Crystallization Behavior of Hdpe

Morphology and Crystallization Behavior ofHDPE/CNT Nanocomposite

STEPHEN L. KODJIE,1 LINGYU LI,1 BING LI,1

WENWEN CAI,1 CHRISTOPHER Y. LI,1

AND MIMI KEATING2

1Department of Materials Science and Engineering and A. J. Drexel

Nanotechnology Institute, Drexel University, Philadelphia, PA, USA2Experimental Station, DuPont, Wilmington, DE, USA

Polymer carbon nanotube nanocomposites (PCNs) represent the first realized majorcommercial application of carbon nanotubes (CNTs). In this study, high density poly-ethylene (HDPE)/CNT PCNs have been prepared using a solution blending technique.Both pristine single-walled nanotubes (SWNT) and polyethylene (PE) single crystaldecorated CNTs (so called nano hybrid shish kebabs, NHSKs) have been used as theprecursors for PCN preparation. Polarized light microscopy, transmission electronmicroscopy, scanning electron microscopy, differential scanning calorimetry, and ther-mogravimetry were used to study the morphology, crystallization behavior, andthermal stability of the resulting PCNs. The PCNs from pristine SWNTs possess amore dense morphology than do the PCNs prepared from NHSKs; PE single crystallamellae are perpendicular or oblique to the CNT axis, leading to relatively open-structured PCNs. Heterogeneous nucleation occurred in both nonisothermal and iso-thermal crystallization of PCNs and the crystallization kinetics are much faster thanthat of the pure HDPE. Thermal stability of PCNs showed dramatic enhancement(as high as 708C/1158C improvement of Tmax in N2/air atmosphere, respectively),which is attributed to the formation of the free radical scavenging CNT network.

Keywords polymer carbon nanotube nanocomposites, carbon nanotubes, single-walled nanotubes (SWNT), crystallization kinetics

Introduction

Due to their extraordinary mechanical, electrical and optical properties, CNTs have

attracted great attention in recent years.[1 6] As the structure and properties of the CNT

have been understood, there is a pressing need to transfer their outstanding properties

from nano- to micro-/macro-scales. One essential step towards this goal is assembling/processing CNTs, which often involves dispersing them in an organic solvent/polymeric matrix to form complex materials such as polymer CNT nanocomposites

Received 31 July 2005; Accepted 19 August 2005.Dedicated to Prof. Phillip H. Geils seventy-fifth birthday.Address correspondence to Christopher Y. Li, Department of Materials Science and Engineer-

ing and A. J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA.E-mail: [email protected]

LMSB152212 LMSB_045_002 Techset Composition Ltd, Salisbury, U.K. 3/7/2006

Journal of Macromolecular Sciencew, Part B: Physics, 45:231245, 2006

Copyright # Taylor & Francis Group, LLCISSN 0022-2348 print/1525-609X online

DOI: 10.1080/00222340500522299

231

(PCNs). The PCNs represent the first realized major commercial application of CNTs.[6]

Incorporation of CNTs into polymers dramatically enhances the mechanical properties of

the polymer while it also imparts attractive properties such as conductivity, electromag-

netic interference shielding, and sensing capability to the otherwise inert polymer

matrix.[6] Multiwalled CNT (MWNT) and SWNT as well as carbon nanofiber (CNF)

have been used. Depending on the targeted properties, a variety of polymers have been

explored. These include amorphous polymers such as polystyrene,[7 9] poly (methyl

methacrylate),[10 13] rigid rod polymers such as poly(p-phenylene benzobisoxazole),[14]

cross-linkable polymers such as epoxy[15 17] and conducting polymers such as polyani-

line, polypyrrole, etc.[18 21]

Most of the studies regarding the crystalline polymer/CNT PCNs focused on theircrystallization behavior. It should also be noted that carbon materials, such as carbon

fibers (CFs) which are 10mm in diameter, have been used to induce polymer crystalliza-tion (transcrystallization). A number of polymers have been investigated including

isotactic polypropylene (iPP), polyethylene (PE), polyhexamethylene adipamide (Nylon

66), poly(phenylene sulfide), polyether ether ketone, etc.[22 24] In the case of iPP/CFcomposite, the pitch-based CF induces a dense population of iPP crystal nuclei along

its surface.[22] As these nuclei grow, they impinge upon one another, such that crystal

growth occurs essentially perpendicular to the fiber axis. This leads to oriented growth

of iPP lamellae away from the fiber outward into the thermoplastic matrix, thus

forming a transcrystalline layer.[22] Epitaxial growth of polymers such as PE, Nylon 6,

polyethers, etc., on graphite surfaces has also been confirmed using scanning tunneling

microscopy (STM) and transmission electron microscopy (TEM) techniques.[25,26]

Polymer chains have been found to possess a trans conformation and align parallel to

the [100] or [110] direction of the graphite lattice. These previous studies suggest that

CNTs, as one type of carbon material, might promote polymer crystallization.

Recently, PCNs formed by CNTs and semicrystalline polymers such as iPP,[27 31]

PE,[32] polyvinyl alcohol (PVA),[33] polyacrylonitrile (PAN),[34 36] thermoplastic

polyimide,[37] and conjugated organic polymer,[38] as well as thermoplastic elastomers

such as polyurethane systems[39 41] have been studied. Using DSC nonisothermal and iso-

thermal crystallization technique, it has been found that t1/2 decreases with increasing

CNT content in PCNs. Crystallization behavior of iPP/CNT PCNs was most extensivelystudied. Grady et al.[28] reported that upon mixing with CNTs, the b form of iPP contentincreases while Assouline et al.[30] showed that MWNTs could act as a nucleation agents.Kelerakis et al.[29] studied the crystallization behavior of elastomeric ethylene/propylene(EP) random copolymer (84.3 wt% P) in the mixture with modified carbon nanofibers

(MCNF).[29] The MCNF was found to nucleate a form iPP. They also reported that thehighly stretched PCN exhibited a higher amount of unoriented crystals, a low degree of

crystal orientation, and a higher amount of g crystals, indicating the effective loadtransfer from the matrix to MCNFs. Chang et al.[27] reported that b form iPP formed inthe iPP/SWNT PCN under strain. In a series of thermoplastic elasotmers (Morthane)/CNT PCNs, it was found that the strain induced crystallization was enhanced with the

addition of CNT (for 15 vol.%), which led to the increase of the rubbery modulus by

a factor of 25 and the shape fixity was also improved.[39,41] When near IR was used to

heat CNTs, leading to the melting of the physical cross-linking points (polymer

crystals) and 50% more recovery stress than the pristine resin were reported.[41]

We recently investigated CNT-induced Nylon 66 and HDPE crystallization.[42] Using

a solution crystallization method,[42,43] it has been found that a unique nano hybrid shish

kebab (NHSK) structure could be obtained. Polymer single crystals were formed around


S. L. Kodjie et al.232

CNTs. In this paper, we report the morphology and crystallization behavior of PE/CNTPCNs. A solution blending method was used to ensure exfoliation of CNT bundles. The

CNT-initiated polymer crystallization was evident. The resulting PCNs showed

dramatic improvement in thermal stability. A detailed mechanism will be discussed.

Experimental

Materials

The high density polyethylene pellets (density 0.937 g/cm3, MFI 12 g/10 min,1908C/2.16 kg, ASTM D 1238) were purchased from Aldrich Co. The SWNTs werepurchased from Carbon Nanotechnologies Inc. and used as received. The MWNTs were

purchased from Aldrich and washed with 2.4 M nitric acid for 0.5 h to remove

amorphous carbon. The resulting MWNTs were then centrifuged, collected, and dried

in a vacuum oven. p-Xylene and methanol were purchased from Aldrich and used as

received.

Preparation of Coprecipitated HDPE/CNT Nanocomposite (CP-PCN)

The HDPE/SWNT nanocomposites with the following wt%: 0.0, 0.1, 0.25, 0.5, and 1.0were prepared by initially sonicating a weighed amount of the SWNT in p-xylene for

about 2 h and adding the content to an already dissolved HDPE/p-Xylene solution in anoil bath at 1208C. Finally a fibrous composite material was precipitated in methanol.The precipitant was filtered, washed with excess methanol, and dried in a vacuum oven

at 608C overnight.

Preparation of HDPE/CNT Nanocomposite from NHSK (N-PCN)

For the solution-crystallized HDPE/SWNT nanocomposites, initially a 0.01 wt% SWNT-HDPE NHSK was prepared by sonicating a known weight of SWNT in p-Xylene and

adding it to a dissolved HDPE-p-Xylene solution at 1208C. The mixture was crystallizedat 1048C for one hour to form the NHSK precursor.[42] Concentrated HDPE/p-Xylenesolution at 1208 was transferred to the oil bath containing NHSK at 1048C for 1 h. Themixture was then transferred to an oil bath at 808C where crystallization of the nanocom-posite took place for 13 h. The contents were filtered and dried overnight in the vacuum

oven at 608C. The final SWNT contents were controlled to be 0.1, 0.25, 0.5, and 1 wt%.The HDPE/MWNT N-PCNs were also prepared using a similar method.

Characterization

The TGA was carried out using a Perkin Elmer TGA 7 under air and nitrogen (N2) atmos-

phere with a flow rate of 40 mL/min. About 36 mg of the sample was heated from 308Cto 6008C at a heating rate of 108C/min. The crystallization and melting behavior of thedifferent concentrations of SWNT in HDPE was studied by DSC using a Perkin Elmer

DSC 7 that had been calibrated with a standard indium and under a N2 flow rate of

20 mL/min. The cooling and heating rate of 108C/min was used. The sample were firstheated to 2008C and then cooled to 308C. The second heating curve was collected byheating the sample at the same rate to 2008C. The crystallinity was determined usingthe enthalpy of fusion of HDPE with 100% crystallinity as 293.6 J/g.[44] Isothermal


HDPE/CNT Nanocomposites 233

crystallization was carried out by quenching the PCNs from 1908C to the presetcrystallization temperatures at a rate of 4008C/min.

The PCN morphology was examined via polarized light microscopy (PLM) (Olympus

BX-51) coupled with a Mettler hot stage (FP-90). The image was captured using an Insight

digital camera. The film was prepared by melt pressing the PCN at 1908C.The TEM experiments were conducted using a JEOL-2000FX microscope with an

accelerating voltage of 120 kV. THE SEM experiments were carried out using a FEI/Phillips XL30 field emission environmental scanning electron microscopy (SEM) with

an acceleration voltage of 15 kV. The PCNs were placed on glass slides and sputtered

with platinum for 30 s to enhance the surface conductivity.

Results and Discussion

Morphology of HDPE/SWNT Nanocomposite

It has been recognized that the structure and morphology of PCNs critically depend on the

fabrication process. Solution blending of polymers and CNTs was employed in the present

study since it should provide better CNT exfoliation compared to the melt blending

method. Two types of precursors were utilized: pristine CNTs and PE single crystal

decorated CNTs (PE NHSK), leading to CP-PCN and N-PCN samples, respectively.

Figure 1 shows a TEM micrograph of the unique NHSK precursor.[42] The SWNTs

formed small bundles and disc-shaped objects on the CNTs can be clearly seen,

spanning along the entire length of SWNTs with a periodicity of 50100 nm. Thesedisc-shaped objects are edge-on views of PE single crystal lamellae and the peculiar mor-

phology is similar to the classical polymer shish-kebab structures formed in an

elongation/shear flow field.[45,46] The CNT/polymer system in this case was not underexternal flow during crystallization and it is the SWNT that induces nucleation of

Figure 1. TEM micrograph of HDPE/SWNT NHSK formed by crystallizing PE at 1048C inp-Xylene at the presence of SWNT.



polymer chains on the SWNT surface. This observation serves as direct evidence that

CNTs can promote heterogeneous nucleation of PE.

Figure 2 shows PLM micrographs of the CP-PCN and N-PCN from both SWNTs and

MWNTs. The samples were sandwiched between two glass slides, melt pressed at 1908C,and then quenched to 1108C for observation. It is evident that 1) CNTs were uniformlydispersed in the PE matrix and large size agglomeration was absent, indicating that

solution blending leads to better CNT dispersion compared to melt blending. 2) The PE spher-

ulites were not observed in all cases, a common case in crystalline PCN since CNTs (or other

inorganic fillers such as nano clay) prevent PE from growing into large size spherulites. The

crystalline nature of the PCN is, however, evident by the strong overall birefringence.

The detailed morphology of the PCN is revealed by SEM micrographs shown in

Fig. 3. Note that these samples were not melt pressed. Figure 3a shows the CP-PCN

sample. Rounded PE aggregates can be clearly seen, which is due to the precipitation/phase separation process. The surface of the PE spheres is rough; small lamellae were

formed within the precipitated spheres. The lamellae are relatively densely packed in

each sphere. The average size of the spheres is 23 micrometers. It should be notedthat these spheres are similar to the PE globs observed by Garber and Geil[47] by

rapid quenching of 0.05% of PE (Marlex 6050) in xylene to temperatures of 86 and

Figure 2. PLM of HDPE/CNT PCNs a) 0.5% SWNT CP-PCN, b) 0.25% SWNT N-PCN, andc) 0.25% MWNT N-PCN.



708C. These authors suggested that the formation of the globs was due to the phase sep-aration occurred during the quenching process.[47] In the present case, CNTs are not

evident in the micrograph, indicating that the fast precipitation kinetics lead to rapid

phase separation of PE from the solution. Microspheres were formed and CNTs might

be buried inside the PE spheres. Figures 3b and 3c show the N-PCN samples of 0.1%

CNT content. Figure 3b was from the SWNT sample while Fig. 3c shows MWNT

Figure 3. SEM of HDPE/CNT PCNs a) 0.1% SWNT CP-PCN, b) 0.1% SWNT CP-PCN, andc) 0.1% MWNT CP-PCN.



PCN. The morphology is clearly different from CP-PCN. The CNTs are wrapped with a

layer of PE single crystals. Although CNTs can not be directly seen, it is evident that in

N-PCNs, PE lamellae were formed on CNTs and the dotted lines in Fig. 3b represent

the orientation of the CNTs. Prominent PE single crystal lamellae can be seen perpendicu-

lar/oblique to the CNT axis. Figures 3b and 3c could thus be viewed as the overgrowthfrom NHSK in Fig. 1. The unique orientation of the PE lamellae leads to the open mor-

phology of the PCN, which could play an important role in the PCN crystallization and

thermal properties, a topic that we will return to in the following section.

Crystallization of HDPE/CNT Nanocomposite

Nonisothermal crystallization behavior of HDPE and HDPE/CNT PCN were studiedusing DSC. The HDPE, CP-PCNs, and N-PCNs were cooled from 2008C to 308C at108C/min and heated up at the same rate; the results are presented in Fig. 4. Tables 1and 2 list the onset temperature (Ton), peak temperature (Tpeak), and heats of fusion

(DH ) as well as the crystallinity (wc) data of CP-PCN and N-PCN. The cooling curvesshow that pure HDPE crystallized at 110.88C and the inclusion of SWNT did not signifi-cantly change the crystallization temperature (Tc). The 1.0 wt% composite increased the

onset of crystallization about 38C while Tcpeak was increased by 1.838C. The increase of

Tc is attributed to the heterogeneous nucleation induced by CNTs. The corresponding

heat release during crystallization was reduced by 11 J/g, corresponding to a reductionin percentage crystallinity of about 4%. Figure 4b shows the second heating curve. Pure

HDPE undergoes crystal melting at 1258C; it was observed that the melting points ofthe composites did not change much, being recorded as 125.28C, 127.48C, 127.78C, and126.28C, corresponding, respectively, to the 0.1, 0.25, 0.5, and 1.0 wt% samples. The cor-responding heat of fusion that depends on the crystallinity was recorded as 176.2 J/g forHDPE and 173.0, 173.2, 174.6, and 169.3 J/g for the 0.1, 0.25, 0.5, and 1.0 wt% PCNs.The observed reduction in percentage crystallinity is different from some of the

reported data, in which case an increase of polymer crystallinity was claimed by adding

Figure 4. DSC thermograms of HDPE/CNT N-PCNs (a) first cooling, and (b) second heating.



CNTs.[28] In the present case, we suggest that the reduction of the crystallinity is because

that the CNTs can break the continuity of the polymer matrix and large, uniform lamellae

can not be formed, as evidenced in Fig. 2. More grain boundaries as well as defects are

formed in the case of PCNs, leading to the crystallinity reduction. It should also be

noted from the DSC data, the HDPE sample that was used might contain a number of

branches per chain since the melting temperature as well as the crystallinity are relatively

low compared to linear HDPE.

For the N-PCN samples, which were prepared from PE-NHSK as shown in Table 2,

similar Tc, and Tm were observed. One noticeable difference is that the heat of fusion of the

first heating is much higher compared to the second heating while the Tm is lower. This

could be attributed to that the first heating curve shows the melting of solution crystal-

lized/precipitated samples while the second heating curve shows the melting of noni-sothermal melt crystallized samples. Compared to melt crystallization, solution

crystallization/precipitation occurred in a more dilute environment, leading to morecomplete crystallization (thus higher DH). Most of the crystals in solution crystallized/precipitated samples, however, were formed during precipitation or crystallization at

808C. The Tc is relatively low compared to nonisothermal melt crystallization; a relativelylower Tm was thus observed in the first heating curves as indicated in Table 2. Note that the

differences tends to diminish as CNT contents increase, possibly due to the CNT network

providing a nano confinement effect (Tm increases while crystallinity decreases with

increasing CNT contents).

Table 1DSC data of HDPE CP-PCNs

Sample Cycle Ton8C Tpeak8C

Heat of

fusion

DH, J/gCrystallinity

(%)

HDPE 1st Heating 116.6 123.0 203.5 69.0

Cooling 112.5 110.8 157.5 54.0

2nd Heating 120.2 125.2 176.2 60.0

0.1wt%

SWCNTHDPE

1st Heating 116.6 124.3 189.2 64.4

Cooling 112.6 109.1 151.1 55.6

2nd Heating 121.0 125.2 173.0 59.1

0.25wt%

SWCNTHDPE

1st Heating 119.7 127.4 173.3 59.1

Cooling 114.7 112.0 151.4 51.5

2nd Heating 119.7 127.4 173.2 59.1

0.5wt%

SWCNTHDPE

1st Heating 121.5 127.7 186.9 63.3

Cooling 113.2 110.1 155.2 55.4

2nd Heating 120.6 127.7 174.5 59.3

1wt%

SWCNTHDPE

1st Heating 120.4 127.2 170.6 58.0

Cooling 115.2 112.6 148.1 50.5

2nd Heating 117.9 126.2 169.3 57.6



Compared to CP-PCN, there were no significant changes in crystallization and

melting temperatures in the case of N-PCN except that a slightly lower impact on crystal-

linity was observed with increasing SWNT. This is probably due to the more open mor-

phology of the PE/CNT hybrid structure as shown in Fig. 3. Since the lamellae in theNHSK PCNs are perpendicular/oblique to the CNT axis, this open structure leads tomore free space for PE crystal growth and the crystallinity reduction due to the CNT con-

finement was, therefore, limited.

Isothermal crystallization was also carried out and Fig. 5 shows the isothermal crystal-

lization of HDPE and PCNs at 1158C and the plots of t1/2 with respect to CNT content. It isevident that crystallization occurred at a much faster rate, for both CP-PCN and N-PCN.

The t1/2 decreased dramatically, from 233 s to 69 s for CP-PCN and 77 s for N-PCN

sample. Again, the slightly higher t1/2 of N-PCN compared to CP-PCN might be due to

the open structure of N-PCN. It is of interest that the Avarmi exponent n did not

change much for the CP-PCN, while it slightly increased for N-PCN (1.59 for HDPE,

1.55 for 1% CP-PCN, and 1.78 for 1% N-PCN), indicating that the growth dimension

of the PE crystal was not significantly affected by CNTs. Comparing the isothermal

curve shapes, one can immediately tell that the pure HPDE curve is very broad, indicating

multiple crystallization behavior, possibly due to the high polydispersity as well as the

nonlinearity of the sample. The isothermal curve of the PCNs, however, is much

narrower and uniform, suggesting that the heterogeneous nucleation is the overwhelming

crystallization mechanism and crystallization occurred at the same time, leading to the

Table 2DSC data of HDPE N-PCNs

Sample Cycle Ton8C Tpeak8C

Heat of

fusion

DH, J/gCrystallinity

(%)

HDPE 1st Heating 116.6 123 203.5 69

Cooling 112.5 110.8 157.5 54

2nd Heating 120.2 125.2 176.2 60

0.1wt%

SWNTHDPE

1st Heating 113.9 121.5 206.5 70.1

Cooling 114.0 112.0 157.4 54.1

2nd Heating 119.2 125.0 175.8 60.1

0.25wt%

SWNTHDPE

1st Heating 115.9 121.2 207.5 70.2

Cooling 115.0 112.0 157.2 53.3

2nd Heating 119.3 125.2 176.6 60.2

0.5wt%

SWNTHDPE

1st Heating 117.0 122.8 206.0 70.4

Cooling 114.3 112.1 155.9 53.3

2nd Heating 119.7 125.9 175.2 60.0

1wt%

SWNT HDPE

1st Heating 118.9 127.8 181.2 61.6

Cooling 114.7 112.0 153.6 52.5

2nd Heating 119.2 126.4 174.5 59.4



narrow distribution of the isotherm curve. Another feature of the isothermal crystallization

is that the crystallinity of the PCNs is lower than that of the pure HDPE, suggesting again

that the CNT network has dramatically hindered the crystal growth.

Thermal Stability of HDPE/CNT Nanocomposites

The degradation of pure HDPE as well as composites was analyzed in air and N2atmosphere. The thermogarmetric analysis (TGA) curves of HDPE and CP-PCN

specimens in air and N2 are shown in Fig. 6 while the N-PCNs sample in air and N2are depicted in Fig. 7. In air HDPE maintained a constant weight until about 240 8Cand the onset of maximum weight loss (Ton) was recorded as 346.98C. The differentialcurves showed a single peak at 350.88C, which can be correlated to the temperature ofmaximum weight loss (Tmax) of the neat polymer (not shown). The temperature of 5%

sample weight loss (T5%) was found to be 315.68C. In N2, the Ton, T5% and Tmax werefound to be at 405.88C, 369.08C, and 446.98C, respectively, much higher than whenrecorded in air atmosphere. At temperatures above 4908C, HDPE had completelydegraded and the weight of the residue remained constant. For both CP-PCN and

N-PCN samples, in air or N2, it is evident that the Ton, T5%, and Tmax have been dramati-

cally increased. Figure 8 shows the plots of Ton, T5%, and Tmax with respect to the CNT

contents for CP-PCN and N-PCN in N2 and air. All three temperatures were dramatically

increased with a small amount of CNT and they then tended to reach a plateau. In air, the

Tmax was increased by1158C for the CP-PCN while the increase was 658C for N-PCN.For all the samples in air atmosphere, the differential curves of the decomposition process

showed several peaks, indicating that the degradation process involves a multistaged

process of separate chemical reactions as a result of carbon-oxygen bond formation and

subsequent decomposition of the bonds at higher temperatures to form carbon

monoxide and carbon dioxide. Regardless of this multistage process, 651158Cincrease of the degradation temperature is evident. Single peaks were found for the

derivatives of the TGA curves for the PCNs in N2 (not shown). With increasing the

Figure 5. Isothermal crystallization at 1158C a) CP-PCNs and b) N-PCNs. c) shows the plot oft1/2 with respect to SWNT contents.



CNT contents, the degradation temperature shifted upward for both CP-PCN and N-PCN.

As high as a 708C increase has been achieved for both samples in N2 atmosphere.It is known that the thermal degradation of HDPE occurs by random chain scission to

form radicals of alkyl and alkyl peroxyl that are susceptible to inhibition reagents capable of

trapping the radicals. The CNTs have high electron affinities similar to C60; as such they have

been proposed to acts as scavengers of free radicals.[48] By incorporating CNTs in polymers

the thermal stability of polymers could be enhanced. Watts et al.[48] recently reported

an 188C increase of the degradation temperature of PE in N2 at a much higher CNTcontent (14% CNT). In the present case, as high as 708C improvements were achieved.The difference could be attributed to two factors: 1) MWNT were used in Watts study

while SWNT were used in our case; and 2) melt blending was used in the previous study

and a solution blending technique was used in our case. In order to further confirm which

is the major reason for the thermal stability enhancement, MWNT were also used for

solution blending. It was found that for 0.25% MWNT/HDPE N-PCN in N2, a Tmax of5058C was achieved, indicating a 598C increase, which is much higher than the previouslyreported 188C improvement. Therefore, more complete CNT exfoliation might hold the keyto the present observation of the high PCN thermal stability.

Solution blending technique ensures more complete CNT exfoliation and less CNT

bundle formation in the PCNs. The radical scavenging efficiency, therefore, can be dra-

matically enhanced at very low CNT content. Figure 8 indicates that Ton, T5%, and Tmaxincrease dramatically as small amounts of CNT are added. The temperatures then

reached a plateau. This indicates that the percolative network of CNTs was achieved

and further increasing CNTs, therefore, did not alter the degradation behavior signifi-

cantly. The threshold CNT content is 0.2%, a very low value due to the high CNTaspect ratio.

It should be noted that the Ton, T5%, and Tmax of coprecipitated CP-PCN samples are

slightly higher than that of the N-PCNs from NHSK (a few degrees difference). This could

Figure 6. TGA of CP-PCNs in (a) air and (b) N2.



be attributed to two possible reasons: 1) since the CP-PCN were prepared immediately

after sonication, CNTs might be dispersed more uniformly while in N-PCNs, 0.51 h crys-

tallization time was allowed, which might result in certain CNT agglomeration. 2) From

Fig. 3, it is evident that N-PCNs possess a more open morphology and PE, therefore, is

Figure 7. TGA of N-PCNs in (a) air and (b) N2.

Figure 8. Ton, T5%, and Tmax vs. CNT contents for CP-PCNs in (a) air, (b) N2, and for N-PCNs in (c)

air, and (d) N2.



not as intimately contacted with CNT as in CP-PCN. The radical scavenging efficiency

might be relatively low, which could lead to a slightly lower degradation temperature.

It is, therefore, again clear that uniform dispersion of SWNT holds the key to the

thermal property enhancement and we are currently investigating using other solvents

(such as 1,2 dichlorobenzene which is a better solvent for SWNT) to achieve better

CNT exfoliation. An even higher degradation temperature of HDPE PCN is envisaged.

Conclusion

The HDPE/SWNT PCNs were prepared using a solution blending technique. Bothpristine SWNT and the unique NHSKs were used as precursors for PCN preparation.

Uniform SWNT dispersion was achieved in both cases. SEM shows that CP-PCNs from

the pristine SWNT possess denser structures while in N-PCNs from NHSK precursors,

single crystal lamellae are perpendicular/oblique to the CNT axis, creating open spacein the PCNs. Crystallization temperatures were found to be slightly increased and the

t1/2 in isothermal crystallization decreased; both suggest that CNTs initiated PE crystal-

lization. Thermal stability of HDPE was dramatically enhanced and as high as 1158Cincreases in Tmax were achieved. The thermal stability enhancement was attributed to

the formation of the radical scavenging SWNT network.

Acknowledgment

This work was supported by the National Science Foundation (NSF CAREER award,

DMR-0239415), DMI-0508407, ACS-PRF, 3M, and DuPont. The ESEM was purchased

through the support of NSF (BES-0216343).

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Crystallization Behavior of Hdpe

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