Strong Strain Sensing Performance of Natural Rubber

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Strong strain sensing performance of natural rubber nanocomposites Tamil Selvan Natarajan, Eshwaran Subramani Bhagavatheswaran, Klaus Werner Stöckelhuber, Sven Wiessner, Petra Potschke, Gert Heinrich, and Amit Das ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13074 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Strong strain sensing performance of natural rubber nanocomposites Tamil Selvan Natarajana,b, Subramani Bhagavatheswaran Eshwarana,b, Klaus Werner Stöckelhubera, Sven Wießnera,b, Petra Pötschkea, Gert Heinricha,b, Amit Dasa,c*. a b

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany Technische Universität Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany c Technical University of Tampere, Korkeakoulunkatu 16, Fi-33101 Tampere, Finland

Abstract A detail study concerning the strain (tensile) dependent electrical conductivity of elastomeric composites is reported in this present paper. Multiwall carbon nanotubes (CNT), conducting carbon black (CB) and their combinations were considered as conducting filler in crosslinked natural rubber matrix. The loadings of the fillers were considered from 3 to 11 phr (filler concentration close to their percolation threshold). Without hindering the elastic nature of the composite (reversible stretchability up to several 100 %), the change of relative resistance, ∆R/R (∆R is the change in the resistance with respect to strain and R is the initial resistance of the sample) of the CB filled composites was found to be as much as ~1300 at around 120 % elongation. This value is much higher than any other reported values obtained from conducting polymeric composites. It was found that CNT offered a strong strain dependent character in the regime 100 % to 150% elongation, whereas, the carbon black filled natural rubber showed strong strain dependencies at 50 % to 100 % elongation strain. The combination of two different fillers could be exploited to tailor and manipulate the sensing operating regime from 50 % to 150 % strain depending on the ratios of the two filler system. Additionally, after several loadingunloading cycles, the conductivity of the sample was very stable for CB filled system but for CNT filled system the conductivity of the sample was altered. This type of elastic materials could be used in structural health monitoring, sensors in different dynamic elastomeric parts like tires, valves, gaskets, engine mounts etc. Key words: piezoresistivity; natural rubber; strain sensing; hybrid nanocomposites; carbon nanotubes; conductivity Corresponding author email: [email protected]

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Introduction Electromechanical properties of polymers have attracted a tremendous surge of interests among researchers over those of ceramics as materials due to light weight, less cost, flexibility and other advantages. In recent years the electromechanical properties and performance of smart polymers are exploited for electricity generation1, energy harvesting2, and for other application like actuators3-4, sensors5-6, transducers7-8 etc. Use of sensors have become very common in modern day life, as their application extend to damage sensing9, liquid sensing10, mass sensing11, motion detection12 artificial skin13 and many more. Stress or strain under loading in many structures need to be detected as they might lead to serious damage during service leading to breakdown or collapse of the entire structure. Thus sensing becomes more essential, when the structure is used to safe guard and transport humans or essential commodities. This has led to the concept of structural health monitoring. Several techniques are being applied for sensing the damage (health monitoring) based on the concepts of piezoelectricity14, piezoresistivity15, dielectrics16, and so on. The key advantages of using the piezoresistive elastomers over other techniques are, for example, higher sensitivity, ease to production, cost effectiveness, very high strain sensitivity (∆R/R) and very stable performance after prolonged and several times use. Piezoresistivity is the ability of a conducting material to cause a change in resistance when a mechanical force is applied17. In general piezoresistivity does not occur in conventional polymers, but can be developed by introduction of electrically conducting particles inside the polymer matrix18 or, by synthesizing intrinsically conductive polymers with desired conjugation or specific structure19. Former method is advantageous in terms of ease in preparation and processing than the latter method. Hence, the polymer composite method has been widely adapted. These kind of conducting polymer composites could be obtained by the incorporation of one or more inorganic fillers like Cu, Ni, Pb, SnO220 or carbon based fillers such as carbon black 21-22

, carbon nanotubes10, 23-29, graphite30 and graphene31-36 into the polymer matrices. However,

the use of metallic or inorganic fillers may lead to a deterioration of the mechanical properties. It is well known that the addition of conductive carbon based fillers to a polymer matrix can improve not only the electrical property but also various mechanical properties37-38. The conductivity of the composite depends on the filler concentration. Above the critical concentration φc, known as percolation threshold, a conducting filler pathway is formed in the


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composite and the polymer composite transformed from insulating to electrically conductive39. Based on literature survey, the percolation threshold depends on many influencing factors, like polymer type, filler type, state of dispersion achieved during mixing. In general the values can be found as ~10 wt. % for carbon fiber, ~20 wt.% for carbon black (CB), ~50 wt. % for graphite filled samples40. Further with the use of nanofillers like graphene and carbon nanotubes (CNTs), significantly lower percolation thresholds can also be achieved41-44. Due to the limitation of nanofillers due to their high cost and inferior processability; hybrid nanocomposites containing a combination of costly nanofillers with other cheap carbon based fillers have gained more attentions22, 25-28, 45-46. Piezoresisitivity of the polymer based nanocomposites mainly depends on the type of filler, the amount of filler as well as the type of the polymer and its molecular weight 22, 26, 47-48. The nature of piezoresistivity is found to be different for different strain directions. Knite et al.49 reported that positive piezoresistivity (PPR) can be observed for elongation strains and negative piezoresistivity (NPR) for compression strains while studying with the poly-isoprene nanocomposites. It is explained that the PPR was due to increase in distance between the conductive fillers on tensile strain and the NPR was due to the decrease in distance between the conductive fillers on compression. Qu et al.50 prepared expanded graphite reinforced maleic anhydride modified polyethylene and reported that the resistivity decreased with compressive stress of 10 MPa and reached a plateau at a higher compressive stress of 30 MPa and further resistivity increased beyond 30 MPa. Their results also showed that the resistivity decreased with time at mechanical stress less than 30 MPa and increased for stress of more than 30 MPa. Piezoresistivity also depends on the amount of fillers used. For example, Knite et al.51 reported that the higher sensitivity of polyisoprene/CNTs nanocomposite occurred on the percolation threshold and also found that the sensitivity was stable within the range of 20° to 60°C. Chen et al.52 prepared piezoresistive samples for finger sensing applications with silicone rubber and graphite nanoplatelets (GN) and found that the nanocomposite with only 1.36 vol% of GN exhibits a supersensitive and higher PPR under very low pressure ( in the finger-pressure range) as compared with highly filled nanocomposites. Use of different types of fillers, has led to the major difference in piezoresistive characteristics. For example, Shui et al.6 prepared a conductive composite based on polyether sulphone (PES) with short carbon fibers of 10 µm and filaments of 0.1 µm diameters. They found that the relationship between resistance and strain during cyclic



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loading to be much more linear and less noisy in case of carbon filament- PES matrix composite compared to carbon fiber-PES matrix composite when the filament and fiber-composites were exposed to 10 cycles of strain. Zhao et al.53 prepared PP/CNT, PP/CB nanocomposites and found that for PP/CB, resistance increases exponentially with strain and for PP/CNT, resistance increases linearly. Piezoresistance is also found to change with time. The time dependency of piezoresistivity has been found to vary with filler, type of filler, deformation mode and the magnitude of deformation. For example, Chen et al.54 investigated the time-dependence of piezoresistance of high density polyethylene (HDPE)-foliated graphite (FG) nanocomposites. Their results show that there exists a critical pressure below which relative resistance decreases with time and above it increases with time. It is found that at lower FG loading, the electrical response with time is sharper than higher FG loading at higher pressures. Zhang et al.44 investigated the creep behavior of polymer matrix and found that the basic reason for time dependence is due to the relaxation of the polymer matrix when it is subjected to a constant strain. In our recent communication55, we prepared natural rubber NR/CNT nanocomposites and studied their piezoresisitive behavior under tensile strain. To understand in detail the piezoresistive behavior of this material, a theoretical scaling model was developed. The mechanisms of piezoresistivity, the effect of filler, plasticizer and curatives on the strain sensitivity had been investigated. It has been found that there are scanty literatures on the works carried out to study some of the important real time electromechanical characteristics like reversibility, repeatability, time dependence of piezoresistivity whose properties are very close to those of end use applications. Thus, the present work deals with the preparation of NR/CNT, NR/CB and NR/CNT/CB nanocomposites and compares the real time sensing properties of these nanocomposites like sensitivity, recoverability, reproducibility and time dependencies aiming to realize the applicability of the developed materials. Materials Natural rubber used in this study was SMR 10 (Standard Malaysian Rubber). Multiwall Carbon Nanotubes (CNT, NC7000) were obtained from Nanocyl S.A. Sambreville, Belgium. This material has an average diameter of 9.5 nm, average length of 1.5 µm, aspect ratio >150, surface area of 250-300 m2/g (BET), metal oxide content of 10% and carbon purity of 90 %. Conducting


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carbon black (CB) used was a highly structured conducting black (Printex XE 2B) with a particle size < 30 nm, surface area 950 m²/g, iodine absorption ~1125 mg/g and DBP absorption number 420 ml/100 g. Other compounding ingredients like ZnO, stearic acid, Tertiary butyl benzothiazole sulfenamide (TBBS) and sulphur used were of industrial grade. Fig. 1 (a, b) shows the SEM image of CNT and CB used. From Fig. 1(a) the large aspect ratio and high entanglement of CNTs can be clearly seen, while from Fig. 1(b) the size of the primary particles and the high structuration of CB can be noted. a)

b)

100 nm

100 nm

Fig.1: Scanning Electron Microscope (SEM) images: a) multiwall carbon nanotubes (CNT, Nanocyl NC 7000), b) carbon black (Printex XE 2B)

Fabrication of Nanocomposites First, percolation sets of nanocomposites containing only CNT (1-6 phr) and CB (1 -15 phr) were prepared, in order to evaluate the individual percolation concentrations. Here ‘phr’ stands for ‘parts per hundred rubber’ by weight. For the investigation of the piezoresistive behavior of nanocomposites containing NR, CNT, and CB, only selected compositions were used whose formulations are listed in Table 1. The selection of these compositions was based on concentration of the fillers close to percolation threshold. The compounding of rubber was



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carried out on a two roll mill (Polymix 110 L, size: 203 × 102 mm, Servitech GmbH, Wustermark, Germany). The friction ratio between the rolls was kept at 1:1.2 throughout the mixing cycle. Before the addition of fillers and other compounding ingredients, the rubber was masticated for 2 min. For the mixing of MWCNT (fluffy mass in the air during mixing), they were first mixed with ethanol in the ratio of 1:10 and ultra-sonicated at 100 W for 1 hour. Then the sonicated mixtures were added into the rubber and mixed at 80 °C for 10 min till the ethanol was completely evaporated. In case of carbon black, the fillers are added as such into the masticated NR. Following that all other compounding ingredients like ZnO, stearic acid, TBBS (sulphur vulcanizing organic accelerator) and sulphur were added sequentially and the mixing was finished in 10 mins and then the compound was allowed to mature for 24 hrs. The compounded rubber was cured in a compression molding press at 150°C for their optimum cure time which was observed from the moving die rheometer (Göttfert elastograph Vario 6703). Table 1: Rubber Compound formulations (in parts per hundred rubber, phr)

Ingredients (phr) CNT 3 CB 11 CNT 2/CB 3 CNT 1.5 /CB 5.5 CNT 0.5/CB 9 100

100

100

100

100

ZnO

3

3

3

3

3

Stearic Acid

2

2

2

2

2

CNT

3

-

2

1.5

0.5

CB

-

11

3

5.5

9

TBBS

1

1

1

1

1

Sulphur

2

2

2

2

2

Natural Rubber

Characterization Techniques The electrical volume resistivity of the nanocomposites was measured on compression molded samples. Two types of samples were used depending on their resistivity. A circular 1 mm thick specimen of 60 mm diameter was used for highly resistive nanocomposites and strips of 20 mm × 3 mm × 1 mm for low resistivity samples. For composites having resistivities >107 Ω.cm, Keithley 8009 Resistivity Test Fixture was used and for composites having < 107 Ω.cm, a two point test fixture connected to Digital Multimeter Model 2000 (Keithley Instruments Inc., USA)


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was used. The resistance of the composites was converted to resistivity (ρ) and conductivity (σ) according to Eq. 1 and Eq. 2, respectively. =

.

(1)

(2)

=

where R (Ω) is resistance, ρ (Ω. m) is resistivity, A (m2) is cross sectional area, d (m) is thickness and σ (S. m-1) is the conductivity. The change in resistance with tensile strain (piezoresistivity) was measured using the in-house fabricated strain sensing instrument as described elsewhere 55. The dumbbell specimen according to DIN S2 standards (25 mm clamping area, 2 mm thick, 4 mm width) are held at their two ends whereby one end is fixed and the other end was moved with the help of a manual screw handle. The strained sample was connected (via stainless steel clamps) to an in-house built multi-meter through wires. The applied voltage was around ~1V and a maximum resistance of up to 20 MΩ can be measured. The maximum possible strain achieved with this set-up is 300 %. The relative resistance change ∆R/R0 was calculated based on the initial resistance R0 and the change in resistance ∆R =Ra-R0 with Ra the actual resistance at a given strain value. For the recoverability studies, the nanocomposites were initially given 40 % strain. Then the relaxation of resistance was measured until it reaches 90 % of initial value or for maximum of 200 sec. In the following cycle, the strain was increased by 10 % and this stepwise increase of strain was performed up to max. 100 % strain, or depending on the nanocomposite’s stretchability. For reproducibility studies, the nanocomposites were strained up to 100 % (under loading and unloading condition) for many cycles and the changes in stress were recorded. Stress and resistance relaxation test were carried out by stretching the nanocomposite to 50% strain and evaluating the change in resistance over a period of 10 min. The stress relaxation studies were carried out with the help of a Universal Tensile Testing Machine, Zwick 1456 (model1456, Z010, Ulm, Germany) with a cross head speed of 200 mm.min-1 (ISO 527). Tensile tests were performed with the use of a universal testing machine (Zwick 1456, Z010, Ulm, Germany) following ISO 527 standards with DIN S2 dumbbell specimens. The strain sweep measurements were done by a dynamic mechanical thermal analyzer (Eplexor 2000 N, Gabo Qualimeter, Germany). This dynamic test was done at room temperature. For this



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experiment a constant frequency of 10 Hz was selected with 60 % static strain and dynamic amplitude was swept from 0.01 % to 30 %. The large number of loading unloading cyclic experiment was also done using the dynamic mechanical analyzer (Gabo Eplexor 2000N) which was coupled with a multi-meter. The measurement was carried out with 1 HZ frequency. The pre-load was given at 30 % strain and the dynamic strain was 60 %. This makes total maximum strain nearly 90 %. The morphology of the nanoparticles in the nanocomposites was characterized by transmission electron microscope (TEM). For TEM, ultra-thin sections of the rubber composites were cut by an ultra-microtome at a temperature of -100˚C and the images were captured by Libra 120 transmission electron microscope (ZEISS, Oberkochen, Germany) with an accelerator voltage of 200 kV.

Results and Discussion The amount of conductive filler in the nanocomposites determines its resistivity and Fig. 2 shows the effect of filler loading on electrical conductivity. A three stage transitional behavior is evident from both CNT and CB nanocomposites; an initial low increase in conductivity, followed by a sudden increase in conductivity known as percolation transition and further a very low increase in conductivity. It is also seen that the percolation occurs at a concentration of around 2.5 - 3 phr for the CNT nanocomposites and between 10 - 11 phr for those based on CB. The lower percolation threshold observed for CNT composites as compared with CB composites is due to the much higher aspect ratio of CNT. It is also seen that the percolation transition region is more narrow for the CNT nanocomposites, while for CB nanocomposites the percolation transition region is broader which may be also due to the lower aspect ratio and the higher primary particle size 40. The exact percolation threshold values of CNT and CB are determined from Fig. 2 using scaling law 27, 39 as 2.65 and 10.6 phr respectively. ( ) = ( − ) >

(3)

( ) = ( − ) <

(4)


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where σ is conductivity at a given volume fraction p, is the proportionality constant, pc is the volume fraction at the percolation threshold and t, s are the critical exponents. From our previous studies, it was clear that the composites having filler concentration close to the percolation threshold showed higher piezoresistivity than the highly filled composites

55

.

Here higher piezoresistivity means rate of change of the resistance with respect to longitudinal strain. Therefore, in this study, we have compared composites having CB, CNT, and three different ratios of the hybrid (CB-CNT) fillers with concentrations close to percolation threshold. For more stable conducting network formation, slightly higher values of 3 phr of CNT and 11 phr of CB are followed for further studies. The percolation threshold of the hybrid (CNTCB)/NR nanocomposites can be estimated theoretically based on the following synergistic effect proposed by Sun et. al.27, 56.

+

=1

(5)

where MCNT, MCB are the weight fractions of CNT and CB respectively, pCNT, pCB are the percolation thresholds of CNT and CB nanocomposites. The CNT and CB contents for the hybrid composites consisting 50:50, 75:25 and 15:85 ratios of CNT/CB are approximated from Eq.(5) as (i) 1.5/5.5, (ii) 2/3 and (iii) 0.5/9 phr respectively. 10-1

Conductivity (S/m)

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10-3

Pc ~ 2.65 phr t ~ 2.28

10-5

Pc ~ 10.6 phr t ~ 1.8

10-7 10-9

CNT CB CNT/CB 2/3 phr CNT/CB 1.5/5.5 phr CNT/CB 0.5/9 phr

10-11 10-13 10-15

0

3

6

9

12

15

Total Filler content (phr)

Fig. 2: Influence of filler concentration on the electrical conductivity of the NR composites with CNT and CB. The experimental data (symbols) are fitted (dotted lines) by scaling law (Eq. 3, where P>Pc) and the respective percolation threshold (Pc) and the critical exponent ‘t’ values are indicated



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a)

b)

c)

d)

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d)

Fig. 3: Transmission electron microscopy (TEM) images of NR nanocomposites containing a) 3 phr CNT, b) 11 phr CB, c) 1.5/5.5 phr CNT-CB, d) schematic presentation of the morphology of the hybrid nanocomposites showing the connecting pathways of electron transfer

The state of dispersion of nanoparticles within the conductive composites as well as their spatial arrangement seems to be of importance for the piezoresistivity behaviour. Therefore, the


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morphology of selected samples was studied using TEM. Fig. 3 presents images of nanocomposites with CNT, CB, hybrid (CNT/CB) and a schematic visualisation of the hybrid filler system. Fig. 3a depicts the morphology of NR filled with 3 phr CNT illustrating high aspect ratio and wavy nature of the CNTs. Fig. 3b represents the morphology of NR composite containing 11 phr of carbon black indicating the highly structured CB aggregates. Fig. 3c shows the TEM image of the hybrid nanocomposite (CNT/CB – 1.5/5.5 phr) which indicates a possibility of formation of filler network in the hybrid nanocomposite. The TEM image proposes different possibilities for electron transfer including hopping and tunnelling effects between neighbouring conductive particles: i) CNT-CNT, ii) CNT-CB, and iii) CB-CB contact. Fig. 3d is a schematic visualization of these possible filler contacts in the hybrid nanocomposite inferred from the TEM image. An interaction between CNT and CB, where CB plays the role of bridging or linking the gap between CNTs forming a conductive path may be assumed, similar to the observations reported by Ma et al. 57 or Hilarius et al 58. So, it can be said that network is formed at lower concentration of CB and CNT hybrid compounds as compared with each individual compound.

a) 40

b) 9

CNT 3 phr CB 11 phr CNT/CB 2/3 phr CNT/CB 1.5/5.5 phr CNT/CB 0.5/9 phr

30


8

E' (MPa)

Stress (MPa)

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20

7 6 5

10

4 0

3 0

100

200

300

400

500

600

700

0.1

1

10

Strain dyn (%)

Strain (%)

Fig. 4: a) Stress-strain plot and b) dynamic strain dependent storage modulus of the NR composites filled with carbon nanotubes and carbon black and their mixture. All the measurements were carried out at room temperature Stress-strain properties of NR composites filled with CNT 3 phr, CB 11 phr, CNT/CB 2/3 phr, 1.5/5.5 phr, 0.5/9 phr are studied and shown in Fig. 4a. All the NR composites exhibited similar



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stress-strain characteristics with high elongation at break values (550-650%). The tensile strength of those composites is ranging from 27-31 MPa indicating the strong reinforcement effect of the fillers present in the natural rubber matrix. It can be mentioned here that all the filler concentrations used are above the percolation points and therefore, all the composites showed very high mechanical performance and behave quite similarly. In order to understand the fillerfiller interaction and rubber-filler interaction strain sweep experiment was performed (Fig. 4b). In general, the unfilled rubber (without any filler) does not exhibit any strain dependent dynamic modulus characteristic but for the filled sample, the modulus decreases at higher strain due to damage of the filler-filler network and this fact is known as Payne effect. In present case the strongest filler-filler network was observed for CNT followed by CB filled sample. The entangled nature and large aspect ratio of CNT are the reasons for this stronger Payne effect59-60. However, the hybrid sample CNT/CB-0.5/9 phr shows lowest filler-filler network as compared with other systems. This result indicates that this particular mixed or hybrid filler system, the overall rubber-filler interactions are higher than all other composites. Fig. 5a shows the variation of relative resistance (∆R/R0) against applied tensile strain, whereas ∆R is the change in the resistance (relative resistance) with respect to strain and R0 is the initial resistance. This figure elucidates the CNT, CB, and hybrid nanocomposites displaying a nonlinear piezoresistive behavior against the elongation strain. The nonlinear piezoresistive behavior can be explained by assuming a discrete deformation behavior of the filler networks at different strain regimes, which is explained in our previous work55. Further, it is also observed that CB composites at 11 phr filling exhibit higher piezoresistive response as compared with the CNT based nanocomposite with 3 phr loading, even if both composites have comparable initial conductivity values of 2 x 10-4 S/m (CNT) and 1.6 x 10-4 S/m (CB). The reason for the higher piezoresistive effect of CB is due to the low aspect ratio of the filler when compared to CNTs. The lower aspect ratio of the carbon black requires higher number of contacts for the formation of percolating conducting network. So, for CB composites at relatively low strain, the possibility of disrupting the network is higher than high aspect ratio CNT composite61. It also can be found from Fig. 5a that the rubber samples have different critical extension range with respect to change of electrical response with strain percent. Beyond this elongation point the samples become non-conducting and the instrument cannot measure the conductivity. In this respect, sample with 3 phr CNT shows maximum stretchability about 180 % whereas CNT/CB


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1.5/5.5 sample offer only less than 80% stretchability range where the rubbers remains conducting. Maximum strain sensing regimes of the CNT 3 phr, CB 11 phr, CNT/CB 2/3 phr, 1.5/5.5 phr, 0.5/9 phr are 185%, 120%, 125%, 75%, 110%. This critical strain of crosslinked elastomers may depend on various factors like size and the shape of the conducting fillers, their amount used in the preparation and finally, the complex interaction between the fillers. Nevertheless, at lower strain regime all the composites showed also strain dependent electrical resistance indicating the strain sensing capability of all the composites at lower strain regime (Fig. 5b). This effect of strain on relative resistance at lower strain regime indicates that the conductive network ruptures in this region.

b)

a) 1500 1200

10


∆R/R o

900 600 300 0 40


8

∆ R/Ro

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0

80

120

160

200

0

10

20

30

40

50

Strain (%)

Strain (%)

Fig. 5: a) Influence of different CNT/CB ratios on the piezoresistive behavior of NR nanocomposites, b) the piezoresistive behavior is shown at lower strain regime

A pictorial demonstration for the number of contacts essential for equidistant conducting network formation for different nanofillers is shown in Fig. 6. A similar explanation was given by Alamusa et. al. while dealing with the CNT- epoxy based materials with different aspect ratios of the tubes

62

. In addition to high aspect ratio, flexible and entangled nature of the

nanotubes could also be the reasons for lower piezoresistivity (change of resistance per unit change of elongation) of CNT when compared to CB. Most probably, the conducting networks formed from large aspect ratio and entangled nanotubes are so stable, that the conducting pathways are not broken at a small strain and after a relatively higher strain, this CNT networks



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are broken resulting in higher resistance. Consequently, the CNT based elastomer composites offer stronger piezoresistive behavior comparatively at higher strain region for example, ~ 100 % elongation. From Fig. 5a it can be observed that the hybrid nanocomposites (filled with CNT and CB) show piezoresistive behavior between CB and CNT response regimes. This is due to the mixed morphology of the system as proposed in Fig. 6. The interaction between CNT and CB is expected to be less than CNT-CNT or CB-CB and so the mixed filler composite requires lower strain for a network breakdown. Due to relatively lower number of contact points in the hybrid composite, it shows lower piezoresistive characteristics when compared to pure CB systems.

Fig. 6: Schematic presentation of number of contact points for NR nanocomposites CNT, CB, and hybrid (CB and CNT) The plausible mechanism for the piezo-resistivity of CNT, CB and hybrid nanocomposites is schematically shown in Fig. 7. The CNT and CB particles (represented as solid rods and spheres) are expected to form conducting networks, which also interact with the polymer chains. Under an applied load, some polymer chains tend to align themselves along the stretching direction; carrying the bound fillers along with them. This alignment and extension of polymer chains disturb the associated filler-filler networks leading to increase in resistance values. From the previous studies, it is evident that CB has better interaction with the polymer chains than CNT 45, 63

. This could also be one of the reasons for CB composites displaying a higher sensitivity to

network breakdown. In case of hybrid system, the CB particles are expected to interact with polymer chains and CNT plays the role of networking, explaining the higher sensitivity of hybrid composites (CNT/CB) than CNT composites.


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a)

b)

Fig. 7: Mechanism of strain dependent electrical conducting properties of a) multiwall carbon nanotube and b) carbon black filled crosslinked natural rubber composites. The aggregated structure of the carbon black particles is marked with yellow and red contour to visualize as single aggregate. The relative size of the fillers is not considered in the picture and should not be compared.

In context with sensory applications, next to piezoresistivity, the aspect of recoverability of the sensors is a very important issue. The recoverability can be defined as the ability of the composite to recover to its initial resistance after release of the strain. The extent of recoverability, i.e., variation of relative resistance (∆R/R0) in different cycles with stepwise increase in strain values is shown in Fig.8 for the composites with 3 phr CNT, 11 phr CB and their mixture. It can be observed that under the selected straining conditions, the resistivity of



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CNT composites (Fig. 8a) is not recoverable even at low strain of 40% within the experimental time scale. The resistance values recorded before the tests (initial) and after a loading-unloading cycle (final) are not same and the difference can be termed as resistance set. This set value would decreases slowly and gradually with time during the relaxation period. It is also found that the resistance set depends on the magnitude of strain applied; higher the strains, higher the set values. It is interesting to note the peculiar behavior of the CB based nanocomposites. Looking carefully into Fig.8b, one could observe that the final resistance values are decreasing after a few loading-unloading cycles. After few cycles the relative resistance change is less than the initial value, indicating a negative resistance set (indicated by red arrow in Fig. 8b). This indicates that for CB filled system during the cyclic deformation; the effective numbers of conducting pathways are possibly increasing, due to mobility and alignment of filler aggregates along the stretching direction after a few loading cycles; resulting in a higher conductivity. This is opposite to the behavior of the CNT based composite where the conducting pathways of CNTs are permanently damaged during multiple loading cycles and the recovery of the conductivity to the initial value was not possible. For CB filled composites the conducting pathways are largely associated with the temporary filler-filler network which can be damaged during stretching and can reform during dynamic loading and unloading of the samples 64. In order to have complete recovery, the destroyed contacts among the conductive fillers need to rebuild. Similarly, Knite et al. 49 observed that regeneration is only possible if the filler-polymer interaction is strong. This means when the polymer chains are relaxing, the associated conductive filler also tries to move according to the movement of the polymeric chains thus forming a new percolating conductive filler network. This reformation of the conducting behavior may be associated with the interaction between polymer chains and fillers (CNT, CB). Fig. 8c reveals that the CNT-CB (0.5/9) hybrid nanocomposites exhibit good electrical recovery up to 90% strain and the resistance set values is similar to that of the initial values. However, in Fig. 8d for the CNT/CB (2/3) samples, a slight increase in the resistance values is observed after few cycles, which is due to the higher percentage of CNTs. It is very important to note here that particularly, the hybrid filler system comprising 0.5 phr of CNT and 9 phr of CB seems to be only an optimum combination of CB and CNT for achieving perfectly recoverable composites and can be used for strain sensing applications with demanding recoverable piezoresistive properties.


Page 17 of 29

a)

b) 900

CNT 3 phr

60

CB 11 phr

100

100

50

300

0

0

0 0

200

400 Time (s)

100

200 Time (s)

300

d)

c) 400

0 0

600

Strain (%)

20

∆R/R0

∆R/R0

50

Strain (%)

600 40

200

CNT/CB 0.5/9 phr

CNT/CB 2/3 phr

100

100

100 50

50

100 0

0 0

100 Time (s)

200

0

Strain (%)

50

Strain (%)

200

∆R/R0

150

300 ∆R/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0

100

Time (s)

200

Fig. 8: Electrical conductance recoverability of NR nanocomposite at different strains (a) CNT 3 phr, (b) CB 11 phr, (c) hybrid CNT/CB 0.5/9 phr, (d) CNT/CB 2/3 phr. The red arrows indicate either increase of resistance (CNT 3, CNT/CB 2/3) or decrease (CB 11) after a few loading unloading cycles.

Some applications demand the sensors to be used repetitively, which requires the sensors to be designed to give reproducible piezoresistive properties. To understand the reproducibility or repeatability, the composites were subjected to 10 recoverable strain cycles (loading and unloading). CNT 3 phr, CNT/CB 1.5/5.5 phr, 2/3 phr composites are not analyzed as these composites show very poor conductivity recovery. Only the composites loaded with CB 11 phr



and CNT/CB 0.5/9 phr are studied and are shown in Fig. 9. It can be observed that CB 11 phr and CNT/CB 0.5/9 phr composite systems exhibit reproducible piezoresistive characteristics which were measured up to 110 % cyclic strain for CB 11 phr and 90 % strains for CNT/CB 0.5/9 phr respectively. a) a)

b) 150

CB 11phr

1200

400

150

CNT/CB (0.5/9 phr)

125

125

75

600

50

400

100

∆R/Ro

100

800

75

200

50

Strain (%)

300

1000

Strain (%)

1400

∆R/Ro

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

100 25

200 0

0 0

75

150

225 300 Time (s)

375

450

25 0

0 0

300

600 Time (s)

900

1200

Fig. 9: (a) Cyclic strain experiment and plots of relative resistance, and strain vs. time for CB 11 phr composite and (b) for CNT/CB 0.5/9 phr hybrid composite

It should be noted here this cyclic loading and unloading experiment has been done manually and very precise loading and unloading cycle time was not controlled but a rough understanding can be possible with this experiment. To further investigate the action of several loading and unloading cycle on the electrical behavior a DMA (see experimental) was used and the conductivity was monitored during the large number of cyclic deformation. Due to limitation of the instrument maximum dynamic strain was given up to 90 %. It can be easily noticed from Fig. 10 that after a large numbers of repeated loading-unloading cycles the conducting properties of the composite is mostly retained indicating very stable but flexible conducting pathways developed by the special arrangement of the filler inside the rubber matrix. However, a slight decrease (marked with red circle) about the absolute conducting pattern (resistance at peak strain) of the composites was noticed at the beginning of the experiment and after a certain loading unloading cycles the effect is disappeared establishing a stable conducting network of the fillers. At the beginning of the cyclic test the unstable filler network structures are


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transformed into more stable network and this typical nature of the materials are further studied by stress-relaxation experiment while monitoring the electrical resistance of the samples. Fig. 11 shows a typical stress relaxation experiment coupled with electrical behavior of the CNT, CB, and hybrid nanocomposites at 50% elongation strain. This study is carried out by stretching the nanocomposite to a particular strain (50%) and measuring the change in resistance over a period of time (10 mins).

a)

b)

120 90 60 30 0

Strain (%)

Stress (MPa)

120 90 60 30 0

Resistance (kOhm)

1.0

Strain (%)

Stress (MPa)

Resistance (kOhm)

1.0

0.5 0.0

500

510

520 530 Time (s)

540

550

0.5 125 100 75 50 25 0

0.0

0

600

1200

1800 2400 Time (s)

3000

3600

125 100 75 50 25 0

Fig. 10: Cyclic stress-strain experiment and monitoring the final resistance response for CNT/CB 0.5/9 phr hybrid composite a) 100 cycles and b) 3600 cycles. The red circle in Fig. b indicates lowering of the peak resistance values during initial loading and unloading cycles

It is seen from Fig.11 that stress and resistance follow the same trend; the initial sharp decrease in stress is followed by a slow relaxation with time. From this comparison, it can be concluded that resistance change is a consequence of stress relaxation 44. Hence this decrease in resistance is basically a phenomenon of stress relaxation as described by Voet et.al.,

65

. It may also be

termed as macromolecular chain relaxation which enhances the formation of percolated conducting network causing a decrease in resistance of the nanocomposites over time (resistance relaxation). The initial rapid decrease in resistance may be due to alignment of the conducting particles along the tensile stress direction and reformation of few broken conducting paths (t >0). Further advancement in time mostly favors the decrease in inter-particle distances between



adjacent fillers (t >>0 ), rather than formation of new conducting networks

66

. The time

dependent behavior was found to be similar for all the nanocomposites. The following expression (Wiechart model) was used to evaluate relaxation times associated with stress and resistance values 67.

CB 11 phr CNT/CB 2/3 phr

1.6

CNT/CB 0.5/9 phr CNT 3 phr

1.2

8

0.8 6 0.4 4 0

200

400

Resistance (MOhm)

10

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

0.0 600

Time (s) Fig. 11: Comparison of stress and resistance relaxation plots at 50% strain for CNT, CB and hybrid nanocomposites (open symbols - stress, closed symbols - resistance; Symbols represent the experimental data and lines represent the fitted Wiechart model) &

= + ! '

"% #$ (6)

where, R is the resistance / stress of the compound at time t, Rs is the residual value, ci are coefficients, τi are the relaxation times; n = 3 for Wiechart’s model. From Table 2 very similar sets of data can be observed with respect to relaxation times ‘τi’ corresponding to mechanical as well as resistance values. Similar pattern of stress relaxation and resistance relaxation profiles confirms that the electrical resistance relaxation is a consequence of stress relaxation. Also, from Table 2, it is observed that CNT filled samples display much higher


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relaxation times than carbon black composites. This signifies that carbon black fillers have much higher mobility and can easily recover its conducting network. Based on the results, a schematic illustration is proposed to explain the mechanism of time dependence of piezoresistivity as shown in Fig. 12. It depicts (i) t0: the breakdown of conductive network immediately upon straining (t=0), (ii) tr: alignment of conductive particles along the tensile direction as well as re-formation of destroyed conductive network (t>0) and (iii) ti: slow and gradual reduction in interparticle distances over longer time, leading to slow decay in resistance (t >>0).

Table: 2: Relaxation times and fit coefficients for various composites Parameters

CB

CNT/CB

CNT/CB

CNT

11 phr

0.5/9 phr

2/3 phr

3 phr

Stress

Resist.

Stress

Resist.

Stress

Resist.

Stress

Resist.

(MPa)

(MΩ)

(MPa)

(MΩ)

(MPa)

(MΩ)

(MPa)

(MΩ)

Rs

4.756

0.795

4.994

0.713

7.619

0.209

8.254

0.151

c1

4.183

8.878

5.224

7.123

5.051

0.159

8.965

0.135

c2

0.200

0.233

0.232

0.316

0.334

0.075

0.388

0.055

c3

0.206

0.328

0.267

0.263

0.427

0.082

0.455

0.058

τ1

5

5

5

5

6

6

5

5

τ2

38

38

47

47

63

63

54

54

τ3

275

275

464

464

651

651

568

568

Finally, one of the developed materials (CNT/CB 0.5/9 phr) is embedded in gloves to understand its efficiency and to get a visual idea about the function of the sensors. As shown in Fig. 13 the sensors show a time responsive behavior in such a way so that it can detect the finger motion during normal typing speed. It can also be realized that the materials is very flexible and it can be



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Page 22 of 29

integrated with textile-based wearable clothing for the detection of human body motion, type of the motion etc.

Fig. 12: Mechanism of time dependence of NR containing CNT, CB, hybrid (CNT/CB) nanocomposites, t0: the breakdown of conductive network immediately upon straining (t=0), tr: alignment of conductive particles along the tensile direction as well as reformation of destroyed conductive network (t>0) and ti: slow and gradual reduction in inter-particle distances over longer time, leading to slow decay in resistance (t >>0)


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Fig. 13: Detection of finger motion and type of the motion by embodiment of conducting elastomer composite (CNT/CB 0.5/9 phr) on latex gloves. Typing the complex stretching and bending motion of sample is directly reflected in the resistance plot.

Conclusions In this article, we examine the efficiency and effectiveness in terms of piezoresistive properties of natural rubber composites based on two different carbon nanofillers and their mixtures (hybrid). The work also demonstrates the easy fabrication process to prepare very sensitive strain sensors based on the raw materials which are normally used by the rubber parts producing industries. This study reveals that the efficiency of the strain sensors is mainly associated with the critical concentration of the fillers. The optimum composition (ratio) of the filler contents in the hybrid composites was evaluated from the percolation thresholds of the corresponding single filler systems. Some of the piezoresistive characteristics which are close to real time applications, like piezoresistivity, recoverability, reproducibility, time dependence behavior are compared for CNT, CB, hybrid nanocomposites under uniaxial strain. Finally, the potential application of this material is also explored by large number of loading-unloading cycle experiments expanding the range of various applications from vehicle tires to human finger motion detections.



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55. Selvan, N. T.; Eshwaran, S.; Das, A.; Stöckelhuber, K.; Wießner, S.; Pötschke, P.; Nando, G.; Chervanyov, A.; Heinrich, G., Piezoresistive natural rubber-multiwall carbon nanotube nanocomposite for sensor applications. Sensors and Actuators A: Physical 2016, 239, 102-113. 56. Sun, Y.; Bao, H.-D.; Guo, Z.-X.; Yu, J., Modeling of the electrical percolation of mixed carbon fillers in polymer-based composites. Macromolecules 2008, 42 (1), 459-463. 57. Ma, P.-C.; Liu, M.-Y.; Zhang, H.; Wang, S.-Q.; Wang, R.; Wang, K.; Wong, Y.-K.; Tang, B.-Z.; Hong, S.-H.; Paik, K.-W., Enhanced electrical conductivity of nanocomposites containing hybrid fillers of carbon nanotubes and carbon black. ACS applied materials & interfaces 2009, 1 (5), 1090-1096. 58. Hilarius, K.; Lellinger, D.; Alig, I.; Villmow, T.; Pegel, S.; Pötschke, P., Influence of shear deformation on the electrical and rheological properties of combined filler networks in polymer melts: Carbon nanotubes and carbon black in polycarbonate. Polymer 2013, 54 (21), 5865-5874. 59. Subramaniam, K.; Das, A.; Steinhauser, D.; Klüppel, M.; Heinrich, G., Effect of ionic liquid on dielectric, mechanical and dynamic mechanical properties of multi-walled carbon nanotubes/polychloroprene rubber composites. European Polymer Journal 2011, 47 (12), 2234-2243. 60. Subramaniam, K.; Das, A.; Stöckelhuber, K. W.; Heinrich, G., Elastomer composites based on carbon nanotubes and ionic liquid. Rubber Chemistry and Technology 2013, 86 (3), 367-400. 61. Ke, K.; Pötschke, P.; Wiegand, N.; Krause, B.; Voit, B., Tuning the Network Structure in Poly(vinylidene fluoride)/Carbon Nanotube Nanocomposites Using Carbon Black: Toward Improvements of Conductivity and Piezoresistive Sensitivity. ACS Applied Materials & Interfaces 2016, 8 (22), 1419014199. 62. Alamusi; Hu, N.; Fukunaga, H.; Atobe, S.; Liu, Y.; Li, J., Piezoresistive strain sensors made from carbon nanotubes based polymer nanocomposites. Sensors (Basel, Switzerland) 2011, 11 (11), 1069110723. 63. Cataldo, F.; Ursini, O.; Angelini, G., MWCNTs Elastomer Nanocomposite, Part 1: The Addition of MWCNTs to a Natural Rubber-based Carbon Black-filled Rubber Compound. Fullerenes, Nanotubes and Carbon Nanostructures 2009, 17 (1), 38-54. 64. Stubler, N.; Fritzsche, J.; Kluppel, M., Mechanical and Electrical Analysis of Carbon Black Networking in Elastomers Under Strain. Polym Eng Sci 2011, 51 (6), 1206-1217. 65. Voet, A.; Cook, F.; Sircar, A., Relaxation of stress and electrical resistivity in carbon-filled vulcanizates at minute shear strains. Rubber Chemistry and Technology 1971, 44 (4), 175-184. 66. Vidhate, S.; Chung, J.; Vaidyanathan, V.; D'Souza, N., Time dependent piezoresistive behavior of polyvinylidene fluoride/carbon nanotube conductive composite. Materials Letters 2009, 63 (21), 17711773. 67. Rudra, R., A curve fitting program to stress relaxation data. Canadian Agricultural Engineering 1987, 29 (2), 209.



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Detection of finger motion and type of the motion by embodiment of conducting elastomer composite (CNT/CB 0.5/9 phr) on latex gloves. Typing the complex stretching and bending motion of sample is directly reflected in the resistance plot. 824x669mm (96 x 96 DPI)


Strong Strain Sensing Performance of Natural Rubber

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