C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

. sc iencedi rec t . com

ava i lab le a t www

journal homepage: www.elsevier .com/ locate /carbon

Processing and properties of polymer composites containingaligned functionalized carbon nanofibers

Chee-Sern Lim, Alejandro J. Rodriguez, Mauricio E. Guzman, Joseph D. Schaefer,Bob Minaie *

Department of Mechanical Engineering, Wichita State University, Wichita, KS 67260, USA

A R T I C L E I N F O

Article history:

Received 23 September 2010

Accepted 5 January 2011

Available online 11 January 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.01.010

* Corresponding author: Fax: +1 316 978 3236E-mail address: bob.minaie@wichita.edu

A B S T R A C T

AC electric field was used to align functionalized carbon nanofibers (CNFs), carboxylic acid-

functionalized CNFs (O-CNFs) and amine-functionalized CNFs (A-CNFs), in an epoxy resin.

The resulting composites were characterized for dispersion and alignment structure as

well as for their mechanical and electrical properties in the CNF alignment direction. Opti-

cal images of the composites revealed uniform distribution and alignment of the CNFs in

the direction of the electric field. Due to the similarity in the alignment structure, it was

observed that alignment of the functionalized CNFs was independent of the functional

groups attached to the CNFs. Compression tests (parallel to the direction of the aligned

A-CNFs) of A-CNF/epoxy composites showed an increase of 19% in compressive modulus

and 9% in compressive strength at a CNF concentration of 4.5 wt.%, with respect to the neat

composite. Electrical resistivity of composites measured parallel to the direction of aligned

CNFs (containing up to 4.5 wt.% O-CNFs and A-CNFs) were found to be approximately three

orders of magnitude lower than composites with non-aligned CNFs. The electrical resistiv-

ity percolation threshold for composites with aligned O-CNFs and A-CNFs occurred at

approximately 0.75 wt.%. Discussion regarding the contribution of CNF type towards the

mechanical and electrical properties is also presented.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanofibers (CNFs) continue to excel as nano-rein-

forcement in polymeric composites due to their high aspect

ratio and unique combination of mechanical [1,2] and electri-

cal properties [3,4]. CNF/polymer composites have been syn-

thesized for several applications such as electromagnetic

interference (EMI) shielding [5], electrostatic dissipation [6],

rubber reinforcement [7] and even biological implants [8]. In

addition, CNFs are also being incorporated into fiber-rein-

forced composites to primarily enhance through-thickness

direction properties [9–13]. Combining their low cost and

availability, CNFs are potential building blocks in fabricating

polymeric composites with improved properties.

er Ltd. All rights reserved

.(B. Minaie).

In order to manufacture CNF/polymer composites with

outstanding properties, the following conditions must be

met: (1) CNFs must be uniformly dispersed in the matrix so

that the external load can be transferred and distributed to

the nanofibers, and (2) the existence of chemical linkage be-

tween CNFs and matrix for better load transfer as well as pre-

venting CNFs from slipping relative to the matrix when

sustaining external load. However, the aforementioned con-

ditions are often challenged by CNF agglomeration due to

their high aspect ratio coupled with van der Waals interac-

tion. Therefore, to uniformly disperse the CNFs in polymer

resins and provide effective chemical compatibility between

them and the matrix, different mixing [14–16] and functional-

ization [17–19] schemes have been employed.

.

1874 C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

In recent years, studies have shown that carbon nanopar-

ticle/polymer composites with outstanding mechanical and

electrical properties can be fabricated at lower nanoparticle

content by selectively orienting them into a preferential direc-

tion through AC electric field [20–24], magnetic field [25,26],

flow induction [27,28], in situ growth [29,30] and mechanical

stretching [31]. Among these methods, the use of AC electric

field is one of the most promising methods since it has proven

to be effective in promoting alignment of carbon nanoparticle

in organic solvent [22] or polymer resins [20,21,23]. Experi-

ments aimed at aligning CNFs using AC electric field in poly-

mer resins have so far been limited to pristine CNFs [23] and

none have reported the study of composites with aligned

functionalized CNFs (carboxylic acid-functionalized and

amine-functionalized CNFs). Since functionalized CNFs, espe-

cially amine-functionalized CNFs, have previously been

shown to disperse uniformly in polymer resins and are capa-

ble of having high interaction with the matrix [18,19], it is

of great interest to pursue the study of the alignment effect

of these CNFs on the mechanical and electrical properties of

composites. Furthermore, no reports have shown the

mechanical properties of composites with aligned functional-

ized CNFs tailored by AC electric field.

In this work, investigation into the effect of functionalized

CNFs aligned in the direction of electric field on the mechan-

ical and electrical properties of composites was performed.

Alignment of the CNFs in the polymer resin was achieved

using AC electric field. Mixing of the CNFs with the resin

was carried out through a novel process involving a combina-

tion of high-speed shear mixing, calendering and planetary

centrifugal mixing. The resulting composites were character-

ized for dispersion and alignment, electrical resistivity, and

compressive strength and modulus parallel to the direction

of the aligned CNFs.

2. Experimental

2.1. Materials

The nanofibers used in this study were PR-24-XT-PS CNFs (Ap-

plied Science Inc.). According to the supplier, the as-received

CNFs (AR-CNFs) had an average diameter of approximately

100 nm and length of 30–100 lm, containing iron impurity of

less than 14,000 ppm and polyaromatic hydrocarbons of less

than 1 mg PAH/g fiber. EPON 862 epoxy resin and EPIKURE

W curing agent (Hexion) were selected as the two-part

epoxy-amine resin system. This system was selected because

it is widely used in aircraft industry due to its excellent

mechanical properties, adhesion and chemical resistance. A

two-part silicon rubber and a Teflon block (McMaster Carr)

were used as molds for curing the composites due to their

high electrical and temperature resistance. All other materi-

als and chemicals were obtained from Fisher Scientific and

used as-received.

2.2. Functionalization of CNFs

The procedures followed for carboxylic acid- and amine-func-

tionalization of CNFs were adapted from the literature [11,12].

Carboxylic acid-functionalized CNFs (O-CNFs) were obtained

through one step nitric acid oxidation process using AR-CNFs

as starting material. In terms of amine-functionalized CNFs

(A-CNFs), previously functionalized O-CNFs were subjected

to a two-stage procedure, which consisted of acylation using

thionyl chloride and amidation using ethylenediamine. After-

ward, both O-CNFs and A-CNFs were grinded and stored in a

desiccator until further processing. The functionalized CNFs

were characterized using Fourier transform infrared (FTIR)

spectroscopy and thermogravimetric analysis (TGA). The de-

tails and confirmation of functional groups on O-CNFs and

A-CNFs can be found in [11,12].

2.3. Preparation of functionalized CNF/epoxy mixture

In this study, composites with six different concentrations of

functionalized CNFs were studied: 0.25, 0.5, 0.75, 1.5, 3 and

4.5 wt.%. Mixing of functionalized CNFs with the resin system

was performed following a novel methodology which con-

sisted of four major steps: sonication, high-speed shear mix-

ing, calendering and planetary centrifugal mixing. To prepare

O-CNF/epoxy mixture, O-CNFs were first sonicated with

150 ml acetone for 10 min at high power followed by evapora-

tion of acetone in a convection oven until it became a semi-

dry mixture. Epoxy resin (185.92 g) was added to the mixture

and hand-stirred to saturate the O-CNFs. Then, the mixture

was homogenized in a water bath using a high-speed homog-

enizer at three different speeds for 15 min to disentangle the

large agglomerates. Subsequently, the mixture was calen-

dered by passing it through a three-roll-mill set at three dif-

ferent combinations of gap size and speed. Afterwards,

49.08 g of curing agent was added to the mixture and further

mixed using a planetary centrifugal mixer for four repetitions

at 2000 rpm (each repetition consisted of 5 min of mixing with

vacuum and 5 min of cooling at room temperature). The cen-

trifugal mixing process provided a non-intrusive method for

mixing the whole blend and removal of air bubbles using vac-

uum simultaneously.

In the case of A-CNF/epoxy mixture, A-CNFs were first son-

icated with 150 ml of ethanol for 3 min at medium power.

Then, 118.7 g of preheated epoxy resin (to reduce its viscosity)

was added to the mixture and sonicated for additional 10 min

followed by evaporation of ethanol with the assistance of

mechanical stirring on a hot plate. This mixing method was

different from the one used to mix O-CNFs due to two reasons.

First, since the sonication input was lower, additional stirring

was required to break large agglomerates. Second, because

ethanol has higher boiling point than acetone, the evaporation

process takes longer and it might induce the formation of

agglomerates in the absence of mechanical stirring. Therefore,

this process was completed with mechanical stirring to accel-

erate evaporation of ethanol and maintain uniform disper-

sion. Afterward, the blend was cooled in a freezer to

drastically increase the viscosity of the blend. In this regard,

high viscosity mixture will experience more shear mixing ef-

fect than low viscosity one during calendering and thus elim-

inate the existence of large agglomerates. The blend was then

homogenized, calendered and centrifugally mixed using the

same profile as the one followed for O-CNFs. The amount of

curing agent for the A-CNF/epoxy mixture was 31.3 g.

C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3 1875

2.4. Preparation of test samples

To prepare mechanical test samples, the mixture was casted

into an in-house made silicone rubber mold followed by addi-

tional degassing process for 15 min to remove air that was

introduced during the casting process. For composites with

aligned CNFs, two stainless steel plates, which were

31.75 mm apart, were used as electrodes to apply AC

electric field. The sinusoidal AC electric field used was

630 Vpeak-to-peak/cm at 2 kHz and was applied using a combi-

nation of function generator (GW Instek) and high voltage

amplifier (Trek Inc.), as shown in Fig. 1. This electric field

was calculated by dividing the applied peak-to-peak voltage

by the separation of electrodes. It was experimentally found

that this level of electric field would yield the optimum align-

ment structure of functionalized CNFs. Furthermore, Zhu

et al. [20] successfully aligned both pristine and amine-

functionalized carbon nanotubes in epoxy using AC voltage

of 2000 Vpeak-to-peak (equivalent to 800 Vpeak-to-peak/cm) with

an electrode separation of 25 mm.

In this study, the electric field was applied while maintain-

ing the oven temperature at 100 �C for 20 min followed by con-

stant temperature ramp (3 �C/min) to 177 �C. Upon reaching

177 �C, the electric field was kept for an additional 20 min to

make sure that the resin gelled before the electric field was re-

moved. Subsequently, the resin was allowed to cure at 177 �Cfor 160 min. The final dimensions of all mechanical test sam-

ples were approximately 63.5 mm · 31.75 mm · 19.05 mm.

For electrical test samples, the mixture was casted into a

Teflon mold and subjected to further degassing process in

vacuum for 15 min. Two aluminum plates, which were

12.7 mm apart, were used as electrodes to apply the same

field strength and frequency as for the mechanical test sam-

ples. Subsequent procedure to manufacture electrical test

samples with electric field was identical to the one followed

for mechanical test samples. The resulting dimensions of

electrical test samples were approximately 25.4 mm · 8 mm ·12.7 mm.

All test specimens were machined using a precision cut-

ting machine. Since the thickness of the sample was high, a

layer-by-layer trimming method was adapted to ensure the

flatness of each specimen. The dimensions of compression

and electrical test specimens were 12.7 mm · 12.7 mm ·

Fig. 1 – Illustration of alignment experiment.

25.4 mm and 6.45 mm · 6.45 mm · 2 mm, respectively. The

two larger surfaces of the electrical test specimens (perpen-

dicular to the CNF alignment direction) were coated with

silver paint to provide better electrical contact between elec-

trodes and surfaces of the specimens.

2.5. Characterization

Optical microscopy images of the composites were obtained

to qualitatively characterize the dispersion and alignment

structure of functionalized CNFs. Compression test was per-

formed according to ASTM D695-02 using a MTS testing ma-

chine. Electrical resistivity test was conducted based on

two-probe method with a high-precision Keithley Digital Mul-

timeter. For composites that were subjected to electric field,

both compression and electrical resistivity were measured

in the direction parallel to the aligned CNFs. Four specimens

were tested for compressive strength and modulus and three

specimens were tested for electrical resistivity at room tem-

perature. Electrical resistance of each specimen was recorded

after allowing the resistance value to stabilize for at least

1 min. The electrical resistivity was then calculated based

on q ¼ RAt , where q, R, A, and t are resistivity, resistance, area,

and thickness of each specimen, respectively.

3. Results and discussion

3.1. Dispersion of functionalized CNFs before and aftercuring

Before looking at the morphology of alignment, it is necessary

to inspect the quality of dispersion of functionalized CNFs be-

fore and after curing of composites. Fig. 2 shows the optical

images of dispersion of 0.5 and 1.5 wt.% O-CNFs in the resin

before (Fig. 2a and c) and after (Fig. 2b and d) curing of sam-

ples, suggesting negligible agglomeration throughout the en-

tire cure cycle. Such quality of dispersion can also be seen

in the case of uncured and cured composites with 0.5 and

1.5 wt.% A-CNFs, as shown in Fig. 3.

Even though the concentrations of O-CNFs and A-CNFs

were increased by threefold (from 0.5 to 1.5 wt.%), uniform

dispersion of O-CNFs and A-CNFs in the resin can still be

attained before curing, confirming the effectiveness of the

mixing and functionalization methodologies. After curing,

evidence of individual O-CNFs and A-CNFs can still be found

in the samples due to their stability in the resin. This finding

is particularly interesting because such quality of dispersion

was derived not just from the mixing method, but also from

the carboxylic acid and amine functional groups grafted on

the surfaces of O-CNFs and A-CNFs, respectively. The repul-

sion caused by the functional groups among adjacent O-CNFs

and A-CNFs led to the stability required to achieve uniform

dispersion.

Microscopically, the average length of functionalized CNFs

was reduced to approximately 10–20 lm as a result of chem-

ical functionalization and rigorous mechanical shear mixing.

Based on this, the aspect ratio of functionalized CNFs was

estimated to be 100–200. In addition, no significant O-CNF

and A-CNF clusters were observed which could be attributed

Fig. 2 – Optical images of dispersion of 0.5 wt.% O-CNFs (a) before, (b) after curing and 1.5 wt.% O-CNFs (c) before, (d) after

curing of composites.

Fig. 3 – Optical images of dispersion of 0.5 wt.% A-CNFs (a) before, (b) after curing and 1.5 wt.% A-CNFs (c) before, (d) after

curing of composites.

1876 C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

to repeated micro-gap shear mixing during homogenization

and calendering process.

During the course of the experiments, it was observed that

the mixing process involving calendering of resin and CNFs

C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3 1877

with concentrations higher than 1.5 wt.% became difficult be-

cause the resulting mixture changed from viscous state to

paste-like state. Hence, the use of three-roll-mill to disperse

CNFs was found to be effective until a CNF concentration of

approximately 1.5 wt.%.

3.2. Morphology of alignment

Figs. 4 and 5 show the alignment structures of composites

containing 0.5 and 1.5 wt.% of O-CNFs and A-CNFs, respec-

tively. These figures were taken approximately at the middle

of mechanical test samples. From the figures, it can be seen

that the functionalized CNFs aligned in the direction of the

electric field and that the alignment morphology varied as

the concentration of functionalized CNFs changed. At

0.5 wt.%, thick individual aligned O-CNF and A-CNF bundles

were formed when the samples were subjected to the electric

field. As the concentration of functionalized CNFs increased,

the alignment structure of O-CNFs and A-CNFs became dense

web-like with thin bundles.

Alignment structure of composites containing 0.5 wt.% O-

CNFs at five different locations is shown in Fig. 6. From the

images in Fig. 6, it can be seen that the aligned network of

O-CNFs was formed continuously between electrodes and

Fig. 4 – Optical images of alignment structure of composites con

arrows lines denote the alignment direction of CNFs along the

colour in this figure legend, the reader is referred to the web ve

Fig. 5 – Optical images of alignment structure of composites con

arrows lines denote the alignment direction of CNFs along the

colour in this figure legend, the reader is referred to the web ve

each image was observed to attain similar alignment mor-

phology. As such, it is suggested that the formation of these

bundles was due to the lateral agglomeration of O-CNFs and

A-CNFs. Similar phenomenon had previously been observed

by Zhu et al. [20] in composites containing aligned amine-

functionalized carbon nanotubes and the explanation em-

ployed by the author is referred herein to explain the mecha-

nism of lateral agglomeration of functionalized CNFs. A

diagram illustrating the evolution of lateral agglomeration

of functionalized CNFs is presented in Fig. 7. When subjected

to external electric field, the functionalized CNFs, which can

be visualized as micro-size cylinders, are polarized and un-

dergo a redistribution of electric charges near the ends of

the CNFs. This effect produces dipole moments on the CNFs,

which eventually becomes rotational motions and force the

CNFs to align along the direction of electric field. At the same

time, new interaction between adjacent CNFs arises due to

the electric charges that are distributed near the ends of

CNFs. As a result of this interaction, opposite electric charges

that are congregated on the ends of adjacent CNFs will attract

each other and force the CNFs to move closer and form micro-

visible bundles, leading to the observed lateral agglomeration.

In the bundles, it is suggested that the CNFs are maintaining

sufficient distance to reach force equilibrium (e.g. electric

taining (a) 0.5 wt.% and (b) 1.5 wt.% O-CNFs. The red two

applied electric field. (For interpretation of the references in

rsion of this article.)

taining (a) 0.5 wt.% and (b) 1.5 wt.% A-CNFs. The red two

applied electric field. (For interpretation of the references in

rsion of this article.)

Fig. 6 – Optical micrograph of alignment structure of composites containing 0.5 wt.% O-CNFs at different locations. The red

two arrows line denotes the alignment direction of CNFs along the applied electric field.

Fig. 7 – Illustration showing the evolution of lateral agglomeration of O-CNF or A-CNF when subjected to electric field. The

oval shape ‘‘ ’’ in the top right diagram shows the possible interaction between opposite electric charge of adjacent CNF at

both ends.

1878 C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

force and van der Waals force) rather than chemically bond-

ing to each other [20].

For samples with higher concentration, since the amount

of O-CNFs and A-CNFs per unit volume was higher, this auto-

matically elevated the interactive activity of adjacent O-CNFs

and A-CNFs upon the application of electric field and thus,

more aligned O-CNFs and A-CNFs bundles can be identified

locally. By comparing the alignment morphologies of samples

with different concentrations of O-CNFs and A-CNFs, it was

observed that the alignment structure for both samples were

similar. This suggests that the alignment morphology of func-

tionalized CNFs is independent of the functional groups

grafted to the CNFs.

3.3. Mechanical properties

3.3.1. O-CNF/epoxy compositesFrom Figs. 8 and 9, it can be seen that all the O-CNF/epoxy

composites showed increase in compressive strength with re-

spect to the neat composite. Interestingly, the strength of

samples with (OCNF-EF) and without (OCNF-NF) electric field

increased exponentially as a function of O-CNF concentra-

tion. The compressive strength of samples containing 4.5

wt.% OCNF-EF and OCNF-NF increased by 8.36% and 6.09%,

respectively, over the neat composite. Throughout the entire

range of O-CNF loading in the resin, the strength of OCNF-

EF samples was higher than OCNF-NF samples. The aligned

Fig. 8 – Compressive strength of samples containing

different loading of O-CNFs with (EF) and without (NF)

electric field. The error bars denote the standard deviation.

Fig. 9 – Percentage change of compressive strength of

samples containing different loading of O-CNFs with respect

to neat composite.

Fig. 10 – Compressive modulus of samples containing

different loading of O-CNFs with (EF) and without (NF)

electric field. The error bars denote the standard deviation.

Fig. 11 – Percentage change of compressive modulus of

samples containing different loading of O-CNFs with respect

to neat composite.

C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3 1879

O-CNFs had the capability of sustaining the compressive load

(parallel to the direction of aligned O-CNFs) better than ran-

domly oriented O-CNFs. At 0.25 wt.%, there was a negligible

enhancement for OCNF-EF and OCNF-NF samples, suggesting

that the amount of material was insufficient to strengthen

the composites despite of the presence of aligned network.

However, beyond 0.25 wt.%, the difference was more pro-

nounced for each corresponding loading.

From Figs. 10 and 11, it can be noticed that the compres-

sive modulus of all O-CNF reinforced composites increased

with respect to neat composite. In contrast to strength, the

changes in modulus for both types of samples increased lin-

early with respect to the concentration of CNFs until 3 wt.%.

The modulus of the composites containing 3 wt.% OCNF-EF

and OCNF-NF samples increased by 11.37% and 7.44%, respec-

tively, over the neat composite. Like strength, the modulus of

OCNF-EF samples was higher than OCNF-NF samples for all

O-CNF concentrations in the resin, which is also attributed

to the presence of alignment network parallel to the direction

of the electric field. At 0.25 wt.%, the change in modulus was

significant for OCNF-EF samples compared to OCNF-NF sam-

ples, indicating the effectiveness of alignment toward the

enhancement of modulus at low O-CNF concentration in

the resin, even though the strength of composites remained

approximately unchanged. In addition, it was found that the

addition of 3 wt.% O-CNFs to the resin was the onset of

achieving higher increase in strength of composites.

3.3.2. A-CNF/epoxy compositesIn general, all A-CNF/epoxy composites attained higher com-

pressive strength with respect to neat composite, as shown in

Figs. 12 and 13. The strength of samples containing 4.5 wt.%

A-CNFs with (ACNF-EF) and without (ACNF-NF) electric field

increased by 8.76% and 5.09%, respectively, over the neat

composite. In addition, the strength of ACNF-EF samples

was higher than ACNF-NF samples, showing the contribution

of alignment parallel to the direction of electric field toward

the strength of the materials. Even though the strength of

ACNF-NF samples increased, the change was minimal as

the concentration of A-CNFs increased. In contrast to ACNF-

NF samples, a gradual increase in strength was observed for

ACNF-EF samples until loading of 0.75 wt.%, then, the change

became more prominent at higher A-CNF concentration.

Figs. 14 and 15 illustrate the compressive modulus and

percentage change of modulus of A-CNF/epoxy composites.

Fig. 12 – Compressive strength of samples containing

different loading of A-CNFs with (EF) and without (NF)

electric field. The error bars denote the standard deviation.

Fig. 13 – Percentage change of compressive strength of

samples containing different loading of A-CNFs with respect

to neat composite.

Fig. 14 – Compressive modulus of samples containing

different loading of A-CNFs with (EF) and without (NF)

electric field. The error bars denote the standard deviation.

Fig. 15 – Percentage change of compressive modulus of

samples containing different loading of A-CNFs with respect

to neat composite.

Fig. 16 – Comparison between percentage change of

compressive strength of 4.5 wt.% O-CNF and A-CNF

reinforced composites with respect to neat composite.

1880 C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

The compressive modulus of samples with 4.5 wt.% ACNF-EF

and ACNF-NF increased by 19% and 10.88%, respectively, over

the neat composite. Like compressive strength, the addition

of A-CNFs also enhanced the modulus of the neat composite.

Furthermore, an interesting observation can also be drawn by

comparing the percentage change of modulus for different

concentrations of ACNF-EF with ACNF-NF samples. From

Fig. 15, it is found that the percentage change of modulus

for ACNF-EF samples was approximately two to three times

greater than ACNF-NF samples. This finding could be

explained by the combination of enhanced compatibility

between A-CNFs and the resin and the presence of aligned

A-CNFs parallel to the direction of electric field.

As mentioned previously, the grafting of amine functional

groups would improve the compatibility between CNFs and

matrix. As such, both compressive strength and modulus of

O-CNF/epoxy and A-CNF/epoxy composites were compared.

From Fig. 16, it is shown that the addition of 4.5 wt.% O-CNFs

and A-CNFs to the resin resulted in similar strength for sam-

ples with and without electric field, respectively. In this re-

gard, it is suggested that the strength of the composites is

independent of the type of functionalized CNFs for the con-

centrations studied. Fig. 17 shows the comparison of com-

pressive modulus of composites containing 4.5 wt.% O-CNFs

Fig. 17 – Comparison between percentage change of

compressive modulus of 4.5 wt.% O-CNF and A-CNF

reinforced composites with respect to neat composite.

C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3 1881

and A-CNFs. In contrast to strength, the modulus of A-CNF

reinforced samples was higher than O-CNF reinforced sam-

ples, especially for ACNF-EF samples. Here, the combination

of amine functional group and alignment effect helped to in-

crease the modulus of ACNF-EF samples, resulting in a 9% dif-

ference over OCNF-EF samples. This suggests that modulus of

the composites is dependent on the type of functionalized

CNFs. Based on these comparisons, it can be seen that the

coupling of amine functional group on A-CNFs and alignment

effect further modified the modulus of neat composite, sug-

gesting better load transfer from the matrix to CNFs. As such,

it is possible that if higher concentration of functionalized

CNFs can be aligned into desired direction in the resin using

any method, composites with higher strength and modulus

along the aligned direction could be achieved provided that

uniform dispersion of CNFs is maintained.

3.4. Electrical properties

Fig. 18 shows the electrical resistivity of OCNF-EF and OCNF-

NF samples. Note that the electrical resistivity of neat com-

Fig. 18 – Electrical resistivity of composites containing O-

CNFs with (EF) and without (NF) electric field as a function of

loading.

posite was obtained from literature [32]. For OCNF-NF sam-

ples, addition of 0.25 wt.% O-CNFs decreased the electrical

resistivity to 109 X cm; however, further increase in O-CNF

loading in the resin did not alter the electrical resistivity sig-

nificantly. This could be due to failure of the sample to reach

electrical resistivity percolation threshold upon the addition

of 4.5 wt.% O-CNFs to the neat composite. In fact, Prasse

et al. [23] reported that the percolation threshold of electrical

resistivity would not occur until 6 wt.% of CNF in the resin

without the application of electric field. In addition, according

to Ahn et al. [18] and Lafdi et al. [32], aspect ratio and oxygen

content of functionalized CNFs are the governing factors of

percolation threshold of electrical conductivity. Particularly,

the lower the aspect ratio, the higher the amount of function-

alized CNFs required to reach percolation threshold; the high-

er the oxygen content, the higher the electrical resistivity of

functionalized CNFs. Based on these relations, it is believed

that the percolation threshold of OCNF-NF samples would oc-

cur at a concentration higher than 6 wt.% owing to the low as-

pect ratio and increased oxygen content of O-CNF during

functionalization process.

In the case of OCNF-EF samples, the electrical resistivity,

which was measured parallel to the direction of electric field,

also decreased to approximately 109 X cm at loading of

0.25 wt.% O-CNFs. As the O-CNF loading in the resin in-

creased, the decrease in resistivity became prominent and

percolation threshold was observed to occur at around

0.75 wt.%. Additionally, the resistivity at 1.5 wt.% decreased

to 107 X cm and continued to decrease to 106 X cm range at

loading of 4.5 wt.% O-CNFs. Note that even though the load-

ing increased by three fold from 1.5 wt.%, the change of resis-

tivity was not significant. This indicates that further increase

in concentration of O-CNFs will modify the resistivity of

OCNF-EF samples at lower rate. Overall, a reduction of four or-

ders magnitude in electrical resistivity was achieved by fabri-

cating composites with 4.5 wt.% O-CNFs aligned in the

direction of electric field.

Fig. 19 shows the electrical resistivity of ACNF-EF and

ACNF-NF samples. In the case of ACNF-NF samples, the

Fig. 19 – Electrical resistivity of composites containing A-

CNFs with (EF) and without (NF) electric field as a function of

loading.

1882 C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3

addition of 0.25 wt.% of A-CNFs led to reduction of electrical

resistivity to 109 X cm range. Similar to OCNF-NF samples,

further increase in loading of A-CNFs did not modify the elec-

trical resistivity of ACNF-NF samples considerably. By taking a

closer look at the resistivity values, it is found that ACNF-NF

samples were slightly less resistive than OCNF-NF samples.

However, because of low aspect ratio and increased func-

tional group content of A-CNFs during functionalization pro-

cess, it is also believed that percolation threshold of ACNF-NF

samples would occur at concentration higher than 6 wt.%.

On the other hand, the electrical resistivity, which was

also measured parallel to the direction of electric field, of

ACNF-EF samples decreased to a value slightly lower than

109 X cm at 0.25 wt.% O-CNF concentration. The resistivity

of 1.5 wt.% ACNF-EF sample dropped to 106 X cm range and

continued to decrease to slightly lower than 106 X cm range

at concentration of 4.5 wt.%. From Fig. 18, it can be seen that

the percolation threshold for ACNF-EF samples also occurred

at around 0.75 wt.%. Similar to OCNF-EF samples, further in-

crease in concentration of A-CNFs, from 1.5 to 4.5 wt.%, did

not yield significant changes in resistivity. Approximately

four orders of magnitude reduction in electrical resistivity

were achieved by fabricating composites containing 4.5 wt.%

of aligned A-CNFs.

The trend of resistivity of OCNF-EF and ACNF-EF samples

herein is similar to the result (the electrical resistivity of com-

posites that was measured parallel to the direction of electric

field) reported by Prasse et al. [23] except with higher resistiv-

ity values at each concentration, which could be attributed to

difference in type of nanofiller, lower aspect ratio, and oxygen

content of O-CNFs. Although the resistivity of 1.5 wt.% ACNF-

EF sample was approximately half an order of magnitude low-

er than 1.5 wt.% OCNF-EF sample, both of them saturated at

approximately same resistivity value at 4.5 wt.%. Further-

more, in terms of electrical properties of composites, it is

found that the use of electric field to prepare OCNF-EF and

ACNF-EF samples with aligned network parallel to the direc-

tion of electric field was beneficial until 1.5 wt.%. Beyond this

concentration, no significant change in electrical resistivity

was obtained.

4. Conclusions

In this study, processing and characterization of composites

with improved mechanical and electrical properties in a pref-

erential direction – having aligned functionalized CNFs – was

presented. All properties of composites containing aligned

CNFs were measured parallel to the CNF alignment direction.

Microscopy images of the composites indicated uniform dis-

tribution and alignment of functionalized CNFs in the direc-

tion of electric field. Even though different functional groups

(carboxylic acid and amine) were grafted to the CNFs, it was

found that the alignment morphology of the functionalized

CNFs was independent of the functional groups studied.

Compressive strength of the composites with 4.5 wt.% of

aligned O-CNFs and A-CNFs attained similar values, 8.36%

and 8.76% higher than the neat composite, respectively. In

contrast to strength, the compressive modulus of the com-

posite with aligned A-CNFs was 9% higher than the composite

with aligned O-CNFs at the same concentration (4.5 wt.%),

resulting in an overall increase of 19% with respect to the neat

composite. These results suggest that strength of the com-

posites is independent of the type of functionalized CNFs

whereas modulus of the composites is dependent on the type

of functionalized CNFs. Coupling of amine functional group

grafted to the CNFs with the epoxy matrix provided better

load transfer from the matrix to CNFs, resulting in an increase

in compressive modulus. In terms of electrical resistivity, a

reduction of more than four orders of magnitude was ob-

served for the composites containing 4.5 wt.% of aligned O-

CNFs or A-CNFs with respect to the neat composite.

Due to their unique properties and cost effectiveness, CNFs

are attractive candidates for the manufacturing of multiscale-

reinforced – hierarchical – composites. Therefore, understand-

ing the fundamental alignment behavior of functionalized

CNFs in the resin and properties of final composites is of great

importance before alignment of nanoparticles can be applied

to manufacture hierarchical structures.

Acknowledgements

The authors gratefully acknowledge financial support from

the Office of Naval Research (Grant No. N000140810893),

National Aeronautics and Space Administration (Grant No.

NNX07A027A), and Kansas Technology Enterprise

Corporation.

R E F E R E N C E S

[1] Ozkan T, Naraghi M, Chasiotis I. Mechanical properties ofvapor grown carbon nanofibers. Carbon 2010;48(1):239–44.

[2] Lawrence JG, Berhan LM, Nadarajah A. Elastic properties andmorphology of individual carbon nanofibers. ACS Nano2008;2(6):1230–6.

[3] Tibbetts GG, Lake ML, Strong KL, Rice BP. A review of thefabrication and properties of vapor-grown carbonnanofiber/polymer composites. Compos Sci Technol 2007;67(7–8):1709–18.

[4] Al-Saleh MH, Sundararaj U. A review of vapor grown carbonnanofiber/polymer conductive composites. Carbon2009;47(1):2–22.

[5] Shuying Y, Lozano K, Lomeli A, Foltz HD, Jones R.Electromagnetic interference shielding effectiveness ofcarbon nanofiber/LCP composites. Compos Part A (Appl SciManuf) 2005;36(5):691–7.

[6] Cho J, Konopka K, Rozniatowski K, Garcia-Lecina E, ShafferMSP, Boccaccini AR. Characterisation of carbon nanotubefilms deposited by electrophoretic deposition. Carbon2009;47(1):58–67.

[7] Pelsoci TM. Composites Manufacturing Technologies:Applications in Automotive, Petroleum, and CivilInfrastructure Industries2004 Contract No.: NIST GCR 04-863.

[8] Shim BS, Starkovich J, Kotov N. Multilayer composites fromvapor-grown carbon nano-fibers. Compos Sci Technol2006;66(Compendex):1171–8.

[9] Kuang-Ting H, Sadeghian R, Gangireddy S, Minaie B.Manufacturing carbon nanofibers toughened polyester/glassfiber composites using vacuum assisted resin transfermolding for enhancing the mode-I delamination resistance.Compos Part A (Appl Sci Manuf) 2006;37(10):1787–95.

C A R B O N 4 9 ( 2 0 1 1 ) 1 8 7 3 – 1 8 8 3 1883

[10] Green KJ, Dean DR, Vaidya UK, Nyairo E. Multiscale fiberreinforced composites based on a carbon nanofiber/epoxynanophased polymer matrix: synthesis, mechanical, andthermomechanical behavior. Compos Part A2009;40(9):1470–5.

[11] Rodriguez A, Guzman M, Lim C, Minaie B, editors.Electrophoretic deposition of CNF onto carbon fiber layers formanufacturing of fiber-reinforced polymer nanocomposites.In: SAMPE ‘09 spring symposium conference proceedings,May 18, 2009–May 21, 2009. Baltimore, MD, United states: Soc.for the Advancement of Material and Process Engineering;2009.

[12] Rodriguez A, Guzman M, Lim C, Minaie B, editors. Hybridcarbon nanofiber-carbon fiber layers for manufacturing offiber-reinforced polymer nanocomposites. In: SAMPE ‘09spring symposium conference proceedings, May 17, 2010–May 20, 2010. Seattle, WA, United States: Soc. for theAdvancement of Material and Process Engineering; 2010.

[13] Rodriguez AJ, Guzman ME, Lim C-S, Minaie B. Synthesis ofmultiscale reinforcement fabric by electrophoretic depositionof amine-functionalized carbon nanofibers onto carbon fiberlayers. Carbon 2010;48(11):3256–9.

[14] Battisti A, Skordos AA, Partridge IK. Monitoring dispersion ofcarbon nanotubes in a thermosetting polyester resin.Compos Sci Technol 2009;69(10):1516–20.

[15] Thostenson ET, Tsu-Wei C. Aligned multi-walled carbonnanotube-reinforced composites: processing and mechanicalcharacterization. J Phys D (Appl Phys) 2002;35(16):77–80.

[16] Tsu-Wei C, Thostenson ET, Ziaee S. Processing and electricalproperties of carbon nanotube/vinyl ester nanocomposites.Compos Sci Technol 2009;69(6):801–4.

[17] Lakshminarayanan PV, Toghiani H, Pittman Jr CU. Nitric acidoxidation of vapor grown carbon nanofibers. Carbon2004;42(12–13):2433–42.

[18] Ahn S-N, Lee H-J, Kim B-J, Tan L-S, Baek J-B. Epoxy/amine-functionalized short-length vapor-grown carbon nanofibercomposites. J Polym Sci. Part A: Polym Chem2008;46(22):7473–82.

[19] Prolongo SG, Campo M, Gude MR, Chaos-Moran R, Urena A.Thermo-physical characterisation of epoxy resin reinforcedby amino-functionalized carbon nanofibers. Compos SciTechnol 2009;69(3–4):349–57.

[20] Zhu Y-F, Ma C, Zhang W, Zhang R-P, Koratkar N, Liang J.Alignment of multiwalled carbon nanotubes in bulk epoxycomposites via electric field. J Appl Phys 2009;105(5):054319-1–6.

[21] Martin CA, Sandler JKW, Windle AH, Schwarz MK, BauhoferW, Schulte K, et al. Electric field-induced aligned multi-wallcarbon nanotube networks in epoxy composites. Polymer2005;46(Copyright 2005, IEE):877–86.

[22] Chen XQ, Saito T, Yamada H, Matsushige K. Aligning single-wall carbon nanotubes with an alternating-current electricfield. Appl Phys Lett 2001;78(23):3714–6.

[23] Prasse T, Cavaille JY, Bauhofer W. Electric anisotropy ofcarbon nanofibre/epoxy resin composites due to electric fieldinduced alignment. Compos Sci Technol 2003;63(13):1835–41.

[24] Cheol P, Wilkinson J, Banda S, Ounaies Z, Wise KE, Sauti G,et al. Aligned single-wall carbon nanotube polymercomposites using an electric field. J Polym Sci Part B (PolymPhys) 2006;44(12):1751–62.

[25] Mahfuz H, Zainuddin S, Rangari VK, Jeelani S, Parker MR, Al-Saadi T, editors. Infusion of carbon nanotubes and carbonnanofibers into SC-15 epoxy; an investigation of the influenceof high magnetic fields. In: 50th international SAMPEsymposium and exhibition, May 1, 2005–May 5, 2005. LongBeach, CA, United states: Soc. for the Advancement ofMaterial and Process Engineering; 2005.

[26] Garmestani H, Al-Haik MS, Dahmen K, Tannenbaum R, Li D,Sablin SS, et al. Polymer-mediated alignment of carbonnanotubes under high magnetic fields. Adv Mater2003;15(22):1918–21.

[27] Abbasi S, Carreau PJ, Derdouri A. Flow induced orientation ofmultiwalled carbon nanotubes in polycarbonatenanocomposites: rheology, conductivity and mechanicalproperties. Polymer 2010;51(4):922–35.

[28] Camponeschi E, Florkowski B, Vance R, Garrett G, GarmestaniH, Tannenbaum R. Uniform directional alignment of single-walled carbon nanotubes in viscous polymer flow. Langmuir2006;22(4):1858–62.

[29] Cheng QF, Wang JP, Wen JJ, Liu CH, Jiang KL, Li QQ, et al.Carbon nanotube/epoxy composites fabricated by resintransfer molding. Carbon 2010;48(1):260–6.

[30] Ci L, Suhr J, Pushparaj V, Zhang X, Ajayan PM. Continuouscarbon nanotube reinforced composites. Nano Lett2008;8(9):2762–6.

[31] Qing W, Jianfeng D, Weixue L, Zhiqiang W, Jinlong J. Theeffects of CNT alignment on electrical conductivity andmechanical properties of SWNT/epoxy nanocomposites.Compos Sci Technol 2008;68(7–8):1644–8.

[32] Lafdi K, Fox W, Matzek M, Yildiz E. Effect of carbon nanofiber –matrix adhesion on polymeric nanocomposites properties-part II. J Nanomater 2008;2008:310126 1–8.