Hydrogen bonds leading nanodiamonds performing different thermal conductance enhancement in different MWCNTs epoxy-based nanocomposites

Progress in Organic Coatings 140 (2020) 105486

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Hydrogen bonds leading nanodiamonds performing different thermal conductance enhancement in different MWCNTs epoxy-based nanocomposites

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Yeming Xian, Zhixin Kang* Guangdong Key Laboratory for Advanced Metallic Materials Processing, School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou, 510640, China

ARTICLE INFO

ABSTRACT

Keywords: Hydrogen bonds Thermal conductance Nanodiamonds MWCNTs Nanocomposites

The lack of thermal conductance (TC) remains to be a challenge for epoxy resin. Fortunately, the thermal conductance of epoxy resin can be effectively improved by filler incorporation. Recently, compositing epoxy resin with hybrid filler system has been the direction for the development of next-generation thermal conductive functional materials. In this paper, epoxy-based nanocomposites containing nanodiamonds (DNDs)/pristine multi-walled carbon nanotubes (p-MWCNTs) and DNDs/KH550 functionalized MWCNTs (MWCNTs-KH) as the hybrid filler system were prepared respectively, and the thermal conductance of the nanocomposites was compared. The addition of DNDs was found to play a dominating role in the MWCNTs-KH filler systems (0.2 g, from 0.30 W/mK to 0.34 W/mK), but hardly has any effect on the p-MWCNTs filler system (0.2 g, from 0.24 to 0.26 W/mK only). Furthermore, the TC of the DNDs/MWCNTs-KH (2 g) epoxy-based nanocomposite increased to 0.45 W/mK, displaying an enhancement of 114.2 %. A shift of 5 cm-1 recorded by FTIR and the shift of C]O revealed by XPS for DNDs/MWCNTs-KH strongly confirmed the existence of hydrogen bonds. Associating with the characterization results of SEM, TEM and dispersion qualitative experiment, the significant improvement in the TC of DNDs/MWCNTs-KH (2 g) epoxy-based nanocomposite was attributed to the hydrogen bond attachment between DNDs and MWCNTs-KH. The mechanism is that the attachment improves the dispersion of the fillers in epoxy, leading to the formation of a more effective thermal conductive network, thus, the enhanced TC. This work may inspire future studies in hybrid filler recognition and self-assemble technology via hydrogen bonds.

1. Introduction As a type of extensively used engineering polymer in aerospace, electronic packaging, manufacturing and adhesion, epoxy resin displays multiple superior has superior properties, such as excellent durability, thermal stability, electronic resistance as well as extraordinary mechanical property. However, the thermal conductance (TC) of epoxy resin is generally low, which restricts its further applications in fields such as electronic industry, and heat supplying and generating. [1]. The branched chain and the lack of crystal structure, which cause phonon scattering, are considered to be the cause for the low TC of epoxy resin. With the rapid growth of the electronic industry and the size reduction of electronic components, the challenge of enhancing the TC of epoxy resin is becoming emergent. Fortunately, compositing with thermal conductive fillers is a potential effective method tackling the challenge [2–4]. However, single fillers, which are presently widely applied to

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enhance the TC of epoxy resin, have hit the ceiling. Single filler, containing only one type of filler (e.g. metal, metallic oxide or carbon material), has induced superior enhancement; however, the lack of hierarchical structure, high viscosity, and isolated enhancement pose limitation on further development. Recently, hybrid filler systems, comprising two or more types of fillers, have attracted research interest due to their synergistic effects, low cost and processability. Multiple studies have been conducted on hybrid fillers, such as SiO2 & Al2O3 nano-micro fillers [5], graphene nanoplatelets & multi-walled carbon nanotubes (MWCNTs) [6,7], SiO2 & Al [8] and SiC & MWCNTs [9]. For example, a 210 % improvement in TC of epoxy resin was achieved by Zhang [10] employing nanodiamond-attached exfoliated hexagonal boron nitride as hybrid filler, while an enhancement of hardness and toughness was achieved with nanodiamonds (DNDs) and MWCNTs hybrid filler system in Subhani’s study [11]. These works indicate that hybrid fillers can achieve enhanced TC and better strengthening

Corresponding author. E-mail address: [email protected] (Z. Kang).

https://doi.org/10.1016/j.porgcoat.2019.105486 Received 16 June 2019; Received in revised form 2 December 2019; Accepted 5 December 2019 0300-9440/ © 2019 Published by Elsevier B.V.

Progress in Organic Coatings 140 (2020) 105486

Y. Xian and Z. Kang

efficiency. Furthermore, some fillers combinations showed a more significant enhancement than expected. In Yang’s work [9], a hybrid filler system of MWCNTs and SiC nanoparticles was embedded in epoxy, leading to a sufficiently improved TC, which was ascribed to the formation of heat conductive net chain induced by the filler. With the same filler volume fraction of 20 %, the TC enhancement induced by hybrid filler system was 30 % higher than that induced by single filler system. Despite the promising achievements, the reason for the formation of heat conductive net chain in the hybrid fillers in Yang’s work is unclear. Therefore, it raises the concern whether the TC of epoxy resin can be enhanced by any combination of two random fillers. In order to provide practical guidance for further work, the dominating factor needs to be identified, thereby, the selection criteria of fillers for hybrid filler system can be established. The segregated structure [12], hierarchical structure [13], 3D network formed by hybrid fillers [14], as well as filler size and chemical bonding with matrix [15] were investigated and regarded as essential factors by researchers around the world. However, the chemical bonding or non-covalent bonding between fillers have not been studied. Herein, we studied the mechanism of nanodiamond (DND) and MWCNT, which might facilitate further discussion about the dominating factor and the selection criteria. In this work, the combination of DND and MWCNT was selected for the hybrid filler system. MWCNT, the one-dimensional carbon material with high aspect ratio [16] and extremely high thermal conductivity (∼3000 W/mK), is regarded as one type of excellent filler for TC enhancement. For application as a filler in polymer-based matrix, MWCNTs are usually functionalized, which is beneficial to the formation of heat conductive net chains [17]. As another type of nanofiller with excellent TC (∼2000 W/mK), extremely high hardness, biocompatibility and chemical resistance, DNDs was applied as the other filler component in this study. Besides the excellent properties, multiple functional groups can be introduced on the surface of DNDs through detonation fabrication. The surface characteristics of DNDs then facilitate the functionalization [18,19], which is well-known to enhance the TC of thermal conductive fillers. With high TC, DNDs are suitable as hybrid filler components, however, not as single filler since the spheroidal structure disables the formation of heat conductive net chain. Before the discussion of the interaction between the fillers, the interaction between fillers and matrix was well investigated in many works around the world. Ma [20] and his co-workers compared the surface energy and wettability of pristine MWCNTs and amino MWCNTs with epoxy. The contact angles in became lower and the total surface energy improved in amino MWCNTs. Amino functionalization improving the interfacial interactions between fillers and matrix was well discussed in plenty of researches and these enhancements would improve the dynamic mechanical property, rheology, and others [21]. Primary amino groups would form covalent bonds with epoxy matrix and involved in the curing stage. Hence, the amino functionalization had influence on the cure. Saeb [22,23]and his team studied about the amino functionalized fillers systematically by thermogravimetric analysis, dynamic scanning calorimetry and other characterization. Almost the pristine MWCNTs tended to hinder the crosslinking but it was improved by MWCNTs-KH because the amino functional groups would improve the curing. With Consideration of the complication of the curing, we focused on the influence of DND in order to clarify the “role” of DND. In this work, the DND/pristine MWCNTs (p-MWCNTs) and DND/3aminopropyltriethoxysilane (KH550) functionalized MWCNTs (MWCNTs-KH) were studied respectively in epoxy-based nanocomposite. In order to explore the mechanism, characterizations, including Xray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM) and dispersion qualitative experiment, were conducted. In addition, a rheometer was employed for further investigation of the filler network in the nanocomposites. The working hypothesis of the filler combination is that the possible

formation of hydrogen bonds between DNDs and MWCNTs-KH will result in the attachment of DNDs to the surface of MWCNTs-KH which will then improve the dispersion of the DNDs/MWCNTs-KH hybrid filler in epoxy, thus, forming an effective heat conductive network for TC enhancement of the corresponding composite. In fact, hydrogen bonds already play an important role in catalysis, arrangement of molecules, crystal engineering and biochemical processes. If the hydrogen bonds effect in the current study can be confirmed, it will provide possibilities to build up supramolecular interactions [24], as well as inspiration to future studies on self-assembly of thermal conductive nanocomposites. 2. Experiment 2.1. Materials Pristine multi-walled carbon nanotubes (p-MWCNTs) with a length between 5 and 20 μm and a diameter between 10 and 30 nm were purchased from Jiechuang New Material Technology Co., Ltd; DNDs were purchased from HeYuan ZhongLian Nanotechnology Co., Ltd., with an average particle size of 3.2 nm; 3-aminopropyltriethoxysilane (KH550) was purchased from Guangzhou Chemical Reagent Factory; E51 Bisphenol A epoxy resin was purchased from Kunshan Jiulimei Electrical Materials Co., Ltd; curing agent, 2-ethyl-4-methylimidazole (EMI-2,4), was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd.. Other reagents were used as received. 2.2. Oxidation and functionalization of MWCNTs MWCNTs were oxidized by a mixture (v/v = 3:1) of sulfuric acid and nitric acid. Specifically, 2 g of MWCNTs and 100 ml of the acid mixture were added into a flask and stirred at 80 °C for 6 h. Afterwards, the suspension was diluted with deionized water till pH≈7. Then, the MWCNTs after oxidation were dried in a vacuum electric furnace for 24 h, yielding MWCNTs−COOH. Finally, MWCNTs−COOH were added into a 2.4 vol% KH550 ethanol solution and stirred at 60 ºC for 4 h, followed by dilution with ethanol and drying in a vacuum electric furnace for 24 h, with MWCNTs-KH as the final product. 2.3. Fabrication of nanocomposites The compositions of nanofillers are listed in Table 1. For a typical fabrication, the nanofiller was added into 40 ml ethanol and stirred for 1 h. The suspension was then introduced into 15 g epoxy resin and stirred at 85 °C for 4 h to disperse the nanofillers and remove the ethanol. The stirring temperature was elevated to lower the viscosity of epoxy and evaporate the ethanol efficiently. Then 0.9 g of curing agent EMI-2, 4 was added into the resin mixture and stirred for 10 min. After degassing by a vacuum pump, the mixture was poured into a PVDF die with simethicone in a vacuum electric furnace, followed by precuring at Table 1 Compositions of nanofillers.

2

Samples

MWCNTs/ g (with/without Functionalization)

DNDs/ g

EP EPDNDs EP0.1 p-MW EP0.2 p-MW EP0.1 MW-KH EP0.2 MW-KH EP0.1 DNDs/p-MW EP0.2 DNDs/p-MW EP0.1 DNDs/MW-KH EP0.2 DNDs/MW-KH EP1 DNDs/MW-KH EP2 DNDs/MW-KH

0 0 0.10 0.20 0.10 0.20 0.10 0.20 0.10 0.20 1.00 2.00

0 0.05 0 0 0 0 0.05 0.05 0.05 0.05 0.25 0.50

(without) (without) (with) (with) (without) (without) (with) (with) (with) (with)

Progress in Organic Coatings 140 (2020) 105486

Y. Xian and Z. Kang

80 °C for 1 h, curing at 120 °C for 1.5 h, and further curing at 140 °C for 2 h.

Interesting results were observed investigating the effect of DNDs in EPDNDs/p-MW and EPDNDs/MW-KH. As shown in Fig. 2, the addition of DNDs in EPp-MW did not lead to any evident enhancement. In contrast, when the MWCNTs were functionalized into MWCNTs-KH, the introduction of DNDs in the system was efficient for improving TC. It indicated that the TC enhancement efficiency of introducing DNDs was dependent on the functionalization of MWCNTs.

2.4. Characterizations FTIR was conducted with VERTEX 70 to determine the surface functional groups of DNDs and MWCNTs. The Raman spectroscopy of pMWCNTs and MWCNTs-KH was obtained by Lab RAM Aramis. XPS was performed with Kratos Axis Ulra DLD for the element determination of the nanofillers. The morphology of the samples was observed by HRTEM (TECNAI G2 S-TWIN F20), while the morphology of nanofillers and the tensile fracture of nanocomposites were analyzed by SEM (ANTA FEG 250). The rheometer (ARES-G2, controlled-strain rheometer, TA instruments) was employed to characterize the network of epoxy and the nanocomposites. As for thermal properties, the thermal diffusivity (α(T)) of the samples were measured by NETZSCH LFA447, and DSC (NETZSCH STA 449) was employed to determine the corresponding heat capacity (CP(T)). In addition, the density (ρ(T)) of each sample was measured by a density balance. The TC (λ(T)) of the nanocomposites was calculated by the following equation [25],

(T) = (T) × CP (T ) × (T )

3.2. Hydrogen bonds between MWCNTs and DNDs The varied effect of introducing DNDs in different hybrid filler systems indicate that there might be interactions between DNDs and MWCNTs-KH but not between DNDs and p-MWCNTs. The surface chemistry which is directly related to the interface of the two fillers was characterized and analyzed via FTIR, XPS, Raman and TEM, as presented in the supplementary materials. Combining the characterization results, the interactions within the hybrid fillers were investigated. FTIR is a conventional and convincing technique to study hydrogen bonds. Specifically, upon formation of hydrogen bonds, the protons move to the lower field, resulting in the corresponding shift [24,35] of the chemical bond vibration positions, as can be observed by FTIR. Therefore, the two types of nanofillers were mixed and analyzed by FTIR. The results are presented in Table 2 and Fig. 3. Compared to MWCNTs-KH550, the deformation vibration of primary amino group shifted to 1581 cm−1 for the DNDs/ MWCNTs-KH550 mixture. A blue shift (from 1704 to 1712 cm−1) was also recorded for the stretching vibration of OeH upon DNDs addition (Fig. 3a). The blue shifts [36,37] implied that the electron cloud of covalent bonds was diluted and confirmed the existence of hydrogen bonds [38,39] between primary amino groups on MWCNTs-KH and C]O on DNDs. The existence of hydrogen bonds between DNDs and MWCNTs was further confirmed by XPS. According to peak separation of C 1s (C]O) spectrum, a single peak was recorded for DNDs at 285.31 eV, with that of MWCNTs-KH at a close position (285.40 eV). As for the DNDs/ MWCNTs-KH mixture, two peaks were observed at 285.32 (peak 1) and 286.62 eV (peak 2), respectively. Peak 1 was located near the peak of MWCNTs-KH, and therefore, assigned to C]O in MWCNTs-KH introduced by the oxidation with mixed acid. Therefore, the presence of Peak 2 was attributed to the addition of DNDs. Assuming the absence of hydrogen bonding, it was expected to be close to the peak observed for DNDs. However, instead of a neighboring peak, peak 2 displayed a significant shift of 1.31 eV from that of DNDs. Therefore, the peak shifting was regarded as a result of the existence of hydrogen bonds. Furthermore, relatively macroscopically, compared with the clear interface between DNDs and p-MWCNTs, a vaguer interface between DNDs and MWCNTs-KH was observed by TEM (Fig. 3d), which was then ascribed to the formation of hydrogen bonds. As consistently suggested by FTIR, XPS and TEM, there were hydrogen bonds between DNDs and MWCNTs-KH. Fig. 3b illustrates the surface chemistry of DNDs and MWCNTs-KH, and the hydrogen bond formation mechanism which was denoted as “powerful combination”. The combination performed well in the epoxy-based nanocomposites. Specifically, the primary amino groups of MWCNTs-KH acted as proton donors and the oxygen groups on the surface of DNDs acted as proton accepters, in which way the hydrogen bonds were formed to attach DNDs to MWCNTs-KH.

(1)

3. Results and discussion 3.1. Thermal conductance and the enhancing effect of DNDs The structure characteristics of epoxy resin (i.e. branched chain and lack of crystal structure) cause phonon scattering, and thereby, a low TC [29]. Researchers have proposed that the TC of epoxy resin can be improved via a heat conductive net chain formed by filler particles [3,30]. However, the agglomeration of filler particles with high-energysurface and poor combination of interfaces poses restrictions on heat conductive net chain formation [31]. The TC of epoxy-based nanocomposites with different fillers was compared, as showed in Fig. 1. For reference, the TC of neat epoxy (EP) was 0.21 W/mK. In comparison, the TC of EP0.1 p-MW was increased to 0.26 W/mK (Fig. 1a), equivalent to an enhancement of 25.76 %. Upon KH550 modification, the TC was further increased to 0.29 and 0.31 W/ mK for EP0.1 MW-KH and EP0.1 DNDs/MW-KH respectively. Increasing the content of MWCNTs to 0.2 g (Fig. 1b), the TC of EP0.2 p-MW, EP0.2 DNDs/pMW, EP0.2 MW-KH and EP0.2 DNDs/MW-KH was 0.24, 0.26, 0.30 and 0.34 W/ mK, respectively. Overall, improved TC was observed for all the fillers, with an observed positive effect of functionalization. The further enhanced TC induced by functionalization was caused by the abundant amino groups introduced on the surface of MWCNTs-KH, which increased the interfacial thermal conductance (ITC) between epoxy and MWCNTs-KH [32–34]. As the DNDs/MWCNTs-KH hybrid filler system showed higher efficiency, the TC of nanocomposites containing 0.1 (EP0.1 DNDs/MW-KH), 0.2 (EP0.2 DNDs/MW-KH), 1 (EP1 DNDs/MW-KH), and 2 g (EP2 DNDs/MW-KH) MWCNTs-KH was compared (Fig. 1c). In contrast to EPp-MW, the TC of which decreased from 0.26 to 0.24 W/mK with increased MWCNTs content to 0.2 g, the TC of EP DNDs/MW-KH improved along with the filler content, achieving a TC of 0.45 W/mK with 2 g of MWCNTs-KH. Compared with other relevant studies (Fig. 1d), this current work achieved significant enhancement of TC with moderate filler content. According to a study carried out by Zhang et al. [10], where the TC of epoxy was enhanced respectively to 0.28 and 0.54 W/mK with 17 and 29 wt% DNDs exfoliated boron nitride (BNs), the function of DNDs was proposed to be preventing BNs from agglomeration by inserting between the layers. To determine if DNDs have played a similar role in the current study, further investigation was carried out to identify the effect of DNDs.

3.3. Attachment between DNDs and MWCNTs As the existence of hydrogen bonds between DNDs and p-MWCNTs was confirmed, further investigation was carried out to identify the relation between the hydrogen bonds and enhanced TC of the corresponding nanocomposites. Possible hypotheses were that the hydrogen bonds could enhance the TC of nanocomposites by improving the

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Fig. 1. TC of nanocomposites: (a) the nanocomposites containing 0.1 g p-MWCNTs or MWCNTs-KH, (b) the nanocomposites containing 0.2 g p-MWCNTs or MWCNTs-KH, (c) the nanocomposites containing MWCNTs-KH and (d), the comparison of TC with other studies [10,26–28].

of several types of chemical bonds, including π-π interaction, hydrogen bonds, amino covalent bonds and covalent bonds with triethylenetetramine, it was confirmed by Shen et al. [15] that the hydrogen bonds were with long distance and low ITC. Therefore, introducing hydrogen bonds into epoxy and MWCNTs-KH could not improve the TC directly. Therefore, the later hypothesis is more likely to be the cause of the unusual effect of DNDs on improving TC. In Song’s work [26], the DNDs were confirmed to be attached to nanofibrillated cellulose films via hydrogen bonds, which then enhanced the TC by 858.8 %. In order to identify if the current study is a similar case, TEM and SEM were conducted to distinguish the interaction between DNDs and p-MWCNTs or MWCNTs-KH. Fig. 4a and c were SEM and TEM images of DNDs/p-MWCNTs (w/ w = 1:1) respectively. As clearly revealed, p-MWCNTs was thin and clean films, with several DNDs randomly stacked on the surface just like “Spaghetti Bolognese”. In contrast, the morphology of DNDs/MWCNTsKH (w/w = 1:1) (Fig. 4b and d) resembled “Cauliflower”, indicating that DNDs were attached densely on the surface of MWCNTs-KH. The attachment between DNDs and MWCNTs-KH provides possibility to improve the dispersion of the hybrid fillers. In the case that the attachment remains effectively in both ethanol and epoxy, the dispersion of the DNDs/MWCNTs-KH hybrid fillers can then be improved.

Fig. 2. TC of Epp-MW and EPMW-KH without/with DNDs respectively.

interfacial thermal conductance (ITC) or the dispersion of MWCNTs-KH for a more effective MWCNTs thermal conductive network. It was suggested by earlier studies [40,41] that the hydrogen bonds enabled the improvement of ITC. However, based on the comparison of the ITC

Table 2 FTIR peak positions of NeH and C]O in MWCNTs-KH, DNDs and DNDs/MWCNTs. Vibration mode

MWCNTs-KH/ cm−1

DNDs/ cm−1

DNDs/MWCNTs/ cm−1

Deformation vibration of primary amino group (N-H) Stretching vibration of C]O

1576 1704

– 1720

1581 1728 (shoulder peak, of DNDs), 1712 (of MWCNTs-KH)

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Fig. 3. (a) FTIR results of MWCNTs-KH and DNDs/MWCNTs-KH (w/w=1:1). (b) illustration of the hydrogen bonds between DNDs and MWCNTs-KH. (c) The peak separations of XPS of C 1s (C]O), O1s (C]O) and N 1s. (d) TEM images of DNDs/p-MWCNTs and DNDs/MWCNTs-KH.

3.4. Dispersion in ethanol and epoxy

bottles with dispersion of MWCNTs and DNDs, respectively (Fig. 5c and d). However, a strong scattering laser appeared in Fig. 5d, implying an excellent affinity of DNDs with ethanol. The green laser did not pass through the dispersions of MWCNTs-KH, p-MWCNTs/DNDs and MWCNTs-KH/DNDs (Fig. 5e–g), with the shortest laser path observed for MWCNTs-KH/DNDs (Fig. 5g). Corresponding to the length of the green light (MWCNTs-KH/DNDs < MWCNTs-KH < DNDs/pMWCNTs < p-MWCNTs), the dispersibility of the fillers in ethanol follows the reverse sequence of DNDs/MWCNTs-KH > MWCNTsKH > DNDs/p-MWCNTs > p-MWCNTs. The SEM images of the tensile fractures of the nanocomposites are shown in Fig. 6. As revealed by Fig. 6a, EP displayed a clean fracture with wide cracks, which is the typical pattern of brittle fracture. The same fracture morphology was observed for EPDNDs (Fig. 6b), which was ascribed to the fact that the DNDs (5 nm) were too fine to influence the fracture of EP. In contrast, aggregation occurred for p-MWCNTs (Fig. 6c), with a large number of p-MWCNTs forming a big cluster and

Results displayed in Fig. 4 serve as a convincing proof for the attachment of DNDs to the surface of MWCNTs-KH via hydrogen bonds. The nanocomposites were fabricated via solution blending method, which requires good dispersion in both ethanol and epoxy. In order to characterize the dispersion of these nanofillers in ethanol and epoxy, a qualitative experiment was designed, and the tensile fractures of the nanocomposites were revealed by the corresponding SEM images. The qualitive experiment was designed according to Beer-Lambert Law to compare the dispersion of different nanofillers by green lasers. The pictures of the mixtures of ethanol and the nanofillers (0.01 g in 10 ml ethanol) are shown in Fig. 5. As can be seen, insufficient dispersion was clearly observed for MWCNTs and DNDs(Fig. 5a and b). As the dispersibility of MWCNTs-KH, p-MWCNTs/DNDs and MWCNTsKH/DNDs was difficult to identify with bare eye, green laser was employed for further determination. The green laser passed through the

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MWCNTs without functionalization could not form interaction with DNDs due to the lack of functional groups. The fracture morphology was also similar with that in Fig. 6c. As for EP0.2 DNDs/MW-KH (Fig. 6f), the morphology was more complicated. Besides river-like patterns, the hilly-like patterns were also observed. Moreover, tips of MWCNTs-KH were identified to be in the middle of the “hills”. The morphology characteristics indicate that MWCNTs-KH were uniformly dispersed in EP0.2 DNDs/MW-KH, which was attributed to the attachment between DNDs and MWCNTs-KH. 3.5. Networks of the nanocomposites In order to discuss the network of MWCNTs from a marco perspective, a rheometer was employed for the network characterizations. The plots of phase angle versus frequency for various nanocomposites were displayed in Fig. 7. Phase angle reflects the relationship between dissipated and stored energy, specifically, an increased phase angle implies that more energy per cycle of deformation will be dissipated. The formation of filler network lowered the phase angle. EPMW-KH and EP DNDs/MW-KH displayed higher phase angle compared with that of EPp-MW and EPDNDs/p-MW, respectively. The increased phase angle for functionalized MWCNT surface was attributed to the flexible molecular chains on the surface. After adding DNDs, the phase angle of EP DNDs/ MW-KH was observed to decrease in the entire frequency range, while the phase angle the p-MW system didn’t decrease at the low frequency range. Compared with EP, all the composites formed network at a certain degree. In order to discuss it further, an experiment was designed and performed to destroy the network of the nanocomposites. For comparison purposes, three steps of scanning (i.e. dynamic scanning, steady scanning and dynamic scanning) were carried out. The phase angle of the nanocomposites before and after network destruction in the range of 0.1 to 100 s−1 was tested during the first and third steps of dynamic scanning, while the network was destroyed by a shear applied during the second step of steady scanning. The comparison of the results is shown in Fig. 7b–f. The high phase angle observed for EP (Fig. 7b) indicated the lack of network structure, thereby, the phase angle hardly changed after steady shearing. The phase angle lowered as

Fig. 4. SEM images of (a) the mixture of p-MWCNTs and DNDs (w/w = 1:1) and (b) the mixture of MWNCTs-KH and DNDs (w/w = 1:1). TEM images of (c) the mixture of p-MWCNTs and DNDs (w/w = 1:1) and (d) the mixture of MWNCTs-KH and DNDs (w/w = 1:1).

the rest dispersing widely on the fracture, indicating poor interfacial bonding. Moreover, some p-MWCNTs were pulled out partially, indicating a weak combination between p-MWCNTs and epoxy matrix. In addition, the fracture morphology was altered from concentrated wide cracks into river-like cracks, implying that more energy was absorbed and the toughness of EP was improved. The same morphology patterns were observed in Fig. 6d. Despite that less MWCNTs-KH were pulled out due to the enhancement of interfacial bonding induced by functionalization, the aggregation still occurred, indicating that poor dispersion still presented. As shown in Fig. 6e, the dispersion of MWCNTs was improved by adding DNDs, however, aggregation still occurred. It was because the

Fig. 5. A comparative experiment on the Tyndall effect of nanofiller dispersions. (a) the dispersion of nanofillers in ethanol Serum bottles, with the nanofiller being pMWCNTs, DNDs, MWCNTs-KH, p-MWCNTs/DNDs (w/w = 1:1), and MWCNTs-KH/DNDs (w/w = 1:1) from left to right; (b) the corresponding dispersions after 1 day stewing and (c–f) the green laser passing through p-MWCNTs (leftest), DNDs, MWCNTs-KH, p-MWCNTs/DNDs, and MWCNTs-KH/DNDs, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 6. SEM images of the fractures of the nanocomposites: (a) EP, (b) EPDNDs, (c) EP0.2

the frequency decreased, implying that there was a percolated filler particle network. The phase angle of EPp-MW increased in most of the frequency range after steady shearing (Fig. 7c). Upon DNDs addition in EPp-MW, lower phase angles were recorded, with expanded increasement after steady shearing. The phase angle decreased at lower frequency, indicating a separation between DNDs and p-MWCNTs. In the MWCNTs-KH systems, the separation did not occur obviously as shown in Fig. 7d and f. After adding DNDs, the phase angle was lower at low frequency, implying the presence of networks in EPDNDs/MW-KH and the improved elasticity by DNDs. EPMW-KH had weaker network for its lower phase angle. The phenomena on phase angle confirms the presence of MWCNTs networks in the nanocomposites. Although more obvious networks were observed for EPp-MW and EPDNDs/p-MW, the networks were rather unstable, which were destroyed easily by steady flow. Hence, the networks were likely to be formed by the aggregation of p-MWCNTs. As for MWCNTs-KH and DNDs/MW-KH, the phase angle remained basically the same without sufficient increase after steady scanning, indicating that the network was from dispersed MWCNTs instead of segregated MWCNTs. Whether the DNDs remained attaching on the p-MWCNTs or MWCNTs- KH was a critical issue and observed by the phase angle as well. Comparing c with e and compare d with f, the roles of DNDs were found different. In pristine MWCNTs system, the phase angle decreases as the frequency decreases and the steady flow made it higher at high frequency. However, in DNDs/p-MWCNTs system, the gap between before and after steady flow was expanding obviously. The two phenomena showed that the network of p-MWCNTs was unstable and the DNDs had more interfacial affinity than p-MWCNTs. In MWCNTs-KH system, the steady flow did not destroy the network as much as that in p-MWCNTs system. DNDs/MWCNTs-KH system did so. It showed that

p-MW,

(d) EP0.2

MW-KH,

(e) EP0.2

DNDs/p-MW,

and (f) EP0.2

DNDs/MW-KH.

there were not obvious movements of both DNDs and MWCNTs-KH. Based on the rheological results it was believed that at least there was no large amount of DNDs detached from the MWCNTs-KH. Above all, in both ethanol and epoxy, the functionalization of MWCNTs enables the MWCNTs dispersion improvement by DNDs. On the basis of these results, considering the chemistry of the nanofillers, mechanism of dispersion was proposed. Fig. 8a and b are schematic diagrams of the dispersion mechanisms. As shown in Fig. 8a, MWCNTs disperse instantly in solution upon physical disaggregation by sonication or stirring. However, due to the π-π interaction and low surface energy of MWCNTs, there are interactions between every MWCNT. Hence, several MWCNTs intertwine and form MWCNTs clusters, followed by the formation of large MWCNTs clusters featuring aggregation. In this case, the DNDs are unable to improve the dispersion due to the lack of attachment interaction, thereby, DNDs and p-MWCNTs disperse independently. Fig. 8b is schematic illustration of the dispersion mechanism of DNDs/MWCNTs-KH. DNDs can disperse well in ethanol solution, whereas MWCNTs-KH display poor dispersion. However, due to the formation of hydrogen bonds between MWCNTs-KH and DNDs, the well dispersed DNDs pull the poorly dispersed MWCNTs-KH apart, hence, the dispersion of MWCNTs-KH is improved. 4. Conclusion The conclusions of this paper were summarized as below: I The TC of the epoxy-based nanocomposites containing single fillers and hybrid fillers were compared. The nanocomposite with DNDs/ MWCNTs-KH as the filler displayed the highest TC, achieving 0.45 W/mK with a filler content of 14.21 %. The addition of DNDs 7

Progress in Organic Coatings 140 (2020) 105486

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Fig. 7. Plots of phase angle versus frequency of EP and the nanocomposites before and after steady flow.

significantly enhanced the TC of the nanocomposites containing DNDs/MWCNTs-KH, which phenomenon was not observed for nanocomposites with DNDs/p-MWCNTs as the filler. II The presence of hydrogen bonds was suggested by the peak shifts of NeH and OeH recorded by FTIR, while the attachment between DNDs and MWCNTs-KH was further confirmed by SEM and TEM. Due to the attachment effect, DNDs/MWCNTs-KH performed better dispersion in both ethanol and epoxy. III The mechanism of DNDs-induced enhancement in the TC of the nanocomposites was proposed. Owing to the attachment between DNDs and MWCNTs-KH and good dispersion of DNDs in both ethanol and epoxy solution, the MWCNTs-KH dispersion was further improved, leading to the formation of efficient thermal conductive network, and therefore, the enhanced TC.

[42–44] studied the curing kinetics of epoxy/MWCNTs nanocomposites employing nonisothermal calorimetric and rheokinetic techniques, the nanocomposites became highly curable upon introducing primary and secondary amino groups. It was expected that the curing ability of hybrid fillers could be further improved. However, due to the complicity of hybrid fillers, the curing properties need to be further discussed in future. Moreover, the effect of hydrogen bonds studied in this work provides probability for nanofiller recognition and self-assembly combining with the functionalization techniques of the nanofillers around the world. In this work, the hydrogen bonds between MWCNTs-KH and DNDs belong to the NeH⋯O systems. Denote A to the proton accepter and D to the proton donor, on the basis of this study (DeH⋯A), more hydrogen bond systems could be considered to graft on the surface of nanofillers, such as dihydrogen bonds (DD⋯AA, AD⋯DA) [45,46] and trihydrogen bonds (AAA⋯DDD, ADA⋯DAD, AAD⋯DDA) [47–49]. As a result, this work enables the recognition function of nanofillers to achieve structural combination in the matrix via hydrogen bonds.

For further research, besides the effect of hydrogen bonds, the effect of curing kinetics of hybrid fillers is also of great importance. Saeb et al.

8

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Fig. 8. Schematics of the dispersing mechanism of (a) DNDs/p-MWCNTs and (b) DNDs/MWCNTs-KH.

CRediT authorship contribution statement Yeming Xian: Conceptualization, Formal analysis, Visualization, Writing - original draft. Zhixin Kang: Writing - review & editing.

[10]

Declaration of Competing Interest

[11]

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed.

[12] [13] [14]

Appendix A. Supplementary data [15]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105486.

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