processing of poly(propylene)/carbon nanotube composites using scco2-assisted mixing

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Processing of Poly(propylene)/Carbon NanotubeComposites using scCO2-Assisted Mixing

Jia Ma, Hua Deng, Ton Peijs*

scCO2 was used to assist in the preparation of PP/CNT composites. Two types of CNTs wereused: MWNTs with and without HDPE coating (cMWNTs). The morphology of the nanocom-posites and their mechanical and thermal properties were investigated and compared withsamples made by traditional melt compounding.The use of cMWNT leads to better dispersion andproperties in melt-compounded nanocomposites.For systems prepared using scCO2-assisted mix-ing, however, better properties were obtainedusing pristine MWNTs, avoiding the additionalcosts of nanotube modification. It was also shownthat observed improvements in the mechanicalproperties for these materials were due to a com-bination of matrix modification and nanotubereinforcement, rather than a reinforcement effectcaused solely by MWNTs.

Introduction

Undoubtedly, polymer nanocomposites have established

themselves now as a new class of composite materials.

Because of their high intrinsic mechanical properties,

nanoscale dimensions and high aspect-ratio, small

amounts of nanofillers such as nanoclays or carbon

J. Ma, Dr. H. Deng, Prof. T. PeijsSchool of Engineering and Materials Science, and Center forMaterials Research, Queen Mary University of London, Mile EndRoad, London E1 4NS, UKE-mail: [email protected]. H. DengCollege of Polymer Science and Engineering, Sichuan University,State Key Laboratory of Polymer Materials Engineering, Chengdu,610065 Sichuan, ChinaProf. T. PeijsEindhoven Polymer Laboratories, Eindhoven University ofTechnology, PO Box 513, 5600 MB Eindhoven, The Netherlands

Macromol. Mater. Eng. 2010, 295, 566–574

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

nanotubes (CNTs) canprovide the resultingnanocomposite

materialwith significantly improved properties. One of the

most promising areas of nanocomposite research involves

the use of CNTs as nanofillers.[1] CNTs are considered to be

ideal candidates for a wide range of applications in

materials science because of their exceptional mechanical,

thermal, and electronic properties.[2] In particular, their

exploitation as conductive fillers for polymer composites

has shownsignificant success.[3–7] The effectiveuse of CNTs

as reinforcements for polymer matrices still presents some

major difficulties.[8–10] The key challenge still remains in

breaking down bundles of aggregated CNTs and reaching a

fine dispersion in the selected polymer matrix. Aggregates

of nanotubes in polymer composites may act as imperfec-

tions and restrain the reinforcing potential of the CNTs.

Undoubtedly, nanofiller dispersion represents the key-step

for producing high-performance polymer nanocompo-

sites.[8] A number of techniques have been employed to

achieve better CNT dispersion in polymer matrices,

including sonication, covalent functionalization, and

DOI: 10.1002/mame.200900405

Processing of Poly(propylene)/Carbon Nanotube . . .

non-covalent functionalization.[3,9,11–13] Often, these mod-

ifications have been successful in combination with

solution or latex-based processes.[3,11] However, difficulty

in achieving good dispersions in polymer melts remains.

For this reason, recently, dissociated CNTs have been

produced by grafting polymer chains directly onto CNTs in

order to achieve a homogeneous polymer coating on the

CNT surface.[14–16] Owing to the de-aggregation of these

CNTs, enhanced dispersion and improved properties, by

comparison with the direct incorporation of CNTs in

polymer melts, have been reported for high-density

polyethylene (HDPE)-coated multi-walled nanotubes

(MWNTs) in thermoplasticmatrices such as ethylene/vinyl

acetate (EVA), polycarbonate (PC), polyamide (PA) and poly

(propylene) (PP).[14–18] Nanocomposites can be prepared

by different methods such as melt compounding, solution,

and dispersion blending and in situ polymerization. Melt

compounding is themost commonmethod used to process

thermoplastic polymer/CNTnanocomposites because it is a

cost-effective technology for polyolefin-based polymer

systems and is most compatible with current industrial

practices.[19]However,melt compounding, especially in the

case of polyolefin matrices, is generally less effective at

dispersing CNTs and is limited to low nanofiller loadings.

This is due to the high viscosity of the composite systems

causedby theaddition of CNTs.[18]Moreover, thehigh shear

rates and high temperatures employed can also cause

thermal instability of the molten polymers.[20] One

approach to promote compatibility includes the use of

polymer-coated CNTs instead of pristine CNTs in polymer

melts or masterbatch processes, in which a pre-mixed

highly loaded CNT composite is dilutedwith fresh polymer

melt.[21]Considering the high viscosities and high tempera-

tures involved in melt compounding processes, super-

critical CO2 (scCO2) processing could be an interesting

alternative as it may overcome some of these issues. scCO2

processing, as one of the new and cleaner processing

methods for polymer nanocomposites, has recently

received increasing attention.[22,23] The use of scCO2 can

induce plasticization of the polymer which decreases the

glass transition temperature (Tg) and viscosity of polymer

melts, because the scCO2 can swell the polymer, causing

an increase in free volume and a reduction in chain

entanglements. Although the role of scCO2 has not been

systematically investigated in the melt compounding of

nanocomposites, earlier studies have indicated clear

improvements in the dispersion of nanoclays in PP

matrices.[24,25] Because of PP’s good balance between

properties and cost and its wide usage in industry, PP/

CNT nanocomposites have been extensively studied in

recentyears forenhancedmechanicalproperties.[4,6,18,19,26–32]

Andrews et al.[19] fabricated PP/MWNT composites by a

shear mixer. They found a modulus increase from 1.0 to

2.4GPawith a relatively high nanotube content of 12.5wt.-



%but at the expense of a reduction in yield stress from30 to

18MPa. Manchado et al.[31] reported a small modulus

increase from 0.85 to 1.19GPa with a nanotube content of

0.75wt.-% and a strength increase from 31 to 36MPwith a

0.5wt.-% nanotube content for PP/single-walled nanotube

(SWNT) composites. Most recently, Deng et al.[18] investi-

gated the effects of a HDPE coating onto MWNTs on the

mechanical properties of PP/MWNT composites produced

by melt compounding. They observed a property increase

from 1.4 to 1.8GPa for Young’s modulus and from 34 to

38MPa for tensile strength at relatively low loadings

(0.5wt.-%) of coatedMWNTs. Some studies such as those by

Ganb et al.[32] did not show any increase in mechanical

properties. Overall, previous research activities on PP/CNT

bulk composites have shown that mechanical property

improvements by CNTs are moderate with the more

effective reinforcing effects observed at lower nanotube

contents. In thiswork, scCO2wasused topreparePP/MWNT

nanocomposites and the potential benefits of using scCO2

were investigated. PristineMWNTs aswell as HDPE-coated

MWNTs were evaluated and the dispersion, physical and

mechanical properties of nanocomposites obtained using

scCO2 processing were compared with those of traditional

melt compounded samples. Micromechanical modeling

was used to analyze the effective properties of the CNTs in

the nanocomposites. The current study also addressed

effects of PP matrix modification by the CNTs and its effect

on mechanical properties of the nanocomposites.

Experimental Part

Materials and Equipment

The PP used was Moplen1 HP500H from Basell [melt flow

index¼ 1.8 g � (10min)�1]. The pristine MWNTs used were Nano-

cyl-7000 (Nanocyl S.A., Belgium), which is a thin MWNT produced

via catalytic chemical vapor deposition (CCVD), achieving a purity

of 90% carbon. Nanocyl-9000 is an HDPE-coated MWNT (cMWNT)

which contains 31.6% of nanotubes pre-dispersed in a polymer

carrierusingaproprietary technology.[16,33]Allmaterialswereused

as received.A liquidwithdrawalCO2cylinderat5MPapressurewas

supplied by BOC Gases. A custom-built stirred high-pressure

autoclave with paddle type stirrer was used to prepare the

nanocomposites.ARondol laboratorybenchtophot-presswasused

to produce the dumbbell-shaped specimens.

Processing of Nanocomposites

MWNTs (or cMWNTs) were directly added to the PP matrix in an

autoclavetoobtainaCNTconcentrationof0.1,0.25,0.5and1.0wt.-%.

CO2waspumped into the autoclavevia an Iscomodel 260Dsyringe

pump after being chilled to �6 8C. The autoclave was filled with

liquid CO2 and held at 15MPa and 200 8C under stirring, using a

pitchedblade turbine impeller for 30min. At the end ofmixing, the

autoclave was cooled in water to room temperature, and then CO2

www.mme-journal.de 567

J. Ma, H. Deng, T. Peijs

Table 1. Composition of PP/CNT nanocomposites.

Sample CNT type Processing method

scCO2 PP/MWNT Nanocyl-7000 scCO2

scCO2.m PP/MWNT Nanocyl-7000 scCO2; masterbatch

scCO2 PP/cMWNT Nanocyl-9000 scCO2

scCO2.m PP/cMWNT Nanocyl-9000 scCO2; masterbatch

melt.m PP/MWNT Nanocyl-7000 melt-compounded; masterbatch

melt.m PP/cMWNT Nanocyl-9000 melt-compounded; masterbatch

568

wasvented. In anotherprocessing route, amasterbatchof 3.0wt.-%

CNT content was prepared using scCO2 assisted mixing under

15MPa and 200 8C for 10min before being diluted into lower

concentrations of 0.1, 0.25, 0.5 and 1.0wt.-% CNT using the same

conditions as described above. The bulk materials obtained from

the autoclave were frozen in liquid nitrogen and then broken into

pieces, which were subsequently used for hot-pressing into

dumbbell-shaped specimens according to ASTM D-638 at 200 8Cunder 40MPa for 5min. The samples were cooled down to room

temperature using the water cooling system of the hot-press.

For comparison, the melt-compounded nanocomposites were

prepared by a two-step blending process in a mini twin-screw

extruder (DSMXplore 15mLmicrocompounder) at 200 8C.Nitrogen

gas flowwas used to avoid degradation of the polymer during the

mixingprocess.Amasterbatchof3wt.-%MWNTs(orcMWNTs)was

prepared at a processing condition of 50 rpm for 5min. This

masterbatchwas subsequently dilutedwith pure PP at 250 rpm for

15min to produce samples with lower nanotube concentrations.

Theextrudedstrandsproducedbytheextruderwerecut intopellets

and then hot-pressed into dumbbell-shaped specimens. The

compositions of the nanocomposite samples are listed in Table 1.

Characterization

Scanning electron microscopy (SEM) was used to investigate the

morphological characteristics using a JEOL 6300 (accelerating

voltage10 kV) innormal secondaryelectron imagingmode. Prior to

the examination, samples were freeze-fractured in liquid nitrogen

and sputter coatedwith a thin layer of gold. Transmission electron

microscopy (TEM) was carried out using a JEOL JEM 2010. MWNTs

were dispersed in ethanol using ultrasonication for 5min and then

drops of suspension were deposited onto a copper grid for TEM

observation.

Differential scanning calorimetry (DSC) was performed using a

Mettler ToledoDSC822e. Samples of about10mgwereheated from

�50 toþ200 8C at a scan rate of 10 8C �min�1 and held for 10min to

eraseanythermalhistory,andthencooledto�50 8Cusingthesame

scan rate of 10 8C �min�1. Thermogravimetric analysis (TGA) was

carried out undernitrogenatmosphere in the temperature rangeof

20–1000 8C by using a TA TGA Q500. The heating rate used was

20 8C �min�1.

Wide angle X-ray scattering (WAXS) spectrawere recordedwith

a Siemens diffractometer D5000, where the X-ray beam was Ni-

filteredCuKa (l¼1.5405 A) and the radiationwasoperatedata rate

of 40 kVwithafilamentcurrentof40mA.Correspondingdatawere



collected from 5 to 308 at a scanning rate of 0.018 �min�1.Tensile

tests were performed using an Instron 5566 machine. Tests were

conducted on dog-bone shaped specimens according to the ASTM

D-638 standard. The Young’s modulus of each specimen was

calculated, and the gradient of the stress versus strain curve was

measured between 0.05 and 0.2% strain. The values presented for

all the properties are the average of five repetitions of each test.

Results and Discussion

Figure 1 displays the SEM micrographs of PP nanocompo-

sites. Nanotubes are observed as bright spots in the images.

In the case of conventional melt compounding, better

dispersions of CNTs were observed for the HDPE coated

MWNTs samples, owing to the de-aggregated state of the

cMWNTs [16] (Figure1aandb).However, in the case of scCO2

assisted mixing, large aggregates of cMWNTs were

observed (Figure 1d) and evenwhen amasterbatchmethod

was applied the cMWNTs were still not well dispersed

(Figure 1f). Interestingly, pristine MWNTs without HDPE

coating provided a better and more homogeneous disper-

sion in the PP matrix. (Figure 1c) Similar improvements in

dispersion, for scCO2 assisted mixing without the use of

compatibilizers were also reported for nanoclays in PP

matrices.[25] A better dispersion of MWNTs was observed

using the masterbatch method (Figure 1e).

The TEM images of MWNTs and cMWNTs, which are

presented in Figure 2 may help to explain why the

dispersion of cMWNTs in PP matrix was unsatisfactory in

scCO2 assisted mixing. Pristine MWNTs could be dispersed

into individual tubes on the grid with no large aggregates

remaining (Figure 2a). However, cMWNTs (Figure 2b)

showed quite a different structure. The polymer coated

MWNTs form large agglomerates and although the HDPE

coating helps to separate the individual nanotubes, the

break up of these agglomerates still requires high shear

forces. Therefore, cMWNTs can be dispersed well in

conventional melt compounding processes in twin-screw

extruders because of the high shear forces employed.

However, a pitched blade turbine impeller, used in scCO2

assisted mixing, generates much lower shear forces, thus

leading to poor dispersion of these agglomerates.

DOI: 10.1002/mame.200900405


Figure 1. SEM images of freeze-fractured samples of PP nanocomposites with 0.5% CNTloading (a) melt.m PP/MWNT (b) melt.m PP/cMWNT (c) scCO2 PP/MWNT (d) scCO2 PP/cMWNT(e) scCO2.m PP/MWNT (f) scCO2.mPP/cMWNT.

Figure 2. TEM images of (a) pristine MWNTs (b) HDPE-coated MWNTs.



The values of Young’s modulus

of the nanocomposites with differ-

ent MWNT loadings are presented

in Figure 3. As a result of the

de-aggregation of the MWNT

bundles by the HDPE coating, the

melt-compounded and master-

batch based PP/cMWNT compo-

sites show a continuous modulus

increase up to 1.7GPa from zero to

1.0wt.-% CNT loading. The same

melt-compounded and master-

batch systems based on uncoated

MWNTs only displayed a Young’s

modulus value of 1.5GPa for

1.0wt.-% CNT loading, due to the

poor dispersion of pristineMWNTs

in the PP matrix. Interestingly,

ScCO2 processed nanocomposites

with pristine MWNTs showed an

increase in Young’s modulus,

which is as good as or even better

than for melt-compounded and

masterbatch systems based on

HDPE coated MWNTs. However,

coated MWNTs showed the lowest

reinforcing efficiency in the case of

ScCO2 processed nanocomposites.

Furthermore, the use of master-

batch in the scCO2 assisted mixing

method did not show improve-

ments in terms of Young’s mod-

ulus,as shownforbothscCO2.mPP/

MWNT and scCO2.m PP/cMWNT

composites. Instead, the long pro-

cessing times needed at these high

temperature conditions may even

have caused degradation of the PP

matrix. Similarly, increases in yield

stress were observed for all scCO2

PP/MWNT samples, with a signifi-

cant 6% increase from zero to

0.1wt.-% CNT loading followed by

roughly constant values across the

rest of the CNT loading range up to

1wt.-% (Figure 4). The degree of

reinforcement of pristine MWNTs

is as good as for melt-compounded

coated PP/cMWNT composites. PP/

cMWNTs composites, showing

poor nanofiller dispersion in the

case of scCO2 assisted mixing, give

lower yield stresses. Although the

HDPE coating indeed helps the



Figure 3. Young’s modulus of PP nanocomposites as a function ofCNT loading, showing that a similar modulus is achieved forscCO2 processed nanocomposites with pristine MWNTs and melt-compounded nanocomposites with coated cMWNTs. Solid linesrepresent results of nanocomposites with pristine MWNTs,whereas dashed lines represent nanocomposites with coatedcMWNTs.

Figure 4. Yield stress of PP nanocomposites as a function of CNTloading, showing that a similar yield stress is achieved for scCO2processed nanocomposites with pristine MWNTs and melt-com-pounded and masterbatch based nanocomposites with HDPEcoated MWNTs. Solid lines represent results of nanocompositeswith pristine MWNTs, whereas dashed lines represent nanocom-posites with coated cMWNTs.

570Macromol. Mater. Eng. 2010, 295, 566–574


dispersion of cMWNTs inmelt compounding processes, the

HDPE coating could also act as a weak interface between

matrix and CNTs, leading to poor stress transfer, thereby

weakening the nanocomposites. This potential effect is

however not found here. ScCO2 assisted mixing achieves a

better dispersion of CNTs without the need of a HDPE

coating, therefore eliminating the potential drawbacks of

the HDPE coating, leading to greater thermal and mechan-

ical reinforcing effects fromtheMWNTs.Also, the relatively

low shear forces involved in scCO2 assisted mixing may

cause less nanotube breakage and damage, while at the

same time avoiding degradation of the polymer.

To calculate the reinforcing efficiency of the CNTs in the

nanocomposites, the rule of mixture is applied to quantify

the effective Young’s modulus of the CNTs. For composites

inwhich discontinuous fibers are not perfectly aligned, the

Young’smodulus of the composite (Ec) canbewritten as: [34]

E ¼ hohLVfEf þ 1� Vfð ÞEm (1)

where hL is the length efficiency factor, ho is the orientation

factor, Em and Ef are the Young’smodulus of thematrix and

the fiber, respectively, and Vf is the volume fraction of the

fiber. hL can vary between 0 and 1. ho is equal to 1 for fully

aligned fibers, 3/8 for random (in-plane) 2D orientation

and 1/5 for random 3D orientation. A similar Equation can

be formulated for the strength, where sm, sm and sf are the

tensile strength of the composites, matrix and fiber,

respectively.

sc ¼ hLhoVfþsf þ 1� Vfð Þsm (2)

Therefore, Equation (1) and (2) canbeused toevaluate the

effective mechanical properties of CNTs in the composites.

In the calculation, the random 3D orientation value of 1/5

was chosen for ho, hL equals 1 for high fiber aspect ratios

such as (1/d)> 100. The volume fractions were deduced

from the weight fractions and densities of the materials.

Figure 5 illustrates the back-calculated effective Young’s

modulus and tensile stress of CNTs. Generally, the effective

properties decrease with increasing nanotube content. The

effective properties of pristine MWNTs in nanocomposites

processed using scCO2 are as good as, or even better than,

those for coated cMWNTs in nanocomposites processed

using melt-compouding and masterbatch. From direct

individual CNT testing, reported in literature, Young’s

moduli of the order of 1 TPa and tensile strength values

ranging from 10 to 150GPa have been reported.[35–38]

However, often back-calculated values of effective nano-

tubeproperties are significantly lower thanexperimentally

measured values of isolated CNTs, presumably due to

difficulties in achieving good dispersions of nanotubes,

strong nanotube/matrix interaction and nanotube align-

ment. Our calculated values of strength are reasonable

DOI: 10.1002/mame.200900405


Figure 5. (a) Effective Young’s modulus and (b) effective strengthof CNTs as calculated from the rule of mixtures, showing thatscCO2 processed nanocomposites incorporating pristine MWNTsresult in nanotube efficiency similar to that of melt-compoundedand masterbatch based nanocomposites based on HDPE coatedMWNTs. The dotted line represents the upper bound for nano-tube moduli.

Table 2. DSC measurement results.

Sample CNT Tm Xc Tc

wt.-% -C % -C

neat PP 0 166.0 37.9 110.3

0.1 164.7 43.7 118.9

scCO2 PP/MWNT 0.25 164.2 43.1 119.0

1.0 165.0 41.0 119.9

0.1 164.4 42.3 119.0

scCO2.m PP/MWNT 0.25 165.0 42.4 119.5

1.0 164.4 43.4 119.9

0.1 164.9 43.3 116.2

scCO2 PP/cMWNT 0.25 164.4 44.6 115.7

1.0 127.0/166.0 42.9 116.5

0.1 164.2 43.7 115.5

scCO2.m PP/cMWNT 0.25 164.7 43.3 116.2

1.0 127.3/165.7 43.7 116.4

melt.m PP/MWNT 1.0 165.0 40.2 121.9

melt.m PP/cMWNT 1.0 165.4 39.8 117.8

among the experimental data for CNTs, however, the

effective modulus values deduced from nanocomposites

with very low nanotube content (0.06 vol.-%) are signifi-

cantly higher than the theoretical modulus values for

nanotubes indicated by the dotted line in Figure 5a. The

reason for this could be related to a modification of the

matrix through the addition of CNTswhich causes errors in

the selectedvalueofEm.Because of thehighmatrix content,

the calculated results are strongly affected by variations of

Em.Small changesofEmvaluesof1.25 to1.40MPawill result

inaneffectivemodulus reduction from2400to1 200GPa in

the calculations. In a previous study, Young’s modulus of

the composites has been reported to increasewith polymer



matrix crystallinity.[39] Similar results were also shown in

many other studies and changes in crystallinity and

crystalline morphology of the polymer matrix can have

pronounced effects on the mechanical properties of

nanocomposites next to mechanical reinforcing effects

from nanofillers.[10,40,41]

DSC data of themelting temperature (Tm), crystallization

temperature (Tc) and crystallinity (Xc) of the PP nanocom-

posites are presented in Table 2. The addition of nanotubes

only slightly decreases the Tm of PP at 166 by 1 8C. A Tm of

around 165 8C indicates the melting of a-crystals of PP.

Additionally, samples with HDPE coated MWNT content

above 0.5wt.-% show an additional broad peak at around

127 8C corresponding to the melting of the HDPE coating

(Figure 6). Interestingly, the scCO2 processed sample before

hot-pressing shows a broad peak in the 150–160 8Ctemperature range apart from the peak of 166 8C. Thislower Tm around 155 8C suggests the melting of PP b-phase

crystals.[41,42] Our X-ray diffraction (XRD) results also

confirm the presence of a g-phase of PP which is discussed

in the text below. All samples after hot-pressing only show

a Tm, at 166 8C, indicating no change to the PP a-crystals

upon the addition of CNTs,which agreeswellwithprevious

reports in the literature.[19,29] Therefore, it is assumed that

b and g-phase crystals formation of PP during scCO2

processing was erased from the final specimens after hot-

pressing. The crystallization temperature (Tc) and the

degree of crystallinity (Xc) are increased by the addition

of MWNTs or cMWNTs into the PP, suggesting that

nanotubes act as nucleating agents which reduce the



Figure 6. DSC heating scan for typical PP/CNT nanocomposites,showing an additional broad peak at around 127 8C for sampleswith cMWNTs.

Figure 7. XRD spectra for typical PP/CNT nanocomposites,confirming the presence of a-phase crystals of PP in the nano-composites.

Figure 8. TGA of PP/CNT nanocomposites processed using scCO2assisted mixing, showing a retarded thermal degradation ofnanocomposites by the presence of MWNTs.

572

spherulites size, induce crystals growth at higher tempera-

ture and enhance crystallization. Furthermore, pristine

MWNTsgivehigherTc of PPnanocomposites thancMWNTs,

which indicates a stronger nucleation effect of MWNTs

than for coated MWNTs. Clearly, the surface of any

nucleating agent plays an important role in the crystal-

lization process.[43,44] MWNTs have high surface energies

which can absorb the polymer preferentially along their

crystal structure and act as nuclei for polymer crystals.

HDPEcoatingsdecrease thissurface freeenergy, resulting in

a reduced nucleating efficiency. Results also show a small

variation in Tc with different CNT loadings. Although

nucleating agents significantly increase the number of

nucleation sites, above certain loadings the introduction of

more CNTs may hinder chain mobility and retard crystal

growth. Thismay explain themore pronounced nucleating

effect observed for very low CNT loadings. Hence, the

mechanical reinforcing effects observed are a combination

of the modification of the PP matrix through increased

crystallinity as well as true reinforcing effects from CNT

fillers.[10]

The melting temperature was calculated from the first

heating, the crystallinity was calculated from the second

heating and the crystallization temperaturewasmeasured

from the first cooling.

Figure 7 shows the XRD patterns for pure PP and scCO2

processed nanocomposites before and after hot-pressing.

The pattern of pure PP shows peaks corresponding to PP a-

phase at 2u¼ 14, 17, 18.5, 21.5, 21.9 and 25.48 for six major

reflections: (110), (040), (130), (111), (041) and (060) plane,

respectively.[45] For scCO2 processed pure PP and PP

nanocomposites, an additional peak at 2u¼ 20.18 is

detected, which corresponds to the characteristic (117)

planeof theg-phaseofPP.[46] Theg-phase isonlyobtained in

some special cases such as under high pressure and by the

crystallization of very low molar mass fractions.[46] In our



case, the high pressure used in the scCO2 processing

promoted the formation of g-phase crystals. Apart from

peaks of a and g-phases, a broad small peak observed

at 2u¼ 168 for the scCO2.m PP/cMWNT 0.5% sample before

hot-pressing corresponds to the (300) plane of the b-phase

of PP.[47]Otherb-phasepeaks suchas2u¼ 16.7 (008) and218(301) are too close to the strong a-phase peaks to be

identified. ThePPnanocomposite sampleafter hot-pressing

only showsana-phaseofPPwhichagreeswellwithourDSC

results, suggesting that the thermal history from scCO2

processing was erased after hot-pressing.

Finally, the thermal stability of the nanocomposites was

investigated by TGA (Figure 8). The thermal degradation of

DOI: 10.1002/mame.200900405


PP is substantially retarded by the presence of CNTs. The

onsetdecomposition temperatures,Tonset, arehigher forPP/

MWNTnanocompositesbecause theCNTshinder thefluxof

degradation product and improve the heat dissipation

within the composites.[48] However, above 470 8C the

composites seem to degrade at a slightly higher rate than

the neat polymer. A similar Tonsetwas observed for both the

PP/cMWNT and PP/MWNT nanocomposites, indicating

that the HDPE coating of cMWNTs had no significant effect

on the thermal degradation behavior.

Conclusion

PP/MWNT nanocomposites were prepared using scCO2-

assisted mixing and compared with those processed by a

traditional melt compounding method. Results showed

that by using scCO2-assisted mixing, both yield stress and

Young’s modulus of the nanocomposites increase with

MWNT loading. However, unlike in melt compounding

processes, with which the use of HDPE coated MWNTs is

essential for improveddispersionandproperties, in thecase

of scCO2-assisted mixing, similar good dispersions and

mechanical properties are achieved with pristine MWNTs,

which in addition to obvious cost benefitsmay also benefit

from a lower viscosity of the polymer melt under scCO2

conditionsandbetterpreservationofnanotube lengths. It is

interesting that techniques designed to achieve high

quality PP nanocomposites, such as the use of master-

batches or polymer coated MWNTs, are strictly not

necessarywhenusingscCO2-assistedmixing. Inaccordance

with many other nanocomposite studies, the addition of

CNTs enhanced the PP nucleation process. The resulting

increase in crystallinity is an important additional factor

responsible for the enhanced mechanical properties of the

final nanocomposites.

Received: December 5, 2009; Revised: February 12, 2010;DOI: 10.1002/mame.200900405

Keywords: carbon nanotubes; nanocomposites; poly(propylene);processing; scCO2

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