Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 339–343

Coiled carbon nanotubes growth and DSC studyin epoxy-based composites

Mei Lua,b,∗, Kin-Tak Laub, Ji-Chuan Xua, Hu-Lin Lia

a Department of Chemistry, Lanzhou University, Lanzhou 730000, Chinab Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

Abstract

Coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nanoparticles supportedon silica gel under reduced pressure and relatively low gas flow rates. The morphology of the coiled carbon nanotubes was examined bytransmission electron microscope (TEM), while the graphitization of coil tube, coil bend and coil node was analyzed by high-resolutiontransmission electron microscope (HRTEM). Compared with the straight single-walled and multi-walled nanotube/epoxy composites, thecoiled nanotube/epoxy composites showed a poor endothermic ability from differential scanning calorimetry (DSC) examination.©

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2004 Elsevier B.V. All rights reserved.

eywords:Coiled carbon nanotubes; Transmission electron microscopy; Differential scanning calorimetry

. Introduction

Micro-coiled carbon fibers (MCCF) have aroused muchnterest due to their unique functional, mechanical andlectrical properties for potential applications in micro-lectromechanical systems (MEMS) and bio-MEMS[1–3].ince the discovery of carbon nanotubes (CNTs) in 1991 by

ijima, CNTs have received considerable attention becausef their unique mechanical, chemical and physical properties,

ncluding high Young’s modulus, the ability to store hydrogennd so on[4,5]. Theoretically, CNTs with regular coils shouldave the combined advantage of CNTs and MCCF[6]. Theoiled carbon nanotubes (CCNTs) may exhibit peculiar me-hanical or chemical properties when they are incorporatednto a polymer matrix.

Among the published techniques for the fabrication ofNTs, catalytic chemical vapor deposition (CCVD), carriedut under atmospheric pressure and high gas flow rate, haseen found to be the only method that can reliably produce he-

ical nanotubes with fine regular coils[7–9]. However, whenhis process is scaled up for the industrial production of coiled

concerns due to the consumption of hydrocarbons, suacetylene. In addition, the yield of the coiled CNTs onbasis of required high gas flow rate for the deposition islow, due to the fact that most of the gases in the CCVDcess are discharged into the atmosphere without participin the chemical reaction.

The objectives of the work presented here are: (1)duction of CCNTs by CCVD acetylene on finely dividCo nanoparticles supported on silica at reduced preand lower gas flow rates; (2) incorporation of 0.5 wt.%single-walled carbon nanotubes (SWNTs), multi-walledbon nanotubes (MWNTs) and CCNTs into pure epoxmake three different nanocomposites; (3) consideringthere is no study on the thermal properties of nanocomites so far, comparisons on the thermal properties of diffeepoxy-based nanocomposites were made through differscanning calorimetry (DSC) examinations.

2. Experimental

NTs, this approach may cause environmental and safety

∗ Corresponding author. Fax: +86 931 891 2582.

The cobalt catalyst supported on silica gel H (5–40�m)was prepared by pH-controlled ion-adsorption precipitation.0.6 g cobalt acetate (Co(CH3COO)2·4H2O) was dissolved in

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927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.colsurfa.2004.10.073

340 M. Lu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 339–343

Fig. 1. CCNT with its projection (right) showing the coil diameter and coilpitch.

50 ml of distilled water. The pH of cobalt acetate solution wasset by liquid ammonia to be 7.5, 8.5 and 9.5 with pH-indicatorstrips checking. The color of the resulting solutions was pink,purple and blue at pH 7.5, 8.5 and 9.5, respectively. A slightprecipitation was observed at pH of 9.5. One gram of silicagel support was added to the cobalt solution and allowed tosonicate for 4 h. The prepared catalyst was then dried in theair overnight at 100◦C. After the drying process, the color ofthe catalyst was shades of light blue (pH 7.5), olive (pH 8.5)and dark grey (pH 9.5). The dried catalysts were ground tothe form of fine powder for the following deposition.

The catalysts were placed in a cylindrical quartz boat po-sitioned in the center of a quartz tube within a horizontaltubular electric furnace. The reactor was evacuated to a basepressure of 100 Pa with a mechanical pump. After the reactorwas heated to 720◦C at a rate of 15◦C/min, acetylene gas(C2H2) of 20 sccm was flowed into the reactor for 30 min.The final product was then purified as follows: cobalt parti-cles were removed by immersion into a 30% HNO3 solutionunder sonication for 30 min and the solution was allowedto settle for 4 h followed by rinsing with distilled water us-ing filtration; the silica gel support was separated via son-ication in an organic solvent mixture (n-heptane:acetone:2-propanol = 1:1:1) and then rinsing with distilled water withfiltration.

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Fig. 2. The longest CCNT with regular pitch found in our experiment.

cord. The observed regular coils have a remarkably constantcoil pitch of 67 nm and coil diameter of 40 nm.

Regular coils in the same sample have varying degreeof tube diameters, coil diameters and coil pitches.Fig. 3ashows CCNTs with tube diameters of∼35 nm but with muchdifferent coil morphologies. These coils have different coilpitches, such as slightly curved tubes, weakly coiled spiralsand spring-like forms. Other CCNTs that are wavy in ap-pearance, around 15 nm in diameter and braided together areshown inFig. 3b. Among the various coiled formations, agood portion is tightly zigzag-shaped nanotubes (Fig. 3c).The appearance of regular and bright circular spots on thenanotube are the coil nodes and are due to the much smallercoil pitch of this particular type of zigzag nanotubes. Anotherobserved coil shape is the loop-wired nanotubes as shownin Fig. 3d, which are formed with periodic curvature andknots during the growth of nanotubes. Unlike double-twistedmicro-coiled carbon fibers[12], CCNTs are generally of sin-gle coils and more importantly, hollow in the center.

3.2. HRTEM analysis of CCNTs

Graphitization of a short straight section in a regularCCNT shown inFig. 4a was observed by HRTEM.Fig. 4bshows that the straight segment in the regular CCNT actu-a g of0 hatu thec agni-fi theC peri-o tiono n-o heret thec dic-i age,t n be

The purified nanotubes were sonicated at low powelcohol and dropped onto a piece of holey carbon grid.onventional transmission electron microscope (TEM) asis and high-resolution TEM (HRTEM) were performedJEOL-2010 microscope at accelerate voltage of 200 kThe preparation process for different types of n

tube/epoxy composites can be referred to the previousications[10,11]. Samples of approximately 10 mg for eaf the composites for non-isothermal tests were heated50 to 250◦C at a scan rate of 10◦C/min, with a differentia

canning calorimeter (DSC) Perkin-Elmer Pyris 1 couith an intercooler.

. Results and discussion

.1. TEM analysis of CCNTs

The geometrical parameters of the CCNT are illustrn Fig. 1. The longest CCNT is shown inFig. 2, at 5.5�m,egularly coiled and shaped as a stretched miniature telep

lly consists of concentric graphitic sheets with a spacin.34 nm, contrasting this with the well-known MCCF tsually consist of amorphous carbon fibers. Althoughurvature appears to be continuous segments at low mcation, the HRTEM images show that the regularity inCNT on a micro-scale does not correspond to a perfectdic repetition of the atomic structures. Detailed examinan one of the periodic bend (Fig. 4c) shows part of the natube displays clear graphitic layers. However, places w

here are buckling to achieve the angle compatibility withoil periodicity are not perfect. Therefore, the true perioty must be determined at a much larger scale. In this imhere are other parts of the coil clearly resolved and ca

M. Lu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 339–343 341

Fig. 3. (a) Four coils with various pitch and diameter, (b) several wavy nanotubes twisted each other, (c) zigzag-shaped nanotube with nodes and (d) loop wireshaped nanotube.

attributed to the 3D shape of the helix and the limited depthof focus in the TEM (typically∼10 nm).

3.3. DSC analysis of different types of nanotube/epoxycomposites

The glass transition temperatures of the composites canbe determined based on the DSC results.Fig. 5shows DSCcurves for different composites and pure epoxy. With the DSCcurve of the pure epoxy as reference, the most noticeable fea-ture in the curves for the three different composites is the dis-appearance of the exothermic peak at 44.33◦C. This indicatesthat nanotubes would prevent the epoxy from releasing heatenergy at this temperature. The endothermic peak located at∼50◦C in each curve represents the glass transition temper-ature range for the corresponding composites. The total areaunder the heat flow peak, based on the extrapolated baselineat the end of the transition, is used to calculate the total heatof transition process.

The initial temperature of the transition (Ti ), the maxi-mum endothermal peak temperature (Tp), the end temper-ature of the transition (Te), the glass transition tempera-ture (Tg) and the heat of the transition (the transition en-thalpy, �H) for each type of composites are reported inT nt xyn oft with

SWNTs and MWNTs, while there is an obvious decreasein Tg in the CCNT/epoxy composites. Meanwhile, the�Hvalue of the SWNT/epoxy composites was higher than thatof the pure epoxy, while the�H values of the MWNT/epoxyand CCNT/epoxy composites each slightly and significantlylower, respectively, than that of the pure epoxy. It is in-ferred that during the glass transition process, SWNTs canact as a heat sink to accelerate the heat absorption of theepoxy, while CCNTs can act as heat-shielding filler andprevents the epoxy from exchanging energy with outsidesystem.

These observed changes reveal that the tubes’ surface con-figuration play an important role in the glass transition be-havior of the epoxy. It has been demonstrated that the incor-poration of carbon fillers can affect the structure of the curedepoxy by restricting the nucleophile–electrophile interactionduring the cure reaction by a steric hindrance effect. Ac-cordingly, nanotubes with different shapes would have differ-ent steric hindrance effects on the cure reaction between the

Table 1Initial temperature, peak temperature, end temperature, glass transition tem-perature and transition enthalpy of different types of nanotube/epoxy com-posites and pure epoxy

Sample Ti (◦C) Tp (◦C) Te (◦C) Tg (◦C) �H (J g−1)

SMCP

able 1. It is obvious thatTi , Tp, Tg and �H decrease ihe order of SWNT/epoxy, MWNT/epoxy and CCNT/epoanocomposites. Compared to the pure epoxy, a shiftTg

o higher temperatures is observed in the composites

WNT/epoxy 52.727 57.333 61.949 57.338 7.852WNT/epoxy 50.218 55.666 60.338 55.278 6.752CNT/epoxy 46.595 50.833 55.291 50.943 0.745ure epoxy 49.979 54.833 58.952 54.466 7.282

342 M. Lu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 339–343

Fig. 4. (a) TEM image of a CCNT, (b) HRTEM image recorded from a straight section in the CCNT and (c) HRTEM image recorded from a bend in the CCNT.

epoxy and hardener. The particular helical shape of CCNTsshould have a more steric hindrance effect than the straightnanotubes, such as SWNTs and MWNTs. As a result, the curereaction of the epoxy would be influenced more by CCNTsthan by the SWNTs and MWNTs.

Fig. 5. DSC curves at a heating rate of 10◦C/min for different types ofnanotube/epoxy nanocomposites and pure epoxy.

Further considering that some functional groups can beintroduced on the tube wall during nanotube purificationby acid treatment, these polar groups can act as a curingagent because of their affinity to the epoxide group. As aresult, the curing reaction should be due to two kinds ofhardeners, amine hardener and polar groups. Epoxy curedby these two hardeners should possess different propertiesfrom that of the pure epoxy cured by a single amine hard-ener. As demonstrated from above, CCNTs have many morepentagon–heptagon pair defects and nodes on the tube side-wall than straight nanotubes (SWNTs and MWNTs); thusCCNTs have more polar groups after acid oxidation treat-ment than straight nanotubes. The polar interaction effect ofthe CCNTs is the most prominent feature among the threecomposites. Because the CCNT/epoxy composites exhib-ited the lowest�H, it can be concluded that if more po-lar groups in the nanotubes contribute to the cure reaction,the �H value of the resulting composites would be evenlower. It should be pointed out that for SWNTs, the po-lar interaction effect is no more important a factor than itsheat sink effect. This is why SWNT/epoxy has the high-est �H value among the three composites and the pureepoxy.

M. Lu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 339–343 343

4. Conclusions

By CCVD method, CCNTs have been prepared on silica-supported Co nanoparticles under reduced pressure and atlower gas flow rates. TEM results show that the CCNTs ex-hibit different morphologies, while HRTEM results indicatea graphitic lattice and polygonization characteristic involvedin the CCNTs. It was also found that the addition of fewpercentages of SWNTs could enhance the heat absorbabil-ity of the epoxy-based composite, while the incorporation ofCCNTs in epoxy-based composites has a great potential inthe development of heat shielding polymer-based compositesstructures. Work along this line is in progress.

Acknowledgments

This work is supported by the National Natural ScienceFoundation of China (G-T 60171004) and the Hong KongPolytechnic University Grants (G-T 684, G-T 688, G-T 861and GT-936).

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