Curing Behavior and Properties of EpoxyNanocomposites With Amine FunctionalizedMultiwall Carbon Nanotubes

Woo Jin Choi,1 Robert L. Powell,2 Dae Su Kim1

1Department of Chemical Engineering, Chungbuk National University, 12 Kaesin-dong Cheongju,Chungbuk 361-763, Korea

2Department of Chemical Engineering and Materials Science, University of California at Davis, Davis,California 95616

Carbon nanotubes (CNTs) with reactive functionalgroups such as amines would affect not only proper-ties but also curing behavior of an epoxy nanocompo-site system comprising them. Therefore, in this study,an amine functionalization of multiwall CNTs (MWNTs)was carried out via treating pristine MWNTs (PMWNT)with 4-aminobenzoic acid in polyphosphoric acid. Thefunctionalization was confirmed by Fourier transforminfrared spectroscopy, thermogravimetric analysis(TGA) and scanning electron microscopy (SEM). Epoxynanocomposites comprising the PMWNT or functional-ized MWNTs (FMWNT) were prepared and their curingbehavior and properties were investigated. Differentialscanning calorimetry (DSC) was used to obtain experi-mental conversion data for curing kinetic analysis. TheFMWNT accelerated the curing rate of the nanocompo-site system. The functionalization induced strong inter-facial bonding between the epoxy matrix and theMWNTs, and resulted in considerable improvements inthe properties of the nanocomposites. The SEM imageshowed strong interfacial bonding between the epoxymatrix and the FMWNT. POLYM. COMPOS., 30:415–421,2009. ª 2008 Society of Plastics Engineers

INTRODUCTION

Engineering applications of polymers are restricted

within narrow limits because they have relatively lower

stiffness and strength compared to ceramics or metals.

One reasonable method that has been used to offset these

deficiencies is incorporating nanosize particles or fibers

into the polymers to fabricate nanocomposites [1].

Carbon nanotube (CNTs), generally classified into two

types of single-wall CNTs (SWNTs) and multiwall CNTs

(MWNTs), are being studied to use in many areas

because they have unique structural, mechanical, thermal,

and electrical properties [2–6]. They have been used to

make polymer nanocomposites because the excellent ma-

terial properties of CNTs could improve the properties of

polymers drastically [7].

However, a few fundamental processing challenges

must be overcome to enable effective reinforcement by

CNTs [8]. Weak interfacial bonding due to the atomically

smooth surface of CNTs limits load transfer from the

polymer matrix to CNTs [9]. Furthermore, homogeneous

dispersions are not easily obtained because CNTs tend to

exist as entangled agglomerates. Therefore, strong inter-

facial bonding between a polymer matrix and CNTs and

dispersing CNTs homogeneously throughout the polymer

matrix would be critical factors in maximizing reinforce-

ment by CNTs [10]. To meet these challenges, ultrasoni-

cation [11], high shear mixing [12], surfactants [13],

chemical treatments using high concentration strong acids

[14], functionalization [10], and polymer chain wrapping

[15] have been explored.

Epoxy resin is one of the most common thermoset res-

ins. They generally show high mechanical strength and

modulus, low shrinkage in curing, high adhesion, and

good chemical and corrosion resistance. Incorporating

CNTs into the epoxy resin would result in advanced func-

tional materials. However, early studies [16–18] reported

that weak interfacial bonding between the epoxy matrix

and CNTs resulted in low performance nanocomposites

with limited load transfer ability. To accomplish strong

interfacial bonding as well as homogeneous dispersion a

functionalization of CNTs has been carried out recently,

and it resulted in a drastic increase in the mechanical

properties of the epoxy/CNT nanocomposites [10].

CNTs with reactive functional groups such as amines

would affect not only the interfacial bonding and disper-

sion of CNTs but also curing behavior of an epoxy/CNT

Correspondence to: D. S. Kim; e-mail: dskim@chungbuk.ac.kr

Contract grant sponsor: Chungbuk National University

DOI 10.1002/pc.20571

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2009

nanocomposite system. An amine functionalization of

CNTs would improve compatibility between an epoxy

resin and CNTs leading to a better dispersion of CNTs

and also lead to strong interfacial bonding through the

chemical reaction between the epoxy groups of the resin

and the amine groups of CNTs affecting the curing

behavior of the epoxy/CNT nanocomposite system. There-

fore, in this study, an amine functionalization of pristine

MWNTs (PMWNT) was carried out, and its effects on

the curing behavior and properties of epoxy/MWNT nano-

composites were investigated.

EXPERIMENTAL

Materials

A diglycidyl ether of bisphenol-A type epoxy resin

(YD128 from Kuk Do Chem., Korea) and an aromatic

amine curing agent, 4,40-methylene dianiline (from Kuk

Do Chem.), were used to formulate a thermoset epoxy

resin system. The epoxy equivalent weight of the epoxy

resin was 185 g/mol and the viscosity of the resin was

12,000 cP at 258C. Figure 1 shows the chemical structures

of the epoxy resin and the curing agent.

Pristine MWNTs were purchased from Iljin Nanotech,

Korea. According to the supplier, the MWNTs were syn-

thesized by chemical vapor deposition process and have

the average diameter and length of 13 nm and 10 lm,

respectively.

Amine Functionalization of the MWNTs

To functionalize the MWNTs, the synthetic method

[19] used to functionalize carbon nanofibers in polyphos-

phoric acid was adopted in this study because the method

seemed relatively simple but quite effective and practical.

In a 250-ml flask equipped with a mechanical stirrer and

a nitrogen inlet and outlet system, 1 g of the MWNTs

was treated for 2 h at 1308C in a solution composed of 4-

aminobenzoic acid (1 g), polyphosphoric acid (84% P2O5

assay, 40 g), and P2O5 (5 g). The resulting FMWNT were

washed with acetone and distilled water each several

times, and then dried using a vacuum freeze dryer for

1 day.

Preparation of Epoxy/MWNT Nanocomposites

At first only the epoxy resin and the PMWNT or

FMWNT, except for the curing agent to prevent prema-

ture curing reaction, were mixed at room temperature for

1 h with a mechanical stirrer. And then, the curing agent

was added to the mixture by stoichiometry, and it was

stirred further for 5 min. The mixture was degassed in a

vacuum oven and used for characterization study. To

make epoxy/MWNT nanocomposite samples for mechani-

cal tests and structural analysis, the mixture was cast into

two silicon rubber molds, respectively, one with dimen-

sions of 35 mm 3 13 mm 3 3.2 mm and the other with

dimensions of 40 mm 3 13 mm 3 4.5 mm, and then

cured with a hot press at 1808C for 1 h, followed by post-

curing at 2008C for 30 min. The amount of the PMWNT

or FMWNT in the epoxy/MWNT nanocomposites was

fixed to 3 phr (parts per hundred of the epoxy resin).

Measurements

Fourier Transform Infrared Spectroscopy. To con-

firm the functionalization of the MWNTs, Fourier trans-

form infrared spectroscopy (FTIR) (Bomem MB100,

Bomem, Quebec, Canada) was used. The FMWNT were

mixed with KBr powder, and a disc-shaped specimen was

prepared for FTIR analysis. The FTIR spectra of the

PMWNT and FMWNT were obtained in the wavenumber

range of 4,000–500 cm21.

Differential Scanning Calorimetry. To investigate the

curing behavior of the epoxy/MWNT nanocomposite sys-

tem differential scanning calorimetry (DSC 2910, TA

Instruments, New Castle, DE) was used. About 10 mg of

each uncured sample was placed in a hermetic aluminum

pan, and tested immediately. Each sample was cured

dynamically at 108C/min under nitrogen gas atmosphere.

The dynamic DSC scanning temperature range was from

10 to 3008C.

Thermogravimetric Analysis. To confirm the function-

alization of the MWNTs, thermogravimetric analysis

(TGA) of the PMWNT and FMWNT was carried out

using the SDT 2960 (TA Instruments). Each measurement

FIG. 1. Chemical structures of the epoxy resin and amine curing agent.

416 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

was carried out under nitrogen gas atmosphere from room

temperature to 9008C at 108C/min.

Dynamic Mechanical Analysis. To investigate the ther-

momechanical properties of the epoxy/MWNT nanocom-

posites, dynamic mechanical analysis (DMA) was carried

out with cured sheet-shaped specimens with dimensions

of 35 mm 3 13 mm 3 3.2 mm using the DMA 2940

(TA Instruments) mounted with a single cantilever. The

frequency was 1 Hz and the scanning rate was 58C/min.

The scanning temperature range was from room tempera-

ture to 3008C.

Field Emission Scanning Electronic Microscopy. The

field emission scanning electronic microscope (FE-SEM,

LEO-1530FE, Carl Zeiss NTS GmbH, Oberkochen, Ger-

many) was used to investigate not only the shape of the

MWNTs before and after the functionalization but also

the structure and morphology of the fracture surfaces of

the nanocomposites. Each sample was coated with Pt by

sputtering prior to SEM image observation.

Universal Testing Machine. The flexural properties of

the epoxy/MWNT nanocomposites were measured by

three-point bending test using the universal testing

machine (LR-30K, LLOYD Instruments, Hampshire, UK)

according to the ASTM D790. For the accuracy of the

measurements, at least five specimens per each nanocom-

posite were prepared and tested. The test was carried out

at a crosshead speed of 1.7 mm/min. The dimensions of

the specimens were 40 mm 3 13 mm 3 4.5 mm.

RESULTS AND DISCUSSION

Characterization of the FMWNT

Figure 2 shows the FTIR spectra of the pristine (named

PMWNT and so forth) and functionalized MWNTs

(named FMWNT and so forth). If the functionalization

was successful, the FMWNT would have amine functional

groups. As circled in Fig. 2, the absorption peak for the

N��H stretching vibrations of amine groups appeared at

3,440 cm21. The other absorption peaks observed at

1,650 and 1,140 cm21 correspond to the stretching vibra-

tions of the carboxylic C¼¼O and C��O groups of ester

linkages, which were formed by the chemical reaction

between the carboxylic groups of the functional molecules

(4-aminobenzoic acid) and the oxidized surface of the

MWNTs. From the FTIR analysis, it was considered that

the amine functionalization of the MWNTs was carried

out successfully.

Figure 3 shows the TGA thermograms of the PMWNT

and FMWNT, comparatively. The initial decomposition

temperature (Ti) of the PMWNT appeared at about

5008C. But, the Ti of the FMWNT appeared at about

2008C, which is significantly low compared to the Ti ofthe PMWNT. This low Ti of the FMWNT was considered

due to the thermal decomposition of the organic mole-

cules bonded to the MWNTs. The TGA results also con-

firmed the functionalization of the MWNTs.

The SEM images of the PMWNT and FMWNT are

shown in Fig. 4. Figure 4a for the PMWNT showed a

random, curled spaghetti-like structure with high aspect

ratios. Figure 4b for the FMWNT showed also almost the

same structure as the PMWNT did except for the slight

increase in diameter, which resulted from the functionali-

zation. The SEM images also confirmed that the function-

alization of the MWNTs was successful.

Curing Kinetics

The change of a specific physical property, which can

be directly related to the chemical conversion of a ther-

moset resin system, can be easily monitored by an instru-

ment during curing and used to investigate curing kinetics

of the resin system. Therefore, DSC [20, 21] that can

monitor the change of reaction exothermic heat during

curing was used to study curing kinetics of the epoxy/

MWNT nanocomposite system. Figure 5 shows the

FIG. 2. FTIR spectra of the PMWNT and FMWNT.

FIG. 3. TGA thermograms of the pristine (PMWNT) and functionalized

MWNTs (FMWNT).

DOI 10.1002/pc POLYMER COMPOSITES—-2009 417

dynamic DSC thermograms for the epoxy/MWNT nano-

composite system obtained at 108C/min.

The epoxy curing reactions by amines are exothermic

and analyzed by somewhat different order kinetics

because they indicate different curing characteristics. In

general, a combination of an epoxy resin and a primary

amine leads to two principal reactions: (a) the addition of

a primary amine hydrogen to an epoxy group to form a

secondary amine and (b) the addition of a secondary

amine hydrogen to an epoxy group to form a tertiary

amine. These epoxy curing reactions by amines are con-

sidered autocatalytic because the OH groups formed dur-

ing the reactions helps the ring opening of other epoxide

rings [22]. Kamal and Sourour [23] proposed the follow-

ing semiempirical kinetic model to describe the autocata-

lytic reaction mechanism of an epoxy curing reaction by

an amine

dX

dt¼ ðk1 þ k2X

mÞð1� XÞn ð1Þ

where, X is conversion, m reaction order related to the

autocatalytic reaction mechanism, n reaction order related

to nonautocatalytic reaction mechanism, and k1 and k2reaction rate constants which have Arrhenius temperature

dependence on temperature

k1 ¼ k11 exp�E1

RT

� �ð2Þ

k2 ¼ k22 exp�E2

RT

� �ð3Þ

where, k11 and k22 are frequency factors, E1 and E2 acti-

vation energies, and R the ideal gas constant.

A mechanistic model, which can be derived from the

balances of reactants with a good understanding of a

reaction mechanism, is better than a phenomenological

one in analyzing curing kinetics of a thermoset resin sys-

tem. However, mechanistic models are rarely feasible

because most thermosetting reactions are rather complex.

Therefore, the phenomenological model, Eq. 1, was used

in this study to analyze curing kinetics of the epoxy/

MWNT nanocomposite system. The overall reaction

order was assumed to be 2 not only to give the kinetic

model some mechanistic aspect but also because this

assumption was reasonable for the other epoxy resin sys-

tems similar to the epoxy system of this work [24]. With

the assumption and by combining Eqs. 1–3, the follow-

ing second order autocatalytic reaction kinetic equation

could be obtained

dX

dT¼ 1

Srk11 exp

�E1

RT

� ��

þk22 exp�E2

RT

� �Xm

�ð1� XÞ2�m ð4Þ

The scanning rate (Sr) was introduced in Eq. 4 to use

directly experimental kinetic data from the dynamic DSC

thermograms in analyzing curing kinetics of the epoxy/

MWNT nanocomposite system.

FIG. 4. SEM images of (a) the pristine (PMWNT) and (b) functional-

ized MWNTs (FMWNT).

FIG. 5. Dynamic DSC thermograms of the nanocomposite systems.

(Scanning rate: 108C/min).

418 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

As shown in Fig. 5, the peak temperature (170.08C) ofthe epoxy/PMWNT system was almost the same as that

(170.18C) of the pure epoxy system. However, the peak

temperature (157.78C) of the epoxy/FMWNT system

shifted considerably to a lower temperature region. This

peak shift means that the curing rate of the epoxy system

increased considerably when the FMWNT were incorpo-

rated. This increase in the curing rate was considered due

to the amine functional groups of the FMWNT, which

could react with the epoxy groups of the resin.

Kissinger’s method [25] or the method suggested by

Ozawa [26] and Flynn [27] has used dynamic DSC ther-

mograms to study curing kinetics of thermoset resin sys-

tems. But, these methods are not sufficient because they

use only limited information from the DSC thermograms.

So a numerical fitting method, which uses all the experi-

mental kinetic information from the DSC thermograms,

was used to study curing kinetics of the epoxy/MWNT

nanocomposite system. To obtain conversion data from

the DSC thermograms the conversion was assumed to be

the ratio of the reaction heat generated until a certain

temperature to the overall heat of reaction at complete

conversion. The overall heat of reaction could be obtained

by integrating each DSC thermogram.

The kinetic parameters were determined by fitting the

experimental conversion data to the kinetic equation,

Eq. 4, using Marquardt’s multivariable nonlinear regres-

sion method and Runge–Kutta integration technique [28].

The values of the kinetic parameters, k1, k2, E1, E2, and

m, determined by the fitting method are listed in Table 1.

The activation energies of the epoxy system decreased

when FMWNT with reactive amine groups were incorpo-

rated. This kind of decrease in activation energies has

been also reported for the epoxy nanocomposite system

comprising CNTs treated with nitric acid [29].

Figure 6 shows that the experimental conversion data

obtained from the DSC thermograms agree well with the

conversion curves calculated from the kinetic equation.

The curing rate of the epoxy/PMWNT system was almost

the same as the pure epoxy system. Therefore, the conver-

sion curves for both systems were almost overlapped each

other. But the curing rate of the epoxy/FMWNT system

was considerably faster than the other two systems due to

the amine groups of FMWNT. The proposed curing ki-

netic model could describe well curing kinetics of the ep-

oxy/MWNT nanocomposite system.

Thermomechanical Properties

Figure 7 shows the storage modulus and tan d of the

epoxy/MWNT nanocomposites. Compared to the pure ep-

oxy system, the storage moduli of the epoxy/MWNT

nanocomposites below their Tgs were significantly

increased, about 32% increase for the epoxy/PMWNT

nanocomposite and about 53% increase for the epoxy/

FMWNT nanocomposite. The tan d peaks for the epoxy/

MWNT nanocomposites were smaller than that for the

pure epoxy system. This shows that the damping property

of the system was decreased by the inclusion of high

TABLE 1. Values of the reaction kinetic parameters.

System k11 (sec21) E1 (cal/mol) k22 (sec

21) E2 (cal/mol) m

Pure epoxy 2.50 3 1010 2.34 3 104 1.18 3 106 1.27 3 104 0.53

Epoxy/3phr PMWNT 2.50 3 1010 2.52 3 104 1.10 3 106 1.25 3 104 0.53

Epoxy/3phr FMWNT 1.17 3 1010 2.04 3 104 2.95 3 104 1.12 3 104 0.27

FIG. 6. Comparison of conversion changes obtained from DSC (points)

and calculated from the kinetic model (curves). (Scanning rate: 108C/min). FIG. 7. Storage modulus and tan d of the nanocomposite systems.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 419

modulus MWNTs. However, it is noteworthy that the tan

d peak for the epoxy/FMWNT nanocomposite is some-

what larger than that for the epoxy/PMWNT nanocompo-

site. This might be caused by the structural difference

between two nanocomposites because the epoxy/FMWNT

nanocomposite would have stronger interfacial bonding

and better dispersion.

The glass transition temperature of each system was

determined by taking the temperature of the most drastic

decrease in storage modulus. The epoxy/FMWNT nano-

composite showed a considerably higher Tg (199.38C)than the epoxy/PMWNT nanocomposite (189.78C) and

pure epoxy system (189.28C). The drastic increase in the

glass transition temperature of the epoxy/FMWNT nano-

composite was considered due to strong interfacial bond-

ing between the epoxy matrix and the MWNTs because

the epoxy matrix and the MWNTs could be connected by

covalent bonding through the chemical reaction between

the amine groups of FMWNTs and the epoxy groups in

the matrix resin. Strong interfacial bonding and homoge-

neous dispersion of the MWNTs would restrict the molec-

ular motion of the polymer chains and network junctions,

and result in a nanocomposite with a high glass transition

temperature as well as a high modulus like the epoxy/

FMWNT nanocomposite.

Figure 8 shows the flexural strength of the epoxy/

MWNT nanocomposites. Compared with the pure epoxy

system the epoxy/PMWNT nanocomposite showed a

slight increase in flexural strength. But the epoxy/

FMWNT nanocomposite showed a considerable increase

in flexural strength, and this was considered due to strong

interfacial bonding because the flexural strength of filler

reinforced composites strongly depend on the extent of

load transfer between the matrix and the filler. This kind

of mechanical property enhancement by incorporating

CNTs into various polymers has also been demonstrated

by others [30–34]. Their results also most likely benefited

from strong interfacial interaction and homogeneous

dispersion.

Structure and Morphology

Figure 9 shows the SEM images for the fracture surfa-

ces of the epoxy/PMWNT and epoxy/FMWNT nanocom-

posites. As shown in Fig. 9a, the fracture surface of the

epoxy/PMWNT nanocomposite shows a morphology indi-

cating that many MWNTs were pulled-out from the epoxy

matrix, rather than fractured, limiting a reinforcement role

of the MWNTs. Only when reinforcing CNTs are bonded

strongly to a polymer matrix can the external load be

effectively transferred from the matrix to the CNTs. The

morphology in Fig. 9a proved that the epoxy/PMWNT

nanocomposite had weak interfacial bonding between the

epoxy matrix and the MWNTs.

On the contrary, as shown in Fig. 9b, the fracture sur-

face of the epoxy/FMWNT nanocomposite clearly shows

less pull-out of MWNTs from the epoxy matrix and many

short broken segments of MWNT ropes indicating that

most of the MWNTs were well embedded and tightly

held to the epoxy matrix due to strong interfacial bond-

ing. This strong interfacial bonding between the epoxy

matrix and the MWNTs made the epoxy/FMWNT nano-

FIG. 8. Flexural strength of the nanocomposite systems.

FIG. 9. SEM images for the fracture surfaces of (a) the epoxy/PMWNT

nanocomposite and (b) epoxy/FMWNT nanocomposite.

420 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

composite capable of transferring the stress load and pre-

venting the sliding of MWNTs at the interfaces.

CONCLUSIONS

The amine functionalization of the PMWNT was car-

ried out successfully via treating them with 4-aminoben-

zoic acid in polyphosphoric acid. The FTIR, TGA, and

SEM results confirmed that the amine functionalization

was successful. The amine FMWNT accelerated the cur-

ing rate of the epoxy/MWNT nanocomposite system.

Curing kinetics of the nanocomposite system could be

described well by the autocatalytic second order reaction

kinetics. The amine functionalization induced strong

interfacial bonding between the epoxy matrix and the

MWNTs, and resulted in considerable improvements in

the glass transition temperature and mechanical proper-

ties of the nanocomposite system. The SEM images for

the fracture surfaces of the nanocomposites showed that

the epoxy/FMWNT nanocomposite had considerably

stronger interfacial bonding than the epoxy/PMWNT

nanocomposite.

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