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The effect of choline-based ionic liquid on CNTs' arrangement in epoxy resin matrix
Materials and Design 91 (2016) 180–185
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The effect of choline-based ionic liquid on CNTs' arrangement in epoxy resin matrix Hormoz Gholami, Hamed Arab, Maryam Mokhtarifar, Morteza Maghrebi, Majid Baniadam ⁎ Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 27 July 2015 Received in revised form 24 November 2015 Accepted 25 November 2015 Available online 30 November 2015 Keywords: Carbon nanotubes Ionic liquid Deep eutectic solvents Bucky gel Nanocomposite Electrical conductivity
a b s t r a c t This work focuses on the effect of ionic liquids (ILs) on the arrangement of carbon nanotubes (CNTs) in CNTs–epoxy composites. Electrical conductivity experiments on epoxy–CNTs–ILs suggested that, compared to Ethaline, Oxaline and Reline, Glyceline had a more distinguishable effect on the CNTs' arrangement in the epoxy–CNTs–Glyceline (CG) composites based on scanning electron microscope results. The electrical conductivity of CG composites was improved by increasing the weight content of Glyceline up to 50 times that of CNTs (Gly/CNTs = 50), and for weight ratios higher than 50, a downward trend was observed. According to the proposed model, the increase in electrical conductivity seems to be a result of the formation of liquid vesicles which push the CNTs closer outside the Glyceline vesicles. Conversely, the drop in the electrical conductivity can be due to the enlargement of the vesicles, which dissolve CNTs and reduce their content in the epoxy. In addition, the tensile strength of CG composites increased by ratio of Gly/CNTs = 10 followed by a significant drop thereafter. Thermal stability of the CG composites was seen to decrease as Glyceline content is increased in the composite. This reduction was attributed to the early decomposition of Glyceline vesicles present inside the epoxy matrix. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Having such excellent properties as low density, thermal stability and high mechanical performance, epoxy resin has received considerable attention in the areas of coating, adhesives, composites, and many other applications [1–3]. However, low electrical and thermal conductivities restrict the use of this polymer in some other applications such as sensors, electrostatic discharge dissipaters [4–6] and heat distributor [7]. Production of composites by incorporating CNTs into epoxy resin is a solution for improving their properties. CNTs are useful in the production of light-weight, resistant polymer composites which are electrically and thermally conductive. However, because CNTs tend to agglomerate, attaining homogeneous distribution and good dispersion are some of the major challenges for producing nanocomposites [5]. One of the solutions proposed to prevent CNTs aggregation is to use ionic liquids. Ionic liquids are organic salt entirely composed of ions which exist in liquid state in temperature lower than 100 °C [8]. Ionic liquids have received considerable attention in the production of electrochemical batteries, actuators, and electrochromic devices due to their unique properties including high ionic conductivity and good solubility [9–11]. Nevertheless, the relatively poor biodegradability and high cost of common ILs impede their widespread exploitation. Recently, a new class of ionic liquids called deep eutectic solvents (DESs) has been
⁎ Corresponding author. E-mail address: [email protected] (M. Baniadam).
http://dx.doi.org/10.1016/j.matdes.2015.11.096 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
introduced. DESs comprise an eutectic mixture with two or more components and have a freezing point much lower than its individual substances [12,13]. DESs have common properties of common ILs such as non-reactivity with water and non-volatility; in addition, easy production, biocompatibility, biodegradability, low cost, and non-toxicity are among their advantages in comparison with traditional ILs [14,15]. Generally, similarities between two classes of molecular ILs and eutectic mixtures lead to call them all ILs. Here, choline chloride-based DESs is used which Abbott et al. reported recently [13,16]. Choline chloride is an organic salt based on quaternary amine, which is sold as a vitamin supplement; therefore, it has been produced in large amounts and can be found with ease and relatively low cost [14,17]. Choline chloride can form hydrogen bonds with different materials including amides, carboxylic acids and alcohols groups to yield ILs with different [13,18,19]. In this study, four types of choline chloride-based ionic liquids were used under the trade names of Glyceline (1 mol ChCl:2 mol glycerol), Ethaline (1 mol ChCl:2 mol ethylene glycol), Reline (1 mol ChCl:2 mol urea) and Oxaline (1 mol ChCl:1 mol oxalic acid). These ratios give combinations with minimum viscosity and freezing point according to Abbott et al. [13,16,19]. After Fukushima et al. found out that ILs have the potential to disperse CNTs, a great deal of research was focused on epoxy–CNTs–ILs (CIL) hybrids [20]. These hybrids fall into covalent (chemical) and non-covalent (physical) categories. Most covalent methods require a multistep reaction. Moreover, they usually lead to defects in the surface of the CNTs and a drop in their mechanical and electrical properties. In
H. Gholami et al. / Materials and Design 91 (2016) 180–185
contrast to covalent methods, non-covalent approaches cause less damage to the surface of the CNTs. Bucky gel is readily produced by rubbing a mixture of CNTs and ILs and it is the most common physical methods to produce CNTs–IL composites [21–23]. Subramanian et al. studied the effects of 1-butyl 3-methyl imidazolium-bis (trifluoromethylsulphonyl) imid (BMI) ionic liquid on MWCNTs–polychloroprene rubber composite properties [24]. They investigated the effects of IL/CNTs weight ratio (in the range of 0 to 20 wt/wt) on the electrical properties of the composite and observed that increasing the IL/CNTs weight ratio can improve dispersion of CNTs and so the electrical conductivity of the composite. However, they did not study composites with higher weight ratios of IL/CNTs. In this study, a much wider range of IL/CNTs were investigated on the electrical conductivity and thermal stability of CIL composites. This range is from 0 to 116 wt/wt, which is much larger than that reported in the earlier studies. Based on the results obtained during this study, it was suggested that IL/CNTs weight ratios could control arrangements of the CNTs. 2. Experimental section 2.1. Materials Epoxy resin Ep 8035 DGEBA (bisphenol F epoxy) and 1150 curing agent were purchased from Epoxiran Company, Iran. Commercial multi-walled carbon nanotubes (MWCNTs) synthesized by chemical vapor deposition (diameter b 30 nm, length of 5–15 μm and purity N 95%) were purchased from Shenzhen Nano-Tech Port Co. Other chemicals were purchased from Merck Inc. All the chemicals were used as received without any further treatment.
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Table 2 Onset temperatures of three weight loss stages in the thermal decomposition of the CG composites. CNT (wt.%)
Glyceline (%)
T1 (°C)
T2 (°C)
T3 (°C)
0 0.3 0.3 0.3 0.3
0 0 3 10 15
152 160 150 147 143
345 356 342 334 330
508 527 520 510 488
poured into rectangular (60 × 10 × 2 mm3) shaped and dumbbell specimen silicone rubber molds and kept at room temperature for 48 h to ensure completion of the curing process. In order to produce CNTs-free ILs-containing (C0IL) samples, the epoxy resin and ILs were mixed without any CNTs and then placed in an ultrasonic bath for 1 h. The other steps were just the same as mentioned above for CIL composites. Electrical conductivity and thermogravimetric analysis were carried out to study the effect of ILs on the produced composite. A high voltage power (max 15 kV) was applied to determine electrical conductivity. The tensile strength of the samples were measured using a Zwick/Z250 tensile testing machine in accordance with ASTM D638 at constant crosshead speed of 1 mm/min, and thermogravimetric analysis was carried out by TG-Shimadzu 50 set at a rate of 10 °C/min. Electron microscope images were also used to observe the morphology of the nanocomposites as well as CNTs arrangements in the epoxy matrix.
3. Results and discussion 3.1. Electrical conductivity
2.2. Synthesis of ionic liquids In order to produce the choline chloride-based ILs, first the complex agent was added to choline chloride in suitable ratios and they were mixed at 80 °C for 30 min. The obtained mixture was a homogeneous colorless liquid with a rather high viscosity. The characteristics of ionic liquids and their components were presented in Table 1 and the onset temperatures of the CG composites in the thermal decomposition is given in Table 2. 2.3. Preparation of CIL composites To produce CIL composites, CNTs and the ionic liquid were first mixed in different ratios and were ground in a mortar for 15 min. The obtained bucky gel was added to the epoxy resin and then sonicated for 3 h. Thereafter, the curing agent was added in stoichiometric ratio to the CNTs–epoxy mixture stirring for 10 min. The final mixture was
Electrical conductivity measurements were performed on the four ILs and their corresponding CIL composites to identify and choose the most suitable IL for the rest of the experiments. Fig. 1 depicts the electrical conductivity (at 25 °C) of four choline-based ILs namely Ethaline, Glyceline, Oxaline and Reline versus the electrical conductivity of their corresponding epoxy composites containing 0.3% CNTs and 10% of mentioned ILs (C0.3IL10). As seen from the figure, Ethaline has the highest conductivity and Glyceline, Oxaline and Reline rank next in a descending order. However, the Glyceline-containing composite displays the highest electrical conductivity among the other comparable composites. Since the electrical conductivity of choline-based ILs is around 1010 × that of cured epoxy, ILs are expected to play a major role in the enhancement of electrical conductivity of both epoxy and CIL composites by contributing to the electrical properties of the epoxy matrix. On the contrary, Glyceline yields a more conductive composite compared to Ethaline. Therefore, it can be inferred that positive role of Glyceline in
Table 1 Characteristics of ionic liquids and their components. Choline chloride (ChCl)
Melting point = 302 °C
Complex agent
Eutectic based ILs
Glycerol Freezing point = 18 °C
Glycerol (Glycerol:ChCl = 2:1) Freezing point = −20 °C Ethaline (Ethylene glycol:ChCl = 2:1) Freezing point = −10 °C Reline (Urea:ChCl = 2:1) Freezing point = 12 °C Oxaline (Oxalic acid:ChCl = 1:1) Freezing point = 13 °C
Ethylene glycol Freezing point = −12 °C Urea Melting point = 133 °C Oxalic acid Melting point = 102
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Fig. 1. The electrical conductivity of C0.3IL10 composites containing Ethaline, Glyceline, Oxaline and Reline vs. the electrical conductivity of the ILs.
the conductivity of the resultant composite is not due to its own conductivity, but rather to an indirect role, such as assisting CNTs to have a desired in the composite. In order to shed some light on the effects of arrangement, composites containing 0.3% CNTs with different mass fractions of Glyceline (C0.3G) and composites without CNTs with different mass fractions of Glyceline (C0G) were investigated. Fig. 2a shows the variation of electrical conductivity in C0G compounds as a function of Glyceline mass fraction. As it can be observed, by adding just 5% Glyceline to the epoxy, the electrical conductivity of epoxy (~10 × 10-13S/cm) is enhanced by 2 orders of magnitude. However, by further increasing of the Glyceline, the
Fig. 2. Variations of electrical conductivity versus Glyceline mass fraction: (a) C0G compounds and (b) C0.3G composites.
conductivity of C0G compounds increases less sharply and reaches a plateau. Finally, by adding 35% Glyceline, the electrical conductivity of C0G35 reaches 1.1×10-10S/cm, which is still 7 orders of magnitude smaller than that of pure Glyceline. Expectedly, the addition of Glyceline to epoxy can increase the conductivity of the epoxy due to the presence of ions of Glyceline. However, the low conductivity of C0G compounds compared to that of pure Glyceline suggests to the discontinuity of Glyceline phase and the lack of mobility of ions to freely move across the epoxy matrix [2]. Fig. 2b shows the variation of electrical conductivity of C0.3G composites versus the mass fraction of Glyceline. As it can be observed, the electrical conductivity of C0.3G composites is much higher than that of cured epoxy while the difference between the electrical conductivity of C0G compounds and the cured epoxy is insignificant. Thus, the major contribution to the enhancement of the electrical conductivity of C0.3G composites can be attributed to the CNTs, as Glyceline alone has had little effect on improving the electrical conductivity of the epoxy [6]. Based on recent studies [5,6,24,25] due to their large aspect ratios and unique graphite structure, CNTs can form a conductive network among themselves and give rise to conductivity of the whole composite. As it can be observed, electrical conductivity of C0.3G composites rises by increasing mass fraction of the Glyceline up to 15% (Gly/CNTs = 50), followed by a decrease in the higher fractions. The former was observed by Subramanian et al. in BMI–CNT composite with BMI/CNTs weight ratios up to 20. Regardless of what may really happened to CNT, the noticeable variations in the conductivity of these composites with an increase in Glyceline content implies a synergy between CNT and Glyceline. Although Glyceline droplets cannot form an electrical conductive pathway throughout the C0.3G composites, they may help CNTs to form a more complete/ incomplete conductive network via the control of the way carbon nanotubes are laid out. In the other words, Glyceline may have a significant indirect effect on the electrical conductivity of the composite through arranging conductive CNTs in the matrix. As suggested by Fukushima [20] studied the effect of ILs on CNTs; based on their findings, ILs may weaken the Van der Waals interactions among CNTs and improve their dispersion via forming cation–π interactions with CNTs. Thus, the increase in electrical conductivity upon the addition of small amounts of Glyceline can be attributed to the improvement in the degree of dispersion of the CNTs, which, in turn, boosts their electrical network [24,26]. This favorable effect of Glyceline on CNT dispersion could be seen in the SEM images (Fig. 3a,b) of the C0.3G composites. While Fig. 3a shows a poor dispersion of CNTs in the composite without Glyceline (i.e., the Glyceline-free composite), an improvement in CNTs dispersion and reduction of aggregates at Gly/CNTs weight ratio of 10 is evident from Fig. 3b. Therefore, SEM images further confirm the improvement of CNTs dispersion in the presence of Glyceline. SEM images of C0.3G15 composite (Gly/CNTs = 50) (Fig. 3c) reveal formation of vesicles of Glyceline (~22 μm) at such high fraction of the Glyceline. These vesicles simply occupy a proportion of the composite's space and could push CNTs closer towards each other outside this occupied space (Fig. 3d). This could enhance the conductive network and a lead to a higher conductivity within the composite. Considering that electrical conductivity decreases for Glyceline contents of over 15% (Gly/CNTs N 50), it can be inferred that CNTs arrangement probably undergoes changes in higher Gly/CNTs weight ratios. SEM image for C0.3G35 composite (Fig. 3e) shows an ~ 50% increase in the size of vesicles. The decline in conductivity in case of Gly/CNTs weight ratios greater than 50 may be associated with the enlargement of Glyceline vesicles. It can be conceivable that larger vesicles start to dissolve CNTs already dispersed in the epoxy matrix, depleting its content in the matrix. Obviously, the lower content of CNT in the continuous matrix of epoxy could be translated as a weaker inter content among CNTs, and so lower conductivity throughout the composite. Fig. 4 schematically presents different arrangements of CNTs and Glyceline vesicles upon increase in mass fraction of Glyceline in the epoxy.
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Fig. 3. SEM images obtained from (a) C0.3G0, (b) C0.3G3, (c, d) C0.3G15, and (e) C0.3G35 composites.
3.2. Tensile strength Fig. 5 depicts the ultimate strength of C0.3G composites based on Glyceline weight percent changes. As it can be observed, at first, an increase in Glyceline weight percent causes to enhance the ultimate strength of C0.3G composites. This ascending trend continues until 3% Glyceline (Gly/CNT = 10), where it reaches to its maximum. However, further addition of Glyceline causes a significant drop in ultimate strength. CNTs tend to agglomerate due to their strong Van der Waals forces, which impede the CNTs dispersibility in resin epoxy matrix [27]. ILs interact with CNTs by means of cation–π interaction, which decrease the strong Van der Waals forces between CNTs and increase their dispersibility as a result [24].
On the other hand, the improved CNTs dispersion and lack of agglomeration between CNTs could enhance the conductance between CNTs inside the epoxy and increase ultimate strength of the composite. Conversely, in high mass ratios, ILs act as a plasticizer agent in the matrix which cause to a significant decline in the ultimate strength of the composite [24]. 3.3. Thermogravimetric analysis Fig. 6 shows the thermograms of Glyceline, cured epoxy (neat), and C0.3G composites for different mass fractions of Glyceline. Obviously all the composites have three steps of weight loss, similar to the neat epoxy. The first step, which occurs relatively slowly, is probably related
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Fig. 4. The proposed schematic representation for the network of CNTs with increasing mass fraction of the IL.
to the burning of the secondary hydroxyl group of the propyl chain in the epoxy resin. The noticeably high weight loss in the next stage could be associated with the decomposition of the group of bisphenol A from the epoxy polymer. Finally, the last weight loss that occurs at temperatures exceeding 500 °C is obviously due to carbon decomposition [28]. Table 1 gives the onset temperature for each of the weight loss steps in the thermograms shown in Fig. 6. According to this Table, T1, T2 and T3 are initial weight loss temperature of first, second and third step of each composite respectively. Compared with cured epoxy, all the steps of weight loss occurring in C0.3G0 sample have a higher onset temperature indicating a slight higher thermal stability of the composite in the presence of CNTs. This can be ascribed to the higher thermal conductivity of the CNTs that results in a more uniform distribution of temperature within the composite. Therefore, formation of hotspots on the surface of the sample is delayed; i.e., the decomposition reactions take place at higher temperatures [29–31]. The results presented in Table 1 show that the thermal stability of C0.3G samples decrease with an increase in the mass fraction of Glyceline. Glyceline decompose sat the temperature of 185 °C as observed in its TGA thermogram. A comparison of the curves of composites that contain Glyceline and those without Glyceline reveals that the first step of weight loss is related to Glyceline decomposition. This weight loss is more noticeable for higher mass fractions of Glyceline. A
Fig. 5. Variations of ultimate strength versus Glyceline mass fraction.
reason for the reduced resistance of C0.3G composites can be the early decomposition of Glyceline compared with epoxy. This could lead to the faster decomposition of epoxy in the vicinity of the Glyceline droplets, and so the lower of thermal stability of the whole composites. A schematic representation of this phenomenon is suggested in Fig. 7. 4. Conclusions In this work, the effects of choline-chloride-based ionic liquids on the properties of polymer nanocomposites reinforced with CNTs are investigated. The results show that epoxy–CNT–ILs (CIL) composites fabricated with Glyceline have the highest electrical conductivity even though Ethaline itself stands first in terms of electrical conductivity compared with the other tested ILs. Moreover, the addition of Glyceline to composites containing 0.3% CNTs with different mass fractions of Glyceline (C0.3G) up to a Gly/CNTs weight ratio of 50 increase the electrical conductivity of the composite to 7 × 10-6 S/cm followed by a decrease in the higher weight ratios of Gly/CNTs. SEM images revealed the fact that the reason for the improvement in conductivity is the formation of small Glyceline vesicles throughout the composites. These vesicles could push CNTs to each others,
Fig. 6. TGA curves of Glyceline, cured neat epoxy, and C0.3G composites for several different mass fractions of Glyceline.
H. Gholami et al. / Materials and Design 91 (2016) 180–185
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Fig. 7. Schematic representation of Glyceline decomposition in the composite.
forming a more interconnected conductive network. However, in the higher weight ratios of Gly/CNTs (N 50) Glyceline vesicles grow larger and probably dissolve CNTs that results in fewer CNTs in the epoxy matrix. Expectedly, this would weaken connectivity among CNTs network and so lower the electrical conductivity. In the case of 0% CNT/Glyceline (C0G) samples, electrical conductivity follows a steady incline in effect of higher mass fraction of Glyceline, due to the improved pathways of ions. In addition, increasing the weight ratio of Glyceline to 3% (Gly/CNTs = 10) leads to an enhancement in ultimate strength of the composite followed with a significant drop thereafter. The ultimate strength enhancement can be attributed to better arrangement (dispersion) of the CNTs by cation–π interactions between Glyceline and CNTs. In contrast, decreasing in ultimate strength in Gly/CNTs higher ratios can be regarded due to plasticizing effect of Glyceline. Finally, an increasing mass fraction of Glyceline reduces the thermal stability of C0.3G composites. This observation can be attributed to an early thermal decomposition of Glyceline compared with the epoxy resin. Acknowledgments The authors are grateful to Iran Nanotechnology Initiative Council for financial support. References [1] Z. Yue, M.Y. Yu, X. Lan, Study on properties of carbon nanotubes/epoxy resin composite prepared by in situ polymerization, Adv. Mater. Res. 750 (2013) 132–135. [2] K. Matsumoto, T. Endo, Confinement of ionic liquid by networked polymers based on multifunctional epoxy resins, Macromolecules 41 (2008) 6981–6986. [3] K. Yang, M. Gu, Y. Jin, Cure behavior and thermal stability analysis of multiwalled carbon nanotube/epoxy resin nanocomposites, J. Appl. Polym. Sci. 110 (2008) 2980–2988. [4] Y.S. Song, J.R. Youn, Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites, Carbon 43 (2005) 1378–1385. [5] F. Gardea, D.C. Lagoudas, Characterization of electrical and thermal properties of carbon nanotube/epoxy composites, Compos. Part B 56 (2014) 611–620. [6] M. Arjmand, M. Mahmoodi, G.A. Gelves, S. Park, U. Sundararaj, Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate, Carbon 49 (2011) 3430–3440. [7] A. Moisala, Q. Li, I. Kinloch, A. Windle, Thermal and electrical conductivity of singleand multi-walled carbon nanotube-epoxy composites, Compos. Sci. Technol. 66 (2006) 1285–1288. [8] B.G. Soares, A.A. Silva, S. Livi, J. Duchet‐Rumeau, J.F. Gerard, New epoxy/jeffamine networks modified with ionic liquids, J. Appl. Polym. Sci. 131 (2014). [9] T. Torimoto, T. Tsuda, K.I. Okazaki, S. Kuwabata, New frontiers in materials science opened by ionic liquids, Adv. Mater. 22 (2010) 1196–1221. [10] Z. Wang, X. Yang, Q. Wang, H.T. Hahn, S.-G. Lee, K.-H. Lee, Z. Guo, Epoxy resin nanocomposites reinforced with ionized liquid stabilized carbon nanotubes, Int. J. Smart Nano Mater. 2 (2011) 176–193.
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The effect of choline-based ionic liquid on CNTs' arrangement in epoxy resin matrix Hormoz Gholami, Hamed Arab, Maryam Mokhtarifar, Morteza Maghrebi, Majid Baniadam ⁎ Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 27 July 2015 Received in revised form 24 November 2015 Accepted 25 November 2015 Available online 30 November 2015 Keywords: Carbon nanotubes Ionic liquid Deep eutectic solvents Bucky gel Nanocomposite Electrical conductivity
a b s t r a c t This work focuses on the effect of ionic liquids (ILs) on the arrangement of carbon nanotubes (CNTs) in CNTs–epoxy composites. Electrical conductivity experiments on epoxy–CNTs–ILs suggested that, compared to Ethaline, Oxaline and Reline, Glyceline had a more distinguishable effect on the CNTs' arrangement in the epoxy–CNTs–Glyceline (CG) composites based on scanning electron microscope results. The electrical conductivity of CG composites was improved by increasing the weight content of Glyceline up to 50 times that of CNTs (Gly/CNTs = 50), and for weight ratios higher than 50, a downward trend was observed. According to the proposed model, the increase in electrical conductivity seems to be a result of the formation of liquid vesicles which push the CNTs closer outside the Glyceline vesicles. Conversely, the drop in the electrical conductivity can be due to the enlargement of the vesicles, which dissolve CNTs and reduce their content in the epoxy. In addition, the tensile strength of CG composites increased by ratio of Gly/CNTs = 10 followed by a significant drop thereafter. Thermal stability of the CG composites was seen to decrease as Glyceline content is increased in the composite. This reduction was attributed to the early decomposition of Glyceline vesicles present inside the epoxy matrix. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Having such excellent properties as low density, thermal stability and high mechanical performance, epoxy resin has received considerable attention in the areas of coating, adhesives, composites, and many other applications [1–3]. However, low electrical and thermal conductivities restrict the use of this polymer in some other applications such as sensors, electrostatic discharge dissipaters [4–6] and heat distributor [7]. Production of composites by incorporating CNTs into epoxy resin is a solution for improving their properties. CNTs are useful in the production of light-weight, resistant polymer composites which are electrically and thermally conductive. However, because CNTs tend to agglomerate, attaining homogeneous distribution and good dispersion are some of the major challenges for producing nanocomposites [5]. One of the solutions proposed to prevent CNTs aggregation is to use ionic liquids. Ionic liquids are organic salt entirely composed of ions which exist in liquid state in temperature lower than 100 °C [8]. Ionic liquids have received considerable attention in the production of electrochemical batteries, actuators, and electrochromic devices due to their unique properties including high ionic conductivity and good solubility [9–11]. Nevertheless, the relatively poor biodegradability and high cost of common ILs impede their widespread exploitation. Recently, a new class of ionic liquids called deep eutectic solvents (DESs) has been
⁎ Corresponding author. E-mail address: [email protected] (M. Baniadam).
http://dx.doi.org/10.1016/j.matdes.2015.11.096 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
introduced. DESs comprise an eutectic mixture with two or more components and have a freezing point much lower than its individual substances [12,13]. DESs have common properties of common ILs such as non-reactivity with water and non-volatility; in addition, easy production, biocompatibility, biodegradability, low cost, and non-toxicity are among their advantages in comparison with traditional ILs [14,15]. Generally, similarities between two classes of molecular ILs and eutectic mixtures lead to call them all ILs. Here, choline chloride-based DESs is used which Abbott et al. reported recently [13,16]. Choline chloride is an organic salt based on quaternary amine, which is sold as a vitamin supplement; therefore, it has been produced in large amounts and can be found with ease and relatively low cost [14,17]. Choline chloride can form hydrogen bonds with different materials including amides, carboxylic acids and alcohols groups to yield ILs with different [13,18,19]. In this study, four types of choline chloride-based ionic liquids were used under the trade names of Glyceline (1 mol ChCl:2 mol glycerol), Ethaline (1 mol ChCl:2 mol ethylene glycol), Reline (1 mol ChCl:2 mol urea) and Oxaline (1 mol ChCl:1 mol oxalic acid). These ratios give combinations with minimum viscosity and freezing point according to Abbott et al. [13,16,19]. After Fukushima et al. found out that ILs have the potential to disperse CNTs, a great deal of research was focused on epoxy–CNTs–ILs (CIL) hybrids [20]. These hybrids fall into covalent (chemical) and non-covalent (physical) categories. Most covalent methods require a multistep reaction. Moreover, they usually lead to defects in the surface of the CNTs and a drop in their mechanical and electrical properties. In
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contrast to covalent methods, non-covalent approaches cause less damage to the surface of the CNTs. Bucky gel is readily produced by rubbing a mixture of CNTs and ILs and it is the most common physical methods to produce CNTs–IL composites [21–23]. Subramanian et al. studied the effects of 1-butyl 3-methyl imidazolium-bis (trifluoromethylsulphonyl) imid (BMI) ionic liquid on MWCNTs–polychloroprene rubber composite properties [24]. They investigated the effects of IL/CNTs weight ratio (in the range of 0 to 20 wt/wt) on the electrical properties of the composite and observed that increasing the IL/CNTs weight ratio can improve dispersion of CNTs and so the electrical conductivity of the composite. However, they did not study composites with higher weight ratios of IL/CNTs. In this study, a much wider range of IL/CNTs were investigated on the electrical conductivity and thermal stability of CIL composites. This range is from 0 to 116 wt/wt, which is much larger than that reported in the earlier studies. Based on the results obtained during this study, it was suggested that IL/CNTs weight ratios could control arrangements of the CNTs. 2. Experimental section 2.1. Materials Epoxy resin Ep 8035 DGEBA (bisphenol F epoxy) and 1150 curing agent were purchased from Epoxiran Company, Iran. Commercial multi-walled carbon nanotubes (MWCNTs) synthesized by chemical vapor deposition (diameter b 30 nm, length of 5–15 μm and purity N 95%) were purchased from Shenzhen Nano-Tech Port Co. Other chemicals were purchased from Merck Inc. All the chemicals were used as received without any further treatment.
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Table 2 Onset temperatures of three weight loss stages in the thermal decomposition of the CG composites. CNT (wt.%)
Glyceline (%)
T1 (°C)
T2 (°C)
T3 (°C)
0 0.3 0.3 0.3 0.3
0 0 3 10 15
152 160 150 147 143
345 356 342 334 330
508 527 520 510 488
poured into rectangular (60 × 10 × 2 mm3) shaped and dumbbell specimen silicone rubber molds and kept at room temperature for 48 h to ensure completion of the curing process. In order to produce CNTs-free ILs-containing (C0IL) samples, the epoxy resin and ILs were mixed without any CNTs and then placed in an ultrasonic bath for 1 h. The other steps were just the same as mentioned above for CIL composites. Electrical conductivity and thermogravimetric analysis were carried out to study the effect of ILs on the produced composite. A high voltage power (max 15 kV) was applied to determine electrical conductivity. The tensile strength of the samples were measured using a Zwick/Z250 tensile testing machine in accordance with ASTM D638 at constant crosshead speed of 1 mm/min, and thermogravimetric analysis was carried out by TG-Shimadzu 50 set at a rate of 10 °C/min. Electron microscope images were also used to observe the morphology of the nanocomposites as well as CNTs arrangements in the epoxy matrix.
3. Results and discussion 3.1. Electrical conductivity
2.2. Synthesis of ionic liquids In order to produce the choline chloride-based ILs, first the complex agent was added to choline chloride in suitable ratios and they were mixed at 80 °C for 30 min. The obtained mixture was a homogeneous colorless liquid with a rather high viscosity. The characteristics of ionic liquids and their components were presented in Table 1 and the onset temperatures of the CG composites in the thermal decomposition is given in Table 2. 2.3. Preparation of CIL composites To produce CIL composites, CNTs and the ionic liquid were first mixed in different ratios and were ground in a mortar for 15 min. The obtained bucky gel was added to the epoxy resin and then sonicated for 3 h. Thereafter, the curing agent was added in stoichiometric ratio to the CNTs–epoxy mixture stirring for 10 min. The final mixture was
Electrical conductivity measurements were performed on the four ILs and their corresponding CIL composites to identify and choose the most suitable IL for the rest of the experiments. Fig. 1 depicts the electrical conductivity (at 25 °C) of four choline-based ILs namely Ethaline, Glyceline, Oxaline and Reline versus the electrical conductivity of their corresponding epoxy composites containing 0.3% CNTs and 10% of mentioned ILs (C0.3IL10). As seen from the figure, Ethaline has the highest conductivity and Glyceline, Oxaline and Reline rank next in a descending order. However, the Glyceline-containing composite displays the highest electrical conductivity among the other comparable composites. Since the electrical conductivity of choline-based ILs is around 1010 × that of cured epoxy, ILs are expected to play a major role in the enhancement of electrical conductivity of both epoxy and CIL composites by contributing to the electrical properties of the epoxy matrix. On the contrary, Glyceline yields a more conductive composite compared to Ethaline. Therefore, it can be inferred that positive role of Glyceline in
Table 1 Characteristics of ionic liquids and their components. Choline chloride (ChCl)
Melting point = 302 °C
Complex agent
Eutectic based ILs
Glycerol Freezing point = 18 °C
Glycerol (Glycerol:ChCl = 2:1) Freezing point = −20 °C Ethaline (Ethylene glycol:ChCl = 2:1) Freezing point = −10 °C Reline (Urea:ChCl = 2:1) Freezing point = 12 °C Oxaline (Oxalic acid:ChCl = 1:1) Freezing point = 13 °C
Ethylene glycol Freezing point = −12 °C Urea Melting point = 133 °C Oxalic acid Melting point = 102
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Fig. 1. The electrical conductivity of C0.3IL10 composites containing Ethaline, Glyceline, Oxaline and Reline vs. the electrical conductivity of the ILs.
the conductivity of the resultant composite is not due to its own conductivity, but rather to an indirect role, such as assisting CNTs to have a desired in the composite. In order to shed some light on the effects of arrangement, composites containing 0.3% CNTs with different mass fractions of Glyceline (C0.3G) and composites without CNTs with different mass fractions of Glyceline (C0G) were investigated. Fig. 2a shows the variation of electrical conductivity in C0G compounds as a function of Glyceline mass fraction. As it can be observed, by adding just 5% Glyceline to the epoxy, the electrical conductivity of epoxy (~10 × 10-13S/cm) is enhanced by 2 orders of magnitude. However, by further increasing of the Glyceline, the
Fig. 2. Variations of electrical conductivity versus Glyceline mass fraction: (a) C0G compounds and (b) C0.3G composites.
conductivity of C0G compounds increases less sharply and reaches a plateau. Finally, by adding 35% Glyceline, the electrical conductivity of C0G35 reaches 1.1×10-10S/cm, which is still 7 orders of magnitude smaller than that of pure Glyceline. Expectedly, the addition of Glyceline to epoxy can increase the conductivity of the epoxy due to the presence of ions of Glyceline. However, the low conductivity of C0G compounds compared to that of pure Glyceline suggests to the discontinuity of Glyceline phase and the lack of mobility of ions to freely move across the epoxy matrix [2]. Fig. 2b shows the variation of electrical conductivity of C0.3G composites versus the mass fraction of Glyceline. As it can be observed, the electrical conductivity of C0.3G composites is much higher than that of cured epoxy while the difference between the electrical conductivity of C0G compounds and the cured epoxy is insignificant. Thus, the major contribution to the enhancement of the electrical conductivity of C0.3G composites can be attributed to the CNTs, as Glyceline alone has had little effect on improving the electrical conductivity of the epoxy [6]. Based on recent studies [5,6,24,25] due to their large aspect ratios and unique graphite structure, CNTs can form a conductive network among themselves and give rise to conductivity of the whole composite. As it can be observed, electrical conductivity of C0.3G composites rises by increasing mass fraction of the Glyceline up to 15% (Gly/CNTs = 50), followed by a decrease in the higher fractions. The former was observed by Subramanian et al. in BMI–CNT composite with BMI/CNTs weight ratios up to 20. Regardless of what may really happened to CNT, the noticeable variations in the conductivity of these composites with an increase in Glyceline content implies a synergy between CNT and Glyceline. Although Glyceline droplets cannot form an electrical conductive pathway throughout the C0.3G composites, they may help CNTs to form a more complete/ incomplete conductive network via the control of the way carbon nanotubes are laid out. In the other words, Glyceline may have a significant indirect effect on the electrical conductivity of the composite through arranging conductive CNTs in the matrix. As suggested by Fukushima [20] studied the effect of ILs on CNTs; based on their findings, ILs may weaken the Van der Waals interactions among CNTs and improve their dispersion via forming cation–π interactions with CNTs. Thus, the increase in electrical conductivity upon the addition of small amounts of Glyceline can be attributed to the improvement in the degree of dispersion of the CNTs, which, in turn, boosts their electrical network [24,26]. This favorable effect of Glyceline on CNT dispersion could be seen in the SEM images (Fig. 3a,b) of the C0.3G composites. While Fig. 3a shows a poor dispersion of CNTs in the composite without Glyceline (i.e., the Glyceline-free composite), an improvement in CNTs dispersion and reduction of aggregates at Gly/CNTs weight ratio of 10 is evident from Fig. 3b. Therefore, SEM images further confirm the improvement of CNTs dispersion in the presence of Glyceline. SEM images of C0.3G15 composite (Gly/CNTs = 50) (Fig. 3c) reveal formation of vesicles of Glyceline (~22 μm) at such high fraction of the Glyceline. These vesicles simply occupy a proportion of the composite's space and could push CNTs closer towards each other outside this occupied space (Fig. 3d). This could enhance the conductive network and a lead to a higher conductivity within the composite. Considering that electrical conductivity decreases for Glyceline contents of over 15% (Gly/CNTs N 50), it can be inferred that CNTs arrangement probably undergoes changes in higher Gly/CNTs weight ratios. SEM image for C0.3G35 composite (Fig. 3e) shows an ~ 50% increase in the size of vesicles. The decline in conductivity in case of Gly/CNTs weight ratios greater than 50 may be associated with the enlargement of Glyceline vesicles. It can be conceivable that larger vesicles start to dissolve CNTs already dispersed in the epoxy matrix, depleting its content in the matrix. Obviously, the lower content of CNT in the continuous matrix of epoxy could be translated as a weaker inter content among CNTs, and so lower conductivity throughout the composite. Fig. 4 schematically presents different arrangements of CNTs and Glyceline vesicles upon increase in mass fraction of Glyceline in the epoxy.
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Fig. 3. SEM images obtained from (a) C0.3G0, (b) C0.3G3, (c, d) C0.3G15, and (e) C0.3G35 composites.
3.2. Tensile strength Fig. 5 depicts the ultimate strength of C0.3G composites based on Glyceline weight percent changes. As it can be observed, at first, an increase in Glyceline weight percent causes to enhance the ultimate strength of C0.3G composites. This ascending trend continues until 3% Glyceline (Gly/CNT = 10), where it reaches to its maximum. However, further addition of Glyceline causes a significant drop in ultimate strength. CNTs tend to agglomerate due to their strong Van der Waals forces, which impede the CNTs dispersibility in resin epoxy matrix [27]. ILs interact with CNTs by means of cation–π interaction, which decrease the strong Van der Waals forces between CNTs and increase their dispersibility as a result [24].
On the other hand, the improved CNTs dispersion and lack of agglomeration between CNTs could enhance the conductance between CNTs inside the epoxy and increase ultimate strength of the composite. Conversely, in high mass ratios, ILs act as a plasticizer agent in the matrix which cause to a significant decline in the ultimate strength of the composite [24]. 3.3. Thermogravimetric analysis Fig. 6 shows the thermograms of Glyceline, cured epoxy (neat), and C0.3G composites for different mass fractions of Glyceline. Obviously all the composites have three steps of weight loss, similar to the neat epoxy. The first step, which occurs relatively slowly, is probably related
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Fig. 4. The proposed schematic representation for the network of CNTs with increasing mass fraction of the IL.
to the burning of the secondary hydroxyl group of the propyl chain in the epoxy resin. The noticeably high weight loss in the next stage could be associated with the decomposition of the group of bisphenol A from the epoxy polymer. Finally, the last weight loss that occurs at temperatures exceeding 500 °C is obviously due to carbon decomposition [28]. Table 1 gives the onset temperature for each of the weight loss steps in the thermograms shown in Fig. 6. According to this Table, T1, T2 and T3 are initial weight loss temperature of first, second and third step of each composite respectively. Compared with cured epoxy, all the steps of weight loss occurring in C0.3G0 sample have a higher onset temperature indicating a slight higher thermal stability of the composite in the presence of CNTs. This can be ascribed to the higher thermal conductivity of the CNTs that results in a more uniform distribution of temperature within the composite. Therefore, formation of hotspots on the surface of the sample is delayed; i.e., the decomposition reactions take place at higher temperatures [29–31]. The results presented in Table 1 show that the thermal stability of C0.3G samples decrease with an increase in the mass fraction of Glyceline. Glyceline decompose sat the temperature of 185 °C as observed in its TGA thermogram. A comparison of the curves of composites that contain Glyceline and those without Glyceline reveals that the first step of weight loss is related to Glyceline decomposition. This weight loss is more noticeable for higher mass fractions of Glyceline. A
Fig. 5. Variations of ultimate strength versus Glyceline mass fraction.
reason for the reduced resistance of C0.3G composites can be the early decomposition of Glyceline compared with epoxy. This could lead to the faster decomposition of epoxy in the vicinity of the Glyceline droplets, and so the lower of thermal stability of the whole composites. A schematic representation of this phenomenon is suggested in Fig. 7. 4. Conclusions In this work, the effects of choline-chloride-based ionic liquids on the properties of polymer nanocomposites reinforced with CNTs are investigated. The results show that epoxy–CNT–ILs (CIL) composites fabricated with Glyceline have the highest electrical conductivity even though Ethaline itself stands first in terms of electrical conductivity compared with the other tested ILs. Moreover, the addition of Glyceline to composites containing 0.3% CNTs with different mass fractions of Glyceline (C0.3G) up to a Gly/CNTs weight ratio of 50 increase the electrical conductivity of the composite to 7 × 10-6 S/cm followed by a decrease in the higher weight ratios of Gly/CNTs. SEM images revealed the fact that the reason for the improvement in conductivity is the formation of small Glyceline vesicles throughout the composites. These vesicles could push CNTs to each others,
Fig. 6. TGA curves of Glyceline, cured neat epoxy, and C0.3G composites for several different mass fractions of Glyceline.
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Fig. 7. Schematic representation of Glyceline decomposition in the composite.
forming a more interconnected conductive network. However, in the higher weight ratios of Gly/CNTs (N 50) Glyceline vesicles grow larger and probably dissolve CNTs that results in fewer CNTs in the epoxy matrix. Expectedly, this would weaken connectivity among CNTs network and so lower the electrical conductivity. In the case of 0% CNT/Glyceline (C0G) samples, electrical conductivity follows a steady incline in effect of higher mass fraction of Glyceline, due to the improved pathways of ions. In addition, increasing the weight ratio of Glyceline to 3% (Gly/CNTs = 10) leads to an enhancement in ultimate strength of the composite followed with a significant drop thereafter. The ultimate strength enhancement can be attributed to better arrangement (dispersion) of the CNTs by cation–π interactions between Glyceline and CNTs. In contrast, decreasing in ultimate strength in Gly/CNTs higher ratios can be regarded due to plasticizing effect of Glyceline. Finally, an increasing mass fraction of Glyceline reduces the thermal stability of C0.3G composites. This observation can be attributed to an early thermal decomposition of Glyceline compared with the epoxy resin. Acknowledgments The authors are grateful to Iran Nanotechnology Initiative Council for financial support. References [1] Z. Yue, M.Y. Yu, X. Lan, Study on properties of carbon nanotubes/epoxy resin composite prepared by in situ polymerization, Adv. Mater. Res. 750 (2013) 132–135. [2] K. Matsumoto, T. Endo, Confinement of ionic liquid by networked polymers based on multifunctional epoxy resins, Macromolecules 41 (2008) 6981–6986. [3] K. Yang, M. Gu, Y. Jin, Cure behavior and thermal stability analysis of multiwalled carbon nanotube/epoxy resin nanocomposites, J. Appl. Polym. Sci. 110 (2008) 2980–2988. [4] Y.S. Song, J.R. Youn, Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites, Carbon 43 (2005) 1378–1385. [5] F. Gardea, D.C. Lagoudas, Characterization of electrical and thermal properties of carbon nanotube/epoxy composites, Compos. Part B 56 (2014) 611–620. [6] M. Arjmand, M. Mahmoodi, G.A. Gelves, S. Park, U. Sundararaj, Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate, Carbon 49 (2011) 3430–3440. [7] A. Moisala, Q. Li, I. Kinloch, A. Windle, Thermal and electrical conductivity of singleand multi-walled carbon nanotube-epoxy composites, Compos. Sci. Technol. 66 (2006) 1285–1288. [8] B.G. Soares, A.A. Silva, S. Livi, J. Duchet‐Rumeau, J.F. Gerard, New epoxy/jeffamine networks modified with ionic liquids, J. Appl. Polym. Sci. 131 (2014). [9] T. Torimoto, T. Tsuda, K.I. Okazaki, S. Kuwabata, New frontiers in materials science opened by ionic liquids, Adv. Mater. 22 (2010) 1196–1221. [10] Z. Wang, X. Yang, Q. Wang, H.T. Hahn, S.-G. Lee, K.-H. Lee, Z. Guo, Epoxy resin nanocomposites reinforced with ionized liquid stabilized carbon nanotubes, Int. J. Smart Nano Mater. 2 (2011) 176–193.
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