MWCNT nanocomposites

Polymer Testing 81 (2020) 106280

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Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest

A facile rheological approach for the evaluation of “super toughness point” of compatibilized HDPE / MWCNT nanocomposites Yifei Wang a, Fucheng Lv a, Yao Song a, Yanyu Yang a, Yanxia Cao a, Jianfeng Wang a, Chenlin Li b, Wanjie Wang a, * a b

College of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, China Energy and Environmental Science and Technology, Idaho National Laboratory, Idaho Falls, 83401, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: HDPE Toughness Rheology Nanocomposites Compatibility

Fast and efficient determination of the optimal mechanical property of a polymer/CNT nanocomposite is crucial to develop polymer conductive nanocomposites. This work establishes a rheological approach to evaluate the super-toughness point of compatibilized high density polyethylene (HDPE)/multi-walled carbon nanotube (MWCNT) nanocomposites. Results illustrate that three types of HDPE/MWCNT nanocomposites exhibit obvious gel plateaus in the dynamic rheological curves and the gel points of nanocomposites with compatibilizer shift to the low MWCNTs loading. The super-toughness points of HDPE/MWCNT nanocomposites with compatibilizers show the correspondence with the gel points acquired from the rheological data, indicating that dynamic rheology is an effective way to determine the super-toughness points of HDPE/MWCNT nanocomposites with compatibilizers. Furthermore, unique network structure at the gel points is directly observed and the new mechanism of toughness is proposed. This study provides new insights for effective control of the structures and properties of polymer/CNT nanocomposites.

1. Introduction Many new nanofillers with perfect properties have been prepared in recent decades and used in many fields [1–9]. Because of high flexibility, low mass density, and large aspect ratio, carbon nanotubes (CNTs) have been widely used as reinforcing agents to produce polymer nano­ composites [10–12]. Polymer/CNT nanocomposites exhibit outstanding conductive percolation behaviors, in which nanotubes will form a network structure to transport electrons throughout the matrix and lead to a decrease of their electrical resistance by several orders of magni­ tude. Many studies indicate that electrical conductivity depends strongly on polymer types, concentrations and properties of CNTs, as well as interactions between polymer and CNTs and distribution of CNTs [13–15]. Melt mixing is a popular way to achieve polymer nanocomposites due to its economic benefit and easy operation for industrial scale pro­ duction of polymer/CNT composites. Recent works have focused on the polymer/CNT nanocomposites fabricated by melt mixing and investi­ gated the ranges of percolation thresholds to obtain polymer/CNT conductive nanocomposites with low CNTs concentrations [13]. For

example, the percolation threshold is about 7.5 wt% for polyethylene/multi-walled carbon nanotube (PE/MWCNT) nano­ composites [16], 1.1 to 2.0 vol% for polypropylene/carbon nanotube (PP/CNT) nanocomposites [17], 1.0 to 3.0 wt% for polycarbonate (PC)/CNT nanocomposites [18], 0.7 wt% for polyamide12 (PA12)/MWCNT nanocomposites [19], and 2 to 5 wt% for HDPE/CNT nanocomposites [20]. The electrical percolation threshold is attained when a conductive path is formed throughout the whole sample. Two mechanisms of electrical conduction have been reported: 1) direct conduction when conductive fillers overlap with each other; 2) electron hopping that electrons ‘‘jump’’ from a conductive filler to another one over a small distance of the order of a few nanometers. At a percolation threshold, the mechanical, thermal, barrier, electrical properties will change greatly because of the formation of filler-filler networks and/or polymer bridging around particles [21]. Rheology is an effective method to characterize the structure and properties of polymer composites. Linear rheological behaviors are sensitive to the network structure and percolation behaviors [22]. Song and Zheng reviewed the linear rheology of nanoparticle filled polymer nanocomposites, and reported the rheological criteria for determining

* Corresponding author. E-mail address: [email protected] (W. Wang). https://doi.org/10.1016/j.polymertesting.2019.106280 Received 20 June 2019; Received in revised form 24 November 2019; Accepted 3 December 2019 Available online 5 December 2019 0142-9418/© 2019 Elsevier Ltd. All rights reserved.

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the so-called liquid-to-solid transition with increasing filler content, the contradictory ideas of four kinds of time-concentration superposition principles proposed for constructing master curves of linear rheology [23]. Du et al. obtained the rheological threshold of poly (methyl methacrylate) (PMMA)/SWNT nanocomposites of 0.12 wt%, which was smaller than the percolation threshold of electrical conductivity, 0.39 wt % [24]. Zhang et al. sprayed SWNT aqueous suspected solution onto HDPE powders, blended the dried SWNTs/powders in a twin-screw mixture to produce nanocomposites with a uniform dispersion of SWNTs, and obtained a rheological percolation threshold of 1.5 wt%, and electrical percolation threshold of 4 wt% of nanotube loading [25]. Wu et al. also found the conductivity threshold value was far higher than the rheological counterpart for carbon black/polylactic acid (CB/PLA) and CNT/PLA composites [26]. Similar results had also been reported on many conductive polymer nanocomposites [27–30]. These studies demonstrate that rheological thresholds are not corresponding to the electrical thresholds of most polymer/CNT nanocomposites. These de­ viations can be attributed to the interphase region structure and in­ teractions between polymers and CNTs, many researches have discussed these issues based on some rheology models [31–38]. CNTs can increase conductivity of origin polymers and improve their mechanical properties. Polymer and CNT types, the distributions of CNTs, chemical pre-treatment and processing strategy are the most important factors to determine the mechanical properties of polymer/ CNT nanocomposites [39–41]. Particularly, the distributions of CNTs have the most critical influence on controlling the mechanical properties of nanocomposites. CNTs are much more effective reinforcement for ductile matrices, whereas they have less impact on the mechanical properties of brittle polymers. Addition of polymeric compatibilizer is an effective way to improve the interfacial strength of polymer/CNT nanocomposites. Hwang et al. reported the storage modulus (G’) of PMMA/PMMA-g-MWCNT nano­ composite increased near 11 times when PMMA-g-MWCNT loading reached 20 wt% [42]. PE-g-MAH was added to HDPE/MWCNT nano­ composites and the mechanical properties increased obviously [43,44]. Other examples included the studies of styrene maleic anhydride copolymer (SMA) in PA12/SWNT nanocomposites [45], maleated styrene/ethylene-butylene/styrene copolymer (mSEBS) in PC/MWCNT nanocomposites [46], etc. Compared with the nanocomposites without compatibilizer, the mechanical properties of nanocomposites with compatibilizer can be improved significantly, and their rheological and conductive percolation thresholds also shifted to the lower CNT con­ centrations [47]. When conductive nanocomposites are used as cables, coating and fibers, toughness is an important parameter to evaluate the properties and determine the applications of polymer/CNT nanocomposites. To date, many works have focused on the tensile and bending properties and neglected the toughness of polymer/CNT nanocomposites [48–50]. It is well-known that mechanical, electrical and rheology behaviors depend on the structure of polymer/CNT nanocomposites. New struc­ ture is an indicator of rheological and conductive thresholds of poly­ mer/CNT nanocomposites. The relationships between rheology and electrical conductivity have been extensively investigated. However, correlations between mechanical properties and rheological behaviors of polymer/CNT nanocomposites have never been encountered. Furthermore, determination of the optimal compositions for poly­ mer/CNT nanocomposites depends usually on many orthogonal exper­ imental design and statistical analysis by varying various complicated factors. It will be beneficial to establish a simple and novel experimental approach for fast and precise determination of the optimal compositions of a polymer/CNT nanocomposite. This work developed a novel dy­ namic rheological method, which could precisely determine the super-toughness point and agilely predict the conductive percolation region by correlating with the dynamic rheological data. With this rheological method, a correlation between mechanical properties and rheological behaviors was established for compatibilized

HDPE/MWCNT nanocomposites. 2. Experimental 2.1. Materials HDPE (5000S, melting flow rate is 0.9 g/10 min at 190 � C and 2.16 kg) is provided by Petro China Co., Ltd., China. Maleated HDPE (HDPEg-MAH, CMG5804) and Maleated LLDPE (LLDPE-g-MAH, CMG5904) are the product of Nantong Sunny Polymer New Material Technology Co., Ltd. The purpose using two compatibilzers is to evaluate the uni­ versality of experiment method. OH-functionalized multi-walled carbon nanotubes (TNSMH3, 10–20 nm diameters, –OH content is 3.06 wt%) are a product of Chinese Academy of Sciences Chengdu Organic Chemical Co., Ltd. The antioxidant (B215) is produced by Ciba-Geigy Co., Switzerland. 2.2. Preparation of polymer nanocomposites HDPE/MWCNT, HDPE/HDPE-g-MAH/MWCNT and HDPE/LLDPE-gMAH/MWCNT nanocomposites were mixed in a torque rheometer (LB100, Shanghai S.R.D. Scientific Instrumental Co., Ltd, China) at 180 � C and 50 rpm for 6 min. Both the concentrations of HDPE-g-MAH and LLDPE-g-MAH are 5 wt%. The rheological samples were compression molded into disks of 25 mm in diameter and 1.2 mm in thickness. The samples for conductive measurements were compression molded into disks of 5 cm in diameter and 2 mm in thickness. The samples for me­ chanical measurements were compression molded into rectangular plates of 8 cm in length, 1 cm in width and 2.5 mm in thickness. All the samples were compressed at 180 � C and 5 MPa. 2.3. Characterizations Notched Izod impact strength was tested on a PTM1251-B pendulum impact tester (Shenzhen Suns Technology Stock Co., Ltd, China) with an impacting rate of 3.5 m/s following Chinese Standard GB/T 1843–2008. An average value was obtained with at least five measurements for each sample. Melt rheology measurements were conducted on a Bohlin Gemini2 Rheometer (Malvern Panalytical Ltd, England) in parallel plates with oscillatory mode at 195 � C. Dynamic frequency sweep tests were per­ formed from 0.025 to 100 rad/s,the strain amplitude was maintained at 1% in order to ensure that the rheological behavior is located in the linear viscoelastic region. The chemical structures of nanocomposites were analyzed with Fourier transform infrared spectrometer (FTIR, NICOLET iS50, USA). The films were obtained through compressing method. Thermogravi­ metric experiments were performed using an analyzer (TG, TA MDSC2910, USA) in nitrogen atmosphere at a heating rate of 10 � C/min. The crystalline structure was evaluated by wide angle X-ray diffraction (WXRD, DX-2700BH, China) using Cu-Kα radiation operated at 40 KV and 100 mA. The data were collected from 5 to 80� at a scanning rate of 5� /min. Raman spectroscopy was obtained by a Thermal Scientific DXRxi (Thermo Fisher Scientific Inc., USA) with an excitation wave­ length of 532 nm. The volume resistivity was measured on a ZC-90G high resistance meter and the samples and electrodes were cleaned with ethanol prior to measurements. Morphologies of the nano­ composites were observed by scanning electron microscopy (SEM, JEOL JSM-7500F, Japan), atomic force microscopy (AFM, Ruker Technology Ltd, USA) and transmission electron microscope (TEM, Fei tecnai f20, USA). The samples were fractured in liquid N2 and coated with a conductive gold layer before SEM analysis. Ultra-thin slice machine was used to cut AFM and TEM samples under ultralow temperature conditions. 2

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3. Results and discussion

Fig. 2. Nevertheless, the corresponding increase in G’ is much higher than that in G00 at a certain nanotube concentration, indicating that G" is insensitive to the interfacial energy or compatibility between polymer and nanofillers [52]. In rheology, the emergence of plateau is considered to be responsible for the existence of ordered structures, such as ag­ glomerations, skeletons, and networks. For polymer/CNT nano­ €tschke et al. found that the plateau can be attributed to composites, Po temporary polymer–polymer, nanotube–nanotube and combined poly­ mer–nanotube networks [27].

3.1. Gelation behaviors of HDPE/MWCNT nanocomposites Dynamic frequency sweep test under small strain amplitude is an effective method to evaluate the gelation behaviors and structure of polymer nanocomposites. Fig. 1 shows G0 varied with frequency (ω) for HDPE/MWCNT, HDPE/HDPE-g-MAH/MWCNT and HDPE/LLDPE-gMAH/MWCNT nanocomposites. The virgin HDPE seems to exhibit typical liquid-like viscoelastic behavior and the slope of G0 curve is near 2 at low ω region. The slopes of G0 curves for three nanocomposites reduce gradually with MWCNTs concentration increasing. When MWCNTs content is higher than 4 wt%, G0 of three nanocomposites show little dependence on ω at terminal region and exhibit solid-like viscoelastic behaviors, which is a symbol of gel plateau corresponding to the results of Nijenhuis and Winter [51]. Therefore, when the slopes of G0 curves reach certain values, the transitions from liquid to solid emerge, and the corresponding gel points of three nanocomposites should be in the range of 1 wt% to 4 wt% MWCNTs concentrations. Although HDPE/MWCNT nanocomposites with compatibilizers show similar transition to that without compatibilizers, the transition points of HDPE/MWCNT nanocomposites with compatibilizer shift to the low MWCNTs loading, their G0 curves at the high frequency region show a little dependence on the MWCNTs content because of the good compatibility between HDPE and MWCNTs. HDPE/HDPE-g-MAH/MWCNT nanocomposite with 1 wt% MWCNTs exhibits similar rheological behaviors to neat HDPE, while HDPE/LLDPE-g-MAH/MWCNT nanocomposite with 1 wt% MWCNTs deviates obviously from neat HDPE. However, the other obvious dif­ ferences of G’ curves for HDPE/MWCNT nanocomposites with HDPE-g-MAH and LLDPE-g-MAH cannot be detected. Compared with G0 curves, a similar trend of G00 curves is observed in

3.2. Determination of gel points It is well-known that when the largest molecular clusters diverge to infinity, polymers will reach chemical gel points, and when polymers lose their chain mobility and have sharp transition from liquid-like to solid-like behavior, they will reach physical gel points. These critical points may be depended on time, temperature, or concentration of fillers [53–55]. Winter and his coworkers provide a method to evaluate the gel point based on the principle that tan δ becomes independent of fre­ quency at the gel point [56,57], Eq. (1) can be obtained as tan δ ¼ G” / G’ ¼ tan (nπ/2)

(1)

where the gel index n can evaluate the difference between the visco­ elastic system and viscous liquid or elastic solid in rheology. If an elastic network is formed in the gel point, n is impossibly equal to 1. Fig. 3 presents tan δ varied with MWCNTs content at different ω for HDPE/MWCNT nanocomposites. Three nanocomposites exhibit the obvious gelation behaviors, different ω curves of tan δ intersect at a constant concentration of MWCNTs, and where tan δ shows no depen­ dence on ω. Therefore, the gel points of three HDPE/MWCNT nano­ composites can be obtained and φ of MWCNTs corresponding to the gel

Fig. 1. Relationships between G0 and ω for three nanocomposites. (a) HDPE/MWCNT nanocomposites; (b) HDPE/HDPE-g-MAH/MWCNT nanocomposites; (c) HDPE/LLDPE-g-MAH/MWCNT nanocomposites. 3

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Fig. 2. Relationships between G00 and ω for three nanocomposites. (a) HDPE/MWCNT nanocomposites; (b) HDPE/HDPE-g-MAH/MWCNT nanocomposites; (c) HDPE/LLDPE-g-MAH/MWCNT nanocomposites.

crystallization, and chain entanglement. As for HDPE/MWCNT nano­ composites with compatibilizer, the strong interactions between hy­ droxyl groups of MWCNTs and maleic anhydride groups of compatibilizer improve the compatibilization and entanglement of HDPE and MWCNTs. Thus, the formation of gels for these nano­ composites would be due to the dynamic entanglement networks, which is consistent with the results of morphological study discussed below.

point are 4.23, 2.75 and 1.95, respectively. Because of weak interaction between HDPE and MWCNTs, the gel formation of HDPE/MWCNTs nanocomposite depends mainly on agglomeration of MWCNTs and entanglement of HDPE matrix, and requires more MWCNTs to involve in the construction of gel network. Introduction of 5 wt% HDPE-g-MAH and LLDPE-g-MAH can improve effectively interactions between HDPE and MWCNTs and stimulate the uniform distribution of MWNCTs. Therefore, it is easy to build perfect percolated structure of HDPE/ MWCNT nanocomposites with two compatibilizers and their gel points shift to the low concentration of MWCNTs. These results are consistent with the previous study [50]. On the other hand, φ corresponding to the gel point of nanocomposite with HDPE-g-MAH is higher than that with LLDPE-g-MAH, which demonstrates that LLDPE-g-MAH have more effective compatibility than HDPE-g-MAH and few LLDPE-g-MAH can induce the transition from liquid to solid. Table 1 presents the gel points and corresponding tan δ and n of three nanocomposites. Many researches illustrate that n is different for different gel systems. n values for physical gels are large and do not vary with the concentration of components, but n values for chemical gels are small and decrease with the increase of crosslinking degree, which are coincided with the results of theoretical predictions [58,59]. The rela­ tively high n values demonstrate that the gels of three nanocomposites are physical gel. Tan δ and n increase greatly with the addition of HDPE-g-MAH or LLDPE-g-MAH, implying that compatibilizers can induce the formation of physical gels for HDPE/MWCNT nano­ composites at low MWCNTs loadings. High n value of the nano­ composite with LLDPE-g-MAH is also a sign of perfect physical gel structure and LLDPE-g-MAH exhibits better compatibilization than HDPE-g-MAH. Previously, lots of researches have been focused on the formation and structure characterization of gels, but the mechanism of gels formation is still not well understood. The gel formation could be attributed to the emergence of crosslinking, phase separation,

3.3. Evaluation on the interfacial interaction The interfacial interaction between HDPE matrix and MWCNTs was investigated by Raman spectroscopy. Fig. 4 represents Raman spectra of HDPE, HDPE/MWCNT nanocomposite, HDPE/HDPE-g-MAH/MWCNT nanocomposite, HDPE/LLDPE-g-MAH/MWCNT nanocomposite and MWCNTs. The spectrum of HDPE exhibits typical Raman modes of PE at 1059, 1125, 1289 and 1429 cm 1. The spectrum of MWCNTs shows peaks at 1338 cm 1 (D band, induced by defects and disorder) and 1570 cm 1 (G band, corresponding to the in-plane vibration of C–C bonds). Results show that D and G band peaks of three nanocomposites shift remarkably to higher wavenumbers. The maximum in the G band peak for HDPE/LLDPE-g-MAH/MWCNT nanocomposite is up-shifted by 13 cm 1. The shifts are more pronounced for the nanocomposites with compatibilizers and LLDPE-g-MAH presents more effective compati­ bility than HDPE-g-MAH. Similar up-shifting of G band peak has been observed in other polymer/CNT nanocomposites [60]. The up-shifting of D and G bands is also a consequence of strong compressive forces associated with HDPE chains on MWCNTs [61]. 3.4. Mechanical and conductivity characterization Based on the above-discussions of rheological data and related studies for other nanocomposites reported, some new morphologies 4

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Fig. 3. Relationships between tan δ and φ of MWCNTs for three nanocomposites. (a) HDPE/MWCNT nanocomposites; (b) HDPE/HDPE-g-MAH/MWCNT nano­ composites; (c) HDPE/LLDPE-g-MAH/MWCNT nanocomposites. Table 1 Gel points and corresponding tan δ and n values of three HDPE/MWNCT nanocomposites. Sample

HDPE/MWCNT nanocomposites

HDPE/HDPE-g-MAH/ MWCNT nanocomposites

HDPE/LLDPE-g-MAH/ MWCNT nanocomposites

ϕ (%) tan δ n

4.23 � 0.05 1.49 � 0.06 0.62

2.75 � 0.02 2.32 � 0.04 0.74

1.95 � 0.02 2.49 � 0.01 0.76

Fig. 5. Relationships nanocomposites.

between

NIIS

and MWCNTs

content

of

three

could be formed at the gel points. Mechanical and conductive properties of polymer nanocomposites depend greatly on their morphologies. Therefore, it is valuable to investigate whether the emergence of gel point has particular relationship with mechanical, conductivity prop­ erties and morphologies. HDPE/MWCNT nanocomposites at the gel points were prepared using the same method and experimental condi­ tions in order to obtain more insights about the mechanical, conduc­ tivity properties and morphologies at the gel points. Figs. 5 and 6 show the notched Izod impact strength (NIIS) and volume resistivity of three HDPE/MWCNT nanocomposites varied with MWCNTs content. NIIS of HDPE/MWCNT nanocomposites without

Fig. 4. Raman spectra of MWCNT (a), HDPE/MWCNT nanocomposites (b), HDPE/LLDPE-g-MAH/MWCNT nanocomposites (c), HDPE/HDPE-g-MAH/ MWCNT nanocomposites (d) and HDPE (e).

5

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Fig. 6. Relationships between the volume resistivity and MWCNT content of three nanocomposites.

Fig. 7. WXRD nanocomposites.

compatibilizer has a peak at 1 wt % MWCNTs where the value of NIIS is two times higher than that of neat HDPE. However, NIIS of this nano­ composite at the gel point (labelled with a red solid mark) is low and follows the general rules of polymer filled with inorganic particles. The poor dispersion and agglomerations of MWCNTs induce a little crazing or shear bands of HDPE matrix, dissipate a little energy and decrease the toughness of HDPE matrix. The volume resistivity of HDPE/MWCNT nanocomposite with 1 wt% MWCNTs decreases by 1 order of magnitude, indicating that the distances between MWCNTs aggregates are not so close that electrons can jump from one conductive filler to another and the perfect conductive network has not been formed. As for the nano­ composites at the gel points (labelled with a red solid mark), the conductive network has been built through filling with more MWCNTs, the volume resistivity decreases by 9 orders of magnitude and attains the conductive threshold. Therefore, it can be inferred that the network structure of HDPE/MWCNT nanocomposite at the gel point is an important reason for the good conductivity. In order to improve the poor dispersion of MWCNTs, 5 wt% HDPE-gMAH and LLDPE-g-MAH were added into HDPE/MWCNT nano­ composites to enhance the compatibility between MWCNTs and HDPE matrix. In Fig. 5, NIIS of two nanocomposites with compatibilizers show peaks at the gel points and the values are about three times higher than that of neat HDPE. Because of the similar structure and properties of two compatibilizers, these two nanocomposites have almost equal NIIS. MWCNTs content of nanocomposite with LLDEP-g-MAH at the gel point is lower than that with HDPE-g-MAH, indicating that LLDPE-g-MAH is more effective compatibility than HDPE-g-MAH. LLDPE exhibits good flexibility and can absorb some impacting energy through deformation except the function of compatibility. Compared with nanocomposites without compatibilizer, the addition of compatibilizer can induce the gel point shift to the low MWCNTs loadings and improve the toughness of HDPE greatly. Therefore, it can be concluded that the super-toughness points of HDPE/MWCNT nanocomposites with compatibilizer are cor­ responding to the gel points acquired from the rheological data. These results suggest that dynamic rheology is an effective approach to determine the super-toughness points of HDPE/MWCNT nano­ composites with compatibilizer. And the applications of dynamic rheology tests could be beneficial to acquire the “super toughness” composition of polymer/CNT nanocomposites through a simple composition design, a set of parallel experiments, and with small amounts of samples. As for the conductive properties, HDPE/MWCNT nanocomposites with compatibilizers exhibit obvious conductive percolation behaviors. Curves of HDPE/MWCNT nanocomposites with and without HDPE-gMAH almost overlap each other, indicating that the addition of HDPEg-MAH has slight effect on conductive threshold of HDPE/MWCNT nanocomposites. However, HDPE/MWCNT nanocomposites with LLDPE-g-MAH show different conductive properties. Because of the effective compatibility, the volume resistivity of this nanocomposite is

diffraction

profiles

of

HDPE

and

HDPE/MWCNTS

lower than those of other two nanocomposites when the content of MWCNTs is less than 3%. 6 orders of magnitude decrease of volume resistivity was observed at the gel points (labelled with a red solid mark), and the gel points are located in the percolation regions. It is worth noting that the gel structure and formation mechanism of the nanocomposites with compatibililizers should be similar because of the close volume resistivity of the nanocomposites with HDPE-g-MAH and LLDPE-g-MAH at the gel point. Because of the same amount of two compatibilizers, the low MWCNTs concentration of nanocomposite with LLDPE-g-MAH at the gel point also testifies LLDPE-g-MAH is a more effective compatibilizer than HDPE-g-MAH for HDPE/MWCNTs nanocomposites. It is well-known that the structure of polymer nanocomposites determine the toughness, rheology and conductive properties. The impact strength tests evaluate the ability of transporting and dissipating energy of their structures under the impacting force, high NIIS depends on yielding of polymer matrix and strong interfacial tension between matrix and fillers. Dynamic rheology explores the response of their structure under the external cyclic loading, the viscoelastic behaviors also are determined by the deformation ability of polymer matrix and interfacial adhesion between matrix and fillers. However, the conduc­ tive property depends strongly on the distribution of fillers and inter­ action between polymer and fillers, and is no direct relationship with deformation ability of polymer matrix. Therefore, the notched Izod impact percolation of HDPE/MWCNT nanocomposites with compati­ bilizer is corresponding to the rheological percolation and has no close relationship with the conductive percolation. The following morpho­ logical evidences confirm this analysis. 3.5. WXRD characterization As shown in Fig. 7, there are two obvious diffraction peaks located at

Fig. 8. FTIR spectra of pure HDPE and HDPE/MWCNT nanocomposites. 6

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in FTIR spectra of HDPE/MWCNT nanocomposites. All TG curves in Fig. 9 show typical one-step degradation mecha­ nism. The thermal stability of HDPE/MWCNTs nanocomposites are better than that of HDPE, suggesting that MWCNTs can increase obvi­ ously the thermal stability of HDPE matrix. TG curves of HDPE/MWCNT nanocomposites have a little difference because of the close content of MWCNTs and compatibilizers hardly affect the thermal stability of HDPE/MWCNTs nanocomposites. 3.7. Morphology characterization The structures of polymer nanocomposites are mainly controlled by the properties of polymer matrix, fillers and their interactions. The rheological behaviors, mechanical and conductive properties of nano­ composites are affected highly by their structures. Fig. 10 illustrates SEM images of the cryo-fracture surfaces of HDPE/MWCNT nanocomposites. MWCNTs have been marked highlightedly with red dash circles. The nanocomposite with 1 wt% MWCNTs shows good distribution of MWCNTs. Although there are small amounts of MWCNTs aggregates, their sizes and quantities are too small to give little influence on the properties of this nanocomposite. MWCNTs with uniform dispersion can transfer impacting stress and induce HDPE matrix crazing and yielding. Crazing and yielding can absorb energy and improve the toughness of HDPE matrix. The average distances estimated between MWCNTs are about 1 μm. Good dispersion and decent distance could be the evidence for high toughness of this nanocomposite. On the other hand, the long average distances between MWCNTs determine high volume resistivity of this nanocomposite. With MWCNTs content increasing, the sizes of MWCNTs aggregates become large and their distributions become nonuniform in Fig. 10b and d, the toughness of these nanocomposites decreases sharply, while the volume resistivity of these nanocomposite increases notably because of the close distances between MWCNTs. However, a clear network can be observed for the nanocomposite at the gel point as shown in Fig. 10c and this morphology is special and different with other nanocomposites.

Fig. 9. TG curves of HDPE and HDPE/MWCNT nanocomposites.

approximately 2θ ¼ 22.0 and 24.3 for neat HDPE, corresponding to the typical orthorhombic unit cell structure of the (110) and (200) crystal planes, respectively. Some high diffraction peaks around 30 and 36 are corresponding to the (210) and (020) crystal planes of HDPE. These characteristic crystal peaks change a little and no new diffraction peaks form for HDPE/MWCNTs nanocomposites with and without compati­ bilizers, indicating MWCNTs do not change the original crystal structure of HDPE matrix. No effect of inorganic particles on the crystal structure of HDPE has been reported in many literatures because of the simplest polymer chain and stable orthorhombic unit cell of HDPE. However, it is worth noting that 2θ values corresponding to four diffraction peaks of nanocomposites increase slightly especial for HDPE/LLDPE-g-MAH/ MWCNTs nanocomposites, demonstrating that MWCNTs can affect the crystal sizes and degrees of HDPE. 3.6. FTIR and TG characterization Fig. 8 presents FTIR spectra of HDPE and HDPE/MWCNT nano­ composites. For neat HDPE, the strong absorption bands attributed to the stretching vibration, bending vibration and rocking vibration of C–H bond (CH2) can be observed at 2915, 2847, 1472, and 720 cm 1, respectively. For MWCNTs reinforced HDPE nanocomposites, compared with HDPE, the same absorption bands without any variation in position are observed. Because the concentration of MWCNTs is low, although addition of compatibilizers can improve the compatibility between HDPE and MWCNTs and increase their interaction, it cannot be detected

Fig. 10. SEM photographs ( � 10000) of the cryo-fracture surfaces of HDPE/ MWCNT nanocomposites with different content of MWCNT. (a) 1 wt%; (b) 3 wt %; (c) 4.2 wt%; (d) 5 wt%.

Fig. 11. SEM and TEM photographs of HDPE/HDPE-g-MAH/MWCNT nano­ composites with different content of MWCNT. (a) 1 wt%; (b) 2 wt%; (c) 2.75 wt %; (d) 5 wt%; (e) 2 wt%; (f) 2.75 wt%. 7

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toughness of this nanocomposite decrease sharply. TEM images are more useful to evaluate the CNT dispersion. Figs. 11e and f show TEM images of HDPE/HDPE-g-MAH/MWCNTs with 2 wt% and 2.75 wt%. It is clear that the distributions of MWCNTs of nanocomposite at the gel point are more uniform than the nanocomposite with 2 wt% MWCNTs and induce the formation of perfect gel network. The slight aggregation of MWCNTs can be seen in the image of nanocomposite with 2 wt% MWCNTs. These results conform the discussions of SEM images in Fig. 11b. In order to confirm the structure of nanocomposite at gel point, AFM measurements were employed. As illustrated in Fig. 12, MWCNTs have good distributions for three nanocomposites at gel points. Combined with SEM photographs, MWCNTs of nanocomposites at the gel points with compatibilizer are coated by compatibilizer and build up a perfect network with HDPE matrix together. Because the additional energy is required for the destruction of the network structure, the nanocomposite at the gel point show a maximum value in NIIS curves. NIIS of HDPE/ MWCNTs nanocomposite at the gel point is low because the gel network has many defects which origin from weak interaction between HDPE and MWCNTs. In order to evaluate deeply the effect of compatibilizer on the mor­ phologies of HDPE/MWCNT nanocomposites, SEM images with high magnification are shown in Fig. 13. The diameters of MWNCTs have been marked. MWCNTs diameters of HDPE/MWCNT nanocomposite in Fig. 13a are about 25–35 nm, while those of the nanocomposites with compatibilizers increase obviously and are about 50–60 nm in Fig. 13b. This increase is likely due to wrapping of MWCNTs by HDPE-g-MAH or LLDPE-g-MAH with the contribution of hydrogen bonding, and thus, a thick coating onto the nanotube surface is formed [62]. These coatings are the evidence of effective compatibility and have influence on the conductivity and toughness of nanocomposites. The morphological ev­ idence of nanocomposites further testifies that the super-toughness points of HDPE/MWCNT nanocomposite with compatibilizers are correlated with the gel points acquired from the rheological data and the gel points are located in the electrical percolation regions. All these results demonstrate that dynamic rheology is an effective way to determine the super-toughness points.

Fig. 12. AFM photographs of three nanocomposites at the gel points. (a) HDPE/MWCNT nanocomposite; (b) HDPE/HDPE-g-MAH/MWCNT nano­ composite; (c) HDPE/LLDPE-g-MAH/MWCNT nanocomposite.

These results indicate that the formation of network is a sign of rheo­ logical percolation and consistent with the definition of gel point. Furthermore, some MWCNTs labelled with red dash circles do not involve in the building of network. The defects of network and weak interactions can be attributed to the low toughness of nanocomposite at the gel point. The joints of carbon nanotube observed in Fig. 10c testify the emergence of conductive threshold. Fig. 11 shows SEM and TEM photographs of HDPE/HDPE-g-MAH/ MWCNT nanocomposites with different content of MWCNT. Compared with HDPE/MWCNT nanocomposites, the distributions of MWCNTs are more uniform and the sizes of aggregates become smaller in Fig. 11a and b. When the samples have been impacted, there are more points to induce the matrix to yield or fabricate craze and absorb more energy. Therefore, the nanocomposites with HDPE-g-MAH exhibit high tough­ ness. As for the nanocomposite at the gel point, the perfect network has been built through the strong interaction between HDPE and MWCNTs and isolated carbon nanotube was hardly observed on the surface of cryo-fracture in Fig. 11c. The nanocomposite with 5 wt% MWCNTs shows coarse and discontinuous network and 5 wt% HDPE-g-MAH cannot undertake the effective dispersion of MWCNTs. Thus, the

3.8. Understanding of toughness mechanism For fiber-reinforced polymer composites, polymer matrix could be reinforced through several energy consuming ways, such as fiber

Fig. 13. SEM photographs ( � 100000) of the cryo-fracture surfaces for three nanocomposites with 1 wt% MWCNTs: (a) HDPE/MWCNT nanocomposite; (b) HDPE/HDPE-g-MAH/MWCNT nanocomposite; (c) HDPE/LLDPE-g-MAH/ MWCNT nanocomposite.

Fig. 14. SEM photographs ( � 50000) of the cryo-fracture surfaces for three nanocomposites with 1 wt% MWCNTs. (a) HDPE/MWCNT nanocomposite; (b) HDPE/HDPE-g-MAH/MWCNT nanocomposite; (c) HDPE/LLDPE-g-MAH/ MWCNT nanocomposite. 8

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aspect ratio of nanotubes would cause mixed matrix-filler interactions during nanotube bridging, breaking and pull-out, which promote the local plastic deformation of matrix [64,65]. Fig. 15 shows the morphologies of three nanocomposites at and near the gel point. Compared with morphologies at the gel point (Fig. 15a), a lot of aggregates can be observed in HDPE/MWCNT nanocomposite with 5 wt% MWCNTs (Fig. 15b), bulky clusters can be found in HDPE/HDPEg-MAH/MWCNT nanocomposite with 4 wt% MWCNTs (Fig. 15d) and HDPE/LLDPE-g-MAH/MWCNT nanocomposite with 3 wt% MWCNTs (Fig. 15f). The aggregates and clusters can induce stress concentration and weaken the toughness of nanocomposites. At the gel point, network structures are built for three nanocomposites (Fig. 15a, c and e) and more elaborate and orderly for nanocomposites with compatibilizers. Many knobs appear in the top of the network, which is assumed to be formed because of springback of nanotube-HDPE structure during breaking process. Hence, the applied load can be successfully trans­ mitted to the network through these knobs. The reinforcement and toughening of HDPE/MWCNT nano­ composites can be attributed to pull-out, elastic deformation of matrix and fracture of the MWCNTs [66]. According to the results of Ajayan [64], nanotube and matrix were pulled out and the fracture surfaces transformed to 3D network structure during the deformation and frac­ ture of the nanocomposites. As shown in Fig. 16, because HDPE lose their high elasticity in extremely low temperature at about 196 � C, HDPE matrix rupture during pulling-out process and form “meshes”. The impacting energy can be successfully transmitted to the network through these knobs. Compared with the MWCNTs pull-out shown in Fig. 14b, the whole network is fully pulled out and destroyed at the gel points, which can absorb more energy. These results indicate that the mechanism of super-toughness at the gel point origins from the pull-out, elastic deformation and destruction of MWCNTs, the destruction of special gel network, and the extensive tearing of matrix.

Fig. 15. SEM photographs ( � 20000) of the brittle fracture surface of three nanocomposites. (a) HDPE/MWCNT nanocomposite at the gel point; (b) HDPE/ MWCNT nanocomposite with 5 wt% MWCNTs; (c) HDPE/HDPE-g-MAH/ MWCNT nanocomposite at the gel point; (d) HDPE/HDPE-g-MAH/MWCNT nanocomposite with 4 wt% MWCNTs; (e) HDPE/LLDPE-g-MAH/MWCNT nanocomposite at the gel point; (f) HDPE/LLDPE-g-MAH/MWCNT nano­ composite with 3 wt% MWCNTs.

4. Conclusion A dynamic rheological approach was developed to precisely deter­ mine the super-toughness point by correlating with the dynamic rheo­ logical data. The gel points acquired from the dynamic rheological data show strong relationships with toughness of HDPE/MWCNT nano­ composites with compatibilizer. The super-toughness points of HDPE/ MWCNT nanocomposite with compatibilizer could be determined by the dynamic rheological tests through simple composition designs with a small amount of samples. The gel point of HDPE/MWCNT nano­ composites without compatibilizer is correlated to its conductive threshold and shows no relationship with its toughness. The addition of MWCNTs can improve the thermal ability of HDPE matrix and has a little influence on its crystal structure. LLDPE-g-MAH exhibits better compatibilization than HDPE-g-MAH. Furthermore, the surfaces of HDPE/MWCNT nanocomposites with compatibilizers at the gel point show unique network morphologies built by MWCNTs coated by com­ patibilizer and HDPE. The mechanism of super-toughness at the gel point origins from the pull-out, elastic deformation and destruction of MWCNTs, the destruction of special gel network, and the extensive tearing of matrix. These results provide a novel approach and valuable insights for composition design and property control of polyolefin/ MWCNT nanocomposites.

Fig. 16. Schematic diagram for toughness mechanism of HDPE/MWCNT nanocomposites with compatibilizer at the gel points.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

fracture, fiber pull-out and fiber bridging [63]. Fig. 14 shows the detailed morphologies for cryo-fracture surfaces of three nano­ composites. MWCNTs can be seen as short fibers, the reinforced mech­ anism can be used for polymer/MWCNTs nanocomposite. Nanotube fracture (marked as A in Fig. 14a), pull-out (marked as B in Fig. 14b) and bridging (marked as C in Fig. 14c) can absorb much energy and the large

CRediT authorship contribution statement Yifei Wang: Methodology, Investigation, Data curation, Writing 9

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Polymer Testing 81 (2020) 106280

original draft. Fucheng Lv: Methodology, Data curation. Yao Song: Data curation, Investigation. Yanyu Yang: Formal analysis, Validation, Funding acquisition. Yanxia Cao: Writing - review & editing. Jianfeng Wang: Data curation, Validation. Chenlin Li: Writing - review & edit­ ing, Data curation. Wanjie Wang: Conceptualization, Formal analysis, Writing - review & editing, Supervision, Funding acquisition.

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