Static and Dynamic Mechanical Characteristics of Ionic Liquid

Jan 22, 2018 - A comparative investigation of composite models found that modified Kelly–Tyson theory gave an excellent fit to tensile strength data...
2 downloads 4 Views 2MB Size
Download PDF
Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

pubs.acs.org/JPCB

Static and Dynamic Mechanical Characteristics of Ionic Liquid Modified MWCNT-SBR Composites: Theoretical Perspectives for the Nanoscale Reinforcement Mechanism Jiji Abraham,† Jince Thomas,† Nandakumar Kalarikkal,†,§ Soney C. George,‡ and Sabu Thomas*,†,∥ †

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, P.D. Hills, Kottayam, Kerala 686560, India ‡ Centre for Nanoscience and Nanotechnology, Department of Basic Sciences, Amal Jyothi College of Engineering, Kottayam, Kerala, India § School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686560, India ∥ School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686560, India ABSTRACT: Well-dispersed, robust, mechanicaly long-term stable functionalized multiwalled carbon nanotube ( f-MWCNT)-styrene butadiene rubber (SBR) nanocomposites were fabricated via a melt mixing route with the assistance of ionic liquid as a dispersing agent. The mechanical properties of f-MWCNT/SBR vulcanizates were compared over a range of loadings, and it was found that the network morphology was highly favorable for mechanical performance with enlarged stiffness. A comparative investigation of composite models found that modified Kelly−Tyson theory gave an excellent fit to tensile strength data of the composites considering the effect of the interphase between polymer and f-MWCNT. Dynamic mechanical analysis highlighted the mechanical reinforcement due to the improved filler−polymer interactions which were the consequence of proper dispersion of the nanotubes in the SBR matrix. Effectiveness of filler, entanglement density, and adhesion factor were evaluated to get an in depth understanding of the reinforcing mechanism of modified MWCNT. The amount of polymer chains immobilized by the filler surface computed from dynamic mechanical analysis further supports a substantial boost up in mechanics. The Cole−Cole plot shows an imperfect semicircular curve representing the heterogeneity of the system and moderately worthy filler polymer bonding. The combined results of structural characterizatrion by Raman spectroscopy, cure characteristics, mechanical properties, and scanning and transmission electron microscopy (SEM, TEM) confirm the role of ionic liquid modified MWCNT as a reinforcing agent in the present system.

1. INTRODUCTION Quasi-one-dimensional (1D) multiwalled and single-walled carbon nanotubes can be considered as an attractive and most promising nanomaterial due to their unprecedented properties such as excellent mechanical properties, outstanding electrical and thermal conductivity, and suitable optical properties.1 The fascinating and attractive properites of carbon nanotubes (CNTs) can be transferred to a wide variety of polymer matrixes to make functional and novel nanocomposites. CNT/polymer nanocomposites have excellent mechanical stability, high stiffness, and remarkable electrical and thermal conductivity at relatively low loading of CNT, and hence, they are useful in various fields such as electromagnetic interference (EMI) shielding, electrostatic dissipation, electrostatic painting, heat sinks, thermal interface materials, highperformance thermal management systems, and optoelectronic devices.2 © XXXX American Chemical Society

Fundamental processing challenges in composite reinforcement are difficulties in proper dispersion of multiwalled carbon nanotubes (MWCNTs) because of the secondary interaction between adjacent carbon nanotubes and inefficiency of proper load transfer between MWCNTs and the polymer due to the atomically smooth surface of MWCNTs.3 Introduction of functional moieties on MWCNTs by emerging chemistry is a common approach to overcome these challenges.4 The reinforcing mechanism and the role of the nanotube−polymer interface are the two key parameters which are very important in the fabrication of nanocomposites with various properties, and the successful load transfer depends on the dispersion and compatibility between filler and polymer.5 Several mechanisms Received: October 27, 2017 Revised: December 18, 2017

A

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

by Lanxess Pvt Limited Germany. The primary and secondary accelerators such as MBTS, tetra methyl thiurium disulfide (TMTD) were purchased locally. The detailed experimental procedure was reported in our previous work.14 The experimental procedure for the preparation of nanocomposites is illustrated in Figure 1, and the samples prepared are displayed in Table 1.

are proposed to explain the reinforcing behavior in polymer nanocomposites. One of the mechanisms is the establishment of hydrodynamic (volumetric) interaction between filler and polymer rather than filler agglomeration or percolation.6 The extent of this interphase region can be improved either by increasing the surface area of the filler or by its surface functionalization. In contrary to the above finding, Dorigato et al. demonstrated that the important parameter responsible for the reinforcement effect is primary nanoparticle aggregation and not the filler polymer interaction at the interface.7 Formation of a filler network which results in transfer of part of the stress to such a network is another possible mechanism for reinforcement. According to Perez-Aparicio, the reinforcement mechanism is based on the filler−matrix interaction at the filler surface and this macromolecular chain adsorption results in an increased amount of immobilized macromolecular chains at the vicinity of fillers (an increased density of trapped entanglements). The mechanical stiffening of the interphase region because of the presence of immobilized macromolecular chains is considered as a further reinforcing component of the system.8 As the temperature reaches the glass transition temperature (Tg) of the polymer, creation of glassy layers around the nano reinforcement also plays a possible role in the reinforcement.9 Gavrilov et al. performed a large-scale dissipative dynamics simulation to study the structural changes in unfilled and filled rubbers during uniaxial deformation to get an idea about fundamental reasons of filler reinforcement mechanisms in rubber nanocomposites.10 Pourhossaini et al. investigated the frictional properties and dynamics of SBR micro- and nanocomposites under the influence of SiO2 particles, and the results emphasize the key impact of the immobilized rubber chains around fillers on the properties of nanocomposites.11 Mujtaba et al. identified the viscoelastic behavior of SBR nanocomposites containing silica particles.12 The effect of particle microstructure on the filler strengthening effect of SBR was investigated by Scott et al., and they found that particles with a higher aspect ratio impart a better reinforcing effect.13 Both the mechanical properties and viscoelastic behavior of polymers can be modified by reinforcing fillers. Proper regulation of both filler−rubber and filler−filler interfaces plays a crucial factor in predicting the filler reinforcing effect. The reinforcement mechanism of functionalized MWCNT (fMWCNT) in styrene butadiene rubber (SBR) matrix is elucidated both by static and dynamic mechanical testing. The experimentally observed mechanical characteristics are related with theoretical calculations. Structural characteristics of nanocomposites are studied by Raman spectroscopy. Improved mechanical performance of the composites with f-MWCNT addition was correlated with the microstructural developments in the composites by various microscopic techniques.

Figure 1. Procedure for the fabrication of ionic liquid modified MWCNT based SBR nanocomposites.

Table 1. Sample Codes of SBR/f-MWCNT Nanocomposites

a

sample designation

MWCNT (phr)

ionic liquid (phr)a

ratio between MWCNT and ionic liquid

T0IL0 T1IL1 T3IL0 T3IL1 T3IL5 T3IL10 T5IL1 T7IL1 T10IL1

0 1 3 3 3 3 5 7 10

0 1 0 3 15 30 1 7 10

0:0 1:1 3:0 1:1 1:5 1:10 1:1 1:1 1:1

1 phr ionic liquid = 1 mmol of ionic liquid.

The tensile properties of the nanocomposites were evaluated using a Universal Testing Machine (Tinius Olsen) with a crosshead rate at 500 mm/min according to ASTM D 412 at room temperature (25 ± 2 °C). Dynamic mechanical properties were analyzed over a wide temperature range from −80 to 80 °C using a dynamic mechanical analyzer (PerkinElmer - DMA 8000). The temperature sweep of the testing was from −80 to +80 °C with a programmed heating rate of +5 °C min−1, and the frequency and dynamic deformation were set to 1 Hz and 0.05%, respectively, according to ASTM D 2231. Rectangular pieces of elastomer nanocomposite with 3.5 cm length, 0.8 cm width, and 2 mm thickness were tested. The surface morphology of the filler and composite was analyzed with a Hitachi SU6600 variable pressure field emission scanning electron microscope (FESEM) with a resolution of 1.2 nm/30 kV. Tensile fractured surfaces were used to analyze the morphology of composites. Transmission electron microscopy (JEOL, JEM-2100) was used to analyze the morphology and dispersion of fillers in composites. Ultrathin sections of Cryocut specimens (∼100 nm thickness) were prepared using an ultramicrotome (Leica, Ultracut UCT) and placed on 300 mesh Cu grids (35 mm diameter), and they were analyzed without staining.

2. EXPERIMENTAL AND CHARACTERIZATION TECHNIQUES Styrene butadiene rubber (Synaprene 1502) with a 25% styrene content was used for this study. MWCNT obtained from Nanocyl (NANOCYL NC7000, Belgium) was used as the filler. Ionic liquid, 1-benzyl-3-methylimidazolium chloride obtained from Reinste Nano ventures, Delhi, is used as a surface modifying agent. All compounding ingredients were of analytical grade. The cross-linking agent used was sulfur which was supplied by Merck India Ltd., Mumbai, India. The activators used were ZnO and stearic acid which were supplied B

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Table 2. Cure Characteristics of f-MWCNT/SBR Nanocomposites: Effect of MWCNT Loading

3. RESULTS AND DISCUSSION 3.1. Structural Characterization by Raman Spectroscopy. Raman spectroscopy is identified to provide information on the structure, crystallinity, and aging of both components of a composite material by the careful evaluation of the associated vibrational features (band frequencies and widths). Variations in MWCNT band spectral features have also been suggested as indicators of their dispersion/loading characterization. In several studies, researchers found a shift of G band depending on the concentration of MWCNT. The shifts observed in the Raman bands of MWCNTs indicate chemical interaction between MWCNTs and the matrix, such as charge transfer or chemical bond changes in the tube−tube interactions due to the van der Waals force of attraction between individual tubes and mechanical compressions from the polymer matrix. In the literature, there are many reports regarding the Raman shift behavior of MWCNTs induced by chemical actions. It has been shown that charge transfer induced by doping, or chemical bonds formed between MWCNTs and the polymer, could induce a shift in the Raman peaks of MWCNTs to higher wavenumbers or to lower wavenumbers.15 In the current system, the phenyl group on the surface of modified MWCNT interacts with the aromatic moiety in the SBR system by aromatic interaction. Raman spectrum is given in Figure 2.

sample

minimum torque (ML) (dNm)

maximum torque (MH) (dNm)

effective torque increment (ETI) ΔM (dNm)

cure rate index (CRI) min−1 100/t90-ts2

T0IL0 T1IL1 T3IL1 T5IL1 T7IL1 T10IL1

0.46 0.56 0.78 1.00 1.31 1.98

10.78 11.78 12.61 14.01 16.56 18.17

10.32 11.22 11.83 13.01 15.25 16.19

12.04 13.45 14.18 15.97 16.31 18.18

MWCNT network in the rubber matrix. The effective torque ΔM is a parameter which indicates the evolution of cross-links in the rubber compounds and is a measure of the cross-link density of the rubber vulcanizate. Thus, the improvement in the effective torque value is because of the increased cross-link density due to the improved filler−filler and rubber−filler interactions. The extent of increment in effective torque is highest for samples with high f-MWCNT loading. The cure rate index (CRI) is used to evaluate the vulcanization rate where increased vulcanization rate leads to higher CRI. It can be inferred that vulcanization rate is promoted by the addition of fMWCNT. Cure characteristics of nanocomposites with the same loading of MWCNT and different amounts of ionic liquid are tabulated in Table 3. Here, the presence of ionic liquid enhances the minimum torque, maximum torque, and effective torque increment (ETI) up to a certain level of ionic liquid and then decreases. A higher amount of ionic liquid reduces the torque due to the plasticizing effect of ionic liquid. Since ionic liquid can act both as a catalyst and a cure accelerator, its presence speeds up the cure reaction. 3.3. Stress Strain Behavior. 3.3.1. Effect of f-MWCNT Loading. Mechanical property measurements examine the reinforcing effect of fillers. Typical stress strain curves of SBR composites having 1 to 10 phr f-MWCNT are shown in Figure 3. As expected, both tensile strength (TS) and modulus at a given strain increase with increase in f-MWCNT loading. However, elongation at break (EB) value decreases with increase in MWCNT loading except for the T1IL1 sample. The improvement in mechanical characteristics is ascribed to (1) the exceptionally high mechanical strength of MWCNT, (2) the huge difference in the mechanical performance of elastomer (MPa in Young’s modulus) and MWCNT, making it an ideal reinforcement in the rubber matrix; (3) the better dispersion of MWCNT in rubber matrix by two roll mill mixing facilitates bonding between polymer and nanotubes. Tensile characteristics of composites are presented in Table 4, and the results indicate the improvement in both tensile strength and modulus values with f-MWCNT loading. This enhancement reveals the very good reinforcing effect of fMWCNT and thereby better surface bonding between the fMWCNT and the SBR. Tensile strength increased with MWCNT loading and reached up to a value of 6.80 MPa (T10I1) from 1.63 MPa (neat SBR), an improvement of ∼318% over the virgin polymer. Ionic liquid aided dispersion of MWCNT results in better load transfer from the matrix to the MWCNTs, causing an even stress distribution and minimizing the presence of stress concentrated centers which led to improved mechanical properties at high loadings of fMWCNTs. The Young’s modulus value of virgin SBR is

Figure 2. Raman spectra of MWCNT and composites containing 1 phr f-MWCNT and 5 phr f-MWCNT.

The ID/IG ratio of SBR/f-MWCNT composites showed significantly different behavior than the MWCNT. For pristine MWCNT, the intensity of the D band is greater than that of the G band. However, in the case of composites, it is noted that the intensity of the G band became stronger as compared to the D band and the G band shifted to the low wavenumber side. The rapid decrease in the ID/IG ratio (from 1.408 for pristine MWCNT to 0.733 for the T1IL1 sample) and the higher intensity of the G band and D band for composites as compared to pristine MWCNT are due to the rearrangement of f-MWCNTs themselves in the SBR/f-MWCNT composites, leading to a better alignment of MWCNTs in the composites and reduction in the degree of disorder or amount of defects. The downward shift of G band frequency is due to the transfer of mechanical compression from the SBR matrix to the MWCNTs, resulting in shrinkage of the f-MWCNTs. 3.2. Cure Characteristics. Cure characteristics of fMWCNT/SBR nanocomposites are tabulated in Table 2. Normally, measured torque is related to the modulus or stiffness of the material. Minimum torque (ML), maximum torque (MH), and their difference (ΔM) increase as the amount of f-MWCNT increases in all composites, which can be attributed to the formation of an ionic liquid mediated C

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Table 3. Cure Characteristics of f-MWCNT/SBR Nanocomposites: Effect of Ionic Liquid Loading sample

ML (dNm)

MH (dNm)

effective torque increment (ETI) ΔM (dNm)

ts2

t90

effective vulcanization time (EVT)

cure rate index (CRI) min−1

T3IL0 T3IL1 T3IL5 T3IL10

0.66 0.78 0.85 0.92

10.82 11.82 12.61 9.49

10.16 11.04 11.76 8.57

0.99 0.96 0.80 0.68

2.68 2.46 2.31 2.00

1.69 1.5 1.49 1.32

59.17 66.67 67.11 75.75

Figure 3. Typical stress strain curves of SBR nanocomposites with 1− 10 phr f-MWCNT.

Figure 4. Typical stress strain curves of SBR nanocomposites having various concentrations of ionic liquid.

around 1.44 MPa, and 10 phr loading of f-MWCNT leads to a Young’s modulus value around 6.45 MPa due to the addition of a stiffer material to the rubber matrix. The stiffening effect without dropping the strength of the material is because of the good adhesion at the filler/matrix interphase which is dedicated by the physical adsorption and the chemical bonding between nanofillers and polymer. Taking into account the values of measurement error specified in Table 4, sample T1IL1 shows a rather “slightly” higher or even comparable elongation at break as compared to neat SBR. Changes in elongation at break between filled samples are not very significant, although a decreasing tendency with increasing nanotube content can be observed. The T1IL1 loaded sample shows a slightly higher elongation at break as compared to neat SBR due to better interaction between modified MWCNT and rubber and also due to the uncurling of the twisted or bent tubes in the stretch direction along with the rubber. Elongation at break value decreases at higher concentrations of modified MWCNT, and it is ascribed to the presence of physical cross-links in SBR composites which would play as blocks to the deformation, resulting in a decrease of EB values. 3.3.2. Effect of Ionic Liquid Loading. Stress−strain curves of SBR composites containing 3 phr MWCNT with different ratios of ionic liquid are depicted in Figure 4. As observed in Figure 4, the dispersion of MWCNT modified with ionic liquid in the SBR matrix was found to increase the elongation-tobreak ratio of the nanocomposites as well as the tensile strength. It can be observed that the ionic liquid on the

MWCNT surface could not only promote the dispersion of MWCNT in SBR, but it also provided a plasticization effect, which resulted in improved elongation at break.16 However, at a higher concentration of ionic liquid (10 phr), a drop in tensile strength, elongation at break, and tensile modulus is observed, which clearly indicates the plasticizing effect of ionic liquid at high concentration. Ionic liquid acts as a coupling agent between MWCNT and rubber. It is believed that the benzyl group present on the ionic liquid is physically linked to the aromatic moiety in SBR rubber by aromatic π−π interaction; it has strong interactions with the π electron cloud of the MWCNTs due to delocalization of the π electrons in the imidazolium cation.17 3.4. Reinforcement Mechanism and Theoretical Modeling. Several micromechanical models have been introduced for predicting the modulus of ionic liquid modified MWCNT based SBR nanocomposites. These models use several factors like the moduli of the filler and the matrix, the bonding between polymer matrix and reinforcement, aspect ratio, shape factor, and volume fraction of filler. 3.4.1. The Halpin−Kardos Model. The ultimate properties of the composite materials can be analyzed by taking account of the behavior of both the matrix and reinforcing phase together with their amount and geometry using some mathematical equations. These empirical relationships are called Halpin− Kardos (HK) equations, and these are applicable for short fibers which are oriented randomly. Here the prime importance is given to the reinforcement effect by rigid inclusions rather

Table 4. Tensile Strength, % Improvement in TS Compared to Unfilled SBR, Elongation at Break and Tensile Modulus Values of Nanocomposites at Various Filler Loadings sample T0IL0 T1IL1 T3IL1 T5IL1 T7IL1 T10IL1

tensile strength (MPa) 1.63 2.55 3.42 4.63 5.14 6.80

± ± ± ± ± ±

0.12 0.20 0.15 0.34 0.36 0.41

% improvement in TS compared to unfilled SBR

elongation at break (%) 118 126 120 118 113 106

56.82 110.33 175.12 216.17 318.26 D

± ± ± ± ± ±

12 08 07 12 11 09

modulus @ 100% elongation (MPa) 1.44 1.93 3.08 4.13 4.68 6.45

± ± ± ± ± ±

0.23 0.18 0.22 0.30 0.42 0.38

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B than filler−filler or filler−matrix interactions. According to this theory, composites can be imagined as a group of layers comprising unidirectional plies focused on at various angles to give a quasi-isotropic composite Ec 3 ⎡ 1 + 2( l d )f ηLVf ⎤ 5 ⎡ 1 + 2ηTVf ⎤ ⎥+ ⎢ ⎥ = ⎢ Em 8 ⎣⎢ 1 − ηLVf ⎥⎦ 8 ⎢⎣ 1 − ηTVf ⎦⎥

nanocomposites can be computed using the equation given below 2 ⎛ ⎛ t⎞ t⎞ σR = (B − 2.04)⎜1 + ⎟ϕf + 1 − ⎜1 + ⎟ ϕf ⎝ ⎝ R⎠ R⎠

where σR is the reduced tensile strength, the B parameter shows a quantitative measurement for filler−matrix adhesion, t is the interphase thickness, and ϕf is the volume fraction of filler. Figure 6A illustrates the experimental tensile strength and the calculations by modified Kelly−Tyson theory. On assuming an appropriate value for interface thickness t, a reasonable agreement is detected between the theoretical and experimental values. Development of a thin interface is evident from t values which are less than that of the diameter of the MWCNT. Herein, we propose a possible microstructure to illustrate the mechanism for the reinforcing effects of ionic liquid modified MWCNT in SBR matrix (Figure 6B). The superior mechanical performance of SBR nanocomposites is because of the following reasons. Here, surface modification of MWCNT by ionic liquid led to the enlargement of the interfacial zone and this also prevents the rebundling of MWCNT. Aromatic π−π interaction between the styrene moiety present in SBR and the benzyl group of ionic liquid further improves the area of the interphase. Because of the strong interfacial interaction, the amount of macromolecular chains adsorbed on MWCNT are substantial, which improves the effective volume fraction of the reinforcement. 3.4.3. Kraus Plot. According to the Kraus equation20

(1)

where ⎡ Ef − 1 ⎤ ⎢E ⎥ ηT = ⎢ Em ⎥ f ⎣ Em + 2 ⎦

(2)

and

⎡ Ef − 1 ⎤ ⎢ E ⎥ ηL = ⎢ E m 2l ⎥ f ⎣ Em + d ⎦

(4)

(3)

where Ec is the Young’s modulus of the nanocomposite, Em is the Young’s modulus of unfilled rubber, Ef is the Young’s modulus of filler, l/d is the aspect ratio of filler, and Vf is the volume fraction of filler. The Young’s moduli estimated by the model are graphed and compared with experimental values in Figure 5 as a function of f-MWCNT content. It is observed that

Vro

⎛ f ⎞ ⎟ Vrf = 1 − m⎜ ⎝1 − f ⎠

(5)

where Vro is the volume fraction of unfilled rubber, Vrf is the volume fraction of filler rubber, and f is the volume fraction of the filler. Vro/Vrf values of all the compositions have been calculated using toluene as the solvent, and it was plotted against corresponding f/(1 − f) values (Figure 7). A positive correlation exists between the reinforcing ability and the swelling resistance caused by the filler.21 Solvent uptake of the sample decreases with an increase in f-MWCNT loading. This causes an increase in Vrf values, which will decrease the ratio of Vro/Vrf, since Vro is a constant. It can be understood from the graph that Vro/Vrf values decrease with f-MWCNT content. This behavior leads to a negative slope representing the reinforcement effect of the f-MWCNT. The low Vro/Vrf value for the T10IL1 sample indicates its better reinforcing ability compared to other samples. 3.5. Dynamic Mechanical Thermal Analysis. 3.5.1. Storage Modulus. Figure 8A shows the dependency of storage modulus obtained from oscillatory tension deformation on temperature. The storage modulus provides information regarding how much energy can be stored in a material, stiffness, and its elastic nature. Figure 8A represents the sigmoidal variation of storage modulus with temperature and the three regions which represent the three physical states of a material glassy region, glass transition region, and rubbery region. In the glassy region, the rise in storage modulus with fMWCNT concentration is due to the reinforcement effect of fMWCNT in the rubber matrix. Addition of ionic liquid functionalized MWCNT to polymer is good enough to improve interfacial adhesion and hence load transfer. The increase in temperature results in the loss of a tight packing

Figure 5. Theoretical modeling of Young’s modulus using a different aspect ratio for f-MWCNTcomparison with experimental data.

the experimentally determined values resemble the random distribution of f-MWCNTs in the polymer matrix, and as the MWCNT content increases, the aspect ratio of filler in the rubber matrix decreases. Interestingly, up to Vf = 0.024, the best prediction obtained by the Halpin−Kardos model was estimated for α = 100−150. However, above Vf = 0.024, the best fitting was obtained for α less than 100. Here, increasing the amount of MWCNT leads to a decrease in l/d value presumably due to a dramatic decrease in the interparticle distance and the consequent aggregation of MWCNT at higher filler loading because of the van der Waals force of attraction between individual tubes. 3.4.2. Modified Kelly−Tyson Theory. It is possible to get tensile strength values of polymer/MWCNT nanocomposites with the help of Kelly−Tyson theory by supposing the influence of the interphase between the matrix and MWCNT.18 Polymer composition, structure, and properties of the thin polymer layer produced near the interface between the filler and polymer have a significant effect on the interfacial adhesion.19 Moreover, even a minute change in the interface condition will lead to variations in viscoelastic properties of nanocomposites. The tensile strength of polymer/f-MWCNT E

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 6. (A) Experimental data and the predictions of the modified Kelly−Tyson theory. (B) Schematic illustration of the reinforcing model.

The values of the cross-link density versus f-MWCNT concentration are plotted in Figure 8B. The cross-linking structures of f-MWCNT/SBR composites are schematically illustrated in Figure 8C. In general, improvement in the crosslink density is observed by incorporating filler into the rubber matrix due to the introduced physical cross-links. Nanocomposites exhibit high cross-link densities as compared to neat rubber due to the improved interfacial interaction and improved dispersion with the aid of ionic liquid which in turn favors the formation of three-dimensional networks. Moreover, the catalytic action of ionic liquid is another reason behind the increase in cross-link density. The physical networks of filler− matrix and filler−filler also show a key part in the cross-link density of the composites which in turn resulted in the overall improvement in the mechanical behavior of composites. The increased cross-link density decreases the conformational freedom and mobility of SBR chains which also benefit the enhanced storage modulus. 3.5.2. Reinforcement Efficiency Factor. The reinforcement efficiency factor (r) of the composites can be calculated by using the equation introduced by Einstein, and it is given below

Figure 7. Plot of Vro/Vrf versus f/1 − f of SBR vulcanizates.

arrangement of components, leading to a significant decrease of storage modulus in the glass transition region. The rubbery plateau modulus (between 0 and 20 °C) of nanocomposites is greater relative to the virgin polymer which may be attributed to the reinforcing effect by the nanotubes, improved cross-link density, and restricted mobility because of the augmented rubber MWCNT interactions.22 An equilibrium swollen experiment was adopted to determine the cross-link by using the Flory−Rehner equation Crosslink density υ = 1 2Mc

Ec = Em(1 + rVf )

Ec and Em represent the storage modulus of the composite and matrix, respectively, r represents the reinforcement efficiency factor, and Vf is the volume fraction of filler. In this study, the reinforcement efficiency factor increases with filler loading. The reinforcement factor is strongly influenced by the dispersion mechanism (formation of filler−filler networks) and interfacial interaction (hydrodynamic effect). The higher content of fMWCNT can build a much stronger bonding between rubber and filler, and the interfacial interaction is significantly improved. In Figure 9B, variation in reinforcement efficiency

(6)

where Mc is the molecular mass between cross-links and is given by Mc = −

(ρr Vsϕ1/3) ln(1 − ϕ) + ϕ + χϕ2

(8)

(7) F

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 8. (A) Variation in storage modulus as a function of temperature. (B) Cross-link density of SBR/f-MWCNT composites. (C) Cross-linking structures of SBR/f-MWCNT composites.

The low value of βf shows the better effectiveness of the nanofiller. Here, E′g and E′r are considered as the modulus values at −60 and 0 °C, respectively. The βf values for different nanocomposites are calculated at a frequency of 10 Hz and are shown in Figure 9C. The highest effectiveness is observed for the T10IL1 sample, and an increasing trend is observed with increasing filler loading. The better effectiveness of T10IL1 is because of the better stiffening effect of f-MWCNT due to the high aspect ratio and uniform dispersion of f-MWCNT in matrix. 3.5.4. Degree of Entanglement. The following equation is used to calculate the degree of entanglement

factor with Ec/Em values again shows the better interfacial interaction in the T10IL1 sample.

N = E′ RT

The degree of entanglement improves with an increase in fMWCNT loading, and the maximum value is observed for T10IL1. Its value mainly depends on the storage modulus value in the rubbery region. In this region, rubber turns soft; the reinforcement effect of the filler particles becomes substantial because of the limited mobility of the SBR chains. Since a high reinforcement efficiency factor is shown by T10IL1, the entanglement density is more for the same. 3.5.5. Loss Modulus. Figure 10 displays the variation in the loss modulus values with temperature for different SBR nanocomposites. A clear peak around −40 °C is observed for all of the samples which is due to the transitions happening in the SBR initiated by micro-Brownian motion.23 At the peak positions, the loss modulus value increases with an increase in MWCNT concentration due to the reinforcement effect and the formation of a percolated network of functionalized MWCNTs. Another possible explanation for the increased loss modulus for composites as compared to virgin rubber is because of the increased release of the heat of friction at the particle−polymer interface.

Figure 9. Variation in reinforcement efficiency factor with amount of fMWCNTs. (B) Ec/Em with reinforcement factor at 0 °C temperature. (C) Effectiveness of filler for different nanocomposites. (D) Variation in the degree of entanglement of MWCNTs in the SBR matrix at 0 °C.

3.5.3. Effectiveness of Filler (βf). The effectiveness of fillers on the modulus of the nanocomposites can be denoted by βf and is expressed as

( E′ E′ )Composite βf = ( E′ E′ )Pure g

r

g

r

(10)

(9) G

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

between SBR and f-MWCNT is the reason behind the decrease in loss tangent. Ionic liquid functionalized MWCNT has successfully restrained the SBR chains in the locality of the polymer−particle interface due to the enhanced interactions with SBR. 3.5.7. Adhesion Factor. Damping parameters of neat rubber matrix and its composites are useful in predicting the adhesion factor. The adhesion factor A can be calculated as follows A= Figure 10. Variation in loss modulus values with temperature for different SBR nanocomposites.

tan δc 1 −1 (1 − ϕf ) tan δp

(11)

Adhesion factors for various composites are shown in Figure 11C. The mobility of macromolecular chains very near to the filler surface is reduced at a very high level of adhesion, which corresponds to a low value for the adhesion factor. The adhesion factor decreases with an increase in the extent of interaction between the reinforcement and the matrix. As shown in Figure 11C, a drastic decrement in adhesion factor is observed for T10IL1 due to the better interlocking of fMWCNT with SBR matrix. 3.5.8. Area under tan δ Peak. The area under the tan δ vs T curve provides the information about the energy liberated during deformation and the viscoelastic behavior of the filled rubber composites. The dramatic reduction in the area under the tan δ curve is interpreted by considering the limited movement of the rubber chains because of the robust rubber− MWCNT interaction or the physical and chemical adsorption of the rubber chains on the surface of MWCNT. The decrease

3.5.6. Damping Parameters (tan δ). It can be seen from Figure 11A that Tg of the SBR/f-MWCNT nanocomposites is shifted to lower temperature (from −32 °C for pure SBR to −35 and −33 °C for T5IL1 and T10IL1, respectively) in agreement with similar reports.24,25 Two competitive effects must be considered: one is that well-dispersed MWCNTs by ionic liquid will restrict the molecular motion, and this will lead to an increase in Tg. On the other hand, plasticization of ionic liquid in the matrix leads to a decrease in Tg.26 This result suggests that the latter plays a more important role in the Tg shift at higher MWCNT loading. The plasticizing effect of ionic liquids on polymers was investigated by Scott et al.27 The loss tangent decreases with an increase in filler loading with simultaneous broadening of the peak. Strong interaction

Figure 11. (A) Plot of tan δ as a function of temperature. (B) Plot of the ratio of the damping factor of the composite and polymer for different composites. (C) Adhesion factor of different SBR nanocomposites. (D) 3D diagram of the adhesion factor and ratio of the damping factor of the composite and polymer for different amounts of f-MWCNTs/SBR composites. The influence of MWCNT alignment on the damping behavior is revealed by the plot of tan δ against temperature in part A. The tan δ values are represented as the ratio of the viscous part to the elastic part (energy lost/energy stored) of the materials. H

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 12. (A) Ratio of the area of theoretical tan δ to experimental tan δ vs MWCNT content (vol %). (B) Plot of Cv vs f-MWCNT content. (C) Constrained polymer model.

in the tan δ peak might be useful in enhancing the dynamic fatigue properties of the materials because of the reduced damping capability and the heat build-up. The volume of the constrained polymer chains (Vcon) and the relative peak height or area is directly related. Experimental tan δ ∝ Vcons Theoretical tan δ

(12)

Area under Experimental tan δ ∝ Vcons Area under Theoretical tan δ

(13)

diminishes the intrinsic motion of the macromolecules. Molecular packing and free volume, crystallinity, and rigid constraints produced on the polymer by cross-linking are some of the critical factors which influence the glass transition temperature of the polymer.28 During glass transition, the height of the tan δ peak is reduced and peak broadening happened due to the restricted mobility of macromolecules by the addition of f-MWCNT. The amount of constrained polymer chains can be calculated from the reduction in the height of the tan δ peak.29 The assessment of the extent of the immobilized section is very important to explain the enhancement of mechanical properties by nano reinforcement. For linear viscoelastic behavior, the energy loss fraction of the polymer nanocomposite W is related to tan δ by the equation given below

Using the integration area under the experimental tan δ peak can be found from the graph (Figure 10A). The theoretical tan δ value can be calculated using the equation given below ⎛ E′ ⎞ tan δc = Vm⎜ m ⎟ tan δm ⎝ Ec′ ⎠

W=

(14)

By combining eqs 13 and 14, the area under theoretical tan δ can be calculated. On plotting the ratio of area of theoretical to experimental tan δ versus vol % of MWCNT, a linear fit is obtained and the ratio increases with an increase in filler loading. The constrained volume has a positive correlation with the volume fraction of filler due to the reduction in segmental mobility of macromolecular chains with filler content. 3.5.9. Constrained Polymer Region in SBR Nanocomposites. Normally, in all polymer nanocomposites, the incorporation of nano reinforcement leads to the creation of immobilized nanoscopic macromolecular chains developed near the filler surface. Formation of a glassy polymer layer

π tan δ π tan δ + 1

(15)

The value of W at the tan δ peak is obtained from the volume fraction of the constrained region (Cv), the energy loss fraction for the neat rubber (W0), and the volume fraction of the immobilized portion of neat rubber (C0) as follows W=

(1 − Cv)W0 1 − C0

(16)

The Cv values of the nanocomposites were calculated and plotted against the volume fraction of f-MWCNT. Figure 12B shows a linear relationship between the volume fraction of filler and Cv. Here, filler−filler networks and the reinforcing effect I

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

interfacial adhesion among two components in the composites. The outstanding compatibility and strong interfacial adhesion between SBR and f-MWCNTs can be credited to the robust contact between the styrene part present on SBR and the benzyl group present on ionic liquid. Thus, the improved mechanical performance with filler loading can be explained on the basis of the above-mentioned fact. As observed in Figure 15, for pristine MWCNT based rubber composites, bundles or agglomerates of tubes are observed due

result in the development of a well-organized immobilized sector. In the constrained region, the benzyl group present on the surface of a functionalized MWCNT interacts with the styrene group present in SBR mediated through aromatic π−π interaction. Due to this strong secondary interaction, movement of rubber chains in the neighborhood of MWCNTs is reduced (Figure 12C). At higher MWCNT loading, a huge amount of polymer chains is immobilized on the surface of MWCNT, leading to an effective interfacial interaction and improved modulus values. 3.5.10. Cole−Cole Plot. The structural alteration in crosslinked polymers with the incorporation of fillers is evaluated by plotting the loss modulus against the storage modulus, and the resulting plot is known as a Cole−Cole plot.30 The shape of the Cole−Cole plot indicates the nature of the system. For a homogeneous polymeric system, a perfect semicircle is obtained, and modified semicircles are obtained for the twophase system. As observed in Figure 13, imperfect semicircles for all of the filled systems show heterogeneity of the systems due to strong interfacial interaction.

Figure 15. High resolution transmission electron microscopy images of (A) T3IL0 and (B) T3IL1.

to the adhesion between individual tubes by some interactions. After surface modification with ionic liquid, agglomerates of pristine MWCNT are dissociated into individual tubes and oriented throughout the SBR matrix. Figure 16 depicts the

Figure 13. Cole−Cole plot.

3.6. Morphological Study by SEM and TEM. Fractured surface analysis of nanocomposites was done by taking SEM images to evaluate the morphology and reinforcing mechanism of composites. Figure 14A displays an almost smooth and uniform freshly fractured surface of virgin SBR. There is a remarkable change in the morphology of filled nanocomposites which might be due to the increase in interfacial area with fMWCNT loading. SEM images of the composites indicate the development of interconnected networks of f-MWCNT within the composites. Moreover, the rougher aspect of fractured surfaces of filled composites is attributed to the better

Figure 16. TEM pictures of (A) T1IL1, (B) T5IL1, and (C) T10IL1.

TEM pictures of composites containing a varying amount of fMWCNT. At low MWCNT loadings (1 phr), the number of MWCNTs is insufficient to create a three-dimensional network structure in the rubber matrix. As the amount of MWCNT reaches 5 phr, individual nanotubes are in contact with each other and the quantity of nanotubes is appropriate for the development of network structure in rubber. The fine

Figure 14. Scanning electron microscopy pictures of the fractured surface of nanocomposites (A) T0IL0, (B) T1IL1, and (C) T5IL1. J

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

(9) Berriot, J.; Montes, H.; Lequeux, F.; Long, D.; Sotta, P. Evidence for the shift of the glass transition near the particles in silica-filled elastomers. Macromolecules 2002, 35 (26), 9756−62. (10) Gavrilov, A. A.; Chertovich, A. V.; Khalatur, P. G.; Khokhlov, A. R. Study of the mechanisms of filler reinforcement in elastomer nanocomposites. Macromolecules 2014, 47 (15), 5400−8. (11) Pourhossaini, M. R.; Razzaghi-Kashani, M. Effect of silica particle size on chain dynamics and frictional properties of styrene butadiene rubber nano and micro composites. Polymer 2014, 55 (9), 2279−84. (12) Mujtaba, A.; Keller, M.; Ilisch, S.; Radusch, H. J.; Beiner, M.; Thurn-Albrecht, T.; Saalwachter, K. Detection of surface-immobilized components and their role in viscoelastic reinforcement of rubber− silica nanocomposites. ACS Macro Lett. 2014, 3 (5), 481−5. (13) Scotti, R.; Conzatti, L.; D’Arienzo, M.; Di Credico, B.; Giannini, L.; Hanel, T.; Stagnaro, P.; Susanna, A.; Tadiello, L.; Morazzoni, F. Shape controlled spherical (0D) and rod-like (1D) silica nanoparticles in silica/styrene butadiene rubber nanocomposites: Role of the particle morphology on the filler reinforcing effect. Polymer 2014, 55 (6), 1497−506. (14) Abraham, J.; Xavier, P.; Bose, S.; George, S. C.; Kalarikkal, N.; Thomas, S. Investigation into dielectric behaviour and electromagnetic interference shielding effectiveness of conducting styrene butadiene rubber composites containing ionic liquid modified MWCNT. Polymer 2017, 112, 102−15. (15) Bokobza, L.; Zhang, J. Raman spectroscopic characterization of multiwall carbon nanotubes and of composites. eXPRESS Polym. Lett. 2012, 6 (7), 601. (16) Xing, C.; Zhao, M.; Zhao, L.; You, J.; Cao, X.; Li, Y. Ionic liquid modified poly (vinylidene fluoride): crystalline structures, miscibility, and physical properties. Polym. Chem. 2013, 4 (24), 5726−34. (17) Das, A.; Stöckelhuber, K. W.; Jurk, R.; Fritzsche, J.; Klüppel, M.; Heinrich, G. Coupling activity of ionic liquids between diene elastomers and multi-walled carbon nanotubes. Carbon 2009, 47 (14), 3313−21. (18) Zare, Y. Effects of interphase on tensile strength of polymer/ CNT nanocomposites by Kelly−Tyson theory. Mech. Mater. 2015, 85, 1−6. (19) Dorigato, A.; Dzenis, Y.; Pegoretti, A. Filler aggregation as a reinforcement mechanism in polymer nanocomposites. Mech. Mater. 2013, 61, 79−90. (20) Kraus, G. Swelling of filler-reinforced vulcanizates. J. Appl. Polym. Sci. 1963, 7 (3), 861−71. (21) Abraham, J.; Maria, H. J.; George, S. C.; Kalarikkal, N.; Thomas, S. Transport characteristics of organic solvents through carbon nanotube filled styrene butadiene rubber nanocomposites: the influence of rubber−filler interaction, the degree of reinforcement and morphology. Phys. Chem. Chem. Phys. 2015, 17 (17), 11217− 11228. (22) Bokobza, L.; Rahmani, M.; Belin, C.; Bruneel, J. L.; El Bounia, N. E. Blends of carbon blacks and multiwall carbon nanotubes as reinforcing fillers for hydrocarbon rubbers. J. Polym. Sci., Part B: Polym. Phys. 2008, 46 (18), 1939−51. (23) Saji, J.; Khare, A.; Choudhary, R. N.; Mahapatra, S. P. Viscoelastic and dielectric relaxation behavior of multiwalled carbonnanotube reinforced silicon elastomer nanocomposites. J. Polym. Res. 2014, 21 (2), 341. (24) Rooj, S.; Das, A.; Heinrich, G. Tube-like natural halloysite/ fluoroelastomer nanocomposites with simultaneous enhanced mechanical, dynamic mechanical and thermal properties. Eur. Polym. J. 2011, 47 (9), 1746−55. (25) Preghenella, M.; Pegoretti, A.; Migliaresi, C. Thermomechanical characterization of fumed silica-epoxy nanocomposites. Polymer 2005, 46 (26), 12065−72. (26) Marzec, A.; Laskowska, A.; Boiteux, G.; Zaborski, M.; Gain, O.; Serghei, A. The impact of imidazolium ionic liquids on the properties of nitrile rubber composites. Eur. Polym. J. 2014, 53, 139−46.

dispersion of nanotubes in rubber might be due to the aromatic π−π interaction of ionic liquid modified nanotubes to the rubber which generates a stable interfacial layer on the nanotube surface. However, for extreme high loading of fMWCNT (10 phr f-MWCNT), a better interconnected network structure of MWCNT is formed in the rubber because of the reduced particle−particle distance.

4. CONCLUSIONS The mechanical and morphological characteristics of ionic liquid modified MWCNT based SBR nanocomposites have been investigated. Tensile strength increased with f-MWCNT loading and reached up to a value of 6.801 MPa (T10IL1) from 1.626 MPa (neat SBR), an improvement of ∼318% over the neat polymer. The plasticizing effect of ionic liquid reduces the mechanical properties at its higher loadings. The quantity of macromolecular chains adsorbed on the filler surface was calculated from DMA, which further supports the substantial enhancement in the mechanics. The Cole−Cole plot confirms the heterogeneous phase behavior of nanocomposites. Superior mechanical performance can be supported by the microstructural developments in the rubber matrix, as evidenced from various microscopic techniques.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sabu Thomas: 0000-0003-4726-5746 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from Council for Scientific and Industrial Research (CSIR), Delhi, India, is greatly acknowledged.

■

REFERENCES

(1) Salvetat, J. P.; Bonard, J. M.; Thomson, N. H.; Kulik, A. J.; Forro, L.; Benoit, W.; Zuppiroli, L. Mechanical properties of carbon nanotubes. Appl. Phys. A: Mater. Sci. Process. 1999, 69 (3), 255−260. (2) Moniruzzaman, M.; Winey, K. I. Polymer nanocomposites containing carbon nanotubes. Macromolecules 2006, 39 (16), 5194− 5205. (3) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Small but strong: a review of the mechanical properties of carbon nanotube− polymer composites. Carbon 2006, 44 (9), 1624−1652. (4) Abraham, J.; Kailas, L.; Kalarikkal, N.; George, S. C.; Thomas, S. Developing highly conducting and mechanically durable styrene butadiene rubber composites with tailored microstructural properties by a green approach using ionic liquid modified MWCNTs. RSC Adv. 2016, 6 (39), 32493−32504. (5) Eitan, A.; Fisher, F. T.; Andrews, R.; Brinson, L. C.; Schadler, L. S. Reinforcement mechanisms in MWCNT-filled polycarbonate. Compos. Sci. Technol. 2006, 66 (9), 1162−1173. (6) Heinrich, G.; Klüppel, M.; Vilgis, T. A. Reinforcement of elastomers. Curr. Opin. Solid State Mater. Sci. 2002, 6 (3), 195−203. (7) Dorigato, A.; Dzenis, Y.; Pegoretti, A. Filler aggregation as a reinforcement mechanism in polymer nanocomposites. Mech. Mater. 2013, 61, 79−90. (8) Pérez-Aparicio, R.; Vieyres, A.; Albouy, P. A.; Sanséau, O.; Vanel, L.; Long, D. R.; Sotta, P. Reinforcement in natural rubber elastomer nanocomposites: breakdown of entropic elasticity. Macromolecules 2013, 46 (22), 8964−72. K

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B (27) Scott, M. P.; Brazel, C. S.; Benton, M. G.; Mays, J. W.; Holbrey, J. D.; Rogers, R. D. Application of ionic liquids as plasticizers for poly (methyl methacrylate). Chem. Commun. 2002, 13, 1370−1. (28) Hande, V. R.; Rath, S. K.; Rao, S.; Praveen, S.; Sasane, S.; Patri, M. Effect of constrained amorphous region on properties of acid−base polyelectrolyte membranes based on sulphonated poly (ether ether ketone) and a non-conjugateddiamine. J. Membr. Sci. 2016, 499, 1−1. (29) Meera, A. P.; Said, S.; Grohens, Y.; Thomas, S. Nonlinear viscoelastic behavior of silica-filled natural rubber nanocomposites. J. Phys. Chem. C 2009, 113 (42), 17997−8002. (30) Harris, B.; Braddell, O. G.; Almond, D. P.; Lefebvre, C.; Verbist, J. Study of carbon fibre surface treatments by dynamic mechanical analysis. J. Mater. Sci. 1993, 28 (12), 3353−66.

L

DOI: 10.1021/acs.jpcb.7b10479 J. Phys. Chem. B XXXX, XXX, XXX−XXX