rubber composites

CARBON

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

Available at www.sciencedirect.com

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

Kinetics of filler wetting and dispersion in carbon nanotube/rubber composites H.H. Le a,*, X.T. Hoang b, A. Das c, U. Gohs c, K.-W. Stoeckelhuber c, R. Boldt c, G. Heinrich c,d, R. Adhikari e,f, H.-J. Radusch a a

Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, D-06099 Halle, Germany University of Technology, National University, HCM City, Vietnam c Leibniz-Institut fu¨r Polymerforschung Dresden e.V. (IPF), D-01069 Dresden, Germany d Institut fu¨r Werkstoffwissenschaft, Technische Universita¨t Dresden, D-01069 Dresden, Germany e Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, Nepal f Nepal Polymer Institute (NPI), P.O. Box 24411, Kathmandu, Nepal b

A R T I C L E I N F O

A B S T R A C T

Article history:

The effects of the surface modification of multi-walled carbon nanotubes (MWCNTs) by an

Received 2 February 2012

ionic liquid, 1-butyl 3-methyl imidazolium bis(trifluoromethyl-sulphonyl)imide (BMI) on

Accepted 20 May 2012

the kinetics of filler wetting and dispersion as well as resulting electrical conductivity of

Available online 27 May 2012

polychloroprene (CR) composites were studied. Two different MWCNTs were used, Baytubes and Nanocyl, which differ in their structure, purity and compatibility to CR and BMI. The results showed that BMI can significantly improve the macrodispersion of Baytubes, and increases the electrical conductivity of the uncured BMI–Baytube/CR composites up to five orders of magnitude. In contrast, the use of BMI slows the dispersion process and the development of conductivity of BMI–Nanocyl/CR composites. Our wetting concept was further developed for the quantification of the bound polymer on the CNT surface. We found that the bonded BMI on the CNT surface is replaced by the CR molecules during mixing as a result of the concentration compensation effect. The de- and re-agglomeration processes of CNTs taking place during the subsequent curing process can increase or decrease the electrical conductivity significantly. The extent of the conductivity changes is strongly determined by the composition of the bound polymer and the curing technique used.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) as new nanofiller in rubber composites have got more and more attention in different areas ranging from rubber hoses, tire components, sensing devices to electrical shielding and electrical heating. A significant progress in the area of the preparation and utilization of nanotube/polymer composite materials in recent years with particular attention to their mechanical and electrical properties can be observed in a number of articles [1–9]. However,

the effect of unmodified CNTs on the reinforcement and dynamic mechanical performance as well as electrical properties of rubber composites is not remarkable and much lower than expectation. The main reason is related to the fact that CNTs are intrinsically bundled and heavily entangled due to van der Waals forces of attraction between adjacent tubes. The fine dispersion of CNTs in rubber has been still the most challenging task for their practical application. To overcome these problems, beside development of new preparation technologies [4,7–9] there has been much progress in the

* Corresponding author: Fax: +49 3461463891. E-mail address: [email protected] (H.H. Le). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.039

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functionalization of CNT surface with various organic molecules for better compatibility and dispersion of CNTs in rubber. For example, the processability and mechanical performance of rubber composites could be improved by introducing carboxylic acid groups [10,11] or multifunctional silane [12–14] onto the CNT surface. Recently, functionalization of CNTs with ionic liquids, a kind of molten salt with nearly zero vapor pressure and high thermal stability, is an interesting topic, because ionic liquids could provide a facile and promising method to control the surface properties of CNTs by means of cation–p interaction [5–17]. Carbon nanotubes/ionic liquid mixture was mixed into silicone elastomer by Sekitani et al. [18] and hydrogenated nitrile rubber (HNBR) by Likozar and Major [19] to produce conductive rubberlike stretchable composites. Das et al. [20,21] used a series of ionic liquids as surfactant for blends of styrene–butadiene rubber (SBR) and polybutadiene rubber (BR) filled with CNTs in order to determine the coupling activity of ionic liquids between diene elastomers and CNTs. Recently, Subramaniam et al. [22,23] described a new simple route to disperse CNTs in polychloroprene (CR) using an ionic liquid, 1-butyl 3-methyl imidazolium bis(trifluoromethyl-sulphonyl)imide (BMI) and found that the usage of BMI and a low concentration (5 phr) of CNTs in CR exhibited an electrical conductivity of 0.1 S/ cm with a stretchability of >500%. In this regard, the present work focuses on the characterization of the kinetics of CNT dispersion in CR nanocomposites and its correlation to the electrical conductivity by means of the method of the online measured electrical conductance. The effectiveness of BMI as surfactant for two different CNTs, Baytubes and Nanocyl, at different conditions of composite fabrication, i.e. compounding and curing processes will be characterized and discussed by taking into consideration the selective wetting behavior of CNTs by BMI and rubber. The success of the work may encourage the application of the method of the online conductance and the wetting concept for the characterization and understanding of the mixing mechanism of CNTs in rubber matrix in the laboratory scale on the one hand, and for monitoring the production of CNT nanocomposites in the industrial scale on the other.

2.

Experimental

2.1.

Materials and composite preparation

Table 1 – Structural parameters of Baytubes and Nanocyl.

Baytubes C150 HP 15 Nanocyl NC7000 9.5

1.0 1.5

140–180 250–300

99 90

3 wt.% according to the thermogravimetric analysis made in the present work. Some important data of both used fillers provided by the manufacturers are given in Table 1. For convenient admixing CNTs into the mixing chamber both CNTs were softly ground with BMI in different ratios by weight of CNTs to BMI, till a black paste BMI/CNT was obtained. The composites were prepared in an internal mixer (Rheocord 300p, ThermoHaake) by keeping the following mixing conditions: initial chamber temperature TA of 25 C, rotor speed of 70 rpm, fill factor of 0.72. The amount of the tubes used in the study was varied in parts per hundred rubber (phr). The black paste BMI/CNT was admixed into the chamber at 3 min mixing time. The first curing package consists of 2.5 phr ZnO, 1 phr stearic acid, 1.4 phr sulfur, and 1 phr CBS (n-cyclohexyl-2-benzothiazole-sulfenamide). The designation of composites containing different CNT and BMI loadings is given in Table 2. The mixing time was varied by taking into account the electrical conductance–time characteristic as described in our previous work [26]. In order to characterize the effect of the preparation technique, two composites were prepared according to Table 3. Baytubes and BMI were separately admixed into the mixing chamber. For these samples 1.5 phr peroxide Luperox 101 (Atofina Chemicals) was used as the second curing agent.

2.2.

Characterization

2.2.1.

Optical microscopy

Optical microscopy was used to characterize the CNT macrodispersion. This method was described for CB filled compounds by Leigh-Dugmore [25] and modified by us [26]. The macrodispersion D is calculated by the ratio of the surface of non-dispersed agglomerates to that of the image. A value D = 0% indicates an image without any agglomerate larger than 6 lm.

2.2.2. Polychloroprene (CR) Baypren 611 (Lanxess, Germany) with Mooney viscosity MU ((ML 1 + 4) 100 C) of 43 ± 6 was used as rubber matrix. Ionic liquid BMI, which is basically made of an asymmetric heterocyclic cation 1-butyl 3-methyl imidazolium (BMI+) and an anion bis(trifluoromethyl-sulphonyl)imide (BMI) (Sigma–Aldrich, Germany) was used as surfactant. According to the chemical structure of BMI a mass ratio BMI+/BMI of 139/280 was calculated. Multi-walled carbon nanotubes Baytubes C150HP (Bayer MaterialScience, Germany) and NanocylTM NC7000 (Nanocyl S.A., Belgium) were used as fillers. Nanocyl nanotubes were found to be longer than Baytubes C150HP [24]. In addition, Nanocyl possesses a broad length distribution with several nanotubes up to 10 lm. The amorphous carbon content of both CNTs is about

CB Average Average Surface diameter length area BET purity (%) nm [24] (lm) (m2/g)

Filler

Transmission electron microscopy (TEM)

Ultra-thin sections with approximately 35 nm thickness cut from compression-molded plates with a diamond knife (35 cut angle, DIATOME, Switzerland) at 140 C on a cryo-micro-

Table 2 – Designation of CNT/CR and BMI-Baytube/CR as well as BMI-Nanocyl/CR composites. Composite Polychloroprene Baytubes Nanocyl BMI (I) (C) (phr) (Ba) (phr) (Na) (phr) (phr) CBa1–20 CBa5I10 CNa1–5 CNa5I10

100 100 100 100

1–20 5

10 1–5 5

10

CARBON

Table 3 – Designation of composites containing Baytubes and BMI separately admixed. Composite

Polychloroprene (C) (phr)

Baytubes (Ba) (phr)

BMI (I) (phr)

CBa5-I5 CBa5-I10

100 100

5 5

5 10

2.2.5.

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) investigations were performed on an Ultra plus microscope (Zeiss, field emission cathode) operated at 2 kV accelerating voltage. To get a plain surface the samples were prepared by cutting a cryo-section using an ultra-microtome (Leica Ultracut UC7) with a diamond knife at 140 C. The cryo-sections were examined without any additional coating to avoid masks over the nanotubes and to allow a charge contrast imaging between the conductive CNT network and the rubber matrix.

2.2.4.

Measuring equipment for the online conductance

A conductivity sensor system was installed in the chamber of the internal mixer to measure the electrical signal of the conductive mixtures between the sensor and the chamber wall. The construction and position of the conductance sensors has been described in our previous works [27–29]. As an example, Nanocyl-filled CR composites CNa5 were prepared for different mixing times of 7, 9, 12, 15, 22 and 32 min. The online conductance was recorded during the mixing process and is presented in Fig. 1 as function of mixing time. The curves show a typical shape quite similar to that of CB-filled rubbers [27,29] or CNT-filled SBR [30]. The onset time tonset of the conductance is observed at about 8 min and the tGmax at 20 min. The reproducibility of the electrical signal is very good.

1 0

-1

10

-4

10

15m

CNa5-7m

-5

tonset

12m

-3

9m

-2

10

Nanocyl

10

32m

22m

10

rubber

on

Online conductance G (mS)

on Gmax

10

10

-6

10

-7

10

1

Extraction experiment

For extraction experiment 0.3 g of each raw mixture obtained directly from the mixing process was stored in 300 ml of an 80/20 toluene/acetone mixture for 4 days at room temperature. The presence of acetone significantly weakens the strong interaction between the cation and anion of the ionic liquid, and initiates ion pair dissociation [31,32]. Once the ions are released, the anion is rapidly saturated with acetone and dissolved, while a part of cations remains bonded to the CNT surface through cation–p interaction [15–17]. After the soluble part was entirely extracted from the raw mixture, the CNT– polymer gel was taken out and dried up to a constant mass mG. The insoluble polymer part is described by the so-called rubber layer L, which can be calculated according to Eq. (1) [33,34]. L¼

mG  mcomp  cCNT mG

ð1Þ

mcomp is the mass of the composite before extraction experiment, cCNT is the mass fraction of the filler in the composite. mG is the sum of the insoluble part of BMI+ mBMIþ and G CR mCR G as well as the mass of the filler: þ mCR mG ¼ mBMIþ G G þ mcomp  cCNT

ð2Þ

The collected solution was also dried up to receive the extracted part mE, which consists of the soluble parts of BMI mEBMI+, mEBMI and CR mECR:  þ mBMI þ mCR mE ¼ mBMIþ E E E ¼ mcomp  mG

10

Mixing time (min) Fig. 1 – Online electrical conductance measured directly during mixing process of CNT and CR in dependence on mixing time.

ð3Þ

According to [30] the rubber layer L is the sum of two conþ tributions, LBMI and LCR: L ¼ LBMIþ þ LCR

ð4Þ

and their ratio can be calculated as follows: LBMIþ mBMIþ ¼ GCR mG LCR

2

10

Measurement of the offline conductivity

Measurement of electrical conductivity of uncured and cured samples was carried out at room temperature by means of a multimeter 2750 (Keithley). The shape of the conductive test specimens was a rectangular strip, whose ends were coated by silver paste in order to receive a good contact with the electrodes.

2.2.6. tome were used for Transmission electron microscopy (TEM) analysis. The slices were collected on a copper grid with a carbon-hole-foil. The specimens were investigated on a Zeiss Libra 200MC (Zeiss, field emission cathode, point resolution 0.2 nm) with an accelerating voltage of 200 kV.

2.2.3.

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5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

2.2.7. Fourier transformed analysis of the extracted part

ð5Þ

infrared (FTIR)

spectroscopy

According to the wetting concept developed in our previous work for silica filled rubber blends [35], it is possible to quantify the composition of the rubber layer bonded to the silica surface by means of the FTIR analysis of the silica-rubber gel. However, in the present work, due to the total absorption of infrared beams by CNTs existing in gel the analysis of the gel by FTIR was not possible. As an alternative, we indirectly characterized the gel composition by investigating the extracted part by use of a FTIR spectrometer S2000 (Perkin–Elmer) equipped with a diamond single Golden Gate ATR cell (Specac). The FTIR spectrum of the composite CNa5I10 is shown in Fig. 2 as an example. The peaks at 1659 cm1, 740 cm1 and

4546

CARBON

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

1192 cm1 are assigned to the C=C stretching mode of CR [36], the vibration of C–H of cyclic BMI+ and –SO2 of BMI [37], respectively. The ratios of peak areas ABMI+/ACR and ABMI+/ ABMI were calculated from the spectra of the extracted part. The ratio mEBMI+/mECR can be determined from the ratio ABMI+/ ACR according to Eq. (6). mBMIþ 1 ABMIþ E ¼ CR f mCR BMIþ=CR A E

ð6Þ

fBMI+/CR is a factor determined from the calibration curve, which describes the correlation between ABMI+/ACR and the given BMI+/CR ratio. The procedure for generation of the calibration curve was described in our previous work [35]. In the present work a value of 8.2 was determined for fBMI+/CR from the calibration curve. Using Eqs. (3) and (6) the value and mCR of mBMIþ E E can be calculated. The sum of BMI mass in gel and extracted part is calculated from mcomp and the mass fraction of BMI in the composite cBMI as follows: mBMIþ þ mBMIþ þ mBMI ¼ cBMI  mcomp G E E

ð7Þ

can be calculated by taking into consideration the mass mBMI E ratio BMI+/BMI and the total BMI used cBMIÆmcomp. The sum of CR mass in gel and extracted part is similarly determined from mcomp and the mass fraction of CR in the composite cCR as follows: þ mCRþ ¼ cCR  mcomp mCRþ G E

ð8Þ

mBMIþ , E

Setting the value of mBMI and mCR E E into Eqs. (7) and and mCR (8) the mass of BMI+ and CR in gel mBMIþ G G , respectively can be determined. and mCR Setting the value of mBMIþ G G into Eq. (5) and combinþ ing with Eq. (4) we can calculate LBMI and LCR.

2.2.8. Scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) Scanning electron microscopy (SEM) (JSM 6300, Fa. JEOL)equipped with Energy dispersive X-ray analysis (EDX) (Voyager 1100, Fa. Noran Instruments) was used for characterization of the CNT-polymer gel of CBa5I10 and CNa5I10 in order to characterize the presence of BMI in the bound layer of the composites.

0.30

BMI -1 1192 cm SO2

+ BMI

Absorbance

0.25

740 cm C-H

-1

CR 1659 cm C=C

0.20

-1

0.15

0.10

800

1000

1200

1600

1700

Wave length (cm-1) Fig. 2 – FTIR spectrum of the extracted part of CNa5I10.

2.2.9. Curing with peroxide, sulfur system and high energy electrons Samples were cured in a compression mold under 100 bar for t90, at 160 C for sulfur-curing and at 190 C for peroxide-curing. In order to characterize the effect of cross-linking by high energy electrons on the electrical conductivity the sample CBa5I10 was treated with high energy electrons using an ELV-2 electron accelerator (Budker Institute of Nuclear Physics, Novosibirsk, Russia). The samples were placed on the conveyor system of irradiation facility and passed under the electron beam exit window with well adjusted velocity in order to apply the desired dose to the samples. The sample was irradiated with a dose of 50 kGy at room temperature in nitrogen atmosphere. The energy of the electrons and the beam current were 1.0 MeV and 4 mA, respectively.

2.2.10. Swelling experiments of cured composites Swelling experiments were performed with cured samples by equilibrating them in toluene at room temperature for 48 h. The swelling degree Q was calculated using Eq. (9): Q¼

Wsw  Wi  100% Wdr

ð9Þ

Wi is the weight of the rubber sample before immersion into the solvent, Wsw and Wdr are the weights of the sample in the swollen state and after dried in an oven at 80 C for 2 h from its swollen state, respectively.

2.2.11. Thermogravimetric analysis (TGA) Thermogravimetric analysis of the neat and coated CNTs was carried out by a thermo-balance (Mettler Toledo) in the temperature range between 30 C and 900 C in air with a heating rate of 20 K/min. The mass change in the temperature range between 310 C and 430 C as well as 500 C and 550 C is attributed to the degradation/oxidation of BMI and amorphous carbon, respectively [38].

2.2.12. Determination of surface energies Wetting experiments (modified Wilhelmy method) were performed, using the dynamic contact angle meter and tensiometer DCAT 21, DataPhysics Instruments GmbH (Filderstadt, Germany). For the Wilhelmy measurements, the CNT particles were put in a shallow plate. In the filler powder a 2 · 1 cm2 piece of a double-face adhesive tape (TESA 55733, Beiersdorf, Hamburg, Germany), was immersed and gently moved, until the tape was uniformly coated by filler particles. The pellets of the granulated Baytubes were pulverized finely in a mortar, before they were attached at the adhesive tape. Surplus particles, which did not stick to the adhesive tape, were blown away by a stream of nitrogen. The CNT particle covered tape was used for Wilhelmy contact angle measurements without further modification. Sessile drop contact angle measurements on a sheet of uncured CR were conducted with the automatic contact angle meter OCA 40 Micro, DataPhysics Instruments GmbH (Filderstadt, Germany). The surface energies were calculated from the results of these wetting experiments. For this purpose a set of test liquids with different surface tension (and polarity) was used: water (Millipore Milli-Q-Quality), formamide (Merck, Darmstadt, Germany), ethylenglycol (Fisher Scientifiy,

10

20 phr CNT

1

15 phr

0.01

Baytubes

0.1 CR

on

Gon max

10 phr

1E-3 1E-4 5 phr 1E-5 1E-6 1

10

100

1000

Mixing time (min)

(b)

100 10

Gon max

1

5 phr

0.1 0.01 1E-3

Nanocyl

The online conductance curves of CR composites filled with different loadings of unmodified Baytubes CBa5-20 are presented in Fig. 3a. They show a typical conductance-time characteristics with tonset and tGmax. At tonset and tGmax the online conductance starts to rise and reaches the maximum value, respectively. According to our previous works [27,28] the macrodispersion of filler and the online conductance correlate closely to each other. The largest change of the size of filler agglomerates, i.e. the dispersion of filler agglomerate into smaller aggregates or even individual tubes, is determined in the period between tonset and tGmax. Thus, tonset and tGmax have been often used as a measure for characterization of the filler dispersion kinetics. Upon tGmax the online conductance decreases slightly that is related to the better distribution of small aggregates throughout the matrix as discussed previously [27]. For the composite CBa5 the tonset was not observed even till a mixing time of up to 200 min. With increasing loading of Baytubes the tonset and tGmax shift to shorter times due to higher shear stress, which results from the increasing viscosity and accelerates the filler dispersion. A linear extrapolation by connecting Gon max of the three curves as shown in Fig. 3a gives a theoretical tGmax of about 1000 min for CBa5. The online conductance of CR filled with different loading of Nanocyl is presented in Fig. 3b. For the composite containing 5 phr Nanocyl CNa5 a tGmax of 20 min was observed that is much shorter compared to that of CBa5. In contrast, the tGmax of the composites CNa2-5 is nearly independent on the filler loading. The value of Gon max of CNa5 is similar to that of CBa15. According to the online conductance measurement it is easy to recognize that Nanocyl is dispersed much faster than Baytubes and imparts higher values of conductivity of the composites. Mu¨ller et al. [40] revealed that a lower mixing energy is needed for dispersion of Nanocyl in PP composite than for Baytubes. By discussion of the different dispersion behavior of both fillers they took into consideration the more compact structure of Baytubes compared to the more loosely packed agglomerates of Nanocyl. Moreover, in our opinion the higher purity of Baytubes causing better filler-filler interaction is surely essential for the resistance of agglomerates against dispersing. The correlation between the offline conductivity and filler loading of CR composites filled with Baytubes and Nanocyl is presented in Fig. 4. The offline conductivity increases exponentially with increasing filler loading. For both series it is obvious that the percolation threshold shifts to lower values with increasing mixing time. Because of the strong conductivity-mixing time dependence, the values determined at tGmax should be chosen for

100

CR

3.1. Dispersion and electrical conductivity of rubber composites filled with unmodified CNTs

Online conductance G (mS)

Results and discussion

(a)

on

Loughborough, UK), dodecane (Merck Schuchardt, Hohenbrunn, Germany), n-hexadecane (Merck, Darmstadt, Germany) and ethanol (Uvasol, Merck, Darmstadt, Germany). Surface energy calculations were performed by fitting the Fowkes equation [39].

3.

4547

5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

Online conductance G (mS)

CARBON

3 phr

1E-4 2 phr

1E-5 1E-6 1

10

100

1000

Mixing time (min) Fig. 3 – Online conductance of CR filled with different loading of Baytubes (a) and Nanocyl (b) in dependence on mixing time.

a reasonable comparison of conductivity properties between different systems. A percolation threshold value of 8 phr for Baytube/CR and 3 phr for Nanocyl/CR composites, respectively, was determined at tGmax.

3.2. Effect of BMI on CNT dispersion and electrical conductivity In Fig. 5a the online conductance of the Baytube composites without and with BMI CBa5 and CBa5I10, respectively, is presented in dependence on mixing time. With addition of BMI the online conductance of CBa5I10 increases faster and reaches a tGmax at about 55 min. It is very short compared to the extrapolated 1000 min of the unmodified CBa5. The macrodispersion of filler in the CR matrix is studied by optical microscopy images. The mixing time and corresponding macrodispersion D of each sample are given on the image. In the images of CBa5 shown in Fig. 6a–d very large agglomerates of Baytubes are still visible even at a very long mixing time. A macrodispersion D of 22% is determined at 220 min mixing time. The addition of BMI leads to a significant improvement of dispersion as seen in the images shown in Fig. 6e–h. The largest change of the size of CNT agglomerates is clearly determined in the range between tonset = 20 min and tGmax = 55 min. The macordispersion D reduces from 37% to

4548

CARBON

Baytube/CR

0.1

Nanocyl/CR

m

t

m in

G

=

=

10 0

tim e

tim e

g M ixi n

M ixi ng

1E-7

10 m in

time

1E-5

Mix ing time =

=t

G ma x

ax

1E-3

Mixin g

off

Offline conductivity G (S/cm)

10

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

= time ing Mix

1E-9

in 20 m

1E-11

0

5

10

15

20

25

CNT loading (phr) Fig. 4 – Offline conductivity of the uncured composites CBa5– 20 and CNa2–5 in dependence on the filler loading and mixing time.

Baytubes

1 0.1 0.01

Gon max CBa5I10

1E-3

Gon max

CR

on

Online conductance G (mS)

(a)

CBa5

1E-4 1E-5 1

10

100

1000

Mixing time (min) 0.1 Gon max

0.01 1E-3 1E-4 1E-5

CBa5I10

1E-6

1E-8 1E-9 1E-10

Baytubes

1E-7

CR

off

Offline conductivity G (S/cm)

(b)

CBa5

1E-11 10

100

1000

Mixing time (min) Fig. 5 – Online conductance (a) and offline conductivity (b) of the uncured composites CBa5 and CBa5I10 as function of mixing time.

20% after 50 min and 14% after 75 min mixing time. Upon tGmax only some small agglomerates are observed. The better macrodispersion of Baytubes by addition of BMI is attributed to the physical cation-p interaction between BMI+ and the tubes and/ or the perturbation of p–p stacking of multi-walls

of the tubes as discussed in literature [15–17,41] and evidently proved by Raman investigation by Subramaniam et al. [22]. Fig. 5b shows that the offline conductivity of the uncured composites CBa5 and CBa5I10 changes correspondently to the online conductance. The one to one relationship between the online and offline conductive values cannot be expected, because the online conductance is measured through an undefined volume during the mixing at changing temperature, while the offline conductivity has to be measured for definite sample geometry by keeping constant the temperature and pressure. The CNT network in the sample is in a steady state during the offline measurement. Thus the online conductance can be used for determine the trend of the off4 S/cm is received for line conductivity. The Goff max of 7.3 · 10 CBa5I10, while CBa5 shows a conductivity of only 109 S/cm all over the mixing time. In Fig. 7a the online conductance of Nanocyl composites without and with BMI CNa5 and CNa5I10, respectively, is shown with mixing time. At mixing times up to tonset the online conductance of CNa5I10 is slightly higher than that of CNa5 that may be related to the contribution of the ionic conductivity of BMI. In the mixing period up to tGmax the online conductance of the unmodified composite CNa5 increases earlier and reaches tGmax at 20 min, while tGmax of CNa5I10 is determined at 30 min. In contrast to CBa5I10, an addition of BMI in CNa5I10 decelerates the dispersion process of Nanocyl. The optical microscopic images shown in Fig. 8 reveal that the dispersion of CNa5 is faster than that of CNa5I10 corresponding to the development of the online conductance. After 30 min mixing time the macrodispersion of Nanocyl in CNa5 is 10% and in CNa5I10 17%. That may be attributed to the fact that the as-produced Nanocyl inherently is a type of CNT, which can be dispersed easily as discussed in [40], and thus, the dispersion of Nanocyl was not accelerated effectively by wetting it with BMI. Furthermore, the shear stress determined by the torque during the mixing process reduces from 20 Nm to 15 Nm by adding the low-viscous BMI leading clearly to the slower dispersion process of Nanocyl in CR. The offline conductivity of the uncured samples of CNa5 and CNa5I10 is presented in Fig. 7b as a function of mixing time. It is obvious that the development of the offline conductivity of both series corresponds well with that of the online conductance and optical microscopic observations, i.e. the offline conductivity of CNa5I10 increases more slowly in presence of BMI. The Goff max of CNa5 and CNa5I10 is in the same order of magnitude, i.e. 103 S/cm, indicating the same dispersion degree of both composites at tGmax regardless of BMI addition. A closer look at the chart of the offline conductivity of CBa5I10 and CNa5I10 reveals that the BMI addition makes the offline conductivity strongly dependent on the mixing time and input of mixing energy. That may cause difficulties for process control of the composite preparation. As seen in Figs. 5b and 7b the offline conductivity decays strongly after reaching tGmax. That is attributed to the better separation/distribution of the filler aggregates throughout the matrix as discussed already in the case of CB filled composites [27,28]. In CNT filled composites the shortening of the tubes during the mixing process becomes essential and should be taken into consideration. The comparison between the pristine and the processed CNTs made by Fu et al. [42],

CARBON

5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

4549

Fig. 6 – Optical microscopic observations of CBa5 (a–d), CBa5I10 (e–h) in dependence on mixing time (image dimension 300 lm · 400 lm).

Chen et al. [43], and Lin et al. [44] demonstrated a strong shortening of CNTs up to 10% of the initial length. According

on

Gmax

on

Gmax

Nanocyl

1

on

0.1

1E-3

CNa5I10

CNa5

0.01 CR

Online conductance G (mS)

(a)

to the work of Krause et al. [24] with the same CNTs as used in the present work, Nanocyl with initially longer nanotubes underwent a more significant shortening of the tube length to about 30% (related to x50-value). The comparison of the tube length distribution of Baytubes before and after melt processing indicated a shortening to about 50% of their initial length (related to x50-value). However, the online conductance values measured upon tGmax are slightly influenced by the shortening of CNT as shown in Figs. 5a and 7a, because the shortened tubes can still touch each other during the mixing and form a non-stationary filler network. After mixing, the shortened tubes exist in a steady state, and they are separated from each other that affects the offline conductivity negatively.

1E-4

3.3.

Selective wetting behavior of CNTs by BMI and rubber

1E-5 10

Mixing time (min)

(b)

1

off

Gmax

0.01

off

Gmax

1E-3

CNa5

1E-4 1E-5

1E-7

CNT

1E-6 CR

off

Offline conductivity G (S/cm)

0.1

CNa5I10

1E-8 10

Mixing time (min) Fig. 7 – Online conductance (a) and offline conductivity (b) of the uncured composites CNa5 and CNa5I10 as function of mixing time.

Fig. 9 represents the extraction experiment and the rubber layer L as a measure for the bound polymer of different composites in dependence on mixing time. In general, the rubber layer L increases with mixing time and reaches a plateau value after a certain time. According to the discussion made by Manas-Zloczower [45,46] and us [34] on the infiltration and dispersion processes, polymer first can infiltrate the outer layer of filler agglomerate and wets its surface. This layer is then eroded and a new filler surface is created. The new surface is progressively wetted by polymer. Therefore wetting and dispersion take place simultaneously. The wetting process of Baytubes in CBa5 was not measurable, because the gel was not formed. In this sample due to the bad filler-rubber interaction and bad dispersion, Baytubes did not form a network, which acts as a skeleton to hold polymer in gel. Thus the wetted agglomerates are separate and swim in the solvent making the flash black as seen in Fig. 9a. In contrast, a clear solution observed for CBa5I10, CNa5 and CNa5I10 demonstrating a formation of the filler-polymer gel after extraction experiment. The rubber layer L of CBa5I10 shown in Fig. 9b increases slowly because of the slow dispersion of Baytubes. The pla-

4550

CARBON

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

Fig. 8 – Optical microscopic images of CNa5 (a–d), CNa5I10 (e–h) in dependence on mixing time (image dimension 300 lm · 400 lm).

teau value LP of CBa5I10 of 0.62 is determined at about 50 min indicating the end of the wetting process at this time, thus no more free tube surface is available for further wetting. Compared to CBa5I10 the plateau value LP of the Nanocyl composites is obtained at much shorter mixing time thanks to the faster dispersion process of Nanocyl. Compared to CNa5, the wetting process of Nanocyl in CNa5I10 becomes more slowly due to the slower filler dispersion process as discussed above. Moreover, Nanocyl has a surface area of 250– 300 m2/g compared to 140–180 m2/g of Baytubes, thus at the same dispersion degree more surface is available for rubber wetting in case of Nanocyl. As a result, the plateau value LP

of CNa5 and CNa5I10 of 0.72 and 0.68, respectively, is higher than that of CBa5I10. A deeper insight into the selective wetting behavior of nanotubes by rubber and BMI can be obtained by taking into consideration the affinity between the components of the composites. On the basis of the Z-model proposed in our previous works [47,48] the CNT surface fraction wetted by CR and BMI in composites at a thermodynamic equilibrium state can be predicted using Eqs. (10)–(13).  2  pffiffiffiffiffiffiffiffiffiffiffiffiffi cBMI þ cF  2 cBMI cF 2 SCR c F ¼ nCR=BMI BMI-F ¼ nCR=BMI ð10Þ pffiffiffiffiffiffiffiffiffiffiffi BMI cCR þ cF  2 cCR cF cCR-F SF BMI SCR ¼1 F þ SF

(a)

SBMI ¼ F

1 nCR=BMI x þ 1

with  pffiffiffiffiffiffiffiffiffiffiffiffiffi cBMI þ cF  2 cBMI cF 2 x¼ pffiffiffiffiffiffiffiffiffiffiffi cCR þ cF  2 cCR cF

(b)

0.9

CNa5

0.8

CNa I

5 10

0.7

Rubber layer L

LP

CBa I

0.6

5 10

0.5 0.4 0.3 0.2 0.1 0.0 1

10

100

Mixing time (min) Fig. 9 – Extraction experiment (a) and rubber layer L of different composites in dependence on mixing time (b).

ð11Þ ð12Þ

ð13Þ

BMI are the surface fractions of the CNT wetted by SCR F and SF the CR and BMI molecules, respectively. cBMI-F and cCR-F are the interfacial tension values between CNT and BMI or CR. cCR, cBMI and cF are the surface tension values of CR and BMI as well as CNT, respectively. nCR/BMI is the blend ratio CR to BMI. In the present work cCR = 35 mN/m, cF = 28.5 mN/m for Baytubes and 30.5 mN/m for Nanocyl were experimentally determined. cBMI = 33.6 mN/m was taken from Ref. [49]. Setting the surface tension values of CR and BMI into Eqs. (10)–(13) a master curve demonstrating the filler surface fraction wetted by the BMI molecules in dependence on the filler surface tension can be created as seen in Fig. 10. Fitting the surface tension of Baytubes and Nanocyl into the master of 0.72 curve with nCR/BMI+ = 1, a CNT surface fraction SBMI F was found for Baytubes and 0.82 for Nanocyl at a thermodynamic equilibrium state. In this case, the selective wetting behavior of CNT is merely dependent on the filler-polymer affinity, and the affinity of both CNTs to BMI is better than to CR.

0.9

(a)

wetting process

0.8 0.7

L LBMI+ LCR

0.6

nCR/BMI+ = 1

0.6 0.5 0.4 0.3 0.2

0.4

0.1

0.2

CR

Rubber layer L

1.0 0.8

replacement process

CNTs + BMI

γNanocyl

4551

5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

γBMI γCR

0.0

nCR/BMI+ = 33

10

0.0

100

Mixing time (min) 26

28

30

32

Filler surface tension

34

36

38

40

γF . mN/m

(b)

wetting process

0.8

Fig. 10 – CNT surface fraction wetted by BMI+ predicted by the Z-model in dependence on the filler surface tension and blend ratio nCR/BMI.

replacement process

Rubber layer L

0.7 0.6 0.5 0.4 0.3 0.2

L LBMI+ LCR

0.1 0.0 10

100

Mixing time (min)

(c) 0.048

BMI-

0.039

/A

0.042

BMI+

0.045

A

For nCR/BMI+ = 33 as used in the present work a CNT surface fraction SBMI of 0.07 was found for Baytubes and 0.11 for NanoF cyl. The CNT surface fraction wetted by BMI is much smaller compared to that wetted by CR, although the affinity of both CNTs to BMI is better than to CR. According to Eq. (10) the CNT surface wetted by BMI is determined by the thermodynamic driving force (cCR-F/cBMI-F) and the concentration compensation effect (nCR/BMI+). In the present work at a high value of nCR/BMI+ the concentration compensation effect dominates the thermodynamic effect and as a result, CR expectedly wets the large amount of CNT surface. The experimental characterization of the selective wetting behavior of CNT by CR and BMI was done by means of FTIR of the extracted parts. The rubber layer L and its contribution þ LBMI and LCR of the sample CBa5I10, which were quantified by Eqs. (4) and (5) are presented in Fig. 11a in dependence on mixing time. It is obvious that in the first mixing period þ up to 50 min both LBMI and LCR increase. In this range BMI and CR concurrently infiltrate the CNT aggregates and wet þ CNT surface. After the wetting process is complete, LBMI decreases while LCR continuously increases. Because the value of L is constant in the second period, it can be concluded that the free CR molecules replaced the bonded BMI+ on the CNT surface as a result of the concentration compensation effect. At 120 min mixing time BMI+ is completely replaced by CR that is corresponding to the prediction made by the Z-model. The similar wetting behavior was also found for CNa5I10 as shown in Fig. 11b. However, the first mixing period ends after 10 min and the second one after 60 min. The faster wetting and replacement process taking place in CNa5I10 are related to the faster dispersion process of Nanocyl compared to Baytubes. Such a replacement process between the blend components on the filler surface was also found in our previous works [30–35] for different rubber blends filled with silica, CB and CNTs. The ratio ABMI+/ABMI of the extracted part is presented in Fig. 11c in dependence on mixing time. In both composites ABMI+/ABMI decreases from 0.045, which is the value deter-

0.9

CNTs + BMI

24

CR

CNT surface fraction wetted by BMI S F

BMI

γBaytubes

CARBON

0.036

neat BMI

0.033

CBa5I10

0.030

CNa5I10 10

100

Mixing time (min) þ

Fig. 11 – Rubber layer L, LBMI and LCR of CBa5I10 (a) and CNa5I10 (b) as well as the ratio ABMI+/ABMI of the extracted part of both composites (c) in dependence on mixing time.

mined from the spectrum of the neat BMI, to a minimum value in the first mixing period. The decrease of ABMI+/ABMI of the extracted part is an evidence for the predominant bonding of BMI+ to the CNT surface in the first mixing period. In the second mixing period ABMI+/ABMI increases because more and more bonded BMI+ are released from the CNT surface and at the end of the mixing process ABMI+/ABMI reaches the value of the neat BMI, i.e. CNTs are wetted completely only by CR.

0

1

2

Fig. 12 – EDX spectrograms of the filler-rubber gel of CBa5I10120 m and CNa5I10-60 m.

The EDX spectrograms of the filler-polymer gel of two composites, CBa5I10-120 m after 120 min mixing time and CNa5I10-60 m after 60 min mixing time, are shown in Fig. 12. No signals of fluorine at 0.67 keV and nitrogen at 0.39 keV of BMI and BMI+, respectively, were found, while a strong peak of chlorine at 2.62 keV of CR can be observed for both composites. That corresponds very well to our FTIR analysis. The unbound part of BMI forms an own phase as seen in SEM images for both composites (Fig. 13). A closer look at the BMI phase does not reveal any CNTs. In Fig. 13a beside the BMI phase some non-dispersed Baytube aggregates are still observed in submicron-scale, which appear in the SEM micrographs as bright domains. The wetting BMI+ cations inside the non-dispersed aggregates could be difficultly replaced by the CR molecules, because CR needs time to infiltrate the aggregates. That is why the wetting and replacement process take place slowly in CBa5I10 as discussed above. In Fig. 13b CNTs were uniformly dispersed in the rubber matrix and form a regular network that enables the fast replacement of BMI+ by CR on the Nanocyl surface as observed in Fig. 11b. Concerning the interaction between ionic liquids and CNTs as well as the molecular structure of their interphase several works have been done recently. Likozar [50] investi-

+ 10 phr BMI

5 phr Nanocyl

sulfur-cured

+ 10 phr BMI

5 phr Baytubes

Samples

3

Binding energy (keV)

cured

1E-9

high energy electrons

3000

1E-7

sulfur-cured

6000

1E-5

5 phr Baytubes

9000

1E-3

+ 10 phr BMI

Intensity

Cl

peroxidic

F

0.1

-cured

N

12000

CR

5 phr Baytubes + 5 phr BMI

BMI

cured

-

+

BMI

uncured cured

10

peroxidic

CNa5I10-60m

15000

Offline conductivity Goff (S/cm)

CBa5I10-120m

5 phr Baytubes

18000

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

10 phr Baytubes

CARBON

20 phr Baytubes

4552

Fig. 14 – Offline conductivity of different composites before and after curing process.

gated the adsorption kinetics of different ionic liquids into CNT/HNBR composites by immersing the cured composites in the ionic liquid/chloroform solvent. He observed a homogeneous distribution of anions in the composites by use of the fluorine signal detected by scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX). By means of fully atomistic molecular simulations Frolov et al. [51] studied the basic mechanisms of carbon nanotube interactions with several different room temperature ionic liquids in their mixtures with acetonitrile. It was found that two distinct layers of cations and anions are formed at the CNT surface. Increase of the length of the non-polar alkyl groups of cations increases the propensity of imidazolium-based cations to lay parallel to the CNT surface. Wang et al. [41] carried out Raman and IR measurements on the mixtures of ionic liquids and single-walled carbon nanotubes (SWCNTs) and found that no strong interaction such as cation–p interaction exists between SWCNTs and imidazolium cations. It could be seen that the fluorine atoms of anions and the hydrogen atoms of the alkyl groups of cations are much closer to the SWCNTs than the nitrogen atoms and carbon atoms of the imidazolium rings. This indicates that the SWCNTs are surrounded by the polar parts of anions and non-polar parts of cations simultaneously. They proposed that the ionic liquids interact with SWCNTs through weak van der Waals interaction and

Fig. 13 – SEM images of CBa5I10-120 m (a) and CNa5I10-60 m (b).

CARBON

5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

the shielding effect of ionic liquids on the p–p stacking interaction among SWCNTs takes the key role in dispersing the SWCNTs. Their conclusion however is in contrast to that made by Fukusima et al. [15,16] and Ma and Dougherty [17], who stated that the specific interaction between the imidazolium ion component and the p-electronic nanotube surface is essential for the excellent dispersion of CNTs in ionic liquids. The time dependent presence of BMI+ cations in the bound polymer regions as found in Fig. 11 can be supported by the interaction mechanism proposed by Fukusima and Ma et al. [15–17]. As discussed above, in the both composites CR bonds to CNT surface directly as well as indirectly through BMI. The difference in composition of the filler-rubber interphase is surely essential for the dynamic mechanical behavior of composites, thus characterization of the effect of CNT wetting on the mechanical properties of composites will be the scope of our future work.

3.4. Effect of curing process on the conductivity of CNT/ rubber composites without and with BMI The offline conductivity of the composites is presented in Fig. 14 in dependence on filler loading, mixing technique and curing conditions. It is clear to observe the positive effect of the BMI loading on the offline conductivity of the uncured composites. Fig. 14 shows that after peroxide-curing the conductivity increases about one order of magnitude for composites without BMI and decreases nearly five orders of magnitude for composites with BMI. The influence of com-

4553

pression molding parameters on the conductivity was also investigated on polycarbonate (PC) composites containing 1 and 2 wt.% Baytubes by Kasalival et al. [52]. By adjusting pressing temperature and time the conductivity of Baytube/ PC composites can be varied over eight orders of magnitude. Zhang and Feng [53] and Gerspacher et al. [54] found the positive effect of curing process on the electrical conductivity of different CB filled rubber composites. The flocculation/reagglomeration of CB aggregates was discussed as the main reason for the increase of conductivity during curing. In contrast, the negative effect of peroxide-curing on the conductivity was observed in our previous works on CB filled rubber composites [55,56]. It was stated that the change of conductivity correlates directly with the state of filler network, which is kinetically determined by the balance of the de- and reagglomeration process of filler aggregates. From the thermodynamic point of view, the re-arrangement of CNT network in the present systems can be described as follows: During the mixing process, rubber molecules are stretched under shear stress, and they infiltrate the filler agglomerates and wet the filler surface. Loss of entropy upon stretching means that there is a retractive force for recovery of molecules, when the external shear stress is removed after the mixing process. The polymer contracts into a state of higher entropy, as if driven by an elastic force. When the curing reaction takes place, the entropic force related to the molecular weight between two chemical cross-links MC according to the entropy elasticity theory [57]. With decreasing MC the entropic force becomes stronger and can separate the CNT aggregate from

Fig. 15 – TEM images of peroxide-cured CBa5I10 (a), (b) and sulfur-cured CNa5I10 (c), (d).

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

1 0.1

without BMI cured uncured

off

off

Gmax

Gmax

0.01 1E-3

with BMI cured uncured

1E-4 1E-5

1E-7

CNT

1E-6 CR

(a)

off

the network. The de-agglomeration of the CNT network during the curing process can result in a decrease of conductivity. Simultaneously, CNT aggregates are also brought together by Brownian motion (re-agglomeration) during the compression molding/curing process to minimize filler surface energy and a new filler network is formed. As a result of the competition between de- and re-agglomeration processes, the electrical conductivity can decrease or increase after curing process. In Fig. 14 the different effect of peroxide- and sulfur-curing on the conductivity of the samples with BMI becomes significant. It is worthy to note that in these composites the ratio LBMI+/LCR is nearly zero. The difference in conductivity between the peroxide-cured and the sulfur-cured composites can be explained by taking into consideration the TEM images shown in Fig. 15. It is obvious that the peroxide-cured sample contains a number of separate CNT aggregates that causes a decrease of conductivity after peroxide-curing (Fig. 15a and b). In contrast, CNTs in the sulfur-cured sample are well dispersed and form a continuous network throughout the CR matrix (Fig. 15c and d), which imparts the composite a high conductivity. At higher magnification (Fig. 15b and d) individual CNTs can be clearly observed in both samples. The difference in reorganization/ reorientation of the network of nanofiller like nanoclay taking place during the peroxide- and sulfur-curing was discussed by Das et al. [58]. He stated that the orientation of clay platelets obtained after mixing process is maintained during peroxide-curing, because a lot of cross-linking sites are available for covalent bond formation (C–C), and thus, no conformational rearrangement of molecules are needed for the curing process. In contrast, sulfur-cured vulcanizates show a non-oriented isotropy of the platelets. It is assumed that during sulfur-curing the conformational re-arrangement of the macromolecular chains takes place for providing the sulfur cross-linking sites close together, and in this way the silicate platelets are forced to move along with rubber chains in different directions. The offline conductivity of the Nanocyl composites without and with BMI before and after sulfur-curing is presented in Fig. 16a. Generally, all the samples show an increase of conductivity after sulfur-curing, i.e. the re-formation of the CNT network is the dominating process during the sulfur-vulcanization of the investigated composites. In order to characterize the extent of the conductivity change after curing the ratio Gcured/Guncured was calculated from Fig. 16a and is shown in Fig. 16b in dependence on the ratio LBMI+/LCR, which was calculated from Fig. 11b. With increasing mixing time, i.e. decreasing LBMI+/LCR from 0.6 to 0.0 the ratio Gcured/Guncured decreases from about 103 to 101, which lies in the same order of that of CNa5. It is noteworthy that the surface energetic properties of the filler surface are determining the extent of flocculation in the rubber mixtures. According to the experimental results it can be concluded that CNTs with a higher value of LBMI+/LCR show a better flocculation tendency during curing. For CNTs wetted only by CR, i.e. LBMI+/LCR = 0, the flocculation is reduced due to a better compatibility between the rubber and the filler surface. Wang [59,60] pointed out that the tendency of filler particles to flocculate is thermodynamically driven by the difference in the work of adhesion DWa between the state of a

Offline conductivity G (S/cm)

CARBON

1E-8 10

Mixing time (min) Mixing time (min)

(b)

15

10

30

60

1000

Gcured /Guncured

4554

100

10

CNa5 CNa 5I10

1 0.6

0.4

0.2

0.0

BMI+ CR

L

/L

Fig. 16 – Offline conductivity of composites of Nanocyl before and after sulfur-curing (a), conductivity change Gcured/ Guncured in dependence on LBMI+/LCR (b).

filler particle in contact with polymer and the flocculated state, where the filler particles are in contact with each other. A close correlation between filler flocculation and DWa in ethylene propylene diene rubber (EPDM) compounds filled with silica and CB was also found in our previous works [61,62]. The influence of the bonded BMI+ on the value of DWa and related filler flocculation will be the focus of our next investigation. Based on the fact that the change of the offline conductivity during the curing process can take place only at high temperatures as a result of the re-arrangement of the tube network, an effective way to suppress this process is to use high energy electrons for curing at room temperature. Swelling experiments of samples CBa5I10 cured by sulfur and alternatively by high energy electrons were carried out. Their swelling of 290% and 340%, respectively, indicate an insignificant difference in the cross-linking degree of two samples. The offline conductivity of CBa5I10 remains unchanged after curing by high energy electrons as seen in Fig. 14. This result confirms our discussion on the re-arrangement of the CNT

CARBON

5 0 ( 20 1 2 ) 4 5 4 3–45 5 6

network taking place during the curing process at high temperature. This process influences the offline conductivity of the product significantly and should be controlled during the composite fabrication.

[7]

[8]

4.

Conclusions

The usage of an ionic liquid, 1-butyl 3-methyl imidazolium bis(trifluoromethyl-sulphonyl)imide (BMI) as a surfactant shows a significant positive effect on the filler dispersion kinetics and electrical conductivity of uncured BMI-Baytube/ CR composites. In contrast, BMI slows the filler dispersion process and development of conductivity of BMI-Nanocyl/CR composites. The re-arrangement of filler network taking place during the curing process leads to significant changes of the conductivity, which is dependent on the used CNTs, addition of BMI and curing methods, i.e. by sulfur, peroxide or high energy electrons. The wetting concept was further developed in order to characterize the selective wetting of CNTs by rubber and BMI. During the mixing process the bonded BMI+ cations are slowly replaced by CR molecules on the Baytube surface. In contrast, the replacement process on the Nanocyl surface takes place much faster. Based on the wetting behavior of both CNTs, which is determined by their different compatibility to rubber and BMI, the difference in filler dispersion kinetics in mixing process, re-arrangement phenomenon of filler network in curing process and related electrical conductivity of the composites was discussed.

[9] [10]

[11]

[12]

[13]

[14]

[15]

Acknowledgements The authors wish to thank the German Research Foundation (DFG) (RA 582/23-1) and the German Academic Exchange Service (DAAD) for the financial support. We also thank Mrs. Cornelia Becker and Mr. Werner Lebek (Martin Luther University Halle-Wittenberg) for EDX analysis and Dr. Andre Wutzler (Polymer service GmbH Merseburg) for TGA.

[16]

[17] [18]

[19] R E F E R E N C E S

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CARBON

5 0 ( 2 0 1 2 ) 4 5 4 3 –4 5 5 6

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