composites with increasing concen-tration of clay was obtained for composites containing as much as10 wt% of nano-clay in the presence of PP-g-MA. The polarity of thefunctional groups of PP-g-MA was believed to promote the inter-action between the nano-clay and polymer which led to betterdispersion of the silicate platelets, an enhanced G0, G00, and jh*j atlow frequencies, and improved mechanical performance. Based onTEM observations, the incorporation of PP-g-MA greatly improvesthe clay dispersion within the polymer matrix at lower clay levels(5 wt%) by direct melt compounding (without the use of CO2), butcombining the PP-g-MA with the technique that used CO2 andsequential mixing is necessary to achieve a high degree of exfoli-ation of the nano-clay at higher clay loading (10 wt%). The Young’smodulus of the 10% MMT/PP/PP-g-MA sample prepared by the

scCO2 aided method and sequential mixing step is found to be2.866 GPa, an 88% increase compared to that of the polymer matrix,without sacrificing other mechanical properties such as tensilestrength and % elongation at break. This modulus is significantlyhigher than that reported in the previous work of Nguyen et al. [36]at a lower clay loading (69% increase), and, of course, the compositeprepared without using PP-g-MA at the same clay loading (63%increase). Mechanical properties of the nanocomposites in thetransverse direction are determined to be lower than the propertiesalong the flow direction, which suggests the orientation of both thepolymer chains and nano-particles in the main flow directionduring mold filling. However, full exfoliation and complete uni-directional orientation of the nano-clay and significant bondingbetween MMT and polypropylene are not achieved and, hence, theYoung’s modulus did not reach the theoretical predictions forsamples produced by any of the processing methods. However, themodulus reported here is higher than that presently reported in theliterature by others.

Acknowledgments

The authors would like to acknowledge the funding supportfrom Institute for Critical Technology and Applied Science (ICTAS)at Virginia Tech and Southern Clay Products for donating the MMTnano-clays. In addition, we would like to thank Steve McCartney atthe Nanoscale Characterization and Fabrication Laboratory in ICTASfor aid in conducting the TEM studies and John Quigley for hisexperimental contributions to this work.

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