Physico-Mechanical, Bound Rubber and Swelling Properties of MWCNT Reinforced Chlorobutyl

Mechanical, dynamic mechanical relaxation and electrical conductivity studies of chlorobutyl nanocomposites

S. K. Tiwari1, B. P. Sahoo2 and S. P. Mahapatra1 1. Department of Chemistry, National Institute of Technology Raipur 492010 India. 2. Rubber Technology Centre, Indian Institute of Technology Kharagpur 721302 India. Abstract The effect of multiwalled carbon nanotube (MWCNT) on the physico-mechanical, dynamic mechanical and electrical properties have been investigated by various characterization techniques. The morphology of nanocomposite samples has been studied from scanning electron microscopy (SEM). The physico mechanical properties of nanocomposites were studied with variation of MWCNT loadings, which revealed that tensile strength, hardness and modulus increases while elongation at break decrease with MWCNT loadings. But at higher filler loading the rate of increase slowly decreases. The bound rubber content (Bdr) of nanocomposites, was found to increase significantly with increasing MWCNT content. Dynamic mechanical properties of nanocomposites have been studied as a function of temperature (-100 to 100oC) at a constant frequency 1Hz and strain 1%. The effect of MWCNT loading on storage modulus, loss modulus, and loss tangent has been studied. The non-linearity in tan delta, storage modulus and loss modulus was explained on the basis of MWCNT-elastomer interaction. The smooth cole-cole plots explain non-linearity in the nanocomposites as well as good dispersion of MWCNT in the chlorobutyl matrix. In addition to this, electrical conductivity has been investigated as a function of frequency (100 to 106 Hz) at different MWCNT loading. The percolation threshold was studied by electrical conductivity measurements and occurred in the vicinity of 6 phr (parts per hundred rubbers) of MWCNT loading.

*Author for Correspondence: Dr. S. P. Mahapatra, E mail: [email protected], [email protected]

Keywords: elastomer, nanocomposite, mechanical, dynamic mechanical, conductivity.

1 INTRODUCTION Conductive polymer composites (CPCs) consist of highly conductive filler and insulating polymer matrix have received a considerable amount of research attention because their characteristic properties of possible applications. A significant part of investigations in this field concern carbon-based composites, using carbon nanotubes (CNTs) [1], graphite [2, 3], carbon black [4] as filler. These composites find applications in electronics and antistatic devices, electro-magnetic shielding, recording media, thermistors, polymer electrolyte membrane fuel cells, gas sensing, etc. [5]. With the development of CNT, many recent researches have been done on the CPCs with CNTs as conductive filler to achieve a low percolation threshold concentration and high electrical conductivity [6, 7]. Researches on carbon nanotubes have increased significantly over the past decade, because besides imparting outstanding mechanical properties, nanotubes also impart distinctive conductivity, which can be used in applications that involve light weight and flexible conductive or semiconductive polymeric composites. Studies have shown that in these nanocomposites, carbon nanotubes exhibit the electrical capability of acting as metallic- like conductors or having characteristics of a semiconductor depending on the chirality of the graphite lattice [8]. The most fundamental feature of the reinforcement of rubbers by filler is the size of the filler particles. The effect of type of fillers on the physico- mechanical properties and reinforcement of elastomer has been studied by many researchers [8-10]. Tripathy et al. [12] have reported the effects of types of carbon black on physical properties of elastomer in bromobutyl vulcanizates. There are some studies on the applicability of carbon nanotubes as reinforcing fillers in rubbers, such as natural rubber [10,11] styrene butadiene rubber [13,], silicone rubber [14] fluoro elastomer [15] The present study deals with the physico mechanical, dynamic mechanical and electrical properties of chlorobutyl elastomer nanocomposites. The dispersion of MWCNT in elastomer matrix is observed from SEM photomicrographs. The effect of MWCNT loading on physico mechanical parameters like tensile strength, elongation at break, modulus and hardness has been studied. The variation of the Bdr content of MWCNT filled chlorobutyl elastomer compounds was studied with the storage time using trichloroethylene, chloroform and benzene as solvent. The effect of MWCNT loading on visco-elastic parameters like storage modulus, loss modulus and loss tangent have also

2 been studied in the temperature range -100 to 100 oC at a frequency of 1 Hz and with 1% strain. Besides, the effect of MWCNT loading on electrical conductivity as a function of frequency (100 to 106 Hz) has also been studied. EXPERIMENTAL Materials

Cholorobutyl elastomer (CIIR, 1.25% chlorine content, Mooney viscosity ML 1+8 at 100°C =38) was procured from Bayer. Multiwalled carbon nanotubes (MWCNT) of purity  95% were purchased from Nanoshel LLC, USA. Before usage, the nanotubes were treated with acid mixture under ultrasonication to remove amorphous carbon and metallic impurities. Other chemicals like Zinc oxide of specific gravity of 5.4, Tetra Methyl Thiouram Disulphide (TMTD) having specific gravity 1.42 and compounding ingredients like sulfur, stearic acid were of chemically pure grade procured from standard suppliers. Compounding and sample preparation The rubber was compounded with the ingredients according to the formulations of the mixes (Table 1). Compounding was done in a brabender plastograph at 60 rpm followed by laboratory size (325x150 mm) mixing mill at a friction ratio of 1:1.25 according to ASTM D 3182 standards while carefully controlling the temperature, nip gap, time of mixing, and uniform cutting operation. The temperature range for mixing was 65–70°C. After mixing the elastomer compositions were molded in an electrically heated Moore Hydraulic Press at a pressure of 10 MPa and at 160 oC using moulding conditions determined by Monsanto Rheometer (R-100) according to ASTM D2084 and ASTM D5289 procedures. Testing Mechanical Studies Hardness of chlorobutyl elastomer nanocomposites were measured using a Shore-A Durometer as per ASTM D 676-59T. Mechanical properties like modulus, tensile strength and elongation at break according to ASTM D 412 procedure using dumbbell specimen were determined using a Hounsfield H10KS Universal Testing Machine. At least five specimens per sample were tested for each property and mean values are reported.

3 Visco-Elastic Studies Visco-elastic parameters were obtained using TA instrument, dynamic mechanical testing analyzer, over a temperature range of -100 to 100 oC, at a frequency of 1 Hz and at 1% strain. Scanning Electron Microscopy The morphology of the compound has been studied by using Scanning Electron Microscope (SEM). Prior to SEM studies, the sample surface was sputter coated with gold. Bound Rubber Measurement The bound rubber contents (Bdr) were determined by extracting the unbound materials such as the ingredients and free rubber with three different solvents for 7 days followed by drying for 2 days at room temperature. The weights of the samples before and after extraction were measured, and the Bdr contents were calculated using the following expression:

  m1  W fg W1   m1  mr  Bdr  100   (1)  mr  W1   m1  mr 

Where Bdr is the bound rubber content, W fg is the weight of filler and gel, W1 is the weight of sample, m1 is the fraction of filler in the compound and mr is the fraction of rubber in the compound. Electrical conductivity Studies The electrical properties of the conductive chlorobutyl elastomer nanocomposites were obtained using a computer-controlled impedance analyzer(LCR meter ) PSM 1735 on application of an alternating electric field across the sample cell with a aluminum foil (blocking electrode) in the frequency range of 100 Hz to 1MHz . The parameters like dielectric permittivity (ε') and dielectric loss tangent (tan δ) were obtained as a function of frequency. The AC conductivity (σac) was calculated from the dielectric data using the relation:

sac = we0 etan d (2)

4 where w is 2p f ( f is frequency), e 0 is permittivity of a vacuum and e is dielectric constant or relative permittivity and is calculated by expression: C e = p (3) C0 where Cp is the observed capacitance of the sample and C0 is vacuum capacitance of the cell and is calculated using the expression: e A C = 0 (4) 0 d Where A is area of the sample and d is thickness of the sample and tand is the dielectric loss. RESULTS AND DISCUSSION Mechanical Studies Mechanical properties like tensile strength, elongation at break, modulus and hardness of the nanocomposites are shown in Table 2. From the table it is observed that Tensile strength, modulus (100%, 200%, 300%), hardness and thickness values increases with MWCNT loading, while elongation at break decreases. Figure 1 shows the stress-strain properties of elastomer with different MWCNT loading. The initial increase of stress is found to be slow for solid elastomer compared to that of MWCNT loaded elastomer, but after a certain strain, the stress increases steeply and with higher filler loading stress increases with strain. The effect of incorporation of a MWCNT (be it a reinforcing filler or not) on the mechanical properties of elastomeric materials can be partially explained as follows: If particles of high elastic-modulus are dispersed through a low elastic-modulus matrix, it is obvious that the modulus of the mixture will be higher than that of the matrix, mainly because of the decrease in volume content of matrix substance. However, for a filler to be really reinforcing in nature, many other factors come into the picture, the most important being the force of adhesion of the matrix to the particle. If the adhesion between the filler and the polymer matrix is very low then no significant increase in modulus (except due to hydrodynamic effect) can be observed. But, if the particle-to-matrix adhesive force is large, then the modulus of the mixture is primarily determined by the magnitude of the polymer filler interactions. Increase in moduli is due to MWCNT-rubber rigid interactions which are confirmed by bound rubber measurement. At 8 phr filler

5 loading chlorobutyl gives the highest value of tensile strength, which means that at 8 phr loading polymer-filler interaction is maximum, but rate of increase decreases above 6 phr MWCNT loading as filler-filler interaction predominates. Unlike conventional filler, MWCNTs have very high surface area and this complex structure of the branched filler aggregates attributes to a strong surface polymer interaction leading to higher bound rubber content. Incorporation of fillers is a major source of energy dissipation thereby increasing the tensile strength of nanocomposites. On addition of filler like carbon nanotube, free space between the chains is filled up thus depriving the chains to straighten thereby reducing elongation. The higher the filler loading the more is the reinforcement and more crosslinks are formed during vulcanization, thereby trapping the free ends of polymer chains. As the degree of crosslinking increases, the hardness progressively increases. The more compact the networks, the shorter are the molecular segments between the crosslinks and hence the tighter is the network, which causes increase in hardness [16]. Very high value of hardness is due to highly reinforcing nanotube having very high surface area. Dynamic Mechanical Studies Figure 2 shows the variation of storage modulus (ε') as a function of temperature for different MWCNT loadings. The higher storage modulus with increase in MWCNT loading is due to formation of more filler network and also because of the decrease in volume content of base polymer. From the figure it is observed that upto 6 phr MWCNT loading storage modulus increases and at 8 phr loading it decreases. This can be attributed towards percolation threshold at 6 phr MWCNT. For a filler to be really reinforcing in nature, many other factors come into the picture, the most important being the adhesion of the matrix to the filler surface, which plays a crucial role. At low temperature the elastomer is in glassy state with high storage modulus, with increasing temperature the elastic modulus abruptly decreases down by 3 orders of magnitude corresponding to the glass- rubber transition. The relaxation process is of course related to an energy dissipation associated with the glass transition phenomenon of the rubber phase. It is also reflected in the corresponding relaxation process where loss tangent passes through a maximum. Figure 3 shows the loss tangent spectra of chlorobutyl nanocomposites as a function of temperature. From the figure it can be observed that the maximum value of loss tangent

(tan δmax) decreases with increase in MWCNT loading. This is due to chlorobutyl-MWCNT

6 interactions which reduce the chain mobility. However the said observation is marked upto 6 phr and above which at 8 phr again loss tangent increases may be due to filler-filler interactions which cause more loss. It is also observed that with MWCNT loading, loss in segmental mobility result an increase in Tg and most probably in a change of thermal expansion of the free volume [17]. And the magnitude of Tg shift is very marginal can be attributed to the sluggish nature of the polyisobutylene relaxation dynamics. The temperature dependence of loss modulus (ε") for various MWCNT loadings is shown in Figure 4. A distinct transition peak at -60 oC may be attributed to the motion of methyl group directly attached to the backbone of CIIR. Incorporation of MWCNT has some effect on the location and intensity but the amount of MWCNT does not affect the temperature at peak, however the intensity slightly increases with MWCNT loadings. Higher filler loadings result a percolated network of filler particles that can influence relaxation on a different scale. The percolation effect is usually considered to be effective for relaxation of longer time scales. Recent dynamic mechanical experiments for composite solids seem to indicate that restriction effects do infact result from the formation of a percolation network [18]. The relaxation dynamics can also be expressed in terms of cole-cole plots i.e. relationship between storage and loss modulus. Cole–Cole plots for multiwalled nanotube reinforced chlorobutyl nanocomposites are shown in Figure 5. Irrespective of MWCNT loading at all concentrations of the filler, the usual depressed semi-circular can be observed which clearly indicates the presence of a reinforcing element. The large change observed on the arc radius in the Cole–Cole plot indicates the relaxation dynamics and a significant alteration of chain conformation due to MWCNT interaction [19]. McLachlan et al. correlated the shape of Cole-Cole plots with the homogeneity of MWCNT dispersion in the polymer matrix. Irrespective of MWCNT concentrations, smooth arcs are observed with no humps indicating good dispersion of MWNT in the polymer matrix [20]. Representative SEM micrographs shown in Figure 6 taken at increasing filler concentration shows excellent distribution of MWCNT in the CIIR matrix. Scanning Electron Microscopy SEM photomicrographs of the razor-cut surfaces of CIIR nanocomposites (2, 4, 6 and 8 phr MWCNT) are presented in Figures 6 (a-d). A comparison of the surface morphology

7 shows a marked change in the surface properties at particular MWCNT loading (8 phr). SEM studies reveals that the dispersion of MWCNT is homogeneous in the polymer matrix but aggregation takes place as filler loading is increased, which is clearly observed from the SEM photomicrographs of samples containing 6 and 8 phr filler loading. This observation appears to be in good agreement with the results of mechanical properties. SEM micrographs shown in Figures 6a ,b, c and d at increasing MWCNT concentration shows good dispersion of MWCNT in the polymer matrix. Figures 7a, b, c and d show the fracture surfaces of the composite loaded with 2, 4, 6, 8 phr nanotube respectively. The fracture surface at lower MWCNT loading is also relatively smooth and featureless, while a pronounced roughness was observed in the case of higher loading composite. Figure 7d shows the fracture surface of the 8 phr MWCNT loaded sample at higher magnification. However it can be seen that the MWCNTs are well dispersed within the insulating elastomer matrix. Most of the fracture surfaces show spherical shaped dimples from pull out MWCNT fillers. There is no obvious phase separation observed, implying good miscibility between elastomer and MWCNT. At higher MWCNT loading agglomerates acted as foreign body and initiate cracks in the composites under stress and also reduce elongation at break. Although the size of agglomerates were big but sphere-like agglomerate has lessen their effect as stress concentrator in composites, so, did not show pronounced effect on tensile properties and enhance the modulus. However, high amount of MWCNT nanoparticles dispersed in elastomer matrix significantly increases the hardness and decreases the impact strength of the composites. Bound Rubber Content Bound rubber can be defined as the rubber portion of uncured compound, which cannot be extracted by a good solvent due to adsorption of rubber molecules on to the filler surface. Bound rubber measurement plays an important role in determination of surface activity of the filler and the degree of reinforcement. It is widely accepted that the formation of bound rubber in a compound involves physical adsorption, chemisorptions and mechanical interaction of which chemisorptions is considered as the crucial one. The adsorption of polymer molecules onto the filler surface leads to two phenomena, which are: the formation of bound rubber and a rubber shell on the filler surface. Many studies have been carried out on the mechanisms and factors affecting the formation of bound rubber [21–

8 24]. The physico-chemical characteristics of the filler surface and filler morphology have a profound effect on the bound rubber content in a compound. The variation of bound rubber content with MWCNT loading has been studied with three different solvents like benzene, trichloroethylene, and chloroform. Figure 8-10 shows effect of MWCNT loadings on the variation of the bound rubber content of chlorobutyl elastomer nanocomposites as a function of the storage time in trichloroethylene, chloroform and benzene respectively. Irrespective of the nature of solvent, bound rubber content increases with MWCNT loading due to increase in degree of reinforcement. The high percentage of Bdr content is attributed to high surface area, high structure and high concentration of oxygen containing surface functional groups. High bound rubber values in sulfur added compounds have been reported earlier by Gessler [25]. During mixing of rubber and carbonaceous filler, free radicals are generated by mechanical breakdown, which is responsible for the higher Bdr content. For chlorobutyl compounds, he explained this phenomenon as a two-step theory: the reaction of sulfur with black and the reaction of this sulfur modified black with the polymer. The surface groups on carbon black react with sulfur and cure accelerators so that ≡CS radicals or ≡CX (X is SH or residues of a cure accelerator) are formed. The effect of type of solvent on the variation of the bound rubber content of chlorobutyl nanocomposites as a function of MWCNT loadings is shown in Figure 11. The higher bound rubber content is observed from benzene than chloroform and trichloroethylene. Electrical Conductivity Studies Figure 12 shows the variation of electrical conductivity with frequency at increasing MWCNT loadings in chlorobutyl nanocomposites. At any given frequency, with MWCNT loading a tremendous increase in conductivity can be observed. From the figure three different regions: frequency independent region (Region 1), exponential growth with increasing frequency (Region 2) and finally a plateau region (Region 3) are observed. The shift from Region 1 to Region 2 depends on the MWCNT concentration. In fact in 8 phr composites, Region 1 is completely absent. It is widely believed that electrical properties of reinforced polymers depend primarily on the way the filler particles are distributed through the polymer matrix also called the mesostructure. At low levels of filler loading, the conductivity of the composite is slightly higher than that of the base polymer, because the filler particles are isolated from each other by the insulating polymer matrix. When

9 concentration of MWCNT is low, the conductivity between the grains of filler is expected to be primarily via hopping and tunneling mechanisms. In this mode of conduction, the electron transport may be coupled strongly with the molecular and ionic processes in the polymer matrix. Usually, hopping transport between localized sites is the main reason for the frequency dependence of conductivity in polymer composites. Percolation Studies The variation of electrical conductivity and dielectric permittivity with MWCNT loadings in chlorobutyl elastomer nanocomposites is shown in Figure 13 and 14 respectively. The electrical conductivity and dielectric permittivity of a composite is generally characterized by its dependence on volume fraction of filler. It can be observed at all frequencies (102,103, 104,105 and 106 Hz), above 6 phr MWCNT loading there is an abrupt increase in parameter implying the occurrence of a percolation limit. Figure 15 also shows the effect of filler loading on the conductivity of MWCNT reinforced chlorobutyl elastomer nanocomposites at 104 Hz frequency. At low level of MWCNT loading, the conductivity of the CIIR composite is slightly higher than that of the base polymer as the filler particles are isolated from each other by the insulating polymer matrix. As the filler loading is increased, mutual contacts between the filler particles are developed and at a critical loading of the filler a sharp increase in conductivity is observed, indicating the “percolation limit”. In case of MWCNT reinforced chlorobutyl elastomer nanocomposites, percolation limit has been observed at around 6 phr filler loading, which depends on the filler characteristics (i.e., surface area, surface activity, particle size, etc). According to Medalia [26] percolation is due to the tunneling of electrons and the conductivity is controlled by the gaps between the carbon black aggregates. Interpretation of dielectric and conductivity performance of such materials has been analyzed through percolation theory [27]. According to Das et al. [28] the variation of conductivity with filler loading can be divided into three regions (Figure 14). (i) inductive region: a small increase in conductivity of the composites with increase in filler loading which is due to the transportation of the small number of charged particles through the system without having any continuous conductive network; (ii) percolation region: the conductivity sharply increases due to the formation of continuous conductive path in the polymer matrix; (iii) saturation region: the marginal effect on σac due to further addition of conducting filler.

10 CONCLUSIONS Morphology of the chlorobutyl elastomer nanocomposite shows good dispersion of MWCNT and formation of agglomerates at higher MWCNT loading. Mechanical properties like tensile strength increases with MWCNT loading up to 8 phr but rate of increase decreases after 6 phr, can be attributed towards percolation threshold at 6 phr MWCNT in chlorobutyl elastomer. Elongation at break gradually decreases with MWCNT loading due to reinforcing nature of MWCNT loading. With MWCNT loading hardness of nancomposite sample gradually increases, due to increase in crosslink density. The Bdr contents of chlorobutyl nanocomposite were studied in three different solvents. Irrespective of the solvent the bound rubber content increases with increase in the MWCNT loading. Very high value of bound rubber is due to high structure and high surface area of carbon nanotube. Solvent with more interaction with the polymer tends to show lower Bdr values. Dynamic mechanical studies show increase in storage modulus / stiffness and decrease in loss tangent spectra occurs with MWCNT loading upto 6 phr due to formation of filler network, decrease in volume fraction of CIIR and reduction of chain mobility. The temperature dependent loss modulus shows a marginal effect on location and intensity, but does not change the temperature of the peak. The complete semicircular cole- cole plots confirm the non-linear/non-debye type of relaxation in CIIR nanocomposite. Irrespective of MWCNT loading smoothness of the curve indicates well dispersion of MWCNT in chlorobutyl matrix. Electrical conductivity increases exponentially with frequency and also increases with MWCNT loading. From the conductivity studies percolation limit of MWCNT has been found to be 6 phr, which is a good agreement with the physico-mechanical studies.

11 References

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5893-5899. 7 Zhang, Q,; Rastogi, S.; Chen, D.; Lippits, D.; Lemstra, P. Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique. Carbon 2006, 44,778-785. 8 Boonstra, B.B. Mixing of carbon black and polymer: Interaction and reinforcement. J. Appl. Polym. Sci 1967, 11, 389-406. 9 Dutta, N.K.; Tripathy, D.K. Effect of plasticizer concentration on the hysteresis, tear strength an stress relaxation characteristics of black-loaded rubber vulcanizate. Colloid and Polymer Science 1991, 269, 331–342. 10 Shanmugharaj, A.M.; Bae, J.H.; Lee Noh, K.Y.; Lee, W.H.; Ryu, S.H.; Comp, S.H. Physical and chemical characteristics of multiwalled carbon nanotubes functionalized with aminosilane and its influence on the properties of natural rubber.

composites Sci. Tech. 2007, 67,1813-1822.

12 11 Fakhrul-Razi, A.; Atieh, M.A.; Girun, N.; Chuah, T.G.; ,El- Sadiq M Biak, D.R.A. Effect of multi-walled carbon nanotubes on the mechanical properties of natural rubber. Comp. Stru, 2006, 75, 496-500. 12 Dutta, N.K.; and Tripathi, D.K. Effects of types of fillers on the molecular relaxation characteristics, dynamic mechanical, and physical properties of rubber vulcanizates. J. Appl. Polym. Sci. 1992, 44, 1635-1648. 13 Bokobza, L,; Diop, A.L.; Fournier, V.; Minne, J.P.; Bruneel, J.L. Spectroscopic investigations of polymer nanocomposites. Macro. Symp. 2005, 87 ,230. 14 Park, I.S,; Kim, K. J.; Nam, D.; Lee, J.; Yim, W. Mechanical, dielectric, and magnetic properties of the silicone elastomer with multi-walled carbon nanotubes as a nanofiller. Polym. Eng 2007, 47, 1396-1405. 15 Pham, T.T.; Sridhar, V.; Kim, J. K. Fluoroelastomer-MWNT nanocomposites-1: Dispersion, morphology, physico-mechanical, and thermal properties. Polym. Com 2009,30,121-130. 16 Chakraborty, S.K.; Bhowmick, A.K.; De, S.K. Mixed. cross-link systems in elastomers. Polymer Reviews 1981, 21, 313-332. 17 Bokobza, L. Multiwall carbon nanotube elastomeric composites: A review, 2007, 48, 4907. 18 Angell, C.A.; Ngai, K.L.; McKenna, G. B.; McMillan, P.F.; Martin, S.W. Relaxation in glass forming liquids and amorphous solids. J. Appl. Phys. 2000, 88, 3113-3157. 19 Valentini, L.; Amentano, I.; Biagotti, J.; Kenny, J.M.; Santucci, S. Frequency dependent electrical transport between conjugated polymer and single-walled Carbon nanotubes. Diamond & Related Materials 2003, 12, 1601. 20 McLachlan, D.S.; Chiteme, C.; Park, C.; Wise, K.E.; Lowther, S.E.; Lillehei, P.T.; Siochi, E.J.; Harrison, J.S. AC and DC percolative conductivity of single wall carbon nanotube polymer composites. J. Polym Sc: part B: Polym Phys 2005 43, 3273- 3287. 21 Stickney, P.B.; Falb, R.D. Carbon black-rubber interactions and bound rubber. Rubber Chemistry and Technology 1964, 37, 1299-1340.

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14 Table- 1 Formulation of unfilled and MWCNT reinforced nanocomposites

Ingredient CB1 CB2 CB3 CB4 CB5

Chlorobutyl Rubber 100 100 100 100 100

MWCNT(phr) 0 2 4 6 8

Each mix contains ZnO 5 phr, Stearic acid 2 phr, TMTD 1 phr and Sulphur 1.5 phr.

15 Table 2 Mechanical properties of unfilled and MWCNT Reinforced Chlorobutyl nanocomposites

Mix Thickness Hardness Tensile Elongation Modulus Modulus Modulus no. (mm) (shore A) strength at break (100%) (200%) (300%)

(Mpa) (%) MPa MPa MPa

CB1 2.32 32 5.73 513.5 1.2 2.3 3.63

CB2 2.38 35 6.75 374.6 1.29 3.0 5.3

CB3 2.50 39 7.89 359 1.65 3.85 6.1

CB4 2.58 40 8.53 327 1.95 4.3 7.1

CB5 2.64 43 8.73 315 2.1 4.6 8.1

Table 3 Bound rubber content as a function of MWCNT loading in chlorobutyl nanocomposites for different solvent after 15 days.

16 MWCNT Trichloroethylene Chloroform Benzene concentration

2 phr 38 45 55

4 phr 42 49 59

6 phr 46.5 52 63.4

8 phr 49.2 54.2 66.5

Figures

17 1 0 8 P h r 6 P h r 8 4 P h r 2 P h r )

2 0 P h r - 6 m m / N (

s 4 s e r t S

0 0 1 0 0 2 0 0 3 0 0 4 0 0 S t r a i n ( % )

Figure 1 Stress-strain plots of chlorobutyl elastomer nanocomposites.

4 10 1 0 0 0 0 0 p h r

1 0 0 0

3 10 1 0 0 ' 

2 10 '  1 - 1 0 0 - 8 0 - 6 0 - 4 0 - 2 0 0 2 0 4 0 6 0 8 0 1 0 0 T e m p e r a t u r e ( o c ) 0 p h r 1 10 2 p h r 4 p h r 6 p h r 8 p h r 0 10 -100 -80 -60 -40 -20 0 20 40 60 80 100 T e m p e r a t u r e , o C

Figure 2 Variation of storage modulus with temperature in chlorobutyl elastomer nanocomposites reinforced with MWCNT at increasing loadings.

18 1.4 0 phr 1.2 2 phr 4 phr 1.0 6 phr 8 phr 0.8 

n 0.6 a T 0.4

-100 -80 -60 -40 -20 0 20 40 60 80 100 Temperature, 0C

Figure 3 Variation of loss tangent (Tan δ) with temperature in chlorobutyl elastomer nanocomposites reinforced with MWCNT at increasing loadings.

104 0 phr 2 phr 103 4 phr 6 phr 8 phr 102 "  101

10-1 -100 -80 -60 -40 -20 0 20 40 60 80 100 Temperature, 0C

Figure 4 Variation of loss modulus with temperature in chlorobutyl elastomer nanocomposites reinforced with MWCNT at increasing loadings.

19 5000 0 phr 2 phr 4000 4 phr 6 phr 8 phr

3000 "  2000

1000 2000 3000 4000 5000 '

Figure 5 Cole-cole plots of chlorobutyl elastmer nanocomposites with increasing MWCNT loadings.

20 (a) 2 phr MWCNT (b) 4 phr MWCNT

(c) 6 phr MWCNT (d) 8 phr MWCNT

Figure 6 State of dispersion of MWCNT in chlorobutyl elastomer: (a) 2 phr MWCNT, (b) 4 phr MWCNT, (c) 6 phr MWCNT, and (d) 8 phr MWCNT.

21 (a) 2 phr MWCNT (b) 4 phr MWCNT

(c) 6 phr MWCNT (d) 8 phr MWCNT

Figure 7 Fractographs of (a) 2 phr (b) 4 phr (c) 6 phr and (d) 8 phr MWCNT loaded filler agglomerates initiate crack during tensile deformation.

) 50 % (

e 45 t n o C

40 r e b b

u 35 R

u 2 p h r o

B 4 p h r 25 6 p h r 8 p h r 20 0 15 30 45 60 S t o r a g e T i m e ( D a y s )

Figure 8 Effect of MWCNT loadings on the variation of the bound rubber content of chlorobutyl elastomer nanocomposites as a function of the storage time in trichloroethylene.

55 ) % (

t 50 n e t n o C

45 r e b b u R

40 d m

u 2 p h r o

B 35 4 p h r 6 p h r 8 p h r 30 0 15 30 45 60 S t o r a g e T i m e ( D a y s )

Figure 9 Effect of MWCNT loadings on the variation of the bound rubber content of chlorobutyl elastomer nanocomposites as a function of the storage time in chloroform.

65 ) % (

t 60 n e t n o C

55 r e b b u R

50 d m

u 2 p h r o

B 45 4 p h r 6 p h r 8 p h r 40 0 15 30 45 60 S t o r a g e T i m e ( D a y s )

Figure 10 Effect of MWCNT loadings on the variation of the bound rubber content of chlorobutyl elastomer nanocomposites as a function of the storage time in benzene.

60 ) % (

t 50 n o C

b 40 u R

o 30 B T r i c h l o r o - e t h y l e n e C h l o r o f o r m B e n z e n e 20 0 2 4 6 8 M W C N T C o n c e n t r a t i o n ( p h r )

Figure 11 Effect of solvent on the variation of the bound rubber content of chlorobutyl elastomer nanocomposites as a function of MWCNT loading.

24 1 E - 3

) 1 E - 4 m c / S (

1 E - 5 y t i v i t

c 1 E - 6 u d n o

c 1 E - 7

l a c i r t 1 E - 8 2 p h r c e

l 4 p h r E 6 p h r 1 E - 9 8 p h r

1 0 2 1 0 3 1 0 4 1 0 5 1 0 6 F r e q u e n c y ( H z )

Figure 12 Effect of MWCNT loadings on electrical conductivity (σ) of chlorobutyl elastomer nanocomposites measured as a function of frequency.

1 E - 3 ) m

c 1 E - 4 / S (

t 1 E - 5 i v i t c

u 1 E - 6 d n

C 1 E - 7 1 0 H z

c 1 0 H z i

t 1 E - 8

c 1 0 H z

e 5 l 1 0 H z E 1 E - 9 1 0 6 H z 2 4 6 8 M W C N T F i l l e r l o a d i g ( p h r )

Figure 13 Effect of MWCNT loading on electrical conductivity of chlorobutyl elastomer nanocomposites at different frequencies.

25 1 . 4 x 1 0 4 1 0 2 H z 3 1 . 2 x 1 0 4 1 0 H z 1 0 4 H z 4 1 . 0 x 1 0 1 0 5 H z y t i 6 v

i 3 1 0 H z t

i 8 . 0 x 1 0 m r

6 . 0 x 1 0 c i r t

e 4 . 0 x 1 0 l e i D 2 . 0 x 1 0 3

0 2 4 6 8 M W C N T F i l l e r l o a d i n g ( p h r )

Figure 14 Effect of MWCNT loading on dielectric permittivity of chlorobutyl elastomer nanocomposites at different frequencies.

1 E - 4 I I I - S a t u r a t i o n

) r e g i o n m c / 1 E - 5 S (

y t i v

i 1 E - 6 t I I - P e r c o l a t i o n r e g i o n c u d

n 1 E - 7

o  = P e r c o l a t i o n c r i t C

l T h r e s o l d a c

i 1 E - 8 r t I - I n d u c t i v e r e g i o n c e l

E 1 E - 9

2 4 6 8 M W C N T L o a d i n g ( p h r )

Figure 15 Variation in ac conductivity (σac ) with MWCNT filler loading in chlorobutyl elastomer nanocomposites at 104 Hz frequency.