Location of dispersing agent in rubber nanocomposites during mixing process

Polymer 54 (2013) 7009e7021

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Location of dispersing agent in rubber nanocomposites during mixing process H.H. Le a, c, *, K. Oßwald b, S. Wießner c, A. Das c, K.-W. Stöckelhuber c, R. Boldt c, G. Gupta d, G. Heinrich c, e, H.-J. Radusch a a

Polymer Service GmbH Merseburg, D-06217 Merseburg, Germany University of Applied Sciences, D-06217 Merseburg, Germany Leibniz-Institut für Polymerforschung (Dresden e.V. (IPF)), D-01069 Dresden, Germany d Institute of Physics, Martin Luther University Halle-Wittenberg, D-06099 Halle, Germany e Institut für Werkstoffwissenschaft, Technische Universität Dresden, D-01069 Dresden, Germany b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2013 Received in revised form 11 September 2013 Accepted 20 October 2013 Available online 28 October 2013

In the present work, the development of morphology and selective wetting of nanoclay and carbon nanotubes (CNTs) in rubber nanocomposites were characterized qualitatively by means of the optical microscopy, TEM and AFM and quantitatively by means of the wetting concept. Carboxylated hydrogenated nitrile butadiene rubber (XHNBR), ionic liquid and ethanol were used as dispersing agent and they show very good effect on the macro- and microdispersion of nanofillers in different rubbers. It was found that the selective wetting of filler surface by the dispersing agent and rubber matrix is controlled by thermodynamic and kinetic factors. A model basing on surface energy data of polymer components (rubber and dispersing agent) and filler was introduced in order to determine the thermodynamic equilibrium state of filler wetting, which is found to be simultaneously determined by the fillerepolymer affinity and the rubber/dispersing agent mass ratio. During the mixing process a replacement process of bound polymer components takes place on the filler surface until the predicted state is reached. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes Dispersing agent Localization

1. Introduction In recent years nanofillers like nanoclay and carbon nanotubes (CNTs) have found application in tire tread compounds aiming at longer life of the compound and lower fuel consumption without negatively affecting the tread grip on the road at low temperature [1e7]. Up to date, four processing methods, including in-situ polymerization, solution mixing, melt compounding, and latex mixing have been developed for preparation of rubber nanocomposites. Among them, melt compounding is the most practical method, because it is economical, more flexible in formulation, and existing rubber processing equipment can be used without consuming organic solvents. The significant factor that determines the improvement of composite properties is the filler dispersion and distribution in the rubber matrix, as it allows to best exploit of the potential of nanofiller. However, the simple melt compounding

* Corresponding author. Leibniz-Institut für Polymerforschung (Dresden e.V. (IPF)), D-01069 Dresden, Germany. Tel.: þ49 3461462741; fax: þ49 3461463891. E-mail address: [email protected] (H.H. Le). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.10.038

of rubber and nanofiller does not often contribute to the good filler dispersion, because the difference in their surface energy is larger. Different concepts were realized in order to overcome this problem. For instance, functionalization of carbon nanotubes (CNTs) with silane, acid groups or ionic liquids [8e17] as well as preintercalation of nanoclay by suitable surfactants [18,19] are known to facilitate dispersion of nanofillers with the formation of stable bonds between filler surface and polymer chains. Another convenient way to enhance the fillererubber interaction and filler dispersion is to use a polymer component as dispersing agent during melt mixing process, which lead to “non-covalent” driven dispersion of nanofiller in rubber matrix. Arroyo et al. [20], Teh et al. [21], Varghese et al. [22], Potiyaraj et al. [23] and Rajasekar et al. [24e26] have used epoxidized natural rubber (ENR) as dispersing agent for nanoclay dispersion in different rubbers like natural rubber (NR), styrene butadiene rubber (SBR), ethylene propylene rubber (EPR) and nitrile butadiene rubber (NBR). Dispersion of halloysite nanotubes [27] in SBR was also improved by use of ENR. The other functional rubbers like carboxylated nitrile butadiene rubber (XNBR) [28] and carboxylated styrene butadiene rubber (XSBR) [29] or poly(ethylene-co-polyvinyl

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acetate) (EVA) statistic copolymer [30] as well as acrylate rubber (AR) [31] were used as dispersing agent for dispersing nanoclay and CNTs in rubber matrix. It was clearly found that the presence of functional rubber acting as dispersing agent improved fillere rubber interaction and filler dispersion as well as performance of the composites. However, in the mentioned works [20e31] the development of composite morphology and especially composition of the fillererubber interphase has not been investigated systematically so far. Some questions can be arising, for example, where is the dispersing agent localized in the composite during and after mixing process? Does it remain bonded on the filler surface and act as coupling agent or isolate filler from the matrix? Or does it leave the filler surface and form an own phase in the matrix? Concerning the development of morphology of nanocomposites, recently we developed the wetting concept for characterization of the kinetics of filler localization in heterogeneous rubber blends [32,33]. In the present work we report on our attempts to disperse nanofiller like organoclay and CNTs in different rubbers in presence of a dispersing agent. By taking into consideration the dispersing agent as a minor blend phase the morphology development of composites and the kinetics of localization of the dispersing agent during the mixing process will be characterized by means of the wetting concept. A new model based on surface tension data of the rubber components and filler is introduced for prediction of the selective wetting of filler by rubber matrix and dispersing agent at an equilibrium state. A comparison between experimental results determined by the wetting concept and that predicted by the surface tension basing model provides a deeper insight into the relocation of the dispersing agent during mixing process. 2. Theoretical prediction of selective wetting and localization of filler in rubber blends at an equilibrium state (Z-model) Generally, the selective filler wetting and localization in a polymer blends is thermodynamically determined by the affinity of the filler to each polymer blend component. Thus, if the surface energy of filler and blend components is known, the filler localization in blend at an equilibrium state can be predicted. The thermodynamic criterion for miscibility of polymer blends is a negative Gibbs free energy of mixing. The Gibbs free energy of mixing is given by the following equation:

DGm ¼ DHm  T DSm

(1)

where DHm is the change in enthalpy of mixing, and DSm the change in entropy of mixing. T is the absolute temperature. Due to the constraints of segmental mobility of polymer chains the change in entropy is usually too small to compensate the change of mixing enthalpy. Thus, the Gibbs free energy of mixing is nearly similar to the change in enthalpy of mixing:

DGm zDHm

(2)

When a filler F is mixed into a binary rubber blend AB it will be wetted and bonded by the molecules A and B. Because polymer molecules can get contact only with the outer layer of the filler particle, the effective particle volume is considered as very small compared to the ineffective particle volume as well as the volume of the blend phase A and B as schematically illustrated in Fig. 1. For the contact between polymer and filler, the change in BF enthalpy of mixing DHAF m of the phase A and F, and DHm of the phase B and F can be described by Eqs. (3) and (4) according to Hildebrand and Scott [34]:

Fig. 1. Effective and ineffective volume of filler particle in a binary polymer blend. AF DHm

VAF

¼ K

FA FA þ

FAF FAF

ðgAF Þ2

(3)

ðgBF Þ2

(4)

FA þ FAF

and BF DHm

VBF

¼ K

FB

FBF

FB þ FBF FB þ FBF

with

FA þ FB þ FF ¼ 1

(5)

and

FF ¼ FAF þ FBF

(6)

where VAF and VBF are the average molar volumes of the two corresponding components. FA and FB are the volume fractions of A and B, respectively. FF is the fraction of the effective volume of the filler F in the ternary system AFB. FAF and FBF are the effective volume fractions of F in A and B, respectively. The inner region (grey area, Fig. 1) considered as ineffective volume of F is not taken into consideration in Eqs. (3) and (4). K is a constant and gets a value of 1 according to Scatchard [35]. At the interface of AB the component F migrates between A and B in order to minimize the Gibbs free energy of mixing DGAF m and DGBF m of the binary system AF and BF, respectively. The filler transfer process goes on until the thermodynamic equilibrium state is reached. Taking Eq. (2) into account a thermodynamic equilibrium state is reached if AF DHm

VAF

¼

BF DHm

(7)

VBF

Setting Eqs. (3) and (4) into Eq. (7) we get:

FA FA þ

FAF FAF

FA þ

FAF

ðgAF Þ2 ¼

FB FB þ

FBF FBF

FB þ FBF

ðgBF Þ2

(8)

Eq. (8) can be rewritten to:

FAF FB FA þ FAF ¼ FA FB þ FBF FBF

!2 

gBF gAF

2 (9)

Because the effective volume fractions FAF and FBF are very small compared to the total filler volume and FA as well as FB of the host blend phases A and B, we can neglect FF A and FBF from the term (FA þ FAF )2/(FB þ FBF )2 of Eq. (9) for simplification purpose. Eq. (9) can be expressed now as follows:



FAF FB FA z FBF FA FB

2 

gBF gAF

2



FAF FA gBF z FBF FB gAF

0

2 (10)

H.H. Le et al. / Polymer 54 (2013) 7009e7021

According to our wetting concept [32,33] describing the relationship between the wetted filler surface and the phase specific filler localization, we can calculate the filler fraction in each blend phase A and B as follows:

4AF 4BF



¼

FAF FA gBF z FBF FB gAF

2

 ¼ nA=B

gBF gAF

2 (11)

4AF and 4BF are the weight fractions of the filler F in the phases A and B, respectively, with 4AF þ 4BF ¼ 1. nA/B ¼ FA/FB is the blend ratio A to B. Using the Girifalco-Good Equation [36] describing the relationship between surface tension and interfacial tension Eq. (11) can be expressed as follows:

4AF 4BF



¼ nA=B

pffiffiffiffiffiffiffiffiffiffiffi2

gB þ gF  2 gB gF pffiffiffiffiffiffiffiffiffiffiffi gA þ gF  2 gA gF

(12)

gA, gB and gF are the surface tension values of the phases A, B and F, respectively. Setting 4BF ¼ 1  4AF into Eq. (12) the weight fraction 4AF can be calculated using Eqs. (13) and (14). 4AF ¼

nA=B u nA=B u þ 1

(13)

with

u ¼



pffiffiffiffiffiffiffiffiffiffiffi2

gB þ gF  2 gB gF pffiffiffiffiffiffiffiffiffiffiffi gA þ gF  2 gA gF

(14)

3. Experimental

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3.2.3. 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 CNTepolymer gel in order to characterize the presence of BMI in the bound layer of the composites. 3.2.4. X-ray diffraction Small-angle X-ray scattering (SAXS) measurements were performed at room temperature using a rotating anode X-ray source RU-3HR (Rigaku) equipped with an X-ray optics device (Confocal Max-Flux, l ¼ 0.154 nm, Osmic Inc.) and a Bruker Hi-Star 2-D detector for detection of the state of exfoliation. The generator voltage was 40 kV and generator current was 60 mA. The scattering vector q is defined by q ¼ 4psin q/l. All samples had a uniform thickness of 1.0 mm, i.e. the obtained peak area corresponds to the amount of ordered structures. 3.2.5. Measurement of the electrical conductivity A conductivity sensor system was installed in the chamber of the internal mixer to measure the online conductance 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 [38,39]. Offline-measurement of electrical conductivity of 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.

3.1. Compounding For the preparation of filled rubber compounds and blends, rubbers were mixed with filler using an internal mixer (Rheocord 300p, ThermoHaake). Along the mixing time samples were taken out for further investigation. Used materials, formulations as well as mixing conditions will be given in details in each part later. 3.2. Characterization 3.2.1. Optical microscopy Optical microscopy has been used to characterize the macrodispersion of filler. This method was first described by Stumpe and Railsback [37], and later becomes a standard method (ASTM D7723). We produced gloss cuts by cutting stretched samples by a razor blade at room temperature and analyzed the cut surface by optical microscopy. If the surface of the cut contents CB agglomerates or aggregates, the light is scattered at this place and its area appears dark. With an image analysis program one can calculate the area of visible filler regions. The macrodispersion A/A0 is calculated by the ratio of the surface of non-dispersed agglomerates to that of the image. A value A/A0 ¼ 0% indicates an image without any agglomerate larger than 6 mm. From each mixture 6 images were made at different positions of the cut surface and six analyses were done for every image. 3.2.2. 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-microtome 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 LibraÒ 200MC (Zeiss, field emission cathode, point resolution 0.2 nm) with an accelerating voltage of 200 kV.

3.2.6. Extraction experiment and Fourier transformed infrared (FTIR) spectroscopy analysis of the fillererubber gel For extraction experiment 0.1 g of each raw mixture obtained directly from the mixing process were stored in 100 ml of a suitable solvent. For each mixture three samples were collected at different position of mixing chamber and used for extraction experiment. For every rubberefiller gel six FTIR spectral were recorded and analyzed. Thus, the phase specific filler localization presented in the manuscript is the mean value of 18 calculations and is characteristic for the whole blend. After the soluble part was entirely extracted from the raw mixture, the fillerepolymer 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. (15) [32,33].

L ¼

mG  mcomp $cF mG

(15)

mcomp is the mass of the composite before extraction experiment, cF is the mass fraction of the filler in the composite. mG is the sum of the insoluble rubber part and filler. When filler F is mixed into a NBR/NR blend, it is wetted by both rubbers. The rubber layer LB of blend is determined using Eq. (15). It consists of two contributions, LB(NBR) and LB(NR), of NBR and NR phase, respectively. LB(NBR) and LB(NR) can be determined by means of a calibration curve. For creation of the calibration curve, blends with different NBR/NR ratios were investigated by FTIR according to the procedure described in our previous work [33]. FTIR spectrograms were recorded by use of a FTIR spectrometer S2000 (Perkin Elmer) equipped with a diamond single Golden Gate ATR cell (Specac). The peaks of the NBR phase at 2237 cm1 and of the NR phase at 1376 cm1 were taken for calculation of the ratio of the surface under peak ANBR/ANR. The correlation between the ANBR/ANR ratio and the given NBR/NR ratio is described by a straight line with

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Table 1 Characteristic FTIR peaks of used materials.

Table 2 Formulation of masterbatch and different clay filled compounds.

Material

Characteristic FTIR peak

NBR XHNBR NR SBR CR Ionic liquid Anion Cation

2237 cm1

(Stretching vibration of eC^N) [40]

1376 cm1 1494 cm1 1659 cm1

(Bending vibration of eCH [41] (Stretching vibration of the phenyl ring) [42] C]C stretching mode of CR [43]

Ethanol

1

1192 cm 740 cm1

Vibration of eSO2 [44] Vibration of CeH of cyclic BMIþ

1045 cm1

Stretching vibration of CeO [45]

Clay/SBR compound

a slope fNBR/NR ¼ 0.89. The ratio LB(NBR)/LB(NR) of the rubberefiller gel can be determined using Eq. (16) with ANBR and ANR being the area under peak of NBR and NR phase, respectively, in the fillererubber gel of the investigated composites.

LBðNBRÞ ðtÞ 1 ANBR ðtÞ ¼ BðNRÞ fNBR=NR ANR ðtÞ L ðtÞ

(16)

SBR XHNBR Clay Clay/XHNBR masterbatch

Clay/XHNBR masterbatch

Clay/XHNBR/SBR compound

(Clay/XHNBR)/ SBR compound

90 10 5

90

100 50

100 5

15

The filler surface fractions SNBR and SNR F F , which are wetted by the NBR and NR phase, respectively, as well as filler loading RNBR and F RNR F localized in each blend phase can be calculated using Eq. (17).

SNBR ðtÞ F SNR F ðtÞ

¼

RNBR ðtÞ F RNR F ðtÞ

¼

LNR LBðNBRÞ ðtÞ P NBR LBðNRÞ ðtÞ LP

(17)

The characteristic peaks of rubbers used in the present work are listed in Table 1. 3.2.7. 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 filler 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 filler 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 filler particle covered tape was used for Wilhelmy contact angle measurements without further modification. Sessile drop contact angle measurements on a sheet of uncured rubber 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, Loughborough, U.K.), 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 [46]. 4. Result and discussion 4.1. Effect of the blend ratio on the phase selective filler localization A dispersing agent has been often used in a minor quantity compared to the matrix that may affect the selective wetting behavior of filler. In order to characterize the effect of the dispersing agent/matrix ratio on the filler localization we used a model system containing NBR, NR and silica. The rubbers used were nitrile Table 3 Formulation of CNT compounds and masterbatch. CNT/CR compound

Fig. 2. Kinetics of filler localization in 50/50 NBR/NR blend (a) and master curve of filler localization in NBR/NR blends with different blend ratios by comparison with experimental results (b).

CR XHNBR CNT CNT/XHNBR masterbatch

CNT/XHNBR masterbatch

CNT/XHNBR/ CR compound

(CNT/XHNBR)/ CR compound

100 10 5

100

100 50

100 5

15

H.H. Le et al. / Polymer 54 (2013) 7009e7021

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Fig. 3. Optical microscopic images of 5/100 clay/SBR compound and 50/100 clay/XHNBR masterbatch.

butadiene rubber (NBR) Perbunan 3445F (Lanxess) with a nitrile content of 34% and natural rubber (NR) SMR 10 (Standard Malaysian Rubber). The modified silica COUPSILÒ 8113 (Evonik) with a specific surface area CTAB of 175 m2/g and a particle size of 20e 54 nm was used as filler. It was produced by means of a presilanization process of 100 part of Ultrasil VN3 by 12.7 part silane Si69. A loading of 50 phr silica was mixed into NBR/NR blends with different blend ratios for a total mixing time of 15 min. The silica fractions in NBR and NR phase 4NBR and 4NR F F , respectively, were experimentally determined by means of the wetting concept and are presented in Fig. 2a as an example for 50/50 NBR/ NR blend in dependence on mixing time. In the first mixing period up to 5 min mixing time, 4NBR and 4NR F F increase rapidly with mixing time. The first mixing period is attributed to the wetting process, in which the filler localization is mainly determined by the selective wetting behavior of the filler by the blend components [33]. According to our previous work [47] the wetting speed of silica by NBR and NR bNBR ¼ 0.5 min1/2 and bNR ¼ 0.45 min1/2, respectively, can be determined for the first mixing period. It indicates that NBR can infiltrate and wet silica faster than NR, because the affinity of silica to NBR is better than to NR. Ziegler et al. [48] detected hydrogen bondings between silanol groups of silica and C^N groups of NBR by means of FTIR, while Ono et al. [49] and Kralevich et al. [41] stated that van der Waal forces are responsible for interaction between silica and NR. That is why in the first mixing period, higher silica loading was distributed into NBR phase. After the wetting process is completed, the filler loading in each blend

phase of all the investigated NBR/NR blends remains unchanged with mixing time. It seems to reach a stationary state. The filler fraction localized in the NBR and NR phase of NBR/NR blends at a thermodynamic equilibrium state can be predicted using Eqs. (12)e(14). Setting the surface tension values of NBR and NR gNBR ¼ 27.5 mN/m [50] and gNR ¼ 22 mN/m [50], respectively, into Eqs. (12)e(14) with nNBR/NR ¼ 1 a Z-shaped master curve demonstrating the filler fraction localized in the NR phase 4NR F in dependence on the filler surface tension can be created as seen in Fig. 2b. Fitting the silica surface tension gF ¼ 45 mN/m [50] into the master curve a filler fraction 4NR of 0.25 was predicted for an F equilibrium state. This value is corresponding very well to the stationary value experimentally determined by means of the wetting concept shown in Fig. 2a. With increasing blend ratio nNBR/NR the shape of the master curves changes and the filler fraction 4NR F decreases correspondingly. The filler fraction 4NR F experimentally determined for all the NBR/NR blends taken out at mixing time 15 min is presented in Fig. 2b in a good correlation with prediction. It is obvious from experiment and theory that when an amount of a blend phase is reduced, it will wet less filler surface, although the filler rubber affinity remains unchanged. In the other word the minor phase should release filler to the mayor phase. Consequently, in a filled rubber compound containing dispersing agent, the location of the dispersing agent on the filler surface is determined by two opposite factors: (1) the higher affinity between dispersing agent and filler surface and (2) the low amount of dispersing agent. In the following parts the competition of two factors determining

Fig. 4. Prediction of clay localization in SBR compound with XHNBR as dispersing agent.

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the location of dispersing agent will be characterized in different systems. 4.2. Clay dispersion in SBR with XHNBR as dispersing agent

Fig. 5. Filler localization in clay/XHNBR/SBR (a) and (clay/XHNBR)/SBR compound (b) in dependence on mixing time.

Solution styrene butadiene rubber (S-SBR) SPRINTAN SLR-4601 (Styron Deutschland GmbH) with a styrene content of 21% and vinyl content of 63% were used as rubber matrix. Carboxylated hydrogenated nitrile butadiene rubber (XHNBR) Therban XT VP KA 8889 (Lanxess) with acrylonitrile content of 33 % was used as dispersing agent. Organoclay Nanofil 9 (Süd-Chemie) modified by stearyl benzyl dimethyl ammonium chloride, an average particle size of about 35 mm, and a weight loss on ignition of 35 wt% was used as filler. Peroxide Luperox 101 (Atofina Chemicals) was used as curing agent for rubber compounds investigated (Tables 2 and 3). Macrodispersion of 5/100 clay/SBR compound and 50/100 clay/ XHNBR masterbatch is characterized by optical microscopy and presented in Fig. 3. It is clearly observed that clay is dispersed well in XHNBR but not in SBR matrix. The poor mechanical properties of clay/SBR compounds related to poor dispersion of clay were reported in different works [18,19,51,52]. The excellent dispersion of clay in XHNBR was also observed in literature [53e55] and in our previous works [56,57]. Pradhan et al. [53] detected strong polar or even ionic interactions of the polar eCN and acidic eCOOH groups of XNBR with the basic eOH functionalities on the clay surface using FTIR technique. The extent of H-bond formation depends on the eCN and eCOOH content as well as the number of the OHgroups available at the edge of clay platelets. So, it is expected that the strong shearing force can be transferred from the rubber to the layered silicate and delaminate the staged layers by overcoming the force between two adjacent silicate layers, and ultimately results in an exfoliated clay structure in the rubber matrix. Making use the good dispersion of clay in XNBR Das et al. [28,54,58] developed a novel method for the preparation of nanocomposites comprising a high performance rubber and nanoclay for tire application. In their works nanocomposites of SBR with nanoclay were prepared with XNBR. A sufficient amount of clay was loaded in XNBR as compatibilizer and this compound was blended as a masterbatch in SBR. A good dispersion of the layered silicate in the SBR matrix was reflected from the physical properties of the nanocomposites, especially in terms of tensile strength and high elongation properties. Using the same technique Malas et al. [29] prepared clay nanocomposites of EPDM by use of clay/XSBR masterbatch. The good dispersion of different nanoclay in the EPDM

Fig. 6. SAXS investigation (a), optical microscopic and AFM images of clay/XHNBR/SBR compound (b,c) and (clay/XHNBR)/SBR compound (d,e).

H.H. Le et al. / Polymer 54 (2013) 7009e7021

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Fig. 7. Optical microscopy of 5/100 CNT/CR compound (a) and 50/100 CNT/XHNBR masterbatch (b).

matrix was observed by wide-angle X-ray diffraction (WAXD) and high resolution transmission electron microscopy (TEM). In the present work the morphology of clay/SBR with XHNBR as dispersing agent can be predicted by means of our model by setting gSBR ¼ 24 mN/m [47], gxHNBR ¼ 36 mN/m and gF ¼ 32 mN/m into Eqs. (12)e(14) as shown in Fig. 4a. With nSBR/xHNBR ¼ 1 clay is entirely localized in the XHNBR phase because of the better fillere rubber interaction. When the amount of XHNBR is reducing as it is used as dispersing agent, the clay migrates to the SBR matrix. However, even at nSBR/xHNBR ¼ 9 almost clay still stays in the phase of dispersing agent as illustrated in Fig. 4b. Thus, in this case, the fillererubber affinity dominates the effect of low amount of dispersing agent. Using the wetting concept, clay loading in dispersing agent and matrix during mixing was determined and presented in Fig. 5. It is clear that when clay is mixed into 10/90 XHNBR/SBR blend, almost clay migrates to the XHNBR phase within 5 min (Fig. 5a). When clay/XHNBR masterbatch is mixed with the fresh SBR, clay is entirely remained in XHNBR (Fig. 5b). Regarding the uneven distribution of clay in heterogeneous polymer blends different works showed that clay preferentially resides in that blend phase having better affinity to clay [59e62]. If clay shows the same affinity to both blend phases it concentrates dominantly at the interphase [63e65].

Fig. 8. Master curve of filler localization in XHNBR/CR blend.

The SAXS analysis in Fig. 6a shows a broad peak representing the interlayer spacing of nanoclay. The data of the clay provider as well as own investigations show that the organoclay used has a basal spacing of 2.0 nm before compounding. When clay/XHNBR masterbatch was mixed with the fresh SBR, the interlayer spacing peak disappears indicating an entire exfoliation of clay in (clay/XHNBR)/ SBR composite. During the clay/XHNBR masterbatch preparation the presence of the carboxyl groups in XHNBR strongly enhanced the rubberefiller interaction as well as the intercalation and

Fig. 9. CNT loading in XHNBR/CR blends using statistic mixing (a) and masterbatch mixing (b).

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H.H. Le et al. / Polymer 54 (2013) 7009e7021

Fig. 10. Optical microscopic and AFM as well as TEM images of CNT/XHNBR/CR compound (a,b,c) and (CNT/XHNBR)/CR compound (d,e,f).

exfoliation process [53,54]. The break-up process of larger agglomerates rapidly establishes permanently a new clay surface that facilitates the diffusion process of rubber chains into the clay galleries. The wetting of clay surface by rubber molecules and their intercalation as well as the exfoliation process is intensified. As a result mixing of clay/XHNBR masterbatch into SBR leads to a composite with an exfoliation structure of clay. In contrast, when clay is mixed into 10/90 XHNBR/SBR blend a broad peak positioned at q ¼ 1.5 nm1 indicating an interlayer spacing of 4.18 nm is observed. The intercalation of clay in clay/XHNBR/SBR composite is owing to the fact that XHNBR encapsulates clay very fast according to Fig. 5a forming clay/XHNBR domains distributed in the SBR

matrix. Because of the poor compatibility between SBR and XHNBR the transfer of shear forces from the matrix to clay is not effective. Consequently, nanoclay tactoids are intercalated but not exfoliated inside the XHNBR domains. Optical microscopic and AFM images in Fig. 6 show clearly clay/ XHNBR domains distributed in the SBR matrix. Domain size of about 20 mm is observed in clay/XHNBR/SBR compound, while (clay/XHNBR)/SBR compound shows domains with a diameter smaller than 2 mm. The large difference in domain sizes of both compounds is related to the well-known compatibilizing effect of clay as observed in literature [59e65] and our previous work for clay/HNBR/NR blends [66]. The compatibilizing effect of clay

Fig. 11. Online conductance (a) and optical microscopic images (b,d) as well as TEM images (c,e) of CNT/CR compounds with and without BMI.

H.H. Le et al. / Polymer 54 (2013) 7009e7021

Fig. 12. CNT surface fraction wetted by BMI predicted in dependence on blend ratio nCR/BMI.

becomes highly effective, if clay is fully exfoliated as seen in (clay/ XHNBR)/SBR compound.

4.3. CNT dispersion in CR with XHNBR as dispersing agent Polychloroprene (CR) Baypren 611 (Lanxess, Germany) with Mooney viscosity MU ((ML 1 þ 4) 100  C) of 43  6 was used as rubber matrix. Carboxylated hydrogenated nitrile butadiene rubber (XHNBR) Therban XT VP KA 8889 (Lanxess) was used as dispersing agent. Multi-walled carbon nanotubes Nanocyl TM NC7000 (Nanocyl S.A., Belgium) was used as filler.

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The macrodispersion of 5/100 CNT/CR compound and 50/100 CNT/XHNBR masterbatch is investigated by examining optical microscopic images shown in Fig. 7. Very large CNT agglomerates are visualized in CNT/CR compound (Fig. 7a), while CNTs are well dispersed in XHNBR (Fig. 7b). Poor dispersion of unmodified CNTs in CR was also found by Das et al. [15] and Subramaniam et al. [17]. Lu et al. [67] found also very good dispersion of CNTs in XHNBR matrix using field emission scanning electron microscopy (FESEM). Verge et al. [68] highlights elements evidencing a possibility for NBR polymer chains to react by a free-radical mechanism and to graft onto CNT surface along the process of mechanical blending of NBR with CNTs. It comes out that the polymer grafting rate onto the CNT surface increases with the nitrile content in NBR. Interestingly, the nanotubes proved more finely dispersed in NBR containing higher nitrile content as evidenced by morphological observations as well as electrical measurements. By preparation of nanocomposites based on CNT/masterbatch and CR a question is now arising, where CNTs is localized in such a heterogeneous rubber matrix. Concerning this issue comprehensive knowledge on structure formation and mechanical as well as electrical properties of CNT filled thermoplastic/thermoplastic blends has been achieved [69e74], however, it is still incomplete in the field of rubber/rubber blends so far [75e79]. Using surface tension data gCR ¼ 35 mN/m, gxHNBR ¼ 36 mN/m and gF ¼ 30 mN/m experimentally determined in the present work the master curve of filler localization of CNTs in XHNBR/CR blend can be created and is present in Fig. 8. With nCR/xHNBR ¼ 1 a CNT fraction of 0.35 is predicted to be localized in the XHNBR phase, and 0.65 in the CR phase. When the amount of XHNBR is reduced, more and more CNTs migrate from the XHNBR to CR phase and at nCR/xHNBR ¼ 10 a CNT fraction of 0.07 is predicted to be localized in the XHNBR phase, and 0.93 in the CR phase.

Fig. 13. Wetting kinetics of CNTs by BMI and CR (a) and EDX spectrogram of rubberefiller gel (b) as well as SEM image (c) of CNT/CR compound with BMI.

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Fig. 14. Online conductance and images made by optical microscopy and TEM of CNT filled SBR/IR blends in dependence on ethanol/CNT ratio.

The CNT loading in XHNBR and CR phase was determined experimentally by the wetting concept is presented in Fig. 9. When CNTs are statistically mixed into 10/100 XHNBR/CR blend, the CNT loading in both phases increases slowly and reaches a plateau value after 30 min mixing (Fig. 9a). The plateau values of CNTs in both phases are tolerably corresponding to the equilibrium values predicted from Fig. 8. In Fig. 9b the CNT loading in XHNBR and CR phase is evaluated upon adding CNT/XHNBR masterbatch into the fresh CR. CNT loading in XHNBR is decreased and in CR increased. After 100 min mixing a loading of 1 phr CNTs was found in the CR phase that indicates a slow transfer process of CNTs from the dispersing agent to the matrix taking place. The very slow transfer of CNTs from the XHNBR to CR phase is owing to the very high viscosity of CNT/XHNBR masterbatch containing 50 phr CNTs. A linear extrapolation to the predicted values reveals an unrealistic mixing time of 1011 min being needed for the compound to reach the thermodynamic equilibrium state. Fig. 10 shows the morphology of CNT/XHNBR/CR and (CNT/ XHNBR)/CR compound at different magnifications. An addition of -1

30

ethanol SBR/IR-7m SBR/IR-22m SBR/IR-50m gel of SBR/IR-7m

0,36

0.1 25

0,34 0,32 0,30

1E-3 20 1E-5 15

1E-7

10

1E-9

5

1050

1000

950

Wave length, cm-1 Fig. 15. FTIR spectral of ethanol and CNT filled SBR/IR blends prepared by wet mixing process with different mixing time as well as fillererubber gel of SBR/IR-7m.

1E-11

0

Ofline conductivity, S/cm

Absorbance, a.u.

Polychloroprene (CR) Baypren 611 (Lanxess, Germany) was used as rubber matrix. Ionic liquid BMI, which is basically made of an asymmetric heterocyclic cation 1-butyl 3-methyl imidazolium (BMIþ) and an anion bis(trifluoromethyl-sulphonyl)imide (BMI)

1045 cm (C-O stretching)

0,38

0,28 1100

4.4. CNT/CR nanocomposites with ionic liquid as dispersing agent

Dispersion A/A0

0,40

CNTs into 10/100 XHNBR/CR blend leads to a poor filler macrodispersion in CNT/XHNBR/CR compound as seen in Fig. 10a. According to Fig. 9a about 3.7 phr CNTs are wetted by the CR matrix, however, CR is not able to disperse CNTs well. The other CNT amount of 1.3 phr migrates to the XHNBR phase and is trapped in XHNBR domains. Due to the poor shear force transfer from the matrix to trapped filler, CNTs are visible microscopically as large agglomerates inside XHNBR domains (Fig. 10c,d). In contrast, (CNT/ XHNBR)/CR compound presents a very good macrodispersion without any CNT agglomerates in optical microscopic image (Fig. 10d). In microscopic level, AFM and TEM images (Fig. 10e,f) show a number of small CNT/XHNBR domains distributed in the CR matrix.

1E-13 1

2

3

4

5

Ethanol/CNT ratio Fig. 16. Macrodispersion and offline conductivity of CNT filled SBR/IR blends in dependence on ethanol/CNT ratio.

H.H. Le et al. / Polymer 54 (2013) 7009e7021

(Sigma Aldrich, Germany) was used as dispersing agent. According to the chemical structure of BMI a mass ratio BMIþ/BMI of 139/280 was calculated. Multi-walled carbon nanotubes Baytubes C150HP (Bayer MaterialScience, Germany) was used as filler. For convenient admixing CNTs into the mixing chamber a filler loading of 5 phr was softly ground with 10 phr BMI, till a black paste BMI/CNT was obtained. The composites were prepared in an internal mixer by keeping the following mixing conditions: initial chamber temperature TA of 25  C, rotor speed of 70 rpm, fill factor of 0.72. The black paste BMI/CNT was admixed into the chamber at 3 min mixing time. In Fig. 11a the online conductance of CNT/CR composites without and with BMI is presented in dependence on mixing time. They show a typical conductanceetime 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 [75] 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. Upon tGmax the online conductance decreases slightly that is related to the better distribution of small aggregates throughout the matrix as discussed previously [75]. For the composite without BMI the tonset was not observed even till a mixing time of up to 200 min. With addition of BMI the online conductance of composite increases faster and reaches a tGmax at about 55 min. The macrodispersion of filler in the CR matrix is studied by optical microscopy images. In the images of unmodified composite shown in Fig. 11d,e large filler agglomerates are still visible even at a very long mixing time. The addition of BMI leads to a significant improvement of dispersion as seen in the images shown in Fig. 11b,c. The better macrodispersion of CNTs by addition of BMI is attributed to the physical cation-p interaction between BMIþ and the tubes and/ or the perturbation of pep stacking of multi-walls of the tubes as discussed in literature [12,13]. The offline conductivity at tGmax of 7.3  104 S/cm is received for the modified composite, while the unmodified composite shows a conductivity of only 109 S/cm all over the mixing time. On the basis of our proposed model the CNT surface fraction wetted by CR and BMI in composite at a thermodynamic equilibrium state can be predicted using Eqs. (12)e(14) with gCR ¼ 35 mN/ m, gF ¼ 28.5 mN/m for Baytubes and gBMI ¼ 33.6 mN/m [80]. 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. 12. Fitting the surface tension of Baytubes into the master curve with nCR/BMI ¼ 1, a CNT surface fraction SBMI of F 0.72 wetted by BMI was found at a thermodynamic equilibrium state. In this case, the selective wetting behavior of CNTs is merely dependent on the fillerepolymer affinity, and the affinity of CNTs to BMI is better than to CR. For nCR/BMI ¼ 10 as used in the present work, a CNT surface fraction SBMI of 0.07 was found. It is much F smaller compared to that wetted by CR, although the affinity of CNTs to BMI is better than to CR. The experimental characterization of the selective wetting behavior of CNTs by CR and BMI was done by means of FTIR of the extracted parts [81]. The rubber layer L and its contribution LBMIþ and LCR are presented in Fig. 13a in dependence on mixing time. It is obvious that in the first mixing period up to 50 min both LBMIþ and LCR increase. In this range BMI and CR concurrently infiltrate the CNT aggregates and wet CNT surface. After the wetting process is complete, LBMIþ 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 BMIþ on the CNT surface. At 120 min mixing time BMIþ is completely

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replaced by CR that is corresponding to the prediction made by the Z-model. The EDX spectrogram of the fillerepolymer gel of composite after 120 min mixing time is shown in Fig. 13b. No signals of fluorine at 0.67 keV and nitrogen at 0.39 keV of BMI and BMIþ, 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 the FTIR analysis. The unbound part of BMI forms an own phase as seen in SEM image (Fig. 13c). A closer look at the BMI phase does not reveal any CNTs. Thus, BMI clearly act as a dispersing agent, while its role as a coupling agent is strongly dependent on the mixing time. The highest BMI concentration bonded to CNT surface is reached at 50 min, thereafter it decreases and reaches zero at 120 min mixing time. 4.5. Ethanol as dispersing agent in rubber nanocomposites The wet mixing of CNTs into polymer matrix in presence of ethanol was carried out by Das et al. [16] and Bal et al. [82]. It promises an effective way to receive well dispersed nanocomposites. In this part, we investigated the morphology development of CNT filled SBR/IR blends during the mixing process in internal mixer assisted by ethanol. Solution styrene butadiene rubber (S-SBR) SPRINTAN SLR-4601 (Styron Deutschland GmbH) and synthetic polyisoprene (IR) Cariflex JR 309 (Shell Chemical Co.) were used as rubber matrix. Multi-walled carbon nanotubes (CNTs) Nanocyl TM NC7000 (Nanocyl S.A., Belgium) was used as filler. For preparation of 75/25 SBR/IR blends filled with 5 phr CNTs an internal mixer (Rheocord 300p, ThermoHaake) was used by keeping the following mixing conditions: initial chamber temperature of 50  C, rotor speed of 75 rpm, fill factor of 0.68. CNTs were first wetted with a certain amount ethanol to a paste. Then, it was added into the mixing chamber with rubber for preparation of the composites. In order to determine the suitable amount of ethanol the online conductance was recorded by adding different ethanol/CNT ratios into SBR/IR blends. Up to an ethanol/CNT ratio of 3.0 the conductivity signal is still very weak as shown in Fig. 14. At a ratio of 3.7 the online conductance curve was recorded at a high level. They show a typical conductanceetime characteristics with tonset and tGmax. Along the mixing process samples of SBR/IR blends with an ethanol/CNT ratio of 3.7 were taken out at 7 min, 22 min and 50 min for further investigation. The images shown in Fig. 14 reveal that the dispersion of SBR/IR blends is corresponding well to the development of the online conductance. On the image of SBR/IR7m taken out at 7 min a number of large agglomerates of CNTs are visible. The macrodispersion A/A0 of CNTs is 20%. The largest change of the size of CNT agglomerates is clearly observed in the range between tonset and tGmax. In this range the macordispersion decreases from 20% to 1.8%. Upon tGmax only some small agglomerates are observed. At 50 min mixing time SBR/IR-50m shows a dispersion value lower than 1.0%. A very good dispersion of CNTs at microscopic level is also evidenced by TEM image with individual tubes. At lower ethanol/CNT ratio of 2.2 or 3.0, on the images made by optical microscopy and TEM some large CNT agglomerates are clearly observed. The better filler dispersion obtained is related to the fact that ethanol can loosen the CNT bundles and helps to deagglomerate nanotubes [82e85]. The physical background of the rapid improvement of CNT dispersion within the ethanol/CNT ratio between 3.0 and 3.7 is the focus of our further investigation. The localization of ethanol remaining in SBR/IR blends was characterized by FTIR. In Fig. 15 FTIR spectra of ethanol, SBR/IR blends taken out at 7 min, 22 min and 50 min mixing time as well as gel of SBR/IR-7m are presented. The peak at 1045 cm1 identified as CeO stretching of ethanol [45] is observed for SBR/IR-7m with a

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high intensity indicating a high amount of ethanol remaining in this blend. During the mixing ethanol is evaporated, thus, the peak intensity decreases with mixing time as observed for SBR/IR-22m. After 50 min mixing SBR/IR-50m does not show any ethanol peak, i.e ethanol is completely evaporated. The FTIR spectrum of the rubberefiller gel of SBR/IR-7m shows also no trace of ethanol. That is an evidence that ethanol found in SBR/IR blends is not bound to the CNT surface, and it was entirely extracted from the blend by cyclohexan. In Fig. 16 the filler dispersion A/A0 and the offline conductivity of SBR/IR blends are presented with increasing ethanol/CNT ratio. Ethanol shows a strong effect on the dispersion A/A0, which decreases strongly when ethanol/CNT ratio increases from 2.6 to 3.7. The offline conductivity of SBR/IR blends is correspondingly dependent on the ethanol/CNT ratio. Up to an ethanol/CNT ratio of 2.6 SBR/IR blends exhibit a very low conductivity value of 2.7  1012 S/cm. Afterwards it increases strongly and reaches a value of 1.0  101 S/cm at a ratio of 3.7. The result of offline conductivity measurement correlates well with the online conductance values presented in Fig. 14. 5. Conclusions In the present work, carboxylated NBR, ionic liquid and ethanol were used as dispersing agent for effective mixing of nanofillers, nanoclay and CNTs, in rubbers in order to prepare nanocomposites. The development of morphology and the localization of the dispersing agent in nanocomposites were characterized qualitatively by means of the optical microscopy, TEM and AFM and quantitatively by means of the wetting concept. It was found that the dispersing agents used show very good effect on the filler dispersion. The selective wetting of filler surface by dispersing agent and by rubber matrix is controlled by thermodynamic and kinetic factors. A model using surface energy data of polymer components (rubber and dispersing agent) and filler was introduced in order to determine the thermodynamic equilibrium state of filler wetting, which is found to be simultaneously determined by the fillerepolymer affinity and the dispersing agent/matrix mass ratio. During the mixing process a replacement process of bound polymer components takes place on the filler surface until the predicted state is reached. Acknowledgment The authors wish to thank the German Research Foundation (DFG) (Project Nr. LE 3202/1-1) for the financial support. We also thank Mrs. Cornelia Becker and Mr. Werner Lebek (Martin Luther University Halle Wittenberg) for EDX analysis. References

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