communications

466

Nanocomposites

High-Strength, High-Toughness Composite Fibers bySwelling Kevlar in Nanotube Suspensions**

Ian O’Connor, Hugh Hayden, Jonathan N. Coleman,* and Yurii K. Gun’ko*

Strong polymer fibers have a broad range of applications and

play a very important role in modern technology and everyday

life. Therefore, the development of new ultra-strong polymer

or composite fibers is of great interest both scientifically and

for industry. Carbon nanotubes have been envisaged as one of

the most promising additives for the fabrication of ultra-strong

polymer composite materials[1] due to their superior mechan-

ical properties. It is well-known that carbon nanotubes have

Young’s modulus and tensile strength above 1 TPa[2] and

60GPa,[3,4] respectively, while their densities can be as low as

�1.3 g cm�1.[3,5] For these reasons, various polymer–nanotube

composites have become a subject of intensive research and

technological development over the last decade.[1] The most

common approaches for the fabrication of polymer–nanotube

composite fibers have been melt processing[6–9] and solution

coagulation spinning.[10–15] In these techniques, nanotubes

must either be incorporated into a polymer solution or molten

polymer before the formation of corresponding polymer–

nanotube composite fibers. However, thesemethods cannot be

applied in the case of insoluble or temperature-sensitive

polymers, which decompose without melting. Kevlar is a well-

known high-strength polymer with a variety of important

applications including bullet-proof vests, protective clothing,

and high-performance composites for aircraft and automotive

industries.[16–18] However, Kevlar is not soluble in any

common solvent and has no melting point, decomposing

above 400 8C.[17] As a result, Kevlar fibers must be produced

by wet-spinning from sulfuric acid solutions.[19,20] While

single-walled nanotube (SWNT) fibers can be produced by

acid-spinning,[21] to the authors knowledge, no reports of

Kevlar–nanotube fibers have appeared in the literature

(however, it should be noted that poly(paraphenylene-2,6-

benzobisoxazole) (PBO, also known as Zylon)–nanotube

composite fibers have been produced by acid dry-jet wet-

[�] Prof. J. N. Coleman

School of Physics and CRANN Institute

Trinity College, University of Dublin

Dublin 2 (Ireland)

E-mail: colemaj@tcd.ie

Prof. Y. K. Gun’ko, I. O’Connor, H. Hayden

School of Chemistry and CRANN Institute

Trinity College, University of Dublin

Dublin 2 (Ireland)

E-mail: igounko@tcd.ie

[��] The authors acknowledge Science Foundation Ireland (RFP pro-gramme) for financial support, and staff members of the ElectronMicroscopy Unit (TCD) for their help with SEM imaging.

DOI: 10.1002/smll.200801102

� 2009 Wiley-VCH Verl

spinning).[22,23] Although it is probably possible to produce

Kevlar–SWNT fibers by acid-spinning, an alternative method,

which would allow proof-of-concept, would be to incorporate

nanotubes in commercially available, pre-existing Kevlar

fibers by a post-production method. Such a method is described

in this paper, where we report the preparation of new,

reinforced Kevlar–nanotube composites. These are prepared

by swelling commercially available Kevlar 129TM fibers in

suspensions of nanotubes in the solvent N-methylpyrrolidone

(NMP). Nanotube uptake of up to 4wt% has been observed,

resulting in significant mechanical enhancement of the

fibers.

Recently, it has been reported that nanotubes can be

efficiently dispersed in NMP.[24,25] It is known that NMP can

be used as a solvent for the synthesis (polymerization) and

processing of Kevlar.[26] We have also found that Kevlar fibers

can be swelled in NMP. This led us to hypothesize that

nanotubes could be incorporated into swelled Kevlar by

soaking Kevlar fibers in a dispersion of nanotubes in NMP. In

our experiments, commercially sourced Kevlar 129 yarns were

placed in stable suspensions of multiwalled carbon nanotubes

(MWNTs), containing various concentrations of nanotubes, in

NMP. These Kevlar–nanotube mixtures in NMP were

sonicated using an ultrasonic bath for various periods of time

at ambient temperature. The Kevlar yarns were then retrieved

from the nanotube dispersions and washed several times. They

were then dried, weighed, and investigated by thermogravi-

metric analysis (TGA), scanning electron microscopy (SEM),

and mechanical testing.

Analysis of the samples showed that swelling of Kevlar

yarn in MWNT dispersions under ultrasound in a sonic bath

resulted in a measurable mass uptake after washing. This

suggests that soaking in NMP induces porosity in the Kevlar,

allowing nanotube infiltration. The nanotube mass uptake was

found to scale with the square root of soak time (Figure 1A),

suggesting that the intercalation of nanotubes into the Kevlar

fibers is diffusion limited.[27] For mass transport by diffusion

into porous thin films, the intercalated mass uptake as a

function of time is given by[28]

mNT

mKev

¼ mNT

mKev

� �Sat

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi16D

b2p

� �t

s(1)

where (mNT/mKev)Sat is the saturated value of the mass uptake,

D is the diffusion coefficient, and b is the sample thickness.

While this equation is not strictly valid for diffusion into a

porous cylinder, we use it here in the expectation that it is

ag GmbH & Co. KGaA, Weinheim small 2009, 5, No. 4, 466–469

Figure 2. SEM images of cross sections of a blade-cut Kevlar fiber after

treatment in 1 mg mL�1 MWNT suspension in NMP.

Figure 1. A) Normalized nanotube mass uptake in Kevlar yarns against

swelling time. The dashed line shows square root behavior allowing

the estimation of the intercalative diffusion coefficient to be

<1.4� 10�17 m2 s�1. B) Graph of nanotube mass uptake (expressed as

nanotube mass fraction) as a function of nanotube concentration in the

swelling suspension.

correct to within a constant factor. In addition, we approx-

imate b by the fiber radius, that is, the furthest the nanotube

can diffuse into the fiber. We did not observe saturation of the

mass uptake in our experiments. However, by normalizing the

mass uptake to the maximum observed value we can get an

upper limit to the diffusion coefficient. This allows Equation 1

to be fitted to the data in Figure 1A. The fit was reasonably

good, indicating the validity of Equation 1. The fit parameters

give an upper limit for the diffusion coefficient (taking

b¼ 5mm): D< 1.4� 10�17 m2 s�1. This is significantly lower

than the value of 1.3� 10�12 m2 s�1 that one can estimate using

the Broersma’s equations[29] for a MWNT dispersed in the

liquid phase. This is unsurprising as diffusion in the restricted

environment of the swelled polymer would be expected to be

considerably slower then diffusion in the solvent.

Extensive tests involving soaking yarns in dispersions with

a nanotube concentration of 0.6mg mL�1 under different

conditions showed that the best mechanical properties were

obtained when soaking MWNTs in a sonic bath for 30min.

Subsequently, a range of yarns were treated by soaking for

30min in nanotube dispersions at a range of nanotube

concentrations. We accept that the true optimum soak time

may vary slightly with concentration, however, we chose to

treat all nanotube concentrations equally in terms of

small 2009, 5, No. 4, 466–469 � 2009 Wiley-VCH Verlag Gmb

processing parameters. Thus, we chose 30min as the soak

time for all samples, accepting that most of the samples may

not be perfectly optimized. In addition, a control sample was

prepared where the Kevlar yarn was soaked under identical

conditions in pure NMP. These yarns were washed, dried, and

weighed and the mass uptake calculated. The mass uptake as a

function of nanotube concentration in the NMP dispersion for

a 30min soak time is shown in Figure 1B. The mass uptake was

found to vary from 0.3 to 4wt%. It should be pointed out that

after a soak time of 30min, the diffusion process had certainly

not reached steady state (Figure 1A). We reiterate that this

time was chosen in order to maximize the mechanical

properties rather than to represent equilibrium.

An image of a Kevlar–MWNT composite fiber that had

been cut using a blade for SEM cross-sectional analysis is

shown in Figure 2. This and other images of the surface of

individually treated Kevlar fibers show a web-like deposit of

nanotubes, even after washing.While the breaks do not tend to

be particularly clean, the presence of nanotubes in the interior

of the fibers can be seen as illustrated in the blow-up on the left

side of Figure 2. This demonstrates nanotube insertion as well

as surface coating.

The yarns treated for 30min in nanotube dispersions of

varying concentrations were used for mechanical testing. The

mass fractions were those shown in Figure 1B. Mechanical

measurements were made on between 8 and 13 individual

fibers extracted from each of the treated yarns. These fibers

were typically �10mm in diameter and were mounted in the

tensile tester with the aid of glue. In all cases the stress–strain

curves (not shown) were linear, demonstrating brittle failure.

From these stress–strain curves, we can obtain fourmechanical

parameters: Young’s modulus, Y; tensile strength, sB; strain at

break, eB; and toughness,T. These data are shown as a functionof nanotube mass fraction in Figure 3A–D. In all cases, the

0 wt% curve is for Kevlar fibers treated in NMP in a fashion

analogous to the composite preparation treatment. This

treatment resulted in very little change in the Kevlar’s

mechanical properties. For each mechanical property, both

mean values, with standard deviations (closed squares), and

maximum observed values (open squares) are plotted. No

significant change in the mean values of either the Young’s

modulus or the strain at break is observed. In fact, hypothesis

testing shows that the data sets for both Young’s modulus and

strain at break are statistically indistinguishable (significance

level 0.01) for all mass fractions. However, an increase in mean

strength from 4GPa for the Kevlar fiber to 5GPa for the

1 wt% composite fiber is observed. In addition, despite the

H & Co. KGaA, Weinheim www.small-journal.com 467

communications

Figure 3. A) Young’s modulus, B) strength, C) strain at break, and

D) toughness as functions of mass fraction for nanotubes in Kevlar

fibers. The toughness is shown on a per mass basis on the right axis of

the lowest panel. This is as converted from the per volume toughness

assuming a density of 1500 kg m�3.

Figure 4. Comparison of fibers prepared in this work with literature

values for various fibers including a PBO–nanotube composite fiber. [22]

Selected fibers are labeled. (For full details, see Chae and Kumar, [18]

Table II).

468

substantial error bars, there is a real increase in mean

toughness from 40 to 70 J g�1 for the 1.75 wt% sample.

Hypothesis testing shows that for both strength and toughness,

both the 1 and 1.75 wt% data are statistically distinguishable

from the pure Kevlar data (significance level 0.01).

However, it is the maximum observed values that are of

most interest as these represent the ultimate mechanical

properties that are potentially achievable. In all cases the

maximum observed value increases from the Kevlar-only

fibers with increasing mass fraction, peaking at either 1 or

1.75 wt%. The observed increases compared to the Kevlar-

only fibers, were (maximum values only): Young’s modulus,

115–207GPa; strength, 4.7–5.9GPa; strain at break, 4.0–5.4%;

and toughness, 63–99 J g�1.

We have demonstrated that significant mechanical

enhancement of Kevlar fibers is possible by the incorporation

of carbon nanotubes. However, it is important to put these

results in the context of the properties of other polymer-based,

high-strength, low-ductility fibers. Shown in Figure 4 is a plot

www.small-journal.com � 2009 Wiley-VCH Verlag Gm

of published data for mechanical properties of various high-

performance fibers (the data have been adapted from Chae

and Kumar, Table II[18]). We have plotted these data in the

form of Young’s modulus versus tensile strength (Figure 4A)

and toughness versus strain at break (Figure 4B). Our data

have been incorporated into this graph as both the set of best

mean values we have observed and the absolute best results

observed for single fibers. (Nota bene: the best results for

single fibers do not necessarily refer to the same fiber. For

example, the best modulus and best toughness were observed

for different fibers.) It is clear that the best mean strength and

toughness values observed for our fibers are better than most

of the fibers reported to date. However, our best observed

results are comparable in terms of strength and modulus and

superior in terms of toughness (albeit at higher strain)

compared to the very best fibers reported. In particular, we

obtain strength and modulus comparable to recent results for

experimental PBO fibers.[30,31] In addition, our best mean

strength and modulus values are comparable to SWNT–PBO

fibers prepared at significantly higher nanotube content (open

triangles).[22] Furthermore, our best mean values for tough-

ness and strain at break are significantly higher than those for

SWNT–PBO fibers.

Finally, we note that while our best results are comparable

to the best polymer-based fibers, they are inferior to the best

nanotube-only fibers. Recently, Windle et al. reported a

method to spin MWNT-only fibers directly from the chemical

vapor deposition (CVD) furnace. This resulted in fibers whose

best values for strength, modulus, strain at break, and

bH & Co. KGaA, Weinheim small 2009, 5, No. 4, 466–469

toughness were �9GPa, �360GPa, 6%, and 300 J g�1,

respectively.[32] However, while we have prepared Kevlar–

nanotube composite fibers by a post-treatment technique, we

feel confident that similar results could be obtained by acid-

spinning from Kevlar–nanotube dopes as has been achieved

for PBO-based fibers.[22] In that case, high-strength, high-

toughness Kevlar–nanotube fibers could be potentially

produced using existing production techniques and possibly

even facilities.

In conclusion, we have developed a new approach for the

preparation of polymer–nanotube composite fibers. This

method is based on the ultrasonic-assisted swelling of the

polymeric fiber in a nanotube suspension with the use of an

appropriate organic solvent. The swelling of Kevlar in a

carbon nanotube suspension in NMP resulted in new Kevlar–

nanotube composites with improved mechanical properties.

Our new approach of incorporating nanomaterials into

polymer macromaterials by swelling could be expanded and

utilized for other nanosystems and polymer materials. We

believe there will be many possible important applications for

this new technique.

Experimental Section

MWNTs grown by CVD were purchased from Nanocyl (product

NC3100) and used as supplied. Kevlar 129 yarn was kindly

provided by du Pont. The MWNTs were dispersed in NMP using an

ultrasonic processor (Model GEX600; 750 W, 20%, 60 kHz) for

5 min followed by 2 h in a sonic bath (grant XB6, 300 W). A

weighed amount of Kevlar 129 yarn was then placed in the

nanotube dispersion in NMP. This yarn consists of a parallel array

of Kevlar fibers, each with diameters of �10mm. The mixture was

treated using the ultrasonic processor for 5 min followed by

30 min in the ultrasonic bath. The Kevlar yarn was then removed

from the solution and washed by sonication in ethanol for 20 s.

The yarn was then washed twice in ethanol without sonication to

remove any residual NMP. The fibers were then straightened and

allowed to dry in a vacuum oven at 60 8C for 1 week. Following this

the Kevlar fibers were weighed again and a net weight change was

calculated. Mechanical measurements were made on individual

fibers using a Zwick 100 tensile tester at a cross-head speed of

15 mm min�1. Eight to thirteen samples were analyzed for each

point on the graph.

Keywords:

carbon nanotubes . fibers . kevlar . nanocomposites .

polymers

[1] J. N. Coleman, U. Khan, W. J. Blau, Y. K. Gun’ko, Carbon 2006, 44,

1624.

[2] E. W. Wong, P. E. Sheehan, C. M. Lieber, Science 1997, 277, 1971.

[3] M. Yu, O. Lourie, M. J. Dyer, T. F. Kelly, R. S. Ruoff, Science 2000,

287, 637.

[4] M. F. Yu, B. S. Files, S. Arepalli, R. S. Ruoff, Phys. Rev. Lett. 2000,

84, 5552.

small 2009, 5, No. 4, 466–469 � 2009 Wiley-VCH Verlag Gmb

[5] J. N. Coleman, W. J. Blau, A. B. Dalton, E. Munoz, S. Collins, B. G.

Kim, J. Razal, M. Selvidge, G. Vieiro, R. H. Baughman, Appl. Phys.

Lett. 2003, 82, 1682.

[6] R. Haggenmueller, H. H. Gommans, A. G. Rinzler, J. E. Fischer, K. I.

Winey, Chem. Phys. Lett. 2000, 330, 219.

[7] J. C. Kearns, R. L. Shambaugh, J. Appl. Polym. Sci. 2002, 86, 2079.

[8] E. M. Moore, D. L. Ortiz, V. T. Marla, R. L. Shambaugh, B. P. Grady, J.

Appl. Polym. Sci. 2004, 93, 2926.

[9] J. K. W. Sandler, S. Pegel, M. Cadek, F. Gojny, M. van Es, J. Lohmar,

W. J. Blau, K. Schulte, A. H. Windle, M. S. P. Shaffer, Polymer 2004,

45, 2001.

[10] A. B. Dalton, S. Collins, E. Munoz, J. M. Razal, V. H. Ebron, J. P.

Ferraris, J. N. Coleman, B. G. Kim, R. H. Baughman, Nature 2003,

423, 703.

[11] A. B. Dalton, S. Collins, J. Razal, E. Munoz, V. H. Ebron, B. G. Kim, J.

N. Coleman, J. P. Ferraris, R. H. Baughman, J. Mater. Chem. 2004,

14, 1.

[12] P. Miaudet, S. Badaire, M. Maugey, A. Derre, V. Pichot, P. Launois,

P. Poulin, C. Zakri, Nano Lett. 2005, 5, 2212.

[13] J. M. Razal, J. N. Coleman, E. Munoz, B. Lund, Y. Gogotsi, H. Ye, S.

Collins, A. B. Dalton, R. H. Baughman, Adv. Funct. Mater. 2007, 17,

2918.

[14] B. Vigolo, A. Penicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet,

P. Bernier, P. Poulin, Science 2000, 290, 1331.

[15] B. Vigolo, P. Poulin, M. Lucas, P. Launois, P. Bernier, Appl. Phys.

Lett. 2002, 81, 1210.

[16] http://www2.dupont.com/Kevlar/en_US/index.html (accessed

November 2008).

[17] D. Tanner, J. A. Fitzgerald, B. R. Phillips, Angew. Chem. Int. Ed.

1989, 28, 649.

[18] H. G. Chae, S. Kumar, J. Appl. Polym. Sci. 2006, 100, 791.

[19] H. Blades, US Patent 3767756, 1973.[20] H. Blades, US Patent 3869429, 1975.[21] L. M. Ericson, H. Fan, H. Q. Peng, V. A. Davis, W. Zhou, J. Sulpizio, Y.

H. Wang, R. Booker, J. Vavro, C. Guthy, A. N. G. Parra-Vasquez, M. J.

Kim, S. Ramesh, R. K. Saini, C. Kittrell, G. Lavin, H. Schmidt, W. W.

Adams, W. E. Billups, M. Pasquali, W. F. Hwang, R. H. Hauge, J. E.

Fischer, R. E. Smalley, Science 2004, 305, 1447.

[22] S. Kumar, T. D. Dang, F. E. Arnold, A. R. Bhattacharyya, B. G. Min, X.

Zhang, R. A. Vaia, C. Park, W. W. Adams, R. H. Hauge, R. E. Smalley,

S. Ramesh, P. A. Willis, Macromolecules 2002, 35, 9039.

[23] C. J. Zhou, S. F. Wang, Y. Zhang, Q. X. Zhuang, Z. W. Han, Polymer

2008, 49, 2520.

[24] S. D. Bergin, V. Nicolosi, P. V. Streich, S. Giordani, Z. Sun, A. H.

Windle, P. Ryan, N. P. P. Niraj, Z.-T. Wang, L. Carpenter, W. J. Blau, J.

J. Boland, J. P. Hamilton, J. N. Coleman, Adv. Mater. 2008, 20,

1876.

[25] S. Giordani, S. D. Bergin, V. Nicolosi, S. Lebedkin, M. M. Kappes,

W. J. Blau, J. N. Coleman, J. Phys. Chem. B 2006, 110, 15708.

[26] W. B. Black, J. Preston, in Science and Technology, Vol. 2(Eds: H. F.

Mark, S. M. Atlas, E. Cernia), Interscience, New York 1968,

pp. 297–301.

[27] A. T. Fiory, S. Martin, L. F. Schneemeyer, R. M. Fleming, A. E. White,

J. V. Waszczak, Phys. Rev. B: Condens. Matter 1988, 38, 7129.

[28] N. Vahdat, V. D. Sullivan, J. Appl. Polym. Sci. 2001, 79, 1265.

[29] S. Badaire, P. Poulin, M. Maugey, C. Zakri, Langmuir 2004, 20,

10367.

[30] T. Kitagawa, K. Yabuki, R. J. Young, Polymer 2001, 42, 2101.

[31] Y. H. So, Prog. Polym. Sci. 2000, 25, 137.

[32] K. Koziol, J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, A.

Windle, Science 2007, 318, 1892.

H & Co. KGaA, Weinheim

Received: July 29, 2008Revised: November 25, 2008

www.small-journal.com 469